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Tsinghua Science and Technology 2023, 28(6): 1115-1127 https://doi.org/10.26599/TST.2023.9010019
Open Access | Issue | Published: 28 July 2023

Joint Beamforming Design for RIS-Assisted Cell-Free Network with Multi-Hop Transmissions

Show Author's Information Decai Shen1,2,  Zijian Zhang1,2,  Linglong Dai1,2( )
Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
Beijing National Research Center for Information Science and Technology (BNRist), Beijing 100084, China
Keywords:
beamforming, Reconfigurable Intelligent Surface (RIS), cell-free network, multi-hop transmissions
Cite this article:
Shen D, Zhang Z, Dai L. Joint Beamforming Design for RIS-Assisted Cell-Free Network with Multi-Hop Transmissions. Tsinghua Science and Technology, 2023, 28(6): 1115-1127. https://doi.org/10.26599/TST.2023.9010019
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Abstract Full text About this article

The collaboration of multiple Reconfigurable Intelligent Surfaces (RISs) and Access Points (APs) enjoys advantages of capacity enhancement, power saving, etc., making the RIS-assisted cell-free network an important architecture for future communications. Similar to most existing works on RIS-assisted communications, the multi-hop link among RISs, i.e., the reflecting link including more than one RISs, is usually ignored in RIS-assisted cell-free networks. In these scenarios, however, since multiple RISs are closely deployed, we find that the multi-hop channels should not be simply ignored due to their potential for capacity improvement. Unfortunately, to the best of our knowledge, there is no work exploring the multi-hop transmission of RIS-assisted cell-free networks. To fill in this blank, we investigate the multi-hop transmission of RIS-assisted cell-free networks, including the multi-hop channel model and the corresponding beamforming design. Specifically, we propose a general multi-hop transmission model, which takes the direct links, single-reflecting links, and multi-hop links into account. Based on this model, we formulate a beamforming design problem in an RIS-assisted cell-free network, which allows us to maximize the multi-user sum-rate with considering the impact of multi-hop channels. To address the non-convexity of the formulated problem, a joint active and passive beamforming scheme is proposed to solve the problem. Particularly, by utilizing fractional programming, we decouple the coupled beamforming parameters in the problem, and then these parameters are alternately optimized until the convergence of the sum-rate. Simulation results verify that the consideration for multi-hop links is necessary, and the capacity performance of the proposed scheme is 20% higher than those of the existing schemes.


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Joint Beamforming Design for RIS-Assisted Cell-Free Network with Multi-Hop Transmissions

Show Author's information Decai Shen1,2, Zijian Zhang1,2, Linglong Dai1,2( )
Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
Beijing National Research Center for Information Science and Technology (BNRist), Beijing 100084, China

Abstract

The collaboration of multiple Reconfigurable Intelligent Surfaces (RISs) and Access Points (APs) enjoys advantages of capacity enhancement, power saving, etc., making the RIS-assisted cell-free network an important architecture for future communications. Similar to most existing works on RIS-assisted communications, the multi-hop link among RISs, i.e., the reflecting link including more than one RISs, is usually ignored in RIS-assisted cell-free networks. In these scenarios, however, since multiple RISs are closely deployed, we find that the multi-hop channels should not be simply ignored due to their potential for capacity improvement. Unfortunately, to the best of our knowledge, there is no work exploring the multi-hop transmission of RIS-assisted cell-free networks. To fill in this blank, we investigate the multi-hop transmission of RIS-assisted cell-free networks, including the multi-hop channel model and the corresponding beamforming design. Specifically, we propose a general multi-hop transmission model, which takes the direct links, single-reflecting links, and multi-hop links into account. Based on this model, we formulate a beamforming design problem in an RIS-assisted cell-free network, which allows us to maximize the multi-user sum-rate with considering the impact of multi-hop channels. To address the non-convexity of the formulated problem, a joint active and passive beamforming scheme is proposed to solve the problem. Particularly, by utilizing fractional programming, we decouple the coupled beamforming parameters in the problem, and then these parameters are alternately optimized until the convergence of the sum-rate. Simulation results verify that the consideration for multi-hop links is necessary, and the capacity performance of the proposed scheme is 20% higher than those of the existing schemes.

Keywords: beamforming, Reconfigurable Intelligent Surface (RIS), cell-free network, multi-hop transmissions

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Publication history

Received: 28 February 2023
Revised: 17 March 2023
Accepted: 18 March 2023
Published: 28 July 2023
Issue date: December 2023

Copyright

© The author(s) 2023.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2020YB1807201), the National Natural Science Foundation of China (No. 62031019), and the European Commission through the H2020-MSCA-ITN META WIRELESS Research Project (No. 956256).

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The articles published in this open access journal are distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).

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