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Secrecy Wireless Information and Power Transfer: Challenges and Opportunities

Xiaoming Chen, Derrick Wing Kwan Ng, Hsiao-Hwa Chen

arXiv:1512.06528v2cs.IT

TL;DR

The paper reviews SWIPT security issues, challenges, and physical-layer security techniques. It proposes massive MIMO for SWIPT and reports improved performance, including a significantly expanded trade-off region and secrecy rate gains for N_T ≥64.

  • Problem

    Security techniques in SWIPT are non-trivial because they must achieve another objective alongside security.

  • Method

    The paper surveys SWIPT scenarios and physical-layer security techniques, then proposes massive MIMO techniques to enhance SWIPT.

  • Results

    Massive MIMO produces performance gains in numerical simulations, with the secrecy rate increasing significantly for N_T ≥64.

  • Takeaways & Limitations

    Massive MIMO can further enhance SWIPT while improving information security and power-transfer performance.

Abstract

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Wireless information and power transfer (WIPT) enables more sustainable and resilient communications owing to the fact that it avoids frequent battery charging and replacement. However, it also suffers from possible information interception due to the open nature of wireless channels. Compared to traditional secure communications, secrecy wireless information and power transfer (SWIPT) carries several distinct characteristics. On one hand, wireless power transfer may increase the vulnerability of eavesdropping, since a power receiver, as a potential eavesdropper, usually has a shorter access distance than an information receiver. On the other hand, wireless power transfer can be exploited to enhance wireless security. This article reviews the security issues in various SWIPT scenarios, with an emphasis on revealing the corresponding challenges and opportunities for implementing SWIPT. Furthermore, we provide a survey on a variety of physical layer security techniques to improve secrecy performance. In particular, we propose to use massive multiple-input multiple-output (MIMO) techniques to enhance power transfer efficiency and secure information transmission simultaneously. Finally, we discuss several potential research directions to further enhance the security in SWIPT systems.

I. INTRODUCTION

SWIPT combines information transmission and wireless power transfer over shared RF signals, but its open channels create leakage risks and a tension between secrecy and energy efficiency. The article surveys these challenges and PHY-security approaches, proposing massive MIMO to improve both objectives.

  • SWIPT carries information and power over the same RF wave, with information receivers decoding data and power receivers harvesting electromagnetic energy.
  • Open wireless channels allow power receivers and other unintended receivers to receive information signals, creating potential information leakage.
  • Physical-layer security exploits fading, noise, and interference, while SWIPT power signals can confuse eavesdroppers and information signals can increase harvested energy.
  • SWIPT security design is a dual-objective problem because secure information transmission and efficient power transfer may conflict.Increasing information-signal power can improve harvested energy but reduce secrecy rate when a power receiver is also an eavesdropper.
  • The article surveys security issues across WIPT scenarios and PHY-security techniques, emphasizing their performance advantages, limitations, and scenario-specific use.
  • The authors propose massive MIMO to enhance information-transmission security and power-transfer efficiency simultaneously, and discuss future research directions.

II. CHALLENGING ISSUES IN SWIPT

SWIPT security must address two equally important objectives: protecting information and transferring power efficiently. Their interaction creates new design challenges because signals may compete for limited system resources.

  • PHY-security requires strengthening the legitimate receiver’s signal while impairing the signal observed by eavesdroppers.
  • SWIPT treats information security and power-transfer efficiency as equally important objectives in PHY-security design.
  • The dual objectives create challenging SWIPT design issues beyond conventional PHY-security.
  • Information and power signals may compete for limited system resources, complicating simultaneous secure transmission and energy transfer.

A. SWIPT in Broadcasting Channels

In broadcasting-channel SWIPT, power receivers can eavesdrop because they are closer and may cooperate, while concurrent information and power transmission creates interference-management trade-offs. However, transmitter CSI of power receivers can support schemes that jointly improve secrecy and satisfy energy-harvesting requirements.

  • Challenges: Power receivers may be closer than information receivers, producing stronger received signals and a high risk of information interception.Power receivers typically require higher received power, so they are placed nearer the transmitter than information receivers.
  • Challenges: Cooperating power receivers can jointly detect signals, potentially obtaining better intercepted-signal quality and causing information leakage.Their cooperation allows multiple receivers to combine observations for detection.
  • Challenges: Broadcasting multiple information and power signals causes strong interference at information receivers, while conventional mitigation can weaken RF signals at power receivers.This trade-off makes conventional inter-user interference mitigation unsuitable for broadcasting-channel SWIPT.
  • Opportunities: Power receivers’ legitimate-user status can provide transmitter CSI for designing transmissions that optimize secrecy while meeting energy-harvesting QoS requirements.The transmitter can adapt schemes using instantaneous CSI of potential eavesdroppers.

