WI-LAN Inc. v. Alcatel-Lucent USA Inc. et al

Filing 167

OPENING CLAIM CONSTRUCTION BRIEF filed by WI-LAN Inc.. (Attachments: # 1 Affidavit DECLARATION OF JEFFREY T. HAN IN SUPPORT OF WI-LANS OPENING CLAIM CONSTRUCTION BRIEF, # 2 Exhibit A-U.S. Patent No. 6,088,326, # 3 Exhibit B-U.S. Patent No. 6,195,327, # 4 Exhibit C-U.S. Patent No. 6,222,819, # 5 Exhibit D-U.S. Patent No. 6,381,211, # 6 Exhibit E-copy of The IEEE Standard Dictionary of Electrical and Electronics Terms (6th ed. 1996), # 7 Exhibit F-copy of Alan Freedman, The ComputerGlossary (7th ed. 1995), # 8 Exhibit G-copy of Harry Newton, Newtons Telecom Dictionary (11th ed. 1996), # 9 Exhibit H-copy of Ramjee Prasad, CDMA for Wireless Personal Communications (1996), # 10 Exhibit I-copy of Theodore S. Rappaport,Wireless Communications (1996), # 11 Exhibit J-copy of Shing-Fong Su, The UMTS Air-Interface in RF Engineering (2007), # 12 Exhibit K-copy of 3GPP TS 25.211,v.6.10.0 (Release 6), # 13 Exhibit L-copy of Jean Conan & Rolando Oliver, Hardware and Software Implementation of the Viterbi Decoding Algorithm for Convolutional Codes, in MIMI 76: Proceedings of the International Symposium on Mini and Micro Computers (M.H. Hamza ed., 1977), # 14 Exhibit M-Definition of Overlay, OxfordDictionaries Online, http://oxforddictionaries.com/definition/overlay?q=overlay, # 15 Exhibit N-copy of the Manual of Patent Examining Procedure (6th ed. rev. 3, July 1997))(Weaver, David)

EXHIBIT C

11111111111111111111111111111I11111911111111111111111111111111
(12)

United States Patent

(to) Patent No.:
US
(45) Date of Patent:

Lysejko et al.

(54) PROCESSING DATA TRANSMITTED AND
RECEIVED OVER A WIRELESS LINK
CONNECTING A CENTRAL TERMINAL AND
A SUBSCRIBER TERMINAL OF A WIRELESS
TELECOMMUNICATIONS SYSTEM
(75) Inventors: Martin Lysejko, Bagshot (GB); Paul F.
Struhsaker, Plano, TX (US)
(73) Assignee: Airspan Networks, Inc., Seattle, WA
(US)
(*)

Notice:

Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 0 days.

(21) Appl. No.: 08/978,970
(22) Filed:
(30)

Nov. 26, 1997

Foreign Application Priority Data

Dec. 20, 1996

(GB)

9626566

(51) Int. C1.7
1104J 11/00
(52) U.S. Cl.
370/209; 370/342
(58) Field of Search
370/203, 208,
370/209, 320, 335, 342, 343, 441, 479;
375/130, 146, 147
(56)

References Cited
U.S. PATENT DOCUMENTS
5,373,502
12/1994 Turban
5,414,728
5/1995 Zehavi
5,764,630 * 6/1998 Natali et al.
5,793,759 * 8/1998 Rakib et al.
5,956,345 * 9/1999 Al'press et al.

370/18
375/200
370/320
370/342
370/480

FOREIGN PATENT DOCUMENTS
0633676
0652650
0730356

1/1995 (EP)
5/1995 (EP)
9/1996 (EP)

HO4J/13/00
HO4B/7/26
HO4L/1/00

2267627
2301744
9314588
9503652
9637066

12/1993
12/1996
7/1993
2/1995
11/1996

6,222,819 B1

(GB)
(GB)
(WO) .
(WO)
(WO)

HO4B/7/00
HO4Q/7/32
HO4B/7/26
HO4L/27/30

* cited by examiner
Primary Examiner Wellington Chin
Assistant Examiner—Kwang B. Yao
(74) Attorney, Agent, or Firm—Baker Botts L.L.P.

(57)

ABSTRACT

The present invention provides a transmission controller and
method for processing data items to be transmitted over a
wireless link connecting a central terminal and a subscriber
terminal of a wireless telecommunications system, a single
frequency channel being employed for transmitting data
items pertaining to a plurality of wireless links. The transmission controller comprises an orthogonal code generator
for providing an orthogonal code from a set of 'm' orthogonal codes used to create 'm' orthogonal channels within the
single frequency channel, and a first encoder for combining
a data item to be transmitted on the single frequency channel
with said orthogonal code from the orthogonal code
generator, the orthogonal code determining the orthogonal
channel over which the data item is transmitted, whereby
data items pertaining to different wireless links may be
transmitted simultaneously within different orthogonal
channels of said single frequency channel. Further, the
transmission controller comprises an overlay code generator
for providing an overlay code from a first set of 'n' overlay
codes which are orthogonal to each other, and a second
encoder arranged to apply the overlay code from the overlay
code generator to said data item, whereby 'n', data items
pertaining to different wireless links may be transmitted
simultaneously within the same orthogonal channel. The
invention also provides a reception controller and method
for processing data items received over a wireless link
32 Claims, 16 Drawing Sheets
113

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Sheet 12 of 16

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FIG. 19A

420

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US 6,222,819 B1
1

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PROCESSING DATA TRANSMITTED AND
RECEIVED OVER A WIRELESS LINK
CONNECTING A CENTRAL TERMINAL AND
A SUBSCRIBER TERMINAL OF A WIRELESS
TELECOMMUNICATIONS SYSTEM

communications system, and as it is desirable for neighbouring cells to use different frequency channels so as to
reduce interference, the demand cannot be met by merely
adding more modem shelves to each central terminal.
5

SUMMARY OF THE INVENTION

TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to wireless telecommunications systems and more particularly to techniques for processing data transmitted and received over a
wireless link connecting a central terminal and a subscriber
terminal of a wireless telecommunications system.

10

BACKGROUND OF THE INVENTION
A wireless telecommunications system has been proposed
in which a geographical area is divided in to cells, each cell
having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber
terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency
channel typically consisting of one frequency for uplink
signals from a subscriber terminal to the central terminal,
and another frequency for downlink signals from the central
terminal to the subscriber terminal.
Due to bandwidth constraints, it is not practical for each
individual subscriber terminal to have its own dedicated
frequency channel for communicating with the central terminal. Hence, techniques need to be applied to enable data
items relating to different wireless links to be passed over the
same frequency channel without interfering with each other.
In current wireless telecommunications systems, this can be
achieved through the use of a 'Code Division Multiple
Access' (CDMA) technique. One way to implement CDMA
is through the application of a set of orthogonal codes to the
data items to be transmitted on a particular frequency
channel, data items relating to different wireless links being
combined with different orthogonal codes from the set. A
suitable set of orthogonal codes is a "Rademacher-Walsh"
(RW) set of sixteen 16-bit codes. Orthogonal codes have the
property that, when perfectly aligned, all codes crosscorrelate to zero, thus making it possible to decode a signal
to which one orthogonal code has been applied while
cancelling interference from signals to which different
orthogonal codes have been applied.
Signals to which an orthogonal code has been applied can
be considered as being transmitted over a corresponding
orthogonal channel within a particular frequency channel.
Hence, considering the example of a set of 16 RW codes, 16
orthogonal channels can be created within a single frequency channel, and hence up to sixteen separate communication signals (corresponding to sixteen separate wireless
links) can be transmitted simultaneously over the single
frequency channel if different RW codes are applied to each
communication signal.
It is known to provide a number of modem shelves within
one central terminal, and for each modem shelf to employ a
different frequency channel. Hence, if a central terminal has
four modem shelves, and the set of 16 RW codes is
employed for each frequency channel, one central terminal
would be able to support wireless links with up to 60
subscriber terminals simultaneously.
However, as more subscribers subscribe to the wireless
telecommunications network, it is becoming desirable to
support more and more subscriber terminals from each
central terminal. There are only a limited number of frequency channels that can be allocated to the wireless tele-

15

20

25

30

35

40

45

50

55

60

65

According to the present invention, there is provided a
transmission controller for processing data items to be
transmitted over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, the transmission controller comprising: an orthogonal
code generator for providing an orthogonal code from a set
of 'm' orthogonal codes used to create 'm' orthogonal
channels within the single frequency channel; a first encoder
for combining a data item to be transmitted on the single
frequency channel with said orthogonal code from the
orthogonal code generator, the orthogonal code determining
the orthogonal channel over which the data item is
transmitted, whereby data items pertaining to different wireless links may be transmitted simultaneously within different
orthogonal channels of said single frequency channel; an
overlay code generator for providing an overlay code from
a first set of 'n' overlay codes which are orthogonal to each
other; and a second encoder arranged to apply the overlay
code from the overlay code generator to said data item,
whereby 'n' data items pertaining to different wireless links
may be transmitted simultaneously within the same orthogonal channel.
Viewed from a second aspect, the present invention
provides a reception controller for processing data items
received over a wireless link connecting a central terminal
and a subscriber terminal of a wireless telecommunications
system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, the receiver controller comprising: an orthogonal code
generator for providing an orthogonal code from a set of 'm'
orthogonal codes used to create 'm' orthogonal channels
within the single frequency channel; a first decoder for
applying, to signals received on the single frequency
channel, the orthogonal code provided by the orthogonal
code generator, in order to isolate data items transmitted
within the corresponding orthogonal channel; an overlay
code generator for providing an overlay code from a first set
of 'n' overlay codes which are orthogonal to each other, the
set of 'n' overlay codes enabling 'n' data items pertaining to
different wireless links to be transmitted simultaneously
within the same orthogonal channel; and a second decoder
for applying, to the data items of the orthogonal channel, the
overlay code from the overlay code generator so as to isolate
a particular data item transmitted using that overlay code.
By using overlay codes in addition to the known set of
orthogonal codes, it is possible for selected orthogonal
channels to be subdivided to form additional orthogonal
channels. For example, if there are originally sixteen
orthogonal channels and a set of four overlay codes are
defined, each orthogonal channel being subject to overlay
codes, then up to 64 orthogonal channels can be defined. By
application of appropriate orthogonal codes and overlay
codes, up to 64 separate communication signals could be
sent simultaneously on the one frequency channel, albeit at
a quarter of the rate that the communication signals could be
transmitted if the overlay codes were not used.
Such an approach has the advantage that it preserves
compatibility with current hardware and software equipment

