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Claims  |
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What is claimed is:
1. A method for communicating in a cellular communication network composed
of a plurality of base stations homing on a central switching office, each
of the base stations serving a plurality of mobile units, the method
comprising the steps of
generating TDMA time slots in each of the base stations and the mobile
units,
synchronizing the TDMA time slots generated in the base stations and the
mobile units by propagating a synch signal from the office to each of the
base stations and, in turn, to the corresponding mobile units,
assigning a CDMA code from a set of CDMA codes to each cell,
assigning each of the mobile units to one of the TDMA time slots,
for downlink communication from an uplink base station to a downlink mobile
unit,
sending an outgoing information signal from the office to the uplink base
station,
converting the outgoing information signal in the uplink base station to an
outgoing CDMA coded signal corresponding to the CDMA code assigned to the
cell containing the uplink base station, and
propagating the outgoing CDMA coded signal to the downlink mobile unit in
the TDMA time slot assigned to the downlink mobile unit,
for uplink communication from a downlink mobile unit to an uplink base
station,
converting an incoming information signal in the downlink mobile unit to an
incoming CDMA coded signal corresponding to the CDMA code assigned to the
cell containing the uplink base station,
propagating the incoming CDMA coded signal to the uplink base station in
the TDMA time slot assigned to the downlink mobile unit, and
converting the incoming CDMA coded signal in the uplink base station to an
incoming information signal for transmission to the office.
2. The method as recited in claim 1 wherein the CDMA codes correspond to
Direct-Sequence, Spread-Spectrum (DS-SS)signals.
3. The method as recited in claim 1 further comprising the step of
detecting the incoming CDMA coded signal and the outgoing CDMA coded
signal each with a matched filter receiver.
4. The method as recited in claim 1 further comprising the step of
detecting the incoming CDMA coded signal and the outgoing CDMA coded
signal each with an interference-suppression receiver.
5. The method as recited in claim 1 further comprising the step of
transmitting an identifier of the CDMA code assigned to each cell from
each of the base stations to corresponding ones of the mobile units served
by each of the base stations in one of the TDMA time slots.
6. A method for communicating in a cellular communication network composed
of a plurality of base stations homing on a central switching office, each
of the base stations serving a plurality of mobile units, the method
comprising the steps of
generating TDMA time slots in each of the base stations and the mobile
units,
synchronizing the TDMA time slots generated in the base stations and the
mobile units by propagating a synch signal from the office to each of the
base stations and, in turn, to the corresponding mobile units,
assigning a CDMA code from a set of CDMA codes to each cell,
transmitting an identifier of the CDMA code assigned to each cell from each
of the base stations to corresponding ones of the mobile units served by
each of the base stations in one of the TDMA time slots,
assigning each of the mobile units to one of the TDMA time slots,
for downlink communication from an uplink base station to a downlink mobile
unit,
sending an outgoing information signal from the office to the uplink base
station,
converting the outgoing information signal in the uplink base station to an
outgoing CDMA coded signal corresponding to the CDMA code assigned to the
cell containing the uplink base station, and
propagating the outgoing CDMA coded signal to the downlink mobile unit in
the TDMA time slot assigned to the downlink mobile unit,
for uplink communication from a downlink mobile unit to an uplink base
station,
converting an incoming information signal in the downlink mobile unit to an
incoming CDMA coded signal corresponding to the CDMA code assigned to the
cell containing the uplink base station,
propagating the incoming CDMA coded signal to the uplink base station in
the TDMA time slot assigned to the downlink mobile unit, and
converting the incoming CDMA coded signal in the uplink base station to an
incoming information signal for transmission to the office.
7. The method as recited in claim 6 wherein the CDMA codes correspond to
Direct-Sequence, Spread-Spectrum (DS-SS)signals.
8. The method as recited in claim 6 further comprising the step of
detecting the incoming CDMA coded signal and the outgoing CDMA coded
signal each with a matched filter receiver.