B. SWIPT in Relaying Systems

Relaying systems introduce risks from untrusted relays and repeated signal exposure, but cooperative relaying can improve secrecy and power transfer through virtual MIMO and coordinated beamforming. SWIPT relays may either split received signals for information relaying and energy harvesting or forward information and power concurrently.

  • Relaying modes: SWIPT relaying has two modes: wireless-powered relays split received signals between information relaying and energy harvesting, while self-powered relays forward information and power concurrently.Both modes support simultaneous information and power transfer through relay operations.
  • Challenges: Untrusted relays may eavesdrop on decoded signals or intentionally inject power-signal interference that weakens the information receiver’s signal.A relay powered by the source may legally receive and decode the signal, while forwarding-only relays can still disrupt reception.
  • Challenges: Two-slot cooperative relaying exposes two signal copies that eavesdroppers can combine, potentially giving them better signal quality than the information receiver.This makes relaying transmission susceptible to eavesdropping, especially when power receivers need strong signals.
  • Opportunities: Multiple-relay cooperation can create virtual MIMO, using information beamforming toward legitimate receivers and power beamforming toward power receivers.Shared relay antennas support separate beamforming roles for information and energy delivery.

C. SWIPT in Interference Networks

Interference-network SWIPT must coordinate geographically separated transmitters despite limited CSI and conflicting interference effects. Properly designed co-channel interference can nevertheless support both energy harvesting and secrecy by supplying energy and confusing eavesdroppers.

  • Challenges: Geographically separated information and power transmitters may struggle to exchange information and coordinate their transmissions.This uncoordinated-transmission problem complicates simultaneous security and power-transfer enhancement.
  • Challenges: Power receivers can harvest energy from multiple signal streams, but concurrent transmissions create severe interference, low SINR, and higher information-leakage risk at information receivers.The same network activity can benefit energy harvesting while degrading information reception.
  • Challenges: Power receivers may lack baseband circuits for channel estimation, preventing transmitters from adapting secure schemes to instantaneous CSI.CSI is usually fed back from receivers, so unavailable receiver-side estimation limits adaptive security design.
  • Challenges: Interference harms information transmission but benefits power transfer, creating a design trade-off between harvested energy and secrecy-rate improvement.Reducing interference at information receivers can improve secrecy while also reducing harvested energy at power receivers.
  • Opportunities: Properly designed co-channel interference can improve both objectives by increasing harvested energy and acting as artificial noise against eavesdroppers.Multiple signal streams contribute energy, while undesired signals can confuse eavesdroppers.

D. SWIPT in Wireless Powered Communication Networks

SWIPT in wireless powered communication networks must jointly manage information transmission, power transfer, and secrecy under constrained harvested energy. Key challenges include non-convex resource allocation and limited power for anti-eavesdropping.

  • Challenges: The harvested power at a power receiver that also acts as an information transmitter affects information-transmission performance.
  • Challenges: Resource allocation must jointly partition time and allocate transmit power because secrecy performance depends on both power transfer and information transmission.
  • Challenges: The resulting resource-allocation optimization problem is generally non-convex, hindering computationally efficient algorithm design.
  • Challenges: Wireless powered communication networks obtain transmit power through RF energy harvesting, leaving only limited available power for information transmission.
  • Challenges: Insufficient power can make artificial noise too weak to interfere effectively with eavesdroppers, producing weak anti-eavesdropping capability.
  • Opportunities: Assigning resource allocation and anti-eavesdropping tasks to the power transmitter may enable more sophisticated security schemes when it has sufficient power.

E. SWIPT in Cognitive Radio Networks

SWIPT in cognitive radio networks combines secret information and power transmission with wireless charging over licensed spectrum. Its opportunities for cooperation are accompanied by security, interference, and degrees-of-freedom constraints.

  • Challenges: The open and dynamic cognitive-radio architecture allows unknown information and power devices to access licensed spectrum opportunistically, creating eavesdropping vulnerability.
  • Challenges: Cooperative spectrum sensing can give eavesdroppers more knowledge of the information transmitter through signal exchanges.
  • Challenges: Spectrum-access preconditions impose interference constraints that restrict artificial-noise transmit direction and power, degrading secure-scheme performance.
  • Challenges: Coexistence with primary networks can reduce secondary receivers’ signal quality and secrecy rate through interference.
  • Scenario and opportunity: A secondary transmitter can send secret information and power to secondary receivers while wirelessly charging energy-limited primary receivers in exchange for licensed-spectrum access.
  • Scenario and opportunity: This cooperation provides incentives for primary and secondary systems to cooperate and improves overall system performance.