US 6,222,819 B1
3

4

which use the set of orthogonal codes, but which do not
reception controller in accordance with the present invensupport the use of overlay codes. By designating certain
tion. Further, the central terminal preferably includes chanorthogonal channels as channels for which overlay codes are
nelisation means for determining which of the orthogonal
not used, the current equipment can communicate over those
channels will be subject to overlay codes, and for transmitchannels without any changes being required to the equip- 5 ting that information to a plurality of subscriber terminals
ment.
within the telecommunications system. This is useful since,
In preferred embodiments, the overlay code generator is
for example, certain orthogonal channels can hence be
arranged to store one or more further sets of overlay codes
designated as being reserved for communications with STs
having different numbers of overlay codes to the first set of
that do not incorporate the features necessary to support
overlay codes. This enables the orthogonal channels to be 10 overlay codes, and which hence require a full 160 kb/s
subdivided differently, depending on which set of overlay
orthogonal channel.
codes is selected. For instance, if an orthogonal channel
In preferred embodiments, the channelisation means also
operates at 160 kb/s, and a set of four overlay codes is used
determines, for those orthogonal channels subject to overlay
to subdivide that orthogonal channel, then four 40 kb/s
codes, which set of overlay codes will apply to each
orthogonal channels can be created from the one original
orthogonal channel. If, alternatively, a set of two overlay 15 orthogonal channel. This gives a great deal of flexibility in
how the channels are used, since some can be subdivided
codes is used, then two 80 kb/s orthogonal channels can be
whilst others are not, and those which are subdivided can be
created from the one orthogonal channel. This flexibility is
subdivided differently to yield differing numbers of differing
useful, since for some communications, eg. fax, a rate of 40
rate channels.
kb/s may not be acceptable, and hence a set of four overlay
20
codes would not be suitable.
As with the central terminal, a subscriber terminal of the
wireless telecommunications system may comprise a transThe orthogonal code generator and overlay code generamission controller and/or a reception controller in accortor may generate orthogonal codes and overlay codes 'on the
dance with the present invention. Unlike the central
fly' using predetermined algorithms. However, alternatively,
terminal, it is preferable for the subscriber terminal to use
the orthogonal code generator may be provided as a storage
arranged to store the set of orthogonal codes, and the overlay 25 overlay codes for all types of channels, whether they be
traffic channels or otherwise. On these uplink traffic
code generator may be provided as a storage arranged to
channels, the pure CDMA approach using overlay codes
store the set of overlay codes. Appropriate orthogonal codes
eliminates the need to time synchronise STs to a TDM frame
and overlay codes could then be read out to the encoders or
reference, and reduces the peak power handling requiredecoders as required.
3
ments in the ST RF transmit chain.
In preferred embodiments, the set of orthogonal codes
comprise a set of Rademacher-Walsh (RW) codes, in preViewed from a third aspect, the present invention provides
ferred embodiments the set comprising a 16x16 matrix of
a wireless telecommunications system comprising a central
RW codes. Further, the set of overlay codes are preferably
terminal and a plurality of subscriber terminals, wherein the
derived from RW codes, each set of 'n' overlay codes 35 central terminal comprises a transmission controller in
preferably comprising an nxn matrix of RW codes.
accordance with the present invention, and at least one of the
subscriber terminal comprises a reception controller in
The transmission controller in accordance with the
accordance with the present invention. Alternatively, or
present invention may be provided within the central termiadditionally, within the wireless telecommunications
nal of a wireless telecommunications system. In preferred
embodiments, a first of the orthogonal channels is reserved 40 system, at least one of the subscriber terminals may comprise a transmission controller in accordance with the
for the transmission of signals relating to the acquisition of
present invention, and the central terminal may comprise a
wireless links, and the transmission controller is provided in
reception controller in accordance with the present inventhe central terminal to enable overlay codes to be applied to
tion.
data items to be sent within said first orthogonal channel
from the central terminal to one of said subscriber terminals. 45
Viewed from a fourth aspect, the present invention proSimilarly, a second of the orthogonal channels is preferably
vides a method of processing data items to be transmitted
reserved for the transmission of signals relating to the
over a wireless link connecting a central terminal and a
control of calls, and the transmission controller in the central
subscriber terminal of a wireless telecommunications
terminal also enables overlay codes to be applied to data
system, a single frequency channel being employed for
items to be sent within said second orthogonal channel from so transmitting data items pertaining to a plurality of wireless
the central terminal to one of said subscriber terminals.
links, the method comprising the steps of: providing an
However, a number of said orthogonal channels are
orthogonal code from a set of 'm' orthogonal codes used to
designated as traffic channels for the transmission of data
create 'm' orthogonal channels within the single frequency
items relating to communication content, and in preferred
channel; combining a data item to be transmitted on the
embodiments a TDM encoder is provided within the central 55 single frequency channel with said orthogonal code, the
terminal arranged to apply time division multiplexing
orthogonal code determining the orthogonal channel over
(TDM) techniques to data items to be sent over a traffic
which the data item is transmitted, whereby data items
channel from said central terminal to said subscriber
pertaining to different wireless links may be transmitted
terminal, so as to enable a plurality of data items pertaining
simultaneously within different orthogonal channels of said
to different wireless links to be sent within one orthogonal 60 single frequency channel; providing an overlay code from a
traffic channel during a predetermined frame period.
first set of 'n' overlay codes which are orthogonal to each
other; and applying the overlay code to said data item,
The use of a CDMA/TDM hybrid approach for downlink
whereby 'n' data items pertaining to different wireless links
traffic channels retains the benefits of CDMA access, ie.
may be transmitted simultaneously within the same orthogointerference is reduced when traffic is reduced, and also
65 nal channel.
reduces receiver dynamic range requirements.
In addition to, or as an alternative to, having a transmisViewed from a fifth aspect, the present invention provides
sion controller, the central terminal may also comprise a
a method of processing data items received over a wireless

US 6,222,819 B1
5

6

link connecting a central terminal and a subscriber terminal
of a wireless telecommunications system, a single frequency
channel being employed for transmitting data items pertaining to a plurality of wireless links, the method comprising
the steps of: providing an orthogonal code from a set of 'm' 5
orthogonal codes used to create 'm' orthogonal channels
within the single frequency channel; applying, to signals
received on the single frequency channel, the orthogonal
code in order to isolate data items transmitted within the
corresponding orthogonal channel; providing an overlay 10
code from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling 'n'
data items pertaining to different wireless links to be transmitted simultaneously within the same orthogonal channel;
and applying, to the data items of the orthogonal channel, the 15
overlay code so as to isolate a particular data item transmitted using that overlay code.
By using overlay codes in addition to the known set of
orthogonal codes, it is possible for selected orthogonal
channels to be subdivided to form additional orthogonal 20
channels, thereby making it possible to support more wireless links on one frequency channel.
BRIEF DESCRIPTION OF THE INVENTION
An embodiment of the invention will be described
hereinafter, by way of example only, with reference to the
accompanying drawings in which like reference signs are
used for like features and in which:
FIG. 1 is a schematic overview of an example of a
wireless telecommunications system in which an example of
the present invention is included;
FIG. 2 is a schematic illustration of an example of a
subscriber terminal of the telecommunications system of
FIG. 1;
FIG. 3 is a schematic illustration of an example of a
central terminal of the telecommunications system of FIG.
1;
FIG. 3A is a schematic illustration of a modem shelf of a
central terminal of the telecommunications system of FIG.
1;
FIG. 4 is an illustration of an example of a frequency plan
for the telecommunications system of FIG. 1;
FIGS. 5A and 5B are schematic diagrams illustrating
possible configurations for cells for the telecommunications
system of FIG. 1;
FIG. 6 is a schematic diagram illustrating aspects of a
code division multiplex system for the telecommunications
system of FIG. 1;
FIGS. 7A and 7B are schematic diagrams illustrating
signal transmission processing stages for the telecommunications system of FIG. 1;
FIGS. 8A and 8B are schematic diagrams illustrating
signal reception processing stages for the telecommunications system of FIG. 1;
FIGS. 9A and 9B are diagrams illustrating the uplink and
downlink delivery methods when the system is fully loaded;
FIG. 10 illustrates the CDMA channel hierarchy in accordance with preferred embodiments of the present invention;
FIG. 11 is a schematic diagram illustrating downlink and
uplink communication paths for the wireless telecommunications system;
FIG. 12 is a schematic diagram illustrating the makeup of
a downlink signal transmitted by the central terminal;
FIGS. 13A and 13B illustrate the structure of the frames
of information sent over the downlink and uplink paths;

25

30

35

40

45

50

55

60

65

FIGS. 14A and 14B illustrate the overhead frame structure for the downlink and uplink paths;
FIGS. 15A and 15B illustrate typical downlink and uplink
channel structures that might occur in a loaded system in
accordance with preferred embodiments of the present
invention;
FIG. 16 illustrates how the available traffic channels are
classified in preferred embodiments of the present invention;
FIG. 17 illustrates the elements used by the central
terminal to perform interference limiting;
FIG. 18 illustrates possible antenna configurations that
can be employed in a wireless telecommunications system in
accordance with the preferred embodiment of the present
invention; and
FIGS. 19A and 19B illustrate how channel switching is
facilitated in preferred embodiments of the present invention.
DETAILED DESCRIPTION OF THE
INVENTION
FIG. 1 is a schematic overview of an example of a
wireless telecommunications system. The telecommunications system includes one or more service areas 12, 14 and
16, each of which is served by a respective central terminal
(CT) 10 which establishes a radio link with subscriber
terminals (ST) 20 within the area concerned. The area which
is covered by a central terminal 10 can vary. For example,
in a rural area with a low density of subscribers, a service
area 12 could cover an area with a radius of 15-20 Km. A
service area 14 in an urban environment where is there is a
high density of subscriber terminals 20 might only cover an
area with a radius of the order of 100 m. In a suburban area
with an intermediate density of subscriber terminals, a
service area 16 might cover an area with a radius of the order
of 1 Km. It will be appreciated that the area covered by a
particular central terminal 10 can be chosen to suit the local
requirements of expected or actual subscriber density, local
geographic considerations, etc, and is not limited to the
examples illustrated in FIG. 1. Moreover, the coverage need
not be, and typically will not be circular in extent due to
antenna design considerations, geographical factors, buildings and so on, which will affect the distribution of transmitted signals.
The central terminals 10 for respective service areas 12,
14, 16 can be connected to each other by means of links 13,
15 and 17 which interface, for example, with a public
switched telephone network (PSTN) 18. The links can
include conventional telecommunications technology using
copper wires, optical fibres, satellites, microwaves, etc.
The wireless telecommunications system of FIG. 1 is
based on providing fixed microwave links between subscriber terminals 20 at fixed locations within a service area
(e.g., 12, 14, 16) and the central terminal 10 for that service
area. Each subscriber terminal 20 can be provided with a
permanent fixed access link to its central terminal 10, but in
preferred embodiments demand-based access is provided, so
that the number of subscribers which can be supported
exceeds the number of available wireless links. The manner
in which demand-based access is implemented will be
discussed in detail later.
FIG. 2 illustrates an example of a configuration for a
subscriber terminal 20 for the telecommunications system of
FIG. 1. FIG. 2 includes a schematic representation of
customer premises 22. A customer radio unit (CRU) 24 is
mounted on the customer's premises. The customer radio

