WI-LAN Inc. v. Apple Inc.

Filing 1

COMPLAINT against Apple Inc. ( Filing fee $ 350 receipt number 0540-3904331.), filed by WI-LAN Inc.. (Attachments: # 1 Exhibit A - USP 6,381,211, # 2 Civil Cover Sheet)(Weaver, David)

EXHIBIT A

111111

1111111111111111111111111111111111111111111111111111111111111
US00638121IBI

(54)

United States Patent

(10)

Lysej ko et ai.

(12)

(45)

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

(75)

Inventors: Martin Lysejko, Bagshot (GB); Paul F.
Struhsaker, Plano, TX (US)

(73)

Assignee: Airspan Networks Inc., Seattle, WA
(US)

( *)

Notice:

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

Patent No.:
US 6,381,211 Bl
Date of Patent:
Apr. 30, 2002

FOREIGN PATENT DOCUMENTS
EP
EP
EP

0633676 A2
0652650 A2
0730356 A2
2267627
2301 744
93/14590
93/15573
95/23464
WO 96/37066

GB
GB
WO
WO
WO
WO

* cited by examiner
Primary Examiner-Ricky Ngo
(74) Attorney, Agent, or Firm-Baker Botts L.L.P.
ABSTRACT

(57)
(21)

Appl. No.: 09/579,349

(22)

Filed:

May 25, 2000
Related U.S. Application Data

(63)
(30)

Continuation of application No. 08/979,408, filed on Nov.
26, 1997, now Pat. No. 6,088,326.
Foreign Application Priority Data

Dec. 20, 1996
(51)
(52)
(58)

(GB) ............................................. 9626567

Int. CI? .......................... H04J ll/OO; H041 13/00;
H04B 7/216
U.S. CI. ....................... 370/209; 370/342; 370/335;
370/345; 370/441; 370/442; 370/479
Field of Search ................................. 370/328, 329,
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
5,373,502
5,414,728
5,481,533
5,572,516
5,805,581
5,894,473

A
A
A
A
A
A

*
*
*
*

12/1994
5/1995
1/1996
11/1996
9/1998
4/1999

Turban .......................
Zehavi .......................
Honing et al. ..............
Miya et al. .................
Uchida et al. ..............
Dent ..........................

370/441
370/206
370/335
370/335
370/335
370/342

1/1995
5/1995
9/1996
12/1993
12/1996
7/1993
8/1993
8/1995
11/1996

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.
10 Claims, 16 Drawing Sheets

162

160

u.s.

Patent

Apr. 30, 2002

US 6,381,211 Bl

Sheet 1 of 16

FIG.

1
14

FIG. 2A
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u.s. Patent

Apr. 30, 2002

US 6,381,211 Bl

Sheet 2 of 16

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Apr. 30, 2002

Sheet 4 of 16

US 6,381,211 Bl

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US 6,381,211 Bl

Sheet 11 of 16

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210

u.s. Patent

Apr. 30, 2002

Sheet 12 of 16

US 6,381,211 Bl

(1)\'--_ _ _ _ _ _ __ _ _ _ _ ___
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01

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u.s.

Patent

(I)

Apr. 30, 2002

FAW

US 6,381,211 Bl

Sheet 13 of 16

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u.s. Patent

Apr. 30, 2002

US 6,381,211 Bl

Sheet 16 of 16

(I)

(III)

FIG. 18
58

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405

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dir

SERVICE
DOMAIN
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US 6,381,211 B1
1

2

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

However, as more subscribers subscribe to the wireless
telecommunications network, it is becoming desirable to
support more and more subscriber terminals from each
central terminal. There are only a limited number of frequency channels that can be allocated to the wireless telecommunications 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

RELATED APPLICATION
This application is a continuation of U.S. application Ser.
No. 08/979,408 filed Nov. 26, 1997 now U.S. Pat. No.
10
6,088,326.
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to wireless telecommunications systems and more particularly to techniques for processing data transmitted and received over a
wireless link connecting a central terminal and a subscriber
terminal of a wireless telecommunications system.

15

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

20

25

30

35

40

45

50

55

60

65

SUMMARY 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 terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, the transmission controller comprising: an orthogonal
code generator for providing an orthogonal code from a set
of 'm' orthogonal codes used to create 'm' orthogonal
channels within the single frequency channel; a first encoder
for combining a data item to be transmitted on the single
frequency channel with said orthogonal code from the
orthogonal code generator, the orthogonal code determining
the orthogonal channel over which the data item is
transmitted, whereby data items pertaining to different wireless links may be transmitted simultaneously within different
orthogonal channels of said single frequency channel; and a
TDM 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.
Viewed from a second aspect, the present invention
provides a reception controller for processing data items
received over a wireless link connecting a central terminal
and a subscriber terminal of a wireless telecommunications
system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
links, and 'm' orthogonal channels being provided within the
single frequency channel, the receiver controller comprising: an orthogonal code generator for providing an orthogonal code from a set of 'm' orthogonal codes used to create
said 'm' orthogonal channels within the single frequency
channel; a first decoder for applying, to signals received on
the single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate data
items transmitted within the corresponding orthogonal channel; and a TDM decoder arranged to extract 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. For
example, if TDM is used to divide one frame period in to
four sub frames, and each orthogonal channel is subject to
the TDM technique, then up to 64 separate communication
signals can be transmitted on the sixteen orthogonal channels during one frame period, albeit at a quarter of the rate
that the communication signals could be transmitted if the
TDM technique was not used.
Such an approach has the advantage that it preserves
compatibility with current hardware and software equipment

US 6,381,211 B1

3

4

which use the set of orthogonal codes, but which do not
ing numbers of time slots per frame period. For instance, if
an orthogonal channel operates at 160 kb/s, and four time
support the use of TDM techniques. By designating certain
orthogonal channels as channels for which TDM is not used,
slots are provided within that orthogonal channel in order to
carry data items pertaining to four different wireless links
the current equipment can communicate over those channels
5 during one frame period, then each ST receiving data from
without any changes being required to the equipment.
said orthogonal channel will receive data at a rate of 40 kb/s
In preferred embodiments, the transmission controller
(since each ST will only read a quarter of the data transfurther comprises: an overlay code generator for providing
mitted on the orthogonal channel during each frame period).
an overlay code from a first set of 'n' overlay codes which
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 10 orthogonal channel, then data items pertaining to only two
different wireless links will be transmitted per frame period,
overlay code from the overlay code generator to said data
and the two STs receiving data will do so at a rate of 80 kb/s
item, whereby 'n' data items pertaining to different wireless
(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: 15 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
In preferred embodiments, a number of said orthogonal
each other, the set of 'n' overlay codes enabling 'n' data
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 20 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
25 downlink traffic channels retains the benefit of CDMA
code.
access, ie. 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 techHowever, a first of the orthogonal channels is preferably
niques whilst data items transmitted within other orthogonal
reserved for the transmission of signals relating to the
channels can be encoded using overlay codes, the reception
controllers including the necessary decoders to decode either 30 acquisition of wireless links, and the second encoder is used
instead of the TDM encoder to enable overlay codes to be
type of encoded data item. A preferred arrangement, where
applied to data items to be sent within said first orthogonal
certain orthogonal channels are subject to TDM techniques
channel from the central terminal to one of said subscriber
whilst others are subject to overlay codes, will be discussed
terminals. Similarly, a second of the orthogonal channels is
in more detail later.
35
preferably reserved for the transmission of signals relating to
The orthogonal code generator may be arranged to genthe 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
set of orthogonal codes. Appropriate orthogonal codes can 40 channel from the central terminal to one of said subscriber
terminals.
then be read out to the encoder or decoder from the storage
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 45 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
embodiments, the central terminal would further comprise 50 approach using overlay codes eliminates the need to time
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- 55 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
60 accordance with the present invention.
period.
Viewed from a fourth aspect, the present invention proIn preferred embodiments, the channelisation means also
vides a method of processing data items to be transmitted
determines, for those orthogonal channels subject to TDM
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 65 system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
in the time dimension whilst others are not, and those which
links, the method comprising the steps of: (a) providing an
are subdivided can be subdivided differently to yield differ-

US 6,381,211 B1
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6

orthogonal code from a set of 'm' orthogonal codes used to
create 'm' orthogonal channels within the single frequency
channel; (b) combining a data item to be transmitted on the
single frequency channel with said orthogonal code, the
orthogonal code determining the orthogonal channel over
which the data item is transmitted, whereby data items
pertaining to different wireless links may be transmitted
simultaneously within different orthogonal channels of said
single frequency channel; 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 'm' 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.