9. The method as recited in claim 6 further comprising the step of
detecting the incoming CDMA coded signal and the outgoing CDMA coded
signal each with an interference-suppression receiver.
10. Circuitry for communicating in a cellular communication network
composed of a plurality of base stations homing on a central switching
office, each of the base stations serving a plurality of mobile units, the
circuitry comprising
means for generating TDMA time slots in each of the base stations and the
mobile units,
means for synchronizing the TDMA time slots generated in the base stations
and the mobile units, the means for synchronizing including means for
propagating a synch signal from the office to each of the base stations
and, in turn, to the corresponding mobile units,
means for assigning a CDMA code from a set of CDMA codes to each cell,
means for assigning each of the mobile units to one of the TDMA time slots,
means for sending an outgoing information signal from the office to the
uplink base station,
means for converting the outgoing information signal in the uplink base
station to an outgoing CDMA coded signal corresponding to the CDMA code
assigned to the cell containing the uplink base station,
means for propagating the outgoing CDMA coded signal to the downlink mobile
unit in the TDMA time slot assigned to the downlink mobile unit,
means for converting an incoming information signal in the downlink mobile
unit to an incoming CDMA coded signal corresponding to the CDMA code
assigned to the cell containing the uplink base station, and
means for propagating the incoming CDMA coded signal to the uplink base
station in the TDMA time slot assigned to the downlink mobile unit, and
means for converting the incoming CDMA coded signal in the uplink base
station to an incoming information signal for transmission to the office.
11. The circuitry as recited in claim 10 wherein the CDMA codes correspond
to Direct-Sequence, Spread-Spectrum (DS-SS) signals.
12. The circuitry as recited in claim 10 further comprising matched filter
means in the downlink mobile unit for detecting the outgoing CDMA coded
signal and matched filter means in the uplink base station for detecting
the incoming CDMA coded signal.
13. The circuitry as recited in claim 10 further comprising interference
suppression filter means in the downlink mobile unit for detecting the
outgoing CDMA coded signal and interference suppression filter means in
the uplink base station for detecting the incoming CDMA coded signal.
14. The circuitry as recited in claim 10 further comprising means for
transmitting the CDMA code assigned to each cell from each of the base
stations to corresponding ones of the mobile units served by each of the
base stations in one of the TDMA time slots.
15. Circuitry for communicating in a cellular communication network
composed of a plurality of base stations homing on a central switching
office, each of the base stations serving a plurality of mobile units, the
circuitry comprising
means for generating TDMA time slots in each of the base stations and the
mobile units,
means for synchronizing the TDMA time slots generated in the base stations
and the mobile units, the means for synchronizing including means for
propagating a synch signal from the office to each of the base stations
and, in turn, to the corresponding mobile units,
means for assigning a CDMA code from a set of CDMA codes to each cell,
means for transmitting the CDMA code assigned to each cell from each of the
base stations to corresponding ones of the mobile units served by each of
the base stations,
means for assigning each of the mobile units to one of the TDMA time slots,
means for sending an outgoing information signal from the office to the
uplink base station,
means for converting the outgoing information signal in the uplink base
station to an outgoing CDMA coded signal corresponding to the CDMA code
assigned to the cell containing the uplink base station,
means for propagating the outgoing CDMA coded signal to the downlink mobile
unit in the TDMA time slot assigned to the downlink mobile unit,
means for converting an incoming information signal in the downlink mobile
unit to an incoming CDMA coded signal corresponding to the CDMA code
assigned to the cell containing the uplink base station,
means for propagating the incoming CDMA coded signal to the uplink base
station in the TDMA time slot assigned to the downlink mobile unit, and
means for converting the incoming CDMA coded signal in the uplink base
station to an incoming information signal for transmission to the office.
16. The circuitry as recited in claim 15 wherein the CDMA codes correspond
to Direct-Sequence, Spread-Spectrum (DS-SS) signals.