US 6,222,819 B1

7

8

unit 24 includes a flat panel antenna or the like 23. The
As an alternative to the RS232 connections 55, which
customer radio unit is mounted at a location on the customextend to a site controller 56, data connections such as an
er's premises, or on a mast, etc., and in an orientation such
X.25 links 57 (shown with dashed lines in FIG. 3) could
that the flat panel antenna 23 within the customer radio unit
instead be provided from a pad 228 to a switching node 60
24 faces in the direction 26 of the central terminal 10 for the 5 of an element manager (EM) 58. An element manager 58 can
service area in which the customer radio unit 24 is located.
support a number of distributed central terminals 10 connected by respective connections to the switching node 60.
The customer radio unit 24 is connected via a drop line 28
The element manager 58 enables a potentially large number
to a power supply unit (PSU) 30 within the customer's
premises. The power supply unit 30 is connected to the local
(e.g., up to, or more than 1000) of central terminals 10 to be
power supply for providing power to the customer radio unit 10 integrated into a management network. The element man24 and a network terminal unit (NTU) 32. The customer
ager 58 is based around a powerful workstation 62 and can
radio unit 24 is also connected via the power supply unit 30
include a number of computer terminals 64 for network
to the network terminal unit 32, which in turn is connected
engineers and control personnel.
to telecommunications equipment in the customer's
FIG. 3A illustrates various parts of a modem shelf 46. A
premises, for example to one or more telephones 34, fac- is transmit/receive RF unit (RFU—for example implemented
simile machines 36 and computers 38. The telecommunicaon a card in the modem shelf) 66 generates the modulated
tions equipment is represented as being within a single
transmit RF signals at medium power levels and recovers
customer's premises. However, this need not be the case, as
and amplifies the baseband RF signals for the subscriber
the subscriber terminal 20 preferably supports either a single
terminals. The RF unit 66 is connected to an analogue card
or a dual line, so that two subscriber lines could be supported 20 (AN) 68 which performs A-D/D-A conversions, baseband
by a single subscriber terminal 20. The subscriber terminal
filtering and the vector summation of 15 transmitted signals
20 can also be arranged to support analogue and digital
from the modem cards (MCs) 70. The analogue unit 68 is
telecommunications, for example analogue communications
connected to a number of (typically 1-8) modem cards 70.
at 16, 32 or 64 kbits/sec or digital communications in
The modem cards perform the baseband signal processing of
accordance with the ISDN BRA standard.
25 the transmit and receive signals to/from the subscriber
FIG. 3 is a schematic illustration of an example of a
terminals 20. This may include 1/2 rate convolution coding
central terminal of the telecommunications system of FIG. 1.
and x16 spreading with "Code Division Multiplexed
The common equipment rack 40 comprises a number of
Access" (CDMA) codes on the transmit signals, and synequipment shelves 42, 44, 46, including a RF Combiner and
chronisation recovery, de-spreading and error correction on
power amp shelf (RFC) 42, a Power Supply shelf (PS) 44 30 the receive signals. Each modem card 70 in the present
and a number of (in this example four) Modem Shelves
example has two modems, and in preferred embodiments
(MS) 46. The RF combiner shelf 42 allows the modem
there are eight modem cards per shelf, and so sixteen
shelves 46 to operate in parallel. If 'n' modem shelves are
modems per shelf. However, in order to incorporate redunprovided, then the RF combiner shelf 42 combines and
dancy so that a modem may be substituted in a subscriber
amplifies the power of 'n' transmit signals, each transmit 35 link when a fault occurs, only 15 modems on a single
signal being from a respective one of the 'n' modem shelves,
modem shelf 46 are generally used. The 16th modem is then
and amplifies and splits received signals 'n' way so that
used as a spare which can be switched in if a failure of one
separate signals may be passed to the respective modem
of the other 15 modems occurs. The modem cards 70 are
shelves. The power supply shelf 44 provides a connection to
connected to the tributary unit (TU) 74 which terminates the
the local power supply and fusing for the various compo- 40 connection to the host public switched telephone network 18
nents in the common equipment rack 40. A bidirectional
(e.g., via one of the lines 47) and handles the signalling of
connection extends between the RF combiner shelf 42 and
telephony information to the subscriber terminals via one of
the main central terminal antenna 52, such as an omnidi15 of the 16 modems.
rectional antenna, mounted on a central terminal mast 50.
The wireless telecommunications between a central terThis example of a central terminal 10 is connected via a
minal 10 and the subscriber terminals 20 could operate on
point-to-point microwave link to a location where an intervarious frequencies. FIG. 4 illustrates one possible example
face to the public switched telephone network 18, shown
of the frequencies which could be used. In the present
schematically in FIG. 1, is made. As mentioned above, other
example, the wireless telecommunication system is intended
types of connections (e.g., copper wires or optical fibres) can
to operate in the 1.5-2.5 GHz Band. In particular the present
be used to link the central terminal 10 to the public switched 50 example is intended to operate in the Band defined by ITU-R
telephone network 18. In this example the modem shelves
(CCIR) Recommendation F.701 (2025-2110 MHz,
are connected via lines 47 to a microwave terminal (MT) 48.
2200-2290 MHz). FIG. 4 illustrates the frequencies used for
A microwave link 49 extends from the microwave terminal
the uplink from the subscriber terminals 20 to the central
48 to a point-to-point microwave antenna 54 mounted on the
terminal 10 and for the downlink from the central terminal
mast 50 for a host connection to the public switched tele- 55 10 to the subscriber terminals 20. It will be noted that 12
phone network 18.
uplink and 12 downlink radio channels of 3.5 MHz each are
provided centred about 2155 MHz. The spacing between the
A personal computer, workstation or the like can be
receive and transmit channels exceeds the required miniprovided as a site controller (SC) 56 for supporting the
mum spacing of 70 MHz.
central terminal 10. The site controller 56 can be connected
to each modem shelf of the central terminal 10 via, for 60
In the present example, each modem shelf supports 1
example, RS232 connections 55. The site controller 56 can
frequency channel (i.e. one uplink frequency plus the corthen provide support functions such as the localisation of
responding downlink frequency). Currently, in a wireless
faults, alarms and status and the configuring of the central
telecommunications system as described above, CDMA
terminal 10. A site controller 56 will typically support a
encoding is used to support up to 15 subscriber links on one
single central terminal 10, although a plurality of site 65 frequency channel (one subscriber link on each modem).
controllers 56 could be networked for supporting a plurality
Hence, if a central terminal has four modem shelves, it can
of central terminals 10.
support 60 (15x4) subscriber links (ie. 60 STs can be

US 6,222,819 B1
10

9
connected to one CT). However, it is becoming desirable for
more than 60 STs to be supported from one central terminal,
and, in preferred embodiments of the present invention,
enhancements to the CDMA encoding technique are provided to increase the number of subscriber links that can be
supported by a central terminal. Both CDMA encoding, and
the enhancements made to the CDMA encoding in accordance with preferred embodiments, will be discussed in
more detail later.
Typically, the radio traffic from a particular central terminal 10 will extend into the area covered by a neighbouring
central terminal 10. To avoid, or at least to reduce interference problems caused by adjoining areas, only a limited
number of the available frequencies will be used by any
given central terminal 10.
FIG. 5A illustrates one cellular type arrangement of the
frequencies to mitigate interference problems between adjacent central terminals 10. In the arrangement illustrated in
FIG. 5A, the hatch lines for the cells 76 illustrate a frequency
set (FS) for the cells. By selecting three frequency sets (e.g.,
where: FS1=F1, F4, F7, F10; FS2=F2, F5, F8, F11; FS3=F3,
F6, F9, F12), and arranging that immediately adjacent cells
do not use the same frequency set (see, for example, the
arrangement shown in FIG. 5A), it is possible to provide an
array of fixed assignment omnidirectional cells where interference between nearby cells can be reduced. The transmitter power of each central terminal 10 is preferably set such
that transmissions do not extend as far as the nearest cell
which is using the same frequency set. Thus, in accordance
with the arrangement illustrated in FIG. 5A, each central
terminal 10 can use the four frequency pairs (for the uplink
and downlink, respectively) within its cell, each modem
shelf in the central terminal 10 being associated with a
respective RF channel (channel frequency pair).
Figure SB illustrates a cellular type arrangement employing sectored cells to mitigate problems between adjacent
central terminals 10. As with FIG. 5A, the different type of
hatch lines in FIG. 5B illustrate different frequency sets. As
in FIG. 5A, FIG. 5B represents three frequency sets (e.g.,
where: FS1=F1, F4, F7, F10; FS2=F2, F5, F8, F11; FS3=F3,
F6, F9, F12) However, in FIG. 5B the cells are sectored by
using a sectored central terminal (SCT) 13 which includes
three central terminals 10, one for each sector 51, S2 and S3,
with the transmissions for each of the three central terminals
10 being directed to the appropriate sector among Sl, 52 and
S3. This enables the number of subscribers per cell to be
increased three fold, while still providing permanent fixed
access for each subscriber terminal 20.
Arrangements such as those in FIGS. 5A and 5B can help
reduce interference, but in order to ensure that cells oper-

ating on the same frequency don't inadvertently decode each
others data, a seven cell repeat pattern is used such that for
a cell operating on a given frequency, all six adjacent cells
operating on the same frequency are allocated a unique
5 pseudo random noise (PN) code. The use of PN codes will
be discussed in more detail later. The use of different PN
codes prevents nearby cells operating on the same frequency
from inadvertently decoding each others data.
As mentioned above, CDMA techniques can be used in a
10 fixed assignment arrangement (ie. one where each ST is
assigned to a particular modem on a modem shelf) to enable
each channel frequency to support 15 subscriber links. FIG.
6 gives a schematic overview of CDMA encoding and
decoding.
In order to encode a CDMA signal, base band signals, for
15
example the user signals for each respective subscriber link,
are encoded at 80-80N into a 160 ksymbols/sec baseband
signal where each symbol represents 2 data bits (see, for
example the signal represented at 81). This signal is then
spread by a factor of 16 using a spreading function 82-82N
20 to generate signals at an effective chip rate of 2.56
Msymbols/sec in 3.5 MHz. The spreading function involves
applying a PN code (that is specified on a per CT basis) to
the signal, and also applying a Rademacher-Walsh (RW)
code which ensures that the signals for respective subscriber
25 terminals will be orthogonal to each other. Once this spreading function has been applied, the signals for respective
subscriber links are then combined at step 84 and converted
to radio frequency (RF) to give multiple user channel signals
(e.g. 85) for transmission from the transmitting antenna 86.
30
During transmission, a transmitted signal will be subjected to interference sources 88, including external interference 89 and interference from other channels 90.
Accordingly, by the time the CDMA signal is received at the
receiving antenna 91, the multiple user channel signals may
3 5 be distorted as is represented at 93.
In order to decode the signals for a given subscriber link
from the received multiple user channel, a Walsh correlator
94-94N uses the same RW and PN codes that were used for
the encoding for each subscriber link to extract a signal (e.g,
40 as represented at 95) for the respective received baseband
signal 96-96N. It will be noted that the received signal will
include some residual noise. However, unwanted noise can
be removed using a low pass filter and signal processing.
The key to CDMA is the application of the RW codes,
these being a mathematical set of sequences that have the
45
function of "orthonormality". In other words, if any RW
code is multiplied by any other RW code, the results are
zero. A set of 16 RW codes that may be used is illustrated in
Table 1 below:
TABLE 1

RWO
RW1
RW2
RW3
RW4
RW5
RW6
RW7
RW8
RW9
RW10
RW11
RW12
RW13
RW14
RW15

1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1

1
-1
1
-1
1
-1
1
-1
1
-1
1
-1
1
-1
1
-1

1
1
-1
-1
1
1
-1
-1
1
1
-1
-1
1
1
-1
-1

1
-1
-1
1
1
-1
-1
1
1
-1
-1
1
1
-1
-1
1

1
1
1
1
-1
-1
-1
-1
1
1
1
1
-1
-1
-1
-1

1
-1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
-1
1

1
1
-1
-1
-1
-1
1
1
1
1
-1
-1
-1
-1
1
1

1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
-1
1
1
-1

1
1
1
1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1

1
-1
1
-1
1
-1
1
-1
-1
1
-1
1
-1
1
-1
1

1
1
-1
-1
1
1
-1
-1
-1
-1
1
1
-1
-1
1
1

1
-1
-1
1
1
-1
-1
1
-1
1
1
-1
-1
1
1
-1

1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
1
1
1
1

1
-1
1
-1
-1
1
-1
1
-1
1
-1
1
1
-1
1
-1

1
1
-1
-1
-1
-1
1
1
-1
-1
1
1
1
1
-1
-1

1
-1
-1
1
-1
1
1
-1
-1
1
1
-1
1
-1
-1
1

US 6,222,819 B1

11
The above set of RW codes are orthogonal codes that
allow the multiple user signals to be transmitted and
received on the same frequency at the same time. Once the
bit stream is orthogonally isolated using the RW codes, the
signals for respective subscriber links do not interfere with
each other. Since RW codes are orthogonal, when perfectly
aligned all codes have zero cross-correlation, thus making it
possible to decode a signal while cancelling interference
from users operating on other RW codes.
In preferred embodiments of the present invention, it is
desired to provide the central terminal with the ability to
support more than 15 subscriber links on each channel
frequency, and to achieve this the above set of 16 RW codes
has been enhanced. In order to maintain compatibility with
former products using the 16 RW codes, it was desirable that
any enhancements should retain the same set of 16 RW
codes.
The manner in which the enhancements have been implemented provides flexibility in the way the frequency channels are configured, with certain configurations allowing a
greater number of subscriber links to be supported, but at a
lower gross bit rate. In preferred embodiments, a channel
can be selected to operate with the following gross bit rates:

160 kb/s
80 kb/s
40 kb/s
10 kb/s

Full rate (Fl)
Half rate (H1, H2)
Quarter rate (Q1, Q2, Q3, Q4)
Low rate (L1, L2, L3, L4), for uplink
acquisition

In preferred embodiments, the manner in which these
channelisations are provided differs for the downlink (CT to
ST) and uplink (ST to CT) communication paths. This is
because it has been realised that different performance
requirements exist for the downlink and uplink paths. On the
downlink all signals emanate from a single source, namely
the central terminal, and hence the signals will be synchronised. However, on the uplink path, the signals will emanate
from a number of independent STs, and hence the signals
will not be synchronised.
Given the above considerations, in preferred
embodiments, on the uplink path full rate (160 kb/s) operation is implemented using the basic set of RW codes
discussed earlier, but half and quarter rates are achieved
through the use of 'Overlay Codes' which comprise RW
coded high rate symbol patterns that are transmitted for each
intermediate rate data symbol. For half rate operation, two
2-bit overlay codes are provide, whilst for quarter rate
operation, four 4-bit overlay codes are provided. WEn
generating a signal for transmission, one of the overlay
codes, where appropriate, is applied to the signal in addition
to the appropriate RW code. When the signal is received,
then at the CDMA demodulator the incoming signal is
multiplied by the channel's PN, RW and Overlay codes. The
correlator integration period is set to match the length of the
Overlay code.
Overlay codes are used extensively to provide variable
rate uplink traffic channels. Overlay codes will also be used
to implement downlink control channels, these control channels being discussed in more detail later. However, as
mentioned earlier, a different approach is taken for providing
flexible channelisations on the downlink traffic channel
paths. Downlink traffic channels will operate in high rate,
160 kb/s, mode, with lower data rates of 80 and 40 kb/s
being supported by 'Time Division Multiplexing' (TDM)
the available bandwidth.