FIGS. 7A and 7B are schematic diagrams illustrating
signal transmission processing stages for the telecommunications system of FIG. 1;
FIGS. 8A and 8B are schematic diagrams illustrating
signal reception processing stages for the telecommunications system of FIG. 1;
FIGS. 9A and 9B are diagrams illustrating the uplink and
downlink delivery methods when the system is fully loaded;
FIG. 10 illustrates the 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

35

DETAILED DESCRIPTION OF THE
INVENTION

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 customer 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;

40

45

50

55

60

65

FIG. 1 is a schematic overview of an example of a
wireless telecommunications system. The telecommunications system includes one or more service areas 12, 14 and
16, each of which is served by a respective central terminal
(CT) 10 which establishes a radio link with subscriber
terminals (ST) 20 within the area concerned. The area which
is covered by a central terminal 10 can vary. For example,
in a rural area with a low density of subscribers, a service
area 12 could cover an area with a radius of 15-20 Km. A
service area 14 in an urban environment where is there is a
high density of subscriber terminals 20 might only cover an
area with a radius of the order of 100 m. In a suburban area
with an intermediate density of subscriber terminals, a
service area 16 might cover an area with a radius of the order
of 1 Km. It will be appreciated that the area covered by a
particular central terminal 10 can be chosen to suit the local
requirements of expected or actual subscriber density, local
geographic considerations, etc, and is not limited to the
examples illustrated in FIG. 1. Moreover, the coverage need
not be, and typically will not be circular in extent due to
antenna design considerations, geographical factors, buildings and so on, which will affect the distribution of transmitted signals.
The central terminals 10 for respective service areas 12,
14,16 can be connected to each other by means of links 13,
15 and 17 which interface, for example, with a public
switched telephone network (PSTN) 18. The links can

US 6,381,211 B1
7

8

include conventional telecommunications technology using
types of connections (e.g., copper wires or optical fibres) can
copper wires, optical fibres, satellites, microwaves, etc.
be used to link the central terminal 10 to the public switched
telephone network 18. In this example the modem shelves
The wireless telecommunications system of FIG. 1 is
are connected via lines 47 to a microwave terminal (MT) 48.
based on providing fixed microwave links between subscriber terminals 20 at fixed locations within a service area 5 A microwave link 49 extends from the microwave terminal
48 to a point-to-point microwave antenna 54 mounted on the
(e.g., 12, 14, 16) and the central terminal 10 for that service
mast 50 for a host connection to the public switched telearea. Each subscriber terminal 20 can be provided with a
phone network 18.
permanent fixed access link to its central terminal 10, but in
preferred embodiments demand-based access is provided, so
A personal computer, workstation or the like can be
that the number of subscribers which can be supported 10 provided as a site controller (SC) 56 for supporting the
exceeds-the number of available wireless links. The manner
central terminal 10. The site controller 56 can be connected
in which demand-based access is implemented will be
to each modem shelf of the central terminal 10 via, for
discussed in detail later.
example, RS232 connections 55. The site controller 56 can
then provide support functions such as the localisation of
FIG. 2 includes a schematic representation of customer
premises 22. FIGS. 2A and 2B illustrate an example of a 15 faults, alarms and status and the configuring of the central
terminal 10. A site controller 56 will typically support a
configuration for a subscriber terminal 20 for the telecomsingle central terminal 10, although a plurality of site
munications system of FIG. 1. A customer radio unit (CRU)
controllers 56 could be networked for supporting a plurality
24 is mounted on the customer's premises. The customer
of central terminals 10.
radio unit 24 includes a fiat panel antenna or the like 23. The
customer radio unit is mounted at a location on the custom- 20
As an alternative to the RS232 connections 55, which
er's premises, or on a mast, etc., and in an orientation such
extend to a site controller 56, data connections such as an
X.25 links 57 (shown with dashed lines in FIG. 3) could
that the fiat panel antenna 23 within the customer radio unit
instead be provided from a pad 228 to a switching node 60
24 faces in the direction 26 of the central terminal 10 for the
service area in which the customer radio unit 24 is located.
of an element manager (EM) 58. An element manager 58 can
The customer radio unit 24 is connected via a drop line 28 25 support a number of distributed central terminals 10 connected by respective connections to the switching node 60.
to a power supply unit (PSU) 30 within the customer's
The element manager 58 enables a potentially large number
premises. The power supply unit 30 is connected to the local
(e.g., up to, or more than 1000) of central terminals 10 to be
power supply for providing power to the customer radio unit
24 and a network terminal unit (NTU) 32. The customer 30 integrated into a management network.
The element manager 58 is based around a powerful
radio unit 24 is also connected via the power supply unit 30
to the network terminal unit 32, which in turn is connected
workstation 62 and can include a number of computer
to telecommunications equipment in the customer's
terminals 64 for network engineers and control personnel.
premises, for example to one or more telephones 34, facFIG. 3A illustrates various parts of a modem shelf 46. A
simile machines 36 and computers 38. The telecommunica- 35 transmit/receive RF unit (RFU-for example implemented
tions equipment is represented as being within a single
on a card in the modem shelf) 66 generates the modulated
customer's premises. However, this need not be the case, as
transmit RF signals at medium power levels and recovers
the subscriber terminal 20 preferably supports either a single
and amplifies the baseband RF signals for the subscriber
or a dual line, so that two subscriber lines could be supported
terminals. The RF unit 66 is connected to an analogue card
by a single subscriber terminal 20. The subscriber terminal 40 (AN) 68 which performs A-D/D-A conversions, baseband
20 can also be arranged to support analogue and digital
filtering and the vector summation of 15 transmitted signals
telecommunications, for example analogue communications
from the modem cards (MCs) 70. The analogue unit 68 is
at 16, 32 or 64 kbits/sec or digital communications in
connected to a number of (typically 1-8) modem cards 70.
accordance with the ISDN BRA standard.
The modem cards perform the baseband signal processing of
FIG. 3 is a schematic illustration of an example of a 45 the transmit and receive signals to/from the subscriber
central terminal of the telecommunications system of FIG. 1.
terminals 20. This may include Y2 rate convolution coding
andx16 spreading with "Code Division Multiplexed Access"
The common equipment rack 40 comprises a number of
equipment shelves 42, 44, 46, including a RF Combiner and
(CDMA) codes on the transmit signals, and synchronisation
power amp shelf (RFC) 42, a Power Supply shelf (PS) 44
recovery, de-spreading and error correction on the receive
and a number of (in this example four) Modem Shelves 50 signals. Each modem card 70 in the present example has two
(MS) 46. The RF combiner shelf 42 allows the modem
modems, and in preferred embodiments there are eight
shelves 46 to operate in parallel. If 'n' modem shelves are
modem cards per shelf, and so sixteen modems per shelf.
However, in order to incorporate redundancy so that a
provided, then the RF combiner shelf 42 combines and
modem may be substituted in a subscriber link when a fault
amplifies the power of 'n' transmit signals, each transmit
signal being from a respective one of the 'n' modem shelves, 55 occurs, only 15 modems on a single modem shelf 46 are
and amplifies and splits received signals 'n' way so that
generally used. The 16th modem is then used as a spare
which can be switched in if a failure of one of the other 15
separate signals may be passed to the respective modem
shelves. The power supply shelf 44 provides a connection to
modems occurs. The modem cards 70 are connected to the
the local power supply and fusing for the various compotributary unit (TV) 74 which terminates the connection to
nents in the common equipment rack 40. A bidirectional 60 the host public switched telephone network 18 (e.g., via one
connection extends between the RF combiner shelf 42 and
of the lines 47) and handles the signalling of telephony
the main central terminal antenna 52, such as an omnidiinformation to the subscriber terminals via one of 15 of the
rectional antenna, mounted on a central terminal mast 50.
16 modems.
The wireless telecommunications between a central. terThis example of a central terminal 10 is connected via a
point-to-point microwave link to a location where an inter- 65 minal 10 and the subscriber terminals 20 could operate on
various frequencies. FIG. 4 illustrates one possible example
face to the public switched telephone network 18, shown
schematically in FIG. 1, is made. As mentioned above, other
of the frequencies which could be used. In the present

