% Frequency error estimation and correction
function [out_signal, freq_est] = rx_frequency_sync(rxsignal, sim_options)

global sim_consts;%在set_sim_consts中定義的全局變量

[n_tx_antennas, n_rx_antennas] = get_n_antennas(sim_options);%獲取天線分集參數

% Estimate the frequency error
if sim_options.FreqSync   %如果選中頻率同步選項，執行語句
   
   % allows for error in packet detection
   pkt_det_offset = 30;  %在分組檢測中允許的錯誤
   
   % averaging length
   rlen = 128;   %計算的窗口長度L
   
   % short training symbol periodicity
   D = 16;    %短訓練符號的周期
      
   phase = rxsignal(:,pkt_det_offset:pkt_det_offset+rlen-D).* ...
      conj(rxsignal(:,pkt_det_offset+D:pkt_det_offset+rlen));%計算rn乘以(rn+D)*,
   
   % add all estimates 
   phase = sum(phase, 2);   %實現窗口長度內的累加

   % with rx diversity combine antennas
   phase = sum(phase, 1);   %考慮到接收分集情況
   
   freq_est = -angle(phase) / (2*D*pi/sim_consts.SampFreq);%根據z與f的關系計算頻率誤差f,1/sim_consts.SampFreq即為采樣時間Ts
   
   radians_per_sample = 2*pi*freq_est/sim_consts.SampFreq;%計算每個符號產生的頻偏
else   %如果沒有選中頻率同步，則賦予默認值
   % Magic number
   freq_est = -sim_options.FreqError;%根據輸入決定默認值
   radians_per_sample = 2*pi*freq_est/sim_consts.SampFreq;
end

% Now create a signal that has the frequency offset in the other direction
siglen=length(rxsignal(1,:));%接收信號的長度
time_base=0:siglen-1;%序列n
correction_signal=repmat(exp(-j*(radians_per_sample)*time_base),n_rx_antennas,1);%獲取頻偏校正矩陣

% And finally apply correction on the signal
out_signal = rxsignal.*correction_signal;%根據頻偏計算正確接收符號