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 A

111111111111111111111111111111111111111111111111111111111111111111111111111
US006088326A

United States Patent

[19]

[11]

Lysejko et al.

[45]

[54]

Inventors: Martin Lysejko, Bagshot, United
Kingdom; Paul F. Struhsaker, Plano,
Tex.

[73]

Assignee: Airspan Communications
Corporation, Wilmington, Del.

0730356
9/1996
2301744 12/1996
9314590 7/1993
9315573
8/1993
9523464 8/1995

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

[75]

[21]

Filed:

[30]

Nov. 26, 1997

Foreign Application Priority Data

Dec. 20, 1996

[51]
[52]
[58]

[GB]

United Kingdom

9626567

Int. CI?

H04J 11/00; H041 13/00;
H04B 7/216
U.S. CI.
370/209; 370/342; 370/345;
370/441; 370/442; 370/479
370/328, 329,
Field of Search
370/330,335,336,337,340,341,342,
343, 345, 347, 441, 442, 465, 468, 479,
498, 203, 208, 209
References Cited

[56]

U.S. PATENT DOCUMENTS
4,688,210 8/1987 Eizenhoffer et al.
4,799,252 1/1989 Eizenhoffer et al.
5,373,502 12/1994 Turban ..
5,592,469 1/1997 Szabo
6,005,854 12/1999 Xu et al.

370/342
370/342
370/18
370/342
370/335

FOREIGN PATENT DOCUMENTS
0652650

5/1995

European Pat. Off

6,088,326
Jui. 11,2000

European Pat. Off
United Kingdom
WIPO
WIPO
WIPO

H04L 1/00
H04Q 7/32
H04N 1/00
H04J 13/00
H04J 3/22

Primary Examiner-Ricky Ngo
Attorney, Agent, or Firm-Baker Botts L.L.P.
[57]

Appl. No.: 08/979,408

[22]

Patent Number:
Date of Patent:

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 a IDM encoder arranged
to apply time division multiplexing (TDM) techniques to the
data item in order to insert the data item within a time slot
of the orthogonal channel, whereby a plurality of data items
relating to different wireless links may be transmitted within
the same orthogonal channel during a predetermined frame
period. The invention also provides a reception controller
and method for processing data items received over a
wireless link.
15 Claims, 16 Drawing Sheets

H04B 7/26

113

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Jui. 11,2000

6,088,326

Sheet 1 of 16

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Sheet 2 of 16

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u.s. Patent

6,088,326

Sheet 13 of 16

Jui. 11,2000

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Sheet 16 of 16

Jul. 11,2000

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6,088,326
1

2

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

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 termi10 nal 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
BACKGROUND OF THE INVENTION
code generator for providing an orthogonal code from a set
A wireless telecommunications system has been proposed 15 of 'm' orthogonal codes used to create 'm' orthogonal
in which a geographical area is divided in to cells, each cell
channels within the single frequency channel; a first encoder
having one or more central terminals (CTs) for communifor combining a data item to be transmitted on the single
cating over wireless links with a number of subscriber
frequency channel with said orthogonal code from the
terminals (STs) in the cell. These wireless links are estaborthogonal code generator, the orthogonal code determining
lished over predetermined frequency channels, a frequency 20 the orthogonal channel over which the data item is
channel typically consisting of one frequency for uplink
transmitted, whereby data items pertaining to different wiresignals from a subscriber terminal to the central terminal,
less links may be transmitted simultaneously within different
and another frequency for downlink signals from the central
orthogonal channels of said single frequency channel; and a
terminal to the subscriber terminal.
TDM encoder arranged to apply time division multiplexing
Due to bandwidth constraints, it is not practical for each 25 (TDM) techniques to the data item in order to insert the data
item within a time slot of the orthogonal channel, whereby
individual subscriber terminal to have its own dedicated
a plurality of data items relating to different wireless links
frequency channel for communicating with the central termay be transmitted within the same orthogonal channel
minal. Hence, techniques need to be applied to enable data
items relating to different wireless links to be passed over the 30 during a predetermined frame period.
same frequency channel without interfering with each other.
Viewed from a second aspect, the present invention
In current wireless telecommunications systems, this can be
provides a reception controller for processing data items
achieved through the use of a 'Code Division Multiple
received over a wireless link connecting a central terminal
Access' (CDMA) technique. One way to implement CDMA
and a subscriber terminal of a wireless telecommunications
is through the application of a set of orthogonal codes to the 35 system, a single frequency channel being employed for
data items to be transmitted on a particular frequency
transmitting data items pertaining to a plurality of wireless
channel, data items relating to different wireless links being
links, and 'm' orthogonal channels being provided within the
combined with different orthogonal codes from the set. A
single frequency channel, the receiver controller comprissuitable set of orthogonal codes is a "Rademacher-Walsh"
ing: an orthogonal code generator for providing an orthogo(RW) set of sixteen 16-bit codes. Orthogonal codes have the 40 nal code from a set of 'm' orthogonal codes used to create
property that, when perfectly aligned, all codes crosssaid 'm' orthogonal channels within the single frequency
correlate to zero, thus making it possible to decode a signal
channel; a first decoder for applying, to signals received on
to which one orthogonal code has been applied while
the single frequency channel, the orthogonal code provided
cancelling interference from signals to which different
by the orthogonal code generator, in order to isolate data
orthogonal codes have been applied.
45 items transmitted within the corresponding orthogonal channel; and a TDM decoder arranged to extract a data item from
Signals to which an orthogonal code has been applied can
a predetermined time slot within said orthogonal channel, a
be considered as being transmitted over a corresponding
plurality of data items relating to different wireless links
orthogonal channel within a particular frequency channel.
being transmitted within the same orthogonal channel during
Hence, considering the example of a set of 16 RW codes, 16
orthogonal channels can be created within a single fre- 50 a predetermined frame period.
quency channel, and hence up to sixteen separate commuBy using TDM techniques in addition to the known set of
nication signals (corresponding to sixteen separate wireless
orthogonal codes, it is possible for selected orthogonal
links) can be transmitted simultaneously over the single
channels to be subdivided in the time dimension. For
frequency channel if different RW codes are applied to each
example, if TDM is used to divide one frame period in to
communication signal.
55 four subframes, and each orthogonal channel is subject to
the TDM technique, then up to 64 separate communication
It is known to provide a number of modem shelves within
signals can be transmitted on the sixteen orthogonal chanone central terminal, and for each modem shelf to employ a
nels during one frame period, albeit at a quarter of the rate
different frequency channel. Hence, if a central terminal has
that the communication signals could be transmitted if the
four modem shelves, and the set of 16 RW codes is
employed for each frequency channel, one central terminal 60 TDM technique was not used.
would be able to support wireless links with up to 60
Such an approach has the advantage that it preserves
subscriber terminals simultaneously.
compatibility with current hardware and software equipment
However, as more subscribers subscribe to the wireless
which use the set of orthogonal codes, but which do not
telecommunications network, it is becoming desirable to
support the use of TDM techniques. By designating certain
support more and more subscriber terminals from each 65 orthogonal channels as channels for which TDM is not used,
central terminal. There are only a limited number of frethe current equipment can communicate over those channels
quency channels that can be allocated to the wireless telewithout any changes being required to the equipment.
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.

