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Wireless Powered Communication: Opportunities and Challenges

Suzhi Bi, Chin Keong Ho, Rui Zhang

arXiv:1408.2335v2cs.NIcs.IT

TL;DR

Limited battery life and recurring replacement or recharging disrupt wireless-device operation. The paper surveys RF-enabled WET and its applications to WPC, focusing on receiver and signal-processing methods, SWIPT tradeoffs, and WPCN challenges; it concludes that WPC could significantly improve battery-powered communication using simple, inexpensive transceivers.

  • Problem

    Limited battery life and frequent manual battery replacement or recharging constrain wireless communication, motivating wireless energy and information transmission designs.

  • Method

    The paper reviews state-of-the-art RF-enabled WET technologies and examines receiver circuits, signal processing, SWIPT tradeoffs, and WPCN design challenges and solutions.

  • Results

    The paper concludes that wireless powered communications could significantly improve battery-powered communication and be practically achieved with simple, inexpensive transceiver structures.

  • Takeaways & Limitations

    SWIPT and WPCN provide the paper's central paradigms for studying opportunities and challenges in wireless powered communications.

Abstract

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The performance of wireless communication is fundamentally constrained by the limited battery life of wireless devices, whose operations are frequently disrupted due to the need of manual battery replacement/recharging. The recent advance in radio frequency (RF) enabled wireless energy transfer (WET) technology provides an attractive solution named wireless powered communication (WPC), where the wireless devices are powered by dedicated wireless power transmitters to provide continuous and stable microwave energy over the air. As a key enabling technology for truly perpetual communications, WPC opens up the potential to build a network with larger throughput, higher robustness, and increased flexibility compared to its battery-powered counterpart. However, the combination of wireless energy and information transmissions also raises many new research problems and implementation issues to be addressed. In this article, we provide an overview of state-of-the-art RF-enabled WET technologies and their applications to wireless communications, with highlights on the key design challenges, solutions, and opportunities ahead.

I. INTRODUCTION

The article presents RF-enabled wireless powered communication as a response to limited battery life and costly battery replacement, using dedicated transmitters to deliver stable energy over the air. It surveys WET technologies, WPC applications, joint energy-information transmission, and the design challenges and opportunities of SWIPT and WPCN.

  • Motivation: Limited battery life and frequent battery replacement or recharging constrain wireless-device operation, especially when many devices are deployed.The issue is particularly problematic for sensors embedded in structures and other difficult-to-access devices.
  • RF-enabled WET: RF-enabled WET remotely transfers microwave energy to wireless devices, offering long operating range, small receivers, low production cost, and energy multicasting.Wireless receivers harvest energy from RF signals radiated by dedicated transmitters through far-field electromagnetic waves.
  • Wireless powered communication: WPC uses harvested RF energy for communication, aiming to avoid energy-depletion interruptions and support more sustainable throughput, lower maintenance cost, and flexible deployment.The article identifies IoT/IoE systems, wireless sensor networks, and smart power grids as application areas.
  • Wireless powered communication: Unlike ambient RF energy harvesting, WET provides a stable energy source with controllable transmit power, waveforms, and occupied time/frequency resources.This controllability supports WPCNs that power multiple communication devices with different physical conditions and service requirements.
  • SWIPT and WPCN: SWIPT jointly transmits information and energy over the same waveform and is more spectrum-efficient than orthogonal time- or frequency-channel transmission.The paper presents SWIPT and WPCN as two paradigms that expose new design opportunities and challenges.
  • Article scope: The article reviews WET receiver circuits and signal processing, SWIPT energy-information tradeoffs, WPCN design challenges and solutions, and future research directions.Its conclusion reports that WPC could significantly improve on battery-powered communication using simple and inexpensive transceiver structures.

II. STATE-OF-THE-ART OF RF-ENABLED WET

RF-enabled WET spans near-field coupling and far-field radiation, with WPC networks supporting energy transfer, information transfer, or both. Its practical advantages include flexible positioning and multi-receiver broadcasting, while distance attenuation and interference remain central challenges.

