Ambient Backscatter-Assisted Wireless-Powered Relaying

Internet-of-Things (IoT) features with low-power communications among a massive number of ubiquitously-deployed and energy-constrained electronics, e.g., sensors and actuators. To cope with the demand, wireless-powered cooperative relaying emerges as a promising communication paradigm to extend data transmission coverage and solve energy scarcity for the IoT devices. In this paper, we propose a novel hybrid relaying strategy by combining wireless-powered communication and ambient backscattering functions to improve the applicability and performance of data transfer. In particular, the hybrid relay can harvest energy from radio frequency (RF) signals and use the energy for active transmission. Alternatively, the hybrid relay can choose to perform ambient backscattering of incident RF signals for passive transmission. For the operation of the hybrid relaying, selecting a proper mode based on the network environment is the key to better relaying performance. To address this issue, we design mode selection protocols to coordinate between the active and passive relaying in the cases with and without instantaneous channel state information (CSI) of active transmission, respectively. In the former case, since the hybrid relay is aware of whether the two relaying modes are applicable for the current time slot based on the CSI, it selects active relaying if applicable due to higher capacity and selects passive relaying otherwise. In the latter case, the hybrid relay first explores the two relaying modes and commits to the mode that achieves more successful transmissions during the exploration period. With different mode selection protocols, we characterize the success probability and ergodic capacity of a dual-hop relaying system with the hybrid relay in the field of randomly located ambient transmitters. The analytical and the numerical results demonstrate the effectiveness of the mode selection protocols in adapting the hybrid relaying into the network environment and reveal the impacts of system parameters on the performance of the hybrid relaying. As applications of our analytical framework which is computationally tractable, we formulate optimization problems based on the derived expressions to optimize the system parameters with different objectives. The optimal solutions exhibit a trade-off between the maximum energy efficiency and target success probability.