1. Requirements

Draw the information generated by the MSK signal, the comparison diagram of the I-channel and Q-channel signals, and the comparison diagram of the information recovery demodulated at the receiving end.

2. Design

1. MSK modulation

2. MSK demodulation

Three, the code

clear all;clc;
N=20;  
T=1;   
fc=10; 
Fs=100;   
bitstream=randi([0,1],1,N);  
bitstream1=2*bitstream-1;  
b0=1;
for i=1:N
    encode_output(i)=b0*bitstream1(i);
    b0=encode_output(i);
end
I=[];Q=[];
for i=1:N
    if mod(i,2)~=0
        I=[I,encode_output(i)];
    else
        Q=[Q,encode_output(i)];
    end
end 
bit_data=[];
for i=1:N
    bit_data=[bit_data,encode_output(i)*ones(1,T*Fs)];
end
 I_data=[];Q_data=[];base_wave=-T:1/Fs:T-1/Fs;
for i=1:N/2
    I_data=[I_data,I(i)*cos(pi*base_wave/(2*T))];
    Q_data=[Q_data,Q(i)*cos(pi*base_wave/(2*T))];
end
% Q delay
number_delay=length(base_wave)/2;
Q_data1=[zeros(1,number_delay),Q_data(1:length(Q_data)-number_delay)];
% plot
figure();
t=0:1/Fs:N*T-1/Fs;
subplot(3,1,1)
plot(t,bit_data);legend('Bitstream')
subplot(3,1,2)
plot(t,I_data);legend('I Bitstream')
subplot(3,1,3)
plot(t,Q_data1);legend('Q Bitstream')
% carrier signal 
bit_t=0:1/Fs:N*T-1/Fs;
I_carrier=cos(2*pi*fc*bit_t);
Q_carrier=cos(2*pi*fc*bit_t+pi/2);
% transmit signal
MSK_signal=I_data.*I_carrier+Q_data1.*Q_carrier;
 
snr=1;  
MSK_receive=awgn(MSK_signal,snr);
% demodulate 
I_output=MSK_receive.*I_carrier;
 
Hd=myfilter;
I_filter_ouput=filter(Hd,I_output);
Q_output=MSK_receive.*Q_carrier;
Q_filter_ouput=filter(Hd,Q_output);
 
for i=1:N/2
    if I_filter_ouput((2*i-1)*number_delay)>0
        I_recover(i)=1;
    else
        I_recover(i)=-1;
    end
    if Q_filter_ouput(2*i*number_delay)>0
        Q_recover(i)=1;
    else
        Q_recover(i)=-1;
    end
end
bit_recover=[];
for i=1:N
    if mod(i,2)~=0
        bit_recover=[bit_recover,I_recover((i-1)/2+1)];
    else
        bit_recover=[bit_recover,Q_recover(i/2)];
    end
end
 
% decode
for i=1:N
    if i==1
        bit_recover1(i)=bit_recover(i);
    else
        bit_recover1(i)=bit_recover(i)*bit_recover(i-1);
    end
end
recover_data=[];
for i=1:N
    recover_data=[recover_data,bit_recover1(i)*ones(1,T*Fs)];
end
 
% plot 
figure();
t=0:1/Fs:N*T-1/Fs;
bit_stream=[];
for i=1:N
    bit_stream=[bit_stream,bitstream1(i)*ones(1,T*Fs)];
end
subplot(2,1,2)
plot(t,bit_stream);legend('original Bitstream')
subplot(2,1,1)
plot(t,recover_data);legend('recover Bitstream')

Fourth, the result

myfilter is a function designed and generated by calling the filter designer through the matlab command line

V. Conclusion

In MSK modulation and demodulation, the generated and received signals can be restored, that is, the modulated I, Q and MSK signals can be restored to the original signal after demodulation. Although the filter design is different, it still does not affect the final result.