12
In preferred embodiments, TDM timeslot bit numbering
will follow the CCITT G. 732 convention with bits transmitted in the sequence bit 1, bit 2 . . . bit 8. Byte orientation
is specified per channel as either most significant bit (MSB)
5 first, least significant bit (LSB) first or N/A.
The provision of a hybrid CDMA/TDM approach for
downlink traffic channels retains the benefits of CDMA
access, ie. interference is reduced when traffic is reduced.
Further, use of TDM ensures that the CDMA signal is
10 limited to a 256 Quadrature Amplitude Modulation' (QAM)
constellation which reduces receiver dynamic range requirements. QAM constellations will be familiar to those skilled
in the art.
On the uplink channels, the pure CDMA approach using
15
overlay codes eliminates the need to time synchronise STs to
a TDM frame reference. This has the advantage of eliminating TDM delays and the 'guard time' in between TDM
frames. Another benefit is reduced peak power handling
requirements in the ST RF transmit chain which would
20
otherwise be needed when transmitting bursty TDM data.
High dynamic range requirement is restricted to the CT
receiver.
The manner in which the transmitted and received signals
are processed in accordance with preferred embodiments of
25
the present invention will be described with reference to
FIGS. 7 and 8. FIG. 7A is a schematic diagram illustrating
signal transmission processing stages as configured in a
subscriber terminal 20 in the telecommunications system of
FIG. 1. In FIG. 7A, an analogue signal from a telephone is
30
passed via an interface such as two-wire interface 102 to a
hybrid audio processing circuit 104 and then via a codec 106
to produce a digital signal into which an overhead channel
including control information is inserted at 108. If the
subscriber terminal supports a number of telephones or other
35
telecommunications equipment, then elements 102, 104 and
106 may be repeated for each piece of telecommunications
equipment.
At the output of overhead insertion circuit 108, the signal
40 will have a bit rate of either 160, 80 or 40 kbits/s, depending
on which channel has been selected for transmission of the
signal.
The resulting signal is then processed by a convolutional
encoder 110 to produce two signals with the same bit rate as
45 the input signal (collectively, these signals will have a
symbol rate of 160, 80 or 40 KS/s). Next, the signals are
passed to a spreader 111 where, if a reduced bit rate channel
has been selected, an appropriate overlay code provided by
overlay code generator 113 is applied to the signals. At the
so output of the spreader 111, the signals will be at 160 KS/s
irrespective of the bit rate of the input signal since the
overlay code will have increased the symbol rate by the
necessary amount.
The signals output from spreader 111 are passed to a
55 spreader 116 where the Rademacher-Walsh and PN codes
are applied to the signals by a RW code generator 112 and
PN Code generator 114, respectively. The resulting signals,
at 2.56 MC/s (2.56 Mega chips per second, where a chip is
the smallest data element in a spread sequence) are passed
60 via a digital to analogue converter 118. The digital to
analogue converter 118 shapes the digital samples into an
analogue waveform and provides a stage of baseband power
control. The signals are then passed to a low pass filter 120
to be modulated in a modulator 122. The modulated signal
65 from the modulator 122 is mixed with a signal generated by
a voltage controlled oscillator 126 which is responsive to a
synthesizer 160. The output of the mixer 128 is then

US 6,222,819 B1
14

13
amplified in a low noise amplifier 130 before being passed
via a band pass filter 132. The output of the band pass filter
132 is further amplified in a further low noise amplifier 134,
before being passed to power control circuitry 136. The
output of the power control circuitry is further amplified in
a power amplifier 138 before being passed via a further band
pass filter 140 and transmitted from the transmission antenna
142.
FIG. 7B is a schematic diagram illustrating signal transmission processing stages as configured in a central terminal
10 in the telecommunications system of FIG. 1. As will be
apparent, the central terminal is configured to perform
similar signal transmission processing to the subscriber
terminal 20 illustrated in FIG. 7A, but does not include
elements 100, 102, 104 and 106 associated with telecommunications equipment. Further, the central terminal
includes a TDM encoder 105 for performing time division
multiplexing where required. The central terminal will have
a network interface over which incoming calls destined for
a subscriber terminal are received. When an incoming call is
received, the central terminal will contact the subscriber
terminal to which the call is directed and arrange a suitable
channel over which the incoming call can be established
with the subscriber terminal (in preferred embodiments, this
is done using the call control channel discussed in more
detail later). The channel established for the call will determine the time slot to be used for call data passed from the
CT to the ST and the TDM encoder 105 will be supplied with
this information.
Hence, when incoming call data is passed from the
network interface to the TDM encoder 105 over line 103, the
TDM encoder will apply appropriate TDM encoding to
enable the data to be inserted in the appropriate time slot.
From then on, the processing of the signal is the same as the
equivalent processing performed in the ST and described
with reference to FIG. 7A, the overlay code generator
producing a single overlay code of value '1' so that the
signal output from spreader 111 is the same as the signal
input to the spreader 111.
As mentioned earlier, in preferred embodiments, overlay
codes, rather than TDM, are used to implement downlink
control channels, and data relating to such channels is passed
from a demand assignment engine (to be discussed in more
detail later) over line 107 through switch 109 to the overhead
insertion circuit 108, thereby bypassing the TDM encoder
105. The processing of the signal is then the same as the
equivalent processing performed in the ST, with the overlay
code generator providing appropriate overlay codes to the
spreader 111. The overlay code generator will be controlled
so as to produce the desired overlay code, in preferred
embodiments, this control coming from the DA engine (to be
discussed in more detail later).
FIG. 8A is a schematic diagram illustrating the signal
reception processing stages as configured in a subscriber
terminal 20 in the telecommunications system of FIG. 1. In
FIG. 8A, signals received at a receiving antenna 150 are
passed via a band pass filter 152 before being amplified in
a low noise amplifier 154. The output of the amplifier 154 is
then passed via a further band pass filter 156 before being
further amplified by a further low noise amplifier 158. The
output of the amplifier 158 is then passed to a mixer 164
where it is mixed with a signal generated by a voltage
controlled oscillator 162 which is responsive to a synthesizer
160. The output of the mixer 164 is then passed via the I/Q
de-modulator 166 and a low pass filter 168 before being
passed to an analogue to digital converter 170. The digital
output of the A/D converter 170 at 2.56 MC/s is then passed
to a correlator 178, to which the same Rademacher-Walsh
and PN codes used during transmission are applied by a RW
code generator 172 (corresponding to the RW code genera-

for 112) and a PN code generator 174 (corresponding to PN
code generator 114), respectively. The output of the correlator 178, at 160 KS/s, is then applied to correlator 179,
where any overlay code used at the transmission stage to
5 encode the signal is applied to the signal by overlay code
generator 181. The elements 170, 172, 174, 178, 179 and
181 form a CDMA demodulator. The output from the
CDMA demodulator (at correlator 179) is then at a rate of
either 160, 80 or 40 KS/s, depending on the overlay code
applied by correlator 179.
10
The output from correlator 179 is then applied to a Viterbi
decoder 180. The output of the Viterbi decoder 180 is then
passed to an overhead extractor 182 for extracting the
overhead channel information. If the signal relates to call
data, then the output of the overhead extractor 182 is then
15
passed through TDM decoder 183 to extract the call data
from the particular time slot in which it was inserted by the
CT TDM encoder 105. Then, the call data is passed via a
codec 184 and a hybrid circuit 188 to an interface such as
two wire interface 190, where the resulting analogue signals
20 are passed to a telephone 192. As mentioned earlier in
connection with the ST transmission processing stages,
elements 184, 188, 190 may be repeated for each piece of
telecommunications equipment 192 at the ST.
If the data output by the overhead extraction circuit 182
25 is data on a downlink control channels, then instead of
passing that data to a piece of telecommunications
equipment, it is passed via switch 187 to a call control logic
185, where that data is interpreted by the ST.
At the subscriber terminal 20, a stage of automatic gain
30
control is incorporated at the IF stage. The control signal is
derived from the digital portion of the CDMA receiver using
the output of a signal quality estimator.
FIG. 8B illustrates the signal reception processing stages
as configured in a central terminal 10 in the telecommuni35
cations system of FIG. 1. As will be apparent from the figure,
the signal processing stages between the RX antenna 150
and the overhead extraction circuit 182 are the as those
within the ST discussed in connection with FIG. 8A.
However, in the case of the CT, call data output from the
40 overhead extraction circuit is passed over line 189 to the
network interface within the CT, whilst control channel data
is passed via switch 191 to the DA engine 380 for processing. The DA engine is discussed in more detail later.
Overlay codes and channelisation plans are selected to
45
ensure signal orthogonality—i.e. in a properly synchronised
system, the contribution of all channels except the channel
being demodulated sum to zero over the correlator integration period. Further, uplink power is controlled to maintain
constant energy per bit. The exception to this is Low rate
50 which will be transmitted at the same power as a Quarter rate
signal. Table 2 below illustrates the overlay codes used for
full, half and quarter rate operations:
TABLE 2
55

Net
Rate
(kb/s)
60

65

160
80
80
40
40
40
40

Channel
designation

ST Tx.
power
relative
to Fl-U
(dB)

-Fl-U
-Hl-U
-H2-U
-Ql-U
-Q2-U
-Q3-U
-Q4-U

0
-3
-3
-6
-6
-6
-6

Overlay Code

Correlator
integration
period (us)

Acquisition
overlay

1
1 1
1 -1
1 1 1 1
1 -1 1 -1
1 1 -1 -1
1 -1 -1 1

6.25
12.5
12.5
25
25
25
25

Ll
Ll
L3
Ll
L2
L3
L4

US 6,222,819 B1

15
In preferred embodiments, a 10 kb/s acquisition mode is
provided which uses concatenated overlays to form an
acquisition overlay; this is illustrated in table 3 below:
TABLE 3
Acquisition
overlay
L1-U
L2-U
L3-U
L4-U

Equivalent high rate pattern
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 —1 —1 1 1 —1 —1 1 1 —1 —1 1 1 —1 —1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 —1 —1 1 1 —1 —1 1 1 —1 —1 1 1 —1 —1 1

FIGS. 9A and 9B are diagrams illustrating the uplink and
downlink delivery methods, respectively, when the system is
fully loaded, and illustrate the difference between the use of
overlay codes illustrated in FIG. 9A and the use of TDM as
illustrated in FIG. 9B. When using overlay codes, an RW
code is split in the RW space domain to allow up to four sub
channels to operate at the same time. In contrast, when using
TDM, an RW code is split in the time domain, to allow up
to four signals to be sent using one RW code, but at different
times during the 125 us frame. As illustrated in FIGS. 9A
and 9B, the last two RW codes, RW14 and RW15, are not
used for data traffic in preferred embodiments, since they are
reserved for call control and acquisition functions; this will
be discussed in more detail later.
The CDMA channel hierarchy is as illustrated in FIG. 10.
Using this hierarchy, the following CDMA channelisations
are possible:
Fl
H1+H2
H1+Q3+Q4
H2+Q1+Q2
Q1+Q2+Q3+Q4
Having discussed how the CDMA codes are enhanced to
enable flexible channelisations to be achieved, whereby the
bit rates can be lowered to enable more subscriber links to
be managed per channel frequency, a general overview of
how the downlink and uplink paths are established will be
provided with reference to FIGS. 11 and 12.
FIG. 11 is a block diagram of downlink and uplink
communication paths between central terminal 10 and subscriber terminal 20. A downlink communication path is
established from transmitter 200 in central terminal 10 to
receiver 202 in subscriber terminal 20. An uplink communication path is established from transmitter 204 in subscriber terminal 20 to receiver 206 in central terminal 10.
Once the downlink and the uplink communication paths
have been established in wireless telecommunication system
1, telephone communication may occur between a user 208,
210 of subscriber terminal 20 and a user serviced through
central terminal 10 over a downlink signal 212 and an uplink
signal 214. Downlink signal 212 is transmitted by transmitter 200 of central terminal 10 and received by receiver 202
of subscriber terminal 20. Uplink signal 214 is transmitted
by transmitter 204 of subscriber terminal 20 and received by
receiver 206 of central terminal 10.
Receiver 206 and transmitter 200 within central terminal
10 are synchronized to each other with respect to time and
phase, and aligned as to information boundaries. In order to
establish the downlink communication path, receiver 202 in
subscriber terminal 20 should be synchronized to transmitter
200 in central terminal 10. Synchronization occurs by performing an acquisition mode function and a tracking mode
function on downlink signal 212. Initially, transmitter 200 of

