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This article presents comprehensive double-directional channel measurements with time-domain channel sounding at 300 GHz that characterize the channel of wireless links in a data center. The channels are classified into three scenario-dependent use cases and are individually analyzed providing channel parameters as a function of the required signal-to-noise ratio of the prospective communication system. The spatial and temporal analysis of the channel reveals relevant propagation effects such as the influence of scattering and derives relations between the channel parameters of the propagation and the radio channel. The analysis shows that multipath propagation becomes relevant for systems with high signal-to-noise ratio requirements despite high-gain directional antennas. A first order approximation of relevant propagation effects complements the analysis. The measurement data of the whole measurement campaign with 18250 calibrated impulse responses including all meta data is published so that the research community has a collective benefit.

Abstract Radio wave propagation in an intra-vehicular (IV) environment is markedly different from other well-studied indoor scenarios, such as an office or a factory floor. While millimetre wave (mmWave)-based intra-vehicular communications promise large bandwidth and can achieve ultra-high data rates with lower latency, exploiting the advantages of mmWave communications largely relies on adequately characterising the propagation channel. Channel characterisation is most accurately done through an extensive channel sounding, but due to hardware and environmental constraints, it is impractical to test channel conditions for all possible transmitter and receiver locations. Artificial neural network (ANN)-based channel sounding can overcome this impediment by learning and estimating the channel parameters from the channel environment. We estimate the power delay profile in intra-vehicular public and private vehicle scenarios with a high accuracy using a simple feedforward multi-layer perception-based ANN model. Such artificially generated models can help extrapolate other relevant scenarios for which measurement data are unavailable. The proposed model efficiently matches the taped delay line samples obtained from real-world data, as shown by goodness-of-fit parameters and confusion matrices.

Many physical-layer security works in the literature rely on purely theoretical work or simulated results to establish the value of physical-layer security in securing communications. We consider the secrecy capacity of a wireless Gaussian wiretap channel using channel sounding measurements to analyze the potential for secure communication in a real-world scenario. A multi-input, multi-output, multi-eavesdropper (MIMOME) system is deployed using orthogonal frequency division multiplexing (OFDM) over an 802.11n wireless network. Channel state information (CSI) measurements were taken in an indoor environment to analyze time-varying scenarios and spatial variations. It is shown that secrecy capacity is highly affected by environmental changes, such as foot traffic, network congestion, and propagation characteristics of the physical environment. We also present a numerical method for calculating MIMOME secrecy capacity in general and comment on the use of OFDM with regard to calculating secrecy capacity. [ABSTRACT FROM AUTHOR]

Radio wave propagation in an intra-vehicular (IV) environment is markedly different from other well-studied indoor scenarios, such as an office or a factory floor. While millimetre wave (mmWave)-based intra-vehicular communications promise large bandwidth and can achieve ultra-high data rates with lower latency, exploiting the advantages of mmWave communications largely relies on adequately characterising the propagation channel. Channel characterisation is most accurately done through an extensive channel sounding, but due to hardware and environmental constraints, it is impractical to test channel conditions for all possible transmitter and receiver locations. Artificial neural network (ANN)-based channel sounding can overcome this impediment by learning and estimating the channel parameters from the channel environment. We estimate the power delay profile in intra-vehicular public and private vehicle scenarios with a high accuracy using a simple feedforward multi-layer perception-based ANN model. Such artificially generated models can help extrapolate other relevant scenarios for which measurement data are unavailable. The proposed model efficiently matches the taped delay line samples obtained from real-world data, as shown by goodness-of-fit parameters and confusion matrices. [ABSTRACT FROM AUTHOR]

In this paper, a newly developed 300 GHz channel sounder is presented followed by a detailed description of test measurements and the subsequent results obtained to validate its working. An elucidation of the high-resolution double-directional channel measurement in a typical conference room scenario precedes the comparison of the results obtained for the current campaign to that obtained from an earlier campaign at 60 GHz for a similar setup. It is observed that a similar number of clusters for both the bands under investigation for all the transmitter (Tx) and receiver (Rx) positions are obtained from the generated power spectra. Any deviation is theorized to be caused either due to the usage of lower measurement bandwidth in the 60 GHz campaign or due to limited elevation expanse in the 300 GHz measurement. To identify the interacting objects (IOs) causing the clusters, environment-embedded angular power spectra (APS) and ray tracing simulations are used. The large-scale parameters (LSPs) are also evaluated for both campaigns. It is observed that signal propagation at 300 GHz is dominated more by the line-of-sight (LoS) path compared to 60 GHz. The results are also compared with other similar results from the literature.

Quantification of uncertainties in the results of channel sounding measurements is important for their interpretation and further usage. In this paper, a novel uncertainty analysis methodology to quantify uncertainties of condensed parameters in measurements of dynamic millimeter-wave channels is presented. The bandwidth limitation and multipath threshold are identified as important impairments. Therefore, the methodology provides three uncertainty metrics for condensed parameters, namely a standard uncertainty to quantify the impact of random variations; a bias due to the multipath threshold; and a total bias including the impact of the bandwidth limitation. These uncertainty metrics are highly channel dependent. Therefore, the proposed methodology creates reference channels, which are representative of corresponding measured channels. Hardware and processing impairments are included in the analysis via a Monte Carlo simulation. This results in a general methodology that can quantify uncertainties in both static and dynamic channel measurements of any wideband channel sounder. The methodology is implemented, verified and demonstrated for the TU/e channel sounder, which exemplifies how it can be used. The proposed methodology can improve the analysis, interpretation and reporting of channel measurement results.

