Performance Analysis of Near-Optimal Energy Buffer Aided Wireless Powered Communication

Abstract

In this paper, we consider a wireless powered communication system, where an energy harvesting (EH) node harvests energy from a radio frequency (RF) signal broadcasted by an access point (AP) in the downlink (DL). The node stores the harvested energy in an energy buffer and uses the stored energy to transmit data to the AP in the uplink (UL). We investigate two simple online transmission policies for the EH node, namely a best-effort policy and an on-off policy, which do not require knowledge of the EH profile nor of the UL channel state information. In particular, for both policies, the EH node transmits in each time slot with a constant desired power if sufficient energy is available in its energy buffer. Otherwise, the node transmits with the maximum possible power in the best-effort policy and remains silent in the on-off policy in order to preserve its energy for future use. For both policies, we use the theory of discrete-time continuous-state Markov chains to analyze the limiting distribution of the stored energy for finite-and infinite-size energy buffers. We provide this limiting distribution in closed form for a Nakagami-m fading DL channel, i.e., for a Gamma distributed EH process and analyze the outage probability for a Nakagami-m fading UL channel. The analytical results derived in this paper are not limited to EH via RF WPT but are applicable for any independent and identically distributed EH process, originating from e.g. solar and wind energy. Our results reveal that, for low-to-medium outage probabilities, the best-effort policy is superior to the on-off policy and the optimal constant UL transmit power of the EH node that minimizes the outage probability is always less than the average harvested power but increases with the capacity of the energy buffer. The opposite behaviour is observed for high outage probabilities, where turning off the transmission in case of insufficient stored energy results in an improved outage performance compared to always transmitting with best effort. Furthermore, we show that despite the low-complexity of the proposed online policies, their minimum outage probability is near-optimal and closely approaches the outage probability of the optimal offline power allocation policy.

A New Interweave Cognitive Radio Scheme for Out-Band Energy Harvesting Systems

“…In this case, for a given Tx power at ST, i.e., M , the corresponding power amplifier consumes a total power ρM , where ρ ≥ 1 denotes the power amplifier inefficiency factor. In addition, the EH circuit consumes a constant power P c during transmission [10]. Hence, according to the proposed mode-of-operation, ST transmits using a constant power M whenever its stored energy reaches M ρM + P c ; otherwise it does not transmit and performs EH instead.…”

“…Notably, the out-band transmission mode is less complex than its corresponding in-band counterpart [9]. The main benefits of the proposed scheme are: (i) no co-channel interference is produced between the two systems; (ii) the CR system is purely EH-enabled since only an EH circuit and a micro-supercapacitor (as an energy storage device) are required, which is a much more cost-effective solution than rechargeable batteries [10]. Moreover, some useful performance metrics are derived in closed-form when the signals undergo κ − µ shadowed fading so as to approach realistic channel conditions [11], [12] along with some impactful engineering insights.…”

Optimal Online Transmission Policy for Energy-Constrained Wireless-Powered Communication Networks

“…However, the temporal correlation of the time-varying channels, which can be exploited to improve the system performance, was not considered in these works. Also, the H-AP in these works, e.g., [8]- [11], was assumed to equip with an infinite power supply and hence energy consumption of the system has not been a consideration in the previous studies.…”

On the performance of wireless powered communication with non-linear energy harvesting

“…For example, [6] and [7] considered a multi-user WPC system that employed a time-division based harvest-thentransmit protocol and jointly optimized the users' time and power allocation to maximize the minimum user throughput and the system energy efficiency, respectively. In addition, several works focused on the performance analysis of WPC systems [9]- [11]. For example, [9] analyzed the outage probability and the average error rate of a WPC system in Nakagamim fading.…”

“…In [10], the average throughput of a WPC system was analyzed for delay-limited and delay-tolerant transmission. Moreover, [11] studied the impact of energy storage on the outage probability of WPC systems.…”