  • RF-enabled WET technologies: RF-enabled WET technologies use inductive coupling, magnetic resonant coupling, or electromagnetic radiation.The first two exploit non-radiative near-field properties, while electromagnetic radiation supports longer-distance transfer.
  • RF-enabled WET technologies: Inductive coupling is standardized for mobile phones and implanted medical devices but typically operates over only several centimeters.Its operating range is limited by the rapid decline of magnetic induction with distance.
  • RF-enabled WET technologies: RF-enabled WET can power devices over moderate to long distances, including RFID tags at 4 meters and harvesting chips with 12-14 meter line-of-sight radii.The cited examples report approximately 0.5 mW and 0.05 mW received RF power, respectively.
  • RF-enabled WET technologies: RF-enabled WET uses inexpensive, tiny, flexibly positioned receivers and can broadcast energy to multiple receivers, but microwave attenuation over distance is its major constraint.Declining device power consumption and MIMO-based efficiency improvements support future applications.
  • Network models and operating modes: WPC networks may separate or co-locate energy and information access points, with devices harvesting energy and using it for uplink or downlink communications.The canonical modes are WET for downlink energy only, SWIPT for simultaneous downlink energy and information, and WPCN for downlink energy followed by uplink information transfer.
  • Network models and operating modes: Energy transmissions can interfere with information receivers sharing the same frequency band, motivating joint energy/information transmission design.The network model includes heterogeneous transmitters, receivers, and operating modes.

IV. WIRELESS ENERGY TRANSFER

RF-enabled WET converts received RF signals into stored energy through receiver circuits and relates harvested energy to received power, transmit power, distance, path loss, and antenna gain. Advanced signal processing, including multiple-antenna beamforming, can substantially improve transfer efficiency.

  • Advanced signal processing: Increasing transmitted energy through antenna techniques can be more cost-effective than raising receiver conversion efficiency from 25% to 99% with more sophisticated rectifying circuits.The comparison concerns the practical cost of improving the transmitter-side energy delivery versus receiver-side conversion.
  • Energy signal design: Wireless energy signals are generally modulated and occupy bandwidth determined by the modulated baseband signal.Signal design can avoid power spectral density spikes and satisfy safety and interference requirements.
  • RF energy receiver model: RF energy receivers use a rectifying circuit with a diode and passive low-pass filter to convert received RF signals into DC energy.The converted energy charges a built-in battery for storage.
  • RF energy transfer model: Harvested energy per symbol time is proportional to received RF power, expressed as Q = η · Pr = η · Pt · D^-α · GA.η is receiver conversion efficiency, Pt transmit power, D normalized distance, α path loss factor, and GA combined antenna gain.
  • RF energy transfer model: The transfer model incorporates receiver efficiency, transmit power, distance-dependent path loss, and combined transmit/receive antenna gain.The path loss factor satisfies α ≥2 and the receiver efficiency satisfies 0 < η < 1.
  • Advanced signal processing: Two antennas at both transmitter and receiver can increase harvested energy by about 4 times, or 6 dB, at the same transmit power.This is compared with single-antenna transmitter and receiver configurations.

B. Energy beamforming

Energy beamforming focuses transmitted power spatially to improve harvesting, but multiple receivers create fairness and channel-state challenges. Proposed solutions include multi-beam, robust, and distributed-antenna designs.

  • Energy beamforming: Multiple antennas sharpen energy beams toward selected spatial directions, maximizing harvested energy for a single receiver.The beam becomes sharper as the number of transmitter antennas increases.
  • Energy beamforming: With multiple energy receivers, a single beam can create severe energy near-far unfairness.Receivers near the transmitter may harvest much more energy than distant receivers.
  • Energy beamforming: Multiple energy beams can balance harvesting performance among receivers in different directions.The transmitter may direct separate beams toward receivers with unequal received energy.
  • Energy beamforming: Accurate CSIT supports efficient energy-beam design, but energy receivers often lack channel-estimation capability and mobility makes tracking difficult.Channel estimation can also consume time and energy that offset refined beamforming gains.
  • Energy beamforming: Robust beamforming under imperfect CSIT can use statistical channel knowledge or received-energy observations to balance energy consumption and beamforming gain.These approaches address uncertainty when instantaneous channel information is unavailable or unreliable.
  • Energy beamforming: Distributed antennas reduce feedback needs and near-far effects, but require efficient coordination.Receivers harvest mainly from nearby transmitters, reducing transmitter-receiver distance.

V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER

SWIPT jointly transmits information and energy over the same waveform, but transmitter and receiver designs must balance their different performance objectives. Practical receiver structures determine the achievable rate-energy tradeoff.

  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: SWIPT saves spectrum by transmitting information and energy jointly over the same waveform.An information waveform also carries energy that the same or another receiver can harvest.
  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: Efficient SWIPT requires a rate-energy tradeoff that balances information decoding and energy harvesting.Information- and energy-optimal waveforms differ, so an imbalanced design can harm one objective.
  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: A multi-antenna HAP can simultaneously serve information-only, energy-only, and joint information-energy receivers.The network model includes receivers with heterogeneous operating modes.
  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: EH and ID receivers have different power sensitivities, so energy-harvesting receivers are generally closer to the transmitter.The cited example gives −10 dBm for EH receivers versus −60 dBm for ID receivers.
  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: Ideal SWIPT receivers can decode information and harvest energy from the same signal, whereas practical circuits cannot realize this directly.The paper introduces TS, PS, IntRx, and AS as practical receiver structures.
  • V. SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER: The rate-energy region is the union of achievable rate-energy pairs, with its boundary giving the maximum data rate for a specified harvested-energy requirement.This characterization is used to compare receiver structures in point-to-point channels.

