trigger.m

这个程序主要是实现空时编码的matlab编解码仿真。

function [index_power_burst,index_block0,w_SNR_vec]=trigger(receive,sync,window_length,fs,T)

%   [index_power_burst,index_block0,w_SNR_vec]=trigger(receive,sync,window_length,fs,T)
%
%	Output:
%	index_power_burst   -   start-index for the block in which the powerburst is found 
%   index_block0        -   start-index for the first block
%   w_SNR_vec           -   vector with the SNR for each window
%
%   Input:
%	receive             -   the received signal after channel 
%   sync                -   the sync vector as sent in the transmitter
%   window_length       -   the length of each block  
%   fs                  -   sampling frequency 
%   T                   -   symbol period
%
%   Short Theoretical Background for the Function:
%
%   The trigger is used to know when a transmission starts and to synchronize the received 
%   blocks with the transmitted block. 
%   Each transmission is split into a number of blocks and begins with a powerburst (a block of ones) 
%   followed by a gard and a known synchronization sequence.
%   The trigger searches for the powerburst and when the powerburst is found it knows in 
%   wich block the powerburst starts.  Then it correlates the following two blocks with the known 
%   synchronization sequence to find the beginning of the first symbolblock. The receiver now knows
%   when the first symbolblock arrives and samples the received signal according to this.
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     
%%% Function part of simulation for Space-Time
%%% coding project, group Grey-2001.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%   Author: Fredrik & Karl
%   Date: 2001-04-27
%   Version: 1.0
%   Revision (Name & Date & Comment):
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


flag =1;
n = 0;
gard_length = window_length;
noise_power = mean(abs(receive(1:window_length).^2)); 
index_power_burst = 0;
index_block0 = 0;
w_SNR_vec =[];
sum_sync1 =[];
sum_sync2 =[];

 ref_burst = ones(1,fs*T*(window_length+10));
 ref = carrier(ref_burst,2500,fs,T);
 treshold = mean(ref.^2)/6;  
   

while n < length(receive)/window_length - 1   % steps through each block 
    
    window = receive(n*window_length+1:(n+1)*window_length);    % the current block 
    window_SNR = (mean(abs(window.^2)))- noise_power;
    w_SNR_vec = [w_SNR_vec window_SNR];
    
    n = n + 1;
    
    if window_SNR > treshold & flag == 1    % in case the powerburst is found
        index_power_burst = (n-1)*window_length;  
        flag = 0;                             % since this loop is to be entered once only
        window_corr1 = [zeros(1,length(sync)) receive((n+1)*window_length+1:(n+2)*window_length) zeros(1,length(sync))]; % the first block with zeros in beginning and end
      
      
        for(m=1:1:(window_length + length(sync)))     % m = sync_start slides from beginning of block to length(block) + length(sync).
            sum_sync1 = [sum_sync1 (window_corr1(m : length(sync) + m-1) * sync')];
        end
    
        window_corr2 = [zeros(1,length(sync)) receive((n+2)*window_length+1:(n+3)*window_length) zeros(1,length(sync))]; % the second block
      
        for(m=1:1:(window_length + length(sync)))     % m = sync_start slides from beginning of block to length(block) + length(sync).
            sum_sync2 = [sum_sync2 (window_corr2(m : length(sync) + m-1) * sync')];
        end
    
        [corr_max1,sync_start1] = max(abs(sum_sync1))
        [corr_max2,sync_start2] = max(abs(sum_sync2))
        ref_sync =  sync(1:window_length)*sync(1:window_length)'
        ref_factor = 3;
        if corr_max1 < (ref_sync / ref_factor) &  corr_max2 < (ref_sync / ref_factor)
            disp('No valid power burst');
            break
        elseif corr_max1 > corr_max2     % if sync is found in the first block                      
            correction = sync_start1;                 % the difference between the start of the transmitted block and the start of the sampled block of the received signal  
            %index_block0 = (n+1)*window_length + correction + gard_length;  % startindex for the first symbolblock 
            index_block0 = (n+1)*window_length + correction + length(sync);     
        else
            correction = sync_start2;                 % if sync is found in the second block  
            %index_block0 = (n+2)*window_length + correction + gard_length; % startindex for the first symbolblock     
            index_block0 = (n+2)*window_length + correction + length(sync);
        end
    end

end