QPSK调制解调完整程序（配有注释）clc;clear all;%假定接收端已经实现载波同步，位同步（盲信号解调重点要解决的问题：载波同步（costas环（未见到相关代码）），位同步（Gardner算法（未见相关代码）），帧同步）% carrier frequency for modulation and demodulationfc=5e6;%QPSK transmitterdata=5000 ; %码数率为5MHZ %原码个数rand_data=randn(1,5000);for i=1:dataif rand_data(i)>=0.5rand_data(i)=1;elserand_data(i)=0;endend%seriel to parallel %同时单极性码转为双极性码for i=1:dataif rem(i,2)==1if rand_data(i)==1I(i)=1;I(i+1)=1;elseI(i)=-1;I(i+1)=-1;endelseif rand_data(i)==1Q(i-1)=1;Q(i)=1;elseQ(i-1)=-1;Q(i)=-1;endendend% zero insertion ，此过程称为成形。

成形的意思就是实现由消息到波形的转换，以便发射，脉冲成形应该是在基带调制之后。

zero=5; %sampling rate 25M HZ ,明白了，zero为过采样率。

它等于采样率fs/码速率。

for i=1:zero*data % 采样点数目=过采样率*原码数目 if rem(i,zero)==1Izero(i)=I(fix((i-1)/zero)+1);Qzero(i)=Q(fix((i-1)/zero)+1);elseIzero(i)=0;Qzero(i)=0;endend%pulse shape filter，接着，将进行低通滤波，因为随着传输速率的增大，基带脉冲的频谱将变宽%如果不滤波（如升余弦滤波）进行低通滤波，后面加载频的时候可能会出现困难。

%平方根升余弦滤波器% psf=rcosfir(rf,n_t,rate,fs,'sqrt') rate:过采样率，rf:滚降因子，n_t:滤波器阶数，fs:采样率%用在调制或发送之前，用在解调或接受之后，用来降低过采样符号流带宽并不引发ISI（码间串扰）NT=50;N=2*zero*NT; % =500fs=25e6;rf=0.1;psf=rcosfir(rf,NT,zero,fs,'sqrt');% psf大小为500Ipulse=conv(Izero,psf);Qpulse=conv(Qzero,psf);%为什么数字信号传输也要过采样，成形滤波？%答：过采样的数字信号处理起来对低通滤波器的要求相对较低，如果不过采样，滤波的时候滤波器需要很陡峭，指标会很严格%成形滤波的作用是保证采样点不失真。

如果没有它，那信号在经过带限信道后，眼图张不开，ISI非常严重。

成形滤波的位置在基带调制之后。

%因为经成形滤波后，信号的信息已经有所损失，这也是为避免ISI付出的代价。

换句话说，成形滤波的位置在载波调制之前，仅挨着载波调制。

%即：（发送端）插值（采样）-成形-滤波（LPF)-加载频(载波调制)-加噪声至（接收端）乘本振-低通-定时抽取-判决。

%modulationfor i=1:zero*data+N %采样点数目改变（因为卷积的缘故） t(i)=(i-1)/(fs); %这里因为假设载频与码速率大小相等，所以用载频fc乘以过采样率=采样率。

