<matalb> CDMA信道仿真-AWGN多徑衰落信道
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%                                                                                                         % 
%  該函數仿真CDMA系統的信道，在輸入所需用戶序列和其他干擾用戶序列后     %
%  在信號上疊加高斯白噪聲，干擾用戶多徑干擾和所需用戶的多徑干擾和衰落。 %  
%  并根據輸入延時曲線產生信道延時，輸出mul_fad_sequence為信道信號輸出  %
%  max_delay為信道最大延遲，fade_sign為瑞利信道中最大幅度分量的衰落量   %
%  mpath_amp為信道多徑的幅度因子                                                        %
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function [mul_fad_sequence,max_delay,fade_sign,rake_rev_sequence,mpath_amp] = cdma_channel_simulation(...
    sequence,sf,mpath_delay,eb_no,k_factor)
% sequence      發射機的輸出序列
% sf            擴頻因子
% mpath_delay   多徑傳輸延時
% snr           信道信噪比
% k_factor      萊斯k因子
mpath_delay = sort(mpath_delay);                                  % 保證多徑延時按照升序排列
eb_no_linear = 10^(eb_no/10);                                     % 轉換信號噪聲比
[signal_row,signal_len] = size(sequence);
if signal_row == 1                                                        % 初始化干擾用戶數目
    interfering_num = 0;
else
    interfering_num = signal_row - 1;
end
%%                                                              
mpath_len = length(mpath_delay);                                % 多徑成分，得到多徑延遲曲線的長度
mpath_amp(2:mpath_len) = rand(1,mpath_len-1);           % 隨機生成多徑成分幅度
scatter_power = mpath_amp*mpath_amp';                    % 多徑成分離散功率
mpath_amp(1) = sqrt(scatter_power*k_factor);              % 視距成分的幅度(LOS)
[max_mpath_amp,max_index] = max(mpath_amp);          % 得到多徑分量的最大幅度
mpath_amp = mpath_amp/max_mpath_amp;           % 歸一化多徑衰落幅度，最大幅度因子為1
 
% 設計衰落信號的IIR濾波器
FilterOrder = 4; Ripple = 0.5; BW = 0.01;               % 濾波器階數為4，通帶衰減為0.5，帶寬0.01
[b,a] = cheby1(FilterOrder,Ripple,BW);                   % 得到切比雪夫1型濾波器的參數
%%
%生成信道噪聲
noise_amplitude = sqrt(sf/(2*eb_no_linear));          
                                 % 計算噪聲的強度，如果擴頻因子越大，信號對擴頻序列的影響也越大
                                 % 信道噪聲與信噪比成反比，信噪比越大，噪聲幅度越小
max_delay = max(mpath_delay);                          % 得到最大延時
len = signal_len;
desired_noise = noise_amplitude*randn(1,len+max_delay);                 
                               % 所需的噪聲序列，長度要比信源序列的長度長最大延時單位
rake_desired_noise = desired_noise(1:len); 
%%
mpath_len = length(mpath_delay);                               % 多徑成分的向量長度
interfering_sequence = sequence(2:(interfering_num+1),:);
if interfering_num > 0                                                  % 如果存在干擾器
    interfering_signal = zeros(interfering_num,len+max_delay);          % 初始化干擾器的信號
    rake_interfering_signal = zeros(mpath_len,len);
    for k = 1:mpath_len                                             % 利用多徑延時曲線成員的數量開始循環
        rake_interfering_signal(k,:) = sum(mpath_amp(k)*...
            interfering_sequence,1);                                    % 多徑疊加，但消除延遲
        index = 1+mpath_delay(k):len+mpath_delay(k);        % 設置索引號
        interfering_signal(:,index) = ...                                % 多徑干擾信號采用加性疊加的方式
            interfering_signal(:,index) + ...
            mpath_amp(k)*interfering_sequence;       % mpath_amp相當于干擾碼序列的權重因子
    end
end
%%
zf = zeros(FilterOrder,mpath_len);
desired_sequence = sequence(1,:);
desired_mpath_signal = zeros(1,len+max_delay);                          % 初始化所需的多徑信號
rake_mpath_signal = zeros(mpath_len,len);
for k = 1:mpath_len                                          
    if k == 1
        fading = ones(1,len);
    else
        fading = randn(1,len/(2^sf-1)) + k*randn(1,len/(2^sf-1));
                                                                                     % 信道模型，瑞利隨機過程發生器
        [fading zf(:,k)] = filter(b,a,fading,zf(:,k));
        fading = interp(fading,(2^sf-1));
        fading = abs(fading/sqrt(mean(fading.*conj(fading))));
    end
    if k == max_index
        fade_sign = sign(fading);
    end
    rake_mpath_signal(k,:) = (mpath_amp(k)*fading).*desired_sequence;   
                                                                        % rake不含延時的所需用戶的多徑衰落分量
    index = 1+mpath_delay(k):len+mpath_delay(k);
    desired_mpath_signal(index) = ...                                   
                                                                  % 利用瑞利隨機過程發生器生成的隨機衰落序列
        desired_mpath_signal(index)+...                                 % 產生衰減的效果
        (mpath_amp(k)*fading).*desired_sequence;
end
%%
if interfering_num > 0                                                  
% 合并用戶多徑信號，干擾用戶多徑信號和信道高斯白噪聲
    mul_fad_sequence = desired_mpath_signal + ...
        sum(interfering_signal,1) + desired_noise;
    for k = 1:mpath_len
        rake_rev_sequence(k,:) = rake_mpath_signal(k,:) + ...
            rake_interfering_signal(k,:) + rake_desired_noise;
    end
else
    mul_fad_sequence = desired_mpath_signal + desired_noise;
    for k = 1:mpath_len
        rake_rev_sequence(k,:) = rake_mpath_signal(k,:) + rake_desired_noise;
    end
end