The availability of large bandwidths in the sub-THz bands appears to be a solution for increasing the capabilities of future wireless communication technologies. However, phase noise generated by oscillators in these bands generates significant signal distortion which needs to be addressed. In this paper, we study the impact of phase noise on OTFS. We propose phase noise compensation scheme and associated frame format. Two dedicated pilot schemes are studied for the phase noise tracking: the first one consists in adding pilots in the delay-Doppler (DD) domain while the other in the delay-Time (DT) domain. We highlight that compensating phase noise in DT domain gives better performance. A comparison between OFDM, DFT-s-OFDM and OTFS according to the 5G-NR numerology is presented. We show that the OTFS waveform is robust to phase noise and may even significantly outperform OFDM and DFT-s-OFDM.

In this work, we study hybrid precoding techniques applied to multi-user Transmitting Reconfigurable Intelligent Surface (T-RIS) systems. The T-RIS considered here is a large array of electronically reconfigurable antenna elements illuminated by a small set of active sources. When it comes to digital signal-processing techniques applied to T-RIS systems, it is necessary to consider realistic models to bridge the gap with theoretical results. For this reason, we propose a multi-beam T-RIS propagation model with strong phase quantization constraints and limited beam codebooks. First, the proposed model is validated by characterizing a Ka-band T-RIS. Then, we optimize the quad-beam T-RIS structure by tuning the focal distance between the lens and the focal sources according to two metrics: (i) the per-user antenna gain (analog-only approach), and (ii) the per-user average rate (hybrid digital/analog approach). For both indicators, the system performance is evaluated in a multi-user scenario by assuming imperfect channel state information. We show that considering only the analog precoder is sufficient to optimize the T-RIS. However, the fully hybrid precoding scheme is required to deal with inter-user interference. We propose a codebook-aware optimization that improves the aperture efficiency of the T-RIS system.

Orthogonal frequency division multiplexing (OFDM) based Cell-free massive multiple-input multiple-output (CF-mMIMO) is the most promising combination that can provide uniformly better quality of service for users than cellular technology in sub-6GHz bands. To make OFDM based CF-mMIMO more cost-effective and more powerefficient, the access points (APs) should be implemented by using low-cost and low-quality transceiver hardware.However, transmitting OFDM signals with high peak-to-average power ratio (PAPR) via such hardware causes severe hardware impairments (HWI), damaging the system performance. Moreover, the PAPR reduction in CF-mMIMO-OFDM has never been studied in the open literature. In this paper, we present a smart and original PAPR reduction technique, referred to as localPAPRfree adequate for cell-free architecture. Interestingly, the proposed technique can be implemented in a distributed and scalable fashion, achieving benefits of CF-mMIMO-OFDM. Specifically, the PAPR reduction scheme is formulated as a simple convex optimization problem solved via an efficient and steepest iterative method. In addition, we present a theoretical study to analyze the downlink (DL) spectral-efficiency (SE) and energy-efficiency (EE) performance of CF-mMIMO-OFDM under power amplifier (PA) non-linearity. In an original way, we derive closed-form expressions for the DL SE and EE for the local full-pilot zero-forcing (FZF) precoding scheme with large-scale fading over independent Rayleigh block fading channels while taking into account channel estimation errors and pilot contamination. This analysis can provide important insights into the practical impact of power-efficient PA. Numerical results show that the proposed localPAPRfree algorithm offers excellent PAPR reduction performance while guaranteeing high transmission quality.

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