The performance of wireless communication is fundamentally constrained by the limited battery life of wireless devices, the operations of which are frequently disrupted due to the need of manual battery replacement/recharging. The recent advance in 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 that need to be addressed. In this article, we provide an overview of stateof- the-art RF-enabled WET technologies and their applications to wireless communications, highlighting the key design challenges, solutions, and opportunities ahead.

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Wireless powered communication: opportunities and challenges

Solar photovoltaic electricity: Current status and future prospects

The idea of wireless power transfer (WPT) has been around since the inception of electricity. In the late 19th century, Nikola Tesla described the freedom to transfer energy between two points without the need for a physical connection to a power source as an ?all-surpassing importance to man? [1]. A truly wireless device, capable of being remotely powered, not only allows the obvious freedom of movement but also enables devices to be more compact by removing the necessity of a large battery. Applications could leverage this reduction in size and weight to increase the feasibility of concepts such as paper-thin, flexible displays [2], contact-lens-based augmented reality [3], and smart dust [4], among traditional point-to-point power transfer applications. While several methods of wireless power have been introduced since Tesla?s work, including near-field magnetic resonance and inductive coupling, laser-based optical power transmission, and far-field RF/microwave energy transmission, only RF/microwave and laser-based systems are truly long-range methods. While optical power transmission certainly has merit, its mechanisms are outside of the scope of this article and will not be discussed.

Harvesting Wireless Power: Survey of Energy-Harvester Conversion Efficiency in Far-Field, Wireless Power Transfer Systems

A novel and simple printed dual-band double-T monopole antenna is proposed. The antenna comprises two stacked T-shaped monopoles of different sizes, which generate two separate resonant modes for the desired dual-band operation. The proposed antenna has a low profile and can easily be fed by using a 50 /spl Omega/ microstrip line. Prototypes of the proposed antenna designed for WLAN operations in the 2.4 and 5.2 GHz bands have been constructed and tested. Good radiation characteristics of the proposed antenna have been obtained. Effects of varying the monopole dimensions and the ground-plane size on the antenna performance have also been studied.

Printed double-T monopole antenna for 2.4/5.2 GHz dual-band WLAN operations

This article discusses the challenges, benefits and approaches associated with realizing largescale antenna arrays at mmWave frequency bands for future 5G cellular devices. Key design considerations are investigated to deduce a novel and practical phased array antenna solution operating at 28 GHz with near spherical coverage. The approach is further evolved into a first-of- a-kind cellular phone prototype equipped with mmWave 5G antenna arrays consisting of a total of 32 low-profile antenna elements. Indoor measurements are carried out using the presented prototype to characterize the proposed mmWave antenna system using 16-QAM modulated signals with 27.925 GHz carrier frequency. The biological implications due to the absorbed electromagnetic waves when using mmWave cellular devices are studied and compared in detail with those of 3/4G cellular devices.

Study and prototyping of practically large-scale mmWave antenna systems for 5G cellular devices

A dual-frequency printed dipole rectenna has been developed for the wireless power transmission at 2.45- and 5.8-GHz (industrial-scientific-medical bands). For operating at dual band, a new uniplanar printed dipole antenna is developed using a coupling method. A GaAs Schottky barrier diode analysis is performed, and a proper device requirement is discussed to have high RF-to-dc conversion efficiencies at both frequencies. A novel coplanar stripline (CPS) low-pass filter integrated with two additional open-ended T-strip CPS bandstop filters effectively block higher order harmonics generated from the diode. The measured conversion efficiencies achieved at free space are 84.4 and 82.7% at 2.45 and 5.8 GHz, respectively.

A high-efficiency dual-frequency rectenna for 2.45- and 5.8-GHz wireless power transmission

A low cost microstrip-fed dual frequency dipole antenna has been developed for personal communication network (PCN) and wireless LAN (WLAN) applications in the 2.4 and 5.2 GHz bands. The antenna radiates bi-directionally and has a double-sided structure to enable transition from a balanced planar stripline to unbalanced microstrip line. The 10 dB bandwidths achieved were 18.75 and 7.7% at 2.4 and 5.2 GHz, respectively.

Low cost microstrip-fed dual frequency printed dipole antenna for wireless communications

A wideband coplanar stripline (CPS)-to-microstrip line was developed. The transition has a simple structure for the ease of fabrication with low cost. The measured performance of two back-to-back transitions exhibits an insertion loss of less than 3 dB and a return loss of better than 10 dB over a bandwidth from 1.3 GHz to 13.3 GHz (1:10.2). For narrower bandwidth, an insertion loss of less than 1 dB with a return loss of better than 10 dB was achieved from 1.4 GHz to 7.3 GHz (1:5.2).

A wideband coplanar stripline to microstrip transition

A circularly polarised rectenna using a truncated-corner square-patch microstrip antenna has been developed for wireless power transmission at 5.8 GHz. The gain and axial ratio achieved were 8.5 dBi and 0.5 dB at 5.82 GHz respectively. A GaAs Schottky barrier diode was employed for microwave to DC conversion with a maximum conversion efficiency of 60%.

Circularly polarised truncated-corner square patch microstrip rectenna for wireless power transmission

A wideband coplanar stripline (CPS)-to-microstrip line was developed. The transition has a simple structure for the ease of fabrication with low cost. The measured performance of two back-to-back transitions exhibits an insertion loss of less than 3 dB and a return loss of better than 10 dB over a bandwidth from 1.3 GHz to 13.3 GHz (1: 10.2). For narrower bandwidth, an insertion loss of less than 1 dB with a return loss of better than 10 dB was achieved from 1.4 GHz to 7.3 GHz (1: 5.2).

Young-Ho Suh

Papers

A high-efficiency dual-frequency rectenna for 2.45- and 5.8-GHz wireless power transmission

Low cost microstrip-fed dual frequency printed dipole antenna for wireless communications

A wideband coplanar stripline to microstrip transition

Circularly polarised truncated-corner square patch microstrip rectenna for wireless power transmission

A wideband coplanar stripline to microstrip transition