Note: Descriptions are shown in the official language in which they were submitted.
CA 02222705 1997-11-28
W 096138938 PCTrU595/~OE~
~ P~U~TUS ANnD ~l~O~ OF CO ~ ~nt-T-T~G TRUiN~ l~a PO~nER A2~D
TR~NS~IT RATE O~ A WT~F,r-F-~IS ~T~ ~TCA~IONS a~
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to
telecommunications systems and more particularly to an
apparatus and method of establishing and maintaining
communication paths in a wireless telecommunications
system.
CA 0222270~ 1997-11-28
W 096/38938 PCTAUS96/08568
~3ACKGROUND OF THE INVENTION
Wireless telecommunications systems require
transmitters and receivers in order to send and receive
information over radio frequency signals in a network
configuration. These transmitters and receivers may suffer
from interference with other transmitters and receivers in
and out of the network configuration. Such interference
may be caused by each transmitter operating at a different
transmitting power level. Control of the transmitting
power of the transmitters has only been performed at each
transmitter and not from a centralized location. Further,
the transmitting power of a transmitter within a wireless
telecommunication system has proved to be difficult to
control with respect to the transmitting powers of other
transmitters in the system. Therefore, it is desirable to
obtain improved control of the transmitting power of each
transmitter within a wireless telecommunications system.
Also, wireless telecommunications systems require
radio links be established between corresponding
transmitters and receivers in order to provide wireless
communication transmissions. Interference and delays occur
in achieving acquisition and establishment of each radio
link. It becomes inefficient to acquire and establish a
radio link every time a call is to be initiated. Problems
with interference also occur if a radio link is maintained
whether or not a call is in progress due to power needed to
maintain the radio link. Therefore, it is desirable to
avoid establishing a radio link for each call and avoid
inserting interference into the wireless telecommunications
system through continuous maintenance of a radio link.
Additionally, transmitters typically transmit signals
at one phase and receivers typically receive signals at a
different phase. The different phases used by transmitters
and receivers in the system may cause problems in
recognizing information from multiple transmitter and
receiver pairs. Further, a receiver having one phase
CA 0222270~ 1997-11-28
3 ~ :
requires a greater amount of circuitry and software support
to identify information from a corresponding transmitter
operating at a different phase. Additionally, a phase
difference between a receiver and transmitter may also be
affected by changes in a path delay between the transmitter
and the receiver. Therefore, it is desirable to be able to
control the phase of transmitters and receivers in a
wireless telecommunications system to provide improved
radio frequency signal transmission.
Transmitters of the originating or destination sources
may send information at one phase and the receivers of the
destination or originating sources may receive information
out of phase. In such a situation, the receivers will not
know at what portion in the data stream the receive process
has started. Information is partitioned into frames and to
adequately process the information the beginning o~ each
frame should be identified. Conventional techniques to
frame align information are cumbersome, slow in frame
identification, and may lose frames of information.
Therefore, it is desirable to quickly and easily identify
the beginning of each frame of information to achieve
proper processing.
WO-A-93/14588 describes a CDMA cellular communication
system in which to permit maximum flexibility within a
predefined structure, the communication of signals from the
mobile station may be characterised in the form of an
access channel or a traffic channel communication, in which
various combinations of user traffic and/or signalling
traffic may be transmitted.
US-A-5, 161,168 discloses a technique for wireless
telephone communications between a central terminal and a
plurality of subscriber terminals, in which a number of
signals associated with the subscriber terminals are
spread, the spread signals are combined, and then the
combined signal is transmitted.
US-A-5,103,459 discloses a spread-spectrum
telecommunications system whereby the mobile units are
AMENDED SHEET
CA 0222270~ 1997-11-28
synchronised using phase offset information contained in a
pilot signal.
US-A-5,383,219 discloses a CDMA telecommunications
system in which a power control bit is multiplexed in place
of another bit in the transmission frame prior to
transmission of that frame to a mobile unit, the mobile unit
then uslng the power control bit in order to determine
whether to increment or decrement its output power level by
a predetermined amount.
SUMM~RY OF THE I~rVE~TION
In accordance wlth one aspect of the present invention,
a method of controlling transmitting power in a subscriber
terminal of a wireless telecommunications systems is provided
which substantially eliminates or reduces disadvantages and
problems associated with conventional wireless systems.
Viewed from one aspect, the present invention provides
a subscriber terminal for communicating with a central
terminal located at a fixed distance from the subscriber
terminal, the subscriber terminal comprising: a spreader
operable to receive an information signal, the spreader
~urther operable to combine the information signal with a
code se~uence signal to generate a spread signal for
transmission as an uplink signal to the central terminal; a
receiver operable to receive a power control signal contained
within an overhead channel inserted in to a downlink signal
transmitted from the central terminal using a wireless link,
the power control signal having a value determined as a
result of comparing within the central terminal the power of
the uplink signal with a desired threshold range; and a
transmitter coupled to the spreader and operable to receive
the spread signal, the transmitter operable to amplify the
spread signal in response to the power control signal, the
power control signal specifying a power adjustment to the
spread signal.
According to an embodiment o~ the present invention, a
method of controlling transmitting power in a subscriber
terminal o~ a wireless telecommunications system that
AME~IDED SHEET
CA 0222270~ 1997-11-28
includes establishing a downlink communication path ~rom a
transmitter of a central terminal to a receiver of the
subscriber terminal. A downlink signal is transmitted from
the transmitter of the central terminal and received at the
receiver of the subscriber terminal. The downlink signal
includes a power control signal that is used to adjust a
transmitting power of the transmitter in the subscriber
terminal in order to establish an uplink communication path
between the transmitter of the subscriber terminal and a
receiver at the central terminal.
One technical advantage of this aspect of the present
invention is to externally control the transmitting power
of the transmitter in the subscriber terminal. Another
technical advantage is to control transmitting power
through an overhead channel in the downlink signal from the
central terminal to the subscriber terminal. Yet another
technical advantage is to provide increasing and decreasing
incremental adjustments to the transmitting power of the
transmitter in the subscriber terminal. Still another
technical advantage is to adjust the transmitting power to
match the transmitting power of other subscriber terminals
serviced by the central terminal.
In accordance with another aspect of the present
invention, there is provided an apparatus and method of
synchronizing a transmitter in a subscriber terminal of a
wireless telecommunications system that substantially
eliminates or reduces disadvantages and problems associated
with conventional wireless communication techniques.
Viewed from another aspect, the present invention
provides a subscriber terminal for communicating with a
central terminal located at a fixed distance from the
subscriber terminal, the subscriber terminal comprising: a
receiver operable to establish a downlink communication
path from the central terminal to the subscriber terminal;
a transmitter for transmitting an uplink signal to the
central terminal; a spreader operable to receive an
information signal, the spreader further operable to
~ lE~lDED SH~
' - CA 0222270~ 1997-11-28
combine the in~ormation signal with a code sequence signal
to generate a spread signal for transmission; the receiver
being operable to receive a code synchronization signal
~rom the central terminal using a wireless link, the code
synchronisation signal having a value determined as a
result of comparing within the central terminal the phase
of the uplink signal with the phase of a receiver in the
central terminal; and a code generator coupled to the
spreader, the code generator operable to generate the code
sequence signal in response to the code synchronization
signal, the code synchronization signal specifying a phase
adjustment to the code sequence signal.
