Ultra-high data rates with low-power consumption wireless communications and low-complexity receivers is one of the key requirements for the next 6-th Generation (6G) of communication networks. Sub-Terahertz (SubTHz) frequency bands can support Ultra-WideBand (UWB) communications and can thus offer unprecedented increase in the wireless network capacity, rendering those bands and relevant research a strong candidate technology for 6G. However, in contrast to millimeter-wave communications, the technological advances in subTHz transceivers are not yet completely mature. In this paper, we focus on the Time Reversal (TR) precoding scheme, which is a computationally simple and robust technique capable of focusing UWB waveforms on both time and space, by exploiting channel diversity when available. We design a novel experimental setup, including a THz link inside a waveguide and operating at the carrier frequency 273.6 GHz with 2 GHz transmission bandwidth, and deploy it to perform multi-user communications with simple modulation. Our results demonstrate large communication rates with only 3 mm space separation of the receiving antennas.

Time Reversal (TR) has been proposed as a competitive precoding strategy for low-complexity wireless devices relying on Ultra-WideBand (UWB) signal waveforms. However, when TR is applied for multiple access, the signals received by the multiple users suffer from significant levels of inter-symbol and inter-user interference, which requires additional processing for mitigation by each receiving user. In this paper, we present an iterative Time-Reversal Division Multiple Access (TRDMA) approach that aims to dim the latter interference levels. The performance of iterative TRDMA is evaluated experimentally in a reverberation chamber that mimics a rich scattering indoor wireless propagation environment. The improved efficiency, in terms of the number of algorithmic iterations, of the proposed approach compared to conventional TRDMA, is demonstrated. We also consider a mobile user configuration, where the position of the receiver changes between the channel estimation and data transmission steps. It is showcased, even for this experimental setup, that the proposed iterative TRDMA approach is more efficient than conventional precoding schemes.

MISO Time-Reversal (TR) communication marks a paradigm shift for ultrawideband (UWB) communications because the processing is mainly carried out by the transmitter, which is ideal for some applications. Thanks to its focusing property, TR is naturally dedicated to multiple access trans-mission (TRDMA). Previous research works have focused on the theoretical performance of TRDMA. In this paper, Multi-user MISO (MU-MISO) TRDMA transmission is evaluated experimentally for the first time to the best of our knowledge, and by simulations. An experimental setup is used to measure the TRDMA signals for different bandwidth in multi-user scenario. The experimental results are then used in a simulation to evaluate the bit error rate and the performance of the system.

In this paper, we introduce a physics-consistent analytical characterization of the free-space path-loss of a wireless link in the presence of a reconfigurable intelligent surface. The proposed approach is based on the vector generalization of Green's theorem. The obtained path-loss model can be applied to two-dimensional homogenized metasurfaces, which are made of sub-wavelength scattering elements and that operate either in reflection or transmission mode. The path-loss is formulated in terms of a computable integral that depends on the transmission distances, the polarization of the radio waves, the size of the surface, and the desired surface transformations. Closed-form expressions are obtained in two asymptotic regimes that are representative of far-field and near-field deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.

Ambient backscatter communication system is an emerging and promising low-energy technology for Internet of Things. In such system, a device named tag, sends a binary message to a reader by backscattering a radio frequency signal generated by an ambient source. Such tag can operate without battery and without generating additional wave. However, the tag-to-reader link suffers from the source-to-reader direct interference. In this paper, for the first time, we propose to exploit a "polarization reconfigurable" antenna to improve robustness of the tag-to-reader link against the source-to-reader direct interference. Our proposed new tag sends its message by backscattering as an usual tag. However, it repeats its message several times, with a different radiation pattern and polarization, each time. We expect one polarization pattern to be better detected by the reader. We show by simulations and experiments, in line-of-sight and in richly scattering environment, that a polarization reconfigurable tag limited to 4 polarization directions outperforms a non-reconfigurable tag and nearly equals an ideally reconfigurable tag in performance.

Reconfigurable intelligent surfaces (RISs) are an emerging transmission technology for application to wireless communications. RISs can be realized in different ways, which include (i) large arrays of inexpensive antennas that are usually spaced half of the wavelength apart; and (ii) metamaterial-based planar or conformal large surfaces whose scattering elements have sizes and inter-distances much smaller than the wavelength. Compared with other transmission technologies, e.g., phased arrays, multi-antenna transmitters, and relays, RISs require the largest number of scattering elements, but each of them needs to be backed by the fewest and least costly components. Also, no power amplifiers are usually needed. For these reasons, RISs constitute a promising software-defined architecture that can be realized at reduced cost, size, weight, and power (C-SWaP design), and are regarded as an enabling technology for realizing the emerging concept of smart radio environments (SREs). In this paper, we (i) introduce the emerging research field of RIS-empowered SREs; (ii) overview the most suitable applications of RISs in wireless networks; (iii) present an electromagnetic-based communication-theoretic framework for analyzing and optimizing metamaterial-based RISs; (iv) provide a comprehensive overview of the current state of research; and (v) discuss the most important research issues to tackle. Owing to the interdisciplinary essence of RIS-empowered SREs, finally, we put forth the need of reconciling and reuniting C. E. Shannon's mathematical theory of communication with G. Green's and J. C. Maxwell's mathematical theories of electromagnetism for appropriately modeling, analyzing, optimizing, and deploying future wireless networks empowered by RISs.

This study is an experimental validation of a new closed-form method for automatic array position calibration, based on time of arrival (TOA) measurements between sources and sensors. An experiment with a large array composed of 121 microphones and a dozen of sources has been set up. We first show that, when considering the whole array, this calibration method gives results on par with a reference state-of-the-art acoustic method. We then show experimentally that the new method provides significantly better results when the number of sources and microphones decreases, confirming numerical simulations. We conclude the paper with a discussion on methodological issues for array position calibration.

This paper investigates the problem of estimating the shape or the boundary impedance of a vibrating membrane from acoustic measurements in a limited sub-domain of the membrane. In acoustics, polygonal room shapes are usually estimated through room impulse response measurements. Impedance values of materials are, in turn, often calculated from the measurement of the acoustic reflection coefficients at the boundaries. In this work, we develop an alternative frequency-domain method to estimate the shape of a convex membrane with generalized Robin boundary conditions, from the measurement of its eigenmodes on a small portion of its surface. Reciprocally, we show that the same model allows to estimate the membrane borders' impedances when its shape is known.

We study the problem of microphone array localization in a strongly reverberant room, where time of arrivals (TOA) or time difference of arrivals (TDOA) cannot always be measured precisely. Instead, we use frequency-domain measurements to calibrate the array position, based on the modes of the room, excited by a wide-band single source, that can be unknown. By using the fact that each measured mode can be decomposed as a sum of model-based polynomials, we build a cost function whose minimum indicates the positions of the microphones. A simple Block Coordinate Descent algorithm can be used to minimize this cost function. Numerical results indicate that this algorithm converges to the right solution, and therefore that using frequency measurements for position calibration is a valid concept for dense arrays, as an alternative to time-domain methods in reverberant domains.