Imod(i)=Ipulse(i)*sqrt(2)*cos(2*pi*fc*t(i));Qmod(i)=Qpulse(i)*(-sqrt(2)*sin(2*pi*fc*t(i)));endsum=Imod+Qmod;%QPSK receiver%demodulationfor i=1:zero*data+NIdem(i)=sum(i)*sqrt(2)*cos(2*pi*fc*t(i));Qdem(i)=sum(i)*(-sqrt(2)*sin(2*pi*fc*t(i)));end%matched filtermtf=rcosfir(rf,NT,zero,fs,'sqrt');Imat=conv(Idem,mtf);Qmat=conv(Qdem,mtf);%data selectionfor i=1:zero*dataIsel(i)=Imat(i+N);Qsel(i)=Qmat(i+N);end%sampler %提取码元for i=1:dataIsam(i)=Isel((i-1)*zero+1);Qsam(i)=Qsel((i-1)*zero+1);end%decision thresholdthreshold=0.2;for i=1:dataif Isam(i)>=thresholdIfinal(i)=1;elseIfinal(i)=-1;endif Qsam(i)>=thresholdQfinal(i)=1;elseQfinal(i)=-1;endend%parallel to serialfor i=1:dataif rem (i,2)==1if Ifinal(i)==1final(i)=1;elsefinal(i)=0;endelseif Qfinal(i)==1final(i)=1;elsefinal(i)=0;endendend% 绘图figure(1)plot(20*log(abs(fft(rand_data))));axis([0 data -40 100]);grid on;title('spectrum of input binary data');figure(2)subplot(221);plot(20*log(abs(fft(I))));axis([0 data -40 140]);grid on;title('spectrum of I-channel data');subplot(222);plot(20*log(abs(fft(Q))));axis([0 data -40 140]);grid on;title('spectrum of Q-channel data');subplot(223);plot(20*log(abs(fft(Izero))));axis([0 zero*data -20 140]);grid on;title('spectrum of I-channel after zero insertion');subplot(224);plot(20*log(abs(fft(Qzero))));axis([0 zero*data -20 140]);grid on;title('spectrum of Q-channel after zero insertion');figure(3);subplot(221);plot(psf);axis([200 300 -0.2 0.6]);title('time domain response of pulse shaping filter');grid on;subplot(222);plot(20*log(abs(fft(psf))));axis([0 N -350 50]);grid on;title('transfer function of pulse shaping filter');subplot(223);plot(20*log(abs(fft(Ipulse))));axis([0 zero*data+N -250 150]);grid on;title('spectrum of I-channel after impulse shaping filter'); subplot(224);plot(20*log(abs(fft(Qpulse))));axis([0 zero*data+N -250 150]);grid on;title('spectrum of Q-channel after pluse shaping filter'); figure(4)subplot(211);plot(20*log(abs(fft(Imod))));axis([0 zero*data+N -250 150]);grid on ;title('spectrum of I-channel after modulation');subplot(212);plot(20*log(abs(fft(Qmod))));axis([0 zero*data+N -250 150]);grid on;title('spectrum of Q-channel after modulation');figure(5)subplot(221);plot(20*log(abs(fft(Idem))));axis([0 zero*data -200 150]);grid on;title('spectrum of I-channel after demodulation');subplot(222);plot(20*log(abs(fft(Qdem))));axis([0 zero*data+N -200 150 ]);grid on;title('spectrum of Q-channel after demodulation');subplot(223);plot(20*log(abs(fft(Imat))));axis([0 zero*data -400 200]);grid on;title('spectrum of I-channel after matched filter'); subplot(224);plot(20*log(abs(fft(Qmat))));axis([0 zero*data -400 200]);grid on;title('spectrum of Q-channel after matched filter');figure(6)subplot(221);plot(20*log(abs(fft(Isam))));axis([0 data -40 150]);grid on;title('spectrum of I-channel after sampler');subplot(222);plot(20*log(abs(fft(Qsam))));axis([0 data -40 150 ]);grid on;title('spectrum of Q-channel after sampler');subplot(223);plot(20*log(abs(fft(Ifinal))));axis([0 data -40 150]);grid on;title('spectrum of I-channel after decision threshold'); subplot(224);plot(20*log(abs(fft(Qfinal))));axis([0 data -40 150]);grid on;title('spectrum of Q-channel after decision threshold'); figure(7)plot(Isel,Qsel);axis([-1.6 1.6 -1.6 1.6]);grid on;title('constellation of matched filter output'); figure(8)plot(Isam,Qsam,'X');axis([-1.2 1.2 -1.2 1.2]);grid on;title('constellation of sampler');figure(9)plot(20*log(abs(fft(final))));axis([0 data 0 100]);grid on;title('aspectrum of final received binary data');。