17. The circuitry as recited in claim 15 further comprising matched filter
means in the downlink mobile unit for detecting the outgoing CDMA coded
signal and matched filter means in the uplink base station for detecting
the incoming CDMA coded signal.
18. The circuitry as recited in claim 15 further comprising interference
suppression filter means in the downlink mobile unit for detecting the
outgoing CDMA coded signal and interference suppression filter means in
the uplink base station for detecting the incoming CDMA coded signal. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates generally to digital systems and, more specifically,
to a methodology for communicating in wireless cellular networks using a
hybrid of Time-Division Multiple-Access/Code-Division Multiple-Access
(TDMA/CDMA).
BACKGROUND OF THE INVENTION
A conventional cellular mobile/radio system is composed of numerous mobile
units, such as handsets operated by individual users, which home-on
associated base stations. A single base station serves a number of mobile
units that lie within a simply connected geographical area-a
cell-identified to that single base unit.
The enormous potential demand for wireless communication services combined
with a limited amount of available radio spectrum has motivated extensive
study of bandwidth-efficient, multiple-access techniques for wireless
applications. Two such techniques that have been widely proposed for
wireless cellular applications, such as mobile cellular and Personal
Communication Services (PCS), are Time-Division Multiple-Access (TDMA) and
Code-Division Multiple-Access (CDMA). Each offers advantages and
disadvantages, which vary in degree of importance, depending on the
application, expected traffic types, channel characteristics, allowed
complexity, and network configuration. However, the prior art is devoid of
teachings or suggestions on an inter-dependent combination of TDMA and
CDMA that advantageously utilizes the primary characteristics of each
technique.
SUMMARY OF THE INVENTION
In accordance with the present invention, a methodology for communicating
in a cellular network with a hybrid, multiple-access arrangement combines
both TDMA and CDMA is disclosed. Specifically, intra-cell multiple-access
is achieved via TDMA, and inter-cell interference is suppressed via CDMA.
Broadly, in accordance with a method aspect of the present invention, a
cellular communication network composed of a plurality of base stations
having numerous mobile units homing on each base station propagates
intra-cellular TDMA signals in synchronized time slots, and propagates
intercellular CDMA signals in the time slots. The CDMA signals are
detected with either a matched filter receiver or an interference
suppression receiver (the latter would be used when maximal network
capacity is required). Exemplary of the CDMA signals are Direct Sequence
Spread-Spectrum (DS-SS) signals.
A feature of the methodology, in contrast to the standard
TDMA/Frequency-Division Multiple-Access (FDMA) technique in which
inter-cell interference is suppressed by making sure that users in
adjacent cells are transmitting on different frequency bands, is that the
hybrid TDMA/CDMA scheme combines the high intra-cell capacity of TDMA with
the inter-cell and multipath interference rejection capabilities of CDMA.
The organization and operation of this invention will be better understood
from a consideration of the detailed description of the illustrative
embodiment thereof, which follows, when taken in conjunction with the
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 depicts an arrangement of communication cells for a general cellular
network;
FIG. 2 depicts a layout of contiguous time frames subdivided into time
slots assigned to individual users;
FIGS. 3(a), 3(b)(i)-(iii), 3(c) and 3(d) illustrate block diagrams of
circuitry for cellular communication in accordance with the present
invention;
FIG. 4 illustrates two cells propagating CDMA signals in TDMA time slots;
FIGS. 5 and 6 show two DS-SS signal shapes propagating during the TDMA
slots of FIG. 4;
FIG. 7 depicts exemplary DS-SS signals assigned to certain cells;
FIG. 8 depicts the relationship between a data symbol stream and the
rate-increased chip bit stream propagated in correspondence to the data
symbol stream;
FIG. 9 illustrates a matched filter to detect a DS-SS CDMA signal in a
receiver having a pre-determined signature sequence;
FIG. 10 illustrates an interference suppression filter to detect a DS-SS
CDMA signal in a receiver having a pre-determined signature sequence; and
FIG. 11 depicts an arrangement for time-sharing an interference suppression
filter structure by uploading adaptive coefficients applicable to each
given time slot.
