Capacity expression and power allocation for arbitrary modulation and coding rates

This paper addresses the dichotomy that exists in the analysis of wireless channels, which exists between employing: the tractable Shannon capacity bound and the accurate simulated capacity of modulation and coding schemes (MCSs). By considering the effects of mutual information saturation in modulation schemes and the efficiency of error correction codes, the paper proposes a new tractable capacity expression that can accurately represent any MCS and adaptive combinations. To demonstrate this methodology, the paper considers the adaptive MCS capacity for the Long-Term-Evolution (LTE) system. The potential benefit is that system-level optimization can employ the more accurate capacity expression. In the second part of the paper, the proposed capacity expression is applied to optimize power allocation to the parallel channels of a MIMO system. The results show that when mutual information saturation occurs, neither the water-filling nor channel inversion schemes are optimal. In fact the optimal power allocation is a combination of the two aforementioned schemes, switching adaptively between their relative merits.