According to an embodiment of the present invention,
a method of synchronizing a transmitter in a subscriber
terminal of a wireless telecommunications system includes
establishing a downlink communication path from a central
terminal to the subscriber terminal. A downlink signal is
transmitted from a transmitter in the central terminal and
received at a receiver of the subscriber terminal. The
receiver of the subscriber terminal extracts a code
synchronization signal from the downlink signal. The code
synchronization signal is used to adjust a phase of an
uplink signal transmitted by a transmitter in the
subscriber terminal. The receiver of the central terminal
monitors the phase of the uplink signal and changes the
code synchronization signal in order to obtain a match of
the phase of the uplink signal to the phase at the receiver
of the central terminal.
One technical advantage of this aspect o~ the present
invention is to remotely adjust a transmitting phase of a
transmitter in a subscriber terminal. Another technical
advantage is to obtain a match of the transmitting phase of
the transmitter in a subscriber terminal at a receiver of
a central terminal. Yet another technical advantage is to
embed a code synchronization signal within a downlink
signal transmitted from a central terminal to incrementally
adjust a transmitting phase of a transmitter in a
~ ENDED SHEET
- CA 0222270~ 1997-11-28
6a ~ ,
subscriber terminal. Still another technical advantage is
to continually monitor a transmitting phase of a
transmitter in a subscrlber terminal in order to maintain
a match with the phase of a receiver in a central terminal.
In accordance with another aspect o~ the present
invention, there is provided an apparatus and/or method of
transmitting and receiving information in a wireless
telecommunications system that substantially eliminates or
reduces disadvantages and problems associated with
conventional wireless communication techniques.
Viewed from another aspect, the present invention
provides a subscriber terminal located at a fixed distance
from a central terminal, comprising: a spreader operable to
receive user traffic and control information, the spreader
operable in an acquisition mode during establishment of an
uplink communication path to generate a ~irst signal at a
first rate representing control information, upon
completion of the acquisition mode, the spreader being
operable in a traffic mode to generate a second signal at
a second rate greater than the first rate representing
control information and user traffic or being operable in
a standby mode when no user traffic is being transmitted to
generate a third signal at said first rate; and a converter
coupled to the spreader, the converter operable to convert
the first, second or third signals into a transmission
signal for communication to the central terminal, the third
signal being transmitted at a lower power than the ~irst or
second signals.
According to an embodiment of the present invention,
a method of transmitting information in a wireless
telecommunications system includes transmitting a downlink
signal at a first transmitting power and first transmitting
rate in an acquisition mode to establish a downlink
communication path. The downlink signal is transmitted at
a second transmitting power and the ~irst transmitting rate
in a standby mode after establishment of the downlink
communication path. The downlink signal is transmitted at
.i'I~C!~ r~ Er~:T
CA 0222270~ 1997-11-28
6b
the first transmitting power and a second transmitting rate
upon a request for a wireless telephone call.
One technical advantage o~ this aspect o~ the present
lnvention is to provide multiple operating modes having
di~~erent transmitting powers and different transmitting
rates. Another technical advantage is to provide low
transmitting power at a low transmit rate during system
idle periods. Yet another technical advantage is to
ef~iciently change between di~erent transmitting powers
and different transmit rates.
In accordance with another aspect of the present
invention, there is provided an apparatus and/or method o~
establishing a downlink communication path in a wireless
telecommunications system that substantially eliminates or
reduces disadvantages and problems associated with
conventional wireless communication techniques.
According to an embodiment of the present invention,
a method of establishing a downlink communication path in
a wireless telecommunications system includes transmitting
a downlink signal having a master code sequence ~rom a
transmitter in a central terminal. The downlink signal is
received at a receiver in a subscriber terminal having a
slave code sequence. The receiver in the subscriber
terminal compares its slave code sequence to the master
code sequence of the downlink signal for a code and phase
CA 0222270~ 1997-11-28
WO 96/38938 PCTJIJS96/08S68
match. The receiver adjusts the phase of its slave code
sequence to match the phase of the master code, determining
a path delay from the transmitter in the central terminal
to the receiver in the subscriber terminal.
One technical advantage of this aspect of the present
invention is to match a receiver slave code sequence to a
master code sequence of a downlink signal. Another
technical advantage is to adjust a phase of a slave code
sequence of a receiver to match a phase of a master code
sequence in a downlink signal. Yet another technical
advantage is to provide fine and coarse incremental
adjustments to a phase of a slave code sequence of a
receiver. Still another technical advantage is to measure
a combined power level of a slave code sequence and a
master code sequence in order to obtain a code sequence
match.
In accordance with another aspect of the present
invention, there is provided an apparatus and/or a method
of frame aligning information in a wireless
telecommunications system that substantially eliminates or
reduces disadvantages and problems associated with
conventional frame alignment techniques.
According to an embodiment of the present invention,
a method of frame aligning information in a wireless
telecommunications system includes receiving at a receiver
of a subscriber terminal a downlink signal carrying the
information transmitted by a transmitter in a central
terminal. A beginning of frame position for a frame of
information is identified from the downlink signal. To
ensure accurate frame alignment, a successive beginning of
frame position of a successive frame of information is
identified. Upon successfully identifying two successive
beginning of frame positions, a downlink communication path
is established from the transmitter of the central terminal
to the receiver of the subscriber terminal.
CA 0222270~ 1997-11-28
W 096/38~38 PCT/U~,'0~568
One technical advantage of this aspect of the present
invention is to accurately identify a beginning of frame
position for a frame of information. Another technical
advantage is to incrementally step through the bit
positions of the downlink signal to identify the beginning
of frame position. Yet another technical advantage is to
decode a frame alignment word representing the beginning of
frame position into an overhead chAnnel of the downlink
signal. Still another technical advantage is to
continuously monitor the beginning of frame position for
subsequent frames of information.
CA 0222270~ 1997-11-28
W O 96/38938 PCTAUS~ U
BRIEF DESCRIPTION OF THE DRAWINGS
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:
Figure 1 is a schematic overview of an example of a
wireless telecommunications system in which an example of
the present invention is includedi
Figure 2 is a schematic illustration of an example of
a subscriber terminal of the telecommunications system of
Figure 1;
Figure 3 is a schematic illustration of an example of
a central terminal of the telecommunications system of
Figure 1;
Figure 3A is a schematic illustration of a modem shelf
of a central terminal of the telecommunications system of
Figure 1;
Figure 4 is an illustration of an example of a
fre~uency plan for the telecommunications system of
Figure 1;
Figures 5A and 5B are schematic diagrams illustrating
possible configurations for cells for the
telecommunications system of Figure 1;
Figure 6 is a schematic diagram illustrating aspects
of a code division multiplex system for the
telecommunications system of Figure 1;
Figure 7 is a schematic diagram illustrating signal
transmission processing stages for the telecommunications
system of Figure 1;
Figure 8 is a schematic diagram illustrating signal
reception processing stages for the telecommunications
system of Figure 1;
Figure 9 is a schematic diagram illustrating downlink
and uplink communication paths for the wireless
telecommunications system;
CA 0222270~ 1997-11-28
W 096/38938 PCTrUS~.'QY~6
Figure 10 is a schematic diagram illustrating the
makeup of a downlink signal transmitted by the central
terminal;
Figure 11 i8 a graphical depiction illustrating the
S phase adjustment to a slave code sequence of the subscriber
terminal;
Figure 12 is a graphical depiction of a signal quality
estimate performed by the receiver in the subscriber
terminal;
Figure 13 is a graphical diagram illustrating the
contents of a frame information signal within the downlink
signal;
Figure 14 is a tabular depiction illustrating overhead
insertion into a data stream of the downlink signal;
Figure 15 is a tabular depiction of a power control
signal in an overhead channel of the downlink slgnal;
Figure 16 is a tabular depiction of a code
synchronization signal in the overhead channel of the
downlink signal;
Figure 17 is a graphical depiction of a transmitting
power and a transmit rate for each mode of operation of the
wireless telecommunications system; and
Figure 18 is a schematic diagram illustrating the
operation of the receiver and transmitter in the subscriber
terminal.
