QPSK调制解调完整程序配有自己的注释.docx

1、QPSK调制解调完整程序配有自己的注释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 demodulation fc=5e6; %QPSK transmitter data=5000 ; %码数率为5MHZ %原码个数 rand_data=randn(1,5000); for i=1:data if rand_data(i)=0.5

2、rand_data(i)=1; else rand_data(i)=0; end end %seriel to parallel %同时单极性码转为双极性码 for i=1:data if rem(i,2)=1 if rand_data(i)=1 I(i)=1; I(i+1)=1; else I(i)=-1; I(i+1)=-1; end else if rand_data(i)=1 Q(i-1)=1; Q(i)=1; else Q(i-1)=-1; Q(i)=-1; end end end % 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

3、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)=1 Izero(i)=I(fix(i-1)/zero)+1); Qzero(i)=Q(fix(i-1)/zero)+1); else Izero(i)=0; Qzero(i)=0; end end %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

4、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; % =500 fs=25e6; rf=0.1; psf=rcosfir(rf,NT,zero,fs,sqrt);% psf大小为500 Ipulse=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

5、? %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。 %modulation for i=1:zero*data+N %采样点数目改变(因为卷积的缘故

6、) 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); end sum=Imod+Qmod; %QPSK receiver %demodulation for i=1:zero*data+N Idem(i)=sum(i)*sqrt(2)*cos(2*pi*fc*t(i); Qdem(i)=sum(i)*(-sqrt(2)*sin(2*pi*fc*t(i); e

7、nd %matched filter mtf=rcosfir(rf,NT,zero,fs,sqrt); Imat=conv(Idem,mtf); Qmat=conv(Qdem,mtf); %data selection for i=1:zero*data Isel(i)=Imat(i+N); Qsel(i)=Qmat(i+N); end %sampler %提取码元 for i=1:data Isam(i)=Isel(i-1)*zero+1); Qsam(i)=Qsel(i-1)*zero+1); end %decision threshold threshold=0.2; for i=1:d

8、ata if Isam(i)=threshold Ifinal(i)=1; else Ifinal(i)=-1; end if Qsam(i)=threshold Qfinal(i)=1; else Qfinal(i)=-1; end end %parallel to serial for i=1:data if rem (i,2)=1 if Ifinal(i)=1 final(i)=1; else final(i)=0; end else if Qfinal(i)=1 final(i)=1; else final(i)=0; end end end % 绘图 figure(1) plot(2

9、0*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-ch

10、annel 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

11、(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(s

12、pectrum 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

13、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

14、(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);