DETAILED DESCRIPTION
In this description, so as to gain an insight into the underlying
principles in accordance with the present invention, the hybrid TDMA/CDMA
method is initially described in overview fashion. Then, the relationship
between TDMA and CDMA parameters for a specific CDMA technique, namely,
Direct-Sequence Spread-Spectrum (DS-SS), is discussed. Finally, the use of
interference suppression techniques to suppress inter-cell interference is
discussed.
The general cellular network arrangement 100 under consideration is
depicted in block diagram form in FIG. 1. Within each cell 101, 102 . . .
, or 110 (cell 1, cell 2, . . . , cell 10, respectively), the standard
TDMA format is employed to communicate between a base station and its
associated mobile units (not shown in FIG. 1, but shown and discussed
shortly with reference to FIG. 3). Thus, as depicted in FIG. 2, each TDMA
corranunication channel between a base station and each of its associated
mobile units is divided into frames 1, 2, . . . (reference numerals 201,
202, . . . , respectively) to support a number of users. In turn, each
frame is subdivided into time slots T1 (reference numeral 211 in frame 1,
221 in frame 2, . . . ), T2 (212 and 222 in frames 1 and 2, . . . ), . . .
, TM (213 and 223 in frames 1 and 2, . . . ) for a maximum of M users;
each time slot is of duration T seconds. Each active user is typically
assigned to one time slot per frame.
With reference to FIG. 3(a), cellular arrangement 100 of FIG. 1 is depicted
from another viewpoint, namely, circuitry 300, in block diagram form, is
used to communicate in both the downlink (base station-to-mobile unit)
direction and the uplink (mobile unit-to-base station) direction. Serving
office 301 is used to switch incoming and outgoing signals from the
cellular arrangement - either inter-cell signals propagating to or from
cell 101 and cell 102 shown in FIG. 3(a), or signals arriving from or
destined for another serving office (not shown) via communication link
302. Base stations 310 and 340 (BS2 and BS1, respectively) are connected
to serving office 301 via communication links 303 and 304, respectively;
these links are usually high bandwidth links. Base station 310 is shown as
serving mobile units 320 and 330 (MU1 and MU2, respectively). Also, base
station 340 serves mobile units 350 and 360 (MU3 and MU4, respectively). A
sequence of frames, commensurate with that shown in FIG. 2, is depicted as
frame sequence 325 associated with base station 310. Similarly, a sequence
of frames associated with base station 340 is shown as frame sequence 355.
Time slots in frames such as frames 325 and 355 are synchronized across all
cells controlled by a given serving office such as office 301.
Synchronization is a conventional function for any system employing TDMA;
synchronization is effected by implementing clock 305 in serving office
301. In turn, the clock signal from clock 305 is propagated to each base
station via its high-bandwidth link. For instance, synch generator 311 in
base station 310 receives the clock signal and regenerates the clock
signal for transmission to mobile units 320 and 330. The synch signal is,
in turn, transmitted to the mobile units from transmitter 312 for timing
purposes.
As will be described in more detail shortly, the CDMA code utilized within
cell 101, designated code 1, is transmitted to mobile units 350 and 360
over message channels 344 and 345, respectively. A message channel is also
deployed in conventional TDMA systems to pass messages between the base
station and the mobile units. Similarly, the CDMA code utilized within
cell 102, designate code 2, is transmitted to mobile units 320 and 330
over message channels 314 and 315, respectively.
