  In this paper, a new spectrum sharing model referred to as riding on the
primary (ROP) is proposed for wireless-powered IoT devices with ambient
backscatter communication capabilities. The key idea of ROP is that the
secondary transmitter harvests energy from the primary signal, then modulates
its information bits to the primary signal, and reflects the modulated signal
to the secondary receiver without violating the primary system's interference
requirement. Compared with the conventional spectrum sharing model, the
secondary system in the proposed ROP not only utilizes the spectrum of the
primary system but also takes advantage of the primary signal to harvest energy
and to carry its information. In this paper, we investigate the performance of
such a spectrum sharing system under fading channels. To be specific, we
maximize the ergodic capacity of the secondary system by jointly optimizing the
transmit power of the primary signal and the reflection coefficient of the
secondary ambient backscatter. Different (ideal/practical) energy consumption
models, different (peak/average) transmit power constraints, different types
(fixed/dynamically adjustable) reflection coefficient, different primary
system's interference requirements (rate/outage) are considered. Optimal power
allocation and reflection coefficient are obtained for each scenario.
