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With the potential of fixed wireless access (FWA) networks as an alternative to optical fiber, it is necessary to determine vegetation loss at millimeter wave frequencies. In this article, we present vegetation loss measurement results for different types of vegetation, including trees, hedges, and forests, at frequencies ranging from 110 to 170 GHz. An experimental method is proposed to determine the average loss per meter vegetation depth (VD) for different vegetation types. Average losses at 140 GHz range from 0.2 dB/m for an open forest, and up to 9.8 dB/m for dense hedges. As there is a large variance of vegetation loss for different vegetation types, we propose a novel vegetation-dependent exponential decay (VED) model, expressing vegetation loss as a function of VD, frequency, and vegetation density, which is expressed by the plant area index (PAI) parameter. The VED vegetation loss model can be used for network design, and to perform accurate link budget calculations.

With the increasing demand for high bandwidthwireless communication systems, and with a congested spectrumin the sub-6 GHz frequency bands, researchers have been lookinginto exploration of millimeter wave (mmWave) and sub-terahertz(sub-THz) frequency bands. Channel modeling is essential forsystem design and performance evaluation of new wirelesscommunication systems. Accurate channel modeling relies onreliable measured channel data, which is collected by high-fidelitychannel sounders. Furthermore, it is of importance to understandto which extent channel parameters are frequency dependent intypical deployment scenario (including both indoor short-rangeand outdoor long-range scenarios). To achieve this purpose, thispaper presents a state-of-art long-range 28 GHz and 300 GHzVNA-based channel sounder using optical cable solutions, whichcan support a measurement range up to 300 m and 600 m inprinciple, respectively. The design, development and validation ofthe long-range channel sounders at mmWave and sub-THz bandsare reported, with a focus on their system principle, link budget,and back-to-back measurements. Furthermore, a measurementcampaign in an indoor corridor is performed using the developed300 GHz system and 28 GHz channel sounding systems. Bothmeasured channels at the 28 GHz and 300 GHz channels areshown to be highly sparse and specular. A higher number ofMulti Path Components (MPC) are observed for the 28 GHzsystem, while the same main MPC are observed for both systems.

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.

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.

In this paper, we present the methods to detect the channel delay profile and the Doppler spectrum of shallow underwater acoustic channels (SUAC). In our channel sounding methods, a short impulse in form of a sinusoid function is successively sent out from the transmitter to estimated the channel impulse response (CIR). A bandpass filter is applied to eliminate the interference from out-of-band (OOB). A threshould is utilized to obtain the maximum time delay of the CIR. Multipath components of the SUAC are specified by correlating the received signals with the transmitted sounding pulse with its shifted phases from 0 to 2Π. We show the measured channel parameters, which have been carried out in some lakes in Hanoi. The measured results illustrate that the channel is frequency selective for a narrow band transmission. The Doppler spectrum can be obtained by taking the Fourier transform of the time correlation of the measured channel transfer function. We have shown that, the theoretical maximum Doppler frequency fits well to that one obtained from measurement results.

Best Measurement Paper Award; International audience; In this paper, we present a small, yet realistic, vehicular cell-free massive MIMO (multiple-input multiple-output) architecture deployed at the University of Lille in a typical suburban environment under both Line-of-Sight (LOS) and obstructed LOS (OLOS) shadowing conditions. The radio channels were acquired with a distributed RF-over-Fiber (RoF) upgrade of the real-time channel sounder MaMIMOSA. The system operates at 5.89 GHz with an 80 MHz bandwidth, which corresponds to the ITS frequency band offered by the ITS-G5 and C-V2X technologies. Four omnidirectional receive antennas were placed on the roof of a van moving at a speed of 25 km/h. The propagation channel was measured for various transmit antenna configurations, ranging from co-located antennas to fully distributed antennas. The measurement results show a significant gain in the signal-to-noise ratio (SNR) as well as a more uniform coverage and smaller delay spread values with the distributed scenarios compared to the centralized ones. Finally, the path loss measurement results obtained for the cell-free network provide deployment guidelines for the distributed antennas.