CA 0222270~ 1997-11-28
W 096/38938 PCTnU~9''~~''~
11
~ETAI~ED DESCRIPTION OF THE INVENTION
Figure 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-
2OKm. 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 lOOm. 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 lKm. 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 Figure 1.
Moreover, the coverage need not be, and typically will not
be circular in extent due to antenna design considerations,
geographical factors, buildings and so on, which will
affect the distribution of transmitted signals.
The central terminals 10 for respective service areas
12, 14, 16 can be connected to each other by means of links
13, 15 and 17 which interface, for example, with a public
switched telephone network (PSTN) 18. The links can
include conventional telecommunications technology using
copper wires, optical fibers, satellites, microwaves, etc.
The wireless telecommunications system of Figure 1 is
based on providing fixed microwave links between subscriber
terminals 20 at fixed locations within a service area
(e.g., 12, 14, 16) and the central terminal 10 for that
service area. In a preferred embodiment each subscriber
terminal 20 is provided with a permanent fixed access link
CA 0222270~ 1997-11-28
W O 96/38938 PCTrUS9
12
to its central terminal lO. However, in alternative
embodiments ~em~n~-based access could be provided, so that
the number of subscribers which can be serviced exceeds the
number of telecommunications links which can currently be
active.
Figure 2 illustrates an example of a configuration for
a subscriber terminal 20 for the telecommunications system
of Figure 1. Figure 2 includes a schematic representation
of customer premises 22. A customer radio unit (CRU) 24 is
mounted on the customer's premises. The customer radio
unit 24 includes a flat panel antenna or the like 23. The
customer radio unit is mounted at a location on the
customer's premises, or on a mast, etc., and in an
orientation such that the flat panel antenna 23 within the
customer radio unit 24 faces in the direction 26 of the
central terminal 10 for the service area in which the
customer radio unit 24 is located.
The customer radio unit 24 is connected via a drop
line 28 to a power supply unit (PSU) 30 within the
customer's premises. The power supply unit 30 is connected
to the local power supply for providing power to the
customer radio unit 24 and a network terminal unit (NTU)
32. The customer radio unit 24 is also connected to via
the power supply unit 30 to the network terminal unit 32,
which in turn is connected to telecommunications equipment
in the customer's premises, for example to one or more
tel~phones 34, facsimile mach;nes 36 and computers 38. The
telecommunications equipment is represented as being within
a single customer's premises. However, this need not be
the case, as the subscriber terminal 20 preferably supports
either a single or a dual line, so that two subscriber
lines could be supported by a single subscriber terminal
20. The subscriber terminal 20 can also be arranged to
support analogue and digital telecommunications, for
example analogue communications at 16, 32 or 64kbits/sec or
CA 0222270~ 1997-11-28
WO 96/38938 PCT~lJS~
13
digital communications in accordance with the ISDN BRA
standard.
Figure 3 is a schematic illustration of an example of
a central terminal of the telecommunications system of
Figure 1. The common equipment rack 40 comprises a number
of equipment shelves 42, 44, 46, including a RF Combiner
and power amp shelf (RFC) 42, a Power Supply shelf (PS) 44
and a number of (in this example four) Modem Shelves (MS)
46. The RF combiner shelf 42 allows the four modem shelves
46 to operate in parallel. It combines and amplifies the
power of four transmit signals, each from a respective one
of the four modem shelves, and amplifies and splits
received signals four way so that separate signals may be
passed to the respective modem shelves. The power supply
shelf 44 provides a connection to the local power supply
and fusing for the various components in the common
equipment rack 40. A bidirectional connection extends
between the RF combiner shelf 42 and the main central
terminal antenna 52, typically an omnidirectional antenna,
mounted on a central terminal mast 50.
This example of a central terminal 10 is connected via
a point-to-point microwave link to a location where an
interface to the public switched telephone network 18,
shown schematically in Figure 1, is made. As mentioned
above, other types of connections (e.g., copper wires or
optical fibres) can be used to link the central terminal 10
to the public switched telephone network 18. In this
example the modem shelves are connected via lines 47 to a
microwave terminal (MT) 48. A microwave link 49 extends
from the microwave terminal 48 to a point-to-point
microwave antenna 54 mounted on the mast 50 for a host
connection to the public switched telephone network 18.
A personal computer, workstation or the like can be
provided as a site controller (SC) 56 for supporting the
central terminal 10. The site controller 56 can be
connected to each modem shelf of the central terminal 10
,
CA 0222270~ 1997-11-28
W 096/38938 PCTnUS~
14
via, for example, RS232 connections 55. The site
controller 56 can then provide support functions such as
the localization of faults, alarms and status and the
configuring of the central terminal 10. A site controller
56 will typically support a single central terminal 10,
although a plurality of site controllers 56 could be
networked for supporting a plurality of central terminals
10 .
As an alternative to the RS232 connections 55, which
extend to a site controller 56, data connections such as an
X.25 links 57 (shown with dashed lines in Figure 3) could
instead be provided from a pad 228 to a switching node 60
of an element manager (EM) 58. An element manager 58 can
support a number of distributed central terminals 10
connected by respective connections to the switching node
60. The element manager 58 enables a potentially large
number (e.g., up to, or more than 1000) of central
terminals 10 to be integrated into a management network.
The element manager 58 is based around a powerful
workstation 62 and can include a number of computer
terminals 64 for network engineers and control personnel.
Figure 3A illustrates various parts of a modem shelf
46. A transmit/receive RF unit (RFU - for example
implemented on a card in the modem shelf) 66 generates the
modulated transmit RF signals at medium power levels and
recovers and amplifies the baseband RF signals for the
subscriber terminals. The RF unit 66 is connected to an
analogue card (AN) 68 which performs A-D/D-A conversions,
baseband filtering and the vector summation of 15
transmitted signals from the modem cards (MCs) 70. The
analogue unit 68 is connected to a number of (typically 1-
8) modem cards 70. The modem cards perform the baseband
signal processing of the transmit and receive signals
to/from the subscriber terminals 20. This includes 1/2
rate convolution coding and x 16 spreading with CDMA codes
on the transmit signals, and synchronization recovery, de-
CA 0222270~ 1997-11-28
WO 96/38938 PCT/US96108S68
spreading and error correction on the receive signals.
