Adaptive MQAM for Energy Harvesting Wireless Communications With 1-Bit Channel Feedback

In this paper, we propose online discrete-rate discrete-power adaptation policies based on 1-bit channel feedback for energy harvesting (EH) wireless communications. The receiver periodically sends 1-bit feedback by comparing the channel power gain with a predetermined threshold. The EH transmitter correspondingly adjusts M-ary modulation level and transmission power based on 1-bit feedback and battery energy. To determine the optimal channel threshold, adaptive modulation levels and corresponding power allocations, we formulate a constrained optimization problem to maximize the throughput within a finite horizon. We show that this problem follows a Markov decision process and can be solved via backward induction method. To alleviate the computational complexity of the backward induction method, we further propose a suboptimal policy that uses the best effort modulation level and the channel threshold that is optimized by maximizing the ergodic spectral efficiency over channel fading and EH processes. Simulation results show that the proposed policies have considerable throughput performance gain compared to other heuristic policies, and the suboptimal policy has negligible performance loss compared to the optimal policy.