cacode.m

GPS C/A code signal genration

function CA= cacode(n, chiprate, fs, n_samples)
%The function CA= cacode(n, chiprate, fs, n_samples) returns the gold code
% for GPS satellite ID n =1...32.
% chiprate = number of chips per second
% fs = sampling frequency
% n_samples = number of samples
% The code is represented at levels : -1 for bit =0 and 1 for bit =1
%phase assignments
phase=[2 6; 3 7; 4 8; 5 9; 1 9; 2 10; 1 8; 2 9; 3 10; 2 3; 3 4; 5 6; 
    6 7; 7 8; 8 9; 9 10; 1 4; 2 5; 3 6; 4 7; 5 8; 6 9; 1 3; 4 6; 5 7; 6 8;
    7 9; 8 10; 1 6; 2 7; 3 8; 4 9];

% Initial state - all ones
G1 = -1*ones(1,10);
G2 = G1;
%select taps for G2 delay
s1 = phase(n,1);
s2 = phase(n,2);
tmp =0;
    for i =1:1023;
        %gold-code
        G(i)= G2(s1)*G2(s2)*G1(10);
        % generator1 -shift reg1
        tmp=G1(1);
        G1(1)=G1(3)*G1(10);
        G1(2:10)=[tmp G1(2:9)];
        
        % Generator2 - shift reg2
        tmp=G2(1);
        G2(1)=G2(2)*G2(3)*G2(6)*G2(8)*G2(9)*G2(10);
        G2(2:10)=[tmp G2(2:9)];
    end;
    
    % Resample - doesn't work for (i*chiprate/fs)>1023
    % but replica chiprate is constant in this implementation
    G=[G G G G G G G G G G];
   %G=[G G];
    %G=[G -1*G G -1*G G ];
    i=1:n_samples;
    CA(i) = G(ceil(i*chiprate/fs));