Channel training in reconfigurable intel-ligent surface(RIS)-assisted communications is usu-ally conducted in an on-off manner,resulting in unaf-fordable training time overhead when the number of RIS elements is large.In this paper,for correlated Rayleigh channels,we compare three typical train-ing overhead reduction schemes,namely RIS element selection(Scheme 1),element grouping(Scheme 2),and statistical CSI-based phase shifts design(Scheme 3).For Scheme 1 and Scheme 2,we propose two algorithms to select RIS elements(or form element groups)and determine the optimal number of acti-vated elements(or formed groups),based on the chan-nel correlation information only;for Scheme 3,we consider a semi-definite programming-based approach in the literature,and propose an alternative domi-nant eigenvector-based method for determining the RIS phase shifts vector.Via extensive simulations,we compare the achievable ergodic rates of these schemes versus the signal-to-noise ratio,the channel correla-tion level,and the element number-to-coherent time ratio,respectively,and discuss possible switching of the three schemes over these system parameters.At last,operation regions of the considered training over-head reduction schemes are shown in the plane charac-terized by the system parameters,which provides use-ful guidelines for practical scheme determination.

Jue Wang;Shuaifeng Lu;Jing Wang;Zhenyu Jiang;Rui Tang;Ruifeng Gao;Yingdong Hu;Ye Li

School of Information Science and Technology,Nantong University,Nantong 226109,China;Nantong Research Institute for Advanced Communication Technologies,Nantong 226109,China;Department of Electronic Engineering,Tsinghua University,Beijing 100084,China;School of Transportation and Civil Engineering,Nantong University,Nantong 226109,China

中国通信（英文版）

[1]Jue Wang;Shuaifeng Lu;Jing Wang;Zhenyu Jiang;Rui Tang;Ruifeng Gao;Yingdong Hu;Ye Li-.RIS-Assisted Communications in Correlated Rayleigh Channels:Comparison of Training Overhead Reduction Schemes)[J].中国通信（英文版）,2022(12):41-53

unaf,fordable

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