Each modem card 70 in the present example has two modems,
each modem supporting one subscriber link (or two lines) to
a subscriber terminal 20. Thus, with two modems per card
and 8 modems per modem shelf, each modem shelf could
support 16 possible subscriber links. However, in order to
incorporate redundancy so that a modem may be substituted
in a subscriber link when a fault occurs, only up to 15
subscriber links are preferably supported by a single modem
shelf 46. The 16th modem is then used as a spare which can
be switched in if a failure of one of the other 15 modems
occurs. The modem cards 70 are connected to the tributary
unit (TU) 74 which terminates the connection to the host
public switched telephone network 18 (e.g., via one of the
lines 47) and handles the signaling of telephony
information to, for example, up to 15 subscriber terminals
(each via a respective one of 15 of the 16 modems).
The wireless telecommunications between a central
terminal 10 and the subscriber terminals 20 could operate
on various frequencies. Figure 4 illustrates one possible
example of the frequencies which could be used. In the
present example, the wireless telecommunication system is
intended to operate in the 1.5-2.5GHz Band. In particular
the present example is intended to operate in the Band
defined by ITU-R (CCIR) Recomm~n~tion F.701 (2025-2110MHz,
2200-229OMHz). Figure 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
3.5MHz each are provided centered about 2155MHz. The
spacing between the receive and transmit channels exceeds
the required minimum spacing of 70MHz.
In the present example, as mentioned above, each modem
shelf will support 1 frequency channel (i.e. one uplink
frequency plus the corresponding downlink frequency). Up
CA 0222270~ l997-ll-28
W O 96/38938 PCTrUS9~'C~r~8
16
to 15 subscriber links may be supported on one frequency
~h~nnel, as will be explained later. Thus, in the present
embodiment, each central terminal 10 can support 60 links,
or 120 lines.
Typically, the radio traffic from a particular central
terminal 10 will extend into the area covered by a
neighboring 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.
Figure 5A illustrates one cellular type arrangement of
the frequencies to mitigate interference problems between
adjacent central terminals 10. In the arrangement
illustrated in Figure 5A, the hatch lines for the cells 76
illustrate a frequency set (FS) for the cells. By
selecting three frequency sets (e.g., where: FS1 = Fl, F4,
F7, F10; FS2 = F2, F5, F8, Fll; FS3 = F3, F6, F9, F12),
and arranging that immediately adjacent cells do not use
the same frequency set (see, for example, the arrangement
shown in Figure 5A), it is possible to provide an array of
fixed assignment omnidirectional cells where interference
between nearby cells can be avoided. The transmitter power
of each central terminal 10 is set such that transmissions
do not extend as far as the nearest cell which is using the
same frequency set. Thus 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).
With each modem shelf supporting one channel frequency
(with 15 subscriber links per channel frequency) and four
modem shelves, each central terminal 10 will support 60
subscriber links (i.e., 120 lines). The 10 cell
arrangement in Figure 5A can therefore support up to 600
ISDN links or 1200 analogue lines, for example. Figure 5B
illustrates a cellular type arrangement employing sectored
CA 0222270~ l997-ll-28
W 096/38938 . PCTAU59C,~FS6
17
cells to mitigate problems between adjacent central
terminals 10. As with Figure 5A, the different type of
hatch lines in Figure 5B illustrate different frequency
sets. As in Figure 5A, Figure 5B represents three
frequency sets (e.g., where: FSl = Fl, F4, F7, F10; FS2 =
F2, F5, F8, Fll; FS3 = F3, F6, F9, F12). However, in
Figure 5B the cells are sectored by using a sectored
central terminal (SCT) 13 which includes three central
terminals 10, one for each sector Sl, S2 and S3, with the
transmissions for each of the three central terminals 10
being directed to the appropriate sector among Sl, 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.
A seven cell repeat pattern is used such that for a
cell operating on a given frequency, all six adjacent cells
operating on the same ~requency are allowed unique PN
codes. This prevents adjacent cells from inadvertently
decoding data.
As mentioned above, each ch~nnel frequency can support
15 subscriber links. In this example, this is achieved
using by multiplexing signals using a Code Division
Multiplexed Access (CDMA) technique. Figure 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 160ksymbols/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 respective Walsh pseudo
random noise (PN) code spreading function 82-82N to
generate signals at an effective chip rate of
2.56Msymbols/sec in 3.5MHz. The signals for respective
subscriber links are then combined and converted to radio
frequency (RF) to give multiple user channel signals (e.g.,
85) for transmission from the transmitting antenna 86.
CA 0222270~ l997-ll-28
W 096/38938 . PCTrUS96/08568
18
During transmission, a transmitted signal will be
subjected to interference sources 88, including external
interference 89 and interference from other ~hAnnels 90.
Accordingly, by the time the CDMA signal 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 pseudo random noise ~PN)
code that was 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 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 Walsh codes,
the signals for respective subscriber links do not
interfere with each other.
Walsh codes are a mathematical set of se~uences that
have the function of "orthonormality". In other words, if
any Walsh code is multiplied by any other Walsh code, the
results are zero.
Figure 7 is a schematic diagram illustrating signal
transmission processing stages as configured in a
subscriber terminal 20 in the telecommunications system of
Figure 1. The central terminal is also configured to
perform equivalent signal transmission processing. In
Figure 7, an analogue signal from one of a pair of
telephones is passed via a two-wire interface 102 to a
hybrid audio processing circuit 104 and then via a codec
106 to produce a digital signal into which an overhead
channel including control information is inserted at 108.
The resulting signal is processed by a convolutional
CA 0222270~ 1997-11-28
W 096/38938 PCTnUS9~J~Pr'~
19
encoder llO before being passed to a spreader 116 to which
the ~ m~cher-walsh and PN codes are applied by a RW code
generator 112 and PN Code generator 114, respectively. The
resulting signals are passed via a digital to analogue
converter 118. The digital to analogue converter 118
shapes the digital samples into an analogue waveform and
provides a stage of baseband power control. The signals
are then passed to a low pass filter 120 to be modulated in
a modulator 122. The modulated signal from the modulator
122 is mixed with a signal generated by a voltage
controlled oscillator 126 which is responsive to a
synthesizer 160. The output of the mixer 128 is then
amplified in a low noise amplifier 130 before being passed
via a band pass filter 132. The output of the band pass
filter 132 is further amplified in a further low noise
amplifier 134, before being passed to power control
circuitry 136. The output of the power control circuitry
is further amplified in a further low noise amplifier 138
before being passed via a further band pass filter 140 and
transmitted from the transmission antenna 142.