US 6,381,211 B1

9

10

example, the wireless telecommunication system is intended
to operate in the 1.5-2.5 GHz Band. In particular the present
example is intended to operate in the Band defined by ITU-R
(CCIR) Recommendation F.701 (2025-2110 MHz,
2200-2290 MHz). FIG. 4 illustrates the frequencies used for
the uplink from the subscriber terminals 20 to the central
terminal 10 and for the downlink from the central terminal
10 to the subscriber terminals 20. It will be noted that 12
uplink and 12 downlink radio channels of 3.5 MHz each are
provided centred about 2155 MHz. The spacing between the
receive and transmit channels exceeds the required minimum spacing of 70 MHz.
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 (ie. 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. 5A illustrates one cellular type arrangement of the
frequencies to mitigate interference problems between adjacent central terminals 10. In the arrangement illustrated in
FIG. 5A, the hatch lines for the cells 76 illustrate a frequency
set (FS) for the cells. By selecting three frequency sets (e.g.,
where: FSl=Fl,F4,F7,FI0;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 adj acent 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,FI0;FS2=F2,FS,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 the number of subscribers per cell to be
increased three fold, while still providing permanent fixed
access for each subscriber terminal 20.
Arrangements such as those in FIGS. 5A and 5B can help
reduce interference, but in order to ensure that cells 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 adj acent 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

55

TABLE 1
RWO
RW1

-1

-1

-1

-1

-1

-1

-1

-1

US 6,381,211 B1
11

12

TABLE 1-continued
RW2
RW3
RW4
RW5
RW6
RW7
RW8
RW9
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

kb/s
kb/s
kb/s
kb/s

-1
-1
-1
-1

-1

-1
-1

Full rate (F1)
Half rate (H1, H2)
Quarter rate (Q1, Q2, Q3, Q4)
Low rate (Ll, L2, L3, L4), for
uplink acquisition

In preferred embodiments, the manner in which these
channelisations are provided differs for the downlink (CT to
ST) and uplink (ST to CT) communication paths. This is
because it has been realised that different performance
requirements exist for the downlink and uplink paths. On the
downlink all signals emanate from a single source, namely
the central terminal, and hence the signals will be synchronised. However, on the uplink path, the signals will emanate
from a number of independent STs, and hence the signals
will not be synchronised.
Given the above considerations, in preferred
embodiments, on the uplink path full rate (160 kb/s) operation is implemented using the basic set of RW codes
discussed earlier, but half and quarter rates are achieved
through the use of 'Overlay Codes' which comprise RW
coded high rate symbol patterns that are transmitted for each
intermediate rate data symbol. For half rate operation, two
2-bit overlay codes are provide, whilst for quarter rate
operation, four 4-bit overlay codes are provided. When

25

30

35

40

45

50

55

60

65

-1
-1
-1

-1

20

-1

-1
-1

-1

-1

-1

-1

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

160
80
40
10

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

-1
-1

-1
-1

-1
-1

generating a signal for transmIssIOn, one of the overlay
codes, where appropriate, is applied to the signal in addition
to the appropriate RW code. When the signal is received,
then at the CDMA demodulator the incoming signal is
multiplied by the channel's PN, RW and Overlay codes. The
correlator integration period is set to match the length of the
Overlay code.
Overlay codes are used extensively to provide variable
rate uplink traffic channels. Overlay codes will also be used
to implement downlink control channels, these control channels being discussed in more detail later. However, as
mentioned earlier, a different approach is taken for providing
flexible channelisations on the downlink traffic channel
paths. Downlink traffic channels will operate in high rate,
160 kb/s, mode, with lower data rates of 80 and 40 kb/s
being supported by 'Time Division Multiplexing' (TDM)
the available bandwidth.
In preferred embodiments, TDM timeslot bit numbering
will follow the CCITT G.732 convention with bits transmitted in the sequence bit 1, bit 2 ... bit S. Byte orientation
is specified per channel as either most significant bit (MSB)
first, least significant bit (LSB) first or N/A.
The provision of a hybrid CDMA/TDM approach for
downlink traffic channels retains the benefits of CDMA
access, ie. interference is reduced when traffic is reduced.
Further, use of TDM ensures that the CDMA signal is
limited to a 256 'Quadrature Amplitude Modulation' (QAM)
constellation which reduces receiver dynamic range requirements. QAM constellations will be familiar to those skilled
in the art.
On the uplink channels, the pure CDMA approach using
overlay codes eliminates the need to time synchronise STs to
a TDM frame reference. This has the advantage of eliminating TDM delays and the 'guard time' in between TDM
frames. Another benefit is reduced peak power handling
requirements in the ST RF transmit chain which would
otherwise be needed when transmitting bursty TDM data.
High dynamic range requirement is restricted to the CT
receIver.
The manner in which the transmitted and received signals
are processed in accordance with preferred embodiments of
the present invention will be described with reference to
FIGS. 7 and S. FIG. 7A is a schematic diagram illustrating
signal transmission processing stages as configured in a
subscriber terminal 20 in the telecommunications system of
FIG. 1. In FIG. 7A, an analogue signal from a telephone is
passed via an interface such as two-wire interface 102 to a
hybrid audio processing circuit 104 and then via a codec 106
to produce a digital signal into which an overhead channel
including control information is inserted at lOS. If the

