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 technology that, compared to cellular technology, provide uniformly better quality of service for users in sub-6GHz bands. To make OFDM based CF-mMIMO more cost-effective and power-efficient, the access points (APs) should be equipped with 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. In this paper, we present an energy-efficient downlink transmission scheme adequate to the emerging CF-mMIMO-OFDM technology. Specifically, we introduce a smart PAPR-aware precoding scheme, referred to as localPAPRfree, adequate for cell-free architecture. Interestingly, the proposed technique can be implemented in a distributed and scalable manner, demonstrating benefits of CF-mMIMO-OFDM. The PAPR-aware precoding scheme is formulated as a simple convex optimization problem solved via an efficient and steepest iterative algorithm. 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, providing important insights into the practical impact of nonlinear PA. Numerical results show that the proposed localPAPRfree algorithm offers excellent PAPR reduction performance, guaranteeing high EE.

Forthcoming wireless systems will have to cope with a heterogeneous and dense traffic. High demands in terms of bandwidth use efficiency and increased flexibility are therefore required for the related air-interface technology. In this context, Filter-Bank MultiCarrier with Offset QAM (FBMC/OQAM) waveform has been recognized as an enabling technology providing strong assets with a well confined spectrum and enhanced spectral efficiency with respect to Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM). Nonetheless, FBMC-OQAM presents a few challenges to overcome: the intrinsic interference must be taken into consideration in the channel estimation processes and multiple antenna schemes design. Those challenges are induced by the reduction of the orthogonality property to the real field. In this paper, OFDM precoding/decoding is proposed as an alternative to the OQAM signaling for filter-bank based waveforms. The OFDM precoding makes the waveforms near-complex orthogonal which enables a straightforward adaptation to channel estimation techniques and MIMO scheme design. Two waveforms based on this precoding, namely Fast Fourier Transform FBMC (FFT-FBMC) and Block Filtered OFDM (BF-OFDM), have been proposed. A common framework is proposed to study the two waveforms. The paper also investigates changing the time structure of the transmitted signal by introducing the rate factor parameter. It is shown that intrinsic interference can be greatly reduced at the price of a slight side-lobe increase.

3GPP release 15 focusing on 5G general outline has been published in December 2017. The major difference with respect to currently deployed LTE is the support of various physical layer numerologies. Making the physical layer scalable allows to properly address new services such as low latency or millimeter communications. However it poses the problem of numerology coexistence. Indeed the orthogonality of the OFDM waveform is broken by the use of different subcarrier spacings and therefore multiplexed communications may interfere with each others. A first and simple solution to limit the distortion is to consider guard bands. In this paper, the authors develop analytical metrics to quantify the level of distortion induced by 5G multi-service multiplexing. Besides, general comments and guard band dimensioning are carried out. It is shown that high interference rejection needs to be associated with side lobe reduction techniques to favor an efficient bandwidth use.

For future wireless communication technologies, an increase in capabilities such as throughput is strongly expected. Transmission in the sub-THz bands (>90 GHz) seems to be the potential solution to meet the ever-increasing capacity demands due to the large unexploited bandwidth. Oscillators used at these frequencies generate phase noise that induces critical distortions in the signal that must be addressed. The correlated nature of PN makes it difficult to overcome. Nowadays, there is a growing interest in considering the extension of multicarrier based waveforms of the 5G new radio for transmissions in the sub-THz bands. In this paper, we introduce a new algorithm called the interpolation filter (IF), which efficiently estimates and compensates PN effects on DFT-s-OFDM systems. Specifically, it is based on the use of stochastic properties of the PN and is compatible with the 3GPP phase tracking reference signal scheme. We highlight a performance improvement over known techniques when using high-order modulation.

This paper presents a detailed analysis of coverage in a factory environment using realistic 3D map data to evaluate the benefits of Distributed MIMO (D-MIMO) over colocalized approach. Our study emphasizes the importance of network densification in enhancing D-MIMO performance, ensuring that User Equipment (UE) remains within range of multiple Access Points (APs). To assess MIMO capacity, we compare two propagation channel models: ray tracing and stochastic. While ray tracing provides accurate predictions by considering environmental details and consistent correlations within the MIMO response, stochastic models offer a more generalized and efficient approach. The analysis outlines the strengths and limitations of each model when applied to the simulation of the downlink (DL) and uplink (UL) single-user capacity in various D-MIMO deployment scenarios.