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4

In preferred embodiments, the transnussion controller
said orthogonal channel will receive data at a rate of 40 kb/s
further comprises: an overlay code generator for providing
(since each ST will only read a quarter of the data transan overlay code from a first set of 'n' overlay codes which
mitted on the orthogonal channel during each frame period).
If, alternatively, two time slots are provided within the
are orthogonal to each other; and a second encoder, selectively operable instead of the TDM encoder, to apply the 5 orthogonal channel, then data items pertaining to only two
overlay code from the overlay code generator to said data
different wireless links will be transmitted per frame period,
item, whereby 'n ' data items pertaining to different wireless
and the two STs receiving data will do so at a rate of 80 kb/s
(since each ST will read half of the data transmitted on the
links may be transmitted simultaneously within the same
orthogonal channel during one frame period). This flexibilorthogonal channel.
Similarly, the reception controller may further comprise: 10 ity is useful, since for some communications, ego fax, a rate
of 40 kb/s may not be acceptable, and hence the use of four
an overlay code generator for providing an overlay code
time slots would not be suitable.
from a first set of 'n ' overlay codes which are orthogonal to
each other, the set of 'n ' overlay codes enabling 'n ' data
In preferred embodiments, a number of said orthogonal
channels are designated as traffic channels for the transmisitems pertaining to different wireless links to be transmitted
simultaneously within the same orthogonal channel; and a 15 sion of data items relating to communication content, and
the TDM encoder is employed to apply time division mulsecond decoder, selectively operable instead of the TDM
tiplexing (TDM) techniques to data items to be sent over a
decoder, to apply to the data items of the orthogonal channel,
traffic channel from said central terminal to said subscriber
the overlay code from the overlay code generator so as to
terminal. The use of this CDMNTDM hybrid approach for
isolate a particular data item transmitted using that overlay
20 downlink traffic channels retains the benefit of CDMA
code.
access, i.e. interference is reduced when traffic is reduced,
By such an approach, data items transmitted within cerand also reduces receiver dynamic range requirements.
tain orthogonal channels can be encoded using TDM techniques whilst data items transmitted within other orthogonal
However, a first of the orthogonal channels is preferably
channels can be encoded using overlay codes, the reception
reserved for the transmission of signals relating to the
controllers including the necessary decoders to decode either 25 acquisition of wireless links, and the second encoder is used
type of encoded data item. A preferred arrangement, where
instead of the TDM encoder to enable overlay codes to be
certain orthogonal channels are subject to TDM techniques
applied to data items to be sent within said first orthogonal
whilst others are subject to overlay codes, will be discussed
channel from the central terminal to one of said subscriber
in more detail later.
terminals. Similarly, a second of the orthogonal channels is
The orthogonal code generator may be arranged to gen- 30 preferably reserved for the transmission of signals relating to
the control of calls, and the second encoder is used instead
erate orthogonal codes 'on the fly' using predetermined
of the TDM encoder to enable overlay codes to be applied
algorithms. However, alternatively, the orthogonal code
to data items to be sent within said second orthogonal
generator may be provided as a storage arranged to store the
channel from the central terminal to one of said subscriber
set of orthogonal codes. Appropriate orthogonal codes can
then be read out to the encoder or decoder from the storage 35 terminals.
as required.
In preferred embodiments, at least one of the subscriber
terminals of a wireless telecommunications system comIn preferred embodiments, the set of orthogonal codes
prises a reception controller in accordance with the present
comprise a set of Rademacher-Walsh (RW) codes, in preferred embodiments the set comprising a 16x16 matrix of 40 invention. However, for transmission of data from subscriber terminals, it is preferable for the ST to have a
RW codes.
transmission controller which employs overlay codes for all
The transmission controller in accordance with the
types of orthogonal channels, whether they be traffic chanpresent invention may be provided within the central terminels or otherwise. On these uplink channels, the pure CDMA
nal of a wireless telecommunications system. In preferred
45 approach using overlay codes eliminates the need to time
embodiments, the central terminal would further comprise
synchronise STs to a TDM frame reference, and reduces the
channelisation means for determining which of the orthogopeak power handling requirements in the ST RF transmit
nal channels will be subject to TDM techniques, and for
chain.
transmitting that information to a plurality of subscriber
Viewed from a third aspect, the present invention provides
terminals within the wireless telecommunications system.
This is useful since, for example, certain orthogonal chan- 50 a wireless telecommunications system comprising a central
terminal and a plurality of subscriber terminals, wherein the
nels can hence be designated as being reserved for commucentral terminal comprises a transmission controller in
nications with STs that do not incorporate the features
accordance with the present invention, and at least one of the
necessary to support TDM techniques, and which hence
subscriber terminal comprises a reception controller in
require the full orthogonal channel for the whole frame
55 accordance with the present invention.
period.
In preferred embodiments, the channelisation means also
Viewed from a fourth aspect, the present invention prodetermines, for those orthogonal channels subject to TDM
vides a method of processing data items to be transmitted
techniques, how many time slots will be provided within
over a wireless link connecting a central terminal and a
each orthogonal channel. This gives a great deal of flexibilsubscriber terminal of a wireless telecommunications
ity in how channels are used, since some can be subdivided 60 system, a single frequency channel being employed for
in the time dimension whilst others are not, and those which
transmitting data items pertaining to a plurality of wireless
are subdivided can be subdivided differently to yield differlinks, the method comprising the steps of: (a) providing an
orthogonal code from a set of 'rn ' orthogonal codes used to
ing numbers of time slots per frame period. For instance, if
an orthogonal channel operates at 160 kb/s, and four time
create 'm' orthogonal channels within the single frequency
slots are provided within that orthogonal channel in order to 65 channel; (b) combining a data item to be transmitted on the
carry data items pertaining to four different wireless links
single frequency channel with said orthogonal code, the
during one frame period, then each ST receiving data from
orthogonal code determining the orthogonal channel over

6,088,326

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6

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; and (c) applying time division
multiplexing (TDM) techniques to the data item in order to
insert the data item within a time slot of the orthogonal
channel, whereby a plurality of data items relating to different wireless links may be transmitted within the same
orthogonal channel during a predetermined frame period.
Viewed from a fifth aspect, the present invention provides
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, and 'm' orthogonal
channels being provided within the single frequency
channel, the method comprising the steps of: (a) providing
an orthogonal code from a set of 'm' orthogonal codes used
to create said 'rn ' orthogonal channels within the single
frequency channel; (b) applying, to signals received on the
single frequency channel, the orthogonal code in order to
isolate data items transmitted within the corresponding
orthogonal channel; and (c) extracting a data item from a
predetermined time slot within said orthogonal channel, a
plurality of data items relating to different wireless links
being transmitted within the same orthogonal channel during
a predetermined frame period. By using TDM techniques in
addition to the known set of orthogonal codes, it is possible
for selected orthogonal channels to be subdivided in the time
dimension, thereby making it possible to support more
wireless links on one frequency channel.

FIG. 10 illustrates the CDMAchannel 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;
FIGS. 14A and 14B illustrate the overhead frame structure for the downlink and uplink paths;
FIGS. 15Aand 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.

5

10

15

20

25

30

BRIEF DESCRIPTION OF IRE 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 representation of a premises; FIGS.
2A and 2B are schematic illustrations 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. 3Ais 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;

35

40

45

50

55

60

65

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 KIn. 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 KIn. 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

6,088,326
7

8

that the number of subscribers which can be supported
central terminal 10. The site controller 56 can be connected
exceeds the number of available wireless links. The manner
to each modem shelf of the central terminal 10 via, for
in which demand-based access is implemented will be
example, RS232 connections 55. The site controller 56 can
discussed in detail later.
then provide support functions such as the localisation of
FIGS. 2A and 2B illustrate an example of a configuration 5 faults, alarms and status and the configuring of the central
terminal 10. A site controller 56 will typically support a
for a subscriber terminal 20 for the telecommunications
single central terminal 10, although a plurality of site
system of FIG. 1. FIG. 2 includes a schematic representation
controllers 56 could be networked for supporting a plurality
of customer premises 22. A customer radio unit (CRU) 24 is
of central terminals
mounted on the customer's premises. The customer radio
unit 24 includes a fiat panel antenna or the like 23. The 10
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
X.25 links 57 (shown with dashed lines in FIG. 3) could
er's premises, or on a mast, etc., and in an orientation such
instead be provided from a pad 228 to a switching node 60
that the fiat panel antenna 23 within the customer radio unit
of an element manager (EM) 58. An element manager 58 can
24 faces in the direction 26 of the central terminal 10 for the
support a number of distributed central terminals 10 conservice area in which the customer radio unit 24 is located.
The customer radio unit 24 is connected via a drop line 28 15 nected by respective connections to the switching node 60.
The element manager 58 enables a potentially large number
to a power supply unit (PSU) 30 within the customer's
(e.g., up to, or more than 1000) of central terminals 10 to be
premises. The power supply unit 30 is connected to the local
integrated into a management network. The element manpower supply for providing power to the customer radio unit
ager 58 is based around a powerful workstation 62 and can
24 and a network terminal unit (NTU) 32. The customer 20 include a number of computer terminals 64 for network
radio unit 24 is also connected via the power supply unit 30
engineers and control personnel.
to the network terminal unit 32, which in turn is connected
FIG. 3A illustrates various parts of a modem shelf 46. A
to telecommunications equipment in the customer's
transmit/receive RF unit (RFU-for example implemented
premises, for example to one or more telephones 34, facon a card in the modem shelf) 66 generates the modulated
simile machines 36 and computers 38. The telecommunica- 25 transmit RF signals at medium power levels and recovers
tions equipment is represented as being within a single
and amplifies the baseband RF signals for the subscriber
customer's premises. However, this need not be the case, as
terminals. The RF unit 66 is connected to an analogue card
the subscriber terminal 20 preferably supports either a single
(AN) 68 which performs A-D/D-A conversions, baseband
or a dual line, so that two subscriber lines could be supported
filtering and the vector summation of 15 transmitted signals
by a single subscriber terminal 20. The subscriber terminal 30 from the modem cards (MCs) 70. The analogue unit 68 is
20 can also be arranged to support analogue and digital
connected to a number of (typically 1-8) modem cards 70.
telecommunications, for example analogue communications
The modem cards perform the baseband signal processing of
at 16, 32 or 64 kbits/sec or digital communications in
the transmit and receive signals to/from the subscriber
accordance with the ISDN BRA standard.
terminals 20. This may include Y2 rate convolution coding
FIG. 3 is a schematic illustration of an example of a 35 and x16 spreading with "Code Division Multiplexed
central terminal of the telecommunications system of FIG. 1.
Access" (CDMA) codes on the transmit signals, and synThe common equipment rack 40 comprises a number of
chronisation recovery, de-spreading and error correction on
equipment shelves 42, 44, 46, including a RF Combiner and
the receive signals. Each modem card 70 in the present
power amp shelf (RFC) 42, a Power Supply shelf (PS) 44
example has two modems, and in preferred embodiments
and a number of (in this example four) Modem Shelves 40 there are eight modem cards per shelf, and so sixteen
(MS) 46. The RF combiner shelf 42 allows the modem
modems per shelf. However, in order to incorporate redunshelves 46 to operate in parallel. If 'n' modem shelves are
dancy so that a modem may be substituted in a subscriber
provided, then the RF combiner shelf 42 combines and
link when a fault occurs, only 15 modems on a single
amplifies the power of 'n ' transmit signals, each transmit
modem shelf 46 are generally used. The 16th modem is then
signal being from a respective one of the 'n ' modem shelves, 45 used as a spare which can be switched in if a failure of one
and amplifies and splits received signals 'n' way so that
of the other 15 modems occurs. The modem cards 70 are
separate signals may be passed to the respective modem
connected to the tributary unit (TU) 74 which terminates the
shelves. The power supply shelf 44 provides a connection to
connection to the host public switched telephone network 18
the local power supply and fusing for the various compo(e.g., via one of the lines 47) and handles the signalling of
nents in the common equipment rack 40. A bidirectional 50 telephony information to the subscriber terminals via one of
connection extends between the RF combiner shelf 42 and
15 of the 16 modems.
the main central terminal antenna 52, such as an omnidiThe wireless telecommunications between a central terrectional antenna, mounted on a central terminal mast 50.
minal 10 and the subscriber terminals 20 could operate on
This example of a central terminal 10 is connected via a
various frequencies. FIG. 4 illustrates one possible example
point-to-point microwave link to a location where an inter- 55 of the frequencies which could be used. In the present
face to the public switched telephone network 18, shown
example, the wireless telecommunication system is intended
schematically in FIG. 1, is made. As mentioned above, other
to operate in the 1.5-2.5 GHz Band. In particular the present
types of connections (e.g., copper wires or optical fibres) can
example is intended to operate in the Band defined by ITU-R
be used to link the central terminal 10 to the public switched
(CCIR) Recommendation F.701 (2025-2110 MHz,
telephone network 18. In this example the modem shelves 60 2200-2290 MHz). FIG. 4 illustrates the frequencies used for
are connected via lines 47 to a microwave terminal (MT) 48.
the uplink from the subscriber terminals 20 to the central
A microwave link 49 extends from the microwave terminal
terminal 10 and for the downlink from the central terminal
48 to a point-to-point microwave antenna 54 mounted on the
10 to the subscriber terminals 20. It will be noted that 12
mast 50 for a host connection to the public switched teleuplink and 12 downlink radio channels of 3.5 MHz each are
phone network 18.
65 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.

