awgnchannel.m

本程序模拟了无线通信中常用的AWGN

fc=50;                    % carrier frequency
Fs=40;   %系统采样频率
Fd=1;    %码速率
num=20;                   % number of symbols
data=rand(1,num)>0.5;    %产生20个（0，1）随机数
source_qpsk=2*data-1;    %把（0，1）变换成（-1，1）
figure(1);               %画出原始信号的阶梯图
stairs(source_qpsk,'g');
    title('the source single of QPSK');

%%%%串并转换
% %%function [Ichannel,Qchannel] = SerialToParallel(source_qpsk)
Ichannel=source_qpsk(1:2:length(source_qpsk));
Qchannel =source_qpsk(2:2:length(source_qpsk));
%%%%通过成形滤波器  
%%%%figure(2）figure(3)画的是是进过滤波器后的信号Ichannel_shape，Qchannel_shape
figure(2);
Ichannel_shape =rcosflt(Ichannel, Fd, Fs, 'fir', 0.5, 3);
plot(Ichannel_shape);
title('the  Ichannel single of QPSK being filtered by shape filter');
 figure(3);
Qchannel_shape=rcosflt(Qchannel, Fd, Fs,' fir', 0.5, 3);
plot(Qchannel_shape);
title('the  Qchannel single of QPSK  being  filtered by shape filter');
%%%%调制信号
t=0:(1/639):1;  %%%t是根据Ichannel_shape的点数确定的，因为乘载波时要点乘，行列要对称
%%% figure(4)， figure(5)画的是经过调制的信号Ichannel_signal，Qchannel_signal
 figure(4);
Ichannel_signal=Ichannel_shape'.*cos(2*pi*fc*t);
plot(Ichannel_signal);
title('the  Ichannel single of QPSK being modulated');
 figure(5);
Qchannel_signal=Qchannel_shape'.*sin(2*pi*fc*t);
plot(Qchannel_signal);
title('the  Qchannel single of QPSK being  modulated');
%%% figure(6)是在要传输的QPSK信号
 figure(6);
qpsk_single=Ichannel_signal+Qchannel_signal;
plot(t,qpsk_single);
title('the   single of QPSK to be translated  t0 AWGN channel' );



%%%%信道具有AWGN干扰的情况.
%%%%算法一：利用awgn()函数，主要通过调制其中的信噪比参数。
%%%%但信噪比大于一定的数值，即当噪声低到某一定值，还是可以正确判决。
%%%%算法二：I和Q信道分别加上（-0.2，0.2）的随机数（此数值可变），观察噪声对判决的影响；
%%%%这里用算法一算。
 figure(7);
Ichannel_awgnsignal= awgn(Ichannel_signal,12,'measured'); % Add white Gaussian noise to  Ichannel_signal
plot(t,Ichannel_awgnsignal);
title('the  Ichannel single of QPSK being added by Gaussian noise in the channel');
  figure(8);
Qchannel_awgnsignal= awgn(Qchannel_signal,12,'measured'); % Add white Gaussian noise to  Qchannel_signal.
plot(t,Qchannel_awgnsignal);
title('the  Qchannel single of QPSK being added by Gaussian noise');
 figure(9);

qpsk_awgnsingle=Ichannel_awgnsignal+Qchannel_awgnsignal;
plot(t,qpsk_awgnsingle);
title('the   single of QPSK being added by Gaussian noise  in the channel');





%%%%解调部分
%%%%%%% figure(10)， figure(11)画的是经过解调后的信号demoIchannel_signal，demoQchannel_signal
 figure(10)
demoIchannel_signal=Ichannel_awgnsignal.*cos(2*pi*fc*t);
plot(demoIchannel_signal);
title('the  Ichannel single of QPSK being demodulated');
 figure(11)
demoQchannel_signal=Qchannel_awgnsignal.*sin(2*pi*fc*t);
plot(demoQchannel_signal);
title('the  Qchannel single of QPSK being demodulated');
%%%%经过低通滤波器 

%%% figure(12)，figure(13)画的是经过低通滤波器的信号demoIchannel_shape，demoQchannel_shape
figure(12);
demoIchannel_shape =rcosflt(demoIchannel_signal, Fd, Fs, 'fir', 0.5, 3);
plot(demoIchannel_shape );
title('the  Ichannel single of QPSK being filtered by LPF');
 figure(13);
demoQchannel_shape=rcosflt(demoQchannel_signal, Fd, Fs,' fir', 0.5, 3);
plot(demoQchannel_shape);
title('the  Qchannel single of QPSK being filtered by LPF');
%%%%判决
s=length(demoIchannel_shape);
%%%%因为经过低通滤波器，必然造成输出信号出现拖尾的残留信号，这些对判决是不利的，应该除去；
%%%%这个步骤是判决能否取得正确结果的关键；
for w=s:-1:1;%%%这个循环主要把信号的末尾部分的残留信号去掉。
  if ((demoIchannel_shape(w)>0.25)||(demoIchannel_shape(w)<-0.25))
 s=w;
break;
  end;
end;
h=0;
for x=1:s;%%%%这个循环主要把信号的开始部分的残留信号去掉。
  if ((demoIchannel_shape(x)>0.25)||(demoIchannel_shape(x)<-0.25))
 h=x;
break;
  end;
  end;
  s=s-h;
  %%开始判决
  %%算法是：因为每个信道有10个码元，所以把有效信号的所以点数分成十段，
  %%对每一段分布判决。这里设门限值为0，设两个计数器m,n，对所有点进行判断，当
  %%某点大于0判为1,否则为-1；
  %%对I信道；
 for i=1:10;
    m=0;
    n=0;
    for j=floor(((i-1)*s/10+x)):floor(((i*s))/10+x);%%%注意这个循环中始末位置的选择
        if  demoIchannel_shape(j)>0;
            m=m+1;
        else if  demoIchannel_shape(j)<0;
                n=n+1;
            end;    
        end;
        if m>n   a(i)=1;
        else a(i)=-1;
        end;
     end;
end;
%%%对Q信道
for i=1:10;
    p=0;
    q=0;
    for j=floor(((i-1)*s/10+x)):floor(((i*s))/10+x);
        if  demoQchannel_shape(j)>0;
           p=p+1;
        else if  demoQchannel_shape(j)<0;
                q=q+1;
            end;    
        end;
        if p>q   b(i)=1;
        else b(i)=-1;
        end;
     end;
end;
%%%% 并串转换
for i=1:10;
    enddata(2*i-1)=a(i);
    enddata(2*i)=b(i);
end
%%%画出最后的解调的最后结果并且与输入值即 figure(1)作比较，即可判断这个程序的成功与否；
 figure(14);
stairs(enddata,'r');
title('the resumed   single of QPSK in AWGN channel ');