A. Multiple Antenna Techniques

Multiple antennas provide spatial degrees of freedom for improving secrecy and harvested energy in SWIPT. Artificial noise and power signals can simultaneously hinder eavesdroppers and support energy harvesting.

  • Multiple-antenna security: Multiple antennas can strengthen information and power signals at intended receivers while weakening eavesdropper signals, improving secrecy and harvested energy.
  • Multiple-antenna security: Transmitting information signals in eavesdropper-channel null spaces can prevent eavesdroppers from overhearing information.
  • Multiple-antenna security: Power signals can confuse eavesdroppers while still meeting power receivers’ energy-harvesting requirements through adaptive adjustment.
  • Artificial noise: Artificial noise must balance interference toward eavesdroppers against interference toward legitimate information receivers.
  • Artificial noise: With full legitimate-receiver CSI, artificial noise can be transmitted in the main channel’s null space; imperfect CSI causes leakage into that channel.
  • Artificial noise: Artificial noise has dual use in SWIPT: it degrades eavesdropper channels while acting as an energy source for power harvesting.

C. Resource Allocation

SWIPT resource allocation must balance secure information transmission and efficient power transfer across coupled receivers and eavesdroppers. Massive MIMO and cooperative relaying offer important opportunities, but CSI and coordination constraints remain.

  • Resource-allocation challenge: Resource allocation affects information receivers, power receivers, and eavesdroppers simultaneously, creating coupled performance metrics and complicating algorithm design.
  • Resource-allocation challenge: Partial or imperfect CSI makes it difficult to design an optimal allocation scheme that simultaneously achieves secure information transmission and efficient power transfer.
  • Relay selection: Cooperative relaying shortens signal-propagation distance and can improve SWIPT performance by assigning relays near receivers or eavesdroppers different roles.
  • Relay selection: Relays near power receivers send power signals, while relays near eavesdroppers transmit artificial noise; relay selection can improve SWIPT performance.
  • Relay selection: Cooperative relaying requires information exchanges between multiple relays, leading to high overhead.
  • Massive MIMO: Massive MIMO improves SWIPT through large array gain, accurate beam steering, spatially focused transmission, and simpler signal processing.
  • Massive MIMO: With sufficiently many antennas, leakage information is expected to become negligible, and channel hardening simplifies performance analysis and optimization.
  • Massive MIMO: In the example, the harvested-power/secrecy-rate trade-off region increases significantly with NT, particularly for massive MIMO with NT ≥64.

V. FUTURE RESEARCH DIRECTIONS

Future SWIPT research must address uncertain channel knowledge, conflicting design goals, and the need for robust secure beamforming. The paper identifies massive MIMO and low-overhead distributed schemes as promising directions.

  • SWIPT remains a critical research topic with challenging future research directions.
  • Perfect global CSI could enable beamformers that jointly support secure information transmission and efficient power transfer.
  • CSI acquisition is difficult because eavesdroppers may be passive, power receivers may lack baseband circuits, and information-receiver CSI may be imperfect.
  • Robust secure beamforming with partial and imperfect CSI is therefore necessary for SWIPT.

B. Multi-Objective System Design

SWIPT system design involves multiple conflicting objectives, including secrecy and power-transfer efficiency, across heterogeneous distributed nodes. The paper reviews security challenges and techniques, proposes massive MIMO, and identifies future directions.

  • Multi-Objective System Design: Secure information transmission and efficient power transfer may not align, creating multiple conflicting system-design objectives.
  • Multi-Objective System Design: Applying single-objective optimization to energy-efficient SWIPT networks may not produce satisfactory performance.
  • Multi-Objective System Design: Multi-objective or vector optimization should be adopted to handle conflicting objective functions.
  • Multi-Objective System Design: Distributed nodes with different tasks require broad system knowledge, which significantly increases signalling overhead.
  • Multi-Objective System Design: Low-overhead distributed transmission schemes are identified as important for realizing the potential of multiple nodes.
  • Multi-Objective System Design: The paper reviews SWIPT scenarios and physical-layer security techniques, proposes massive MIMO, reports numerical performance gains, and identifies research directions.
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