Source-linked AI summary
Full-Duplex Wireless-Powered Relay with Self-Energy Recycling
Yong Zeng, Rui Zhang
TL;DR
The paper addresses wireless-powered relaying with an energy-constrained relay that must support source-to-destination communication using harvested energy. It proposes full-duplex two-phase AF relaying with simultaneous harvesting and transmission, including self-energy recycling, and reports significant throughput gains over TSR.
Problem
Wireless-powered relaying uses an energy-constrained relay powered by source energy, while existing protocols rely mainly on half-duplex TSR or PSR operation.
Method
The paper proposes a two-phase full-duplex AF protocol with simultaneous relay energy harvesting and information transmission, and derives optimal MISO relay power allocation and beamforming.
Results
Significant throughput gains are achieved over the existing TSR scheme, particularly with sufficiently large source transmit power.
Takeaways & Limitations
Self-energy recycling and simultaneous harvesting and information transmission provide the proposed relay design's reported throughput advantage while maintaining uninterrupted information transmission.
Abstract
from arXiv · showhide
This letter studies a wireless-powered amplify-and-forward relaying system, where an energy-constrained relay node assists the information transmission from the source to the destination using the energy harvested from the source. We propose a novel two-phase protocol for efficient energy transfer and information relaying, in which the relay operates in full-duplex mode with simultaneous energy harvesting and information transmission. Compared with the existing protocols, the proposed design possesses two main advantages: i) it ensures uninterrupted information transmission since no time switching or power splitting is needed at the relay for energy harvesting; ii) it enables the so-called self-energy recycling, i.e., part of the energy (loop energy) that is used for information transmission by the relay can be harvested and reused in addition to the dedicated energy sent by the source. Under the multiple-input single-output (MISO) channel setup, the optimal power allocation and beamforming design at the relay are derived. Numerical results show a significant throughput gain achieved by our proposed design over the existing time switching-based relay protocol.
I. INTRODUCTION
The paper proposes a two-phase full-duplex AF wireless-powered relaying protocol that combines simultaneous energy harvesting with information transmission. Its self-energy recycling mechanism and relay optimization yield uninterrupted transmission and significant throughput gains over TSR.
- Wireless-powered relaying assists source-to-destination communication through an energy-constrained relay powered by harvested source energy.
- The proposed two-phase AF protocol transmits information from source to relay, then amplifies and forwards it while the source sends dedicated energy signals for harvesting.
- The full-duplex design enables uninterrupted information transmission because relay energy harvesting requires neither time switching nor power splitting.
- Self-energy recycling harvests and reuses part of the relay's transmission energy alongside dedicated energy sent by the source.
- Under the MISO setup, the paper derives optimal relay power allocation and beamforming and optimizes TSR time division for benchmarking.
- Significant throughput gains are reported over the existing TSR protocol, particularly for sufficiently large source transmit power, due to self-energy recycling and simultaneous relay operations.
II. SYSTEM MODEL AND PROPOSED PROTOCOL
The system uses an energy-constrained full-duplex relay with separate transmission and reception resources. The proposed protocol divides each block into two equal phases, combining AF forwarding with source-driven relay energy harvesting.
- System model: The source transmits information to the destination through a relay powered solely by energy harvested from the source.
- System model: The source and relay have M and N +1 antennas, respectively, while the destination has one antenna.
- System model: The relay uses N RF chains for information transmission, one RF chain for reception, and one rectifier for energy harvesting.
- System model: At each time instant, at most N relay antennas transmit information while one antenna handles information or energy reception.
- System model: The model neglects the direct source-to-destination link and assumes quasi-static channels with perfect CSI at the source and relay.
- Proposed protocol: The first phase lasts T/2 for source-to-relay information transmission, and the second lasts T/2 for relay forwarding and source energy transmission.
III. RELAYING WITH SELF-ENERGY RECYCLING
The proposed two-phase full-duplex AF protocol lets the relay receive information, transmit it, and harvest source and recycled relay energy concurrently. Optimal relay beamforming balances information transmission with self-energy recycling, enabling relay transmit power beyond direct source-harvested energy.
- Energy harvesting: The source uses maximal ratio transmission based on channel h, and the source energy-bearing signal can be phase-aligned with the information signal to attain the harvested-energy upper bound.The phase relation is x_s,2[k] = x_s,1[k]e^j∠f^Hv_r.
