PMID- 35890978
OWN - NLM
STAT- PubMed-not-MEDLINE
LR  - 20220731
IS  - 1424-8220 (Electronic)
IS  - 1424-8220 (Linking)
VI  - 22
IP  - 14
DP  - 2022 Jul 15
TI  - Characteristics of Channel Eigenvalues and Mutual Coupling Effects for 
      Holographic Reconfigurable Intelligent Surfaces.
LID - 10.3390/s22145297 [doi]
LID - 5297
AB  - As a prospective key technology for the next-generation wireless communications, 
      reconfigurable intelligent surfaces (RISs) have gained tremendous research 
      interest in both the academia and industry in recent years. Only limited 
      knowledge, however, has been obtained about the channel eigenvalue 
      characteristics and spatial degrees of freedom (DoF) of systems containing RISs, 
      especially when mutual coupling (MC) is present between the array elements. In 
      this paper, we focus on the small-scale spatial correlation and eigenvalue 
      properties excluding and including MC effects, for RISs with a quasi-continuous 
      aperture (i.e., holographic RISs). Specifically, asymptotic behaviors of 
      far-field and near-field eigenvalues of the spatial correlation matrix of 
      holographic RISs without MC are first investigated, where the counter-intuitive 
      observation of a lower DoF with more elements is explained by leveraging the 
      power spectrum of the spatial correlation function. Second, a novel metric is 
      proposed to quantify the inter-element correlation or coupling strength in RISs 
      and ordinary antenna arrays. Furthermore, in-depth analysis is performed 
      regarding the MC effects on array gain, effective spatial correlation, and 
      eigenvalue architectures for a variety of element intervals when a holographic 
      RIS works in the radiation and reception mode, respectively. The analysis and 
      numerical results demonstrate that a considerable amount of the eigenvalues of 
      the spatial correlation matrix correspond to evanescent waves that are promising 
      for near-field communication and sensing. More importantly, holographic RISs can 
      potentially reach an array gain conspicuously larger than conventional arrays by 
      exploiting MC, and MC has discrepant impacts on the effective spatial correlation 
      and eigenvalue structures at the transmitter and receiver.
FAU - Sun, Shu
AU  - Sun S
AUID- ORCID: 0000-0001-8655-3746
AD  - Department of Electronic Engineering, School of Electronic Information and 
      Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
AD  - Shanghai Key Laboratory of Digital Media Processing and Transmissions, Shanghai 
      200240, China.
FAU - Tao, Meixia
AU  - Tao M
AD  - Department of Electronic Engineering, School of Electronic Information and 
      Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
AD  - Shanghai Key Laboratory of Digital Media Processing and Transmissions, Shanghai 
      200240, China.
LA  - eng
GR  - WH220403031/Shanghai Jiao Tong University/
PT  - Journal Article
DEP - 20220715
PL  - Switzerland
TA  - Sensors (Basel)
JT  - Sensors (Basel, Switzerland)
JID - 101204366
SB  - IM
PMC - PMC9316737
OTO - NOTNLM
OT  - eigenvalue
OT  - holographic communications
OT  - mutual coupling
OT  - reconfigurable intelligent surface (RIS)
OT  - spatial correlation
OT  - spatial degrees of freedom
COIS- The authors declare no conflict of interest. The funders had no role in the 
      design of the study; in the collection, analyses, or interpretation of data; in 
      the writing of the manuscript, or in the decision to publish the results.
EDAT- 2022/07/28 06:00
MHDA- 2022/07/28 06:01
PMCR- 2022/07/15
CRDT- 2022/07/27 01:42
PHST- 2022/06/13 00:00 [received]
PHST- 2022/07/03 00:00 [revised]
PHST- 2022/07/11 00:00 [accepted]
PHST- 2022/07/27 01:42 [entrez]
PHST- 2022/07/28 06:00 [pubmed]
PHST- 2022/07/28 06:01 [medline]
PHST- 2022/07/15 00:00 [pmc-release]
AID - s22145297 [pii]
AID - sensors-22-05297 [pii]
AID - 10.3390/s22145297 [doi]
PST - epublish
SO  - Sensors (Basel). 2022 Jul 15;22(14):5297. doi: 10.3390/s22145297.