Figure 8 is a schematic diagram illustrating the
equivalent signal reception processing stages as configured
in a subscriber terminal 20 in the telecommunications
system of Figure l. The central terminal is also
configured to perform equivalent signal reception
processing. In Figure 8, signals received at a receiving
antenna 150 are passed via a band pass filter 152 before
being amplified in a low noise amplifier 154. The output
of the amplifier 154 is then passed via a further band pass
filter 156 before being further amplified by a further low
noise amplifier 158. The output of the amplifier 158 is
then passed to a mixer 164 where it is mixed with a signal
generated by a voltage controlled oscillator 162 which is
responsive to a synthesizer 160. The output of the mixer
164 is then passed via the de-modulator 166 and a low pass
filter 168 before being passed to an analogue to digital
CA 0222270~ l997-ll-28
W 096/38938 PCT~US96/08S68
converter 170. The digital output of the A/D converter 170
is then passed to a correlator 178, to which the same
Rademacher-Walsh and PN codes used during transmission are
applied by a RW code generator 172 (corresponding to the RW
code generator 112) and a PN code generator 174
(corresponding to PN code generator 114), respectively.
The output of the correlator is applied to a Viterbi
decoder 180. The output of the Viterbi decoder 180 is then
passed to an overhead extractor 182 for extracting the
overhead channel information. The output of the overhead
extractor 182 is then passed via a codec 184 and a hybrid
circuit 188 to a two wire interface 190 where the resulting
analogue signals are passed to a selected telephone 192.
At the subscriber terminal 20, a stage of automatic
gain control is incorporated at the IF stage. The control
signal is derived from the digital portion of the CDMA
receiver using the output of a signal quality estimator to
be described later.
Figure 9 is a block diagram of downlink and uplink
communication paths between central terminal 10 and
subscriber terminal 20. A downlink communication path is
established from transmitter 200 in central terminal 10 to
receiver 202 in subscriber terminal 20. An uplink
communication path is established from transmitter 204 in
subscriber terminal 20 to receiver 206 in central terminal
10. Once the downlink and the uplink communication paths
have been established in wireless telecommunication system
1, telephone communication may occur between a first user
208 or a second user 210 of subscriber terminal 20 and a
user serviced through central terminal 10 over a downlink
signal 212 and an uplink signal 214. Downlink signal 212
is transmitted by transmitter 200 of central terminal 10
and received by receiver 202 of subscriber terminal 20.
Uplink signal 214 is transmitted by transmitter 204 of
subscriber terminal 20 and received by receiver 206 of
CA 0222270~ l997-ll-28
W 096/38938 PCT~USS~08r~8
21
central terminal 10. Downlink signal 212 and uplink signal
214 are transmitted as CDMA spread spectrum signals.
Receiver 206 and transmitter 200 within central
terminal 10 are synchronized to each other with respect to
time and phase, and aligned as to information boundaries.
In order to establish the downlink communication path,
receiver 202 in subscriber terminal 20 should be
synchronized to transmitter 200 in central terminal 10.
Synchronization occurs by performing an acquisition mode
function and a tracking mode function on downlink signal
212. Initially, transmitter 200 o~ central terminal 10
transmits downlink signal 212. Figure 10 shows the
contents of downlink signal 212. Downlink signal 212
includes a code sequence signal 216 for central terminal 10
combined with a frame information signal 218. Code
sequence signal 216 is derived from a combination of a
pseudo-random noise code signal 220 and a Rademacher-Walsh
code signal 222. Although Figure 10 relates specifically
to the makeup of the downlink signal, the uplink has the
same makeup.
Each receiver 202 of every subscriber terminal 20
serviced by a single central terminal 10 operate off of the
same pseudo-random noise code signal as central terminal
10. Each modem shelf 46 in central terminal 10 supports
one radio frequency channel and fifteen subscriber
term; n~ 1 S 20, each subscriber terminal having a first user
208 and a second user 210. Each modem shelf 46 selects one
of sixteen Rademacher-Walsh code signals 222, each
Rademacher-Walsh code signal 222 corresponding to a unique
subscriber terminal 20. Thus, a specific subscriber
terminal 20 will have an identical code sequence signal 218
as downlink signal 212 transmitted by central terminal 10
and destined for the specific subscriber terminal 20.
Downlink signal 212 is received at receiver 202 of
subscriber terminal 20. Receiver 202 compares its phase
and code sequence to a phase and code sequence within code
CA 0222270~ l997-ll-28
W 096/38938 PCTrU~g~8'~8
22
sequence signal 216 of downlink signal 212. Central
terminal 10 is considered to have a master code sequence
and subscriber terminal 20 iS considered to have a slave
code sequence. Receiver 202 incrementally adjusts the
phase of its slave code sequence to recognize a match to
master code sequence and place receiver 202 of subscriber
terminal 20 in phase with transmitter 200 of central
terminal 10. The slave code sequence of receiver 202 is
not initially synchronized to the master code sequence of
transmitter 200 and central terminal 10 due to the path
delay between central terminal 10 and subscriber terminal
20. This path delay is caused by the geographical
separation between subscriber terminal 20 and central
terminal 10 and other environmental and technical factors
affecting wireless transmission.
Figure 11 illustrates how receiver 202 of subscriber
terminal 20 adjusts its slave code sequence to match the
master code sequence of transmitter 200 in central terminal
10. Receiver 202 increments the phase of the slave code
sequence throughout the entire length of the master code
sequence within downlink signal 212 and determines a signal
quality estimate by performing a power measurement on the
combined power of the slave code sequence and the master
code sequence for each incremental change in the phase of
the slave code sequence. The length of the master code
sequence is approximately 100 microseconds based on a chip
period of 2. 56 MegaHertz. The phase of the slave code
sequence is adjusted by one half of a chip period for each
incremental interval during the acquisition phase.
Receiver 2 02 completes a first acquisition pass when it
identifies a correlation peak where the combined power
reaches a maximum value. Receiver 202 performs a second
acquisition pass throughout the entire length of the code
sequence to verify identification of the maximum value of
the combined power at the correlation peak. The
approximate path delay between subscriber terminal 20 and
CA 0222270~ 1997-11-28
WO 96t38938 PCT/USS.*'C -S~
23
central terminal 10 is determined when the correlation peak
position is identified in the acquisition mode.
Once acquisition of downlink signal 212 is achieved at
receiver 202, fine adjustments are made to the phase of the
slave code sequence in order to maintain the phase matching
of the slave code sequence with the master code sequence in
the tracking mode. Fine adjustments are made through one
sixteenth of a chip period incremental changes to the phase
of the slave code sequence. Fine adjustments ma~ be
performed in either forward (positive) or backward
(negative) directions in response to the combined power
measurements made by receiver 202. Receiver 202
continuou~cly monitors the master code sequence to ensure
that subscriber terminal 20 is synchronized to central
terminal 10 for the downlink communication path.
Figure 12 shows a graph of the combined power curve
measured by receiver 202 during the acquisition mode and
the tracking mode. The maximum value of the combined power
occurs at the correlation peak 219 of the combined power
curve. It should be noted that the peak 219 may not be as
well defined as in Figure 12, but may be flattened at the
top, more in the form of a plateau. This is the point
where the slave code sequence of receiver 202 iS in phase
with and matches the master code sequence of transmitter
200. Measurements resulting in combined power values that
occur off of correlation peak 219 require incremental
adjustments to be made to the slave code sequence. A fine
adjustment window is established between an early
correlator point 221 and a late correlator point 223. An
average power measurement is performed at early correlator
point 221 and at late correlator point 223. Since early
correlator point 221 and late correlator point 223 are
spaced one chip period apart, an error signal is produced
upon calculating the difference between the average powers
of early correlator point 221 and late correlator point 223
CA 0222270~ 1997-11-28
W 096/38938 PCTrUS96/08568
24
that is used to control the fine adjustments to the phase
of the slave code sequence.
