Yifan Gu

An adaptive transmission protocol for wireless-powered cooperative communications

In this paper, we consider a wireless-powered cooperative communication network, which consists of one hybrid access point (AP), one source and one relay to assist information transmission. Unlike conventional cooperative networks, the source and relay are assumed to have no embedded energy supplies in the considered system. Hence, they need to first harvest energy from the radio-frequency (RF) signals radiated by the AP in the downlink (DL) before information transmission in the uplink (UL). Inspired by the recently proposed harvest-then-transmit (HTT) and harvest-then-cooperate (HTC) protocols, we develop a new adaptive transmission (AT) protocol. In the proposed protocol, at the beginning of each transmission block, the AP charges the source. AP and source then perform channel estimation to acquire the channel state information (CSI) between them. Based on the CSI estimate, the AP adaptively chooses the source to perform UL information transmission either directly or cooperatively with the relay. We derive an approximate closed-form expression for the average throughput of the proposed AT protocol over Nakagami-m fading channels. The analysis is then verified by Monte Carlo simulations. Results show that the proposed AT protocol considerably outperforms both the HTT and HTC protocols.

Distributed Multi-Relay Selection in Accumulate-Then-Forward Energy Harvesting Relay Networks

This paper investigates a wireless-powered cooperative network (WPCN) consisting of one source-destination pair and multiple decode-and-forward relays. We develop an energy threshold based multi-relay selection (ETMRS) scheme for the considered WPCN. The proposed ETMRS scheme can be implemented in a fully distributed manner as the relays only need local information to switch between energy harvesting and information forwarding modes. By modeling the charging/discharging behaviors of the finite-capacity battery at each relay as a finite-state Markov chain, we derive an analytical expression for the system outage probability of the proposed ETMRS scheme over mixed Nakagami-

A Discrete Time-Switching Protocol for Wireless-Powered Communications with Energy Accumulation

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Distributed multi-relay selection in wireless-powered cooperative networks with energy accumulation

and Rayleigh fading channels. Based on the derived expression, the optimal energy thresholds for all the relays corresponding to the minimum system outage probability can be obtained via an exhaustive search. However, this approach becomes computationally prohibitive when the number of relays and the associated number of battery energy levels are large. To resolve this issue, we propose a heuristic approach to optimize the energy threshold for each relay. To gain some useful insights for practical relay design, we also derive the upper bound for system outage probability corresponding to the case that all relays are equipped with infinite-capacity batteries. Numerical results validate our theoretical analysis. It is shown that the proposed heuristic approach can achieve a near-optimal system performance and our ETMRS scheme outperforms the existing single-relay selection scheme and common energy threshold scheme.

Ultra-Reliable Short-Packet Communications: Half-Duplex or Full-Duplex Relaying?

This paper investigates a wireless-powered communication network (WPCN) setup with one multi- antenna access point (AP) and one single-antenna source. It is assumed that the AP is connected to an external power supply, while the source does not have an embedded energy supply. But the source could harvest energy from radio frequency (RF) signals sent by the AP and store it for future information transmission. We develop a discrete time-switching (DTS) protocol for the considered WPCN. In the proposed protocol, either energy harvesting (EH) or information transmission (IT) operation is performed during each transmission block. Specifically, based on the channel state information (CSI) between source and AP, the source can determine the minimum energy required for an outage-free IT operation. If the residual energy of the source is sufficient, the source will start the IT phase. Otherwise, EH phase is invoked and the source accumulates the harvested energy. To characterize the performance of the proposed protocol, we adopt a discrete Markov chain (MC) to model the energy accumulation process at the source battery. A closed-form expression for the average throughput of the DTS protocol is derived. Numerical results validate our theoretical analysis and show that the proposed DTS protocol considerably outperforms the existing harvest-then-transmit protocol when the battery capacity at the source is large.

Accumulate then forward: An opportunistic relaying protocol for wireless-powered cooperative communications

This paper investigates a wireless-powered cooperative network (WPCN) consisting of one source-destination pair and multiple decode-and-forward (DF) relays. We consider that the DF relays are wireless-powered such that they rely only on the harvested energy from the source to perform information forwarding. Furthermore, these relays are equipped with separate energy and information receivers as well as energy storage to accumulate the harvested energy. In this paper, we develop an energy threshold based multi-relay selection (ETMRS) scheme for the considered WPCN, in which each relay determines to switch between energy harvesting and information forwarding modes in a fully distributed manner. By modeling the charging/discharging of the discrete-level battery at each relay as a finite state Markov Chain (MC), we derive a closed-form expression for the system outage probability of the proposed ETMRS scheme over independent but not necessarily identical Rayleigh fading channels. Numerical results validate our theoretical analysis and show that the proposed ETMRS scheme can outperform the existing single-relay selection scheme, especially when the sources transmission rate is relatively high.

Throughput analysis of wireless-powered communications with energy beamforming and adaptive time switching

This letter analyzes and compares the performance of full-duplex relaying (FDR) and half-duplex relaying (HDR) for ultrareliable short-packet communications. Specifically, we derive both approximate and asymptotic closed-form expressions of the block error rate (BLER) for FDR and HDR using short packets with finite blocklength codes. We define and attain a closed-form expression of a critical BLER, which can be used to efficiently determine the optimal duplex mode for ultrareliable low latency communication scenarios. Our results unveil that FDR is more appealing to the system with relatively lower transmit power constraint, less stringent BLER requirement, and stronger loop interference suppression.

Wireless-Powered Two-Way Relaying with Power Splitting-Based Energy Accumulation

This paper investigates a wireless-powered cooperative communication network consisting of a source, a destination and a multi-antenna decode-and-forward relay. We consider the relay as a wireless-powered node that has no external power supply; but it is equipped with an energy harvesting (EH) unit and a rechargeable battery such that it can harvest and accumulate energy from radio-frequency signals broadcast by the source. By fully incorporating the EH feature of the relay, we develop an opportunistic relaying protocol, termed accumulate-then-forward (ATF), for the considered WPCCN. We then adopt the discrete Markov chain to model the dynamic charging and discharging behaviors of the relay battery. Based on this, we derive a closed-form expression for the exact outage probability of the proposed ATF protocol. Numerical results show that the ATF scheme can outperform the direct transmission one, especially when the amount of energy consumed by relay for information forwarding is optimized.

Multiuser MIMO Short-Packet Communications: Time-Sharing or Zero-Forcing Beamforming?

This paper studies a wireless-powered communication network consisting of a hybrid access-point (AP) equipped with multiple antennas and a single-antenna user. The user is assumed to have no embedded power supply. Thus, it needs to first harvest energy from RF signals sent by the AP in the downlink (DL) before transmitting its information to the AP in the uplink (UL). We consider the scenario that the user transmits with fixed rate and the full channel state information is available at the AP side. This enables the AP to adaptively adjust the time duration of DL energy transfer to maximize the system throughput. We then characterize the average throughput of the considered network by deriving a closed-form expression for the probability density function of the adaptive DL energy transfer time. The asymptotic analysis of the system throughput is also performed to gain more insights. Numerical results are finally provided to verify the theoretical analysis, which show that the proposed adaptive time switching scheme always outperforms its fixed time switching counterpart.