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

11.9 MATLAB Exercises 659
y_out=x_in+awgnois;
Y_out=reshape (y_out ,Lc,Ldata) .';
clear y_out awgnois;
% Despread first
z_out=Y_out*pcode ;
% Decision based on the sign of the samples
dec=sign (real ( z_out ))+j * sign ( imag ( z_out) );
% Now compare against the original data to compute BER
BERl= [BERl ;sum ( [real (data_syrn (:,l) )~=real (dec (:,l) ) ; ...
imag ( data_syrn (:,l))~=imag (dec ( : ,l))J ) / (2*Ldata) J ;
BER2=[BER2 ;sum ( [real (data_syrn ( : ,2))~=real (dec ( : ,2));...
imag ( data_syrn (:,2)) - =imag (dec (:,2) ) ] ) / (2*Ldata) ];
BER3=[BER3 ;sum([real (data_syrn(:,3) )~=real (dec ( : ,3));...
imag (data_syrn ( : ,3)) - =imag (dec ( : ,3) ) ] ) / (2*Ldata) J ;
BER4= [BER4 ;sum ( [real (data_syrn (:,4) ) -=real (dec ( : ,4));...
imag ( data_syrn (:,4)) - =imag (dec ( : ,4) ) ] ) / (2* Ldata) ];
BER_az= [ BER_az ;0.5*erfc ( sqrt (Eb2N_num) )l;
%analytical
end
BER= [ BERl BER2 BER3 BER4 ] ;
figure (l)
figber=semilogy (Eb2N, BER_az, 'k-',Eb2N, BER1 , 'k-o' ,Eb2N,BER2 , 'k-s',...
Eb2N,BER3 , ' k-v ' , Eb2N, BER4 , 'k- * ');
legend ('Single-user (analysis)','User 1 BER' , 'User 2 BER' ,
'User 3 BER' , 'User 4 BER' )
axis ([O 12 0.99e-5 1.eO ] );
set ( figber, 'LineWidth ' , 2) ;
xlabel ('E_b /N (dB) ');ylabel ('QPSK bit error rate')
title ('4-user CDMA BER with Gold code of length 31' );
COMPUTER EXERCISE 11 .4: MULTIUSER CDMA DETECTION
IN NEAR-FAR ENVIRONMENT
We can now modify the program in Computer Exercise l 1.3 to include the near-far effect. Among the
four users, user 2 and user 4 have the same power and are the weaker users from far transmitters. User
l is l O dB stronger, while user 3 is 7 dB stronger. In this near-far environment, both users 2 and 4 suffer
from strong interference (users 1 and 3) signals due to the lack of code orthogonality. Note that the two
weak users do not have the same level of multiuser interference (MUI) from other users because of the
difference in their correlations.
MATLAB program Exl 1_ 4a. mcompares the performance of the conventional single-user receiver
with the performance of the decorrelator multiuser detector (MUD) described in Sec. 11. 7. We show the
performance results of user 2 and user 4 in Fig. 11.26.
% MATLAB PROGRAM <Exll 4a .m>
% This program provides simulation for multiuser CDMA system
% that experiences the near-far effect due to user Tx power
% variations .
%
% Decorrelator receivers are
% applied to mitigate the near-far effect
%
%clear;clf