On the Power Allocation for Relay Networks with Finite-Alphabet Constraints

In this paper, we investigate the optimal power allocation scheme for relay networks with finite-alphabet constraints. It has been shown that the previous work utilizing various design criteria with the Gaussian inputs assumption may lead to significant loss for a practical system with finite constellation set constraint, especially when signal-to-noise ratio (SNR) is in medium-to-high regions, or when the channel coding rate is medium to high. An optimal power allocation scheme is proposed to maximize the mutual information for the relay networks under discrete-constellation input constraint. Numerical examples show that significant gain can be obtained compared to the conventional counterpart for nonfading channels and fading channels. At the same time, we show that the large performance gain on the mutual information will also represent the large gain on the bit-error rate (BER), i.e., the benefit of the power allocation scheme predicted by the mutual information can indeed be harvested and can provide considerable performance gain in a practical system.