  In this work, we employ multiple energy harvesting relays to assist
information transmission from a multi-antenna hybrid access point (HAP) to a
receiver. All the relays are wirelessly powered by the HAP in the
power-splitting (PS) protocol. We introduce the novel concept of hybrid relay
communications, which allows each relay to switch between two radio modes,
i.e., the active RF communications and the passive backscatter communications,
according to its channel and energy conditions. We envision that the complement
transmissions in two radio modes can be exploited to improve the overall relay
performance. As such, we aim to jointly optimize the HAP's beamforming,
individual relays' radio mode, the PS ratio, and the relays' collaborative
beamforming to enhance the throughput performance at the receiver. The
resulting formulation becomes a combinatorial and non-convex problem. Thus, we
firstly propose a convex approximation to the original problem, which serves as
a lower bound of the relay performance. Then, we design an iterative algorithm
that decomposes the binary relay mode optimization from the other operating
parameters. In the inner loop of the algorithm, we exploit the structural
properties to optimize the relay performance with the fixed relay mode in the
alternating optimization framework. In the outer loop, different performance
metrics are derived to guide the search for a set of passive relays to further
improve the relay performance. Simulation results verify that the hybrid
relaying communications can achieve 20% performance improvement compared to the
conventional relay communications with all active relays.