1) Time switching (TS) receiver:

The receiver structures separate or combine energy harvesting and information decoding in different ways, producing distinct rate-energy regions. TS and PS expose explicit design controls, while IntRx trades capacity for energy efficiency.

  • 1) Time switching (TS) receiver:: TS divides each transmission block into orthogonal energy-transfer and data-transmission slots.The receiver switches periodically between harvesting energy and decoding information.
  • 1) Time switching (TS) receiver:: Varying the energy-transfer slot length changes the achievable rate-energy tradeoff for TS.The transmitter can optimize waveforms separately for energy or information in the corresponding slot.
  • 1) Time switching (TS) receiver:: PS splits the received signal into EH and ID streams with power ratios ρ and (1 −ρ), respectively.Adjusting ρ produces different rate-energy tradeoffs.
  • 1) Time switching (TS) receiver:: IntRx converts RF to baseband using a passive rectifier, reducing circuit power relative to the active mixer in TS/PS information decoders.Its information receiver instead performs noncoherent detection on the DC-current signal.
  • 1) Time switching (TS) receiver:: IntRx uses energy modulation rather than conventional PAM, which reduces capacity, but is superior to PS/TS when more harvested energy is required.The advantage follows from avoiding active frequency down conversion.
  • 1) Time switching (TS) receiver:: The ideal receiver has a box-shaped rate-energy region, while IntRx has a similar region because only an infinitesimal DC current is used for ID.TS forms a straight line between optimal EH and ID operating points, whereas PS has a strictly larger region than TS.
  • 1) Time switching (TS) receiver:: The optimal EH-ID receiver remains unknown, including whether the non-trivial region between ideal and practical receivers is achievable.The open problem may require physics, circuit theory, and information theory.

C. Design challenges and opportunities

Wireless powered networks face fading, interference, multi-receiver fairness, spectrum-sharing, and channel-estimation challenges. Adaptive receivers, beamforming, relaying, and WPCN extensions provide supported avenues for addressing them.

  • C. Design challenges and opportunities: Channel fading degrades both information decoding and energy harvesting, while strong interference harms ID but can help EH.Receivers can adapt their strategies to channel conditions and interference levels.
  • C. Design challenges and opportunities: TS should favor ID under weak received signals and sufficiently high SNR, whereas PS should allocate more power to ID in poor channels.More received power can be allocated to EH when channel conditions are better.
  • C. Design challenges and opportunities: CSIT-aware power adaptation avoids transmitting during deep channel fading and improves information and energy transfer efficiency.MIMO further mitigates fading through energy beamforming, spatial diversity, and multiplexing.
  • C. Design challenges and opportunities: Multi-antenna HAPs focus radiation at intended locations, enhancing harvested energy while mitigating interference to unintended information receivers.Antenna switching provides a lower-complexity alternative to power splitting at each receive antenna.
  • C. Design challenges and opportunities: The rate-energy region of antenna switching approaches that of power splitting when the number of receive antennas is large.Antenna switching assigns antenna subsets to EH and ID instead of splitting power at every antenna.
  • C. Design challenges and opportunities: Relaying lets a node harvest energy, receive a message, and forward it in another time slot to extend HAP coverage.The relay supports communication with a receiver located farther from the HAP.
  • C. Design challenges and opportunities: The paper frames WPCN as a complementary architecture in which devices use harvested energy to transmit information.It introduces basic operations, design challenges, solutions, and extensions to practical network models.

A. Harvest-then-transmit protocol

The harvest-then-transmit protocol separates downlink wireless energy transfer from uplink data transmission, creating a trade-off in selecting the harvesting duration. WPCNs must also coordinate multiple access and resource allocation to address severe near–far unfairness.

