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Enhancing Wireless Information and Power Transfer by Exploiting Multi-Antenna Techniques

Xiaoming Chen, Zhaoyang Zhang, Hsiao-Hwa Chen, Huazi Zhang

arXiv:1501.02429v1cs.IT

TL;DR

WIPT must combat fading effects while balancing spectral and energy efficiencies, and obtaining instantaneous CSI at the transmitter is nontrivial. This paper presents a tutorial on multi-antenna WIPT, emphasizing parameter optimization, protocol design, and simulations of proposed schemes.

  • Problem

    WIPT requires effective countermeasures against fading effects, while instantaneous CSI acquisition at the transmitter remains nontrivial.

  • Method

    The paper gives a comprehensive tutorial on multi-antenna WIPT, analyzing tradeoffs through spatial beam design, parameter optimization, protocol design, full-duplex, and LS-MIMO techniques.

  • Results

    Simulations validate the WIPT tradeoffs and the effectiveness of the proposed schemes in two typical multi-antenna scenarios.

  • Takeaways & Limitations

    Multi-antenna techniques provide a framework for improving WIPT efficiency over fading channels and addressing its theoretical and design issues.

Abstract

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This paper reviews emerging wireless information and power transfer (WIPT) technique with an emphasis on its performance enhancement employing multi-antenna techniques. Compared to traditional wireless information transmission, WIPT faces numerous challenges. First, it is more susceptible to channel fading and path loss, resulting in a much shorter power transfer distance. Second, it gives rise to the issue on how to balance spectral efficiency for information transmission and energy efficiency for power transfer in order to obtain an optimal tradeoff. Third, there exists a security issue for information transmission in order to improve power transfer efficiency. In this context, multi-antenna techniques, e.g., energy beamforming, are introduced to solve these problems by exploiting spatial degree of freedom. This article provides a tutorial on various aspects of multi-antenna based WIPT techniques, with a focus on tackling the challenges by parameter optimization and protocol design. In particular, we investigate the WIPT tradeoffs based on two typical multi-antenna techniques, namely limited feedback multi-antenna technique for short-distance transfer and large-scale multiple-input multiple-output (LS-MIMO, also known as massive MIMO) technique for long-distance transfer. Finally, simulation results validate the effectiveness of the proposed schemes.

I. INTRODUCTION

WIPT uses multi-antenna spatial degrees of freedom to address fading, transfer-efficiency, spectral–energy-efficiency, and security challenges. The paper reviews parameter optimization and protocol design across diverse multi-antenna WIPT forms.

  • Challenges: WIPT is more sensitive to channel fading and path loss than information decoding, shortening practical power-transfer distance.The paper identifies effective fading mitigation as necessary to improve transfer efficiency and distance.
  • Multi-antenna approach: Multi-antenna beamforming adapts transmit signals to channel states and aligns RF energy with power receivers.The transmitter can select beams, power, and accessing users using full or partial CSI.
  • Challenges: Information and power compete for shared RF resources, creating a tradeoff between spectral efficiency and energy efficiency.Spatial beams can be designed to balance these efficiencies.
  • Multi-antenna approach: Multi-antenna techniques can concurrently support multiple information and power streams, improving the corresponding efficiencies.They can also separate information and power spatially and support adaptive transmission over fading channels.
  • CSI acquisition: CSI acquisition presents a central design tradeoff: larger FDD feedback codebooks improve accuracy but increase overhead, whereas TDD saves feedback resources but can lose performance through hardware impairments.Estimated-CSI beamforming also suffers performance degradation relative to instantaneous-CSI beamforming.
  • Scope and organization: The article surveys transmission frameworks spanning traditional and LS-MIMO systems, single- and multi-user access, CSI feedback and estimation, and SWIPT and WPC protocols.It analyzes parameter optimization, protocol design, and the effects of CSI accuracy and antenna number using representative traditional and LS-MIMO cases.

II. MULTI-ANTENNA BASED WIPT TECHNIQUE

Multi-antenna WIPT addresses transfer efficiency, distance, tradeoff, and security constraints in wireless channels. The section motivates diversity, multiplexing, beamforming, and protocol choices while distinguishing SWIPT from WPC.

  • Fundamental metrics: RF attenuation, noise, interference, and interception create fundamental efficiency, distance, tradeoff, and security challenges for WIPT.Power receivers are especially sensitive to weak RF signals, and wireless power transfer has stringent distance limitations.
  • Fundamental metrics: Shared power, spectrum, and time resources create information–power tradeoffs that require optimization of their allocation.SWIPT distributes transmit power, whereas WPC divides time slots between information and power transfer.
  • Multi-antenna solutions: Multi-antenna diversity combats fading and can improve transfer efficiency and distance, while multiplexing separates information and power spatially.Spatial separation can support transfer tradeoffs and security simultaneously.
  • Multi-antenna solutions: Transmitting information in the power-transfer channel’s null space can support physical-layer security even when the power receiver is close to the transmitter.This example uses spatial structure to protect information while power transfer continues.
  • Scope: The paper provides a detailed investigation of single-hop multi-antenna WIPT and covers SWIPT, WPC, and their integration.Multi-hop transmission is identified as future work, while the study presents protocol designs and parameter optimizations for WPC.