As the number of Wi-Fi network users is growing, the problem of increasing the network capacity in a scenario with many user devices becomes relevant. One of the approaches to increasing network capacity is multiple input, multiple output (MIMO) algorithms. Their performance relies heavily on information about the channel. This paper compares the explicit and implicit sounding mechanisms in Wi-Fi networks and discusses the accuracy of sounding when using each mechanism and protocol overhead. Using simulation, it was shown that the use of the implicit channel sounding mechanism increases the total network capacity by more than 40%. [ABSTRACT FROM AUTHOR]

In this study, an investigation into the small-scale and multipath fading of radio wave propagation in a complex building environment has been conducted by using Afe Babalola University Ado-Ekiti Nigeria, College of Engineering building as a case study. The study utilizes sliding correlator channel sounding measurement techniques, implemented on NI USRP 2920 modules by using GNU radio software. Moreover, single-input and single-output configurations have been employed by setting one module as a transmitter and the other one as a receiver, propagating at a carrier frequency of 1.2 GHz. Five scenarios within the first floor of the building have been considered and various cases in each one of the scenarios are either outdoor-to-indoor, indoor-to-outdoor, indoor or outdoor measurement. The analysis of the scatter plot of the maximum delay spread against the distance of experiments for the line-of-sight and non-line-ofsight cases show that analytical expression cannot be obtained due to the complexity of the building structure. The result obtained also shows that the maximum delay spread values are between 575 µs and 1023 µs, which points to the existence of a frequency selective fading condition that causes intersymbol interference and data rate reduction. [ABSTRACT FROM AUTHOR]

The low terahertz band (0.1–1 THz) offers very high data rates for novel mobile applications and constitutes a promising candidate to meet the requirements in aviation, enabling future wireless in-flight entertainment (IFE) systems. An extensive knowledge of the propagation effects in the radio channel in complex scenarios is a prerequisite to successfully design a communication system. This article presents a measurement campaign in a real aircraft investigating the radio channel at 300 GHz for different deployments of a wireless IFE system. The propagation effects as well as temporal and spatial channel characteristics are analyzed for likely communication links between an access point and a user equipment (UE). In the same way, interference links between two UEs are studied, reporting transmission and reflection losses for various components of the aircraft. A deterministic and geometry-based channel model is derived and extended to cover the effect of human blockage that suggests an integration of the access point into the passenger service unit. The measurement and realistic path loss models can be used in further link and system level simulations and contribute to the design of future IFE systems. [ABSTRACT FROM AUTHOR]

Many physical-layer security works in the literature rely on purely theoretical work or simulated results to establish the value of physical-layer security in securing communications. We consider the secrecy capacity of a wireless Gaussian wiretap channel using channel sounding measurements to analyze the potential for secure communication in a real-world scenario. A multi-input, multi-output, multi-eavesdropper (MIMOME) system is deployed using orthogonal frequency division multiplexing (OFDM) over an 802.11n wireless network. Channel state information (CSI) measurements were taken in an indoor environment to analyze time-varying scenarios and spatial variations. It is shown that secrecy capacity is highly affected by environmental changes, such as foot traffic, network congestion, and propagation characteristics of the physical environment. We also present a numerical method for calculating MIMOME secrecy capacity in general and comment on the use of OFDM with regard to calculating secrecy capacity.

Millimeter-wave (mmWave) communications promise Gigabit/s data rates thanks to the availability of large swaths of bandwidth between 10–100 GHz. Although cellular operators prefer the lower portions of the spectrum due to popular belief that propagation there is more favorable, the measurement campaigns to confirm this – conducted by ten organizations thus far – report conflicting results. Yet it is not clear whether the conflict can be attributed to the channel itself – measured in different environments and at different center frequencies – or to the differences in the organizations’ channel sounders and sounding techniques. In this paper, we propose a methodology to measure mmWave frequency dependence, using the 26.5–40 GHz band as an example. The methodology emphasizes calibration of the equipment so that the measurement results represent the channel alone (and not the channel coupled with the channel sounder). Our results confirm that free-space propagation is indeed frequency invariant – a well understood phenomena but to our knowledge reported nowhere else at mmWave to date. More interestingly, we found that specular paths – the strongest after the line-of-sight path and so pivotal to maintaining connectivity during blockage – are the least invariant compared to weaker diffracted and diffuse paths.

