仿真思路：
step1:生成随机bit作为待调制信号sig；
step2:对生成bit stream做信道编码，编码方式：咬尾卷积1/3编码，可观察经编码后的码率变化；
step3:将信道编码后的bit stream经QPSK调制；
step4:调制信号过awgn信道，改变snr，观察过信道后信号变化；
step5:接收端QPSK解调；
step6:对QPSK解调后的输出，经维特比译码还原至1/3码率的bit stream，采用硬判决方式译码，取译码后的原数据长度为原始bit 流，即为系统bit。
step6:计算误码率；
备注：源程序略做修改，只是观察中间信号变化
%% Constants FRM=2048; MaxNumErrs=200;MaxNumBits=1e7; EbNo_vector=0:10;BER_vector=zeros(size(EbNo_vector)); %% Initializations Modulator = comm.QPSKModulator('BitInput',true); AWGN = comm.AWGNChannel; DeModulator = comm.QPSKDemodulator('BitOutput',true); BitError = comm.ErrorRate; ConvEncoder=comm.ConvolutionalEncoder(... 'TerminationMethod','Terminated'); Viterbi=comm.ViterbiDecoder('InputFormat','Hard',... 'TerminationMethod','Terminated'); %% Outer Loop computing Bit-error rate as a function of EbNo for EbNo = EbNo_vector snr = EbNo + 10*log10(2); AWGN.EbNo=20; numErrs = 0; numBits = 0;results=zeros(3,1); %% Inner loop modeling transmitter, channel model and receiver for each EbNo % while ((numErrs < MaxNumErrs) && (numBits < MaxNumBits)) % Transmitter u = randi([0 1], FRM,1); % Generate random bits encoded = ConvEncoder.step(u); % Convolutional encoder mod_sig = step(Modulator, encoded); % QPSK Modulator scatterplot(mod_sig); % Channel rx_sig = step(AWGN, mod_sig); % AWGN channel % rx_sig = awgn(mod_sig,20,'measured'); % Receiver scatterplot(rx_sig); demod = step(DeModulator, rx_sig); % QPSK Demodulator decoded = Viterbi.step(demod); % Viterbi decoder y = decoded(1:FRM); % Compute output bits results = step(BitError, u, y); % Update BER numErrs = results(2); numBits = results(3); % end % Compute BER ber = results(1); bits= results(3); %% Clean up & collect results reset(BitError); BER_vector(EbNo+1)=ber; end