A channel sounding and communications experiment was performed in the Oslofjord, using eight bottom-mounted instrument units deployed in a network configuration. Five units were equipped with a software-defined 4–8-kHz acoustic modem, programmed to transmit probe signals and communication packets in a round-robin fashion. All transmitted waveforms were recorded by all units over 35 horizontal links and 5 vertical links. The channels reveal a reverberant environment with long and dense multipath arrival patterns. Measured power-delay profiles and delay-Doppler spread functions are used to predict receiver output signal-to-noise ratio (SNR) over a signaling period of 26 h. To this end, the channel quantities are first calibrated for the propagation loss. The prediction examines the effect of ambient noise, reverberation of previously transmitted packets, the packet’s own reverberation, and Doppler spread. The delay profile can be used under calm conditions, and results in a mean prediction error (averaged over all links) of about 3 dB, even hours after the measurement of the profiles. The mean error on individual links (averaged over time) is reduced to 1–2 dB by using up-to-date channel information. The relevance of predicting output SNR is finally illustrated by establishing a relationship between output SNR and the probabilities of bit and packet error.

In industrial applications, the large comprehensive wireless channel impulse response (CIR) reference dataset, measured by National Institute of Standards and Technology (NIST), has been a useful tool for understanding propagation within factory environments. The NIST CIR reference dataset is obtained using a precision channel sounder instrument where transmitter and receiver are time-synchronized by two rubidium clocks. While the accuracy of the NIST CIRs is much higher than the CIRs measured by general commercial digital receiver, two types of system errors have been discovered within the dataset from the perspective of signal processing. These errors are significant for wireless localization, physical layer security, and related applications. To calibrate the CIR, two channel sounder error calibration methods (CSEC) is proposed: the CSEC based on phase compensation and carrier frequency offset recovery. Our results reveal that the CSEC method can improve the accuracy of the CIR to the accuracy that precise instruments cannot achieve. To demonstrate the consequence of these systemic errors, a case study involving physical layer authentication is investigated showing a marked improvement in authentication accuracy after the systemic errors in the dataset are removed. Moreover, the CSEC method may be used to correct other CIR datasets with similar systemic errors.

We propose a 28.5-GHz channel sounder that switches through all antennas of multiple dual-polarized $8 \times 8$ phased arrays at the transmitter and receiver and performs beamforming in postprocessing through digital weights to synthesize a sweepable beam. To our knowledge, we are the first to implement—what we refer to as— switched beamforming with phased arrays for millimeter-wave channel sounding, realized through highly stable Rubidium clocks and local oscillators coupled with precision over-the-air calibration techniques developed in house. By circumventing the time-consuming programming of analog weights that is associated with analog beamforming—what phased arrays are designed for—we can sweep a 3-D double-omnidirectional dual-polarized channel in just 1.3 ms, for real-time sounding. By in turn circumventing the coarse precision of analog weights, we can synthesize ideal beam patterns thanks to the effectively infinite precision of digital weights, enabling fine weight calibration for the nonidealities of the system hardware and fine weight tapering for sidelobe suppression. This translates to average estimation errors of 0.47° in 3-D double-directional angle, 0.48 dB in co-polarized path gain, and 0.18 ns in delay, as substantiated by field measurements.

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.

The traditional channel sounding protocol requires that the whole sounding process must be completed within a coherent time. When the communication delay is close to or greater than the coherent time, the protocol will not work properly. In order to solve this problem, a novel Non-Time-Limited channel sounding protocol is proposed. The proposed channel sounding protocol does not need to guarantee the channel reciprocity, has no limit on the communication delay and has strong multi-relay scalability. At the same time, a relay-assisted security extension mechanism is proposed to prevent the collaborative eavesdropping of the neighboring attackers. Simulation experiments show that the Non-Time-Limited channel sounding protocol can get rid of the influence of channel change speed on key generation, effectively improve key consistency in high-speed mobile scenarios, and still maintain good key agreement rate performance in underwater acoustic communication.

