Source-linked AI summary
Cutting Last Wires for Mobile Communications by Microwave Power Transfer
Kaibin Huang, Xiangyun Zhou
TL;DR
Mobile devices remain tethered for recharging because battery advances have lagged behind wireless communication, motivating MPT as a route to remove the “last wires.” The paper introduces WPC by examining MPT efficiency, mobile energy harvesting, SWIPT architectures, and safety and interference challenges. It identifies beam efficiency as the bottleneck for long-distance MPT and describes practical range and design constraints for wirelessly powered devices.
Problem
Mobile devices require periodic grid tethering because battery advances have lagged, while integrating wireless power with communications raises efficiency, architecture, safety, and interference challenges.
Method
The article introduces WPC through MPT fundamentals, mobile energy-harvester architecture, SWIPT configurations, and key design issues with possible solutions.
Results
Beam efficiency is the bottleneck in end-to-end MPT over long distances, while practical WPC operation is constrained by device power requirements, transfer range, safety, and interference.
Takeaways & Limitations
WPC requires seamless integration of information and power transfers, including network architectures and techniques for efficient and safe mobile power delivery.
Takeaways & Limitations
Pilot contamination can reduce both information-transfer performance and power-transfer efficiency by steering reflected beams toward unintended mobiles.
Abstract
from arXiv · showhide
The advancements in microwave power transfer (MPT) over past decades have enabled wireless power transfer over long distances. The latest breakthroughs in wireless communication, namely massive MIMO, small cells and millimeter-wave communication, make wireless networks suitable platforms for implementing MPT. This can lead to the elimination of the "last wires" connecting mobile devices to the grid for recharging, thereby tackling a long-standing ICT grand challenge. Furthermore, the seamless integration between MPT and wireless communication opens a new area called wirelessly powered communications (WPC) where many new research directions arise e.g., simultaneous information-and-power transfer, WPC network architectures, and techniques for safe and efficient WPC. This article provides an introduction to WPC by describing the key features of WPC, shedding light on a set of frequently asked questions, and identifying the key design issues and discussing possible solutions.
1. Introduction
Wireless communications have advanced rapidly while battery technology has lagged, leaving mobile devices dependent on charging cables. MPT could remove these “last wires” by combining long-range wireless power with communication techniques, if propagation loss and safety challenges are addressed.
- Wireless access has reached gigabit speeds through advances including MIMO, millimeter-wave communications, and small-cell networks.
- Slower battery advances leave mobile devices with short battery lives that require periodic tethering to the grid for recharging.
- Non-radiative wireless power technologies are commercialized but typically transfer no more than a meter and do not support mobility.
- MPT supports long propagation ranges, mobility, and multicasting, making it a candidate for eliminating charging cables if high propagation loss and safety concerns are overcome.
- Microwave power transfer and wireless communication share techniques such as beamforming, propagation modeling, channel estimation, power control, and adaptive beamforming.
2. Key Features of Wirelessly Powered Communications
WPC combines microwave power transfer with mobile energy harvesting and simultaneous information-and-power transfer. Its design depends on efficient beamforming, suitable receiver architectures, and system configurations that balance range, efficiency, and interference.
- 2.1 Microwave Power Transfer: Power beamforming concentrates radiated power toward a target mobile, with sharper beams obtained from larger arrays or higher carrier frequencies.
- 2.1 Microwave Power Transfer: The end-to-end MPT efficiency is the product of DC-to-RF, beam, and RF-to-DC efficiencies, with beam efficiency identified as the long-distance bottleneck.
- 2.1 Microwave Power Transfer: At a fixed beam efficiency, doubling transmit-array radius or carrier frequency doubles transfer distance; scaling from 2.4 GHz to 60 GHz increases distance 25 times.
- 2.2 Mobile Architecture for WPC: WPC mobiles add an RF energy harvester, typically using a rectenna to convert incident microwave energy into DC power stored in a battery or supercapacitor.
- 2.2 Mobile Architecture for WPC: Receiver design trades many small communication-antenna elements against a large rectenna aperture under form-factor constraints.
- 2.3 Simultaneous Wireless Information-and-Power Transfer: SWIPT includes integrated, closed-loop, and decoupled configurations; closed-loop SWIPT has double attenuation, while decoupled SWIPT separates PT and IT by frequency bands or time slots.