Spatial modulation (SM) as a multiple-input multiple-output (MIMO) technique is a solution suitable which offers, with low system complexity and cost, improved spectral efficiency compared to single-input-single-output (SISO) systems. Moreover, the transmission chain is simplified which decreases energy consumption. This paper aims to analyze the SM system transmission under different line-of-sight/non-line-of-sight (LOS/NLOS) propagation scenarios. The analysis of the SM system performance is based on both simulations and experimental results and the bit error rate (BER) is tackled. Concerning the experimental results, two strategies are considered in order to configure the propagation channel. One method consists in constructing a controlled propagation environment. More precisely, the multipath channel’s Rician K-factor is imposed by configuring the power levels of the LOS and of the NLOS components. The second method consists in performing over-the-air transmissions on a realistic propagation channel. A channel sounding method allows us to measure the channel characteristics. The experimental results are confronted with the system-level simulation results, good agreement between experimental and simulation results are obtained. The results show that the SM system can maintain the performance in multipath propagation in the presence of indirect paths for both two proposed propagation environments.

Design and assessment of THz communications systems, which will form an essential part of 6G, require an understanding of the propagation channels the systems will operate in. This paper presents investigations of the channel characteristics in various scenarios at 145 GHz, which is the band currently envisioned for the first round of deployments. In particular, we review several extensive measurement campaigns performed by the University of Southern California in both outdoor and indoor environments. We present the measurement and evaluation methodology and sample results that illustrate the dominant propagation effects in different environments. We then summarize the parameters of the statistical channel models for path loss, delay dispersion, and angular dispersion. Based on these results, we find that even in NLoS (non-line-of-sight) situations, Gbit/s communications can be sustained over a 100 m distance; that (for an antenna gain of 20 dB), there is considerable delay dispersion, requiring tens of equalizer taps, and that angular dispersion is significant in both LoS and NLoS situations. The channel parameters can be thus used as a basis for system design and evaluation under realistic operating conditions.

This paper developed a 60-GHz millimeter-wave cluster channel model for various office environments and investigated the impact of cluster characteristics on wireless data transmission. We conducted double-directional channel measurement campaigns in a university laboratory office environment to develop the channel model using a custom-developed channel sounder with an angular resolution of 6° and a delay resolution of $\mathrm {2.5~ns}$ at $\mathrm {58.32~GHz}$ for the measurements. The super-resolution multipath parameter estimation algorithm extracts multipath components from the measured data with a delay resolution of $\mathrm {0.1~ns}$ and an azimuth resolution of 0.1°, and the $K$ -PowerMeans algorithm identified multipath clusters. Based on the conventional 3rd Generation Partnership Project (3GPP) model, a scenario-categorized site-general model, we attempted to develop a highly accurate cluster model that can more accurately reproduce small-scale fading fluctuations in site-specific environments. The large- and small-scale parameters obtained from the measured data were extracted and compared with the 3GPP map-based hybrid channel model, a quasi-deterministic channel model. The results revealed strong site dependency in the inter-cluster and intra-cluster properties. Moreover, we assessed the impact of different channel parameters on system evaluation using single-user and multi-user multiple-input- multiple-output channel capacities.

This article presents a novel and cost-effective vector network analyzer (VNA)-based phase-compensated channel sounder operating in the frequency range of 10–50 GHz using radio-over-fiber (RoF) techniques, which can support multilink/channel long-range phase-coherent measurements. The optical cable enables long-range channel measurements with a dynamic range of 115.7 dB at 30 GHz (for the back-to-back connection). The phase compensation scheme is utilized for stabilizing the inherent phase variations introduced by the optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. A novel optical delay line and combiner scheme is proposed and implemented to separate the signals, thereby saving the port resource on the VNA for multilink/channel measurements. The proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with the presence of thermal changes and mechanical stress. The phase change could be maintained within 3° at 10–30 GHz and 7° in 30–50 GHz compared to the over 80° phase variation introduced by the cable effects at 10–50 GHz, demonstrating the robustness and effectiveness of the developed channel sounder in practice. With the proposed optical delay line and combiner scheme, multiple channels can be measured simultaneously with minimal VNA ports, significantly reducing the measurement cost and time for multilink channel measurements. Two multilink indoor channel measurements are conducted and analyzed using a virtual uniform rectangular array (URA) at 28-GHz bands. The results demonstrate the capability of the proposed channel sounder to perform high-fidelity long-range ultrawideband multilink channel measurements. [ABSTRACT FROM AUTHOR]

Human blockage and its dynamics are potential challenges for millimeter-wave (mm-wave) mobile communication. This article presents the results of wideband measurements at 27 GHz with one human blocker close by a dynamic mobile terminal (MT) as well as one or multiple dynamic human blockers further away from an MT. The measured human blockage loss is largest when the direct path (DP) in a line-of-sight (LOS) is blocked, but this loss is limited by other multipath components (MPCs). For nonline-of-sight (NLOS) channels, it is shown that human blockage loss is typically negligible. The presented measurement results show that human blockage loss in multipath channels is much smaller than that reported in diffraction- and measurement-based models, which neglect or minimize the contribution of all MPCs other than the DP. This suggests that the multipath nature of the indoor wireless channel highly limits the impact of human blockage. [ABSTRACT FROM AUTHOR]