Terahertz (THz) communications are envisioned as a key technology for the sixth-generation wireless communication system (6G). However, it is not practical to perform large-scale channel measurements with high degrees of freedom at THz frequency band. This makes empirical or stochastic modeling approaches relying on measurements no longer stand. In order to break through the bottleneck of scarce full-dimensional channel sounding measurements, this paper presents a novel paradigm for THz channel modeling towards 6G. With the core of high-performance ray tracing (RT), the presented paradigm requires merely quite limited channel sounding to calibrate the geometry and material electromagnetic (EM) properties of the three-dimensional (3D) environment model in the target scenarios. Then, through extensive RT simulations, the parameters extracted from RT simulations can be fed into either ray-based novel stochastic channel models or cluster-based standard channel model families. Verified by RT simulations, these models can generate realistic channels that are valuable for the design and evaluation of THz systems. Representing two ends of 6G THz use cases from microscopy to macroscopy, case studies are made for close-proximity communications, wireless connections on a desktop, and smart rail mobility, respectively. Last but not least, new concerns on channel modeling resulting from distinguishing features of THz wave are discussed regarding propagation, antenna array, and device aspects, respectively.

As a vital vertical industry empowered by the sixth-generation mobile communication system (6G), the vision of “smart rail mobility” requires a seamless wireless connectivity with the ultra high-data rate on the order of 100 Gbps or even more. This forms a strong driving force for exploiting the terahertz (THz) band where the available spectrum is massively abundant. In this paper, the smart rail mobility channel is characterized through ultra-wideband (UWB) channel sounding and ray tracing (RT) at 300 GHz band with an 8 GHz bandwidth in the five scenarios – Train-to-infrastructure (T2I), Inside station, Train-to-train (T2T), Infrastructure-to-infrastructure (I2I), and Intra-wagon. Corresponding channel characteristics such as path loss, shadow fading, Rician $K$ -factor, root-mean-square delay spread, azimuth/elevation angular spread of arrival/departure, and cross-polarization ratio are extracted and analyzed. Based on realistic channel information, the performance of THz communication is investigated in terms of channel capacity for all above five scenarios with various antenna patterns and weather conditions (except Intra-wagon) – sunny, rainy, and snowy. The results provide valuable insights into the system design and evaluation for THz communication enabled Smart Rail Mobility.

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.

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.

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

This paper presents a novel channel estimation technique for the multi-user massive multiple-input multiple-output (MU-mMIMO) systems using angular-based hybrid precoding (AB-HP). The proposed channel estimation technique generates group-wise channel state information (CSI) of user terminal (UT) zones in the service area by deep neural networks (DNN) and fuzzy c-Means (FCM) clustering. The slow time-varying CSI between the base station (BS) and feasible UT locations in the service area is calculated from the geospatial data by offline ray tracing and a DNN-based path estimation model associated with the 1-dimensional convolutional neural network (1D-CNN) and regression tree ensembles. Then, the UT-level CSI of all feasible locations is grouped into clusters by a proposed FCM clustering. Finally, the service area is divided into a number of non-overlapping UT zones. Each UT zone is characterized by a corresponding set of clusters named as UT-group CSI, which is utilized in the analog RF beamformer design of AB-HP to reduce the required large online CSI overhead in the MU-mMIMO systems. Then, the reduced-size online CSI is employed in the baseband (BB) precoder of AB-HP. Simulations are conducted in the indoor scenario at 28 GHz and tested in an AB-HP MU-mMIMO system with a uniform rectangular array (URA) having 16x16=256 antennas and 22 RF chains. Illustrative results indicate that 91.4% online CSI can be reduced by using the proposed offline channel estimation technique as compared to the conventional online channel sounding. The proposed DNN-based path estimation technique produces same amount of UT-level CSI with runtime reduced by 65.8% as compared to the computationally expensive ray tracing., 18 pages, 21 figures

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.

International audience; 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 singleinput-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-lineof-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 thesystem-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.

