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Adjustable-delay RIS is capable of improving OFDM systems

Adjustable-delay RIS is capable of improving OFDM systems
Adjustable-delay RIS is capable of improving OFDM systems

Reconfigurable intelligent surfaces (RIS) demonstrate the potential to improve the spectrum and energy efficiency of wireless networks. In this paper, we investigate multiple-RIS-assisted orthogonal frequency division multiplexing (OFDM) communications. Specifically, we generalize the existing RIS concept conceived for frequency-flat channels to the adjustable-delay RIS by introducing varactor diodes. In contrast to conventional reflecting elements, each adjustable-delay RIS element is capable of storing and retrieving the impinging electromagnetic waves upon dynamically controlling its electromagnetically induced transparency (EIT), thus imposing an extra delay onto the incident signals. This allows for aligning multiple signal copies via multiple RISs. To this end, we formulate a rate-maximization problem by jointly optimizing the transmit power allocation and the RIS reflection coefficients as well as the RIS delays. To address the coupling issue between these optimization variables, we propose a computationally efficient algorithm to find a high-quality solution to the non-convex design problem by alternately optimizing the transmit power allocation and the RIS reflection pattern, including both the reflection coefficients and the delays. Furthermore, we conceive a low-complexity reflection optimization scheme upon aligning the strongest taps of all reflected channels, while ensuring that the maximum delay spread introduced by extra RIS delays does not exceed the length of the cyclic prefix. Our simulation results demonstrate that the proposed design significantly improves the achievable rate as well as the RIS's adaptability to wideband signals compared to various benchmark schemes operating without adjustable-delay RIS. Moreover, it is shown that there exists a fundamental trade-off between the adjustable delay margin to align different reflected channels and the practical component's power decay caused by delay.

adjustable-delay metasurface, Delays, MISO communication, OFDM, Optimization, orthogonal frequency division multiplexing (OFDM), power allocation, Reconfigurable intelligent surface (RIS), Reflection, Reflection coefficient, reflection pattern optimization, Resource management
0018-9545
1-16
An, Jiancheng
5fa38cfb-6010-4404-a39c-f03c68f248c5
Xu, Chao
5710a067-6320-4f5a-8689-7881f6c46252
Ng, Derrick Wing Kwan
8e2a32d3-cb0d-4c38-b05c-03ef16a5c707
Yuen, Chau
0dd04333-bade-4812-b3df-a416597f1325
Hanzo, Lajos
66e7266f-3066-4fc0-8391-e000acce71a1
An, Jiancheng
5fa38cfb-6010-4404-a39c-f03c68f248c5
Xu, Chao
5710a067-6320-4f5a-8689-7881f6c46252
Ng, Derrick Wing Kwan
8e2a32d3-cb0d-4c38-b05c-03ef16a5c707
Yuen, Chau
0dd04333-bade-4812-b3df-a416597f1325
Hanzo, Lajos
66e7266f-3066-4fc0-8391-e000acce71a1

An, Jiancheng, Xu, Chao, Ng, Derrick Wing Kwan, Yuen, Chau and Hanzo, Lajos (2024) Adjustable-delay RIS is capable of improving OFDM systems. IEEE Transactions on Vehicular Technology, 1-16. (doi:10.1109/TVT.2024.3362953).

Record type: Article

Abstract

Reconfigurable intelligent surfaces (RIS) demonstrate the potential to improve the spectrum and energy efficiency of wireless networks. In this paper, we investigate multiple-RIS-assisted orthogonal frequency division multiplexing (OFDM) communications. Specifically, we generalize the existing RIS concept conceived for frequency-flat channels to the adjustable-delay RIS by introducing varactor diodes. In contrast to conventional reflecting elements, each adjustable-delay RIS element is capable of storing and retrieving the impinging electromagnetic waves upon dynamically controlling its electromagnetically induced transparency (EIT), thus imposing an extra delay onto the incident signals. This allows for aligning multiple signal copies via multiple RISs. To this end, we formulate a rate-maximization problem by jointly optimizing the transmit power allocation and the RIS reflection coefficients as well as the RIS delays. To address the coupling issue between these optimization variables, we propose a computationally efficient algorithm to find a high-quality solution to the non-convex design problem by alternately optimizing the transmit power allocation and the RIS reflection pattern, including both the reflection coefficients and the delays. Furthermore, we conceive a low-complexity reflection optimization scheme upon aligning the strongest taps of all reflected channels, while ensuring that the maximum delay spread introduced by extra RIS delays does not exceed the length of the cyclic prefix. Our simulation results demonstrate that the proposed design significantly improves the achievable rate as well as the RIS's adaptability to wideband signals compared to various benchmark schemes operating without adjustable-delay RIS. Moreover, it is shown that there exists a fundamental trade-off between the adjustable delay margin to align different reflected channels and the practical component's power decay caused by delay.

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Adaptive_Delay_RIS_is_Capable_of_Improving_OFDM_Systems - Accepted Manuscript
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Accepted/In Press date: 4 February 2024
e-pub ahead of print date: 6 February 2024
Published date: 6 February 2024
Additional Information: L. Hanzo would like to acknowledge the financial support of the Engineering and Physical Sciences Research Council projects EP/W016605/1, EP/X01228X/1 and EP/Y026721/1 as well as of the European Research Council’s Advanced Fellow Grant QuantCom (Grant No. 789028). Publisher Copyright: IEEE
Keywords: adjustable-delay metasurface, Delays, MISO communication, OFDM, Optimization, orthogonal frequency division multiplexing (OFDM), power allocation, Reconfigurable intelligent surface (RIS), Reflection, Reflection coefficient, reflection pattern optimization, Resource management

Identifiers

Local EPrints ID: 486785
URI: http://eprints.soton.ac.uk/id/eprint/486785
ISSN: 0018-9545
PURE UUID: aacde19f-868c-4961-a46c-f59ff6725d0d
ORCID for Chao Xu: ORCID iD orcid.org/0000-0002-8423-0342
ORCID for Lajos Hanzo: ORCID iD orcid.org/0000-0002-2636-5214

Catalogue record

Date deposited: 06 Feb 2024 17:38
Last modified: 30 May 2024 04:01

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Contributors

Author: Jiancheng An
Author: Chao Xu ORCID iD
Author: Derrick Wing Kwan Ng
Author: Chau Yuen
Author: Lajos Hanzo ORCID iD

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