function matrix = MAC_layer (Data_users, SNR_users)

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%%                                                                       %
%%      File: MAC_layer.m                                                %
%%                                                                       %
%%      Function: This routine simulate the MAC layer of the system      % 
%%                                                                       %
%%      Input:                                                           %  
%%        SNR_users = We don't have the channel estimation process       %
%%         so we calculate these values on a random process.             %
%%        Data_users = Amount of data users (coded & mapped: ready)      %
%%         to be sent. We only have to map them onto a matrix.           %
%%                                                                       %
%%                                                                       %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% You have to input these parameters:
user1 = randint (1,10000);
user2 = randint (1,10000);
user3 = randint (1,10000);

%We iniciate every parameters we are going to use.

Data_users = [user1; user2; user3];    % Data_users has to be a matrix 
% I realize that it's probably that each user wants to transmit a
% different amount of data...It has to fix it.

% We have to know, how many users are on the system. We've to know how many
% bits they want to transmit, too. These bits have been coded and mapped so
% they are ready to transmit through the channel.

N_users = size(Data_users,1);

% We want to know which mapping has been applied in each user. We need to
% know what is the SNR of each one. When we mapped the signal, we had to do
% the same thing !!! It has be the same result !!!
% You can change the way we choose the signal mapping...but you've to do
% the same before mapping the signal.

Users_segment = zeros (1,N_users)
for i=1:N_users
    if ((SNR_users(i) >= 1) && (SNR_users(i) <= 9))
        Users_segment(i) = 0;                       % We use QPSK mapping
    elseif ((SNR_users(i) >= 10) && (SNR_users(i) < 22))
        Users_segment(i) = 1;                       % We use 16-QAM mapping
    elseif (SNR_users(i) >= 22) 
        Users_segment(i) = 2;                       % We use 64-QAM mapping
    end
end

% We Know how many bits are going to transmit each user. We have a FOR
% bucle to do that, because we could have more users on the system.
Amount_Data_users = [];
for i=1:N_users
    s = 'user';
    Each_user = [s , ('i')];   %we need to concatenate each value...
    L = length (Each_user);
    Amount_Data_users = [Amount_Data_Users L];
end

Amount_Data_users = [10000 10000 10000];  %You need to cancel this sentence once you do the bule right


% The most important thing is the acknowledgement of the area where each
% user is going to transmit. (Data Region)
[Rest_amount_data_users, data_matrix] = data_region (Amount_Data_users, Users_segment, N_users); 
 
% Now, we have to prepare each subcarrier where we've to put the
% information we want. It is recommendable insert the pilot subcarriers
% later, but we want to know its values now.

% We have to calculate hte values of the pilots subcarriers....

xxxxxxxxx CALCULATE THAT...It Is so easy, because standard give us the way we
    have to do that!!

% Now we are going to fill the whole matrix, so the result is going to be a
% matrix which dimensions are 2048x15;

Transmision_Matrix = Filling_matrix (Data_users, data_matrix, pilot_subcarriers);

% Now, we have the whole matrix ready to be send. Before that, we have to
% do the permutation and renumbering stages.

xxxxxxx It isn't so hard. In fact You have already done part of this.

% Finally, we convert our OFDMA symbol data into frequency domain, 
% so we've to do an FFT.

OFDMA_symbol = frequency (Transmission_Matrix);

% Frequency has to be a routine where you select each OFDMA symbol, do the
% IFFT process and send it.  Remember you have to add the cyclic prefix before send it!!

matrix = OFDMA_symbol;