After acquiring and initiating tracking on the central
terminal 10 master code sequence of code sequence signal
216 within downlink signal 212, receiver 202 enters a frame
alignment mode in order to establish the downlink
communication path. Receiver 202 analyzes frame
information within frame information signal 218 of downlink
signal 212 to identify a beginning of frame position for
downlink signal 212. Since receiver 202 does not know at
what point in the data stream of downlink signal 212 it has
received information, receiver 202 must search for the
beginning of frame position in order to be able to process
information received from transmitter 200 of central
terminal 10. Once receiver 202 has identified one further
beginning of frame position, the downlink communication
path has been established from transmitter 200 of central
terminal 10 to receiver 202 of subscriber terminal 20.
Figure 13 shows the general contents of frame
information signal 218. Frame information signal 218
includes an overhead ch~nnel 224, a first user channel 226,
a second user channel 228, and a signaling channel 230 for
each frame of information transported over downlink signal
212. Overhead channel 224 carries control information used
to establish and maintain the downlink and uplink
communication paths. First user channel 226 is used to
transfer traffic information to first user 208. Second
user channel 228 is used to transfer traffic information to
second user 210. Signaling channel 230 provides the
signaling information to supervise operation of subscriber
terminal 20 telephony functions. Overhead channel 224
occupies 16 kilobits per second of a frame of information,
first user channel 226 occupies 64 kilobits per second of
a frame of information, second user channel 228 occupies 64
kilobits per second of a frame of information, and
-
CA 0222270~ l997-ll-28
W 096/38938 PCTAUS~5'
signaling channel 230 occupies 16 kilobits per second of a
frame of information.
Figure 14 shows how overhead channel 224 iS inserted
into the data stream of downlink signal 212. The data
stream of downlink signal 212 iS partitioned into twenty
bit subframes. Each twenty bit subframe has two ten bit
sections. A ~irst ten bit section includes an overhead
bit, a signaling bit, and eight first user bits. A second
ten bit section includes an overhead bit, a signaling bit,
and eight second user bits. This twenty bit subframe
format is repeated throughout an entire four millisecond
frame of information. Thus, an overhead bit occupies every
tenth bit position of frame information in the data stream
of downlink signal 212.
Overhead channel 224 includes eight byte fields - a
frame alignment word 232, a code synchronization signal
234, a power control signal 236, an operations and
maintenance channel signal 238, and four reserved byte
fields 242. Frame alignment word 232 identifies the
beginning of frame position for its corresponding frame of
information. Code synchronization signal 234 provides
information to control synchronization of transmitter 204
in subscriber terminal 20 to receiver 206 in central
terminal 10. Power control signal 236 provides information
to control transmitting power of transmitter 204 in
subscriber terminal 20. Operations and maintenance channel
signal 238 provides status information with respect to the
downlink and uplink commlln-cation paths and a path from the
central terminal to the subscriber terminal on which the
communication protocol which operates on the modem shelf
between the shelf controller and the modem cards also
extends.
In order to identify two successive beginning of frame
positions, receiver 202 of subscriber terminal 20 searches
through the ten possible bit positions in the data stream
of downlink signal 212 for overhead channel 224 and frame
CA 0222270~ l997-ll-28
W 096/38938 PCT/U~3CI~85C8
26
alignment word 232. Receiver 202 initially extracts a
first bit position of every ten bit section of frame
information to determine if overhead channel 224 has been
captured. If frame alignment word 232 has not been
identified after a predetermined period of time from the
extraction of the first bit position, receiver 202 will
repeat this procedure for the second bit position of each
ten bit section and subsequent bit positions until frame
alignment word 232 has been identified. An example of a
frame alignment word 232 which receiver 202 would search
for is binary 00010111. Once the correct bit position
yields frame alignment word 232, receiver 202 attempts to
identify two successive beginning of frame positions. A
downlink communication path is established upon the
successful identification of two successive beginning of
frame positions in response to recognition of successive
frame alignment words 232 in the data stream of downlink
signal 212.
Receiver 202 continues to monitor the appropriate bit
position in order to recognize subsequent frame alignment
words 232 for subsequent frames of information. f
receiver 202 fails to recognize a frame alignment word 232
for three successive frames, then receiver 202 will return
to the search process and cycle through each of the bit
positions of the ten bit section until identifying two
successive beginning of frame positions through recognition
of two successive frame alignment words 232 and
reestablishing frame alignment. Failure to recognize three
successive frame alignment words 232 may result from a
change in the path delay between central terminal 10 and
subscriber terminal 20. Receiver 202 will also return to
the search process upon an interruption in the downlink
communication path from transmitter 200 in central terminal
10 to receiver 202 in subscriber terminal 20.
Acquisition of a downlink communication path may
also be verified to ensure that a subscriber te~m; n~31 20 is
CA 0222270~ 1997-11-28
WO 96/38938 PCT/US9G'. ~5~S
27
locked on to the appropriate portion of an appropriate
modem card 70. Situations may arise where a modem card 70
assigned to a particular subscriber terminal 20 may be out
of service for any of a number of reasons. The particular
subscriber terminal 70 may continue to attempt to acquire
a signal and such acquisition may occur with respect to a
signal from one of the other modem cards 70 within the
central terminal 10. Though the particular subscriber
terminal 20 should not be able to read signals from other
modem cards 70, the particular subscriber terminal 20 is
still locked to another modem card 70 and will not lock to
the appropriate modem card when it comes back into service.
Therefore, an acquisition verification technique is used to
ensure that the particular subscriber terminal 20 does not
lock onto a non-associated modem card 70 in central
terminal 10.
The acquisition verification technique uses the
reserved byte fields 242 in overhead channel 224. A
channel identifier field occupies one of reserved byte
fields 242. The channel identifier field includes eight
bits - an inversion bit, a three bit pseudo random noise
code identifier, and a four bit Rademacher-Walsh code
identifier, in the form of INV P P P R R R R. The three
bit pseudo random noise code identifier corresponds to the
sequence associated with central terminal 10 and its
associated subscriber terminals 20. The four bit
Raddemacher-Walsh code identifier corresponds to the
specific code associated with one of fifteen subscriber
terminals 20. The channel identifier field prevents a
subscriber terminal 20 from establishing comm1lnications
~ with an incorrect central terminal 10 and also prevents a
subscriber terminal 20 from establishing c~mm11n;cations
CA 0222270~ 1997-11-28
W 096/38938 PCTrUS96/08S68
28
with an incorrect modem card 70 in a correct central
terminal 10.
Receiver 202 in establishing a downlink c~mmllnication
path through identification of two successive frame
alignment words 232 also monitors the channel identifier
field within overhead channel 224. Though a frame
alignment may occur, the downlink communication path will
not be established unless an appropriate match occurs on
the channel identifier field. Since the pseudo random
noise code identifier and the Raddemacher-Walsh code
identifier are constant for each subscriber terminal 20,
receiver 202 should avoid confusing a channel identifier
field with a frame alignment word. The inversion bit
changes state at each frame of information along with the
first and third bits of the pseudo random noise code
identifier and the first and third bits of the Raddemacher-
Walsh code identifier within the channel identifier field
as shown by italics above. This prevents the channel
identifier field from being recognized as the frame
alignment word.