16
central terminal 10 transmits downlink signal 212. FIG. 12
shows the contents of downlink signal 212. A frame information signal 218 is combined with an overlay code 217
where appropriate, and the resultant signal 219 is combined
5 with a code sequence signal 216 for central terminal 10 to
produce the downlink 212. Code sequence signal 216 is
derived from a combination of a pseudo-random noise code
signal 220 and a Rademacher-Walsh code signal 222.
Downlink signal 212 is received at receiver 202 of
subscriber terminal 20. Receiver 202 compares its phase and
io
code sequence to a phase and code sequence within code
sequence signal 216 of downlink signal 212. Central terminal 10 is considered to have a master code sequence and
subscriber terminal 20 is considered to have a slave code
15 sequence. Receiver 202 incrementally adjusts the phase of
its slave code sequence to recognize a match to master code
sequence and place receiver 202 of subscriber terminal 20 in
phase with transmitter 200 of central terminal 10. The slave
code sequence of receiver 202 is not initially synchronized
20 to the master code sequence of transmitter 200 and central
terminal 10 due to the path delay between central terminal
10 and subscriber terminal 20. This path delay is caused by
the geographical separation between subscriber terminal 20
and central terminal 10 and other environmental and tech25 nical factors affecting wireless transmission.
After acquiring and initiating tracking on the central
terminal 10 master code sequence of code sequence signal
216 within downlink signal 212, receiver 202 enters a frame
alignment mode in order to establish the downlink commu30 nication path. Receiver 202 analyzes frame information
within frame information signal 218 of downlink signal 212
to identify a beginning of frame position for downlink signal
212. Since receiver 202 does not know at what point in the
data stream of downlink signal 212 it has received
35 information, receiver 202 must search for the beginning of
frame position in order to be able to process information
received from transmitter 200 of central terminal 10. Once
receiver 202 has identified one further beginning of frame
position, the downlink communication path has been estab40 lished from transmitter 200 of central terminal 10 to receiver
202 of subscriber terminal 20.
The structure of the radio frames of information sent over
the downlink and uplink paths will now be discussed with
reference to FIGS. 13 and 14. In FIGS. 13 and 14, the
45 following terms are used:
Bn Customer payload, 1x32 to 2x64 Kb/s
Dn Signalling Channel, 2 to 16 kb/s
OH Radio Overhead Channel
16 kb/s Traffic Mode
50
10 kb/s Acquisition/Standby Mode
Both FIGS. 13A and 13B show a 125 us subframe format,
which is repeated throughout an entire radio frame, a frame
typically lasting for 4 milliseconds (ms). FIG. 13A illustrates
the radio frame structures that are used in preferred embodi55 ments for the downlink path. Subframe (i) in FIG. 13A
shows the radio frame structure used for low rate, 10 Kb/s,
acquisition mode (Ln-D) during which only the overhead
channel is transmitted. Subframe (ii) in FIG. 13A shows the
radio frame structure employed for the call control channel
60 operating in quarter rate, 40 Kb/s, mode (Qn-D), whilst
subframe (iii) of FIG. 13A illustrates the radio frame structure used for traffic channels operating in full rate, 160 kb/s,
mode (F1-D).
Similarly, subframe (i) of FIG. 13B shows the radio frame
65 structure used for the uplink path when operating in low rate
acquisition or call control mode (Ln-U). Sub-frames (ii) to
(iv) show the radio frame structure used for traffic channels

US 6,22 2,819 B1
17

18

when operating in quarter rate mode (Qn-U), half rate mode
assignment arrangements using the set of 16 RW codes
(Hn-U), and full rate mode (Fl-U), respectively.
discussed earlier are still supported, as well as demand
Considering now the overhead channel in more detail,
access services that are available when using a system
FIGS. 14A and 14B show the overhead frame structure
in accordance with the preferred embodiment. FIGS.
employed for various data rates. The overhead channel may 5
15A and 15B illustrate typical downlink and uplink
include a number of fields—a frame alignment word (FAW),
channel structures that might occur in a loaded system
a code synchronization signal (CS), a power control signal
in accordance with preferred embodiments of the
(PC), an operations and maintenance channel signal (OMC),
present invention. As illustrated in FIG. 15A, on the
a mixed OMC/D-Channel (HDLC) signal (OMC/D), a chandownlink path, some signals may be at 160 kb/s and
nel identifier byte (Ch.ID), and some unused fields.
utilise an entire RW channel. An example of such
0
The frame alignment word identifies the beginning of
signals would be those sent over fixed assignment links
frame position for its corresponding frame of information.
to products which do not support the CDMA enhanceThe code synchronization signal provides information to
ments provided by systems in accordance with precontrol synchronization of transmitter 204 in subscriber
ferred embodiments of the present invention, as illusterminal 20 to receiver 206 in central terminal 10. The power
trated for RW1 and RW2 in FIG. 15A. Alternatively, a
control signal provides information to control transmitting 15
user may have authority to utilise a whole RW channel,
power of transmitter 204 in subscriber terminal 20. The
for example when sending a fax, as illustrated by RW12
in FIG. 15A.
operations and maintenance channel signal provides status
As illustrated by RW5 to RW11, TDM can be used on the
information with respect to the downlink and uplink comdownlink traffic channels to enable more than one CT to ST
munication paths and a path from the central terminal to the
subscriber terminal on which the communication protocol 20 communication to take place on the same RW channel
during each frame. Further, as illustrated for RW3 and RW4,
which operates on the modem shelf between the shelf
in preferred embodiments, certain channels can be locked to
controller and the modem cards also extends. The OMC/D
limit interference from other nearby cells, as will be dissignal is a combination of the OMC signal and a signalling
cussed in more detail later.
signal (D), whilst the Ch. ID signal is used to uniquely
Similar channelisations can be achieved for the uplink
identify an RW channel, this Ch. ID signal being used by the 25
paths, but as illustrated in FIG. 15B, overlay codes are used
subscriber terminal to ensure that the correct channel has
instead of TDM to enable more than one ST to CT combeen acquired.
munication to take place on the same RW channel during
In preferred embodiments, the subscriber terminal will
each frame (as shown in FIG. 15B for RW5 to RW11). It
receive downlink traffic channel data at a rate of 160 kb/s.
30 should be noted that, in both FIGS. 15A and 15B, the
Depending on the B-channel rate, the ST will be allocated an
channels RW14 and RW15 are reserved as a call control
appropriate share of the radio overhead. The following TDM
channel and an link acquisition channel, respectively, and
mappings are created:
overlay codes are employed on these channels, irrespective
of whether the path is a downlink or an uplink path. These
TABLE 4
35 two channels will be discussed in more detail below.
Acquisition/net entry will take place via the Link AcquiOverRate Channel
head
sition Channel (LAC). Following power-up an ST will
(kb/s) designation
Bearer
CS
PC
OMC
rate
automatically attempt downlink acquisition of the LAC on a
pre-determined 'home' RF channel. The LAC downlink
160
—Fl—D—T1/1 Bl, B2, B3, CS1, PC1, OMC1, OMC3
4 ms
B4
CS3
PC3
40 channel (eg. RW15 in preferred embodiments) will operate
80
—Fl—D—T2/1 Bl, B2
CS1, PC1, OMC1, OMC3
4 ms
at 10 kb/s, full single user power. Downlink acquisition will
CS3
PC3
be simultaneous for all STs.
80
—Fl—D—T2/2 B3, B4
CS2, PC2, OMC2, OMC4
4 ms
Each CT Modem Shelf will maintain a database holding
CS4
PC4
the serial numbers of all STs that could possibly be sup40
—Fl—D—T4/1 B1
CS1
PC1
OMC1
8 ms
40
—F1—D—T4/2 B2
CS2
PC2
OMC2
8 ms
45 ported by that CT. The state of each ST will recorded with
8 ms
40
—Fl—D—T4/3 B3
CS3
PC3
OMC3
top level states as follows:
40
—Fl—D—T4/4 B4
CS4
PC4
OMC4
8 ms
cold
idle
In the above chart, the scheme used to identify a channel
call in progress
is as follows. Rate code 'F1' indicates full rate, 160 kb/s, 'D' 50
Transition states will also be defined. An ST is considered
indicates that the channel is a downlink channel, and 'Tn/t.'
cold if the ST is newly provisioned, the CT has lost
indicates that the channel is time division multiplexed
management communications with the ST or the CT has
between STs,
been power cycled. Over the LAC, the CT broadcasts
`n' indicating the total number of TDM timeslots, and T
individual ST serial numbers and offers an invitation to
indicating the selected traffic timeslot.
55 acquire the LAC uplink Cold uplink acquisition will be
All ST's operating on a traffic channel will receive
carried out on the Link Acquisition Channel at low rate. The
D-channel information at the 16 kb/s rate. The D-channel
CT will invite specific ST's to cold start via the management
protocol includes an address field to specify which ST is to
channel.
process the contents of the message.
Assuming an uplink channel is available, the appropriate
The channel structure was illustrated earlier in FIGS. 9A 60 acquisition overlay will be selected, and acquisition will be
and 9B. In preferred embodiments, the channel structure is
initiated.
flexible but comprises:
`Rapid' downlink RW channel switching may be supAt least one Link Acquisition Channel (LAC)
ported at rates other than Ln-D. Rapid means that coherent
At least one Call Control Channel (CCC)
demodulation is maintained, and only convolutional decodTypically one Priority Traffic Channels (PTC)
65 ing and frame synchronisation processes need be repeated.
1 to 13 Traffic Channels (TC) The manner in which the
On acquisition, management information will be
channelisation is provided ensures that former fixed
exchanged. The ST will be authenticated and allocated a

US 6,222,819 B1
19
short ST identifier (between 12 and 16 bits) which will be
used for subsequent addressing. The ST uplink will operate
for long enough for the uplink to be parametised by the ST
in terms of code phase and transmit power. These parameters
will be used by the ST for subsequent warm start acquisitions and will also be held by the CT to allow the CT to force
a cold ST to warm start. On successful completion of net
entry, the ST will be placed in the idle state and instructed
to cease uplink communications and move to the Call
Control Channel (CCC) (RW14 in preferred embodiments).
The time taken for net entry to be achieved can be
monitored, and the following techniques can be used to
decrease net entry time if desired:
(i) Prioritise so that high GOS (Grade Of Service) users
are offered net entry first.
(ii) Convert Traffic Channels to LACs.
(iii) In the event of a CT restart, invite STs to attempt
uplink warm start. A reduction in net entry time of a
factor of 4 could be achieved. This mechanism would
need to be safeguarded against possible deterioration of
uplink warm start parameters—i.e. it should only be
allowed provided no CT RF related parameters have
been modified. The CT would need to broadcast an ID
to allow an ST to validate that the uplink warm start
parameters were valid for this CT.
(iv) ST restart—the CT will keep copies of the ST warm
start parameters so that a cold ST may have warm start
parameters downloaded in the invitation to acquire and
then be instructed to warm start.
Following Net Entry, all STs listen to the CCC. This
channel broadcasts management and call control information via a 32 kb/s HDLC channel. In order to maintain
management communication, the CT polls each ST in
sequence. Each poll comprises a broadcast invitation for an
addressed ST to acquire the CCC Uplink followed by an
exchange of management information (authentication, ST
alarm update, warm start parameters, downlink radio performance data etc).
A Management Poll may fail for one of the following
reasons:
(i) The ST is or has been powered down. An EM alarm
may be flagged if this persists and the database for that
ST should be marked cold. The Net Entry process will
follow.
(ii) The ST is either making a call or in the process of
making a call. The poll cycle may be suspended and
management communications effected on the appropriate traffic channel.
When a Management Poll fails it should be followed up
by a number of faster polls until either the ST responds or
it is marked cold. The CCC is required to transmit all copies
of the invitations to acquire the LAC so that an ST can be
forced to acquire the LAC uplink
Traffic Channel Uplink Acquisition Procedure
The basic acquisition process from the ST side is as
follows;
(i) Switch the downlink (receiver) circuitry to 10 kb/s rate,
and select the appropriate Traffic Channel RW and
Overlay codes. Acquisition of the TC downlink is
limited to achieving frame alignment.
(ii) The downlink PC/CS channel will be decoded to
create a busy/idle flag. If PC/CS reports busy, then this
means that another ST is using that traffic channel and
the ST aborts the acquisition process.
(iii) Switch uplink to 10 kb/s rate, and select the appropriate Traffic Channel RW and Overlay codes.

