PHY4 – RIS and precoding

Tuesday, 4 June 2023, 16:00-17:30, room Darwin Hall

Session Chair: Mikko A. Uusitalo (Nokia Bell Labs, FI)

On the Performance of RIS-Assisted Networks with HQAM
Thrassos Oikonomou (Aristotle University of Thessaloniki, Greece); Dimitrios Tyrovolas (Aristotle University of Thessaloniki & Technical University of Crete, Greece); Sotiris A. Tegos and Panagiotis D. Diamantoulakis (Aristotle University of Thessaloniki, Greece); Panagiotis Sarigiannidis (University of Western Macedonia, Greece); Christos Liaskos (University of Ioannina, Greece & Foundation of Research and Technology Hellas, Greece); George K. Karagiannidis (Aristotle University of Thessaloniki, Greece)
In this paper, we investigate the application of hexagonal quadrature amplitude modulation (HQAM) in reconfigurable intelligent surface (RIS)-assisted networks, specifically focusing on its efficiency in reducing the number of required reflecting elements. Specifically, we present analytical expressions for the average symbol error probability (ASEP) and propose a new metric for conditioned energy efficiency, which assesses the network’s energy consumption while ensuring the ASEP remains below a certain threshold. Additionally, we introduce an innovative detection algorithm for HQAM constellations, which demonstrates a substantial reduction in computational complexity. Finally, our study reveals that HQAM significantly enhances both the ASEP and energy efficiency compared to traditional quadrature amplitude modulation (QAM) schemes.

Spider RIS: Mobilizing Intelligent Surfaces for Enhanced Wireless Communications
Ibrahim Yildirim (Istanbul Technical University, Turkey); Mobeen Mahmood (McGill University, Canada); Ertugrul Basar (Koc University, Turkey); Tho Le-Ngoc (McGill University, Canada)
In this study, we introduce Spider RIS technology, which offers an innovative solution to the challenges encountered in movable antennas (MAs) and unmanned aerial vehicle (UAV)-enabled communication systems. By combining the dynamic adaptation capability of MAs and the flexible location advantages of UAVs, this technology offers a dynamic and movable RIS, which can flexibly optimize physical locations within the two-dimensional movement platform. Spider RIS aims to enhance the communication efficiency and reliability of wireless networks, particularly in obstructive environments, by elevating the signal quality and achievable rate. The motivation of Spider RIS is based on the ability to fully exploit the spatial variability of wireless channels and maximize channel capacity even with a limited number of reflecting elements by overcoming the limitations of traditional fixed RIS and energy-intensive UAV systems. Considering the geometry-based millimeter wave channel model, we present the design of a three-stage angular-based hybrid beamforming system empowered by Spider RIS: First, analog beamformers are designed using angular information, followed by the generation of digital precoder/combiner based on the effective channel observed from baseband stage. Subsequently, the joint dynamic positioning with phase shift design of the Spider RIS is optimized using particle swarm optimization, maximizing the achievable rate of the systems.

A Sub-Band Precoding Scheme for Wideband Massive MIMO-OFDM Systems
Kelvin Kuang-Chi Lee (Tamkang University, Taiwan); Chiao-En Chen (National Chung Hsing University, Taiwan)
Sub-band precoding (SBP) stands out as a promising technology for orthogonal frequency division multiplexing (OFDM)-based wireless communication systems where the frequency selective channels exhibit gradual fading characteristics. Under such environment, the complexity of coefficient design can be significantly reduced by segmenting subcarriers into smaller sub-bands and applying identical precoding coefficients to all subcarriers within each sub-band. In this paper, a SBP algorithm is proposed for wideband millimeter wave (mmWave) and terahertz (THz) massive multiple-input multiple-output OFDM (M-MIMO-OFDM) systems. By leveraging fractional programming, quadratic transformation and first-order Taylor approximation, we formulate sum-rate maximization as a convex optimization problem, enabling efficient derivation of SBP coefficients. To provide justification for its effectiveness, simulation results are presented and compared with other benchmark precoding schemes.

Enhancing the WLAN OFDM-PHY by OTFS Precoding
Muhammad Nauman (IHP Leibniz-Institut Für Innovative Mikroelektronik, Germany); Lukasz Lopacinski (IHP, Germany); Nebojsa Maletic (IHP – Leibniz-Institut für Innovative Mikroelektronik, Germany); Matthias Scheide (IHP – Leibniz Institut für Innovative Mikroelektronik, Germany); Jesús Gutiérrez (IHP – Leibniz-Institut für Innovative Mikroelektronik, Germany); Milos Krstic (IHP, Germany); Eckhard Grass (IHP & Humboldt-University Berlin, Germany)
Reliable communication in high-mobility scenarios is considered a significant challenge. For such a problematic task, the Orthogonal Time Frequency Space (OTFS) modulation technique has come up as an efficient solution, providing promising results in doubly selective time and frequency channels. In this paper, we have simulated and compared OTFS and a complete Orthogonal Frequency-Division Multiplexing (OFDM) system in a multipath environment at different mobile speeds under the presence of a synchronization impairment, i.e., timing offset (TO). Firstly, we have modelled the OTFS system according to the active subcarriers in the OFDM system and then modified the OTFS frame in the time-frequency (TF) plane in compliance with the OFDM symbol. This modification kept a similar bandwidth for both systems, eliminated the need for multi-rate processing and allowed OTFS to be used as a precoder to the targeted OFDM transceiver. Our simulation results show that, with increased mobility, the OFDM Error Vector Magnitude (EVM) performance is degraded abruptly, whereas the OTFS link remains stable.

Post-Computing Analog Beams After User Selection in a Hybrid Beamforming System
Tugce Kobal (Nokia Bell Labs, United Kingdom (Great Britain)); François Durand (Nokia Bell Labs France, France); Arndt Ryo Koblitz (Nokia Bell Labs, United Kingdom (Great Britain)); Matthew Andrews (Nokia Bell Labs, USA)
We investigate the benefits of recomputing analog beams after users are selected in a Hybrid Beamforming (HBF) system. In the standard HBF procedure, each user selects an analog beam, the effective channels are computed, users are selected, and finally the digital precoder is computed. However, this means that the analog beams are picked for users in isolation, without consideration of the joint interference characteristics. A natural way to address this is for the analog beams to be recomputed after the user selection step, since then the interference for the simultaneously transmitting users is known. We study this scheme and demonstrate that for specific configurations of selected users, it can lead to significantly higher SINR and therefore system performance. However, we also show that in a full simulation, the gains are marginal. This is because the user selection step typically selects users that are far apart in beam space, and hence have the least interference. We view this as a positive result since it means that the extra effort involved in the beam recomputation step is not generally needed.

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