A. Simultaneous Wireless Information and Power Transfer

SWIPT simultaneously transfers information and power, using channel-adapted spatial beamforming when the transmitter has multiple antennas. Information and power receivers may be combined or separated.

  • SWIPT operation: SWIPT transmits information and power simultaneously over wireless signals.The paper considers multi-antenna transmitters whose spatial beamforming adapts to channel states.
  • Receiver configurations: In multi-antenna SWIPT, information and power receivers can be either combined in one node or separated across nodes.These receiver configurations define distinct SWIPT subcases.

1) Combined Case:

The combined SWIPT case requires receiver protocols and resource allocation because information decoding and energy harvesting cannot occur simultaneously. Multiple receivers add scheduling, beam-design, tradeoff, and security challenges.

  • Receiver protocols: A combined receiver cannot decode information and harvest energy simultaneously because of physical constraints, so a protocol must separate the two operations.The section considers time division and power splitting protocols.
  • Receiver protocols: Time division separates information and power transfer into durations, while power splitting separates the received signal into decoding and harvesting parts.Time division switches receiver roles across durations; power splitting performs separation at baseband.
  • Receiver protocols: Time division requires two RF receive modules, whereas power splitting requires one because separation occurs at baseband.The protocols therefore differ in receiver hardware requirements.
  • Tradeoff optimization: The WIPT tradeoff can be formulated as maximizing information rate subject to minimum harvested power, or maximizing harvested power subject to minimum information rate.Time or power resources are constrained and must be allocated according to an optimization objective.
  • Multiple receivers: Multiple combined receivers require urgency-based scheduling, receiver-specific allocations, and beam designs that address conflicting information and power objectives.Information beams mitigate inter-user interference, whereas power transfer can benefit from inter-user interference.
  • Multiple receivers: Eavesdropping by one or more receivers creates additional security problems, and TDMA may be suboptimal relative to SDMA.Designing an optimal multiple-access protocol remains an open issue.

2) Separated Case:

In the separated SWIPT case, information and power receivers occupy different nodes, so the transmitter uses spatial beamforming and power allocation to balance transfer objectives and security.

  • Information and power receivers are separated into different nodes, with the transmitter sending RF signals for information and power transfer.
  • The transmitter uses beamforming to separate information and power transfer spatially and reduce information leakage to power receivers.
  • Transmit power is allocated between information and power beams to achieve a tradeoff between the two transfers.
  • The secrecy-rate problem maximizes secrecy rate subject to minimum harvested power, but zero-forcing beamforming and maximum transmit power may be suboptimal.These choices can reduce legitimate-channel capacity or increase information leakage.
  • With multiple power receivers, individual interception is governed by the strongest interceptor, whereas cooperative interception uses combined eavesdropper signal quality.
  • SDMA creates inevitable inter-user interference, especially with imperfect transmitter CSI, leaving transmit-beam design more complicated and open.

B. Wireless Powered Communication

Wireless powered communication uses harvested power to transmit information, coupling energy acquisition and communication performance. The paper describes protocol and optimization designs, while LS-MIMO supports longer-distance, lower-power transfer and improved security.

  • WPC uses harvested power for information transmission, so power acquisition is closely coupled to the information rate.
  • Time-division WPC optimizes the tradeoff by choosing a switching point between power transfer and information transfer.
  • The time-division objective can maximize information rate for given transmit power or minimize transmit power subject to a minimum rate.
  • LS-MIMO generates high-resolution spatial beams using tens or hundreds of antennas.
  • Large-scale MIMO can significantly improve transfer efficiency and distance through array gain, enabling long-distance WPC with low power.
  • High-resolution beams can reduce information leakage to unintended nodes, while performance gain becomes larger as antenna count increases.

C. Integration of SWIPT and WPC

General WIPT integrates SWIPT and WPC: harvested power enables subsequent information transmission, and full-duplex operation can improve efficiency while multi-antenna processing cancels self-interference.