20
Enable the ST transmitter at a level of nominal full rate
power minus 18 dB. While PC/CS reports idle the ST will
continue uplink fast codesearch, stepping the uplink power
level by +2 dB at the end of each search. The uplink should
5 acquire at nominal full rate power minus 6 dB. Uplink
acquisition is aborted if maximum transmit level is reached
and PC/CS continues to report idle.
(iv) PC/CS reports busy. At this point the ST may have
genuinely acquired the traffic channel, or instead may
10
be observing PC/CS go busy because another ST has
acquired the traffic channel. The ST is sent an authentication request and responds with it's ST identifer.
The CT grants uplink access by returning the
ST identifier. The ST aborts the acquisition process if
15
the returned ST identifier is not recognised (ie. is not
the ST identifer that it sent). This authentication process arbitrates between two STs contending for outgoing access and it also keeps STs from acquiring TCs
that have been reserved from incoming access.
20 Incoming Call
A number of TCs will be reserved for incoming calls, and
incoming call processing is as follows:
(i) Check the CT database—if the ST is in the call in
progress state the call is rejected.
25
(ii) Check that an uplink TC of the required bandwidth is
available. If there is bandwidth then a TC is reserved.
(iii) An incoming call setup message is broadcast over the
CCC to inform the addressed ST of the incoming call
and specify the TC on which to receive the call. If no
30
TC is available but the CT forms part of a Service
Domain, then the incoming call setup message is sent
with a null TC otherwise the call is rejected. Service
domains will be discussed in more detail later. The
incoming call setup message is repeated a number of
35
times.
(iv) The ST attempts uplink acquisition. The ST listens to
the downlink and keeps trying for uplink acquisition
until the CT sends a message to the ST to return the ST
40
to the CCC. The ST will also run a timer to return it
back to the CCC in the event of an incoming call failing
to complete.
(v) On successful uplink acquisition, the CT authenticates
the ST.
45
(vi) Rate switching is originated from the CT modem. A
command is sent via the PC/CS to switch the downlink
to the required bandwidth. The ST returns the rate
switch command via the uplink PC/CS. The link is now
of the required bandwidth.
50
Outgoing Call
Outgoing calls are supported by allowing slotted random
access to the TC uplinks. The outgoing call processing is as
follows:
(i) The CT publishes a 'free list' of available Traffic
55
Channels and Priority Traffic Channels with their
respective bandwidths. This list is published periodically (in preferred embodiments, every 500 ms) and is
used to mark uplink access slots.
(ii) An off-hook condition is detected by the ST. The ST
60
starts a call setup timer.
(iii) The ST waits for the next free list to be received over
the CCC. If the Free list is empty the outgoing call is
blocked. The ST will generate a congestion tone.
65
(iv) If the Free list has available channels, the ST picks a
channel from the free list at random. The algorithm that
the ST uses to pick a channel will need to be specified

US 6,222,819 B1

21

22

in the free list. For example, the ST may be required to
In preferred embodiments, the CT will control the number
always choose from a pool of minimum bandwidth
of Traffic Channels to minimise access noise. TCs will be
channels so that high bandwidth channels remain availclassified as:
able for high GOS users. Alternatively the ST may be
(i) Busy—carrying traffic;
allowed to choose any channel regardless of bandwidth 5
(ii) Access, Incoming (Access In)—reserved for incomfor minimum blocking. In preferred embodiments, STs
ing access;
will not choose low bandwidth channels and negotiate
(iii) Access, Outgoing (Access Out)—reserved for outthe rate up.
going access—such TCs appear on the Free list;
(v) The ST attempts uplink acquisition on the specified
(iv) Priority—reserved for priority outgoing access—such
TC, this process having been described earlier. If 10
TCs appear in the Free list;
acquisition is successful then the outgoing call is processed. Otherwise the ST returns to the CCC and waits
(v) Free—available for any purpose; and
for the next available free list. To avoid a number of STs
(vi) Locked—not available due to interference limiting.
repetitively attempting to acquire the same TC, and
This classification scheme is illustrated in FIG. 16. The
blocking each other, a suitable protocol can be 15 CT will allocate traffic on the following basis:
employed to govern how individual STs will act upon
(i) The CT will monitor incoming and outgoing call
receipt of the free list.
setup-times and convert Access TCs from Free TCs in
(vi) The ST may be unable to acquire a TC by the time the
order to achieve a required grade of service.
call setup timer expires. The ST may in such cases
(ii) When a call is setup, an Access TC is converted to a
cease attempting outgoing access and generate conges- 20
Busy TC. If a Free TC is available, it is converted to a
tion tone.
new Access TC. If there are no Free TCs then the
Outgoing Priority Call
Access TC is lost until a call clears.
It is recognised that the random access protocol used to
(iii) When a call clears the Busy TC is converted to a Free
setup normal outgoing calls could lead to blocking. In
TC. If a previous call setup resulted in a lost Access TC
preferred embodiments, access to a largely non-blocking 25
then the Busy TC is converted back into an Access TC.
Priority Traffic Channel will be allowed. Priority calling is
(iv) When the PTC is accessed, a new PTC is created by
complicated because the ST must:
converting a Free, Access or Busy (normal call) TC.
(i) Capture and decode dialled digits.
(v) The CT will monitor the Busy TC downlink and uplink
(ii) Regenerate digits when a blocking condition occurs.
soft error counts in an attempt to establish link quality.
(iii) Allow transparent network access in a non-blocking 30
If the CT records a lower than average soft error count
condition.
and long call setup times are being recorded, a Locked
(iv) Categorise all outgoing calls as priority or normal so
TC may be converted to a Free TC. Conversely, if the
that normal calls are dropped in favour of priority calls.
CT records a higher than average soft error count, a
The priority call procedure in preferred embodiments is as
35
Free or Access TC may be converted to a Locked TC.
follows:
FIG. 17 illustrates how the central terminal performs the
(i) The CT will publish Directory Numbers (DNs) for a
above interference limiting function. When incoming call
number of emergency services over the CCC.
data arrives at a central terminal modem 320, encoder 325
(ii) The ST will attempt uplink access according to the
encodes the data for transmission over the wireless link 300
normal algorithms. If the outgoing access is successful 40 to the subscriber terminal 20. At the subscriber terminal 20,
then the customer is able to dial as normal. All dialled
the decoder 326 decodes the data, and passes the decoded
digits are check against the emergency DN list so that
user data over line 328 to the subscriber telecommunications
calls may be categorised normal or priority at the CT.
equipment. As the decoder 326 decodes the data, it is able to
(iii) If congestion tone is returned the customer is allowed
establish a bit error rate (BER) estimate 330 associated with
to dial the emergency number into the ST. If the ST 45 the signal transmission over the wireless link 300, which can
detects an emergency DN sequence then uplink access
be passed to the multiplexer 332 for combining with other
via the Priority Traffic Channel (PTC) is attempted.
signals, such as those from a call control function 336 or user
(iv) On PTC acquisition, the ST relays the dialled digit
data on line 338, before being passed to an encoder 334.
sequence to the CT for dialling into the PSTN.
Here, the BER estimate is encoded and passed on the OMC
(iv) The CT converts the PTC to a TC and reallocates 50 channel over the wireless link 310 to the decoder 340 within
another TC to become the PTC, dropping a normal call
the central terminal modem 320. Once decoded by the
in progress if necessary.
decoder 340, the signal passes to the multiplexer 345, where
Interference Limiting (Pool Sizing)
the BER estimate from the subscriber terminal is detected
Across a large scale deployment of cells, optimum capacand passed over line 355 to the dynamic pool sizing function
ity is achieved by minimising radio traffic while maintaining 55 360.
an acceptable grade of service. Lowest possible radio traffic
Further, as at the subscriber terminal 20, the decoder 340
results in improved 'carrier to interference' (C/I) ratios for
within the central terminal modem 320 is able to establish a
users within the cell of interest and to co-channel users in
bit error rate estimate 350 associated with the signal transnearby cells. The C/I ratio is a measure (usually expressed
mission over the wireless link 310. This BER estimate 350
in dB) of how high above interference the transmitted signal 60 is also passed over line 355 to the dynamic pool sizing
needs to be to be decoded effectively. In preferred
function 360. The dynamic pool sizing function 360 is
embodiments, the central terminal is provided with the
provided on the CT modem shelf 302, and receives BER
ability to trade traffic for C/I, thereby allowing network
estimates from each of the modems on that shelf indicated
planning to be carried out less rigidly. This feature can be
by the lines entering the bottom of the dynamic pool sizing
realised by a system using CDMA as in preferred embodi- 65 function 360.
ments of the present invention, and is a benefit that CDMA
In addition to BER estimates, grade of service (GOS) data
offers over TDMA and FDMA systems.
is obtained from two sources. Firstly, at each subscriber

