  Internet-of-Things (IoT) is featured 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
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. To
efficiently utilize the ambient RF resource, we design mode selection protocols
to coordinate between the active and passive relaying in circumstances with and
without instantaneous channel gain. 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 gain 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 tradeoff between the
maximum energy efficiency and target success probability.
