function H_freq = to_freq_channel(h_time,ch ,N_subc,N_sym, N_Tx_ant, N_Rx_ant,ChannelEffectTest)

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% 得到頻域信道響應,為自適應調制算法提供數據,也為接收機的信道估計器提供已知的頻域信道響應.
% 這兩類算法,在我們的B3G系統設計中,都是假設一幀/數據包中信道不變進行的. 因此我們只需要提供
% 第一個OFDM符號內的信道響應即可.
% 然而在頻域加信道響應時, 最好把時變也考慮進去, 所以此處還是產生了所有OFDM符號內的頻域響應.

% 時域信道響應H_freq的產生方法如下:
% 1) 按照ch.Delay_sample調整列元素的位置,并使得列長為N_subc
% 2) 矩陣第三維仍然表示不同天線對之間的信道參數, 共N_Tx_ant * N_Rx_ant個
% 3) 把列變換到頻域, 并按照頻域信道位置, 用fftshift進行重排, 得到頻移信道響應

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    H_freq = zeros(N_subc ,N_sym , N_Tx_ant * N_Rx_ant );

    H_freq(ch.Delay_sample + 1,:,:) = h_time;
    
    % 我們假設頻域的樣點是在[-fs/2  fs/2]中的, fs是采樣頻率
    % fftshift目的是使得變換后的頻域樣點在[-fs/2  fs/2]中,而不是[0 2*pi]中
    % 以保證頻域信道響應的零頻率點，對應OFDM的零號（中心）子載波
    H_freq = fftshift( fft( H_freq ), 1 ) ;
    
    if ChannelEffectTest == 2
        H_freq = ones(size(H_freq));
    end