Upon establishing the downlink commllnication path from
central terminal 10 to subscriber terminal 20 through
proper code sequence phase synchronization and frame
alignment, wireless telecommlln;cation system 1 performs
procedures to establish the uplink communication path from
transmitter 204 in subscriber terminal 20 to receiver 206
in central terminal 10. Initially, transmitter 204 is
powered off until the downlink communication path has been
established to prevent transmitter interference of central
terminal communications with other subscriber terminals.
After the downlink communication path is established,
transmitting power of transmitter 204 is set to a minimum
value on command from the central terminal CT via power
CA 0222270~ 1997-11-28
W 096/38938 rcT/u~ C~5C~
29
control channel 236 of overhead channel 224. Power control
signal 236 controls the amount of transmitting power
produced by transmitter 204 such that central terminal 10
receives approximately the same level of transmitting power
from each subscriber terminal 20 serviced by central
terminal 10.
Power control signal 236 is transmitted by transmitter
200 of central termi n~ 1 10 in overhead channel 224 of frame
information signal 218 over downlink signal 212. Receiver
202 of subscriber terminal 20 receives downlink signal 212
and extracts power control signal 236 therefrom. Power
control signal 236 is provided to transmitter 204 of
subscriber terminal 20 and incrementally adjusts the
transmitting power of transmitter 204. Central terminal 10
continues to incrementally adjust the transmitting power of
transmitter 204 until the transmitting power falls within
a desired threshold range as determined by receiver 206.
Adjustments to the transmitting power initially occur in a
coarse adjustment mode having one decibel increments until
the transmitting power falls within the desired threshold
range. Upon turning transmitter 204 on, the transmitting
power is gradually ramped up in intensity through
incremental adjustments in order to avoid interference of
central term; n~l commlln; cations with other subscriber
terminals.
Figure 15 shows an example decoding scheme for power
control signal 236. After the transmitting power of
transmitter 204 in subscriber terminal 20 reaches the
desired threshold range, receiver 206 in central terminal
10 continues to monitor the amount of transmitting power
from transmitter 204 for any changes resulting from power
fluctuations, and variations in the path delay between
central terminal 10 and subscriber terminal 20, et al. If
CA 0222270~ 1997-11-28
W 096/38938 PCTrUS96108S68
the transmitting power falls below or exceeds the desired
threshold range, central terminal 10 will send an
appropriate power control signal 236 to increase or
decrease the transmitting power of transmitter 204 as
needed. At this point, adjustments made to return the
transmitting power to the desired threshold range may occur
in a fine adjustment mode having 0.1 decibel increments.
Upon an interruption in the downlink or uplink
communication paths, central terminal 10 may command
transmitter 204 to return to a previous transmitting power
level through recovery of parameters stored in a memory in
subscriber terminal 20 in order to facilitate
reestablishment of the appropriate comml7nication path.
To fully establish the uplink commllnication path from
subscriber terminal 20 to central terminal 10, transmitter
204 in subscriber termi n~l 20 should be synchronized to
receiver 206 in central terminal 10. Central terminal 10
controls the synchronization of transmitter 204 through
code synchronization signal 234 in overhead channel 224 of
frame information signal 218. Code synchronization signal
234 incrementally adjusts a phase of the slave code
sequence of transmitter 204 to match the phase of the
master code sequence of receiver 206. Synchronization of
transmitter 204 is performed in a substantially similar
manner as synchronization of receiver 202.
Code synchronization signal 234 is transmitted by
transmitter 200 in central terminal 10 in overhead channel
224 of frame information signal 218 over downlink signal
212. Receiver 202 of subscriber terminal 20 receives
downlink signal 212 and extracts code synchronization
signal 234 therefrom. Code synchronization signal 234 is
provided to transmitter 204 for incrementally adjustment of
the phase of the slave code sequence of transmitter 204.
CA 0222270~ l997-ll-28
W 096/38938 PCTAUS9~08S68
31
Central terminal 10 continues to incrementally adjust the
phase of the slave code sequence of transmitter 204 until
receiver 206 recognizes a code and phase match between the
slave code sequence of transmitter 204 and the master code
sequence of central terminal 10.
Receiver 206 performs the same power measurement
technique in determ;ning a phase and code match for
transmitter 204 synchronization as performed for receiver
202 synchronization. Adjustments to the phase of the slave
code sequence of transmitter 204 initially occur in a
coarse adjustment mode having one half of a chip rate
increments until receiver 206 identifies the m~ximllm power
position of the co-mbined power of the master code sequence
and the slave code sequence of transmitter 204.
Figure 16 shows an example decoding scheme for code
synchronization signal 234. After identification and
verification of a phase and code match of the slave code
sequence to the master code sequence, receiver 206
continues to monitor uplink signal 214 for changes in the
phase of the slave code sequence of transmitter 204
resulting from variations in the path delay between central
terminal 10 and subscriber terminal 20. If further
adjustments are needed to the phase of the slave code
sequence of transmitter 204, central terminal 10 will send
appropriate code synchronization signals 234 to increase or
decrease the phase of the slave code sequence of
transmitter 204 as needed. At this point, adjustments made
to the phase of the slave code sequence of transmitter 204
may occur in a fine adjustment mode having one sixteenth of
a chip rate increments. Upon an interruption in the
downlink or uplink commllnication paths, central terminal 10
may command transmitter 204 to return to a previous slave
code sequence phase value through recovery of parameters
CA 0222270~ 1997-11-28
W 096/38938 PCTrUS~C,'-~OE8
32
stored in a memory in subscriber terminal 20 in order to
facilitate reestablishment of the appropriate commllnication
path.
After synchronization of transmitter 204 is achieved,
receiver 206 performs frame alignment on uplink signal 214
in a similar manner as frame alignment is performed by
receiver 202 during establishment of the downlink
commllnication path. Once receiver 206 recognizes two
successive frame alignment words and obtains frame
alignment, the uplink commllnication path has been
established. Upon establishing both the downlink and the
uplink commllnication paths, information transfer between
first user 208 or second user 210 of subscriber terminal 20
and users coupled to central terminal 10 may commence.
Wireless telecommllnication system 1 is capable of
ad~usting the transmitting power level and the transmit
rate to one of two settings for each of three different
system operating modes. The system operating modes are
acquisition, standby and traffic. Adjustments in the
transmitting power and the transmit rate make it possible
to reduce and minim; ze interference with other subscriber
terminals. Improvements in link establishment time are
also achieved. The transmitting power level is decoded in
power control signal 236 and the transmit rate is decoded
in code synchronization signal 234.
The transmitting power for both downlink signal 212
and uplink signal 214 can be set to either a nominal 0
decibel high power level or a reduced -12 decibel low power
level. The transmit rate for both downlink signal 212 and
uplink signal 214 can be set to a low rate of 10 kilobits
per second or a high rate of 160 kilobits per second. When
switched to the high rate of 160 kilobits per second, user
traffic and overhead information are spread such that one
CA 0222270~ 1997-11-28
W 096/38938 pcT/u~r ~ r~
33
information symbol results in the transmission of 16 chips.
