A Cross-Layer Perspective on Energy-Harvesting-Aided Green Communications Over Fading Channels

In this paper, we consider the power allocation of the physical layer and the buffer delay of the upper application layer in energy harvesting green networks. The total power required for reliable transmission includes the transmission power and the circuit power. The harvested power (which is stored in a battery) and the grid power constitute the power resource. The uncertainty of data generated from the upper layer, the intermittence of the harvested energy, and the variation of the fading channel are taken into account and described as independent Markov processes. In each transmission, the transmitter decides the transmission rate and the allocated power from the battery, and the rest of the required power will be supplied by the power grid. The objective is to find an allocation sequence of transmission rate and battery power to minimize the long-term average buffer delay under the average grid power constraint. A stochastic optimization problem is formulated accordingly to find such transmission rate and battery power sequence. Furthermore, the optimization problem is reformulated as a constrained Markov decision process (MDP) problem whose policy is a 2-D vector with the transmission rate and the power allocation of the battery as its elements. We prove that the optimal policy of the constrained MDP can be obtained by solving the unconstrained MDP. Then, we focus on the analysis of the unconstrained average-cost MDP. The structural properties of the average optimal policy are derived. Moreover, we discuss the relations between elements of the 2-D policy. Next, based on the theoretical analysis, the algorithm to find the constrained optimal policy is presented for the finite-state-space scenario. In addition, heuristic policies (two deterministic policies and a mixed policy) with low complexity are given for the general state space. Finally, simulations are performed under these policies to demonstrate their effectiveness.