rx_find_packet_edge.m

MATLAB Simulation of OFDM System

function [flag, thres_idx, detected_packet] = rx_find_packet_edge(rx_signal, sim_options)

global sim_consts;

search_win = 800;  %注意在接收信号之前加入了sim_consts.ExtraNoiseSamples（=500)的噪声
D = 16;%短训练符号的周期。
flag=1;  %用于判决是否正确地进行了分组检测

if sim_options.PacketDetection
   rx_len = length(rx_signal);
   
   % 计算延迟相关。
   delay_xcorr = rx_signal(:,1:search_win+2*D).*conj(rx_signal(:,1*D+1:search_win+3*D));
   
   % 延迟相关的滑动平均
   ma_delay_xcorr = abs(filter(ones(1,2*D), 1, delay_xcorr, [], 2));
   %  L=2*D 参考《OFDM无线局域网》第36页（2.8)式。
   % 计算互相关系数窗口期间接收信号的能量。此值用于判决统计的归一化。
 %  ma_rx_pwr = filter(ones(1,2*D), 1, abs(rx_signal(:,1*D+1:search_win+3*D)).^2,[], 2);
   ma_rx_pwr = filter(ones(1,2*D), 1,abs(rx_signal(:,1:search_win+2*D)).^2+abs(rx_signal(:,1*D+1:search_win+3*D)).^2,[], 2);
  %//////////////////////////////////////////////////
   %Y = FILTER(B,A,X) filters the data in vector X with the
    %filter described by vectors A and B to create the filtered
    %data Y.  The filter is a "Direct Form II Transposed"
  %  implementation of the standard difference equation:
    %a(1)*y(n) = b(1)*x(n) + b(2)*x(n-1) + ... + b(nb+1)*x(n-nb)
     %                     - a(2)*y(n-1) - ... - a(na+1)*y(n-na)
     %FILTER(B,A,X,[],DIM) or FILTER(B,A,X,Zi,DIM) operates along the
    %dimension DIM.
    %故此处实际上是计算窗口L＝32长度内的延迟相关的累加，即(2.8)式。
     %//////////////////////////////////
   % 统计判决变量
   delay_len = length(ma_delay_xcorr);
%   ma_M = ma_delay_xcorr(:,1:delay_len)./ma_rx_pwr(:,1:delay_len);
  ma_M = 2*ma_delay_xcorr(:,1:delay_len)./ma_rx_pwr(:,1:delay_len); 
         
   % 移除延迟的部分
   ma_M(:,1:2*D) = [];
   % 如果采用了接收分集
   ma_M = sum(ma_M, 1);
   
   if ~sim_options.UseRxDiv
      threshold = 0.75; %如果没有使用接收分集，则设门限值为0.75
   else
      threshold = 1.5;
   end
   
   thres_idx = find(ma_M > threshold);
   if isempty(thres_idx)
      thres_idx = 1;% 如果thres_idx为空，则设thres_idx = 1。
   else
      thres_idx = thres_idx(1);%定义数据分组开始时刻为第一个大于门限值的ma_M的下标。
   end
   
else
   thres_idx = sim_consts.ExtraNoiseSamples;
end;

% 检查分组是否检测得太迟或者由于噪声而使判决变量超过门限
if (thres_idx > sim_consts.ExtraNoiseSamples + 35) | (thres_idx < 475)
   thres_idx = 1;
end

detected_packet = rx_signal(:,thres_idx:length(rx_signal));

if thres_idx ==1  %没有正确地检测到分组
    flag=0;
end
    
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