  • During WPC, power receivers use harvested power to send information to next-hop receivers.
  • General WIPT concatenates SWIPT and WPC, with each slot divided into SWIPT and WPC durations.
  • With time division at the SWIPT stage, each slot contains three non-overlapped durations for the integrated transfers.
  • Full-duplex technology permits simultaneous information transmission and reception at the power receiver and can improve efficiency.
  • Full-duplex operation introduces self-interference, which multi-antenna technology can cancel by exploiting spatial degrees of freedom.
  • The paper characterizes multi-antenna WIPT as a solution for efficient, reliable, secure, and long-distance transfer.

III. WIRELESS INFORMATION AND POWER TRANSFER TRADEOFF

WIPT design must balance information and power objectives that can be inconsistent or contradictory. The paper addresses this through parameter optimization and protocol design, including CSI feedback, transfer duration, beamforming, and constraints.

  • Information transfer emphasizes rate, delay, and security, whereas power transfer emphasizes efficiency and distance, creating an important WIPT tradeoff.
  • Design parameters include transmit beams, transmit power, transfer duration, user scheduling, channel selection, and transfer protocol.
  • Tradeoffs can use multi-objective optimization, a utility function such as a weighted efficiency sum, or one metric constrained by another.
  • A common formulation maximizes information rate subject to a minimum harvested-power constraint, while WPC directly links harvested power to information transmission.
  • In a basic multi-antenna WPC model, harvested power increases with CSI feedback, transfer duration, and transmit power but decreases with transfer distance.
  • For fixed CSI feedback and transmit power, optimizing average information rate determines the power-transfer duration, while varying feedback produces different tradeoffs.
  • Energy beamforming directs RF signals according to the current channel state to overcome channel fading and propagation loss.
  • QoS, minimum-rate, power-splitting, and secrecy constraints extend the optimization, but fixed transmit power and feedback may yield no feasible transfer duration.

B. Energy Efficiency Maximization in LS-MIMO Systems

LS-MIMO WIPT uses large antenna arrays and channel reciprocity to support efficient long-distance transfer while optimizing information-per-energy tradeoffs. Its feasibility and performance depend on antenna count and CSI accuracy.

  • LS-MIMO generates high-resolution spatial beams with large antenna arrays, enabling substantial power-transfer efficiency and distance.
  • Exact CSI is difficult to obtain through feedback because the required feedback grows with the number of antennas, so LS-MIMO commonly uses TDD channel reciprocity.
  • Transceiver hardware impairment makes reciprocity-based CSI estimates imperfect, causing performance loss and making CSI accuracy decisive.
  • Harvested power depends on transmit power, transfer duration, CSI accuracy, and transmit-antenna number; information rate additionally depends on receiver-antenna number.
  • Energy efficiency maximization defines the tradeoff by dividing transmitted information by total energy consumption, including amplifier and circuit energy.
  • Adding antennas can make extended QoS and secrecy-constrained problems feasible and improve LS-MIMO WIPT performance.

IV. PERFORMANCE ANALYSIS AND SIMULATIONS

Simulations evaluate multi-antenna WIPT tradeoffs for short-distance traditional systems and long-distance LS-MIMO systems. Limited feedback can approach full-feedback performance, while more LS-MIMO antennas improve energy efficiency without increasing transmit power.

  • Traditional multi-antenna WPC: For Nt = Nr = 4 and d = 10 m, feedback amount B strongly affects the information-rate tradeoff.
  • Traditional multi-antenna WPC: B = 2 remarkably increases information rate over no feedback, but additional gains diminish as feedback increases.
  • Traditional multi-antenna WPC: With B = 4, the performance gap to ideal full feedback is small, showing that limited CSI feedback can effectively enhance WIPT.
  • LS-MIMO WPC: For Nr = 100, ρ = 0.9, and d = 50 m, LS-MIMO supports long-distance transfer without consuming more transmit power than traditional multi-antenna techniques.
  • LS-MIMO WPC: The LS-MIMO energy-efficiency tradeoff is strongly affected by antenna number, and adding antennas further improves energy efficiency.
  • LS-MIMO WPC: Higher antenna counts enable high QoS for WPC with affordable power even in the presence of one node.

V. CONCLUSION AND FUTURE WORKS

The paper reviews multi-antenna WIPT technologies, emphasizing parameter optimization and protocol design for managing WIPT tradeoffs. Simulations validate the proposed schemes, while several multiuser and full-duplex issues remain open.

  • The paper introduces and analyzes WIPT tradeoffs based on multi-antenna techniques, including limited-feedback and LS-MIMO approaches.
  • The tutorial covers multi-antenna WIPT through both parameter optimization and protocol design.
  • Simulation results validate the tradeoffs using proposed schemes based on two typical multi-antenna techniques.
  • Open issues include multiuser scheduling, careful beam construction under imperfect CSI, and optimal use of full-duplex self-interference.Full-duplex self-interference harms information transmission but can enhance power transfer, so complete cancellation is not necessarily optimal.
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