I understand that you are trying to execute the “nrPolarDecode” example in https://www.mathworks.com/help/5g/ref/nrpolardecode.html. The code will result in zero if you use a default AWGN channel as follows:
chan = comm.AWGNChannel;
instead of
chan = comm.AWGNChannel('NoiseMethod','Variance','Variance',nVar);
Regarding the scenario where your system only had the encoder and decoder: The input “rxLLR” represents the log-likelihood ratios per bit with a negative bipolar mapping. Here, a 0 should be mapped to 1, and a 1 should be mapped to -1. Therefore, a zero error can be obtained by defining the input to the decoder as mentioned below:
rxLLR = 1.0 - 2.0*double(enc);