Millimeter-wave (mm-Wave) band channel modeling and characterization are essential for system design and analysis in the fifth generation (5G) and future communication systems. Reliable channel sounding in the deployment scenarios is required for accurate and realistic channel modeling and characterization. In the state of the art, directional scanning sounding (DSS) and virtual antenna array (VAA) sounding are two popular methods due to their simplicity and cost-effectiveness. The DSS and the VAA methods are typically based on mechanically rotatable directional antennas and mechanically movable omnidirectional antennas, respectively. However, the spatial resolution is limited by the directivity and high sidelobes of the directional antennas used in the DSS method. The conventional VAA method also has limitations in terms of the low signal-to-noise ratio (SNR) and the unavailability of suitable omnidirectional antennas that support mmWave (above 60 GHz) band measurements and wideband horizontal polarization measurements. In this article, a novel directional antenna-based VAA framework in combination with the associated beamforming algorithm is proposed. Compared to the state-of-the-art methods, the proposed framework can achieve high angular resolution and high SNR for mm-Wave channel measurements without introducing additional cost and measurement time. Furthermore, it is a generic solution that can be applicable for arbitrary frequency bands and polarizations, unlike the conventional VAA method. To validate the effectiveness and robustness of the proposed method, experiments in two scenarios (a clean anechoic chamber and a realistic indoor meeting room) were conducted over 28–30 GHz with two types of directional antennas (i.e., a horn antenna and a corrugated antenna). Besides, the advantages of the proposed method are highlighted with a comparison to the conventional VAA and DSS methods. [ABSTRACT FROM AUTHOR]

In this paper, we present the development of a multicarrier modulation system of low complexity for broadband underwater acoustic communications (UAC), whose frequency band is located in the ultrasonic range, specifically between 32 kHz and 128 kHz. Underwater acoustic channels are recognized among the most hostile communication channels due to their strong time and frequency selectivity and, hence, the design of high-performance systems is a challenge that is difficult to resolve at the present time with state-of-art technology. The aim of the proposed system is to reach a reasonable bit rate, between 40 and 50 Kbps, over these channels that allows, for instance, the transmission of video signals of limited quality. We describe an orthogonal frequency division multiplexing (OFDM) modem prototype with a parameter setting and design specifically adapted to the channel nature. For this purpose, actual measurements carried out at the Mediterranean sea, on shallow waters, have been used to evaluate the system performance and to optimize the design. A discussion on several modulations and OFDM configurations is presented that leads to the selection of differential and non-differential quadri-phase shift keying (QPSK) as good candidates depending on synchronization capabilities. [ABSTRACT FROM AUTHOR]

This article presents an evaluation of the performance of two different approaches for beamforming (BF) in a fifth generation (5G) handset, based on measured channels from a dual-polarized transmitter (Tx) to a mockup handset with a seven-element receiver (Rx) array. The measurements at 21.5 GHz include both small-scale and large-scale channel changes and were conducted for different users holding the mockup at different locations in an indoor corridor environment. The unique measurements enable novel results based on the statistics of the power achieved with BF, using channel state information (CSI) measured at a 90 Hz rate, and both when using beam scanning (BS) and the equal gain combining (EGC) reference case. For the case of ideal CSI knowledge, BF gains of up to about 18 dB were found. However, when the density of the angle scanning grid is reduced and the CSI is delayed to realistic values, the average performance decreases by 8 dB. More results are given in the article. [ABSTRACT FROM AUTHOR]

This paper investigates the wideband channel characteristics obtained in a classroom and a hall indoor scenarios at 2-4, 9-11 and 27-29GHz. A virtual uniform circular array (UCA) based channel sounding system was utilized to capture wideband spatial channel characteristics. The propagation parameters of multipath components (MPCs) were estimated by using a high resolution parameter estimation (HRPE) algorithm. The estimated MPCs are further grouped into clusters via a novel clustering identification algorithm based on the KPowerMeans algorithm. The comparison of the composite and cluster-level characteristics at multiple frequency bands in both scenarios is investigated. Moreover, the impact of the indoor room environment, i.e. dimension and furniture, on the propagation channel is also analyzed. The statistics of channel parameters at multiple frequency bands extracted constitute a stochastic clustered spatial channel model.

This paper presents a multipath component (MPC) parameter estimator that uses a double-directional (D-D) channel sounding system in WiGig millimeter-wave bands. It employs low-cost commercial-off-the-shelf (COTS) RF transceivers capable of beam steering within an azimuth angle range of ±45°; the transmit power is approximately 31 dBm, equivalent isotropic radiated power (EIRP), including a total antenna gain of approximately 19 dBi. The half-power beam widths in the azimuth and elevation planes are 6° and 45°, respectively. A fast D-D channel acquisition in 360° full azimuth range can be achieved by developing a $4 \times 4$ multiple-input-multiple-output (MIMO) time division-multiplexing (TDM) scheme that enables simultaneous use of four antenna arrays at both sides of the transmitter and receiver. The beam switching for all combinations ( $48 \times 48 = 2304$ ) with 12 beams at each array requires less than 200 ms, excluding optional procedure related to data storage. A high-resolution MPC extraction method was developed from the D-D angular delay power spectra obtained using the sub-grid CLEAN algorithm, which is applicable when phase coherence among multiple snapshots obtained via the beam-switching measurement cannot be guaranteed. Furthermore, the operation of the developed system was validated via a measurement at 58.32 GHz in an office floor of a university laboratory. From the data analysis, the cluster properties and the corresponding scattering processes were identified.