As shown in frame sequence 325, an exemplary assignment of mobile units to
time slots is such that mobile unit 320 is assigned to time slot 2 whereas
mobile unit 330 is assigned to time slot K. Similarly, as shown in frame
sequence 355, an exemplary assignment of mobile units to time slots is
such that mobile unit 350 is assigned to time slot 3 whereas mobile unit
360 is assigned to time shot L (perhaps different than K). Transmissions
from base station 310 are accomplished via transmitter 312, coupled to an
antenna (not shown), whereas received signals are detected at base station
310 over receiver 313, which is also coupled to the antenna. Similarly,
transmissions from base station 340 are accomplished via transmitter 342,
coupled to an antenna (not shown), whereas received signals are detected
at base station 340 over receiver 343, which is also coupled to the
antenna. Each mobile unit in the system has a commensurate
transmitter-receiver pair (e.g., transmitter 351 and receiver 352 of
mobile unit 350). Since the downlink and uplink directions may basically
use the same type of transmitter-receiver combination, the focus of the
immediate discussion is on one direction, say the uplink direction, and in
particular, the type of receiver (e.g., receiver 313 or 343) to provide
for interference suppression, as discussed in more detail shortly. By way
of elucidating aspects of FIG. 3(a) in terms of known communications
techniques, reference is now made to FIG. 3(b), which shows the details of
an exemplary base station 340 in FIG. 3(b)(i). TDMA/CDMA frames required
of the inventive subject matter may be translated directly to or from
TDMA/FDMA frames. At base station transmitter 342, each TDMA times slot's
symbols generated in generator 3421 can be modulated by the CDMA waveform
serving as an in to modulator 3422. The modulated output is then
transmitted on channels 344 and 345. At the base station receiver 343,
each incoming CDMA waveform received over channels 344 and 345 can be
demodulated in demodulator 3432, and then converted into the symbol for
that TDMA time slot in generator 3431. With this arrangement, it is
immediately apparent that the base station 340 can internally handle TDMA
frames with familiar TDMA/FDMA techniques, and need only convert to or
from CDMA at the front end (elements 3422 and 3432). The frame of FIG.
3(b)(ii) depicts the signals appearing at points A in FIG. 3(b)(i),
whereas the frame of FIG. 3(b)(iii) depicts the modulated signals
appearing at points n in FIG. 3(b)(i). The same framing and
interconnection structure of any conventional TDMA/FDMA radio system could
be used. This structure was alluded to earlier in this section.
With reference to FIG. 3(c), a transmitter such as transmitter 342
modulates the baseband TDMA/CDMA signals to a radio frequency (RF) band
via modulator 3423, and then propagates the modulated signals on a radio
link through antenna 3424. Each receiver, such as receiver 352, detects
the incoming RF signal from antenna 3521 and demodulates the RF signal,
via modulator 3522, to baseband TDMA/CDMA signals.
With reference to FIG. 3(d), there is shown the propagation path of each RF
modulated TDMA/CDMA signal. In keeping with the protocol of conventional
systems, the uplink and downlink signals use two different carrier
frequencies. These RF signals therefore unambiguously propagate from the
base station antenna to a mobile antenna and vice-versa in the arrangement
of FIG. 3(d).
The control and management functions of system 300 are generally
independent of the type of multiplexing involved in the circuitry of FIGS.
3(a)-3(d), that is, these functions are managed on a different layer than
the multiplexing level; such functions are therefore generic to both the
conventional techniques as well as the present invention.
It is further noted that there is no implicit difference between CDMA code
identification for the TDMA/CDMA of the present invention and CDMA code
identification for presently existing systems. When a mobile unit is
assigned a given call, a copy of the CDMA code sequence for that cell is
transmitted to that mobile unit from the corresponding base station. A
typical arrangement is for each base station to continuously transmit its
CDMA code sequence on a dedicated "pilot" or message time-slot. Such
message time-slots with accompanying message signals are already
implemented in conventional systems for handoff and power control purposes
in that each mobile unit can then determine the received power from each
base station for level control. The utility of the message channel was
also alluded to in the foregoings.
In general terms, within a particular cell (say cell m) it is assumed that
the i.sup.th user in cell m transmits the following pulse-amplitude
modulated waveform during the user's particular time slot,
##EQU1##
where {b.sub.k.sup.(i) } is the sequence of symbols corresponding to user
i, p.sub.m (t) is the transmitted pulse shape assigned to cell m, and 1/T
is the symbol rate.