6,088,326

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10

In the present example, each modem shelf supports 1
frequency channel (i.e. one uplink frequency plus the corresponding downlink frequency). Currently, in a wireless
telecommunications system as described above, CDMA
encoding is used to support up to 15 subscriber links on one
frequency channel (one subscriber link on each modem).
Hence, if a central terminal has four modem shelves, it can
support 60 (15x4) subscriber links (i.e. 60 STs can be
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 terminall0 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. SA 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: FSl=Fl,F4, F7, FlO; FS2=F2,F5,F8,Fll;FS3=F3,
F6, F9, FI2), 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).
FIG. 5B 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: FSl=Fl,F4, F7, FlO; FS2=F2,F5,F8,Fll;FS3=F3,
F6, F9, FI2). 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 SI, S2 and S3,
with the transmissions for each of the three central terminals
10 being directed to the appropriate sector among SI, S2 and
S3. This enables he 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 operating 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
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
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
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
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
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.
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 CDMAsignal is received at the
receiving antenna 91, the multiple user channel signals may
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,
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
function of "orthonormality". In other words, if any RW
code is multiplied by any other RW code, the results are
zero. Aset of 16 RW codes that may be used is illustrated in
Table 1 below:

5

10

15

20

25

30

35

40

45

50

TABLE 1
RWO
RW1
RW2
RW3
RW4
RW5
RW6
RW7
RW8
RW9

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

6,088,326
11

12

TABLE l-continucd
RW10
RWll
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

10

Overlay codes are used extensively to provide variable
The above set of RW codes are orthogonal codes that
rate uplink traffic channels. Overlay codes will also be used
allow the multiple user signals to be transmitted and
to implement downlink control channels, these control chanreceived on the same frequency at the same time. Once the
nels being discussed in more detail later. However, as
bit stream is orthogonally isolated using the RW codes, the
signals for respective subscriber links do not interfere with 15 mentioned earlier, a different approach is taken for providing
each other. Since RW codes are orthogonal, when perfectly
flexible channelisations on the downlink traffic channel
paths. Downlink traffic channels will operate in high rate,
aligned all codes have zero cross-correlation, thus making it
160 kb/s, mode, with lower data rates of 80 and 40 kb/s
possible to decode a signal while cancelling interference
being supported by 'Time Division Multiplexing' (TDM)
from users operating on other RW codes.
In preferred embodiments of the present invention, it is 20 the available bandwidth.
desired to provide the central terminal with the ability to
In preferred embodiments, TDM timeslot bit numbering
will follow the CCITT G.732 convention with bits transsupport more than 15 subscriber links on each channel
frequency, and to achieve this the above set of 16 RW codes
mitted in the sequence bit 1, bit 2... bit 8. Byte orientation
has been enhanced. In order to maintain compatibility with
is specified per channel as either most significant bit (MSB)
former products using the 16 RW codes, it was desirable that 25 first, least significant bit (LSB) first or N/A.
any enhancements should retain the same set of 16 RW
The provision of a hybrid CDMA/TDM approach for
codes.
downlink traffic channels retains the benefits of CDMA
The manner in which the enhancements have been impleaccess, ie. interference is reduced when traffic is reduced.
mented provides flexibility in the way the frequency chanFurther, use of TDM ensures that the CDMA signal is
nels are configured, with certain configurations allowing a 30 limited to a 256 'Quadrature Amplitude Modulation' (QAM)
greater number of subscriber links to be supported, but at a
constellation which reduces receiver dynamic range requirements. QAM constellations will be familiar to those skilled
lower gross bit rate. In preferred embodiments, a channel
can be selected to operate with the following gross bit rates:
in the art.
35
On the uplink channels, the pure CDMA approach using
overlay codes eliminates the need to time synchronise STs to
a TDM frame reference. This has the advantage of elimi160 kb/s
Full rate (F1)
nating TDM delays and the 'guard time' in between TDM
80 kb/s
Half rate (H1, H2)
Quarter rate (Q1, Q2, Q3, Q4)
40 kb/s
frames. Another benefit is reduced peak power handling
10 kb/s
Low rate (Ll, L2, L3, L4), for uplink acquisition
40 requirements in the ST RF transmit chain which would
otherwise be needed when transmitting bursty TDM data.
High dynamic range requirement is restricted to the CT
In preferred embodiments, the manner in which these
recerver.
channelisations are provided differs for the downlink (CT to
The manner in which the transmitted and received signals
ST) and uplink (ST to CT) communication paths. This is
because it has been realised that different performance 45 are processed in accordance with preferred embodiments of
the present invention will be described with reference to
requirements exist for the downlink and uplink paths. On the
FIGS. 7 and 8. FIG. 7A is a schematic diagram illustrating
downlink all signals emanate from a single source, namely
signal transmission processing stages as configured in a
the central terminal, and hence the signals will be synchrosubscriber terminal 20 in the telecommunications system of
nised. However, on the uplink path, the signals will emanate
from a number of independent STs, and hence the signals 50 FIG. 1. In FIG. 7A, an analogue signal from a telephone is
passed via an interface such as two-wire interface 102 to a
will not be synchronised.
hybrid audio processing circuit 104 and then via a codec 106
Given the above considerations, in preferred
to produce a digital signal into which an overhead channel
embodiments, on the uplink path full rate (160 kb/s) operaincluding control information is inserted at 108. If the
tion is implemented using the basic set of RW codes
discussed earlier, but half and quarter rates are achieved 55 subscriber terminal supports a number of telephones or other
telecommunications equipment, then elements 102, 104 and
through the use of 'Overlay Codes' which comprise RW
106 may be repeated for each piece of telecommunications
coded high rate symbol patterns that are transmitted for each
equipment.
intermediate rate data symbol. For half rate operation, two
At the output of overhead insertion circuit 108, the signal
2-bit overlay codes are provide, whilst for quarter rate
operation, four 4-bit overlay codes are provided. When 60 will have a bit rate of either 160, 80 or 40 kbits/s, depending
on which channel has been selected for transmission of the
generating a signal for transmission, one of the overlay
signal.
codes, where appropriate, is applied to the signal in addition
to the appropriate RW code. When the signal is received,
The resulting signal is then processed by a convolutional
then at the CDMA demodulator the incoming signal is
encoder 110 to produce two signals with the same bit rate as
multiplied by the channel's PN, RW and Overlay codes. The 65 the input signal (collectively, these signals will have a
symbol rate of 160, 80 or 40 KS/s). Next, the signals are
correlator integration period is set to match the length of the
passed to a spreader 111 where, if a reduced bit rate channel
Overlay code.