Firstly, the different characteristics of terahertz waves compared to lower frequency bands were summarized, including the influence of meteorological factors and rough surfaces of materials on THz waves. Next, it was proposed to use ray-tracing (RT) technology to calibrate the three-dimensional (3D) environment model and electromagnetic parameters of materials in the target scenarios through very limited channel measurement data. Then, the reversed parameters were utilized to perform in a similar but more general scenario, instead of a large amount of channel measurement data. Finally, the channel characteristics were analyzed by extraction of the channel key parameters, such as path loss, shadow fading, Rician K-factor, delay spread, angular spread, and doppler parameters in a mobile scenario. Representing two ends of 6G THz use cases from indoor scenario to outdoor scenario, case studies were made for wireless connections on a desktop and vehicular communications, respectively. As for the outdoor scenario, the influences of meteorological factors were additionally considered. The work is of great significance for the design and evaluation of the THz communication system. [ABSTRACT FROM AUTHOR]

This letter presents the first vector network analyzer based subterahertz (sub-THz) phase-compensated channel sounder at 220–330 GHz using radio-over-fiber techniques that could enable long-range phase-coherent measurements. The optical cable solution enables long-range channel measurements at sub-THz bands, since it can effectively minimize the cable loss. This letter also proposes a novel phase compensation scheme to stabilize the phase variations introduced by optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. This proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with presence of thermal changes and mechanical stress. The phase variation introduced by the cable effects in the system is shown to be over $400^\circ$ in 220–330 GHz, compared to $15^\circ$ at 220–288 GHz and $37^\circ$ in 288–330 GHz after compensation, respectively, demonstrating the robustness and effectiveness of the developed channel sounder in practice. The developed system, which has a dynamic range of 106.7 dB, can support measurement range up to 300 m (limited by the optical cable length in our system and subject to over-the-air signal transmission loss in practical environment). [ABSTRACT FROM AUTHOR]

Proper characterization of the millimeter-wave (mmWave) propagation channel requires measuring the power angular-delay profile of the channel which includes angle-of-departure and angle-of-arrival of the multipath components (MPCs). In this paper, we first describe in detail our rotating directional antennas-based 28 GHz channel sounder. Then, for this specific sounder class, we describe and address the following two problems in extracting the MPCs from the measurements: 1) For long-distance channel measurements, triggering signal cannot be generated for the TX and the RX using a single clock (SICL). This necessitates the use of separate clocks (SECLs) which introduces a random timing drift between the clocks. 2) As positions of the antennas change during scanning, total distance traveled by the same MPC differs at each measurement. These problems together cause missing some of the MPCs and detecting MPCs that do not exist in reality. We propose an algorithm to correct the clock drift and MPC delay errors due to the rotation of the antennas. We compare the MPCs from the SICL measurement and the corrected SECL measurements using a Hungarian algorithm based MPC matching method. We show that the percentage of the matched MPCs increases from 28.36% to 74.13% after the correction process.

Untethered channel sounders are used to measure mobile radio propagation channels. As high data rate, millimeter-wave and multiple-input multiple output (MIMO) are considered in the next generation communications systems, knowledge of the propagation channel needs to be studied in high accuracy. Measuring time stability of the clocks in both transmitter and receiver is essential when characterizing relative delay between multipath components (MPCs), absolute delay, angle of arrival (AoA) and angle of departure (AoD) of MPCs. This paper describes timing circuit design of untethered electronic switched MIMO millimeter-wave channel sounders, proposes practical method to remove a constant initial time error, and experimentally estimates time stability of the channel sounders. Time stability is classified in three categories: short-term (in order of microseconds) that impacts relative delay, AoA and AoD; medium-term (in order of minutes) that impacts absolute delay; and long-term stability (in order of days) that can be calibrated. This is the first time to apply standard parameters for measuring clock stability, such as timing Allan deviation (TDEV) and time interval error (TIE), to channel sounding and modeling. Novel methods are developed for measuring jitter and short-term noise. It was found that TDEV for short-term measurements was less than 1 ps and the medium-term timing errors due to clock noise could be maintained at 0.4 ns/min.

This paper analyzes the performance of well-known precoding schemes for massive multiple-input multiple-output (MMIMO) systems. The investigations are based on extensive measurements made with a sounding system capable of capturing the dynamic channels towards users moving in many different outdoor scenarios. Assuming ideal channel state information (CSI), results show that the mean sum-rate of the maximum ratio transmission (MRT) precoder varies considerably with the scenario, e.g., from 6.5 to 14.5 bit/s/Hz (10%- and 90%-percentiles) for a 64 element uniform linear array (ULA) at the base station (BS), while the zero-forcing (ZF) and signal to leakage and noise ratio (SLNR) precoders are more robust and higher performing with variation from 13.4 to 16.3 bit/s/Hz in the same conditions. However, when the CSI is non-ideal the performance drops. With the CSI delayed corresponding to movement of about 1/5 of a wavelength, the ZF and SLNR mean sum-rate is 60-92% of that achieved with ideal CSI (10%- and 90%-percentiles). More statistics for different massive array sizes with both delay and frequency offset CSI are given in the paper.

