function fine_time_est = rx_fine_time_sync(input_signal, sim_options)
%符號定時算法
global sim_consts;

[n_tx_antennas, n_rx_antennas] = get_n_antennas(sim_options);%獲取分集天線數量

if sim_options.FineTimeSync
   %timing search window size，定時搜索窗口長度
   start_search=130;
   end_search=200;
   
   % get time domain long training symbols，獲得時域長訓練符號
   long_tr_symbols = tx_freqd_to_timed(sim_consts.LongTrainingSymbols);%以sim_consts.LongTrainingSymbols為輸入參數調用函數tx_freqd_to_timed
   % 'LongTrainingSymbols', [1 1 -1 -1 1 1 -1 1 -1 1 1 1 1 1 1 -1 -1 1 1 -1 1 -1 1 1 1 1 ...
    %  1 -1 -1 1 1 -1 1 -1 1 -1 -1 -1 -1 -1 1 1 -1 -1 1 -1 1 -1 1 1 1 1], ...
  
   if sim_options.UseTxDiv   %是否使用發射分集，若使用，則執行
      long_trs = [long_tr_symbols(49:64) long_tr_symbols(1:48)];%？？？？？？？
   else                      %沒使用，則執行
      long_trs = [long_tr_symbols(33:64) long_tr_symbols(1:32)];%？？？？？
   end
   
   time_corr_long = zeros(n_rx_antennas,end_search-start_search+1);%zeros(接收天線數量，200-130+1）
   
   for k=1:n_rx_antennas   
      % calculate cross correlation ，計算互相關     
      for idx=start_search:end_search  %130:200
         time_corr_long(k,idx-start_search+1) = sum((input_signal(k,idx:idx+63).*conj(long_trs)));%從信號的第130列開始計算p39的式子，到第200列結束計算，每次循環計算的窗口長度L為64，搜索長度為71.
      end
   end
   
   % combine, if we had two antennas
   time_corr_long = sum(abs(time_corr_long),1);%取絕對值后，按列相加，得行向量
   %figure(3);
   %plot(time_corr_long)
   %grid on
   [max_peak_long,long_search_idx] = max(abs(time_corr_long));%將行向量中的最大值賦予max_peak_long，其索引賦予long_search_idx
   
   if sim_options.UseTxDiv
      fine_time_est = start_search-1 + long_search_idx+16;%定時時間計算，加上開始搜索的時間
   else
      fine_time_est = start_search-1 + long_search_idx+32;
   end
   
else    %如果沒有選中定時同步選項，則設置默認值
   % Magic numbers
   if ~sim_options.UseTxDiv
      fine_time_est = 194;
   else
      fine_time_est = 194-16;
   end
end