Modulation Optimization for Achieving Energy Efficient Communications over Fading Channels

It is commonly assumed that the energy consumption of wireless communications is minimized when low-order modulations such as BPSK are used. Nevertheless, the literature provides some evidence that low-order modulations are suboptimal for short transmission distances. A thorough analysis on how the modulation scheme and transmission power must be chosen as a function of distance in order to achieve energy-efficient communications over fading channels has not been reported yet. In this paper we provide this analysis by presenting a model that determines the energy consumed per payload bit transferred without error over correlated or uncorrelated random channels. We find that each modulation scheme has a single optimal signal- to-noise ratio (SNR) at which the energy consumption is minimized. We also find that if all modulations are operated at their optimal SNR, BPSK and QPSK are the optimal choices for long transmission distances, but as the transmission distance shortens the optimal modulation size grows to 16-QAM and even to 64- QAM. This result leads to showing that for short-range communications the lifetime of a typical low-power transceiver can be increased by up to 600% by selecting the optimal constellation rather than BPSK.