US 6,222,819 B1
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24

terminal 20, the call control function 336 will note how
RF Channel Switching
readily it is able to establish traffic channels for transmitting
In preferred embodiments, it has been realised that if an
and receiving data, and from this can provide a GOS
ST is allowed to operate from more than one CT Modem
estimate to the multiplexer 332 for encoding by the encoder
Shelf/RF Channel then the following benefits may be rea334 for subsequent transmission over the wireless link 310 5 lised:
to the central terminal modem 320. Here, the GOS estimate
(i) Fault tolerance—should a CT Modem Shelf subis decoded by decoder 340, passed through multiplexer 345,
system fault occur, an ST may switch to an alternative
and then the GOS estimate is passed over line 355 to the
frequency for service.
dynamic pool sizing function 360.
(ii) Call blocking—an ST denied service from one CT
Additionally, incoming call information to the central 10
shelf may choose to switch to an alternative frequency
terminal, other than call information from the subscriber
for service.
terminals 20 connected to the central terminal, is provided
over the concentrated network interface 390 to the DA
(iii) Traffic load balancing—the Element Manager may on
engine 380. The DA engine 380 includes a call control
the basis of call blocking statistics choose to move STs
function, similar to the call control function 336 in each of
between CT shelves.
5
the subscriber terminals 20, for each of the modems on the
(iv) Frequency diversity—in the presence of channel
modem shelf. Hence, in a similar fashion to the call control
selective fading (slow multipath) an ST may operate on
function 336 at the subscriber terminals 20, the call control
the frequency channel offering highest signal strength
functions within the DA engine 380 are also able to provide
and lowest soft error count.
GOS estimates for incoming calls, and these GOS estimates
RF channel switching is only possible where there are two
are passed over line 395 to the dynamic pool sizing function 20
or more co-located CT shelves serving the same geographi360.
cal area on different RF frequency channels within the same
At set up, the management system 370 within the element
RF band. A deployment that meets this criterion may be
manager will have connected to the central terminal, and
configured as a 'Service Domain'. Possible deployment
provided the dynamic pool sizing function 360 within the
modem shelf with data identifying a BER goal, a GOS goal, 25 scenarios are illustrated in FIG. 18. FIG. 18(i) shows an
arrangement where omni antennae are used to provide the
and a pool size limit (i.e. the number of channels that can be
entire cell with four frequency channels, eg Fl, F4, F7, F10.
used for data traffic). The dynamic pool sizing function 360
FIG. 18(ii) shows an arrangement where sectored antennae
then compares this data from the management system with
are used to provide six separate sectors within a cell, each
the actual BER, actual GOS, and the actual pool size
information that it receives. A suitable algorithm can be 30 sector being covered by two frequency channels. FIG. 18(iii)
shows an alternative arrangement where three sectored
provided within the dynamic pool sizing function 360 to
antennae are used to divide the cell in to three sectors, each
determine, based on this information, whether pool sizing is
sector being covered by a separate frequency channel, and
appropriate. For example, if the actual bit error rate exceeds
then an omni antenna is used to provide an 'umbrella'
the BER goal provided by the management system 370, then
the dynamic pool sizing function 360 may be arranged to 35 coverage for the entire cell, this coverage employing a
frequency channel different to the three frequency channels
send a pool sizing request to the demand assignment engine
used by the sectored antennae.
380.
For the system to work effectively, the STs must be able
The demand assignment engine 380 provides modem
to switch channels quickly, and fast channel switching
enable signals over lines 400 to each of the modems on the
CT modem shelf. If the dynamic pool sizing function 360 40 necessitates that CT shelf synchronisation be provided at the
following levels:
has requested that the DA engine 380 perform pool sizing,
(i) CDMA PN code. This preserves uplink code phase
then the DA engine 380 can disable one or more of the
across RF channels during warm start; and
modems, this causing the interference, and hence the actual
BER, to be reduced. Apart from being used for interference
(ii) RF carrier frequency. This eliminates the need for the
limiting, the DA engine is also responsible, in preferred 45
coarse frequency search on a downlink RF channel
embodiments, for providing the encoders 325 with instrucswitch.
tions on which set of overlay codes or how many TDM slots
On installation, an ST will be programmed with an RF
to be used for signals to be transmitted to the STs 20.
channel and PN code, these codes specifying the ST's initial
The dynamic pool sizing function can store the BER and
home channel.
GOS information received in the storage 365, and periodi- 50
The manner in which channel switching is facilitated in
cally may pass that data to the management system 370 for
preferred embodiments will be described with reference to
analysis. Further, if the system is unable to attain the BER
FIGS. 19A and 19B. A service domain controller 400 is
or GOS goal with the allocated pool size, the dynamic pool
preferably provided to act as an interface between the
sizing function can be arranged to raise an alarm to the
exchange connected to the service domain controller over
management system. The receipt of this alarm will indicate 55 path 405 and a number of central terminals 10 connected to
to personnel using the management system that manual
the service domain controller over paths 410. The central
intervention may be required to remedy the situation, eg by
terminals connected to the service domain controller form a
the provision of more central terminal hardware to support
`service domain' of central terminals that may be used by a
the STs.
subscriber terminal 20 for handling communications.
The CDMA approach used in preferred embodiments 60
In preferred embodiments, the service domain controller
exhibits the property that the removal of any of the orthogo400 is used to provide each CT 10 with appropriate infornal channels (by disabling the modem) will improve the
mation about the other CTs within the service domain. Each
resistance of the other channels to interference. Hence, a
CT can then broadcast a 'Service Domain' message comsuitable approach for the demand assignment engine 380,
prising a list of RF frequencies and CT Identifiers that form
upon receipt of pool sizing request from the dynamic pool 65 a Service Domain to be used by the STs for subsequent RF
sizing function 360, is to disable the modem that has the
switching functions. The ST then stores this information for
least traffic passing through it.
future reference when establishing a link with one of the

US 6,222,819 B1

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26

CTs. It is preferable for each CT to broadcast the service
supplied over line 455 to the call control function 336 and
domain message since an ST may be listening to any of the
the channel selection controller 440 for processing. The
CTs at the time that the message is broadcast.
incoming call setup message will typically specify a TC on
Each CT database will hold an entry for every ST located
the current frequency channel which should be used to
within the Service Domain. Each database entry describes 5 access the incoming call, and the channel selection controlhow the CT views it's relationship with the ST and may be
ler will attempt to establish a link on that TC. The channel
marked as:
selection controller will in such cases instruct the radio
sub-system 420 over line 465 to use the current frequency
(i) Primary service provider—the CT is the ST's home
channel to establish the required link If, on the other hand,
channel. All management communication with an ST is
the traffic channel specified in the call setup message is
via it's home CT.
0
`null', the channel selection controller has the option to
(ii) Supplying backup service the CT is providing service
change RF frequency using the information stored in storage
to the ST.
445 about the other CTs in the service domain.
(iii) Available for backup service—the CT will provide
To enable the channel selection controller 440 to receive
service to the ST if required.
information about the status of links, a link operating status
It should be noted that the ST need not switch to an
entirely different CT, but can instead switch to a different CT is signal can be supplied over line 470 from the radio subsystem. This signal will give an indication of the radio link
shelf (and hence different RF frequency channel) within the
quality, and may be a simple 'OK' or 'failed' indication, or
same CT. However, in preferred embodiments, the ST will
alternatively may include extra information such as BER
typically switch to a different CT, since some errors expevalues for the link This information can be used by the
rienced by one CT shelf may also affect other shelves within
the same CT, and so for fault tolerance (described in more 20 channel selection controller to determine whether a particular frequency channel should be used or not.
detail below), it is preferable for the ST to switch to a
To enable the call control function to specify a specific
separate CT.
Access-Out channel for outgoing calls, a line 460 is proDatabase consistency across CT shelves is preferably
vided between the call control function 336 and the channel
supported through the service domain controller 400. Database consistency needs to be real-time so that an ST entering 25 selection controller 440. The call control function 336 may
choose an access-out channel from the free list in storage
the network is allowed full Service Domain access imme445, and instruct the channel selection controller over line
diately (the Service Domain message is broadcast to all STs,
460 to attempt acquisition of that channel.
and so a new ST will expect access across the full Service
The following examples indicate how the above described
Domain).
structure may be used to perform channel switching in
Incoming access via backup CTs requires some function 3 particular circumstances.
to be provided to broadcast duplicate incoming call setup
RF Channel Switching for Fault Tolerance
messages to all CTs that form a Service Domain. Preferably
Should one RF channel suffer complete loss of downlink,
this is handled by the service domain controller 400, which
the following process takes place in preferred embodiments:
forwards incoming call setup messages to each CT operating
(i) The ST will attempt downlink re-acquisition for a
in the service domain. All CTs will allocate Access In 35
period of time, say 20 seconds.
Traffic Channels and relay the incoming call setup message
(ii) If acquisition fails, the channel selection controller
via the Call Control Channel. On successful uplink access,
440 of the ST will select the next available channel
one CT will respond to the service domain controller with a
from the Service Domain information in storage 445
call accepted message, the other CTs will eventually respond
and attempt downlink acquisition. This process will be
with call setup failed messages. Outgoing access via a 40
repeated until a downlink signal is acquired.
backup CT is similar to normal outgoing access.
(iii) Once a backup RF channel is located, the ST will
Another job which can be performed by the service
`camp' on the Call Control Channel and may subsedomain controller is to assist the element manager 58 in
quently be granted traffic access.
reconfiguring equipment in the event of a fault. For example,
(iv) If the CT fault persists, the EM 58 may use the service
if one CT is taken out of commission because of a fault, a 45
domain controller 400 to reconfigure the Service
different CT can be brought 'on-line', and the service
Domain so that the functioning CT shelves become
domain controller can provide that new CT with the necesprimary service providers for the pool of 'homeless'
sary information about the other CTs in the service domain.
STs.
FIG. 19B illustrates those elements of the subscriber
A fault that does not result in complete loss of downlink
terminal used to implement RF channel switching. The radio 50
signal will not result in RF channel switching 'en mass'.
subsystem 420, which incorporates the transmission and
Rather, a fault may result in excessive or total call blocking,
reception signal processing stages, will pass any data
as discussed below.
received on the call control channel over line 425 to the
RF Channel Switching for Call Blocking
message decoder 430. If the decoder 430 determines that the
If Incoming access traffic channels are being blocked, the
data on the call control channel forms a service domain 55
following process is employed in preferred embodiments:
message, then this is passed over line 435 to the channel
(i) The call setup message sent over the Call Control
selection controller 440, where the information within the
Channel will specify a TC on which to access the call.
service domain message is stored in storage 445.
(ii) In the case of incoming access being blocked, the CT
Similarly, if the message decoder identifies the data as a
will specify a null TC. The channel selection controller
`free list' identifying the available traffic channels on a 60
440 of the ST will in such cases switch to the next RF
particular RF frequency, then this data is passed to the call
channel from the Service Domain information in storcontrol function 336 and the channel selection controller 440
age 445 and monitor the Call Control Channel.
over path 450. The call control function 336 stores the free
(iii) If the ST receives a call setup message with a valid
list in the storage 445 for subsequent use by the call control
TC, then the call is processed as normal.
function 336 and the channel selection controller 440.
65
If the message decoder 430 determines that the data forms
(iv) When the call clears, the ST downlink preferably
an incoming call setup message, then that information is
switches back to the home CT.