US 6,381,211 B1

13

14

subscriber terminal supports a number of telephones or other
From then on, the processing of the signal is the same as the
equivalent processing performed in the ST and described
telecommunications equipment, then elements 102, 104 and
with reference to FIG. 7A, the overlay code generator
106 may be repeated for each piece of telecommunications
producing a single overlay code of value '1' so that the
equipment.
At the output of overhead insertion circuit 108, the signal 5 signal output from spreader 111 is the same as the signal
input to the spreader 111.
will have a bit rate of either 160,80 or 40 kbits/s, depending
As mentioned earlier, in preferred embodiments, overlay
on which channel has been selected for transmission of the
codes, rather than TDM, are used to implement downlink
signal.
control channels, and data relating to such channels is passed
The resulting signal is then processed by a convolutional 10 from a demand assignment engine (to be discussed in more
encoder 110 to produce two signals with the same bit rate as
detail later) over line 107 through switch 109 to the overhead
the input signal (collectively, these signals will have a
insertion circuit 108, thereby bypassing the TDM encoder
symbol rate of 160, 80 or 40 KS/s). Next, the signals are
105. The processing of the signal is then the same as the
passed to a spreader 111 where, if a reduced bit rate channel
equivalent processing performed in the ST, with the overlay
code generator providing appropriate overlay codes to the
has been selected, an appropriate overlay code provided by
overlay code generator 113 is applied to the signals. At the 15 spreader 111. The overlay code generator will be controlled
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
discussed in more detail later).
overlay code will have increased the symbol rate by the
FIG. 8A is a schematic diagram illustrating the signal
necessary amount.
20 reception processing stages as configured in a subscriber
The signals output from spreader 111 are passed to a
terminal 20 in the telecommunications system of FIG. 1. In
spreader 116 where the Rademacher-Walsh and PN codes
FIG. 8A, signals received at a receiving antenna 150 are
are applied to the signals by a RW code generator 112 and
passed via a band pass filter 152 before being amplified in
PN Code generator 114, respectively. The resulting signals,
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 25 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
where it is mixed with a signal generated by a voltage
analogue waveform and provides a stage of baseband power
controlled oscillator 162 which is responsive to a synthesizer
control. The signals are then passed to a low pass filter 120 30 160. 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
output of the AID converter 170 at 2.56 MC/s is then passed
synthesizer 160. The output of the mixer 128 is then
to a correlator 178, to which the same Rademacher-Walsh
amplified in a low noise amplifier 130 before being passed 35 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
code generator 114), respectively. The output of the corroutput of the power control circuitry is further amplified in
elator 178, at 160 KS/s, is then applied to correlator 179,
a power amplifier 138 before being passed via a further band 40 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
181 form a CDMA demodulator. The output from the
FIG. 7B is a schematic diagram illustrating signal transCDMA demodulator (at correlator 179) is then at a rate of
mission processing stages as configured in a central terminal
10 in the telecommunications system of FIG. 1. As will be 45 either 160, 80 or 40 KS/s, depending on the overlay code
applied by correlator 179.
apparent, the central terminal is configured to perform
The output from correlator 179 is then applied to a Viterbi
similar signal transmission processing to the subscriber
decoder 180. The output of the Viterbi decoder 180 is then
terminal 20 illustrated in FIG. 7A, but does not include
elements 100, 102, 104 and 106 associated with telecompassed to an overhead extractor 182 for extracting the
munications equipment. Further, the central terminal 50 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
from the particular time slot in which it was inserted by the
a network interface over which incoming calls destined for
a subscriber terminal are received. When an incoming call is
CT TDM encoder 105. Then, the call data is passed via a
received, the central terminal will contact the subscriber 55 codec 184 and a hybrid circuit 188 to an interface such as
two wire interface 190, where the resulting analogue signals
terminal to which the call is directed and arrange a suitable
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
connection with the ST transmission processing stages,
is done using the call control channel discussed in more
elements 184, 188, 190 may be repeated for each piece of
detail later). The channel established for the call will deter- 60 telecommunications equipment 192 at the ST.
mine the time slot to be used for call data passed from the
If the data output by the overhead extraction circuit 182
CT to the ST and the TDM encoder 105 will be supplied with
is data on a downlink control channels, then instead of
this information.
passing that data to a piece of telecommunications
equipment, it is passed via switch 187 to a call control logic
Hence, when incoming call data is passed from the
network interface to the TDM encoder 105 over line 103, the 65 185, where that data is interpreted by the ST.
TDM encoder will apply appropriate TDM encoding to
At the subscriber terminal 20, a stage of automatic gain
enable the data to be inserted in the appropriate time slot.
control is incorporated at the IF stage. The control signal is

US 6,381,211 B1
15

16

derived from the digital portion of the CDMAreceiver using
The CDMAchannel hierarchy is as illustrated in FIG. 10.
Using this hierarchy, the following CDMA channelisations
the output of a signal quality estimator.
are possible:
FIG. 8B illustrates the signal reception processing stages
F1
as configured in a central terminal 10 in the telecommuniH1+H2
cations system of FIG. 1. As will be apparent from the figure, 5
H1+Q3+Q4
the signal processing stages between the RX antenna 150
H2+Q1+Q2
and the overhead extraction circuit 182 are the as those
Q1+Q2+Q3+Q4
within the ST discussed in connection with FIG. 8A.
However, in the case of the CT, call data output from the
Having discussed how the CDMA codes are enhanced to
overhead extraction circuit is passed over line 189 to the 10 enable flexible channelisations to be achieved, whereby the
network interface within the CT, whilst control channel data
bit rates can be lowered to enable more subscriber links to
be managed per channel frequency, a general overview of
is passed via switch 191 to the DA engine 380 for processhow the downlink and uplink paths are established will be
ing. The DA engine is discussed in more detail later.
provided with reference to FIGS. 11 and 12.
Overlay codes and channelisation plans are selected to
FIG. 11 is a block diagram of downlink and uplink
ensure signal orthogonality-i.e. in a properly synchronised 15
communication paths between central terminal 10 and subsystem, the contribution of all channels except the channel
scriber terminal 20. A downlink communication path is
being demodulated sum to zero over the correlator integraestablished from transmitter 200 in central terminal 10 to
tion period. Further, uplink power is controlled to maintain
receiver 202 in subscriber terminal 20. An uplink commuconstant energy per bit. The exception to this is Low rate
which will be transmitted at the same power as a Quarter rate 20 nication path is established from transmitter 204 in subscriber terminal 20 to receiver 206 in central terminal 10.
signal. Table 2 below illustrates the overlay codes used for
Once the downlink and the uplink communication paths
full, half and quarter rate operations:
have been established in wireless telecommunication system
1, telephone communication may occur between a user 208,
TABLE 2
25 210 of subscriber terminal 20 and a user serviced through
STTx.
central terminal 10 over a downlink signal 212 and an uplink
power
signal 214. Downlink signal 212 is transmitted by transmitNet
relative
Correlator
ter 200 of central terminal 10 and received by receiver 202
to F1-U
Rate Channel
integration Acquisition
of subscriber terminal 20. Uplink signal 214 is transmitted
(kb/s) designation
period (us) overlay
(dB) Overlay Code
30 by transmitter 204 of subscriber terminal 20 and received by
160 -F1-U
0
6.25
Ll
receiver 206 of central terminal 10.
-3
80 -H1-U
1 1
12.5
Ll
Receiver 206 and transmitter 200 within central terminal
-3
1 -1
80 -H2-U
12.5
L3
-6
40 -Q1-U
1 111
25
Ll
10 are synchronized to each other with respect to time and
-6
1 -11 -1
40 -Q2-U
25
L2
phase, and aligned as to information boundaries. In order to
-6
1 1 -1 -1
40 -Q3-U
25
L3
35 establish the downlink communication path, receiver 202 in
-6
1 -1 -11
40 -Q4-U
25
L4
subscriber terminal 20 should be synchronized to transmitter
200 in central terminal 10. Synchronization occurs by performing an acquisition mode function and a tracking mode
In preferred embodiments, a 10 kb/s acquisition mode is
function on downlink signal 212. Initially, transmitter 200 of
provided which uses concatenated overlays to form an
acquisition overlay; this is illustrated in table 3 below:
central terminal 10 transmits downlink signal 212. FIG. 12
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

FIGS. 9A and 9B are diagrams illustrating the uplink and
shows the contents of downlink signal 212. A frame information signal 218 is combined with an overlay code 217
downlink delivery methods, respectively, when the system is
fully loaded, and illustrate the difference between the use of 55 where appropriate, and the resultant signal 219 is combined
with a code sequence signal 216 for central terminal 10 to
overlay codes illustrated in FIG. 9A and the use of TDM as
produce the downlink 212. Code sequence signal 216 is
illustrated in FIG. 9B. When using overlay codes, an RW
derived from a combination of a pseudo-random noise code
code is split in the RW space domain to allow up to four sub
signal 220 and a Rademacher-Walsh code signal 222.
channels to operate at the same time. In contrast, when using 60
Downlink signal 212 is received at receiver 202 of
TDM, an RW code is split in the time domain, to allow up
subscriber terminal 20. Receiver 202 compares its phase and
to four signals to be sent using one RW code, but at different
code sequence to a phase and code sequence within code
times during the 125 us frame. As illustrated in FIGS. 9A
sequence signal 216 of downlink signal 212. Central termiand 9B, the last two RW codes, RW14 and RW15, are not
nal 10 is considered to have a master code sequence and
used for data traffic in preferred embodiments, since they are 65 subscriber terminal 20 is considered to have a slave code
reserved for call control and acquisition functions; this will
sequence. Receiver 202 incrementally adjusts the phase of
its slave code sequence to recognize a match to master code
be discussed in more detail later.