- Protocol: The first phase receives the source information signal, while the second phase amplifies and forwards it from the relay to the destination.The relay uses multiple transmitting antennas with beamforming vector v_r.
- Protocol: During second-phase information transmission, the relay concurrently harvests dedicated source energy and recycles part of its own transmitted energy through a loop channel.The harvested energy includes source-dedicated and loop-recycled components.
- Optimization: The optimal relay beamforming vector is a linear combination of g and f, balancing information transmission toward the destination with self-energy recycling.The effective angle between f and g determines the channel alignment used in the design.
- Special case: With a single relay transmitting antenna, self-energy recycling allows relay transmit power to exceed the power available from direct source energy harvesting.The comparison is stated as P_r > ηP_s∥h∥^2.
IV. TIME-SWITCHING BASED RELAYING
The benchmark time-switching relaying protocol separates energy harvesting, source-to-relay information transfer, and relay-to-destination forwarding into three phases. Its relay transmit power is determined by energy harvested during the dedicated harvesting phase.
- Protocol: The TSR benchmark uses three phases: source-to-relay energy harvesting, source-to-relay information transmission, and AF forwarding from relay to destination.The information and forwarding phases have equal duration.
- Protocol: During the first phase, the source sends an energy signal for duration αT, with 0 < α < 1.The remaining time is divided equally between information reception and forwarding.
- Power allocation: The relay harvests E_r = αTηP_s∥h∥^2 and transmits with P_r = 2αηP_s∥h∥^2/(1 − α).These expressions follow from using the harvested energy during the two equal-length information phases.
- Beamforming: For TSR, the relay's optimal transmit beamforming matches the relay-to-destination MISO channel, v_r = g/∥g∥.The resulting received SNR at the destination is then used in the throughput expression.
V. NUMERICAL RESULTS
The numerical evaluation compares the proposed protocol with an optimized TSR benchmark under specified MISO channel and noise assumptions. The proposed protocol achieves significant throughput gains at sufficiently large source transmission power, attributed to self-energy recycling and simultaneous harvesting and transmission.
- Comparison: Throughput in bps/Hz is plotted against source transmission power Ps for the proposed design and TSR with optimal time allocation α⋆.
- Results: Significant throughput gains are achieved by the proposed protocol with sufficiently large Ps.The reported gain is attributed to self-energy recycling and simultaneous energy harvesting and information transmission at the relay.
- Results: Simultaneous harvesting and information transmission ensures continuous information transmission without interruption from energy transfer.
VI. CONCLUSION
The paper proposes a full-duplex wireless-powered relaying protocol that combines simultaneous energy harvesting and information transmission with self-energy recycling. Under the MISO AF setting, it derives optimal relay power allocation and beamforming, and reports significant throughput gains over the existing scheme.
- The proposed protocol enables simultaneous energy harvesting and information transmission at the relay, achieving uninterrupted information transmission and self-energy recycling.
- Under the MISO channel setup and AF relaying, the paper derives the relay’s optimal power allocation and beamforming design.
- Numerical results show significant throughput gains for the proposed design over the existing scheme.
APPENDIX A
Appendix A establishes an equivalent optimization formulation and derives its optimal solution through phase normalization, constraint redundancy, and a transformed vector optimization problem.
- Lemma 1 converts (P1.1) into an equivalent problem whose transformed variables include F, b, and β.
- The appendix reformulates (P1.1) by restricting f̂^H v to a real number without loss of generality.This follows from the arbitrary phase rotation of an optimal vector v.
- The constraint Im(f̂^H v) = 0 is shown to be redundant in the relaxed problem.A phase-adjusted, slightly scaled vector would otherwise improve the objective while remaining feasible, contradicting optimality.
- The optimal transformed vector is obtained by attaining the stated upper bound, after which F^−1 is evaluated to recover the solution to the original problem.
APPENDIX B
Appendix B characterizes the solution of (P2) as a convex optimization problem whose stationary point is represented by the unique root of f(z) in a specified interval.
- Problem (P2) is convex, and its solution is given by the stationary point in the relevant interval.
- With z = 1 + γ1α/[α + C(1−α)], the variable satisfies 1 < z < 1 + γ1 and equation (19) becomes f(z) = 0.