6,088,326

13

14

code generator providing appropriate overlay codes to the
has been selected, an appropriate overlay code provided by
spreader 111. The overlay code generator will be controlled
overlay code generator 113 is applied to the signals. At the
so as to produce the desired overlay code, in preferred
output of the spreader 111, the signals will be at 160 KS/s
embodiments, this control coming from the DAengine (to be
irrespective of the bit rate of the input signal since the
overlay code will have increased the symbol rate by the 5 discussed in more detail later).
necessary amount.
FIG. 8A is a schematic diagram illustrating the signal
The signals output from spreader 111 are passed to a
reception processing stages as configured in a subscriber
spreader 116 where the Rademacher-Walsh and PN codes
terminal 20 in the telecommunications system of FIG. 1. In
are applied to the signals by a RW code generator 112 and
FIG. 8A, signals received at a receiving antenna 150 are
PN Code generator 114, respectively. The resulting signals, 10 passed via a band pass filter 152 before being amplified in
a low noise amplifier 154. The output of the amplifier 154 is
at 2.56 MC/s (2.56 Mega chips per second, where a chip is
then passed via a further band pass filter 156 before being
the smallest data element in a spread sequence) are passed
further amplified by a further low noise amplifier 158. The
via a digital to analogue converter 118. The digital to
output of the amplifier 158 is then passed to a mixer 164
analogue converter 118 shapes the digital samples into an
analogue waveform and provides a stage of baseband power 15 where it is mixed with a signal generated by a voltage
controlled oscillator 162 which is responsive to a synthesizer
control. The signals are then passed to a low pass filter 120
160. The output of the mixer 164 is then passed via the I/Q
to be modulated in a modulator 122. The modulated signal
de-modulator 166 and a low pass filter 168 before being
from the modulator 122 is mixed with a signal generated by
passed to an analogue to digital converter 170. The digital
a voltage controlled oscillator 126 which is responsive to a
synthesizer 160. The output of the mixer 128 is then 20 output of the AID converter 170 at 2.56 MC/s is then passed
to a correlator 178, to which the same Rademacher-Walsh
amplified in a low noise amplifier 130 before being passed
and PN codes used during transmission are applied by a RW
via a band pass filter 132. The output of the band pass filter
code generator 172 (corresponding to the RW code genera132 is further amplified in a further low noise amplifier 134,
tor 112) and a PN code generator 174 (corresponding to PN
before being passed to power control circuitry 136. The
output of the power control circuitry is further amplified in 25 code generator 114), respectively. The output of the correlator 178, at 160 KS/s, is then applied to correlator 179,
a power amplifier 138 before being passed via a further band
where any overlay code used at the transmission stage to
pass filter 140 and transmitted from the transmission antenna
encode the signal is applied to the signal by overlay code
142.
generator 181. The elements 170, 172, 174, 178, 179 and
FIG. 7B is a schematic diagram illustrating signal transmission processing stages as configured in a central terminal 30 181 form a CDMA demodulator. The output from the
CDMA demodulator (at correlator 179) is then at a rate of
10 in the telecommunications system of FIG. 1. As will be
either 160, 80 or 40 KS/s, depending on the overlay code
apparent, the central terminal is configured to perform
applied by correlator 179.
similar signal transmission processing to the subscriber
The output from correlator 179 is then applied to a Viterbi
terminal 20 illustrated in FIG. 7A, but does not include
elements 100, 102, 104 and 106 associated with telecom- 35 decoder 180. The output of the Viterbi decoder 180 is then
passed to an overhead extractor 182 for extracting the
munications equipment. Further, the central terminal
overhead channel information. If the signal relates to call
includes a TDM encoder 105 for performing time division
data, then the output of the overhead extractor 182 is then
multiplexing where required. The central terminal will have
passed through TDM decoder 183 to extract the call data
a network interface over which incoming calls destined for
a subscriber terminal are received. When an incoming call is 40 from the particular time slot in which it was inserted by the
CT TDM encoder 105. Then, the call data is passed via a
received, the central terminal will contact the subscriber
codec 184 and a hybrid circuit 188 to an interface such as
terminal to which the call is directed and arrange a suitable
two wire interface 190, where the resulting analogue signals
channel over which the incoming call can be established
are passed to a telephone 192. As mentioned earlier in
with the subscriber terminal (in preferred embodiments, this
is done using the call control channel discussed in more 45 connection with the ST transmission processing stages,
elements 184, 188, 190 may be repeated for each piece of
detail later). The channel established for the call will detertelecommunications equipment 192 at the ST.
mine 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
If the data output by the overhead extraction circuit 182
this information.
is data on a downlink control channels, then instead of
Hence, when incoming call data is passed from the 50 passing that data to a piece of telecommunications
equipment, it is passed via switch 187 to a call control logic
network interface to the TDM encoder 105 over line 103, the
185, where that data is interpreted by the ST.
TDM encoder will apply appropriate TDM encoding to
enable the data to be inserted in the appropriate time slot.
At the subscriber terminal 20, a stage of automatic gain
From then on, the processing of the signal is the same as the
control is incorporated at the IF stage. The control signal is
equivalent processing performed in the ST and described 55 derived from the digital portion of the CDMAreceiver using
with reference to FIG. 7A, the overlay code generator
the output of a signal quality estimator.
producing a single overlay code of value '1' so that the
FIG. 8B illustrates the signal reception processing stages
signal output from spreader 111 is the same as the signal
as configured in a central terminal 10 in the telecommuniinput to the spreader 111.
cations system of FIG. 1. As will be apparent from the figure,
As mentioned earlier, in preferred embodiments, overlay 60 the signal processing stages between the RX antenna 150
codes, rather than TDM, are used to implement downlink
and the overhead extraction circuit 182 are the as those
control channels, and data relating to such channels is passed
within the ST discussed in connection with FIG. 8A.
from a demand assignment engine (to be discussed in more
However, in the case of the CT, call data output from the
detail later) over line 107 through switch 109 to the overhead
overhead extraction circuit is passed over line 189 to the
insertion circuit 108, thereby bypassing the TDM encoder 65 network interface within the CT, whilst control channel data
105. The processing of the signal is then the same as the
is passed via switch 191 to the DA engine 380 for processequivalent processing performed in the ST, with the overlay
ing. The DA engine is discussed in more detail later.

6,088,326
15

16

Overlay codes and channelisation plans are selected to
scriber terminal 20. A downlink communication path is
established from transmitter 200 in central terminal 10 to
ensure signal orthogonality-i.e. in a properly synchronised
receiver 202 in subscriber terminal 20. An uplink commusystem, the contribution of all channels except the channel
nication path is established from transmitter 204 in subbeing demodulated sum to zero over the correlator integration period. Further, uplink power is controlled to maintain 5 scriber terminal 20 to receiver 206 in central terminal 10.
Once the downlink and the uplink communication paths
constant energy per bit. The exception to this is Low rate
have been established in wireless telecommunication system
which will be transmitted at the same power as a Quarter rate
1, telephone communication may occur between a user 208,
signal. Table 2 below illustrates the overlay codes used for
210 of subscriber terminal 20 and a user serviced through
full, half and quarter rate operations:
10 central terminal 10 over a downlink signal 212 and an uplink
signal 214. Downlink signal 212 is transmitted by transmitTABLE 2
ter 200 of central terminal 10 and received by receiver 202
STTx.
of subscriber terminal 20. Uplink signal 214 is transmitted
power
by transmitter 204 of subscriber terminal 20 and received by
Net Channel relative
Correlator
Rate designa- to F1-U
integration
Acquisition 15 receiver 206 of central terminal 10.
Receiver 206 and transmitter 200 within central terminal
(kb/s)
tion
(dB)
Overlay Code
period (us)
overlay
10 are synchronized to each other with respect to time and
-F1-U
160
0
1
6.25
Ll
phase, and aligned as to information boundaries. In order to
-H1-U
-3
80
1
1
12.5
Ll
establish the downlink communication path, receiver 202 in
-H2-U
-3
80
1 -1
12.5
L3
-Q1-U
-6
40
1
1
1
25
Ll
20 subscriber terminal 20 should be synchronized to transmitter
-Q2-U
-6
-1
40
1 -1
25
L2
200 in central terminal 10. Synchronization occurs by per-Q3-U
-6
40
1 -1 -1
25
L3
forming an acquisition mode function and a tracking mode
-Q4-U
-6
-1 -1
40
1
25
L4
function on downlink signal 212. Initially, transmitter 200 of
central terminal 10 transmits downlink signal 212. FIG. 12
In preferred embodiments, a 10 kb/s acquisition mode is 25 shows the contents of downlink signal 212. A frame inforprovided which uses concatenated overlays to form an
mation signal 218 is combined with an overlay code 217
acquisition overlay; this is illustrated in table 3 below:
where appropriate, and the resultant signal 219 is combined

TABLE 3
Acquisition overlay
Ll-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

with a code sequence signal 216 for central terminal 10 to
FIGS. 9A and 9B are diagrams illustrating the uplink and
downlink delivery methods, respectively, when the system is
produce the downlink 212. Code sequence signal 216 is
fully loaded, and illustrate the difference between the use of 40 derived from a combination of a pseudo-random noise code
signal 220 and a Rademacher-Walsh code signal 222.
overlay codes illustrated in FIG. 9A and the use of TDM as
illustrated in FIG. 9B. When using overlay codes, an RW
Downlink signal 212 is received at receiver 202 of
code is split in the RW space domain to allow up to four sub
subscriber terminal 20. Receiver 202 compares its phase and
channels to operate at the same time. In contrast, when using
code sequence to a phase and code sequence within code
TDM, an RW code is split in the time domain, to allow up 45 sequence signal 216 of downlink signal 212. Central termito four signals to be sent using one RW code, but at different
nal 10 is considered to have a master code sequence and
times during the 125 us frame. As illustrated in FIGS. 9A
subscriber terminal 20 is considered to have a slave code
and 9B, the last two RW codes, RW14 and RWI5, are not
sequence. Receiver 202 incrementally adjusts the phase of
used for data traffic in preferred embodiments, since they are
its slave code sequence to recognize a match to master code
reserved for call control and acquisition functions; this will 50 sequence and place receiver 202 of subscriber terminal 20 in
be discussed in more detail later.
phase with transmitter 200 of central terminal 10. The slave
The CDMAchannel hierarchy is as illustrated in FIG. 10.
code sequence of receiver 202 is not initially synchronized
Using this hierarchy, the following CDMA channelisations
to the master code sequence of transmitter 200 and central
are possible:
terminal 10 due to the path delay between central terminal
Fl
55 10 and subscriber terminal 20. This path delay is caused by
Hl+H2
the geographical separation between subscriber terminal 20
Hl+Q3+Q4
and central terminal 10 and other environmental and techH2+Ql+Q2
nical factors affecting wireless transmission.
Ql+Q2+Q3+Q4
After acquiring and initiating tracking on the central
Having discussed how the CDMA codes are enhanced to 60 terminal 10 master code sequence of code sequence signal
enable flexible channelisations to be achieved, whereby the
216 within downlink signal 212, receiver 202 enters a frame
bit rates can be lowered to enable more subscriber links to
alignment mode in order to establish the downlink commube managed per channel frequency, a general overview of
nication path. Receiver 202 analyzes frame information
how the downlink and uplink paths are established will be
within frame information signal 218 of downlink signal 212
provided with reference to FIGS. 11 and 12.
65 to identify a beginning of frame position for downlink signal
FIG. 11 is a block diagram of downlink and uplink
212. Since receiver 202 does not know at what point in the
communication paths between central terminal 10 and subdata stream of downlink signal 212 it has received

