%Problem 3.28
%Demonstration low detection probability of spread spectrum signals in
%comparison with plain ones; 
clear all; close all;
t=[0:0.001:3]; N=length(t); %time scale; 
WD=[0,50]; %two deviatiobns of LFM signals;
for k=1:2 %loop repeating the same for both signals;
    Sk=exp(i*pi*WD(k)*(0:0.001:2).^2); Sk=[zeros(1,500) Sk zeros(1,500)]; %signal complex envelope;
    Skbp=imag(Sk.*exp(i*2*pi*25.*t)); %bandpass sinal;
    subplot(3,2,k); plot(t/2,Skbp); ylim([-1.2 1.2]); ylabel('signal');  grid; %plotting signal;
    NM=normrnd(0,3,10,N); Y=kron(ones(10,1),Skbp)+NM; %10 observations: signal+Gaussian noise;
    subplot(3,2,k+2); plot(t/2,Y(1,:));  xlabel('t/T'); ylabel('signal+noise'); ylim([-20 20]); grid; %plotting selected observation;
    SpY=(abs(fft(Y')/norm(Sk))).^2; %power spectrum of all observatios;
    subplot(3,2,k+4); plot(SpY); xlim([0 150]); ylim([-0.01,1]*0.7e3); xlabel('f'); ylabel('spectrum'); grid; %plotting radiometer output;
end;