This paper presents the verification of indoor propagation channel simulations based on an intelligent ray launching algorithm (IRLA) in the Ka‐band of the millimeter‐wave (mmWave) spectrum in various indoor environments, including a classroom, an office, and a corridor, by radio channel measurements. Power delay profiles (PDPs), path loss, and root‐mean‐square (RMS) delay spreads were obtained from both the simulation results and the measurement results. Moreover, parameters of two site‐general path loss models, the close‐in free space reference distance path loss model and the floating‐intercept path loss model, were estimated based on the measured and simulated path loss. The comparison between the simulation results and the measurement results indicates that the IRLA‐based simulation can accurately describe the main characteristics of the indoor propagation channel in the Ka‐band. Key Points: Intelligent ray launching algorithm (IRLA) based channel simulations in three‐dimensional (3‐D) indoor environment modelsRadio channel measurements in indoor environments in the Ka‐band of the millimeter‐wave (mmWave) spectrumComparison of power delay profiles (PDPs), path loss, and root‐mean‐square (RMS) delay spreads between simulation and measurement results [ABSTRACT FROM AUTHOR]

Simple, low-power, reliable backscatter communication systems must overcome the severe, double-fading nature of the backscatter channel. In this article, we show experimentally that when a conventional transponder is replaced by a retrodirective transponder in backscatter systems, those deep fading nulls are reduced by as much as two orders-of-magnitude. We consider three multipath scenarios all of which show a pronounced link-reliability improvement with a retrodirective transponder compared to its standard non-retrodirective counterpart. The measurements also reveal that by comparing the best case scenario of the two transponder designs under 0.001 outage probability, the retrodirective transponder requires only a 13-dB fade margin while the conventional tag requires a 34-dB fade margin—a significant improvement that would be a key enabler for next-generation backscatter systems. [ABSTRACT FROM AUTHOR]

MU-MIMO and OFDMA are two key techniques in IEEE 802.11ax standard. Although these two techniques have been intensively studied in cellular networks, their joint optimization in Wi-Fi networks has been rarely explored as OFDMA was introduced to Wi-Fi networks for the first time in 802.11ax. The marriage of these two techniques in Wi-Fi networks creates both opportunities and challenges in the practical design of MAC-layer protocols and algorithms to optimize airtime overhead, spectral efficiency, and computational complexity. In this paper, we present DeepMux, a deep-learning-based MU-MIMO-OFDMA transmission scheme for 802.11ax networks. DeepMux mainly comprises two components: deep-learning-based channel sounding (DLCS) and deep-learning-based resource allocation (DLRA), both of which reside in access points (APs) and impose no computational/communication burden on Wi-Fi clients. DLCS reduces the airtime overhead of 802.11 protocols by leveraging the deep neural networks (DNNs). It uses uplink channels to train the DNNs for downlink channels, making the training process easy to implement. DLRA employs a DNN to solve the mixed-integer resource allocation problem, enabling an AP to obtain a near-optimal solution in polynomial time. We have built a wireless testbed to examine the performance of DeepMux in real-world environments. Our experimental results show that DeepMux reduces the sounding overhead by 62.0% ~ 90.5% and increases the network throughput by 26.3% ~ 43.6%. [ABSTRACT FROM AUTHOR]

The received signal strength (RSS) is widely used in range-based localization because of its practical advantages, such as low cost and low complexity. For accurate ranging based on the RSS in indoor environments, well-defined propagation models (PMs) that consider the distortion of the RSS due to abrupt obstructions are required. This article proposes an exponential PM for indoor hallways whose structures can be modeled as a slab waveguide. This waveguide-shaped structure has two main power loss mechanisms: wave penetration and scattering. These loss mechanisms are quantified as exponential coefficients according to the law of conservation of energy. The surface-scattering theory and the multimode waveguide concept are utilized for the quantification of scattering and penetration, respectively. The proposed model is verified through actual site measurements using the vector network analyzer (VNA)-based channel-sounding system at IEEE 802.11ax frequency bands. The estimated range obtained from the measured data indicates that the proposed model leads to the ranging estimation performance that could not be achieved with existing sounding-based indoor channel models. [ABSTRACT FROM AUTHOR]

The upcoming fifth-generation wireless technology application areas bring new communication performance requirements, mainly in terms of reliability and latency, but also in terms of radio planning, where the further detailed characterization of the wireless channel is needed. To address these demands, we developed an agile multi-node multi-antenna wireless channel sounding system, using multiple software-defined radio devices. The system consists of 12 testbed nodes which are controlled from a centralized testbed server. Each node features a control host computer and two multi-antenna universal software radio peripheral boards. By managing the transmission and reception of reference signals among all the distributed testbed nodes, the system can measure the channel conditions of all multiple independent radio links. At the same time, the distributed architecture of the testbed allows a large number of spatially distributed locations to be covered with only a few redeployments of the testbed nodes. As a consequence of this, the system favors the collection of a large number of distributed channel samples with limited effort within a short dedicated measurement time. In this paper, we detail the general testbed design considerations, along with the specific sounding signal processing implementations. As further support to the system design, we also include the results from different verification and calibration tests, as well as a real measurement application example.

