Slotted ALOHA for Wireless Powered Communication Networks
Centralized controls in wireless-powered communication networks (WPCNs) induce considerable overhead for channel estimation and high complexity for optimization, as the number of wireless devices (WDs) increases. To tackle this problem, we apply slotted ALOHA protocol to WPCNs and design a slotted ALOHA-based energy-harvesting medium access control protocol. In this protocol, the WD randomly selects one of the given random access (RA) slots and continuously harvests the energy from the hybrid access point (HAP) until it has access. We analyze the average channel throughput and obtain the optimal number of RA slots allocated ( $m^{*}$ ) to maximize it. Thereafter, we present a prioritized access control to alleviate the doubly near-far problem in the WPCN. Considering the near and far WDs from the HAP, we assign the far WDs a high priority and make them access at the later part of the frame in order to allow them to have a longer energy harvesting time than the near WDs. In terms of Jain’s fairness index, we obtain the optimal ratio of RA slots allocated for the low- and high-priority WDs ( $\alpha ^{*}$ ) to maximize the fairness. Through an asymptotic analysis in the high signal-to-noise ratio (SNR) environment with a sufficient number of accessing WDs, it is shown that there are unique $m^{*}$ and $\alpha ^{*}$ that maximize the channel throughput and user fairness, respectively, and both depend only on the average of the minimum SNRs of the WDs without the knowledge of full channel state information.