Far- and Near-Field Channel Measurements and Characterization in the Terahertz Band Using a Virtual Antenna Array

Extremely large-scale antenna array (ELAA) technologies consisting of ultra-massive multiple-input-multiple-output (UM-MIMO) or reconfigurable intelligent surfaces (RISs), are emerging to meet the demand of wireless systems in sixth-generation and beyond communications for enhanced coverage and extreme data rates up to Terabits per second. For ELAA operating at Terahertz (THz) frequencies, the Rayleigh distance expands, and users are likely to be located in both far-field (FF) and near-field (NF) regions. On one hand, new features like NF propagation and spatial non-stationarity need to be characterized. On the other hand, the transition of properties near the FF and NF boundary is worth exploring. In this paper, a complete experimental analysis of far- and near-field channel characteristics using a THz virtual antenna array is provided based on measurement of the multi-input-single-output channel with the virtual uniform planar array (UPA) structure of at most 4096 elements. In particular, non-linear phase change is observed in the NF, and the Rayleigh criterion regarding the maximum phase error is verified. Then, a new cross-field path loss model is proposed, which characterizes the power change at antenna elements in the UPA and is compatible with both FF and NF cases.Comment: 5 pages, 10 figure

Low-Complexity Distance-Based Scheduling for Multi-User XL-MIMO Systems

© 2021 IEEE. This version of the article has been accepted for publication, after peer review. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. The Version of Record is available online at: https://doi.org/10.1109//LWC.2021.3101940[Abstract]: We introduce Distance-Based Scheduling (DBS), a new technique for user selection in downlink multi-user communications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interference while largely reducing the computational burden. Results show that ( i ) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; ( ii ) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; ( iii ) SW propagation brings additional degrees of freedom, which allows for increasing the number of served users.This work was funded by the Xunta de Galicia (ED431G2019/01), the Agencia Estatal de Investigación of Spain (TEC2016-75067-C4-1-R, PID2020-118139RB-I00), ERDF funds of the EU (AEI/FEDER, UE), the Junta de Andalucia and the European Fund for Regional Development FEDER (project P18-RT-3175

Leakage Subspace Precoding and Scheduling for Physical Layer Security in Multi-User XL-MIMO Systems

We investigate the achievable secrecy sum-rate in a multi-user XL-MIMO system, on which user distances to the base station become comparable to the antenna array dimensions. We show that the consideration of spherical-wavefront propagation inherent to these set-ups is beneficial for physical-layer security, as it provides immunity against eavesdroppers located in similar angular directions that would otherwise prevent secure communication under classical planar-wavefront propagation. A leakage subspace precoding strategy is also proposed for joint secure precoding and user scheduling, which allows to improve the secrecy sum-rate compared to conventional zero-forcing based strategies, under different eavesdropper collusion strategies.Comment: 5 pages and 4 figures. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

Low-Complexity Distance-Based Scheduling for Multi-User XL-MIMO Systems.

Política de acceso abierto tomada de: https://v2.sherpa.ac.uk/id/publication/37970We introduce Distance-Based Scheduling (DBS), a new technique for user selection in downlink multi-user communications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interference while largely reducing the computational burden. Results show that (i) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; (ii) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; (iii) SW propagation brings additional degrees of freedom, which allows for increasing the number of served user

Non-Stationarities in Extra-Large Scale Massive MIMO

Massive MIMO, a key technology for increasing area spectral efficiency in cellular systems, was developed assuming moderately sized apertures. In this paper, we argue that massive MIMO systems behave differently in large-scale regimes due to spatial non-stationarity. In the large-scale regime, with arrays of around fifty wavelengths, the terminals see the whole array but non-stationarities occur because different regions of the array see different propagation paths. At even larger dimensions, which we call the extra-large scale regime, terminals see a portion of the array and inside the first type of non-stationarities might occur. We show that the non-stationarity properties of the massive MIMO channel changes several important MIMO design aspects. In simulations, we demonstrate how non-stationarity is a curse when neglected but a blessing when embraced in terms of computational load and multi-user transceiver design

Accelerated Randomized Methods for Receiver Design in Extra-Large Scale MIMO Arrays

Recent interest has been cast on accelerated versions of the randomized Kaczmarz (RK) algorithm due to the increase in applications that consider sparse linear systems. In particular, considering the context of massive multiple-input-multiple-output (M-MIMO) communication systems, a low complexity naive RK-based receiver has recently been proposed. This method can take advantage of non-stationarities emerging from extra-large M-MIMO systems, but it performs poorly on highly spatially correlated channels. To address this problem, in this paper, we propose a new class of accelerated RK-based receiver designs, where convergence acceleration is based on the residual information. However, we show that the cost of obtaining this knowledge on an iteration basis is not worth it due to the lousy convergence effects caused by system and channel parameters. Inspired by this observation, we further propose a RK-based receiver with sampling without replacement, referred to as RK-RZF. This simple technique is more effective in performing signal detection under reduced complexity. Future works suggest advantage of RK-based receivers to improve current 5G commercial systems and solve the problem of signal detection in other paradigms beyond 5G.Comment: 11 pages, 4 figures, submitted to IEEE TV