Imperial College London

Yang Zhao

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

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Publication Type

3 results found

Zhao Y, Clerckx B, 2022, RIScatter: unifying backscatter communication and reconfigurable intelligent surface, Publisher: arXiv

Backscatter Communication (BackCom) nodes harvest energy from and modulateinformation over an external electromagnetic wave. Reconfigurable IntelligentSurface (RIS) adapts its phase shift response to enhance or attenuate channelstrength in specific directions. In this paper, we show how those two seeminglydifferent technologies (and their derivatives) can be unified to leverage theirbenefits simultaneously into a single architecture called RIScatter. RIScatterconsists of multiple dispersed or co-located scatter nodes, whose reflectionstates can be adapted to partially engineer the wireless channel of theexisting link and partially modulate their own information onto the scatteredwave. This contrasts with BackCom (resp. RIS) where the reflection pattern isexclusively a function of the information symbol (resp. Channel StateInformation (CSI)). The key principle in RIScatter is to render the probabilitydistribution of reflection states (i.e., backscatter channel input) as a jointfunction of the information source, CSI, and Quality of Service (QoS) of thecoexisting active primary and passive backscatter links. This enables RIScatterto softly bridge, generalize, and outperform BackCom and RIS; boil down toeither under specific input distribution; or evolve in a mixed form forheterogeneous traffic control and universal hardware design. For a single-usermulti-node RIScatter network, we characterize the achievableprimary-(total-)backscatter rate region by optimizing the input distribution atthe nodes, the active beamforming at the Access Point (AP), and the backscatterdetection regions at the user. Simulation results demonstrate RIScatter nodescan exploit the additional propagation paths to smoothly transition betweenbackscatter modulation and passive beamforming.

Working paper

Feng Z, Clerckx B, Zhao Y, 2022, Waveform and beamforming design for intelligent reflecting surface aided wireless power transfer: single-user and multi-user solutions, IEEE Transactions on Wireless Communications, Vol: 21, Pages: 5346-5361, ISSN: 1536-1276

In this paper, we study the waveform and passive beamforming design for intelligent reflecting surface (IRS)-aided wireless power transfer (WPT). Generalized multi-user and low complexity single-user algorithms are demonstrated based on alternating optimization (AO) framework to maximize the weighted sum output direct current (DC), subject to the transmit power constraints and passive beamforming modulus constraints. The input signal waveform and IRS passive beamforming phase shifts are jointly designed as a function of users’ individual frequency-selective channel state information (CSI). The energy harvester nonlinearity is explored and two IRS deployment schemes, namely frequency selective IRS (FS-IRS) and frequency flat IRS (FF-IRS), are modeled and analyzed. This paper highlights the fact that IRS can provide an extra passive beamforming gain on output DC power over conventional WPT designs and significantly influence the waveform design by leveraging the benefit of passive beamforming, frequency diversity and energy harvester nonlinearity. Even though FF-IRS exhibits lower output DC than the ideal FS-IRS, it still achieves substantially increased DC power over conventional WPT designs. Performance evaluations confirm the significant benefits of a joint waveform and passive beamforming design accounting for the energy harvester nonlinearity to boost the performance of single-user and multi-user WPT systems.

Journal article

Zhao Y, Clerckx B, Feng Z, 2022, IRS-aided SWIPT: Joint waveform, active and passive beamforming design under nonlinear harvester model, IEEE Transactions on Wireless Communications, Vol: 70, Pages: 1345-1359, ISSN: 1536-1276

The performance of Simultaneous Wireless Information and Power Transfer (SWIPT) is mainly constrained by the received Radio-Frequency (RF) signal strength. To tackle this problem, we introduce an Intelligent Reflecting Surface (IRS) to compensate the propagation loss and boost the transmission efficiency. This paper proposes a novel IRS-aided SWIPT system where a multi-carrier multi-antenna Access Point (AP) transmits information and power simultaneously, with the assist of an IRS, to a single-antenna User Equipment (UE) employing practical receiving schemes. Considering harvester nonlinearity, we characterize the achievable Rate-Energy (R-E) region through a joint optimization of waveform, active and passive beamforming based on the Channel State Information at the Transmitter (CSIT). This problem is solved by the Block Coordinate Descent (BCD) method, where we obtain the active precoder in closed form, the passive beamforming by the Successive Convex Approximation (SCA) approach, and the waveform amplitude by the Geometric Programming (GP) technique. To facilitate practical implementation, we also propose a low-complexity design based on closed-form adaptive waveform schemes. Simulation results demonstrate the proposed algorithms bring considerable R-E gains with robustness to CSIT inaccuracy and finite IRS states, and emphasize the importance of modeling harvester nonlinearity in the IRS-aided SWIPT design.

Journal article

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