6,088,326
17

18

information, receiver 202 must search for the beginning of
munication paths and a path from the central terminal to the
frame position in order to be able to process information
subscriber terminal on which the communication protocol
received from transmitter 200 of central terminal 10. Once
which operates on the modem shelf between the shelf
receiver 202 has identified one further beginning of frame
controller and the modem cards also extends. The OMC/D
position, the downlink communication path has been estab5 signal is a combination of the OMC signal and a signalling
lished from transmitter 200 of central terminal 10 to receiver
signal (D), whilst the Ch.ID signal is used to uniquely
202 of subscriber terminal 20.
identify an RW channel, this Ch.ID signal being used by the
The structure of the radio frames of information sent over
the downlink and uplink paths will now be discussed with
subscriber terminal to ensure that the correct channel has
reference to FIGS. 13 and 14. In FIGS. 13 and 14, the 10 been acquired.
following terms are used:
In preferred embodiments, the subscriber terminal will
Bn Customer payload, lx32 to 2x64 Kb/s
receive downlink traffic channel data at a rate of 160 kb/s.
Dn Signalling Channel, 2 to 16 kb/s
Depending on the B-channel rate, the ST will be allocated an
OR Radio Overhead Channel
appropriate share of the radio overhead. The following TDM
16 kb/s Traffic Mode
mappings are created:
10 kb/s Acquisition/Standby Mode
TABLE 4
Rate
(kb/s)

Channel
designation

160

-Fl-D-Tljl

80

-Fl-D-T2jl

80

-Fl-D-T2j2

40
40
40
40

-Fl-D-T4jl
-Fl-D-T4j2
-Fl-D-T4j3
-Fl-D-T4j4

Bearer

CS

Bl, B2, B3, B4 CS1,
CS3
Bl, B2
CS1,
CS3
B3, B4
CS2,
CS4
Bl
CSl
B2
CS2
B3
CS3
B4
CS4

PC

OMC

Overhead rate

PC1,
PC3
PC1,
PC3
PC2,
PC4
PCl
PC2
PC3
PC4

OMC1,OMC3

4 ms

OMC1,OMC3

4 ms

OMC2,OMC4

4 ms

OMCl
OMC2
OMC3
OMC4

8
8
8
8

ms
ms
ms
ms

In the above chart, the scheme used to identify a channel
Both FIGS. 13A and 13B show a 125 us subframe format,
is as follows. Rate code 'Fl' indicates full rate, 160 kb 'D'
which is repeated throughout an entire radio frame, a frame
typically lasting for 4 milliseconds (ms). FIG. 13Aillustrates 35 indicates that the channel is a downlink channel, and "Tn/t'
the radio frame structures that are used in preferred embodiindicates that the channel is time division multiplexed
ments for the downlink path. Subframe (i) in FIG. 13A
between STs, 'n' indicating the total number of TDM
shows the radio frame structure used for low rate, 10 Kb/s,
timeslots, and 't' indicating the selected traffic timeslot.
acquisition mode (Ln-D) during which only the overhead
All ST's operating on a traffic channel will receive
channel is transmitted. Sub frame (ii) in FIG. 13Ashows the 40 D-channel information at the 16 kb/s rate. The D-channel
radio frame structure employed for the call control channel
protocol includes an address field to specify which ST is to
operating in quarter rate, 40 Kb/s, mode (Qn-D) , whilst
process the contents of the message.
sub frame (iii) of FIG. 13A illustrates the radio frame strucThe channel structure was illustrated earlier in FIGS. 9A
ture used for traffic channels operating in full rate, 160 kb/s,
and 9B. In preferred embodiments, the channel structure is
mode (FI-D).
45 flexible but comprises:
Similarly, subframe (i) of FIG. 13B shows the radio frame
At least one Link Acquisition Channel (LAC)
when operating in low rate
structure used for the uplink path
At least one Call Control Channel (CCC)
acquisition or call control mode (Ln-V). Sub-frames (ii) to
Typically one Priority Traffic Channels (PTC)
(iv) show the radio frame structure used for traffic channels
when operating in quarter rate mode (Qn-V), half rate mode
1 to 13 Traffic Channels (TC)
(Rn-V), and full rate mode (FI-V), respectively.
50
The manner in which the channelisation is provided
Considering now the overhead channel in more detail,
ensures that former fixed assignment arrangements using the
FIGS. 14A and 14B show the overhead frame structure
set of 16 RW codes discussed earlier are still supported, as
well as demand access services that are available when using
employed for various data rates. The overhead channel may
include a number of fields-a frame alignment word (FAW),
a system in accordance with the preferred embodiment.
a code synchronization signal (CS), a power control signal 55 FIGS. 15A and 15B illustrate typical downlink and uplink
channel structures that might occur in a loaded system in
(PC), an operations and maintenance channel signal (OMC),
a mixed OMC/D-Channel (RDLC) signal (OMC/D), a chanaccordance with preferred embodiments of the present
nel identifier byte (Ch.ID), and some unused fields.
invention. As illustrated in FIG. 15A, on the downlink path,
some signals may be at 160 kb/s and utilise an entire RW
The frame alignment word identifies the beginning of
frame position for its corresponding frame of information. 60 channel. An example of such signals would be those sent
The code synchronization signal provides information to
over fixed assignment links to products which do not support
control synchronization of transmitter 204 in subscriber
the CDMA enhancements provided by systems in accorterminal 20 to receiver 206 in central terminal 10. The power
dance with preferred embodiments of the present invention,
control signal provides information to control transmitting
as illustrated for RWI and RW2 in FIG. 15A. Alternatively,
power of transmitter 204 in subscriber terminal 20. The 65 a user may have authority to utilise a whole RW channel, for
operations and maintenance channel signal provides status
example when sending a fax, as illustrated by RW12 in FIG.
information with respect to the downlink and uplink com15A.

6,088,326

19

20

(iii) In the event of a CT restart, invite STs to attempt
As illustrated by RW5 to RWll, TDM can be used on the
downlink traffic channels to enable more than one CT to ST
uplink warm start. A reduction in net entry time of a
communication to take place on the same RW channel
factor of 4 could be achieved. This mechanism would
during each frame. Further, as illustrated for RW3 and RW4,
need to be safeguarded against possible deterioration of
in preferred embodiments, certain channels can be locked to 5
uplink warm start parameters-i.e. it should only be
limit interference from other nearby cells, as will be disallowed provided no CT RF related parameters have
cussed in more detail later.
been modified. The CT would need to broadcast an ID
Similar channelisations can be achieved for the uplink
to allow an ST to validate that the uplink warm start
paths, but as illustrated in FIG. 15B, overlay codes are used
parameters were valid for this CT.
instead of TDM to enable more than one ST to CT com- 10
(iv) ST restart-the CT will keep copies of the ST warm
munication to take place on the same RW channel during
start parameters so that a cold ST may have warm start
each frame (as shown in FIG. 15B for RW5 to RWll). It
parameters downloaded in the invitation to acquire and
should be noted that, in both FIGS. 15A and 15B, the
then be instructed to warm start.
channels RW14 and RW15 are reserved as a call control
Following Net Entry, all STs listen to the CCC. This
channel and an link acquisition channel, respectively, and 15 channel broadcasts management and call control informaoverlay codes are employed on these channels, irrespective
tion via a 32 kb/s HDLC channel. In order to maintain
of whether the path is a downlink or an uplink path. These
management communication, the CT polls each ST in
two channels will be discussed in more detail below.
sequence. Each poll comprises a broadcast invitation for an
Acquisition/net entry will take place via the Link Acquiaddressed ST to acquire the CCC Uplink followed by an
sition Channel (LAC). Following power-up an ST will 20 exchange of management information (authentication, ST
automatically attempt downlink acquisition of the LAC on a
alarm update, warm start parameters, downlink radio perpre-determined 'home' RF channel. The LAC downlink
formance data etc.).
channel (eg. RW15 in preferred embodiments) will operate
A Management Poll may fail for one of the following
at 10 kb/s, full single user power. Downlink acquisition will
reasons:
25
be simultaneous for all STs.
(i) The ST is or has been powered down. An EM alarm
Each CT Modem Shelf will maintain a database holding
may be flagged if this persists and the database for that
the serial numbers of all STs that could possibly be supST should be marked cold. The Net Entry process will
ported by that CT. The state of each ST will recorded with
follow.
top level states as follows:
(ii) The ST is either making a call or in the process of
30
cold
making a call. The poll cycle may be suspended and
idle
management communications effected on the appropriate traffic channel.
call_in_proogress
When a Management Poll fails it should be followed up
Transition states will also be defined. An ST is considered
cold if the ST is newly provisioned, the CT has lost 35 by a number of faster polls until either the ST responds or
it is marked cold. The CCC is required to transmit all copies
management communications with the ST or the CT has
of the invitations to acquire the LAC so that an ST can be
been power cycled. Over the LAC, the CT broadcasts
forced to acquire the LAC uplink.
individual ST serial numbers and offers an invitation to
Traffic Channel Uplink Acquisition Procedure
acquire the LAC uplink. Cold uplink acquisition will be
The basic acquisition process from the ST side is as
carried out on the Link Acquisition Channel at low rate. The 40
follows;
CT will invite specific ST's to cold start via the management
channel.
(i) Switch the downlink (receiver) circuitry to 10 kb/s rate,
Assuming an uplink channel is available, the appropriate
and select the appropriate Traffic Channel RW and
acquisition overlay will be selected, and acquisition will be
Overlay codes. Acquisition of the TC downlink is
limited to achieving frame alignment.
45
initiated.
'Rapid' downlink RW channel switching may be sup(ii) The downlink PC/CS channel will be decoded to
ported at rates other than Ln-D. Rapid means that coherent
create a busy/idle flag. If PC/CS reports busy, then this
demodulation is maintained, and only convolutional decodmeans that another ST is using that traffic channel and
ing and frame synchronisation processes need be repeated.
the ST aborts the acquisition process.
On acquisition, management information will be 50
(iii) Switch uplink to 10 kb/s rate, and select the approexchanged. The ST will be authenticated and allocated a
priate Traffic Channel RW and Overlay codes. Enable
short ST_identifier (between 12 and 16 bits) which will be
the ST transmitter at a level of nominal full rate power
used for subsequent addressing. The ST uplink will operate
minus 18 dB. While PC/CS reports idle the ST will
for long enough for the uplink to be parametised by the ST
continue uplink fast codesearch, stepping the uplink
in terms of code phase and transmit power. These parameters 55
power level by +2 dB at the end of each search. The
will be used by the ST for subsequent warm start acquisiuplink should acquire at nominal full rate power minus
tions and will also be held by the CT to allow the CT to force
6 dB. Uplink acquisition is aborted if maximum transa cold ST to warm start. On successful completion of net
mit level is reached and PC/CS continues to report idle.
entry, the ST will be placed in the idle state and instructed
(iv) PC/CS reports busy. At this point the ST may have
to cease uplink communications and move to the Call 60
genuinely acquired the traffic channel, or instead may
Control Channel (CCC) (RWI4 in preferred embodiments).
be observing PC/CS go busy because another ST has
The time taken for net entry to be achieved can be
acquired the traffic channel. The ST is sent an authenmonitored, and the following techniques can be used to
tication request and responds with it's ST_identifer.
decrease net entry time if desired:
The CT grants uplink access by returning the
(i) Prioritise so that high GOS (Grade Of Service) users 65
ST_identifier. The ST aborts the acquisition process if
are offered net entry first.
the returned ST_identifier is not recognised (i.e. is not
the ST_identifer that it sent). This authentication pro(ii) Convert Traffic Channels to LACs.

