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This paper underscores the paramount significance of through-wall communications by providing a comprehensive exploration of pivotal aspects like wall characterization, relevant technologies, versatile applications, and future prospects. Through-wall communication has the potential of being adopted in diverse fields, from indoor wireless communications, ground penetrating radar, through-wall radar imaging, to even power transmission. To augment the efficiency and reliability of signal detection, a whole variety of tools, such as antenna elements, frequency-selective structures, metasurfaces, and lenses, are exploited. By surveying the landscape of relevant technologies and applications in the existing literature, this paper serves as a beacon for researchers and engineers alike. Furthermore, it extends an invitation for pioneering exploration in this field, advocating the examination of tunable and adaptive systems, as well as the untapped potential of 3D printing in reshaping the future of through-wall communications.

In this paper, a path loss characterization at millimeter-wave (mmWave) frequencies is performed in a typical indoor office environment. Path loss results were derived from propagation channel measurements collected in the 25–40 GHz frequency band, in both line-of-sight (LOS) and obstructed-LOS (OLOS) propagation conditions. The channel measurements were performed using a frequency-domain channel sounder, which integrates an amplified radio over fiber (RoF) link to avoid the high losses at mmWave. The path loss was analyzed in the 26 GHz, 28 GHz, 33 GHz and 38 GHz frequency bands through the close-in free space reference distance (CI) and the floating-intercept (FI) models. These models take into account the distance dependence of the path loss for a single frequency. Nevertheless, to jointly study the distance and frequency dependence of the path loss, multi-frequency models were considered. The parameters of the ABG (A-alpha, B-beta and G-gamma) and the close-in free space reference distance with frequency path loss exponent (CIF) models were derived from the channel measurements in the whole 25–40 GHz band under the minimum mean square error (MMSE) approach. The results show that, in general, there is some relationship between the model parameters and the frequency. Path loss exponent (PLE) values smaller than the theoretical free space propagation were obtained, showing that there are a waveguide effect and a constructive interference of multipath components (MPCs). Since the measurements were obtained in the same environment and with the same configuration and measurement setup, it is possible to establish realistic comparisons between the model parameters and the propagation behavior at the different frequencies considered. The results provided here allow us to have a better knowledge of the propagation at mmWave frequencies and may be of interest to other researchers in the simulation and performance evaluation of future wireless communication systems in indoor hotspot environments. [ABSTRACT FROM AUTHOR]

The propagation of signal and its strength in an indoor area have become crucial in the era of fifth-generation (5G) and beyond-5G communication systems, which use high bandwidth. High millimeter wave (mmWave) frequencies present a high signal loss and low signal strength, particularly during signal propagation in indoor areas. It is considerably difficult to design indoor wireless communication systems through deterministic modeling owing to the complex nature of the construction materials and environmental changes caused by human interactions. This study presents a methodology of data-driven techniques that will be applied to predict path loss using artificial intelligence. The proposed methodology enables the prediction of signal loss in an indoor environment with an accuracy of 97.4%. [ABSTRACT FROM AUTHOR]

The sterilization properties of the C-band of the ultraviolet (UV) light have been well-known for many years; however, their application has become more vital and widespread today due to the Covid-19 pandemic. Nevertheless, minimal work has been devoted to simulating UV-C propagation for surface disinfection in indoor environments. UV light has also been proposed for non-line-of-sight (NLOS) outdoor communication applications, where a number of simulators have been developed, but with several simplifications. The current work presents a fully-featured simulator for UV light propagation, called LaUV, designed mainly for indoor settings, capable of calculating measurable parameters that can be used for indoor surface disinfection and communication applications. LaUV considers atmospheric absorption and scattering according to Rayleigh and Mie theory, and reflections from rough surfaces. The scattering cross-section and probability density function of the scattering angles (phase function) are calculated accurately for any wavelength, concentration, and diameter distribution of spherical scattering particles (aerosols and droplets). Examples are presented for indoor environments, including empty rooms and rooms with furniture, for different environmental conditions and surface albedos. Interesting initial conclusions can be drawn, considering the spatial distribution of UV exposure for surface disinfection applications and channel modeling for line-of-sight (LOS) and NLOS communication, demonstrating the versatility and the potential applications of the proposed simulator.

In this letter, we present a new real-time blind signal detection technique for indoor optical communications using a supervised learning framework in an artificial neural network (ANN). We model the optical channel as a time-varying doubly-stochastic Poisson process. To compare the performance of the proposed ANN network, we present an iterative method for joint channel estimation and symbol detection using expectation-maximization (EM) and Viterbi decoding and demonstrate that the bit error rate (BER) performance of the proposed technique is better than that of the Viterbi decoding. The proposed scheme is found to be resilient to indoor channel dynamics and can achieve a good BER performance over a wide range of channel variations.

