%
% FUNCTION 2.11 : "cp0203_OFDM_qpsk"
%
% Simulation of a transmitter implementing 
% the OFDM transmission chain with QPSK modulation
% on each sub-carrier
%
% 'numbits' is the number of bits generated by the source.
% 'fp' is the carrier frequency of the generated signal
% 'fc' is the sampling frequency
% 'T0' is the block lenght in [s], i.e. 1/T0 is the carrier
%  separation
% 'TP' is the length of the cyclic prefix [s]
% 'TG' is guard time
% 'A' is the amplitude of the rectangular impulse response
%  [V]
% 'N' is the number of carriers (tones) used in the OFDM
%  system
%
% The function returns:
% 1) the generated stream of bits ('bits')
% 2) the corresponding stream of QPSK symbols ('S')
% 3) the I component of the generated signal ('SI')
% 4) the Q component of the generated signal ('SQ')
% 5) the generated OFDM signal ('Stx')
% 6) the value of the sampling frequency ('fc')
% 7) the value of the carrier frequency ('fp')
% 8)9)10) The values of T0, TP, and TG
% 11) the number of tones used for transmission
%
% Programmed by Guerino Giancola
%

function [bits,S,SI,SQ,Stx,fc,fp,T0,TP,TG,N] = ...
   cp0203_OFDM_qpsk;

% ----------------------------
% Step Zero - Input parameters
% ----------------------------
 
numbits = 128;      % number of bits to be transmitted

fp = 1e9;            % central frequency

fc = 50e9;           % sampling frequency   

T0 = 100e-9;       % information length
TP = 0;        % cyclic prefix
TG = 50e-9;        % total guard time

A = 1;               % amplitude of the rectangular impulse
                     %  response

N = 4;             % number of carriers of the OFDM
                     %  system

% -------------------------
% Step One - OFDM modulator
% -------------------------

tc = T0 / N;            % chip time
ntcp = floor(TP/tc);    % number of tones of the cyclic
                        %  prefix
n = (-ntcp+1:1:N);      % tone counter
NT = length(n);         % total number of tones per symbol

% Bit generation
[bits] = cp0201_bits(numbits);


% QPSK modulator
[S,Sc,Ss] = cp0203_qpsk_mod(bits);

% OFDM modulator

nb = ceil(length(S)/N);      % number of OFDM blocks to be
                             %  transmitted
S0 = zeros(1,nb*N);          % zero padding
S0(1:length(S))=S;

dt = 1 / fc;                 % sampling period

if ntcp>0
    tc = (T0+TP)/NT;         % tone duration
end
tonesamples = floor(tc/dt);  % samples per tone
toneres = floor((TG-TP)/dt); % samples for the residual
                             %  part   

symsamp = (tonesamples*NT)+toneres;
% number of samples representing one OFDM symbol

totsamp = symsamp * nb;
% number of samples representing the transmitted signal

X = [zeros(1,totsamp)'];

for b = 1 : nb
    
    c = S0((1+(b-1)*N):(N+(b-1)*N));    % block extraction

    % S/P conversion and zero padding
    A = length(c);
    a1 = floor(A/2);
    a2 = A - a1;
    FS = 2*A;
    Czp=zeros(FS,1);
    Czp(1:a1)=[c(1:a1).'];
    Czp(FS-a2+1:FS)=[c(A-a2+1:A).'];
       
    C = ifft(Czp);  % IFFT of the zero-padded input
    
    
    if ntcp>0 % Insertion of the cyclic prefix
        C1=zeros(length(C)+2*ntcp,1);
        C1(1:(2*ntcp))=C(2*N+1-(2*ntcp):2*N);  
        C1(2*ntcp+1:length(C1))=C;
    else
        C1=C;
    end
    %     
    
    zp = floor(tonesamples/2);
    C2 = [C1.';zeros((zp-1),length(C1))];
    C3 = C2(:);
    g = ones(1,zp);
    C4 = conv(g,C3);
    
    C4 = C4(1:(zp*NT*2));
    
    ics = 1 + (b-1)*symsamp + toneres;
    X(ics:ics+length(C4)-1)=C4;
    
            
end % for b = 1 : nb

XM = X';                % P/S conversion
XM = XM(1:totsamp);

I = real(XM);
Q = imag(XM);

% Carrier modulation
time = linspace(0,totsamp*dt,length(I));
SI = I.*(cos((2*pi*fp).*time));
SQ = Q.*(sin((2*pi*fp).*time));

Stx = SI - SQ;