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软解调，输出LLR；
step6:将LLR结果导入维特比输入，经维特比译码还原至1/3码率的bit stream，采用软判决方式译码，取译码后的原数据长度为原始bit 流，即为系统bit。代码中LLR结果取了乘以-1，取决于实现时LLR实现时分子分母的含义。
step6:计算误码率；
%% 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,... 'DecisionMethod','Log-likelihood ratio',... 'VarianceSource', 'Input port'); BitError = comm.ErrorRate; ConvEncoder=comm.ConvolutionalEncoder(... 'TerminationMethod','Terminated'); Viterbi=comm.ViterbiDecoder(... 'InputFormat','Soft',... 'SoftInputWordLength', 4,... 'OutputDataType', 'double',... 'TerminationMethod','Terminated'); Quantizer=dsp.ScalarQuantizerEncoder(... 'Partitioning', 'Unbounded',... 'BoundaryPoints', -7:7,... 'OutputIndexDataType','uint8'); %% Outer Loop computing Bit-error rate as a function of EbNo % for EbNo = EbNo_vector % snr = EbNo + 10*log10(2); snr = 20; noise_var = 10.^(-snr/10); AWGN.EbNo=snr; 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 = DeModulator.step(rx_sig, noise_var); % Soft-decision QPSK Demodulator llr = Quantizer.step(-demod); % Quantize Log-Likelihood Ratios decoded = Viterbi.step(llr); % Viterbi decoder with LLRs 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
