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(yīng),為自適應(yīng)調(diào)制算法提供數(shù)據(jù),也為接收機(jī)的信道估計(jì)器提供已知的頻域信道響應(yīng).
% 這兩類(lèi)算法,在我們的B3G系統(tǒng)設(shè)計(jì)中,都是假設(shè)一幀/數(shù)據(jù)包中信道不變進(jìn)行的. 因此我們只需要提供
% 第一個(gè)OFDM符號(hào)內(nèi)的信道響應(yīng)即可.
% 然而在頻域加信道響應(yīng)時(shí), 最好把時(shí)變也考慮進(jìn)去, 所以此處還是產(chǎn)生了所有OFDM符號(hào)內(nèi)的頻域響應(yīng).

% 時(shí)域信道響應(yīng)H_freq的產(chǎn)生方法如下:
% 1) 按照ch.Delay_sample調(diào)整列元素的位置,并使得列長(zhǎng)為N_subc
% 2) 矩陣第三維仍然表示不同天線對(duì)之間的信道參數(shù), 共N_Tx_ant * N_Rx_ant個(gè)
% 3) 把列變換到頻域, 并按照頻域信道位置, 用fftshift進(jìn)行重排, 得到頻移信道響應(yīng)

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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(shè)頻域的樣點(diǎn)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(diǎn)在[-fs/2  fs/2]中,而不是[0 2*pi]中
    % 以保證頻域信道響應(yīng)的零頻率點(diǎn)，對(duì)應(yīng)OFDM的零號(hào)（中心）子載波
    H_freq = fftshift( fft( H_freq ), 1 ) ;
    
    if ChannelEffectTest == 2
        H_freq = ones(size(H_freq));
    end