On the optimal duplexing strategy for wireless-powered communication networks

Abstract

Due to its simplicity and lack of channel state information (CSI) feedback requirements, time division duplexing (TDD) has been the preferred duplexing method in wireless-powered communication networks (WPCNs), while the advantages of frequency-division duplexing (FDD) has remained largely unexplored. Yet, the decision between TDD and FDD goes beyond CSI considerations, as it depends on various system parameters and operational trade-offs not previously considered. In FDD, the transmitter remains active throughout the entire frame duration, enabling more effective utilization of the maximum instantaneous transmit power of the hybrid access point (HAP). In contrast, TDD exploits the full bandwidth (BW), thereby making better use of the feasible maximum power spectral density (PSD). Finally, while the constraint of maximum time-averaged transmit power in FDD closely resembles the effect of the maximum instantaneous transmit power constraint, they have different effects on the operation of the TDD-WPCN.

To analyze these effects, we thoroughly investigate both TDD-WPCN and FDD-WPCN and characterize their respective operating regions. Our extensive theoretical and simulation results reveal that selecting between the two schemes involves a complex, multidimensional trade-off, warranting careful consideration in system design. We demonstrate that, under certain assumptions, the throughput of an FDD-WPCN can be substantially greater than that of the same WPCN operating in TDD mode. Furthermore we prove that, under certain conditions, a single-node FDD-WPCN can achieve a two-fold increase in throughput compared to a single-node TDD-WPCN.