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

11 . 9 MATLAB Exercises 661
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
y_out=x_in+awgnois;
Y_out=reshape (y_out ,Lc,Ldata) .'; clear y_out awgnois;
% Despread first and apply decorrelator Rinv
z_out= (Y_out*pcode );
% despreader (conventional ) output
clear Y_out ;
z_dcr=z_out*Rinv;
% decorrelator output
% Decision based on the sign of the single receivers
decl=sign (real ( z_out) )+j*sign ( imag ( z_out) );
dec2=sign (real ( z_dcr) )+j*s ign ( imag ( z_dcr) );
% Now compare against the original data to compute BER of user 2
% and user 4 (weaker ones) .
BERa2= [BERa2 ;sum ( [real (data_sym( : ,2) )~=real (decl (:,2) ) ; ...
imag ( data_sym (:,2) )~=imag (decl (:,2 ))])/ (2*Ldata) J ;
BERa4= [BERa4 ;sum ( [real (data_sym( : ,4) )~=real (decl (:,4) ) ; ...
imag ( data_sym (:,4) )~=imag (decl (:,4) ) ] )/(2 *Ldata) J ;
BERb2 =[BERb2 ;sum ( [real (data_sym( : ,2) )~=real (dec2 (:,2)) ; ...
imag ( data_sym (:,2) )~=imag (dec2 (:,2)) ] )/(2 *Ldata) J ;
BERb4= [BERb4 ;sum ( [real (data_sym( : ,4))~=real (dec2 (:,4) ) ; ...
imag ( data_sym( :,4 ))~=imag (dec2 (:,4 ))])/ (2*Ldata) ] ;
BER_az= [ BER_az ;0.S*erfc ( sqrt (Eb2N_num ) )];
%analytical
end
figure (l)
figber=semil ogy (Eb2N, BER_az , 'k~' ,Eb2N, BERa2 , 'k~o ' ,Eb2N,BERa4 , ' k~s', ...
Eb2N, BERb2 , 'k~~o' ,Eb2N, BERb4 , 'k--s');
legend('Single~user (analysis)', 'User 2 (single user detector) ', ...
'User 4 (single user detector) ', 'User 2 (decorrelator) ', ...
'User 4 (decorrelator) ')
axis ( [ 0 12 0.99e-5 1.e0] );
set ( figber , 'LineWidth ' , 2) ;
xlabel ('E_b /N (dB) ') ;ylabel ('QPSK bit error rate ')
title ('Weak~user BER comparisons ');
We also implement the decision feedback MUD of Sec. 11.7 in MATLAB program Exl 1_ 4b . m.
The decision feedback MUD performance of the two users is shown in Fig. 11.27.
% MATLAB PROGRAM <Exll 4b .m>
% This program provides simulation for multiuser CDMA
% systems . The 4 users have different powers to illustrate the
% near-far effect in single user conventional receivers
%
% Decision feedback detectors are tested to show its
% abi lity to overcome the near~far problem .
%
%clear;clf
Ldata=l00000;
Lc=31;
%User number = 4;
% Generate QPSK modulation symbols
% data length in simulation; Must be divisible by 8
% spreading factor vs data rate