US 6,381,211 B1
17

18

sequence and place receiver 202 of subscriber terminal 20 in
power of transmitter 204 in subscriber terminal 20. The
phase with transmitter 200 of central terminal 10. The slave
operations and maintenance channel signal provides status
information with respect to the downlink and uplink comcode sequence of receiver 202 is not initially synchronized
munication paths and a path from the central terminal to the
to the master code sequence of transmitter 200 and central
terminal 10 due to the path delay between central terminal 5 subscriber terminal on which the communication protocol
which operates on the modem shelf between the shelf
10 and subscriber terminal 20. This path delay is caused by
controller and the modem cards also extends. The OMCID
the geographical separation between subscriber terminal 20
signal is a combination of the OMC signal and a signalling
and central terminal 10 and other environmental and techsignal (D), whilst the Ch. ID signal is used to uniquely
nical factors affecting wireless transmission.
After acquiring and initiating tracking on the central 10 identify an RW channel, this Ch. ID signal being used by the
subscriber terminal to ensure that the correct channel has
terminal 10 master code sequence of code sequence signal
been acquired.
216 within downlink signal 212, receiver 202 enters a frame
In preferred embodiments, the subscriber terminal will
alignment mode in order to establish the downlink commureceive downlink traffic channel data at a rate of 160 kb/s.
nication path. Receiver 202 analyzes frame information
Depending on the B-channel rate, the ST will be allocated an
within frame information signal 218 of downlink signal 212
15 appropriate share of the radio overhead. The following TDM
to identify a beginning of frame position for downlink signal
mappings are created:
212. Since receiver 202 does not know at what point in the
data stream of downlink signal 212 it has received
TABLE 4
information, receiver 202 must search for the beginning of
frame position in order to be able to process information 20
Overreceived from transmitter 200 of central terminal 10. Once
Rate
Channel
head
Bearer
CS
PC
OMC
rate
(kb/s) designation
receiver 202 has identified one further beginning of frame
position, the downlink communication path has been estab160
-Fl-O-Tl/l Bl, B2, B3, CS1, PC1, OMC1,OMC3 4 ms
lished from transmitter 200 of central terminal 10 to receiver
B4
CS3 PC3
-Fl-O-T2/1
Bl, B2
CS1, PC1, OMC1,OMC3 4 ms
80
202 of subscriber terminal 20.
25
CS3 PC3
The structure of the radio frames of information sent over
80
-Fl-O-T2/2
B3, B4
CS2, PC2, OMC2,OMC4 4 ms
the downlink and uplink paths will now be discussed with
CS4 PC4
reference to FIGS. 13 and 14. In FIGS. 13 and 14, the
40
-Fl-O-T4/1
Bl
CSl PCl
OMCl
8 ms
40
-Fl-O-T4/2
B2
CS2 PC2
OMC2
8 ms
following terms are used:
40
-Fl-O-T4/3
B3
CS3 PC3
OMC3
8 ms
Bn Customer payload, 1x32 to 2x64 Kb/s
30
40
-Fl-O-T4/4
B4
CS4 PC4
OMC4
8 ms
Dn Signalling Channel, 2 to 16 kb/s
OR Radio Overhead Channel
In the above chart, the scheme used to identify a channel
16 kb/s Traffic Mode
is as follows. Rate code 'F1' indicates full rate, 160 kb/s, 'D'
10 kb/s Acquisition/Standby Mode
Both FIGS. 13A and 13B show a 125us subframe format, 35 indicates that the channel is a downlink channel, and 'Tn/t'
indicates that the channel is time division multiplexed
which is repeated throughout an entire radio frame, a frame
between STs, 'n' indicating the total number of TDM
typically lasting for 4 milliseconds (ms). FIG. 13Aillustrates
timeslots, and 't' indicating the selected traffic timeslot.
the radio frame structures that are used in preferred embodiAll ST's operating on a traffic channel will receive
ments for the downlink path. Subframe (i) in FIG. 13A
shows the radio frame structure used for low rate, 10 Kb/s, 40 D-channel information at the 16 kb/s rate. The D-channel
protocol includes an address field to specify which ST is to
acquisition mode (Ln-D) during which only the overhead
process the contents of the message.
channel is transmitted. Sub frame (ii) in FIG. 13Ashows the
The channel structure was illustrated earlier in FIGS. 9A
radio frame structure employed for the call control channel
and 9B. In preferred embodiments, the channel structure is
operating in quarter rate, 40 Kb/s, mode (Qn-D), whilst
sub frame (iii) of FIG. 13A illustrates the radio frame struc- 45 flexible but comprises:
At least one Link Acquisition Channel (LAC)
ture used for traffic channels operating in full rate, 160 kb/s,.
At least one Call Control Channel (CCC)
mode (F1-D).
Typically one Priority Traffic Channels (PTC)
Similarly, subframe (i) of FIG. 13B shows the radio frame
structure used for the uplink path when operating in low rate
1 to 13 Traffic Channels (TC)
acquisition or call control mode (Ln-V). Sub-frames (ii) to 50
The manner in which the channelisation is provided
(iv) show the radio frame structure used for traffic channels
ensures that former fixed assignment arrangements using the
when operating in quarter rate mode (Qn-V), half rate mode
set of 16 RW codes discussed earlier are still supported, as
well as demand access services that are available when using
(Rn-V), and full rate mode (F1-V), respectively.
Considering now the overhead channel in more detail,
a system in accordance with the preferred embodiment.
FIGS. 14A and 14B show the overhead frame structure 55 FIGS. 15A and 15B illustrate typical downlink and uplink
channel structures that might occur in a loaded system in
employed for various data rates. The overhead channel may
accordance with preferred embodiments of the present
include a number of fields-a frame alignment word (FAW),
a code synchronization signal (CS), a power control signal
invention. As illustrated in FIG. 15A, on the downlink path,
some signals may be at 160 kb/s and utilise an entire RW
(PC), an operations and maintenance channel signal (OMC),
a mixed OMC/D-Channel (RDLC) signal (OMCID), a chan- 60 channel. An example of such signals would be those sent
nel identifier byte (Ch.ID), and some unused fields.
over fixed assignment links to products which do not support
The frame alignment word identifies the beginning of
the CDMA enhancements provided by systems in accordance with preferred embodiments of the present invention,
frame position for its corresponding frame of information.
The code synchronization signal provides information to
as illustrated for RW1 and RW2 in FIG. 15A. Alternatively,
control synchronization of transmitter 204 in subscriber 65 a user may have authority to utilise a whole RW channel, for
terminal 20 to receiver 206 in central terminal 10. The power
example when sending a fax, as illustrated by RW12 in FIG.
control signal provides information to control transmitting
15A.

US 6,381,211 B1

19

20

As illustrated by RWS to RWll, TDM can be used on the
(iii) In the event of a CT restart, invite STs to attempt
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
uplink warm start parameters-i.e. it should only be
in preferred embodiments, certain channels can be locked to 5
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
addressed ST to acquire the CCC Uplink followed by an
Acquisition Channel (LAC). Following power-up an STwill 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:
be simultaneous for all STs.
25
(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_progress
When a Management Poll fails it should be followed up
Transition states will also be defined. An ST is considered
by a number of faster polls until either the ST responds or
cold if the ST is newly provisioned, the CT has lost
management communications with the ST or the CT has 35 it is marked cold. The CCC is required to transmit all copies
of the invitations to acquire the LAC so that an ST can be
been power cycled. Over the LAC, the CT broadcasts
forced to acquire the LAC uplink.
individual ST serial numbers and offers an invitation to
acquire the LAC uplink. Cold uplink acquisition will be
Traffic Channel Uplink Acquisition Procedure
carried out on the Link Acquisition Channel at low rate. The 40
The basic acquisition process from the ST side IS as
CT will invite specific ST's to cold start via the management
follows;
channel.
Assuming an uplink channel is available, the appropriate
(i) Switch the downlink (receiver) circuitry to 10 kb/s rate,
acquisition overlay will be selected, and acquisition will be
and select the appropriate Traffic Channel RW and
45
initiated.
Overlay codes. Acquisition of the TC downlink is
'Rapid' downlink RW channel switching may be suplimited to achieving frame alignment.
ported at rates other than Ln-D. means that coherent
(ii) The downlink PC/CS channel will be decoded to
demodulation is maintained, and only convolutional decodcreate a busy/idle flag. If PC/CS reports busy, then this
ing and frame synchronisation processes need be repeated.
means that another ST is using that traffic channel and
On acquisition, management information will be 50
the ST aborts the acquisition process.
exchanged. The ST will be authenticated and allocated a
(iii) Switch uplink to 10 kb/s rate, and select the approshort ST_identifier (between 12 and 16 bits) which will be
priate Traffic Channel RW and Overlay codes. Enable
used for subsequent addressing. The ST uplink will operate
the ST transmitter at a level of nominal full rate power
for long enough for the uplink to be parametised by the ST
minus 18 dB. While PC/CS reports idle the ST will
in terms of code phase and transmit power. These parameters 55
continue uplink fast codesearch, stepping the uplink
will be used by the ST for subsequent warm start acquisipower level by +2 dB at the end of each search. The
tions and will also be held by the CT to allow the CT to force
uplink should acquire at nominal full rate power minus
a cold ST to warm start. On successful completion of net
6 dB. Uplink acquisition is aborted if maximum transentry, the ST will be placed in the idle state and instructed
mit level is reached and PC/CS continues to report idle.
to cease uplink communications and move to the Call 60
(iv) PC/CS reports busy. At this point the ST may have
Control Channel (CCC) (RWI4 in preferred embodiments).
genuinely acquired the traffic channel, or instead may
The time taken for net entry to be achieved can be
be observing PC/CS go busy because another ST has
monitored, and the following techniques can be used to
acquired the traffic channel. The ST is sent an authendecrease net entry time if desired:
tication request and responds with it's ST_identifer.
(i) Prioritise so that high GOS (Grade Of Service) users 65
The CT grants uplink access by returning the
are offered net entry first.
ST_identifier. The ST aborts the acquisition process if
the returned ST_identifier is not recognised (ie. is not
(ii) Convert Traffic Channels to LACs.

US 6,381,211 B1

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22

the ST_identifer that it sent). This authentication process arbitrates between two STs contending for outgoing access and it also keeps STs from acquiring TCs
that have been reserved from incoming access.
5

Incoming Call

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.

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_ 10
Prcgress state the call is rejected.
Outgoing Priority Call
(ii) Check that an uplink TC of the required bandwidth is
It is recognised that the random access protocol used to
available. If there is bandwidth then a TC is reserved.
setup normal outgoing calls could lead to blocking. In
(iii) An incoming call setup message is broadcast over the
preferred embodiments, access to a largely nonCCC to inform the addressed ST of the incoming call 15
blocking Priority Traffic Channel will be allowed. Priand specify the TC on which to receive the call. If no
ority calling is complicated because the ST must:
TC is available but the CT forms part of a Service
(i) Capture and decode dialled digits.
Domain, then the incoming call setup message is sent
(ii) Regenerate digits when a blocking condition occurs.
with a null TC otherwise the call is rejected. Service
domains will be discussed in more detail later. The 20
(iii) Allow transparent network access in a non-blocking
incoming call setup message is repeated a number of
condition.
times.
(iv) Categorise all outgoing calls as priority or normal so
(iv) The ST attempts uplink acquisition. The ST listens to
that normal calls are dropped in favour of priority calls.
the downlink and keeps trying for uplink acquisition
The priority call procedure in preferred embodiments is as
until the CT sends a message to the ST to return the ST 25 follows:
to the CCC. The ST will also run a timer to return it
(i) The CT will publish Directory Numbers (DNs) for a
back to the CCC in the event of an incoming call failing
number of emergency services over the CCC.
to complete.
(ii) The ST will attempt uplink access according to the
(v) On successful uplink acquisition, the CT authenticates
normal algorithms. If the outgoing access is successful
the ST.
30
then the customer is able to dial as normal. All dialled
(vi) Rate switching is originated from the CT modem. A
digits are check against the emergency DN list so that
command is sent via the PCjCS to switch the downlink
calls may be categorised normal or priority at the CT.
to the required bandwidth. The ST returns the rate
(iii) If congestion tone is returned the customer is allowed
switch command via the uplink PCjCS. The link is now 35
to dial the emergency number into the ST. If the ST
of the required bandwidth.
detects an emergency DN sequence then uplink access
via the Priority Traffic Channel (PTC) is attempted.
Outgoing Call
(iv) On PTC acquisition, the ST relays the dialled digit
Outgoing calls are supported by allowing slotted random
sequence to the CT for dialling into the PSTN.
access to the TC uplinks. The outgoing call processing is as 40
(iv) The CT converts the PTC to a TC and reallocates
follows:
another TC to become the PTC, dropping a normal call
in progress if necessary.
(i) The CT publishes a 'free list' of available Traffic
Channels and Priority Traffic Channels with their
Interference Limiting (Pool Sizing)
respective bandwidths. This list is published periodically (in preferred embodiments, every 500 ms) and is 45
Across a large scale deployment of cells, optimum capacused to mark uplink access slots.
ity is achieved by minimising radio traffic while maintaining
an acceptable grade of service. Lowest possible radio traffic
(ii) An off-hook condition is detected by the ST. The ST
results in improved 'carrier to interference' (CII) ratios for
starts a call setup timer.
(iii) The ST waits for the next free list to be received over 50 users within the cell of interest and to co-channel users in
nearby cells. The CII ratio is a measure (usually expressed
the CCC. If the Free list is empty the outgoing call is
in dB) of how high above interference the transmitted signal
blocked. The ST will generate a congestion tone.
needs to be to be decoded effectively. In preferred
(iv) If the Free list has available channels, the ST picks a
embodiments, the central terminal is provided with the
channel from the free list at random. The algorithm that
the ST uses to pick a channel will need to be specified 55 ability to trade traffic for CII, thereby allowing network
planning to be carried out less rigidly. This feature can be
in the free list. For example, the ST may be required to
realised by a system using CDMA as in preferred embodialways choose from a pool of minimum bandwidth
ments of the present invention, and is a benefit that CDMA
channels so that high bandwidth channels remain availoffers over TDMA and FDMA systems.
able for high GOS users. Alternatively the ST may be
60
In preferred embodiments, the CT will control the number
allowed to choose any channel regardless of bandwidth
of Traffic Channels to minimise access noise. TCs will be
for minimum blocking. In preferred embodiments, STs
classified as:
will not choose low bandwidth channels and negotiate
(i) Busy---carrying traffic;
the rate up.
(ii) Access, Incoming (Access_In)-reserved for incom(v) The ST attempts uplink acquisition on the specified
ing access;
TC, this process having been described earlier. If 65
acquisition is successful then the outgoing call is pro(iii) Access, Outgoing (Access_Out)-reserved for outcessed. Otherwise the ST returns to the CCC and waits
going access-such TCs appear on the Free list;