In this letter, we present a new real-time blind signal detection technique for indoor optical communications using a supervised learning framework in an artificial neural network (ANN). We model the optical channel as a time-varying doubly-stochastic Poisson process. To compare the performance of the proposed ANN network, we present an iterative method for joint channel estimation and symbol detection using expectation-maximization (EM) and Viterbi decoding and demonstrate that the bit error rate (BER) performance of the proposed technique is better than that of the Viterbi decoding. The proposed scheme is found to be resilient to indoor channel dynamics and can achieve a good BER performance over a wide range of channel variations. [ABSTRACT FROM AUTHOR]

In this paper, optimization of diffuse spots' parameters in indoor optical wireless communications (OWC) system is carried out by using the particle swarm optimization (PSO) algorithm. By simultaneously optimizing the diffusion spots' locations and intensities, we show an improvement in the signal-to-noise ratio (SNR) and the delay spread at the receivers, while considering both the background noise and the multipath dispersion. A comparison is made between different optimization scenarios, to illustrate the effect of varying the parameters that are being optimized. We show that the optimization of both intensities and locations of diffuse spots resulted in improvement up to 42% and 23% in the average delay spread and the average SNR, respectively, compared with the centrally located position of diffuse spots' distribution, with respect to the receivers' locations, which has a uniform distribution of power. [ABSTRACT FROM AUTHOR]

Along with the evolution of wireless technologies, users are expected to have high-throughput and low-latency indoor mobile applications, such as augmented reality and virtual reality. Therefore, the use of millimeter-wave for low-latency communications becomes attractive. Under low-latency criteria, a fast multi-link establishment is desired. In this paper, we present a novel concurrent multi-beam training jointly with a proposed multi-beam codebook. First, we propose a progressive multi-beam estimation (PMBE). This algorithm makes multiple coarse beams emerge within a few training steps. Second, we propose a Fast Fourier Transform (FFT)-based codebook which can be considered as a Discrete Fourier Transform based codebook built in a bit-reversal scrambler, thus forming a scattering multi-beam codebook. This FFT-based codebook design is optimally implemented on a sub-connected architecture, a hardware-efficient architecture. For fast multi-link establishment, we design our concurrent multi-beam training by combining the proposed PMBE jointly with the proposed FFT-based codebook, so as to further improve multiplexing gain. Since multiplexing gain can occur immediately, even under strictly-limited training steps, the high data rate can be achieved. Simulation results show that when reaching more than 95% capacity gain of exhaustive channel estimation, our work only spends 1% training steps of the exhaustive case. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel optimization of the locations of the diffusion spots in indoor optical wireless local area networks, based on the central force optimization (CFO) scheme. The users’ performance uniformity is addressed by using the CFO algorithm, and adopting different objective function’s configurations, while considering maximization and minimization of the signal to noise ratio and the delay spread, respectively. We also investigate the effect of varying the objective function’s weights on the system and the users’ performance as part of the adaptation process. The results show that the proposed objective function configuration-based optimization procedure offers an improvement of 65 % in the standard deviation of individual receivers’ performance. [ABSTRACT FROM AUTHOR]

Although the broadcast television (TV) spectrum is currently open for unlicensed operation in the USA, a considerably large geographic area still remains excluded from the unlicensed operation due to potential interference to the licensed users. However, it might be possible to reuse primary spectrum within the protection contour if the frequency reuse occurs inside a building that shields radio signals and reduces interference to the primary system. Interference to outdoor licensed users from the indoor operations can be minimized if the unlicensed users adjust their transmit power according to their locations in the building. This paper presents an analysis and effectiveness evaluation of a novel cognitive radio (CR) system which enables CRs to access the licensed spectrum inside a building in the area within the protection contour. The system utilizes an indoor sensor network for (i) interference sensing, (ii) CR transmit power control, to limit the interference to the outdoor primary receiving antennas. Power control model of the indoor system has been developed to estimate safe transmit power for the indoor users. Two cases have been considered; single-user single-sensor (single indoor user and single sensor), and multi-user multi-sensor. Based on the power control model, a power control algorithm has been developed and its effectiveness is assessed through simulations. The algorithm is effective in realistic propagation scenarios, e.g. when internal partition walls and multipath fading are present. The outage probabilities in these propagation scenarios are found and the procedure of determining the transmit powers for CRs is presented. [ABSTRACT FROM AUTHOR]

This article shows an experimental study that aims to determine the dielectric constant and the cement paste loss tangent. The cement paste is a common element in masonry construction, whether in the mortar or in the composition of constructive elements, such as walls made of clay bricks, concrete blocks, precast slabs, etc., that affect wireless communications in indoor environments of buildings. An accurate analysis of the reduction of signal levels caused by these artificial barriers has performed in this study, in which a band in the 2.45 GHz range (wireless networks operation frequency) was used. Results show that the perform method is appropriate to characterize the permittivity of cement pastes. [ABSTRACT FROM AUTHOR]