In standard TDMA/FDMA, the baseband pulse shapes corresponding to different
cells are the same. Inter-cell interference is suppressed by assigning
different carrier frequencies to different cells. Furthermore, the
frequency bands assigned to adjacent cells are assumed to be
nonoverlapping. For instance, with reference to FIG. 1, cell 101 would be
assigned a frequency designated f1, whereas cells surrounding cell 101,
namely, cells 102-107 have frequencies f2, . . . , f7, respectively.
In TDMA/CDMA, however, inter-cell interference is suppressed by selecting
different baseband pulse shapes for each cell. As an example, the baseband
pulse p.sub.m (t) is considered to be a DS-SS waveform
##EQU2##
where {a.sub.k.sup.(m) } is a pseudo-random sequence of length N assigned
to cell m, and .PSI.(t) is the chip waveform, which typically has duration
T.sub.c =T/N.
In TDMA/CDMA, different DS-SS spreading codes (as opposed to frequency
channels in FDMA)) are therefore assigned to different cells. Within each
cell, all users take turns (as in TDMA) transmitting the type of signal
set forth in equation (1) where the pulse shape is associated with the
particular spreading sequence assigned to that cell. In addition, when a
user is handed off to an adjacent cell, the spreading code (as opposed to
carrier frequency in TDMA/FDMA) must therefore be changed.
To ensure adequate interference rejection, it is important that the pulse
shapes assigned to different cells have low cross-correlations. In
general, there may be many ways of assigning pulse shapes p.sub.m (t) with
good cross-correlation properties to different cells. For instance, with
reference to FIG. 4, the shape p.sub.1 (t) propagated for a user in time
slot 401 in cell 101 and the shape p.sub.2 (t) for a user in the
corresponding time slot 402 in cell 102 are considered. Each such signal
is illustratively of the type that propagates both positive and negative
electrical signals having amplitudes which fall within a given dynamic
range. Now with reference to FIG. 5, which is an enlargement of time
interval 401, the p.sub.1 (t) propagated in interval 401 provides a bit
stream of rate-increased two-level level signals; without loss of
generality, one level is +1 and the other level is -1 on a normalized
basis. This rate-increased waveform may also be referred to with the
shorthand notation "101100" (where "1" corresponds to +1 and "0"
corresponds to -1). Also, a rate-increased stream corresponding to p.sub.2
(t) ("110111") shown in FIG. 6 is propagated during interval 402. A
primary function of each mobile unit for uplink propagation is that of
converting each data symbol (which may represent, for example, a sampled
and coded voice signal of a user) generated by the mobile unit to the
predetermined rate-increased bit stream corresponding to given data
symbol, as generally depicted by signals of FIGS. 5 or 6.
The signal set selected for p.sub.m (t), m=1, 2, . . . , M is such that the
set has appropriate auto-correlation and cross-correlation properties. In
this case, the auto-correlation property is such that, for example, if a
matched filter is used in a base station receiver (say receiver 343 of
FIG. 3(a)) for detection, the output of the matched filter will be above a
pre-selected threshold if the incoming signal is the signal from the set
for which the filter was designed, whereas the cross-correlation property
means the output of the matched filter will be below a preselected
threshold if the incoming signal is not the signal from the set for which
the filter was designed. To complete the understanding of the code
assignment, the pictorial representation of FIG. 7 shows the cellular
arrangement, commensurate with FIG. 1, wherein the two codes "101100" and
"10111" from an exemplary set of p.sub.m (t) are assigned to cells 101 and
102, respectively.
In analogy with frequency reuse in TDMA/FDMA, "code reuse" is possible in
TDMA/CDMA. That is, the same code can be assigned to cells sufficiently
far apart (e.g, cell 109 has the same code assignment as cell 102 in FIG.
