A Wireless Powered Feedback Protocol for Opportunistic Beamforming Using Rectenna Arrays

This paper proposes an opportunistic beamforming (OBF) protocol with limited feedback in the context of wireless powered communication networks. The terminals are randomly deployed around an access point (AP) and are equipped with multiple antennas as well as a rectenna array to harvest energy. In the OBF scheme, the AP acquires channel related feedback from the network's terminals and allocates the orthonormal beams to the users with the best link. We consider the case, where the feedback is solely powered by energy harvested from electromagnetic radiation. Specifically, each terminal adjusts the length of its feedback based on the amount of harvested energy; when the energy is not sufficient to feed back at least one bit, the terminal is considered to be in outage. We study two fundamental rectenna architectures, the direct-current combiner and the radio-frequency combiner (coherent and non-coherent), as well as a hybrid architecture of these two. Furthermore, to alleviate the doubly near-far problem, we consider a scheme where a terminal adapts the number of rectenna elements according to its distance from the origin. By using higher order statistics and stochastic geometry, the beam outage probability is derived in closed form for all considered architectures and scenarios.