function [ H_est , MSE ] = estimator_mmse( Recv , Training , Method , N_subc_Remain , H_ideal)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 進行基本的MMSE和MMSE-DFT信道估計
% 計算出均方誤差.

% 輸入:   Recv, 頻域接收到的各個子載波上的復數信號, N_subc(子載波數) 行, N(訓練OFDM符號個數)列的復數向量
%         Training , 發送的訓練序列 , N_subc 行, N 列的向量 
%                    注意: 訓練序列中不能有零, 否則出錯.
%         Method, 信道估計的方法 ,正整數,  1--- MMSE估計, 2 ---MMSE-DFT估計的DFT改進
%         N_subc_Remain, LS估計的DFT改進方法,所保留的子載波數. 為正整數值.
%                        建議選取改值等于最大多徑時延對應的時域樣點數
%         
%
% 輸出:   H_est, 估計得到的各個子載波上的頻域響應, N_subc 行, 1列的復數向量
%         MSE, 本次估計的均方誤差
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


N_subc = size(Recv,1);  % 子載波數
H_est = zeros(N_subc ,1);
N = size(Recv,2);
% 接收信號乘上訓練序列的共軛, 除以訓練序列的模值的平方,得到LS估計的信道響應
H_ls = Recv.*conj(Training)./abs(Training).^2;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
% 如果僅輸入兩個參數
if nargin == 2   
    H_est = H_ls;   %  LS估計
    MSE = [];
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
    % 如果輸入五個參數
elseif nargin == 5
    if  Method == 1 
        H_est = H_ls; %  LS估計
    elseif  Method == 2  %  LS估計DFT改進
        % 首先變換到時域
        time_domain = ifft(fftshift(H_ls,1)) * sqrt(N_subc);
        time_cut = zeros(N_subc,N);
        leakage = 0;       % 能量泄漏的子載波數
        % 保留時域能量集中的N_subc_Remain個樣點
        time_cut( 1: N_subc_Remain,: ) = time_domain( 1: N_subc_Remain,: );
        % 保留能量泄漏的leakage個樣點
        time_cut( N_subc - leakage : N_subc,: ) = time_domain( N_subc - leakage : N_subc,: );
        % 變換到頻域
        H_ls_dft = fftshift(fft(time_cut) * 1/sqrt(N_subc),1);
        H_est = H_ls_dft;
    else
        error('子程序estimator輸入參數不匹配!');
    end
    % 計算均方誤差
    diff = H_ideal - H_est;
    MSE = sum(sum( abs(diff).^2 )) / (N_subc*N) ;
end