function [transmit_signal, known_training] = ofdm_mod(st_coded,PrefixRatio,N_subc,N_sym,...
                            N_used,Idx_used,N_Tx_ant,N_tran_sym)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 實現OFDM的基本調制

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

transmit_signal = zeros(1,N_subc*N_sym*(1+PrefixRatio),N_Tx_ant);
cp_len = round(PrefixRatio*N_subc);

for ant = 1:N_Tx_ant
    % ifft乘sqrt(N_subc)以保證變換前后能量不變
    % 我們假設頻域的樣點是在[-fs/2  fs/2]中的, fs是采樣頻率
    % 使用fftshift函數目的是使得變換前的頻域樣點轉換到[0 fs]中,以滿足IFFT變換的要求
    ofdm_frame = sqrt(N_subc) * ifft( fftshift( st_coded(:,:,ant), 1 ) );
    cp = ofdm_frame(N_subc - cp_len + 1:N_subc ,:);
    ofdm_frame = [cp;ofdm_frame];
    % 加窗處理
    
    
    % 轉換為串行信號
    transmit_signal(:,:,ant) = reshape( ofdm_frame, 1, N_subc*N_sym*(1+PrefixRatio) );    
    
end
   


% 加前導序列,兩個OFDM符號,作為定時同步和信道估計使用
[transmit_signal, known_training] = add_training(transmit_signal,PrefixRatio,N_subc,N_used,...
                        Idx_used,cp_len,N_Tx_ant,N_tran_sym);