In a wireless communication system, the radio propagation channel is a complicated component to characterise. Channel sounding equipment thus has to meet specific criteria to extract the desired channel parameters. In this study, the authors outline the development and validation of a vector network analyser (VNA) based channel sounder for the frequency range from 1 to 50 GHz using radio-over-fibre techniques. Three methods of de-embedding phase errors due to hardware impairments are demonstrated and validated via back-to-back measurements. The bidirectional scheme utilising optical circulators is shown to have a superior performance over the two-branch unidirectional and the two-branch bidirectional schemes. Therefore, the bidirectional scheme utilising optical circulators is proposed to achieve a long-range ultra-channel sounder based on the VNA.

We develop a beamforming scheme to enhance the physical layer security in dual-polarized millimeter wave (mmWave) systems. We aim to compute a transmit beamformer that maximizes the secrecy rate of the desired user. Assuming the transmitter has a statistical distribution of a dominant radio path, we compute a single-polarization beamforming vector that makes directional transmissions at mmWave frequencies more secure. Moreover, we propose a channel sounding scheme to obtain a weight vector that combines the dual-polarized antennas for improving the secrecy rate. Simulation results are presented to verify the secrecy rate performances of the proposed beamforming algorithm.

In this letter, we address the problem of the beam tracing in the line-of-sight massive multiple-input multiple-output (MIMO) systems. In order to enhance the accuracy of the closed-loop channel sounding, by maximizing the predicted beamforming gain (PBFG), a new approach to the beam tracing is proposed to optimize the sounding beam sequence. The approximate expression of the PBFG measure is derived based on the perturbation analysis of the maximum likelihood (ML) cost function. At each channel use, the receiver feeds back the preferred sounding beam to the transmitter to be used at the next channel use. The performance advantage of the proposed beam tracing rule over the other existing rules and the open-loop channel sounding scheme is evidenced by the simulation results.

This study proposes a new strategy of extracting parameters for tapped delay line (TDL) channel models from vehicle to infrastructure channel measurements. The proposed approach is based on an already existing method to derive parameters for a non-stationary model using first-order Markov chains. It will be shown that with the proposed method, the number of taps necessary to regenerate the delay spread of a channel can be significantly reduced. An approach will be discussed to evaluate the performance of the model. The feasibility of the method will be confirmed using channel sounding measurements.

The user traffic in the mobile communication area has rapidly increased owing to the widespread of smartphones and various cloud services. To handle the increasing traffic, in the fifth-generation mobile communication system (5G), the millimeter-wave multiple-input and multiple-output (MIMO) communication technology is under development. Because the MIMO transmission performance heavily depends on the radio propagation characteristics, various MIMO channel measurements are needed for the performance evaluation and system design. The accurate and efficient parameter estimation algorithm which estimates the propagation delays and angle of arrivals (AoA) of radio waves is also indispensable for the purpose. In this paper, we extended the joint delay and azimuth estimation (JADE) method based on multiple signal classification (MUSIC) algorithm. In our proposal, the drawback of the MUSIC that the performance degrades for the estimation of coherent waves was solved by applying the smoothing technique in the frequency domain. It also makes the antenna calibration simpler. We implemented the proposed algorithm for the channel sounding system in the 66 GHz band, which is one of the candidate frequency bands for the International Mobile Telecommunications (IMT) system and evaluated the effectiveness through the experiment in an anechoic chamber. The result showed that our proposed method can de-correlate the signal components of coherent waves, and improved the parameter estimation accuracy significantly. The root means square error (RMSE) of the propagation delay estimation was improved from 2.7 ns to 0.9 ns, and the RMSE of the AoA estimation was improved from 20.3 deg. to 7.2 deg. The results are expected to be utilized for the millimeter wave band MIMO channel modeling.

In the future, fifth‐generation communications system (5G) is expected to provide reasonable mobile broadband experience to high‐density users in stadium, gym, shopping mall, etc. As the fundamental limitation is attributed to the propagation environments, it is essential to understand profoundly the high‐density channels, say, the propagation channel in the environments where the user density could be very high. In this article, seven different environments are considered. The targeted environments include stadium (or gymnasium), shopping mall, airport, hall, street canyon, indoor (or indoor office), and crowded area. The propagation investigations in the literature for the seven environments are reviewed in terms of channel sounding, parameter estimation, channel characterization and modeling, etc. The review of the high‐density channel facilitates the “great service in a crowd” defined in 5G. (Less)

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.