In this paper, two recent global optimisation techniques: particle swarm optimisation and central force optimisation are considered for optimising the location of the diffusion spots, which are used as transmitters in an indoor optical wireless communications system. The diffusion spots are uniformly distributed along a circle on the ceiling of the communications environment. The location of the diffusion spots is optimised for maximising the signal-to-noise ratio and minimising the root-mean-square delay spread at the receivers in the presence of noise sources. A performance comparison between the central located position of the spots and the optimised positions for three different scenarios is considered to validate and show the strengths of the applied optimisation techniques in solving this sort of problems. The used techniques can provide the communication system with the ability to adapt to changes in the surrounding environment, which will benefit future switching towards real-time intelligent systems. [ABSTRACT FROM AUTHOR]

We present a study on evaluating the effects of multipath dispersion on indoor optical code division multiple access (CDMA) systems over optical wireless communication. The indoor propagation channel model which takes multiple reflecting surfaces into account is addressed. Theoretical analysis of the pulse response of direct light and reflected light is given. The bit error rate (BER) of this system is analyzed considering reflected light, background light, avalanche photo-diode noise, thermal noise, and multi-user interference. The results prove that the BER of the system is influenced by reflected light and that the effect of reflected light is related to room size and receiver position. With an increase in the number of users, the effect of reflected light becomes stronger and is a significant source of inter-symbol interference. It becomes clear that when optical CDMA is used in an indoor optical wireless system, the effect of reflected light is a key issue to be considered. [ABSTRACT FROM PUBLISHER]

A multidisciplinary-research effort encompassing compact broadband antenna design, indoor channel characterization, and an RF microelectromechanical-system (RF MEMS) technology with its application to a diversity system for indoor wireless communications is presented in this paper. We first introduce a coplanar waveguide-fed compact wideband antenna and characterize its impedance and radiation characteristics. The antenna has a 42% impedance bandwidth covering the frequency band of 4–6 GHz. Subsequently, a series of experiments are conducted to characterize the diversity performance of a system using three elements of the proposed antenna for nonline-of-sight indoor multipath environments. From these experiments, we evaluate the performance of the diversity system by calculating power correlation coefficients, power imbalance, and diversity gain. Microwave-laminate-compatible RF MEMS process technology, which has been recently developed in the author's group, is used to construct a microelectromechanically switched pattern-diversity antenna on RT/Duroid5870 microwave laminate. Radiation and impedance characteristics of this antenna are provided, and its reconfigurability capabilities for mobile environment are discussed. [ABSTRACT FROM AUTHOR]

The wireless indoor communications system employing an optical backboneprovides an economic and flexible approach for local area networks in officebuildings. In this approach the wireless communication serves small picocellsthat are interconnected by optical highways. The performance of the system isenhanced by assigning specific subcarriers for the individual picocells whichenables the transmission of the radio channels without altering theirmodulation format. The reception is simplified by a direct optical-microwavemixing method providing simultaneous optical detection and frequency shifting.This method allows for simple and cost-effective radio nodes. [ABSTRACT FROM AUTHOR]

This letter presents a broadband corner reflector based on a monopole-type excitation structure. Using a moment method to optimize the different parameter structures, it is possible to use this structure from 1.8 to 6 GHz, with an average gain of 9 dBi and a matching always better than 10 dB. The structure has been specially optimized for two frequencies of wireless communications, the 2.4 and 5 GHz bands. It has been used as a base station for indoor experimental communication systems in the above bands. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 21: 196–199, 1999. [ABSTRACT FROM AUTHOR]

This paper presents a modified receiver based on the conventional Rake receiver for Ultra-Wide Band (UWB) indoor channels of femtocell systems and aims to propose a new solution to mitigate the multipath phenomenon. Furthermore, this work proposes an upgrade for the conventional Rake receiver to fulfill the needs of 5G wireless systems through a new concept named "hybrid femtocell" that joins UWB with millimeter wave (mmWave) signals. The modified receiver is considered to be a part of the UWB/mmWave hybrid femtocell system, where it is developed for confronting the indoor multipath channels and to ensure a flexible transmission based on an Intelligent Controlling System (ICS). Hence, we seek to exploit the circumstances when the channel is less complex to switch the transmission to a higher data rate through higher M-ary Pulse Position Modulation (PPM). Furthermore, an ICS algorithm is proposed and an analytical model is developed followed by performance studies through simulation results. The results show that using the UWB technology through the modified receiver in femtocells could aid in mitigating the multipath effects and ensuring high throughputs. Thus, the UWB based system promotes Internet of Things (IoT) devices in indoor multipath channels of future 5G. [ABSTRACT FROM AUTHOR]

Driven by a new generation of wireless user equipment and the proliferation of bandwidth-intensive applications, user data traffic and the corresponding network load are increasing in an exponential manner. Most of this new data traffic is being generated indoors, which requires increased link budget and coverage extension to provide satisfactory user experience. As a result, current cellular networks are reaching their breaking point, and conventional cellular architectures that are devised to cater to large coverage areas and optimized for homogeneous traffic are facing unprecedented challenges to meet these user demands. [ABSTRACT FROM PUBLISHER]