  • Harvest-then-transmit protocol: During the first phase, devices harvest energy for τ0T, then use the remaining block time to transmit uplink data.The half-duplex HAP supports this two-phase operation within a block of duration T.
  • Harvest-then-transmit protocol: A larger τ0 increases harvested energy and potential uplink rate but leaves less time for data transmission, so its optimum depends on channel conditions.The optimal τ0 is smaller when users are close to the HAP and larger when far users need more harvesting time.
  • Fairness challenge: The doubly-near-far problem can leave far users with 100 times less data rate than nearby users without careful multiple-access design.Far users harvest less downlink energy and require more uplink transmit energy because of signal attenuation.
  • Multiple access: TDMA can allocate longer uplink times to far users, whereas SDMA lets users transmit simultaneously and generally provides higher spectrum efficiency.With multiple HAP antennas, joint decoding uses multi-user detection in the uplink.
  • Fairness enhancement: Energy beamforming, uplink power control, and user cooperation can mitigate near–far unfairness by directing resources toward disadvantaged users.A nearby relay can forward a far user's messages; greater relay resource use improves the far user's throughput, while both users can benefit under suitable allocation.

C. Extensions

WPCN extensions address channel-estimation costs, antenna scaling, multi-cell deployment, and applications beyond single-cell networks. These extensions improve throughput, range, or network-level design analysis while introducing additional allocation and interference considerations.

  • Channel estimation: Channel estimation consumes device energy, creating a trade-off because more accurate CSI improves beamforming, transmission rate, reliability, and subsequent harvested energy.Energy for estimation and information transmission must be allocated jointly.
  • Massive MIMO: Massive MIMO supports simultaneous uplink spatial multiplexing and sharper downlink energy beams, with more than 20 dB power gain reported.Its application to WPCN can produce multiple-fold throughput improvement and longer operating range.
  • Massive MIMO: Large user-to-HAP channel orthogonality can simplify beamforming, multiple-access control, and power-control solutions despite many antennas.The high antenna count does not necessarily imply proportionally high processing complexity and cost.
  • Multi-cell WPCN: In multi-cell WPCNs, stochastic geometry is used to study capacity scaling and relate base-station and power-beacon densities and transmit powers to outage probability.Single-cell resource-allocation methods are often insufficient for deriving multi-cell network capacity.
  • Other applications: WPCN also extends to cognitive radio and energy-constrained multi-hop systems, where harvesting opportunities and interference constraints shape transmission design.A secondary transmitter may harvest from a nearby primary transmitter while transmitting only when sufficiently far away to avoid interference.

relays, systems with densely deployed HAPs using millimeter-wave technologies, and distributed antenna

Future WPC research must address coexistence between wireless energy and communication networks, cross-layer coordination, and application-specific scheduling. These challenges arise from asymmetric interference, device energy constraints, and the need to jointly optimize physical and medium-access layers.

  • Network coexistence: Overlapping spectrum creates one-way interference from energy networks to communication networks, with WET interference generally much stronger than information signals.Scheduling, energy beamforming, and spectrum sensing are identified as mitigation approaches.
  • Network coexistence: Cognitive radio technology can support coexistence by enabling opportunistic wireless energy transfer based on spectrum sensing.This approach is presented alongside information/energy transfer scheduling and energy beamforming design.
  • Cross-layer design: WPC design should span PHY and MAC layers because MAC determines system fairness and efficiency in practical operation.Cross-layer beam steering can consider both channel strength and queued data rather than channel conditions alone.
  • Cross-layer design: Energy scheduling should account for residual battery life, wake-up or sleep schedules, and expected device energy consumption.These factors complement physical-channel information in allocating wireless energy.

C. Hardware implementation

Practical deployment requires hardware prototyping, safety-aware antenna design, and validation of proposed WPC technologies under real wireless conditions. The article concludes that RF-enabled WET, SWIPT, and WPCN are important building blocks for future systems combining energy and information transfer.

  • Hardware implementation: Existing WPC studies are mainly theoretical, and achievable throughput with off-the-shelf harvesting and communication modules in practical environments remains unknown.Hardware prototypes and testbeds are needed to evaluate feasibility and application scenarios.
  • Hardware implementation: Testbeds can evaluate joint energy and information transmission technologies including massive MIMO, millimeter wave, and distributed antenna systems.They can also help identify suitable technologies and application scenarios.
  • Safety: Strong microwave intensity can create human-health and safety concerns, motivating real-time sensing that can stop harmful energy transmission.The concern is especially relevant when advanced beamforming concentrates energy spatially.
  • Safety: Distributed antennas can keep each antenna's radiation relatively weak while producing constructive interference at the destination and destructive interference elsewhere.This design satisfies EIRP constraints and reduces the risk of radiation burns from random human blockage.
  • Conclusion: The article surveys RF-enabled WET technologies and communications applications, using SWIPT and WPCN to illustrate design opportunities and challenges.It argues that simple, inexpensive transceiver structures could make wireless powered communications practically achievable and improve on battery-powered systems.
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