3. Wirelessly Powered Communications: Frequently Asked Questions
The FAQ section examines practical WPC constraints, including power-transfer distance, safety, RF scavenging, SWIPT coverage, and interference with communications. It concludes that dedicated, dense infrastructure and intelligent control are needed for practical deployment.
- How far can a mobile device be wirelessly powered?: A PB transmitting tens of watts can power sensors, smartphones, and tablets at around 10 meters.Similar ranges for small and medium devices result because larger antennas can harvest more power, compensating for higher consumption.
- How far can a mobile device be wirelessly powered?: Under practical constraints, MPT distance is 3-15 meters for typical mobile devices, depending on radiated power.These ranges remain shorter than the 50-100 meter cell radiuses of 5G small-cell networks.
- Is WPC safe?: Power beamforming can create safety concerns within 15 meters, especially for people standing between a PB and a mobile for too long.Because exposure limits are averaged over a half-hour, intelligent WPC systems can adapt transmission power within milliseconds to improve safety.
- Is SWIPT practical?: Full network-wide SWIPT coverage using integrated or closed-loop designs would require impractical 10-15 meter cell radiuses, whereas decoupled SWIPT can use low-cost PBs for dense deployment.MPT can also interfere with communications through strong signals, receiver saturation, quantization noise, and generator harmonics unless countermeasures are implemented.
- Is it possible to power mobile devices by RF energy scavenging?: RF energy scavenging produces only peak power of tens of µW and average power in the order of µW for typical mobile-sized devices.It is sufficient for small sensors with sporadic activity, while larger devices require dedicated PBs.
4. Wirelessly Powered Communications: Designs and Challenges
WPC integrates wireless access and microwave power transfer through coordinated network architectures, beam control, safety measures, and redesigned communication protocols. Key challenges include pilot contamination, coverage planning, interference management, fairness, and joint resource allocation.
- Efficient and Safe WPC: Retrodirective beam control steers power beams toward mobiles by conjugating antenna phase shifts derived from received pilot signals.The receiver sends a pilot, the transmitter compares antenna outputs with a local reference, and conjugated phase shifts control transmission.
- Efficient and Safe WPC: Pilot contamination degrades information-transfer performance, reduces power-transfer efficiency, and can direct beams toward unintended mobiles, creating safety threats.Non-orthogonal or identical pilots across cells cause the beamformer to reflect multiple beams toward intended and unintended devices.
- Efficient and Safe WPC: Safety requires measures beyond automatic beam de-phasing, including guard zones, microwave life detection, surveillance cameras, radar tracking, and network localization.Automatic de-phasing does not protect people near a target mobile who do not intercept the beam.
- Efficient and Safe WPC: Coordinated power beacons can form coherently combined incoming beams from multiple directions around a target mobile.The passage identifies coordinated beacons as an approach for efficient and safe power transfer in dense-PB networks.
- WPC Network Architecture: WPC networks combine BSs, PBs, and mobile devices to provide wireless-access and microwave-power coverage.BSs primarily provide wireless-access coverage, while dense PB deployments support full MPT coverage.
- WPC Protocols and Techniques: WPC design must balance BS and PB densities, cooperation, spectrum selection, relaying, and adaptive joint scheduling of power and information transfer.These techniques address coverage, interference, fairness, and changing energy, queue, channel, and beam-efficiency states.
5. Towards Truly Mobile Communications
Wirelessly powered communications aims to eliminate the last wires connecting mobile devices to the grid by integrating information and power transfer. The resulting research agenda spans network architecture, safe and efficient MPT, and redesigned communication techniques.
- Towards Truly Mobile Communications: WPC targets elimination of the last wires connecting mobile devices to the grid for recharging.The paper associates this goal with greater user convenience, improved reliability of widespread mobile services, and market opportunities.
- Towards Truly Mobile Communications: Achieving this goal requires seamless integration between information transfer and power transfer rather than straightforward MPT implementation.The paper identifies SWIPT-capable architectures, safe and efficient MPT, and redesigned cooperation, cognitive-radio, and adaptive-transceiver techniques as research challenges.