Throughput maximization in wireless powered communication networks

This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with a constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have fixed energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocated to the DL wireless power transfer and the UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. Our result reveals an interesting "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than the nearer users in the DL, has to transmit with more power in the UL for reliable communication. As a result, the sum-throughput maximization solution allocates substantially more time to the near users than the far users, thus resulting in unfair rate allocation among users. To overcome this problem, we furthermore propose to maximize a new metric so-called common-throughput with an additional constraint that all users should be allocated with equal rates regardless of their distances to the H-AP. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the uniquely new doubly near-far problem in wireless powered communication networks.