6,088,326

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cess arbitrates between two STs contending for outgoing access and it also keeps STs from acquiring TCs
that have been reserved from incoming access.
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 calLin
progress state the call is rejected.
(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
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
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
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.
(vi) Rate switching is originated from the CT modem. A
command is sent via the PCjCS to switch the downlink
to the required bandwidth. The ST returns the rate
switch command via the uplink PCjCS. The link is now
of the required bandwidth.
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
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
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.
(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
in the free list. For example, the ST may be required to
always choose from a pool of minimum bandwidth
channels so that high bandwidth channels remain available for high GOS users. Alternatively the ST may be
allowed to choose any channel regardless of bandwidth
for minimum blocking. In preferred embodiments, STs
will not choose low bandwidth channels and negotiate
the rate up.
(v) The ST attempts uplink acquisition on the specified
TC, this process having been described earlier. If
acquisition is successful then the outgoing call is processed. Otherwise the ST returns to the CCC and waits
for the next available free list. To avoid a number of STs
repetitively attempting to acquire the same TC, and
blocking each other, a suitable protocol can be
employed to govern how individual STs will act upon
receipt of the free list.

(vi) The ST may be unable to acquire a TC by the time the
call setup timer expires. The ST may in such cases
cease attempting outgoing access and generate congestion tone.
Outgoing Priority Call
It is recognised that the random access protocol used to
setup normal outgoing calls could lead to blocking. In
preferred embodiments, access to a largely non-blocking
Priority Traffic Channel will be allowed. Priority calling is
complicated because the ST must:
(i) Capture and decode dialed digits.
(ii) Regenerate digits when a blocking condition occurs.
(iii) Allow transparent network access in a non-blocking
condition.
(iv) Categorise all outgoing calls as priority or normal so
that normal calls are dropped in favor of priority calls.
The priority call procedure in preferred embodiments is as
follows:
(i) The CT will publish Directory Numbers (DNs) for a
number of emergency services over the CCC.
(ii) The ST will attempt uplink access according to the
normal algorithms. If the outgoing access is successful
then the customer is able to dial as normal. All dialed
digits are check against the emergency DN list so that
calls may be categorised normal or priority at the CT.
(iii) If congestion tone is returned the customer is allowed
to dial the emergency number into the ST. If the ST
detects an emergency DN sequence then uplink access
via the Priority Traffic Channel (PTC) is attempted.
(iv) On PTC acquisition, the ST relays the dialed digit
sequence to the CT for dialling into the PSTN.
(iv) The CT converts the PTC to a TC and reallocates
another TC to become the PTC, dropping a normal call
in progress if necessary.
Interference Limiting (Pool Sizing)
Across a large scale deployment of cells, optimum capacity is achieved by minimising radio traffic while maintaining
an acceptable grade of service. Lowest possible radio traffic
results in improved 'carrier to interference' (CII) ratios for
users within the cell of interest and to co-channel users in
nearby cells. The CII ratio is a measure (usually expressed
in dB) of how high above interference the transmitted signal
needs to be to be decoded effectively. In preferred
embodiments, the central terminal is provided with the
ability to trade traffic for CII, thereby allowing network
planning to be carried out less rigidly. This feature can be
realised by a system using CDMA as in preferred embodiments of the present invention, and is a benefit that CDMA
offers over TDMA and FDMA systems.
In preferred embodiments, the CT will control the number
of Traffic Channels to minimise access noise. TCs will be
classified as:
(i) Busy---earrying traffic;
(ii) Access, Incoming (Access_In)-reserved for incoming access;
(iii) Access, Outgoing (Access_Out)-reserved for outgoing access-such TCs appear on the Free list;
(iv) Priority-reserved for priority outgoing access-such
TCs appear in the Free list;
(v) Free-available for any purpose; and
(vi) Locked-not available due to interference limiting.
This classification scheme is illustrated in FIG. 16. The
CT will allocate traffic on the following basis:
(i) The CT will monitor incoming and outgoing call
setup-times and convert Access TCs from Free TCs in
order to achieve a required grade of service.

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(ii) When a call is setup, an Access TC is converted to a
GOS estimates for incoming calls, and these GOS estimates
Busy TC. If a Free TC is available, it is converted to a
are passed over line 395 to the dynamic pool sizing function
new Access TC. If there are no Free TCs then the
360.
Access TC is lost until a call clears.
At set up, the management system 370 within the element
(iii) When a call clears the Busy TC is converted to a Free 5 manager will have connected to the central terminal and
provided the dynamic pool sizing function 360 within the
TC. If a previous call setup resulted in a lost Access TC
modem shelf with data identifying a BER goal, a GOS goal,
then the Busy TC is converted back into an Access TC.
and a pool size limit (i.e. the number of channels that can be
(iv) When the PTC is accessed, a new PTC is created by
used for data traffic). The dynamic pool sizing function 360
converting a Free, Access or Busy (normal call) TC.
(v) The CTwill monitor the Busy TC downlink and uplink 10 then compares this data from the management system with
the actual BER, actual GOS, and the actual pool size
soft error counts in an attempt to establish link quality.
information that it receives. A suitable algorithm can be
If the CT records a lower than average soft error count
provided within the dynamic pool sizing function 360 to
and long call setup times are being recorded, a Locked
determine, based on this information, whether pool sizing is
TC may be converted to a Free TC. Conversely, if the
CT records a higher than average soft error count, a 15 appropriate. For example, if the actual bit error rate exceeds
the BER goal provided by the management system 370, then
Free or Access TC may be converted to a Locked TC.
the dynamic pool sizing function 360 may be arranged to
FIG. 17 illustrates how the central terminal performs the
send a pool sizing request to the demand assignment engine
above interference limiting function. When incoming call
380.
data arrives at a central terminal modem 320, encoder 325
The demand assignment engine 380 provides modem
encodes the data for transmission over the wireless link 300 20
enable signals over lines 400 to each of the modems on the
to the subscriber terminal 20. At the subscriber terminal 20
CT modem shelf. If the dynamic pool sizing function 360
the decoder 326 decodes the data, and passes the decoded
has requested that the DA engine 380 perform pool sizing,
user data over line 328 to the subscriber telecommunications
then the DA engine 380 can disable one or more of the
equipment. As the decoder 326 decodes the data, it is able to
establish a bit error rate (BER) estimate 330 associated with 25 modems, this causing the interference, and hence the actual
BER, to be reduced. Apart from being used for interference
the signal transmission over the wireless link 300, which can
limiting, the DA engine is also responsible, in preferred
be passed to the multiplexer 332 for combining with other
embodiments, for providing the encoders 325 with instrucsignals, such as those from a call control function 336 or user
tions on which set of overlay codes or how many TDM slots
data on line 338, before being passed to an encoder 334.
Here, the BER estimate is encoded and passed on the OMC 30 to be used for signals to be transmitted to the STs 20.
The dynamic pool sizing function can store the BER and
channel over the wireless link 310 to the decoder 340 within
GOS information received in the storage 365, and periodithe central terminal modem 320. Once decoded by the
cally may pass that data to the management system 370 for
decoder 340, the signal passes to the multiplexer 345, where
analysis. Further, if the system is unable to attain the BER
the BER estimate from the subscriber terminal is detected
?OS goa~ with the allocated pool size, the dynamic pool
and passed over line 355 to the dynamic pool sizing function 35
sizing function can be arranged to raise an alarm to the
360.
management system. The receipt of this alarm will indicate
Further, as at the subscriber terminal 20, the decoder 340
to personnel using the management system that manual
within the central terminal modem 320 is able to establish a
intervention may be required to remedy the situation, eg by
bit error rate estimate 350 associated with the signal transmission over the wireless link 310. This BER estimate 350 40 the provision of more central terminal hardware to support
the STs.
is al~o passed over line 355 to the dynamic pool sizing
The CDMA approach used in preferred embodiments
function 360. The dynamic pool sizing function 360 is
exhibits the property that the removal of any of the orthogoprovided on the CT modem shelf 302, and receives BER
nal channels (by disabling the modem) will improve the
estimates from each of the modems on that shelf indicated
by the lines entering the bottom of the dynamic pool sizing 45 resistance of the other channels to interference. Hence, a
suitable approach for the demand assignment engine 380,
function 360.
u?~n receip~ of pool sizing request from the dynamic pool
. In ad.dition to BER estimates, grade of service (GOS) data
sizing function 360, is to disable the modem that has the
IS obtamed from two sources. Firstly, at each subscriber
least traffic passing through it.
terminal 20, the call control function 336 will note how
readily it is able to establish traffic channels for transmitting 50 RF Channel Switching
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
ShelflRF Channel then the following benefits may be rea334 for subsequent transmission over the wireless link 310
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, 55
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
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 60
(iii) Traffic load balancing-the Element Manager may on
over the concentrated network interface 390 to the DA
the basis of call blocking statistics choose to move STs
engine 380. The DA engine 380 includes a call control
between CT shelves.
function, similar to the call control function 336 in each of
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 65
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.