7). Because the codes used in TDMA/CDMA can be designed to have small
auto-correlations, interference from cells using the same code will be
suppressed by a matched filter receiver. Consequently, the distance
between cells using the same code in TDMA/CDMA in general need not be as
great as the distance between cells assigned the same frequency band in
TDMA/FDMA.
Because each cell in a hexagonal tiling illustrated in FIG. 1 has at most
six nearest neighbors, it is likely that the number of significant
interferers in the TDMA/CDMA scheme will be no more than six, assuming
omni-directional antennas. If the interfering signals were always
significantly attenuated relative to the desired intra-cell signal, then
the processing gain N in equation (2) needed to maintain a desired
signal-to-interference ratio (SIR) is generally small, even assuming a
matched filter receiver. However, if the cells are physically adjacent,
then because of shadow fading, there is a significant chance that at least
one interferer is stronger than the desired intra-cell signal. In this
situation a relatively large processing gain is needed to maintain the
desired SIR.
The matched filter receiver is the simplest choice for the TDMA/CDMA
scheme. The receiver corresponding to mobile units and the base station in
cell m then makes decisions based on samples of the output of a filter
with impulse response p.sub.m (-t). However, as indicated in the preceding
paragraph, the processing gain (discussed in detail shortly) needed to
maintain an adequate SIR with high probability for TDMA/CDMA in the
presence of shadow fading would most likely be larger than the frequency
reuse factor which would be used in a conventional TDMA/FDMA system.
Consequently, without power control, the capacity of TDMA/CDMA with
matched filter receivers is not likely to be greater than that of
TDMA/FDMA. Since the capacity of TDMA/CDMA is limited by inter-cell
interference, however, interference reduction techniques such as antenna
sectorization, and interference suppression techniques such as described
later, can be used to reduce the processing gain relative to that required
for the matched filter receiver, and thereby increase capacity. (It is
noted that the near-far problem, which may impair the performance of CDMA
systems, is not as severe in the TDMA/CDMA system described because
multiple-access interference can originate only from outside the
receiver's cell. However, power control, as is used in conventional DS-SS
CDMA, can be used in TDMA/CDMA to increase system capacity, either with
the matched filter receiver, or with interference suppression, as will be
described shortly.
As already alluded to above, a primary function of each mobile unit (say
mobile unit 350 for concretehess) is that of converting each data symbol
generated within the mobile unit to a predetermined rate-increased bit
stream corresponding to given data symbol, as generally depicted by signal
800 in FIG. 8. Line (i) in FIG. 8 depicts three contiguous data symbols,
namely, the +1,-1,+1 symbol stream produced within mobile unit 350 at the
symbol rate of 1/T, that is, the time duration of a symbol is designated
as duration 801 in FIG. 8 and is denoted by T.
Line (ii) in FIG. 8 represents a rate-increased output pulse stream, say
p.sub.1 (t) from mobile unit 350 (also referred to as the signature stream
of the associated mobile unit), corresponding to the line (i) symbol
stream. As shown, a rate-increased signature stream of
(+1,-1,+1,+1,-1,-1)-level pulses is propagated for each +1 in the low-rate
data symbol stream; however, the negative of this signature stream is
propagated for each -1 in the low-rate symbol stream.
In the rate-increased data stream shown in line (ii) of FIG. 8, the time
interval of each +1 or -1 level in the rate-increased stream is designated
the chip duration 802 and is denoted T.sub.c. The ratio T/T.sub.c is
called the "processing gain" and the ratio is denoted by N
(N.dbd.T/T.sub.c). Therefore, each frame is composed of a fixed number N
of so-called "chips"; in FIG. 8, N.dbd.6, so six +1 and -1 chips emanate
from mobile unit 350 during each frame. Thus, the signature for mobile
unit 350 is the ordered set (+1,-1,+1,+1,-1,-1).
