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1、第一题clearechoofft0=0.15;ts=0.001;fc=250;snr=20;fs=1/ts;df=0.3;t=(0:ts:t0);snr_lin=10^(snr/10);m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)];c=cos(2.*pi.*fc.*t);u=[2+0.85*m].*c;[M,m,df1]=fftseq(m,ts,df);M=M/fs;[U,u,df1]=fftseq(u,ts,df);U=U/fs;[C,c,df1]=
2、fftseq(c,ts,df);f=[0:df1:df1*(length(m)-1)]-fs/2;dt=0.01;signal_power=sum(u.*u)*dt/(length(t).*dt);noise_power=signal_power/snr_lin;noise_std=sqrt(noise_power);n=noise_std*randn(1,length(u));r=u+n;[N,n,df1]=fftseq(n,ts,df);%对噪声进行fftN=N/fs;%频率采样[R,r,df1]=fftseq(r,ts,df)
3、;%对最后的信号进行fftR=R/fs;signal_powernoise_powerclffigure(1)plot(t,m(1:length(t)))xlabel('Time')title('Themessagesignal')figure(2)subplot(2,1,1)plot(t,c(1:length(t)))xlabel('Time')title('Thecarrier')subplot(2,1,2)plot(t,u(1:length(t)))xlabel('Time')title('Themodulatedsignal
4、')figure(3)subplot(2,1,1)plot(f,abs(fftshift(M)))xlabel('frequency')title('spectrumofthemessagesignal')subplot(2,1,2)plot(f,abs(fftshift(U)))title('spectrumofthemodulatedsignal')xlabel('frequency')figure(4)subplot(2,1,1)%噪声时域显示plot(t,n(1:length(t)))title('采样的噪声')subplo
5、t(2,1,2)%噪声频域显示plot(f,abs(fftshift(N)))axis([-20020000.15])title('噪声频谱')figure(5)subplot(2,1,1)%加噪调制信号时域显示plot(t,r(1:length(t)))title('信号加噪声')axis([00.15-33])subplot(2,1,2)%加噪调制信号频域显示plot(f,abs(fftshift(R)))title('信号加噪声频谱')axis([-50050000.15])M文件1fftseq:function[M,m,df
6、]=fftseq(m,ts,df)fs=1/ts;n1=fs/df;n2=length(m);n=2^(max(nextpow2(n1),nextpow2(n2)));M=fft(m,n);m=[m,zeros(1,n-n2)];df=fs/n;M文件2spower：functionp=spower(x)p=(norm(x)^2/length(x));第二题clearechooffb=3000;sn0=-20;sn1=30;ns=0.01;N=(sn0:ns:sn1);snrlin=10.^(N/10);ct=b.*log2(1+s
7、nrlin.*(1/b));clfplot(snrlin,ct)xlabel('s/n0')title('高斯信道容量')clearechooffn1=2.5;sn1=10.^n1;b0=300000;bs=10;B=(3000:bs:b0);b=1./B;ct=B.*(log2(1+sn1.*b));plot(B,ct)xlabel('B')title('信道容量')ct=(1/log(2)*sn1)ct=456.2202第三题clearall;clf;Tb=1;f1=1000/Tb;f2=f1+1/Tb;phi=pi/4;N=1
8、2000;n=N/4;t=0:Tb/(n-1):Tb;%t=0:1/(n-1):1;T=0:4*Tb/(N-1):4*Tb;%T=0:4*1/(N-1):4*1;s1=[cos(2*pi*f1*t+phi)cos(2*pi*f1*t+