0:

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26

RF channel switching is only possible where there are two
or more co-located CT shelves serving the same geographical area on different RF frequency channels within the same
RF band. A deployment that meets this criterion may be
configured as a 'Service Domain'. Possible deployment
scenarios are illustrated in FIG. 18. FIG. 18(i) shows an
arrangement where omni antennae are used to provide the
entire cell with four frequency channels, eg FI, F4, F7, FlO.
FIG. 18(ii) shows an arrangement where sectored antennae
are used to provide six separate sectors within a cell, each
sector being covered by two frequency channels. FIG. 18(iii)
shows an alternative arrangement where three sectored
antennae are used to divide the cell in to three sectors, each
sector being covered by a separate frequency channel, and
then an omni antenna is used to provide an 'umbrella'
coverage for the entire cell, this coverage employing a
frequency channel different to the three frequency channels
used by the sectored antennae.
For the system to work effectively, the STs must be able
to switch channels quickly, and fast channel switching
necessitates that CT shelf synchronisation be provided at the
following levels:
(i) CDMA PN code. This preserves uplink code phase
across RF channels during warm start; and
(ii) RF carrier frequency. This eliminates the need for the
coarse frequency search on a downlink RF channel
switch.
On installation, an ST will be programmed with an RF
channel and PN code, these codes specifying the ST's initial
home channel.
The manner in which channel switching is facilitated in
preferred embodiments will be described with reference to
FIGS. 19A and 19B. A service domain controller 400 is
preferably provided to act as an interface between the
exchange connected to the service domain controller over
path 405 and a number of central terminals 10 connected to
the service domain controller over paths 410. The central
terminals connected to the service domain controller form a
'service domain' of central terminals that may be used by a
subscriber terminal 20 for handling communications.
In preferred embodiments, the service domain controller
400 is used to provide each CT 10 with appropriate information about the other CTs within the service domain. Each
CT can then broadcast a 'Service Domain' message comprising a list of RF frequencies and CT Identifiers that form
a Service Domain to be used by the STs for subsequent RF
switching functions. The ST then stores this information for
future reference when establishing a link with one of the
CTs. It is preferable for each CT to broadcast the service
domain message since an ST may be listening to any of the
CTs at the time that the message is broadcast.
Each CT database will hold an entry for every ST located
within the Service Domain. Each database entry describes
how the CT views it's relationship with the ST and may be
marked as:
(i) Primary service provider-the CT is the ST's home
channel. All management communication with an ST is
via it's home CT.
(ii) Supplying backup service-the CT is providing service to the ST.
(iii) Available for backup service-the CT will provide
service to the ST if required.
It should be noted that the ST need not switch to an
entirely different CT, but can instead switch to a different CT
shelf (and hence different RF frequency channel) within the
same CT. However, in preferred embodiments, the ST will

typically switch to a different CT, since some errors experienced by one CT shelf may also affect other shelves within
the same CT, and so for fault tolerance (described in more
detail below), it is preferable for the ST to switch to a
separate CT.
Database consistency across CT shelves is preferably
supported through the service domain controller 400. Database consistency needs to be real-time so that an ST entering
the network is allowed full Service Domain access immediately (the Service Domain message is broadcast to all STs,
and so a new ST will expect access across the full Service
Domain).
Incoming access via backup CTs requires some function
to be provided to broadcast duplicate incoming call setup
messages to all CTs that form a Service Domain. Preferably
this is handled by the service domain controller 400, which
forwards incoming call setup messages to each CT operating
in the service domain. All CTs will allocate Access_In
Traffic Channels and relay the incoming call setup message
via the Call Control Channel. On successful uplink access,
one CT will respond to the service domain controller with a
call accepted message, the other CTs will eventually respond
with call setup failed messages. Outgoing access via a
backup CT is similar to normal outgoing access.
Another job which can be performed by the service
domain controller is to assist the element manager 58 in
reconfiguring equipment in the event of a fault. For example,
if one CT is taken out of commission because of a fault, a
different CT can be brought 'on-line', and the service
domain controller can provide that new CT with the necessary information about the other CTs in the service domain.
FIG. 19B illustrates those elements of the subscriber
terminal used to implement RF channel switching. The radio
subsystem 420, which incorporates the transmission and
reception signal processing stages, will pass any data
received on the call control channel over line 425 to the
message decoder 430. If the decoder 430 determines that the
data on the call control channel forms a service domain
message, then this is passed over line 435 to the channel
selection controller 440, where the information within the
service domain message is stored in storage 445.
Similarly, if the message decoder identifies the data as a
'free list' identifying the available traffic channels on a
particular RF frequency, then this data is passed to the call
control function 336 and the channel selection controller 440
over path 450. The call control function 336 stores the free
list in the storage 445 for subsequent use by the call control
function 336 and the channel selection controller 440.
If the message decoder 430 determines that the data forms
an incoming call setup message, then that information is
supplied over line 455 to the call control function 336 and
the channel selection controller 440 for processing. The
incoming call setup message will typically specify a TC on
the current frequency channel which should be used to
access the incoming call, and the channel selection controller will attempt to establish a link on that TC. The channel
selection controller will in such cases instruct the radio
sub-system 420 over line 465 to use the current frequency
channel to establish the required link. If, on the other hand,
the traffic channel specified in the call setup message is
'null', the channel selection controller has the option to
change RF frequency using the information stored in storage
445 about the other CTs in the service domain.
To enable the channel selection controller 440 to receive
information about the status of links, a link operating status
signal can be supplied over line 470 from the radio subsystem. This signal will give an indication of the radio link