US 6,381,211 B1
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(iv) Priority-reserved for priority outgoing access-such
Additionally, incoming call information to the central
terminal, other than call information from the subscriber
TCs appear in the Free list;
terminals 20 connected to the central terminal, is provided
(v) Free-available for any purpose; and
over the concentrated network interface 390 to the DA
(vi) Locked-not available due to interference limiting.
5 engine 380. The DA engine 380 includes a call control
This classification scheme is illustrated in FIG. 16. The
function, similar to the call control function 336 in each of
CT will allocate traffic on the following basis:
the subscriber terminals 20, for each of the modems on the
(i) The CT will monitor incoming and outgoing call
modem shelf. Hence, in a similar fashion to the call control
setup-times and convert Access TCs from Free TCs in
function 336 at the subscriber terminals 20, the call control
order to achieve a required grade of service.
functions within the DA engine 380 are also able to provide
(ii) When a call is setup, an Access TC is converted to a 10 GOS estimates for incoming calls, and these GOS estimates
are passed over line 395 to the dynamic pool sizing function
Busy TC. If a Free TC is available, it is converted to a
360.
new Access TC. If there are no Free TCs then the
At set up, the management system 370 within the element
Access TC is lost until a call clears.
manager will have connected to the central terminal, and
(iii) When a call clears the Busy TC is converted to a Free
15 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.
then compares this data from the management system with
(v) The CTwill monitor the Busy TC downlink and uplink 20 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
appropriate. For example, if the actual bit error rate exceeds
CT records a higher than average. soft error count, a 25 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 30 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
modems, this causing the interference, and hence the actual
establish a bit error rate (BER) estimate 330 associated with 35 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.
to be used for signals to be transmitted to the STs 20.
Here, the BER estimate is encoded and passed on the OMC 40
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
or GOS goal with the allocated pool size, the dynamic pool
and passed over line 355 to the dynamic pool sizing function 45 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 transthe provision of more central terminal hardware to support
mission over the wireless link 310. This BER estimate 350 50 the STs.
is also 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
resistance of the other channels to interference. Hence, a
by the lines entering the bottom of the dynamic pool sizing 55 suitable approach for the demand assignment engine 380,
function 360.
upon receipt of pool sizing request from the dynamic pool
In addition to BER estimates, grade of service (GOS) data
sizing function 360, is to disable the[]modem that has the
is obtained from two sources. Firstly, at each subscriber
least traffic passing through it.
terminal 20, the call control function 336 will note how
RF Channel Switching
readily it is able to establish traffic channels for transmitting 60
In preferred embodiments, it has been realised that if an
and receiving data, and from this can provide a GOS
ST is allowed to operate from more than one CT Modem
estimate to the multiplexer 332 for encoding by the encoder
Shelf/RF Channel then the following benefits may be rea334 for subsequent transmission over the wireless link 310
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, 65
system fault occur, an ST may switch to an alternative
and then the GOS estimate is passed over line 355 to the
dynamic pool sizing function 360.
frequency for service.

US 6,381,211 B1

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26

(ii) Call blocking-an ST denied service from one CT
(ii) Supplying backup service-the CT is providing sershelf may choose to switch to an alternative frequency
vice to the ST.
for service.
(iii) Available for backup service-the CT will provide
(iii) Traffic load balancing-the Element Manager may on
service to the ST if required.
the basis of call blocking statistics choose to move STs 5
It should be noted that the ST need not switch to an
between CT shelves.
entirely different CT, but can instead switch to a different CT
(iv) Frequency diversity-in the presence of channel
shelf (and hence different RF frequency channel) within the
selective fading (slow multipath) an ST may operate on
same CT. However, in preferred embodiments, the ST will
the frequency channel offering highest signal strength
typically switch to a different CT, since some errors expeand lowest soft error count.
10 rienced by one CT shelf may also affect other shelves within
RF channel switching is only possible where there are two
the same CT, and so for fault tolerance (described in more
or more co-located CT shelves serving the same geographidetail below), it is preferable for the ST to switch to a
cal area on different RF frequency channels within the same
separate CT.
RF band. A deployment that meets this criterion may be
Database consistency across CT shelves is preferably
configured as a 'Service Domain'. Possible deployment
15 supported through the service domain controller 400. Datascenarios are illustrated in FIG. 18. FIG. 18(i) shows an
base consistency needs to be real-time so that an ST entering
arrangement where omni antennae are used to provide the
the network is allowed full Service Domain access immeentire cell with four frequency channels, eg F1, F4, F7, FlO.
diately (the Service Domain message is broadcast to all STs,
FIG. 18(ii) shows an arrangement where sectored antennae
and so a new ST will expect access across the full Service
are used to provide six separate sectors within a cell, each
sector being covered by two frequency channels. FIG. 18(iii) 20 Domain).
Incoming access via backup CTs requires some function
shows an alternative arrangement where three sectored
to be provided to broadcast duplicate incoming call setup
antennae are used to divide the cell in to three sectors, each
messages to all CTs that form a Service Domain. Preferably
sector being covered by a separate frequency channel, and
then an omni antenna is used to provide an 'umbrella'
this is handled by the service domain controller 400, which
coverage for the entire cell, this coverage employing a 25 forwards incoming call setup messages to each CT operating
frequency channel different to the three frequency channels
in the service domain. All CTs will allocate Access_In
used by the sectored antennae.
Traffic Channels and relay the incoming call setup message
For the system to work effectively, the STs must be able
via the Call Control Channel. On successful uplink access,
to switch channels quickly, and fast channel switching
one CT will respond to the service domain controller with a
necessitates that CT shelf synchronisation be provided at the 30 call accepted message, the other CTs will eventually respond
following levels:
with call setup failed messages. Outgoing access via a
(i) CDMA PN code. This preserves uplink code phase
backup CT is similar to normal outgoing access.
across RF channels during warm start; and
Another job which can be performed by the service
(ii) RF carrier frequency. This eliminates the need for the
domain controller is to assist the element manager 58 in
coarse frequency search on a downlink RF channel 35 reconfiguring equipment in the event of a fault. For example,
switch.
if one CT is taken out of commission because of a fault, a
On installation, an ST will be programmed with an RF
different CT can be brought 'on-line', and the service
channel and PN code, these codes specifying the ST's initial
domain controller can provide that new CT with the neceshome channel.
sary information about the other CTs in the service domain.
The manner in which channel switching is facilitated in 40
FIG. 19B illustrates those elements of the subscriber
preferred embodiments will be described with reference to
terminal used to implement RF channel switching. The radio
FIGS. 19A and 19B. A service domain controller 400 is
subsystem 420, which incorporates the transmission and
preferably provided to act as an interface between the
reception signal processing stages, will pass any data
exchange connected to the service domain controller over
received on the call control channel over line 425 to the
path 405 and a number of central terminals 10 connected to 45 message decoder 430. If the decoder 430 determines that the
the service domain controller over paths 410. The central
data on the call control channel forms a service domain
terminals connected to the service domain controller form a
message, then this is passed over line 435 to the channel
'service domain' of central terminals that may be used by a
selection controller 440, where the information within the
subscriber terminal 20 for handling communications.
service domain message is stored in storage 445.
In preferred embodiments, the service domain controller 50
Similarly, if the message decoder identifies the data as a
400 is used to provide each CT 10 with appropriate infor'free list' identifying the available traffic channels on a
mation about the other CTs within the service domain. Each
particular RF frequency, then this data is passed to the call
CT can then broadcast a 'Service Domain' message comcontrol function 336 and the channel selection controller 440
prising a list of RF frequencies and CT Identifiers that form
over path 450. The call control function 336 stores the free
a Service Domain to be used by the STs for subsequent RF 55 list in the storage 445 for subsequent use by the call control
switching functions. The ST then stores this information for
function 336 and the channel selection controller 440.
future reference when establishing a link with one of the
If the message decoder 430 determines that the data forms
CTs. It is preferable for each CT to broadcast the service
an incoming call setup message, then that information is
domain message since an ST may be listening to any of the
supplied over line 455 to the call control function 336 and
CTs at the time that the message is broadcast.
60 the channel selection controller 440 for processing. The
Each CT database will hold an entry for every ST located
incoming call setup message will typically specify a TC on
within the Service Domain. Each database entry describes
the current frequency channel which should be used to
how the CT views it's relationship with the ST and may be
access the incoming call, and the channel selection controlmarked as:
ler will attempt to establish a link on that TC. The channel
(i) Primary service provider-the CT is the ST's home 65 selection controller will in such cases instruct the radio
channel. All management communication with an ST is
sub-system 420 over line 465 to use the current frequency
via it's home CT.
channel to establish the required link. If, on the other hand,