In order to communicate effectively within system 300 of FIG. 3(a), each
baseband pulse p.sub.i (t) ,i.dbd.1, . . . , M, as produced by its
assigned mobile unit in response to each input symbol, may not be selected
arbitrarily, but must be carefully chosen to achieve efficient, error-free
communication. This means basically that each p.sub.i (t) must be selected
in view of all the other p.sub.i (t)'s based on such considerations as
number of mobile units and the effective bandwidth of the time slots.
These considerations, in turn, depend on the system requirements and
transmission characteristics. Procedures for the set of baseband signals
p.sub.i (t) which effect efficient information interchange for a given
number of chips and mobile units are known in the art. (E.g., see U.S.
Pat. No. 4,779,266). An example of another baseband signature, generated
with reference to the above-identified signature (namely,
(+1,-1,+1,+1,-1,-1)) is the signature given by the ordered set
(+1,+1,-1,+1,+1,+1) depicted in FIG. 6.
The essential function of each receiver, such as receiver 343 in base
station 340, is that of discriminating within each time slot and from
among the composite of all signals arriving at the antenna of base station
340 the signature preassigned to the time slot; in the case of mobile unit
350, this would be time slot L and the signature would be the ordered set
(+1,-1,+1,+1,-1,-1).
In one arrangement, receiver 343 is implemented by a matched filter, as now
discussed with reference to FIG. 9. In FIG. 9, there is shown standard
matched filter 900 for the specific case of six chip positions in a
rate-increased data stream. The input, which appears on lead 901,
represents the input received by the antenna of base station 340. The
signal appearing on lead 901, designated r(t), in the absence of
interference from other cells, is equal to the signal p.sub.1 (t) plus,
generally, additive noise at the input. In general, r(t) is a continuous
time signal. This continuous signal serves as an input to chip matched
filter 905, that is, filter 905 is a filter matched to the chip shape. The
output of filter 905 is converted to a sampled data signal by sampler 910
which samples r(t) at the chip rate T.sub.c ; for the specific example
under consideration, six samples of r(t) are taken in each frame - the
samples are denoted by the set r(k), r(k-1), r(k-2), r(k-3), r(k-4), and
r(k-5), with r(k) being the latest sample taken and r(k-5) corresponding
to the earliest sample in a frame. In order to have access to all six
samples for demodulation purposes, the samples are stored in shift
register 920 composed of five delay elements 921-925 wherein each delay
element provides a delay of T.sub.c seconds between its input and output.
To generate the overall filter output y.sub.1, appearing on lead 961: (i)
the samples r(k) (k.dbd.0, . . . ,5) are each multiplied by a
pre-specified coefficient (a.sub.1 [k], k.dbd.0, . . . ,5 shown by
reference numerals 931-936, respectively) in multipliers 941-946,
respectively, to obtain resultant products; (ii) the resultant products
are summed in summer 950, with the resultant sum appearing on lead 951;
and (iii) the resultant sum is sampled by sampler 960 at the frame rate
1/T to produce the output y.sub.1. (Typically, y.sub.1 is processed by a
threshold detector (not shown) to yield a bit decision corresponding to
the estimate of the received symbol; in the remainder of the discussion,
such a conventional threshold detector is presumed to exist, although it
is not shown for sake of clarity.) In the implementation of filter 900,
sample r(k)is multiplied by coefficient a.sub.1 [5], r(k-1) by a.sub.1
[4], . . . , and r(k-5) by a.sub.1 [0]. In general, the a.sub.1 [i]'s
correspond to the signature sequence assigned to the given receiver. If it
is assumed that matched filter receiver 900 is configured to demodulate
the first above-identified signature sequence (+1,-1,+1,+1,-1,-1), then
the a.sub.1 [i]'s are assigned in reverse order to the signature sequence,
that is, a.sub.1 [0].dbd.-1, a.sub.1 [2].dbd.-1, a.sub.1 [2].dbd.+1, . . .
, a.sub.1 [5]=+1. Thus, whenever a data symbol | | |
