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1、QPSK调制与解调在 MATLAB 平台上的实现QPSK即四进制移向键控(Quaternary Phase Shift Keying ),它利用载波的四种不同相位来表示数字信息，由于每一种载波相位代表两个比特信息， 因此每个四进制码元可以用两个二进制码元的组合来表示。两个二进制码元中的前一 个码元用 a表示，后一个码元用 b表示。QPSK信号可以看作两个载波正交2PSK信号的合成，下图表示QPSK正交调制器。由QPSK信号的调制可知，对它的解调可以采用与2PSK信号类似的解调方 法进行解调。解调原理图如下所示，同相支路和正交支路分别采用相干解调方式 解调，得到I(tDQ(t),经过抽样判决和并
2、/用交换器，将上下支路得到的并行 数据恢复成申行数据。输入带通滤波器%调相法clear allclose all t=-1:0.01:7-0.01;tt=length(t);x1=ones(1,800);for i=1:ttif (t(i)=-1 & t(i)=5& t(i)=0 & t1(i)=4& t1(i)0data_recover_a(i:i+19)=1;bit_recover=bit_recover 1;elsedata_recover_a(i:i+19)=-1;bit_recover=bit_recover -1;endenderror=0;dd = -2*bit_in+1;ddd
3、=dd;ddd1=repmat(ddd,20,1);for i=1:2e4ddd2(i)=ddd1(i);endfor i=1:1e3if bit_recover(i)-=ddd(i)error=error+1;endendp=error/1000;figure(1)subplot(2,1,1);plot(t2,ddd2);axis(0 100 -2 2);title(原序歹I );subplot(2,1,2);plot(t2,data_recover_a);axis(0 100 -2 2);title(解调后序列);效果图:原序列%设定T=1,不加噪声clear allclose all%调
4、制bit_in = randint(1e3, 1, 0 1);bit_I = bit_in(1:2:1e3);bit_Q = bit_in(2:2:1e3);data_I = -2*bit_I+1;data_Q = -2*bit_Q+1;data_I1=repmat(data_I,20,1);data_Q1=repmat(data_Q,20,1);for i=1:1e4data_I2(i)=data_I1(i);data_Q2(i)=data_Q1(i);end;t=0:0.1:1e3-0.1;f=0:0,1:1;xrc=0.5+0.5*cos(pi*f);data_I2_rc=conv(da
5、ta_I2,xrc)/5.5;data_Q2_rc=conv(data_Q2,xrc)/5.5;f1=1;t1=0:0.1:1e3+0.9;I_rc=data_I2_rc.*cos(2*pi*f1*t1);Q_rc=data_Q2_rc.*sin(2*pi*f1*t1);QPSK_rc=(sqrt(1/2).*I_rc+sqrt(1/2).*Q_rc);%解调I_demo=QPSK_rc.*cos(2*pi*f1*t1);Q_demo=QPSK_rc.*sin(2*pi*f1*t1);I_recover=conv(I_demo,xrc);Q_recover=conv(Q_demo,xrc);I
6、=I_recover(11:10010);Q=Q_recover(11:10010);t2=OO05:1e3-0.05;t3=OO1:1e3-O1;data_recover=;fo门=1:20:10000data_recover=data_recover I(i:1:i+19) Q(i:1:i+19);end;ddd = -2*bit_in+1;ddd1=repmat(ddd,10,1);for i=1:1e4ddd2(i)=ddd1(i);endfigure(1)subplot(4,1,1);plot(t3,I);axis(0 20 -6 6);subplot(4,1,2);plot(t3,
7、Q);axis(0 20 -6 6);subplot(4,1,3);plot(t2,data_recover);axis(0 20 -6 6);subplot(4,1,4);plot(t,ddd2);axis(0 20 -6 6);效果图：% QPSK误码率分析SNRindB1=0:2:10;SNRindB2=0:0.1:10;for i=1:length(SNRindB1)pb,ps=cm_sm32(SNRindB1(i);smld_bit_err_prb(i)=pb;smld_symbol_err_prb(i)=ps;end;for i=1:length(SNRindB2)SNR=exp(
8、SNRindB2(i)*10g(10)/10);theo_err_prb(i)=Qfunct(sqrt(2*SNR);end;title(QPSK误码率分析);semilogy(SNRindB1,smld_bit_err_prb,*);axis(0 10 10e-8 1);hold on;% semilogy(SNRindB1,smld_symbol_err_prb,o);semilogy(SNRindB2,theo_err_prb);legend(仿真比特误码率,理论比特误码率);hold off;functiony=Qfunct(x)y=(1/2)*erfc(x/sqrt(2);funct
9、ionpb,ps=cm_sm32(SNRindB)N=10000;E=1;SNR=10A(SNRindB/10);sgma=sqrt(E/SNR)/2;s00=1 0;s01=0 1;s11=-1 0;s10=0 -1;for i=1:Ntemp=rand;if (temp0.25)dsource1(i)=0;dsource2(i)=0;elseif (temp0.5)dsource1(i)=0;dsource2(i)=1;elseif (temp0.75)dsource1(i)=1;dsource2(i)=0;elsedsource1(i)=1;dsource2(i)=1;end;end;n
10、umofsymbolerror=0;numofbiterror=0;for i=1:Nn=sgma*randn(size(s00);if(dsource1(i)=0)&(dsource2(i)=0)r=s00+n;elseif(dsource1(i)=0)&(dsource2(i)=1)r=s01+n;elseif(dsource1(i)=1)&(dsource2(i)=0)r=s10+n;elser=s11+n;end;c00=dot(r,s00);c01=dot(r,s01);c10=dot(r,s10);c11=dot(r,s11);c_max=max(c00 c01 c10 c11);
11、if (c00=c_max)decis1=0;decis2=0;elseif(c01=c_max)decis1=0;decis2=1;elseif(c10=c_max)decis1=1;decis2=0;elsedecis1=1;decis2=1;end;symbolerror=0;if(decis1=dsource1(i)numofbiterror=numofbiterror+1;symbolerror=1;end;if(decis2=dsource2(i)numofbiterror=numofbiterror+1;symbolerror=1;end;if(symbolerror=1)numofsymbolerror=numofsymbolerror+1;end;end;ps=numofsymbolerror/N;pb=numofbiterror/(2*N);效果图: