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

11.9 MATLAB Exercises 655
% Add jamming sinusoid sampling frequency is fc = Le
jam_mod=kron (jam_data,ones (Lc,1 )); clear jam_data;
jammer= sqrt (Pj/2)*jam_mod .*exp (j *2*pi*0.12* (1:Ldata*Lc) ).'; %fj/fc=0 .12.
clear jam_mod;
[P,x] =pwelch (x_in, [] , [] , [4096] ,Le , 'twoside');
figure (1) ;
semil ogy (x-Lc/2, fftshift (P) );
axis ([-Lc/2 Lc /2 l.e-2 l.e2] );
grid;
xfont=xlabel ('frequency (in unit of 1/T_s ) ');
yfont=ylabel ('CDMA signal PSD' );
set (xfont , 'FontSize' , 11) ;set (yfont , 'FontSize' ,11) ;
[P,x] =pwelch (jammer+x_in, [] , [] , [4096] ,Le , 'twoside' );
figure (2) ;semi logy (x-Lc/2, fftshift (P));
grid;
axis ([-Lc/2 Lc/2 l.e-2 l.e2] );
xfont=xlabel ('frequency (in unit of 1/T_s ) ');
yfont=ylabel ('CDMA signal + narrowband jammer PSD' );
set (xfont, 'FontSize' , ll) ;set (yfont, 'FontSize ' , 11) ;
BER=[];
BER_az= [];
for i=l:10,
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
y_out=x_in+awgnois+jammer;
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
decl=sign (real ( z_out ))+j *sign ( imag ( z_out) );
% Now compare against the original data to compute BER
BER= [BER;sum ( [real (data_sym) ~=real (decl) ; ...
imag ( data_sym) -=imag (decl)])/(2*Ldata) ];
BER_az= [ BER_az ;0.5*erfc ( sqrt (Eb2N_num ) )];
%analytical
end
figure (3)
figber=semi logy (Eb2N, BER_az, 'k-',Eb2N,BER, 'k- o' ) ;
legend ('No jamming ','Narrowband jamming (-10 dB ) ');
set ( figber , 'LineWidth ' , 2) ;
xfont=xlabel ('E_b/N (dB) ');
yfont=ylabel ('Bit error rate');
title ('DSSS (CDMA ) wi th spreading gain 11' ) ;
Because the spreading factor in this case is L = 11, the DSSS signal occupies a bandwidth approximately
11 times wider. From the user signal carrier, we add a narrowband QPSK jamming signal with a
carrier frequency offset of 1.32/T. The signal-to-interference ratio (SIR) can be adjusted. In Fig. 11.23,