In this paper, we present the results of the measurements and modeling of the propagation channel for ultra wide band (UWB) communications in large indoor environments. The selected environment is a sports center of large dimensions ($51 \times 26$ m, 7.4m height) without intermediate obstacles so that there is always direct path (DP) between TX and RX and UWB technology can be used for critical communications of sensors. On this environment, narrow band and ultra wide band measurements have been made with a high resolution pulse channel sounder (8ns) at the nominal frequency of 3.9 GHz. Narrow band measurements have been used to model path loss for the DP, while for UWB measurements, the power delay profile have been used to obtain the statistics of the multipath of the channel. Combining both models has been possible to obtain a detailed characterization of the environment. The results constitute a complete characterization of this type of indoor environments for UWB signals and can be easily extrapolated to other environments with similar characteristics, allowing the use of UWB technology for applications of critical communications for sensors and other devices.

Operating remotely piloted aircraft is not imaginable without a continuous data exchange between the air vehicle and the remote pilot. This data exchange requires reliable data links. One approach for such a data link discussed in the community is a terrestrial system deployed in C-band. A good knowledge of the physical conditions of the communication channel, in this case the air-ground/ground-air channel, is indispensable for the development of wireless data links. Therefore we carried out a 50 MHz bandwidth channel sounding campaign with a terrestrial transmitter and an airborne receiver. In this paper we give a detailed description of our campaign setup and the processing of the collected data. The campaign covered several flight scenarios, such as take-off, taxiing, and multiple en-route maneuvers. We furthermore present results on the received power and the amplitude distribution of the dominant component of the received signal for the different flight scenarios. We observed significant drops in reception power during certain maneuvers that need to be considered in the design process of a data link for unmanned aviation. Additionally, we show that the amplitude distribution follows the distributions commonly used in statistical channel modeling of wireless channels to some extent. We finally present parameter sets for multiple flight scenarios for scaling the amplitude distributions to allow a statistical channel modeling of the reception power of the first resolvable signal path. [ABSTRACT FROM AUTHOR]

In order to realize the vision of “smart rail mobility,” a seamless high-data rate wireless connectivity with up to dozens of gigahertz bandwidth will be required. This forms a strong motivation for exploring the underutilized millimeter wave (mmWave) and sub-mmWave bands. In this article, the wireless channel in one “smart rail mobility” scenario—the intrawagon scenario—is characterized through ultrawideband (UWB) channel sounding and ray tracing (RT) at mmWave and sub-mmWave bands. To begin with, a series of horizontal directional scan-sounding measurements are performed inside a real high-speed train wagon at 60 and 300 GHz frequency bands with a bandwidth of 8 GHz. Correspondingly, the channel characteristics such as Rician $K$ -factor, root-mean-square (rms) delay spread (DS), azimuth spread of arrival, and azimuth spread of departure are extracted and analyzed. Based on the measurements, a self-developed RT simulator is validated through the reconstruction of the three-dimensional wagon model and the calibration of the electromagnetic (EM) properties of the main materials. This gives the chance to physically interpret the measurement results and characterize the intrawagon mmWave and sub-mmWave channels more comprehensively through extensive RT simulations. [ABSTRACT FROM AUTHOR]

Wireless networks are the preferred future access networks for both defense and civilian deployments as part of telecommunication networks. The successful implementation of long term evolution (LTE) networks and applications such as the Internet of Things (IoT) in the telecommunication infrastructure has guaranteed rates of up to 100 Mbps while supporting ultra-dense wireless access network. With the incorporation of LTE-Advanced and fifth-generation wireless protocols, the data rates are expected to reach upto 1 Gbps. Hence, there is a pertinent requirement to carry out channel measurements at sub 1 GHz, 2 GHz, and 3 GHz bands to enable the design and implementation of optimum transceivers for pico-cells of LTE and future wireless networks. For the first time measurements and comparison with standard models of channel impulse response models have also been carried out in five different terrains namely Urban, Semi-Urban, Forest, Rural, and Desert terrains in the Indian sub-continent to effectively cover a variety of deployments of future wireless access networks for defense wireless networks. [ABSTRACT FROM AUTHOR]

Industrial wireless channel is a challenge for the design of communication systems, due to non-Line-of-Sight transmission, caused by the presence of many highly reflective obstacles, and machines in operation, which are a source of the increased noise level. The main effect, which must be analyzed, is multipath propagation. In this article, a low-cost sounding system is proposed, based on Software Defined Radio (SDR) equipment, with the intention of making sounding devices more accessible to a larger group of researchers. Likewise, the mathematical foundations and the software/hardware implementation of the wireless channel sounding system are presented, and the solutions to mitigate the synchronization issues and SDR limitations are also introduced. The performance of the proposed sounder is validated through a measurement campaign in an industrial workshop, considering the 2.4 GHz Industrial, Scientific, Medical (ISM) band. Channel sounding measurements corroborate the accuracy of the results, which converge with the channel mathematical models proposed for several industrial environments and reported in the state-of-the-art literature. In this sense, the proposed channel sounder can be used to investigate the wireless propagation environments.

