function [s_out] = channel(s_in, SNR)
%% 信道函數
% 模擬多徑信道
% 使用濾波器來模擬信道，也可從時域上進行

global fade;

Len = length(s_in);
f_len = length(fade);
sch = s_in;
for m = 1:f_len
    sch(1+m:Len) = sch(1+m:Len) + fade(m)*(s_in(1:Len-m));
end

%h = [1,1/2,1/4];
%sch = filter(h,1,s_in);

% 高斯信道
Tx_signal_power = var(sch);  % 方差
linear_SNR = 10^( SNR /10) ;
noise_sigma = Tx_signal_power / linear_SNR;
noise_scale_factor = sqrt(noise_sigma) ;
noise = randn(1, length(sch) )*noise_scale_factor;          %模擬信道噪聲,為隨機數,即高斯白噪
s_out = sch + noise;