Fairness-Aware Intragroup Cooperative Transmission in Wireless Powered Communication Networks

This article investigates fairness-aware transmission in a wireless powered communication network (WPCN), in which a group of adjacent users receive radio frequency energy and then cooperatively transmit information to a remote access point (AP) by using the harvested energy with time division multiple access (TDMA) scheme. Considering the constraints of practical energy harvesting electric circuits, a non-linear energy harvesting model is adopted in this paper. To guarantee both throughput fairness and signal-to-noise-ratio (SNR) fairness among users in the group, we propose a cooperative transmission protocol which contains three phases: wireless energy transmission (WET) phase, single information transmission phase and cooperative information transmission phase. Each user harvests wireless energy in WET phase, and then divides the harvested energy into two parts to be used in the rest two phases. In single information transmission phase, each user broadcasts its information to the AP and all the other users. In cooperative information transmission phase, all the users transmit information jointly to the AP. At the AP, the same signals in cooperative information transmission phase are combined with these in single information transmission phase by using maximum ratio combining (MRC) method. The common SNR of the received signals at the AP from all the users is achieved in both single information transmission phase and cooperative information transmission phase. Furthermore, the throughput fairness is achieved by common SNR and equal time allocation for information transmission at each user. Besides, we optimize the max-min throughput by joint WET time and harvested energy allocation. The optimal time allocation ratio and harvested energy allocation ratio are derived in closed forms. The cooperative diversity gain is achieved and numerical results demonstrate that the proposed protocol has low complexity and high reliability. Moreover, it also achieves a higher minimum throughput and is more energy-efficient in harsh transmission condition than other transmission protocols in the literature.

I. Introduction

In recent years, energy harvesting (EH) based on radio frequency (RF) signals has drawn significant attention in both academic institutes and industrial communities since it solves the problems of power limited wireless devices by harvesting RF energy from the transmitted signals [1], [2]. Besides, compared with other energy resources, such as wind, solar, thermoelectric and mechanical vibration power, RF waves are more sustainable and stable to supply energy. Hence, RF energy harvesting is able to provide stable and continuous power to prolong the lifetime of low-power networks in communication systems such as wireless sensor network (WSN) and wireless body area network (WBAN) for monitoring physical parameters, such as temperature, pressure, humidity, light, vibrations and seismic events [3], [4]. Linear EH model is usually used in theoretical studies for simplicity. But considering the circuit constraints, this paper applies a non-linear mathematical model for RF energy harvesting, which is more accurate and practical [5].