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quality, and may be a simple 'OK' or 'failed' indication, or
(iv) When the call clears, the ST downlink preferably
alternatively may include extra information such as BER
switches back to the home CT.
values for the link. This information can be used by the
RF Channel Switching for Traffic Load Balancing
channel selection controller to determine whether a particuTraffic load balancing is, in preferred embodiments, proIar frequency channel should be used or not.
S vided by static configuration via the EM 58. Call blocking
To enable the call control function to specify a specific
and setup time statistics may be forwarded to the EM where
Access-Out channel for outgoing calls, a line 460 is proan operator may decide to move an ST to another RF
channel.
vided between the call control function 336 and the channel
RF Channel Switching for Frequency Diversity
selection controller 440. The call control function 336 may
choose an access-out channel from the free list in storage 10
Frequency diversity is, in preferred embodiments, provided by static configuration via the EM 58. Radio link
445, and instruct the channel selection controller over line
statistics may be forwarded to the EM where an operator
460 to attempt acquisition of that channel.
may decide to move an ST to another RF channel.
The following examples indicate how the above described
Although a particular embodiment has been described
structure may be used to perform channel switching in
herein, it will be appreciated that the invention is not limited
particular circumstances.
15 thereto and that many modifications and additions thereto
RF Channel Switching for Fault Tolerance
Should one RF channel suffer complete loss of downlink,
may be made within the scope of the invention. For example,
the following process takes place in preferred embodiments:
various combinations of the features of the following dependent claims could be made with the features of the indepen(i) The ST will attempt downlink re-acquisition for a
dent claims without departing from the scope of the present
period of time, say 20 seconds.
(ii) If acquisition fails, the channel selection controller 20 invention.
What is claimed is:
440 of the ST will select the next available channel
1. A transmission controller for processing data items to
from the Service Domain information in storage 445
be transmitted over a wireless link connecting a central
and attempt downlink acquisition.
terminal and a subscriber terminal of a wireless telecomThis process will be repeated until a downlink signal is
25 munications system, a single frequency channel being
acquired.
employed for transmitting data items pertaining to a plural(iii) Once a backup RF channel is located, the ST will
ity of wireless links, the transmission controller comprising:
'camp' on the Call Control Channel and may subsean orthogonal code generator for providing an orthogonal
quently be granted traffic access.
code from a set of 'rn ' orthogonal codes used to create
(iv) If the CT fault persists, the EM 58 may use the service
'm' orthogonal channels within the single frequency
domain controller 400 to reconfigure the Service 30
channel;
Domain so that the functioning CT shelves become
a first encoder for combining a data item to be transmitted
primary service providers for the pool of 'homeless'
on the single frequency channel with said orthogonal
STs.
code from the orthogonal code generator, the orthogoA fault that does not result in complete loss of downlink
nal code determining the orthogonal channel over
signal will not result in RF channel switching 'en mass'. 35
which the data item is transmitted, whereby data items
Rather, a fault may result in excessive or total call blocking,
pertaining to different wireless links may be transmitted
simultaneously within different orthogonal channels of
as discussed below.
said single frequency channel; and
RF Channel Switching for Call Blocking
a TDM encoder arranged to apply time division multiIf Incoming access traffic channels are being blocked, the
plexing (TDM) techniques to the data item in order to
following process is employed in preferred embodiments: 40
insert the data item within a time slot of the orthogonal
(i) The call setup message sent over the Call Control
channel, whereby a plurality of data items relating to
Channel will specify a TC on which to access the call.
different wireless links may be transmitted within the
(ii) In the case of incoming access being blocked, the CT
same orthogonal channel during a predetermined frame
will specify a null TC. The channel selection controller
period.
440 of the ST will in such cases switch to the next RF 45
2. A transmission controller as claimed in claim 1, further
channel from the Service Domain information in storcomprising:
age 445 and monitor the Call Control Channel.
an overlay code generator for providing an overlay code
(iii) If the ST receives a call setup message with a valid
from a first set of 'n' overlay codes which are orthogoTC, then the call is processed as normal.
nal to each other; and
50
(iv) When the call clears, the ST downlink preferably
a second encoder, selectively operable instead ofthe TDM
switches back to the home CT.
encoder, to apply the overlay code from the overlay
If Outgoing access traffic channels are being blocked, the
code generator to said data item, whereby 'n' data items
following process is employed in preferred embodiments:
pertaining to different wireless links may be transmitted
(i) The ST registers an off-hook. The Free List in storage 55
simultaneously within the same orthogonal channel.
445 is checked and if a traffic channel is available, then
3. A transmission controller as claimed in claim 1,
the call control function 336 asserts a channel request
wherein the orthogonal code generator is a storage arranged
on line 460 to the channel selection controller 440 and
to store the set of orthogonal codes.
normal uplink access is attempted.
4. A transmission controller as claimed in claim 1,
(ii) If the Free List shows no Access_Out channels are 60 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
5. A central terminal of a wireless telecommunications
ST to the next RF channel in the Service Domain,
system, comprising a transmission controller having:
whereupon the ST will wait for the next Free List.
an orthogonal code generator for providing an orthogonal
code from a set of 'rn ' orthogonal codes used to create
(iii) When the ST finds a Free List with an available 65
Access_Out channel, then uplink access is attempted
'm' orthogonal channels within the single frequency
and the call is processed as normal.
channel;

6,088,326

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a first encoder for combining a data item to be transmitted
code from the orthogonal code generator, the orthogoon the single frequency channel with said orthogonal
nal code determining the orthogonal channel over
code from the orthogonal code generator, the orthogowhich the data item is transmitted, whereby data items
nal code determining the orthogonal channel over
pertaining to different wireless links may be transmitted
which the data item is transmitted, whereby data items 5
simultaneously within different orthogonal channels of
pertaining to different wireless links may be transmitted
said single frequency channel; and
simultaneously within different orthogonal channels of
a TDM encoder arranged to apply time division multisaid single frequency channel;
plexing (TDM) techniques to the data item in order to
a TDM encoder arranged to apply time division multiinsert the data item within a time slot of the orthogonal
plexing (TDM) techniques to the data item in order to 10
channel, whereby a plurality of data items relating to
insert the data item within a time slot of the orthogonal
different wireless links may be transmitted within the
channel, whereby a plurality of data items relating to
same orthogonal channel during a predetermined frame
different wireless links may be transmitted within the
period;
same orthogonal channel during a predetermined frame
period;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogoan overlay code generator for providing an overlay code 15
from a first set of 'n ' overlay codes which are orthogonal to each other;
nal to each other;
a second encoder, selectively operable instead ofthe TDM
a second encoder, selectively operable instead of the TDM
encoder, to apply the overlay code from the overlay
encoder, to apply the overlay code from the overlay
code generator to said data item, whereby 'n' data items
code generator to said data item, whereby 'n' data items 20
pertaining to different wireless links may be transmitted
pertaining to different wireless links may be transmitted
simultaneously within the same orthogonal channel,
simultaneously within the same orthogonal channel,
wherein the orthogonal code generator is a storage
wherein the orthogonal code generator is a storage
arranged to store the set of orthogonal codes and
arranged to store the set of orthogonal codes and
wherein the set of orthogonal codes comprise a set of
wherein the set of orthogonal codes comprise a set of 25
Rademacher-Walsh (RW) codes;
Rademacher-Walsh (RW) codes.
and wherein at least one subscriber terminal comprises a
6. A central terminal as claimed in claim 5, further
reception controller having:
comprising channelisation means for determining which of
an orthogonal code generator for providing an orthogothe orthogonal channels will be subject to TDM techniques,
nal code from a set of 'm' orthogonal codes used to
and for transmitting that information to a plurality of sub- 30
create said 'm' orthogonal channels within the single
scriber terminals within the wireless telecommunications
frequency channel;
system.
a first decoder for applying, to signals received on
7. A central terminal as claimed in claim 6, wherein the
the single frequency channel, the orthogonal code
channelisation means also determines, for those orthogonal
provided by the orthogonal code generator, in
channels subject to TDM techniques, how many time slots 35
will be provided within each orthogonal channel.
order to isolate data items transmitted within the
8. A central terminal as claimed in claim 7, wherein a
corresponding orthogonal channel; and
number of said orthogonal channels are designated as traffic
a TDM decoder arranged to extract a data item from a
channels for the transmission of data items relating to
predetermined time slot within said orthogonal
communication content, and the TDM encoder is employed 40
channel, a plurality of data items relating to different
to apply time division multiplexing (TDM) techniques to
wireless links being transmitted within the same
data items to be sent over a traffic channel from said central
orthogonal channel during a predetermined frame
terminal to said subscriber terminal.
period;
9. A central terminal as claimed in claim 5, wherein a first
an overlay code generator for providing an overlay
code from a first set of 'n ' overlay codes which are
of the orthogonal channels is reserved for the transmission 45
orthogonal to each other, the set of 'n ' overlay codes
of signals relating to the acquisition of wireless links, and
enabling 'n' data items pertaining to different wirethe second encoder is used instead of the TDM encoder to
less links to be transmitted simultaneously within the
enable overlay codes to be applied to data items to be sent
same orthogonal channel;
within said first orthogonal channel from the central terminal
a second decoder, selectively operable instead of the
to one of said subscriber terminals.
50
TDM decoder, to apply to the data items of the
10. A central terminal as claimed in claim 5, wherein a
second of the orthogonal channels is reserved for the transorthogonal channel, the overlay code from the overmission of signals relating to the control of calls, and the
lay code generator so as to isolate a particular data
second encoder is used instead of the TDM encoder to
item transmitted using that overlay code, wherein the
enable overlay codes to be applied to data items to be sent 55
orthogonal code generator is a storage arranged to
within said second orthogonal channel from the central
store the set of orthogonal codes and wherein the set
terminal to one of said subscriber terminals.
of orthogonal codes comprise a set of Rademacher11. A wireless telecommunications system comprising a
Walsh (RW) codes.
central terminal and a plurality of subscriber terminals,
12. A method of processing data items to be transmitted
wherein the central terminal comprises a transmission con- 60 over a wireless link connecting a central terminal and a
subscriber terminal of a wireless telecommunications
troller having:
system, a single frequency channel being employed for
an orthogonal code generator for providing an orthogonal
transmitting data items pertaining to a plurality of wireless
code from a set of 'm' orthogonal codes used to create
links, the method comprising steps of:
'm' orthogonal channels within the single frequency
channel;
65
providing an orthogonal code from a set of 'm' orthogonal
codes used to create 'm' orthogonal channels within the
a first encoder for combining a data item to be transmitted
single frequency channel;
on the single frequency channel with said orthogonal

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combining a data item to be transmitted on the single
applying the overlay code to said data item, whereby 'n'
data items pertaining to different wireless links may be
frequency channel with said orthogonal code, the
transmitted simultaneously within the same orthogonal
orthogonal code determining the orthogonal channel
channel.
over which the data item is transmitted, whereby data
14. A method as claimed in claim 12, further comprising
items pertaining to different wireless links may be 5
steps of:
transmitted simultaneously within different orthogonal
determining which of the orthogonal channels will be
channels of said single frequency channel; and
subject to TDM techniques; and
applying time division multiplexing (TDM) techniques to
transmitting that information to a plurality of subscriber
the data item in order to insert the data item within a
terminals within the wireless telecommunications systime slot of the orthogonal channel, whereby a plurality 10
tem.
of data items relating to different wireless links may be
15. A method as claimed in claim 14, further comprising
transmitted within the same orthogonal channel during
a step of:
a predetermined frame period.
determining, for those orthogonal channels subject to
13. A method as claimed in claim 12, wherein said
TDM techniques, how many time slots will be provided
applying step is selectively replaced by steps of:
15
within each orthogonal channel.
providing an overlay code from a first set of 'n ' overlay
codes which are orthogonal to each other; and
* * * * *