Advanced public safety communication (PSC) services call for fast, reliable and low-latency communication technologies, capable of supporting diverse communication modes (aerial, unmanned, vehicular, and peer-to-peer), fast channel dynamics, and ad hoc or mesh structures. For this reason, PSC has been identified as one of the key potential uses cases for the next generation of communication systems, the so-called 5G. In this scenario, the millimeter wave (mmWave) bands and other frequencies above 6 GHz are particularly interesting, since they are largely untapped and offer vastly more spectrum than current cellular allocations in the highly congested bands below 6 GHz, thus enabling orders of magnitude greater data rates and reduced latency. For example, new PSC networks in the mmWave bands could support high-definition video, virtual reality, and other broadband data to large numbers of first responders. Surveillance drones or ambulances could also be provided high-speed connectivity along with machine-type communication for remotely controlled robotic devices entering dangerous areas. However, the way towards this ambitious goal is hindered by a number of open research challenges. In this paper, after a brief introduction to PSC services and requirements, we illustrate the potential of the frequencies above 6 GHz for PSC and discuss the open problems that need to be solved in order to pave this way. Finally, we describe the main components of a test platform for mmWave systems that is functional to the study of such complex scenarios and that we plan to develop as an invaluable tool for realizing mmWave PSC networks.

Virtual antenna arrays are versatile and cost-effective tools for millimeter-wave channel characterization. Massive sampling of the channel in space with virtual antenna arrays enables high spatial resolution in channel sounding. In this paper, a uniform cubic array is used with a vector network analyzer to characterize an indoor propagation channel. The power angle delay profile under line-of-sight (LOS) and obstructed-LOS (OLOS) scenarios is then extracted from the measurement data. The analysis is extended to a multi-user scenario using a virtual uniform rectangular array, which acts as the base station and with users under LOS and OLOS conditions. Results show interference suppression with zero-forcing beamforming that performs well even in critical multi-user scenarios based on the measured channels, i.e., a scenario with closely spaced users in LOS conditions and a scenario with a strong interfering user and a weak desired user.

In this paper, the power angular profile (PAP) of the dense multipath components (DMCs) was analyzed and characterized based on a channel sounding measurement campaign at 11 GHz in an indoor environment. The specular multipath components (SMCs) were estimated with the SAGE algorithm, and the RiMAX framework was applied for the estimation of the DMC in both the time-delay and the angular domain. After careful inspection of the spectrum of the residual signal components in the angular domain, we found that the DMC should be modeled by taking into account multiple angular clusters at different angles, to better characterize the diffuse scattering between transmitter and receiver, originating from multiple reflections in an environment. Therefore, we propose to extend the maximum likelihood estimation of the angular DMC parameters in the RiMAX framework so that the PAP of the DMC can be modeled with a multimodal von Mises distribution. We also validate our proposed method with the results of the measured channel sounding data.

The increasing requirement for the mobile data traffic accelerates the research of millimeterwave (mm-wave) for future wireless systems. Accurate characterization of the mm-wave propagation channel is fundamental and essential for the system design and performance evaluation. In this paper, we conducted measurement campaigns in various indoor scenarios, including classroom, office, and hall scenarios, at the frequency bands of 27-29 GHz. The spatial channel characteristics were recorded by using a large-scale uniform circular array. A high-resolution parameter estimation algorithm was applied to estimate the mm-wave spherical propagation parameters, i.e., the azimuth angle, elevation angle, delay, source distance, and complex amplitude of multipath components. With the same measurement system, the channel parameters including decay factor, delay spread, angular spread, and line of sight power ratio are investigated thoroughly in individual indoor scenarios and compared in different indoor scenarios. Furthermore, the impact of the furniture richness level and indoor geometry on the propagation parameters are also investigated.

This paper proposes a two dimensional (2-D) statistical channel model for Terahertz (THz) chip-to-chip wireless communication in desktop size metal enclosures. This model differs from traditional statistical channel models as it models both traveling and resonant waves that exist inside metal enclosures. Based on the cavity environment and the statistical properties of the channel inside the metal cavity, the geometrical model which describes propagation in resonant cavity as a superposition of LoS, single bounced (SB), double bounced (DB), and multi-bounced (MB) rays is proposed. Based on the geometrical model, a parametric reference model is proposed. Furthermore, the path loss model that captures signal strength variation in a resonant cavity is proposed. Frequency correlation functions (FCF) and power delay profiles (PDP) for different possible chip-to-chip communication scenarios are derived and compared with the measured ones. The results show a good agreement between the simulated and measured statistics. [ABSTRACT FROM AUTHOR]