Throughput-Optimal Link-Layer Design in Power Constrained Hybrid OW/RF Systems

The aim of this paper is to develop link layer transmission schemes for hybrid optical wireless (OW)/radio frequency (RF) systems, with constraints on both per-link and total average power consumption at the transmitter. In this context, we adopt a timeslot structure with a queue for storing the data packets for transmission and model the hybrid channel as an erasure channel with parameters varying along the time-slots according to a Markov chain. Then, a stochastic optimization problem is formulated, where intelligent decisions regarding the number of the packets admitted in the queue and the power levels used in every link, are taken by the hybrid transmitter in each slot. The objective of this formulation is to design a control policy that maximizes the transmitter throughput, while satisfying the power constraints as well. A solution is offered by using the Lyapunov optimization framework and an on-line transmission algorithm is developed. The proposed transmission algorithm takes decisions based only on the status of the queue and the statistical parameters of the OW/RF channel in each time-slot, without requiring any knowledge of the underlying Markov chain of the channel process, or the statistics of the packet arrival process. Furthermore, in order to alleviate the requirement for full feedback at the transmitter, i.e., feedback for every successfully received packet, which is critical for the accurate queue update, we extend our analysis and incorporate reduced-feedback coding schemes. The proposed transmission policy in this scenario still meets the throughput objective and satisfies the power consumption constraints, while a tradeoff between transmission delays and feedback requirements is revealed.