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CODE DIVISION MULTIPLE ACCESS COMMUNICATIONS

Page 2

CODE DIVISION

MULTIPLE ACCESS
COMMUNICATIONS

Edited by

SAVO G. GLISIC
and
PEN1TI A. LEPPANEN
University of Oulo,
Oulo, Finland

SPRINGER SCIENCE+BUSINESS MEDIA, B.V.

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175

subtract off all of the users' signals from all of the others. It is found that
when all of the users are received with equal strength, the parallel method
outperforms the successive scheme (Figure 4). When the received signals
are of distinctly different strengths (the more important case), the successive
method is superior in performance (Figure 5). The important thing to note is
that in both cases, both outperform the conventional detector.

I.~~----~r------n~----~------~----~~

Figure 4 Parallel vs. Successive Cancellations (Ideal Power Control)

I

N"'lfthM-of
Figure 5 Parallel vs. Successive Cancellations (Rayleigh Fading)

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Sensitivities and Robustness

Most of the discussion and analyses of multiuser detection have assumed a
number of idealizations. For example, in canceling out a chip sequence, it
has been assumed that there is perfect synchronization of each user. Clearly,
if there are tracking errors, the chip sequence being canceled will be offset
and doing an imperfect cancellation. The pertinent question is whether the
tracking error tolerable for the conventional detector is tolerable for the
cancellation, or how much tighter it must be for the interference
cancellation. For the conventional detector, the synchronization must be
within a fraction of a chip duration. This problem is analyzed in [27]. For
numerical results presented here, the interference cancellation scheme was
subjected to pessimistic conditions for the interference canceller:

(i) Did not use averaging of the correlator outputs for amplitude
estimates which significantly improves cancellation performance.

(ii) Assumed equal received powers (perfect power control). The
improvement over the conventional detector is greater in the more
realistic case of unequal received powers.

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,o~~----~~----~----~~----~------~----~ • to 12 '4 .. " ID
NUMBER OF ACTIVE USERS'

Figure 6 Analysis Results of Conventional Detector vs. IC
Scheme for Asynchronous, Random Phase and NO =0.

The processing gain of spread spectrum is set to N=31 and total numbers of
users are ranged from 10 to 24. Figure 6 shows a result for no AWGN.
There are 3 curves each for the interference cancellation scheme and the
conventional detector. Each curve represents different standard deviation e's

Page 364

S
satellite

COMA.33 I
consteUations,336
mobile communications33 1,337
networks.20

scattering,333
semi-Markov decision process

(SMDp),299,30I,309
sequence

duaI-BCR, 73
GoId,74
Gold-like,74
Kasami,7S
m-sequence,72

sequential state estimation,238
serial-search strategies,238
shadowing,332,333,339
Shannon Ciaude,I4,l7
Shannon

bandwidth,31,32,38
SFH 900 system,20S,220
signature

sequence,S3,72
waveform,98

single-user
receiver,l21

slow FHlCOMA,SS
slow frequency hop (SFH),203,204,207
spatial filtering.1l8,130,131
spatial spectral estimation,139
specialized mobile radio systems

(SMRS),351
spread-spectrum,283

communication,284
history of,3
multiple access (SSMA),117
signal,29,30,32,33,141

spreading factor,32
standardization,3S 1
standards,3S2,3S3,357

PCS •• 3S3.354
successive interference

cancellation, 161. 166, 168
suppression filter. 150.152

symbol error probability,306
symbol rate.40

T
tapped delay line (TDL) array

antenna, 133,139
telephone,S
temporal filtering, 124
terrestrial

COMA,337
mobile communication,9

threshold model,30S

363

time division duplex (TD0),29S,331
time-division multiple access

(TDMA),6,117,203,207
time resolution,12
time-correlation function, 186
time-shared (tau-dither) loop,239
total delay spread,186
traffic

data,299,303.315
voice,299,302,308,309

training sequence, 1 03
TV broadcasting spread spectrum

modulation,283,289

U
uncertainty region,226

V
value-iteration a1gorithm,299,30 1 ,31 0
vehicle to vehicle communication and

ranging,283.289
Viterbi algorithm, 1 00, 123
voice traffic,299,302,308,309

W
wideband PM,II
Wiener filter,1S0,1SS

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364

wire communication.28S
wireless

communication,8,28S,3S 1,3S7
environment,3S 1
local area networks

(LANs),117,118

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