A pr 2 01 3 Throughput Maximization for Wireless Powered Communication Networks

This paper studies the newly emerging wireless powered comm unication network in which one hybrid access point (H-AP) with constant power supply coordinates the wir eless energy/information transmissions to/from a set of distributed users that do not have other 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-divisionmultiple-access (TDMA). First, we study the sumthroughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users’ UL information transmissions given a tota l time constraint based on the users’ DL and UL channels as well as their average harvested energy values. B y applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocatio n maximize the sum-throughput. Our solution 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 e nergy than a nearer user in the DL, has to transmit with more power in the UL for reliable communication. As a res ult, the maximum sum-throughput is achieved by allocating substantially more time to the near user than the far user, thus resulting in unfair rate allocation among users. To overcome this problem, we furthermore propose a ne w performance metric so-called common-throughput with the additional constraint that all users should be allo c ted with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the commo n-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throu g p t approach for solving the uniquely new doubly near-far problem in wireless powered communication networ ks. Index Terms Wireless power, energy harvesting, throughput maximizati on, doubly near-far problem, TDMA, convex optimization.