US 6,222,819 B1
27

28

If Outgoing access traffic channels are being blocked, the
3. A transmission controller as claimed in claim 1,
following process is employed in preferred embodiments:
wherein the orthogonal code generator is a storage arranged
(i) The ST registers an off-hook. The Free List in storage
to store the set of orthogonal codes.
445 is checked and if a traffic channel is available, then
4. A transmission controller as claimed in claim 1,
the call control function 336 asserts a channel request 5 wherein the overlay code generator is a storage arranged to
on line 460 to the channel selection controller 440 and
store the set of overlay codes.
normal uplink access is attempted.
5. A transmission controller as claimed in claim 1,
(ii) If the Free List shows no Access Out channels are
wherein the set of orthogonal codes comprise a set of
available on the current frequency channel, then the
Rademacher-Walsh (RW) codes.
channel selection controller will be used to switch the 10
6. A transmission controller as claimed in claim 5,
ST to the next RF channel in the Service Domain,
wherein the set of overlay codes are derived from RW codes,
whereupon the ST will wait for the next Free List.
each set of 'n' overlay codes comprising an nxn matrix of
(iii) When the ST finds a Free List with an available
RW codes.
Access Out channel, then uplink access is attempted
7. A central terminal of a wireless telecommunications
and the call is processed as normal.
15 system, comprising:
(iv) When the call clears, the ST downlink preferably
a transmission controller having:
switches back to the home CT.
an orthogonal code generator for providing an orthogonal
RF Channel Switching for Traffic Load Balancing
code from a set of 'm' orthogonal codes used to create
Traffic load balancing is, in preferred embodiments, pro`m' orthogonal channels within the single frequency
vided by static configuration via the EM 58. Call blocking
and setup time statistics may be forwarded to the EM where 20
channel, wherein 'm' is a positive integer;
an operator may decide to move an ST to another RF
a first encoder for combining a data item to be transmitted
channel.
on the single frequency channel with said orthogonal
RF Channel Switching for Frequency Diversity
code from the orthogonal code generator, the orthogoFrequency diversity is, in preferred embodiments, pronal code determining the orthogonal channel over
vided by static configuration via the EM 58. Radio link 25
which the data item is transmitted, whereby data items
statistics may be forwarded to the EM where an operator
pertaining to different wireless links may be transmitted
may decide to move an ST to another RF channel.
simultaneously within different orthogonal channels of
Although a particular embodiment has been described
said single frequency channel;
herein, it will be appreciated that the invention is not limited
an overlay code generator for providing an overlay code
thereto and that many modifications and additions thereto 30
from a first set of 'n' overlay codes which are orthogomay be made within the scope of the invention. For example,
various combinations of the features of the following depennal to each other, wherein 'n' is a positive integer;
dent claims could be made with the features of the indepena second encoder arranged to apply the overlay code from
dent claims without departing from the scope of the present
the overlay code generator to said data item, whereby
invention.
35
'n' data items pertaining to different wireless links may
What is claimed is:
be transmitted simultaneously within the same orthogo1. A transmission controller for processing data items to
nal channel, wherein the overlay code generator is
be transmitted over a wireless link connecting a central
arranged to provide overlay codes from one or more
terminal and a subscriber terminal of a wireless telecomfurther sets of overlay codes having different numbers
munications system, a single frequency channel being 40
of overlay codes to said first set of overlay codes,
employed for transmitting data items pertaining to a pluralwherein the orthogonal code generator is a storage
ity of wireless links, the transmission controller comprising:
arranged to store the set of orthogonal codes, wherein
an orthogonal code generator for providing an orthogonal
the overlay code generator is a storage arranged to store
code from a set of 'm' orthogonal codes used to create
the set of overlay codes, wherein the set of orthogonal
`m' orthogonal channels within the single frequency 45
codes comprise a set of Rademacher-Walsh (RW)
channel, wherein 'm' is a positive integer;
codes, and wherein the set of overlay codes are derived
a first encoder for combining a data item to be transmitted
from RW codes, each set of 'n' overlay codes comprison the single frequency channel with said orthogonal
ing an nxn matrix of RW codes.
code from the orthogonal code generator, the orthogo8. A central terminal as claimed in claim 7, wherein a first
nal code determining the orthogonal channel over 50 of the orthogonal channels is reserved for the transmission
which the data item is transmitted, whereby data items
of signals relating to the acquisition of wireless links, and
pertaining to different wireless links may be transmitted
the transmission controller is provided in the central termisimultaneously within different orthogonal channels of
nal to enable overlay codes to be applied to data items to be
said single frequency channel;
sent within said first orthogonal channel from the central
an overlay code generator for providing an overlay code 55 terminal to one of said subscriber terminals.
from a first set of 'n' overlay codes which are orthogo9. A central terminal as claimed in claim 8, wherein a
nal to each other, wherein 'n' is a positive integer; and
second of the orthogonal channels is reserved for the transa second encoder arranged to apply the overlay code from
mission of signals relating to the control of calls, and the
the overlay code generator to said data item, whereby
transmission controller is provided in the central terminal to
`n' data items pertaining to different wireless links may 60 enable overlay codes to be applied to data items to be sent
be transmitted simultaneously within the same orthogowithin said second orthogonal channel from the central
nal channel.
terminal to one of said subscriber terminals.
2. A transmission controller as claimed in claim 1,
10. A central terminal as claimed in claim 7, further
wherein the overlay code generator is arranged to provide
comprising channelisation means for determining which of
overlay codes from one or more further sets of overlay codes 65 the orthogonal channels will be subject to overlay codes, and
having different numbers of overlay codes to said first set of
for transmitting that information to a plurality of subscriber
overlay codes.
terminals within the wireless telecommunications system.

US 6,222,819 B1
29
11. A central terminal as claimed in claim 7, wherein a
number of said orthogonal channels are designated as traffic
channels for the transmission of data items relating to
communication content, said central terminal further comprising:
a TDM encoder arranged to apply time division multiplexing (TDM) techniques to data items to be sent over
a traffic channel from said central terminal to said
subscriber terminal, so as to enable a plurality of data
items pertaining to different wireless links to be sent
within one orthogonal traffic channel during a predetermined frame period.
12. A reception controller for processing data items
received over a wireless link connecting a central terminal
and a subscriber terminal of a wireless telecommunications
system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, the receiver controller comprising:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
`m' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first encoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal code;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
`n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
channel, wherein 'n' is a positive integer; and
a second encoder for applying, to the data items of the
orthogonal channel, the overlay code from the overlay
code generator so as to isolate a particular data item
transmitted using that overlay code.
13. A reception controller as claimed in claim 12, wherein
the overlay code generator is arranged to provide overlay
codes from one or more further sets of overlay codes having
different numbers of overlay codes to said first set of overlay
codes.
14. A reception controller as claimed in claim 12, wherein
the orthogonal code generator is a storage arranged to store
the set of orthogonal codes.
15. A reception controller as claimed in claim 12, wherein
the overlay code generator is a storage arranged to store the
set of overlay codes.
16. A reception controller as claimed in claim 12, wherein
the set of orthogonal codes comprise a set of RademacherWalsh (RW) codes.
17. A controller as claimed in claim 12, wherein the set of
overlay codes are derived from RW codes, each set of 'n'
overlay codes comprising an nxn matrix of RW codes.
18. A central terminal of a wireless telecommunications
system, comprising:
a reception controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
`m' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal channel;

30
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
`n' data items pertaining to different wireless links to be
5
transmitted simultaneously within the same orthogonal
channel, wherein 'n' is a positive integer; and
a second decoder for applying, to the data items of the
orthogonal channel, the overlay code from the overlay
code generator so as to isolate a particular data item
10
transmitted using that overlay code.
19. A central terminal as claimed in claim 18, further
comprising channelisation means for determining which of
the orthogonal channels will be subject to overlay codes, and
for transmitting that information to a plurality of subscriber
15 terminals within the wireless telecommunications system.
20. A central terminal as claimed in claim 19, wherein the
channelisation means also determines, for those orthogonal
channels subject to overlay codes, which set of overlay
codes will apply to each orthogonal channel.
20
21. A subscriber terminal of a wireless telecommunications system, comprising:
a transmission controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
25
'm' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first encoder for combining a data item to be transmitted
on the single frequency channel with said orthogonal
code from the orthogonal code generator, the orthogo30
nal code determining the orthogonal channel over
which the data item is transmitted, whereby data items
pertaining to different wireless links may be transmitted
simultaneously within different orthogonal channels of
said single frequency channel;
35
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, wherein 'm' is a positive integer;
a second encoder arranged to apply the overlay code from
the overlay code generator to said data item, whereby
40
`n' data items pertaining to different wireless links may
be transmitted simultaneously within the same orthogonal channel, wherein the overlay code generator is
arranged to provide overlay codes from one or more
further sets of overlay codes having different numbers
45
of overlay codes to said first set of overlay codes,
wherein the orthogonal code generator is a storage
arranged to store the set of orthogonal codes, wherein
the set of orthogonal codes comprise a set of
Rademacher-Walsh (RW) codes, and wherein the set of
50
overlay codes are derived from RW codes, each set of
`n' overlay codes comprising an nxn matrix of RW
codes; the transmission controller operable to enable
overlay codes to be applied to data items sent from the
subscriber terminals to the central terminal.
55
22. A subscriber terminal of a wireless telecommunications system, comprising:
a reception controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
60
`m' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
65
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal channel;

US 6,222,819 B1
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32

an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
`n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
channel, wherein 'n' is a positive integer;
a second decoder for applying, to the data items of the
orthogonal channel, the overlay code from the overlay
code generator so as to isolate a particular data item
transmitted using that overlay code, wherein the overlay code generator is arranged to provide overlay codes
from one or more further sets of overlay codes having
different numbers of overlay codes to said first set of
overlay codes, wherein the orthogonal code generator
is a storage arranged to store the set of orthogonal
codes, wherein the overlay code generator is a storage
arranged to store the set of overlay codes, wherein the
set of orthogonal codes comprise a set of RademacherWalsh (RW) codes, and wherein the set of overlay
codes are derived from RW codes, each set of 'n'
overlay codes comprising an nxn matrix of RW codes.
23. A wireless telecommunications system comprising a
central terminal and a plurality of subscriber terminals,
wherein the central terminal comprises:
a transmission controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
`m' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first encoder for combining a data item to be transmitted
on the single frequency channel with said orthogonal
code from the orthogonal code generator, the orthogonal code determining the orthogonal channel over
which the data item is transmitted, whereby data items
pertaining to different wireless links may be transmitted
simultaneously within different orthogonal channels of
said single frequency channel;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, wherein 'n' is a positive integer; and
a second encoder arranged to apply the overlay code from
the overlay code generator to said data item, whereby
`n' data items pertaining to different wireless links may
be transmitted simultaneously within the same orthogonal channel; and
at least one of the subscriber terminal comprises:
a reception controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
`m' orthogonal channels within the single frequency
channel;
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal channel;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
`n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
channel; and
a second decoder for applying, to the data items of the
orthogonal channel, the overlay code from the overlay
code generator so as to isolate a particular data item
transmitted using that overlay code.

24. A wireless telecommunications system comprising a
central terminal and a plurality of subscriber terminals,
wherein at least one of the subscriber terminals comprises:
a transmission controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
`m' orthogonal channels within the single frequency
channel, wherein 'm' is a positive integer;
a first encoder for combining a data item to be transmitted
on the single frequency channel with said orthogonal
code from the orthogonal code generator, the orthogonal code determining the orthogonal channel over
which the data item is transmitted, whereby data items
pertaining to different wireless links may be transmitted
simultaneously within different orthogonal channels of
said single frequency channel;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, wherein 'n' is a positive integer; and
a second encoder arranged to apply the overlay code from
the overlay code generator to said data item, whereby
`n' data items pertaining to different wireless links may
be transmitted simultaneously within the same orthogonal channel; and
the central terminal comprises:
a reception controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
'm' orthogonal channels within the single frequency
channel;
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal channel;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
`n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
channel; and
a second decoder for applying, to the data items of the
orthogonal channel, the overlay code from the overlay
code generator so as to isolate a particular data item
transmitted using that overlay code.
25. A method of processing data items to be transmitted
over a wireless link connecting a central terminal and a
subscriber terminal of a wireless telecommunications
system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, the method comprising steps of:
providing an orthogonal code from a set of 'm' orthogonal
codes used to create 'm' orthogonal channels within the
single frequency channel, wherein 'm' is a positive
integer;
combining a data item to be transmitted on the single
frequency channel with said orthogonal code, the
orthogonal code determining the orthogonal channel
over which the data item is transmitted, whereby data
items pertaining to different wireless links may be
transmitted simultaneously within different orthogonal
channels of said single frequency channel;
providing an overlay code from a first set of 'n' overlay
codes which are orthogonal to each other, wherein 'n'
is a positive integer; and

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US 6,222,819 B1
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applying the overlay code to said data item, whereby 'n'
data items pertaining to different wireless links may be
transmitted simultaneously within the same orthogonal
channel.
26. A method as claimed in claim 25, further comprising
a step of:
providing one or more further sets of overlay codes
having different numbers of overlay codes to said first
set of overlay codes.
27. A method as claimed in claims 25, further comprising
steps of:
determining which of the orthogonal channels will be
subject to overlay codes; and
transmitting that information to a plurality of subscriber
terminals within the wireless telecommunications system.
28. A method as claimed in claim 27, further comprising
a step of:
determining, for those orthogonal channels subject to
overlay codes, which set of overlay codes will apply to
each orthogonal channel.
29. A method of processing data items received over a
wireless link connecting a central terminal and a subscriber
terminal of a wireless telecommunications system, a single
frequency channel being employed for transmitting data
items pertaining to a plurality of wireless links, the method
comprising the steps of providing an orthogonal code from
a set of 'm' orthogonal codes used to create 'm' orthogonal
channels within the single frequency channel, wherein 'm' is
a positive integer;
applying, to signals received on the single frequency
channel, the orthogonal code in order to isolate data
items transmitted within the corresponding orthogonal
channel;

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providing an overlay code from a first set of 'n' overlay
codes which are orthogonal to each other, the set of 'n'
overlay codes enabling 'n' data items pertaining to
different wireless links to be transmitted simultaneously within the same orthogonal channel, wherein
`n' is a positive integer; and
applying, to the data items of the orthogonal channel, the
overlay code so as to isolate a particular data item
transmitted using that overlay code.
30. A method as claimed in claim 29, further comprising
a step of:
providing one or more further sets of overlay codes
having different numbers of overlay codes to said first
set of overlay codes.
31. A method as claimed in claim 29, further comprising
steps of:
determining which of the orthogonal channels will be
subject to overlay codes; and

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transmitting that information to a plurality of subscriber
terminals within the wireless telecommunications system.
32. A method as claimed in claim 31, further comprising
a step of:

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determining, for those orthogonal channels subject to
overlay codes, which set of overlay codes will apply to
each orthogonal channel.