Channel input adaptation via natural type selection

Abstract: We consider a channel-independent decoder which is for i.i.d. random codes what the maximum mutual-information decoder is for constant composition codes. We show that this decoder results in exactly the same i.i.d. random coding error exponent and almost the same correct-decoding exponent for a given codebook distribution as the maximum-likelihood decoder. We propose an algorithm for computation of the optimal correct-decoding exponent which operates on the corresponding expression for the channel-independent decoder. The proposed algorithm comes in two versions: computation at a fixed rate and for a fixed slope. The fixed-slope version of the algorithm presents an alternative to the Arimoto algorithm for computation of the random coding exponent function in the correct-decoding regime. The fixed-rate version of the computation algorithm translates into a stochastic iterative algorithm for adaptation of the i.i.d. codebook distribution to a discrete memoryless channel in the limit of large block length. The adaptation scheme uses i.i.d. random codes with the channel-independent decoder and relies on one bit of feedback per transmitted block. The communication itself is assumed reliable at a constant rate $R$. In the end of the iterations the resulting codebook distribution guarantees reliable communication for all rates below $R + \Delta$ for some predetermined parameter of decoding confidence $\Delta > 0$, provided that $R + \Delta$ is less than the channel capacity.