B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

658 SPREAD SPECTRUM COMMUNICATIONS
Figure 11.25
Performance of
DS-CDMA
conventional
single-user
detection without
the near-far
effect.
Four-user CDMA BER for QPSK with Gold code of length 31
1 0 ° ----------------------
-- Single user (analysis)
--e-- User 1 BER
10-l
-e- User 2 BER
--3ir- User 3 BER
--+- User 4 BER
10-4
1 0-S ----------------
0 2 4 6 8 1 0 12
Eb/N, dB
% Generate QPSK modulation symbols
data_sym=2*round (rand ( Ldata,4) ) -l+j*(2 *round (rand ( Ldata,4) )-1) ;
% Select 4 spreading codes (Gold Codes of Length 11)
gold31code ;
pcode=GPN ;
% Spreading codes are now in matrix pcode of 31x4
PowerMat=diag (sqrt ([l 1 1 1] ));
pcodew=pcode* PowerMat ;
% Now spread
x_in=kron ( data_sym( :,l) ,pcodew ( :,l) )+kron ( data_sym( :,2) ,pcodew( :,2 )) + . ..
kron ( data_sym( :,3) ,pcodew( :,3)) +kron ( data_sym ( :,4) ,pcodew ( :,4 ));
% Signal power of the channel input is 2*Lc
% Generate noise (AWGN )
noiseq=randn (Ldata*Lc ,l)+j*randn (Ldata*Lc ,l) ; % Power is 2
BERl= [];
BER2 = [];
BER]=[];
BER4= [];
BER_az= [];
for i=l :12,
Eb2N (i)= (i-1);
%(Eb/N in dB )
Eb2N_num=10 A (Eb2N(i) /10) ;
% Eb/N in numeral
Var_n=Lc/ (2*Eb2N_num) ;
%1/SNR is the noise variance
signois=sqrt (Var_n) ;
% standard deviation
awgnois=signois*noiseq ;
% AWGN
% Add noise to signals at the channel output