This paper investigates the capability of millimeter-wave (mmWave) channel sounders with phased arrays to perform super-resolution parameter estimation, i.e., determine the parameters of multipath components (MPC), such as direction of arrival and delay, with resolution better than the Fourier resolution of the setup. We analyze the question both generally, and with respect to a particular novel multi-beam mmWave channel sounder that is capable of performing multiple-input-multiple-output (MIMO) measurements in dynamic environments. We firstly propose a novel two-step calibration procedure that provides higher-accuracy calibration data that are required for Rimax or SAGE. Secondly, we investigate the impact of center misalignment and residual phase noise on the performance of the parameter estimator. Finally we experimentally verify the calibration results and demonstrate the capability of our sounder to perform super-resolution parameter estimation.

In this article, a novel method for passive indoor localization using LED luminaires is proposed where explicit user participation is not required. This approach measures changes in the impulse response between sources and receivers and estimates a location based on optical channel sounding data. An exponential integrating-sphere model is used to represent object impulse response (OIR) from each luminaire source-receiver pair, which is obtained by subtracting impulse response (IR) of the room background (i.e., without an object) from IR when the object is present inside the room. This model is represented as a function of 3D position of the object and depends on both source and receiver parameters and the physical geometry of the room. An analytical expression of the Cramer–Rao lower bound (CRLB) on the proposed passive indoor localization method is derived. The position is also estimated by using a maximum likelihood (ML) estimator which gives the position estimate by maximizing the log-likelihood function of the received noisy OIR waveforms. The results show that the signal-to-noise ratio (SNR) and number of source-receiver pairs used in the estimation, play a crucial role in performance. Typical localization root-mean squared error is less than 10 cm over a broad range of light intensities and object locations.

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.

Millimeter-wave (mmWave) bands will be used for the fifth generation communication systems to support high data rates. For the proper characterization of the mmWave propagation channel, it is essential to measure the power angular-delay profile (PADP) of the channel which includes angle-of-departure (AoD) and angle-of-arrival (AoA) of the multipath components (MPCs). In this paper, we first describe in detail our 28 GHz channel sounder where directional horn antennas are placed on rotating gimbals. Then, for this specific sounder class, we describe and address the following two problems in extracting the MPCs from the measurements: 1) For the channel measurements at large distances between the transmitter (TX) and the receiver (RX), it is not possible to generate the triggering signal 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 the positions of the antennas change during the scanning process, total distance traveled by the same MPC differs at each measurement. These two errors 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 the errors in the MPC delays 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., 15 pages. Submitted to IEEE Transactions on Antennas and Propagation

While customers rivet their eyes on Wi-Fi 6, in the bowels of the IEEE 802.11 Working Group that creates Wi-Fi standards, the next generation Wi-Fi is being developed. At the very first sight, the new IEEE 802.11be amendment to the Wi-Fi standard is nothing but scaled 11ax with doubled bandwidth and the increased number of spatial streams, which together provide data rates as high as 40 Gbps. A bit deeper dive into the 802.11 activities reveals that 11be will support real-time applications. In reality, 11be introduces many more revolutionary changes to Wi-Fi, which will form a basement for further Wi-Fi evolution. Although by now (May 2020), the development process is at the very early phase without any draft specification, the analysis of the discussion in the 802.11 Working Group gives insights into the main innovations of 11be. In addition to the ones above, they include native multi-link operation, channel sounding optimization that opens the door for massive MIMO, advanced PHY and MAC techniques, the cooperation of various access points. The paper analyzes hundreds of features proposed for the new technology, focusing on the open problems that can be solved by the researchers who want to contribute to the development of 802.11be.