US 6,381,211 B1
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the traffic channel specified in the call setup message is
the call control function 336 asserts a channel request
'null', the channel selection controller has the option to
on line 460 to the channel selection controller 440 and
change RF frequency using the information stored in storage
normal uplink access is attempted.
445 about the other CTs in the service domain.
(ii) If the Free List shows no Access_Out channels are
To enable the channel selection controller 440 to receive 5
available on the current frequency channel, then the
information about the status of links, a link operating status
channel selection controller will be used to switch the
signal can be supplied over line 470 from the radio subST to the next RF channel in the Service Domain,
system. This signal will give an indication of the radio link
whereupon the ST will wait for the next Free List.
quality, and may be a simple 'OK' or 'failed' indication, or
(iii) When the ST finds a Free List with an available
alternatively may include extra information such as BER 10
Access-Out channel, then uplink access is attempted
values for the link. This information can be used by the
and the call is processed as normal.
channel selection controller to determine whether a particu(iv) When the call clears, the ST downlink preferably
1ar frequency channel should be used or not.
switches back to the home CT.
To enable the call control function to specify a specific
Access-Out channel for outgoing calls, a line 460 is pro- 15
RF Channel Switching for Traffic Load Balancing
vided between the call control function 336 and the channel
Traffic load balancing is, in preferred embodiments, proselection controller 440. The call control function 336 may
vided by static configuration via the EM 58. Call blocking
choose an access-out channel from the free list in storage
and setup time statistics may be forwarded to the EM where
445, and instruct the channel selection controller over line
an operator may decide to move an ST to another RF
460 to attempt acquisition of that channel.
20 channel.
The following examples indicate how the above described
structure may be used to perform channel switching in
RF Channel Switching for Frequency Diversity
particular circumstances.
Frequency diversity is, in preferred embodiments, provided by static configuration via the EM 58. Radio link
RF Channel Switching for Fault Tolerance
25 statistics may be forwarded to the EM where an operator
Should one RF channel suffer complete loss of downlink,
may decide to move an ST to another RF channel.
the following process takes place in preferred embodiments:
Although a particular embodiment has been described
(i) The ST will attempt downlink re-acquisition for a
herein, it will be appreciated that the invention is not limited
period of time, say 20 seconds.
30 thereto and that many modifications and additions thereto
may be made within the scope of the invention. For example,
(ii) If acquisition fails, the channel selection controller
various combinations of the features of the following depen440 of the ST will select the next available channel
dent claims could be made with the features of the indepenfrom the Service Domain information in storage 445
dent claims without departing from the scope of the present
and attempt downlink acquisition. This process will be
35 invention.
repeated until a downlink signal is acquired.
What is claimed is:
(iii) Once a backup RF channel is located, the ST will
1. A reception controller for processing data items
'camp' on the Call Control Channel and may subsereceived over a wireless link connecting a central terminal
quently be granted traffic access.
and a subscriber terminal of a wireless telecommunications
(iv) If the CT fault persists, the EM 58 may use the service
domain controller 400 to reconfigure the Service 40 system, a single frequency channel being employed for
transmitting data items pertaining to a plurality of wireless
Domain so that the functioning CT shelves become
links, and 'm' orthogonal channels being provided within the
primary service providers for the pool of 'homeless'
single frequency channel, the receiver controller comprisSTs.
ing:
A fault that does not result in complete loss of downlink
an orthogonal code generator for providing an orthogonal
signal will not result in RF channel switching 'en mass'. 45
code from a set of 'm' orthogonal codes used to create
Rather, a fault may result in excessive or total call blocking,
said 'm' orthogonal channels within the single freas discussed below.
quency channel;
RF Channel Switching for Call Blocking
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
If Incoming access traffic channels are being blocked, the 50
by the orthogonal code generator, in order to isolate
following process is employed in preferred embodiments:
data items transmitted within the corresponding
(i) The call setup message sent over the Call Control
orthogonal channel; and
Channel will specify a TC on which to access the call.
a TDM decoder arranged to extract a data item from a
(ii) In the case of incoming access being blocked, the CT 55
predetermined time slot within said orthogonal
will specify a null TC. The channel selection controller
channel, a plurality of data items relating to different
440 of the ST will in such cases switch to the next RF
wireless links being transmitted within the same
channel from the Service Domain information in stororthogonal channel during a predetermined frame
age 445 and monitor the Call Control Channel.
period.
(iii) If the ST receives a call setup message with a valid 60
2. A reception controller as claimed in claim 1, further
TC, then the call is processed as normal.
comprising:
(iv) When the call clears, the ST downlink preferably
an overlay code generator for providing an overlay code
switches back to the home CT.
from a first set of 'n' overlay codes which are orthogoIf Outgoing access traffic channels are being blocked, the
nal to each other, the set of 'n' overlay codes enabling
following process is employed in preferred embodiments: 65
'n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
(i) The ST registers an off-hook. The Free List in storage
445 is checked and if a traffic channel is available, then
channel; and

US 6,381,211 B1

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a second decoder, selectively operable instead of the TDM
decoder, to apply to the data items of the orthogonal
channel, the overlay code from the overlay code generator so as to isolate a particular data item transmitted
using that overlay code.
3. A reception controller as claimed in claim 1, wherein
the orthogonal code generator is a storage arranged to store
the set of orthogonal codes.
4. A reception controller as claimed in claim 1, wherein
the set of orthogonal codes comprise a set of RademacherWalsh (RW) codes.
5. A subscriber terminal of a wireless telecommunications
system, comprising a reception controller having:
an orthogonal code generator for providing an orthogonal
code from a set of 'm' orthogonal codes used to create
said 'm' orthogonal channels within the single frequency channel;
a first decoder for applying, to signals received on the
single frequency channel, the orthogonal code provided
by the orthogonal code generator, in order to isolate
data items transmitted within the corresponding
orthogonal channel;
a TDM decoder arranged to extract 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;
an overlay code generator for providing an overlay code
from a first set of 'n' overlay codes which are orthogonal to each other, the set of 'n' overlay codes enabling
'n' data items pertaining to different wireless links to be
transmitted simultaneously within the same orthogonal
channel;
a second decoder, selectively operable instead of the TDM
decoder, to apply to the data items of the orthogonal
channel, the overlay code from the overlay code generator so as to isolate a particular data item transmitted
using that overlay code, wherein the set of orthogonal
codes comprise a set of Rademacher-Walsh (RW)
codes and wherein the set of orthogonal codes comprise
a set of Rademacher-Walsh (RW) codes.
6. 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 steps of:

providing an orthogonal code from a set of 'm' orthogonal
codes used to create said 'm' orthogonal channels
within the single frequency channel;
applying, to signals received on the single frequency
channel, the orthogonal code in order to isolate data
items transmitted within the corresponding orthogonal
channel; and
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.
7. A method as claimed in claim 6, wherein said extracting
step is selectively replaced by steps of:
providing an overlay code from a first set of 'n' overlay
codes which are orthogonal to each other, the set of 'n'
overlay codes enabling 'n' data items pertaining to
different wireless links to be transmitted simultaneously within the same orthogonal channel; and
a second decoder, selectively operable instead ofthe TDM
decoder, to apply to the data items of the orthogonal
channel, the overlay code so as to isolate a particular
data item transmitted using that overlay code.
8. A method as claimed in claim 7, further comprising
steps of:
determining which of the orthogonal channels will be
subject to TDM techniques, and transmitting that information to a plurality of subscriber terminals within the
wireless telecommunications system; and
applying the overlay code to the data items of the orthogonal channel so as to isolate a particular data item
transmitted using its particular overlay code.
9. A method as claimed in claim 6, further comprising
steps of:
determining which of the orthogonal channels will be
subject to TDM techniques; and
transmitting that information to a plurality of subscriber
terminals within the wireless telecommunications system.
10. A method as claimed in claim 9, further comprising a
step of:
determining, for those orthogonal channels subject to
TDM techniques, how many time slots will be provided
within each orthogonal channel.

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