Traditionally energy-constrained wireless networks, such as sensor networks, are powered by fixed energy sources, e.g. batteries, which have limited operation time. Although the lifetime of the network can be extended by replacing or recharging the batteries, it may be inconvenient, costly, dangerous (e.g., in a toxic environment) or even impossible (e.g., for sensors implanted in human bodies). As an alternative solution to prolong the network’s lifetime, energy harvesting has recently drawn significant interests since it potentially provides unlimited power supplies to wireless networks by scavenging energy from the environment. In particular, radio signals radiated by ambient transmitters become a viable new source for wireless energy harvesting. It has been reported that 3.5mW and 1uW of wireless power can be harvested from radio-frequency signals at distances of 0.6 and 11 meters, respectively, using Powercast RF energy harvester operating at 915MHz 1. Furthermore, the recent advance in designing highly efficient rectifying antennas will enable more efficient wireless energy harvesting from RF signals in the near future 2. It is worth noting that there has been recently a growing interest in studying wireless powered communication networks, where energy harvested from ambient RF signals is used to power wireless terminals in the network, e.g., 3-5. In 3, a wirelessly powered sensor a network was investigated, where a mobile charging vehicle moving in the network is employed as the energy transmitter to wirelessly power the sensor nodes. In 4, the wireless the powered cellular network was studied in which dedicated power-beacons are deployed in the cellular network to charge mobile terminals. Moreover, the wireless powered cognitive the radio network has been considered in 5, where the active primary users are utilized as energy transmitters for charging their nearby secondary users that are not allowed to transmit over the same channel due to strong interference. Furthermore, since radio signals carry energy as well as information at the same time, a joint investigation of simultaneous wireless information and power transfer has recently drawn a significant attention (see e.g. 6-11 and the references therein). In this paper, we study a new type of WPCN as shown in Fig. 1, in which one hybrid access point with constant power supply (e.g. battery) coordinates the wireless energy/information transmissions to/from a set of distributed users that are assumed to have no other energy sources. All users are each equipped with a rechargeable battery and thus can harvest and store the wireless energy broadcast by the hybrid access point. Unlike prior works on SWIPT 6-11, which focused on the simultaneous energy and information transmissions to users in the downlink, in this paper we consider a different setup where the hybrid access point broadcasts only wireless energy to all users in the downlink while the users transmit their independent information using their individually harvested energy to the H-AP in the uplink. We are interested in maximizing the uplink throughput of the aforementioned WPCN by optimally allocating the time for the downlink wireless energy transfer by the and the uplink wireless information transmissions by different users.

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