schmidl.m

ofdm符号同步算法-schmidl&cox算法的matlab实现

%********************schmidl algorithm*******************
%*******************by:sk 2008-5-22**********************
close all;
clear all;
clc;
%参数定义
N=256;       %FFT/IFFT 变换的点数或者子载波个数（Nu<=N）
Ng=N/8;      %循环前缀的长度 (保护间隔的长度)
Ns=Ng+N;     %包括循环前缀的符号长度

%************利用查表法生成复随机序列**********************
QAMTable=[7+7i,-7+7i,-7-7i,7-7i];
buf=QAMTable(randint(N/2,1,4)+1);

%*************在奇数子载波的位置插入零*********************
x=zeros(N,1);
index = 1;
for n=1:2:N
     x(n)=buf(index);
     index=index+1;
end;

%**************利用IFFT变换生成Schmidl训练符号***************
sch = ifft(x);   %[A A]的形式

%*****************添加一个空符号以及一个后缀符号*************
src = QAMTable(randint(N,1,4)+1).';
sym = ifft(src);
sig =[zeros(N,1) sch sym];

%**********************添加循环前缀*************************
tx =[sig(N - Ng +1:N,:);sig];

%***********************经过信道***************************
recv = reshape(tx,1,size(tx,1)*size(tx,2));
recv1 = awgn(recv,1,'measured');
recv2 = awgn(recv,5,'measured');
recv3 = awgn(recv,10,'measured');
%*****************计算符号定时*****************************
P=zeros(1,2*Ns);
R=zeros(1,2*Ns);
P1=zeros(1,2*Ns);
R1=zeros(1,2*Ns);
P2=zeros(1,2*Ns);
R2=zeros(1,2*Ns);
P3=zeros(1,2*Ns);
R3=zeros(1,2*Ns);
for d = Ns/2+1:1:2*Ns
    for m=0:1:N/2-1 
        P(d-Ns/2) = P(d-Ns/2) + conj(recv(d+m))*recv(d+N/2+m); 
        R(d-Ns/2) = R(d-Ns/2) + power(abs(recv(d+N/2+m)),2);
        P1(d-Ns/2) = P1(d-Ns/2) + conj(recv1(d+m))*recv1(d+N/2+m); 
        R1(d-Ns/2) = R1(d-Ns/2) + power(abs(recv1(d+N/2+m)),2);
        P2(d-Ns/2) = P2(d-Ns/2) + conj(recv2(d+m))*recv2(d+N/2+m); 
        R2(d-Ns/2) = R2(d-Ns/2) + power(abs(recv2(d+N/2+m)),2);
        P3(d-Ns/2) = P3(d-Ns/2) + conj(recv3(d+m))*recv3(d+N/2+m); 
        R3(d-Ns/2) = R3(d-Ns/2) + power(abs(recv3(d+N/2+m)),2);
    end
end
M=power(abs(P),2)./power(abs(R),2);
M1=power(abs(P1),2)./power(abs(R1),2);
M2=power(abs(P2),2)./power(abs(R2),2);
M3=power(abs(P3),2)./power(abs(R3),2);

%**********************绘图******************************
figure('Color','w');
d=1:1:400;
plot(d,M(d),d,M1(d),d,M2(d),d,M3(d));
grid on;
axis([0,400,0,1.1]);
title('schmidl algorithm');
xlabel('Time (sample)');
ylabel('Timing Metric');
legend('no noise','SNR=1dB','SNR=5dB','SNR=10dB');