Correlation is performed over 16 chips, yielding a
processing gain of 12 decibels. When switched to the low
rate of 10 kilobits per second, only overhead information
is spread such that one overhead symbol results in the
transmission of 256 chips. Correlation is performed over
256 chips, yielding a processing gain of 24 decibels.
Figure 17 show the transmitting power and transmit
rate for each of the three system operating modes. At
power up or whenever the downlink or uplink comml~n;cation
paths are lost, wireless telecommllnication system 1 enters
the acquisition mode. During the acquisition mode, the
transmitting power of the downlink and uplink transmitters
are maximized as well as the correlator processing gain.
This m~x;mizes the signal to noise ratio at the correlator
output, increasing the amplitude of the correlation peak
219 for easier identification and min;m~l risk of false
acquisition. Since only overhead information is needed in
the acquisition mode, the transmit rate is at the low rate
level of 10 kilobits per second.
When the downlink and the uplink commllnication paths
are acquired, wireless telecommllnication system 1 enters
the standby mode. In the standby mode, the transmitting
power of the downlink and uplink transmitters are reduced
by 12 decibels. This reduction in transmitting power
minimizes the interference to other subscriber terminals
while still maintaining synchronization. The transmit rate
r~m~ins at the low rate level to allow exchange of control
information between central terminal 10 and subscriber
termin~l 20 over overhead channel 224.
When either an incoming or outgoing call is detected,
a message is sent from the originating terminal to the
destination terminal indicating that the downlink and
CA 0222270~ 1997-11-28
W 096138938 PCTrUS96 !~ ~r68
34
uplink commllnication paths are required for the
transmission of user traffic information. At this point,
wireless telec~mmlln;cation system 1 enters into the traffic
mode. During the traffic mode, the transmitting power of
both the downlink and uplink commllnication paths is
increased to the high power level and the transmit rate is
increased to the high rate level of 160 kilobits per second
to facilitate information transfer between originating and
destination termin~ls. Upon detection of call termination,
a message is sent from the terminating terminal to the
other termin~l indicating that the downlink and uplink
c~mm-lnication paths are no longer required. At this point,
wireless telecommllnication system 1 reenters the standby
mode. Code synchronization and frame alignment tracking is
performed in both the standby mode and the traffic mode.
Figure 18 is a detailed block diagram of receiver 202
and transmitter 204 in subscriber termin~l 20. Receiver
202 receives downlink signal 212 at an RF receive interface
250. RF receive interface 250 separates the spread
spectrum signal into I and Q signal components. RF receive
interface 250 band pass filters each of the I and Q signal
components by removing portions above approximately half of
receiver 202 bandwidth of 3.5 MegaHertz. RF receive
interface 250 low pass filters the I and Q signal
components to reject image frequencies and prevent signal
aliasing. The I and Q signal components are placed into
digital format by an analog to digital converter 252. The
sampling frequency of analog to digital converter 252 is
four times the chip period, or 10.24 MegaHertz, with an
eight bit resolution.
The digital I and Q signal components are stepped to
a rate of 5.12 MegaHertz by a down converter 254. A code
generator and despreader 256 performs the synchronization
CA 0222270~ 1997-11-28
W 09613893~ PCT~U~5
acquisition and tracking functions previously described to
synchronize the phase of the Rademacher-Walsh and pseudo-
random noise code sequence of receiver 202 to that of
downlink signal 212. A digital signal processor 258
controls the phase of the slave code sequence through a
code tracker 260 and a carrier tracker 262. An automatic
gain control unit 264 produces an automatic gain control
signal to control the gain of RF receive interface 250.
Code generator and despreader 256 generates the I and Q 160
kilobits per second of frame information for further
synchronization by a node sync interface 266 under the
control of a node sync logic unit 268. Node sync interface
266, through node sync logic unit 268, determines whether
the I and Q channels should be swapped, as they may be
received in four different ways.
Viterbi decoder 270 provides forward error correction
on the I and Q channels and generates an error corrected
160 kilobits per second data signal after a 71 symbol
delay. The error corrected signal is processed by a frame
aligner and extractor 272 determines frame alignment and
extracts power control signal 236, code synchronization
234, and operations and maintenance channel signal 238.
Frame aligner and extractor 272 also extracts first user
channel 226 and second user channel 228 for traffic
transmission towards first user 208 an second user 210, and
signaling channel 230 for processing by high level data
link controller 274 and a microcontroller 276. Frame
aligner and extractor 272 also provides alarm and error
indications upon detecting a loss in frame alignment. A
non-volatile random access memory 278 stores system
parameter information for subsequent insertion through an
arbitrator 280 in the event of link loss in order to
facilitate link reestablishment. Arbitrator 280 also
-
CA 0222270~ 1997-11-28
W 096/38938 PCTrUSgf~8'6Y
36
provides an interface between digital signal processor 258
and microcontroller 276.
In the transmit direction, a frame inserter 282
receives first user traffic and second user traffic from
first user 208 and-second user 210, signaling channel 230
information from high level data link controller 274, and
operations and maintenance channel 238 information from
microcontroller 276. Frame inserter generates frame
information signal 218 for uplink signal 214 for processing
by a convolutional encoder 284. Convolutional encod-er 284
doubles the data rate of frame information signal 218 to
provide forward error correction. A spreader 286 splits
the 320 kilobits per second signal of convolutional encoder
284 into two 160 kilobits per second I and Q signals and
exclusively ORs these signals with the spreading sequence
generated by a code generator 288 in response to a system
clock generated by clock generator 290 as adjusted by code
synchronization signal 234. Code generator 288 generates
one of sixteen Rademacher-Walsh functions exclusive ORed
with a pseudo-random sequence having a pattern length of
256 with a chip rate of 2.56 MegaHertz. The pseudo-random
sequence should match that of central terminal 10, but is
adjustable under software control to provide reliable
rejection of signals from other bands or other cells.
Spreader 286 supplies the I and Q signals to an analog
transmitter 290. Analog transmitter 290 produces pulsed I
and Q signals for an RF transmit interface 292. Transmit
power is generated by first establishing a control voltage
from a digital to analog converter in response to power
control signal 236 extracted from overhead channel 224.
This control voltage is applied to the power control inputs
of analog transmitter 290 and RF transmit interface 292.
Power control of 35 decibels is obtainable in both analog
CA 0222270~ 1997-11-28
WO 96/38938 PCT~US5~ F56
37
transmitter 290 and RF transmit interface 292. RF transmit
interface 292 includes a step attenuator that provides 2
decibel steps of attenuation over a 30 decibel range. This
attenuator is used to switch between high and low power
levels. On power up, maximum attenuation is selected to
minimi ze the transmitting power of transmitter 204.
In summary, a wireless telecommllnication system
provides wireless telephone type commllnications for users
remote from the public telephone switching network. The
wireless telecommllnication system includes a central
terminal that comm~n;cates through CDMA spread spectrum
radio frequency transmissions to a plurality of users
serviced by subscriber term; n~ 1 s .
Although a particular embodiment has been described
herein, it will be appreciated that the invention is not
limited thereto and that many modifications and additions
thereto may be made within the scope of the invention.