Forschung und Industrie arbeiten mit Hochdruck an der Nutzbarmachung von Teilen des Millimeterwellenfrequenzbereichs, um den steigenden Bedarf an höheren Datenraten zukünftiger Mobilfunknetze decken zu können. Für diese Frequenzbereiche können Mehrantennensysteme mit hunderten von Antennen kompakt realisiert werden, welche eine Steigerung der spektralen Effizienz durch die Ausschöpfung der räumlichen Mehrwegeausbreitung ermöglichen. Um den wirtschaftlichen Erfolg dieser Mehrantennensysteme zu sichern, gilt es, effiziente architektonische Lösungen mit einer niedrigen hardware-technischen Komplexität zu finden. Eine solche architektonische Lösung stellen hybride Beamformingsysteme dar, die den Strahlformungsprozess in ein analoges Beamformingnetzwerk bestehend aus Phasenschiebern sowie einen Digitalteil geringer Größe unterteilen. Diesen hybriden Beamformingsystemen widmet sich die vorliegende Dissertation. Dabei fokussieren sich die Betrachtungen auf den von der Internationalen Fernmeldeunion für die nächste Generation des Mobilfunks definierten Frequenzbereich um 28 GHz. Ziel dieser Arbeit ist es, hybride Beamformingarchitekturen niedriger Komplexität zu untersuchen und die für die erfolgreiche Anwendung notwendigen Verfahren zur Strahlformung und Kanalschätzung zu entwickeln. Als Grundlage der Untersuchung dienen ein pfadbasiertes Kanalmodell sowie die im Rahmen dieser Arbeit durchgeführten Kanalmessungen bei 28 GHz. Für die Messungen wird ein eigens entwickeltes Kanalmesssystem bestehend aus vier Sendern und 16 Empfängern verwendet. Die durchgeführten Outdoor-Messkampagnen in drei verschiedenen Szenarien mit insgesamt 162 Messpositionskonstellationen von Basisstation und mobiler Sendeeinheit liefern einen bisher unerreichten Einblick in das Ausbreitungsverhalten der elektromagnetischen Wellen innerhalb von Small Cell-Szenarien bei 28 GHz. Zur Realisierung hybrider Beamformingsysteme niedriger Komplexität muss der Schaltungs- und Hardwareaufwand innerhalb des analogen Beamformingnetzwerks auf ein Minimum begrenzt werden. Als besonders geeignet kristallisiert sich in dieser Arbeit die untersuchte SBHB-Architektur heraus. Für die Bestimmung der Beamformingmatrizen am Sender und Empfänger werden abhängig von den bekannten Kanalinformationen der SIC-Algorithmus und der BA-Algorithmus diskutiert. Zur Steigerung der Energieeffizienz in Zeiten einer geringer Mobilfunkzellauslastung wird für die SBHB-Architektur eine Methodik zur intelligenten Abschaltung von Antennenelementen verwirklicht. Im Gegensatz dazu wird in Szenarien mit einem dauerhaft hohen Nutzeraufkommen die maximal erreichbare Übertragungsrate durch das Hinzufügen zusätzlicher Freiheitsgrade in Form von Phasenschiebern gesteigert. Die durchgeführten Analysen verdeutlichen den Zusammenhang zwischen dem notwendigen Hardwareaufwand innerhalb des analogen Beamformingnetzwerks und der maximal erreichbaren Übertragungsrate und legen damit den Grundstein für die bedarfsorientierte Auslegung hybrider Beamformingsysteme in der Praxis. Ein weiterer Kernbeitrag dieser Dissertation sind die entwickelten Verfahren zur Kanalschätzung für hybride Beamformingsysteme. Diese stehen im Spannungsfeld aus notwendiger Kanalschätzungsdauer und erreichbarer Schätzgenauigkeit. Zur Reduktion der Kanalschätzungsdauer bei der Suche der räumlichen Austritts- und Einfallswinkel am Sender bzw. Empfänger wird ein hierarchisches Codebuchverfahren konstruiert und analysiert. Darüber hinaus wird eine eigene Klasse von Kanalschätzungsverfahren basierend auf der Schätzung mittels dünnbesetzter Antennenarrays eingeführt. Das daraus resultierende SABA-Verfahren erreicht eine deutliche Reduktion der Kanalschätzungsdauer, während dem MSAM-Verfahren eine vollständige Rekonstruktion der Kanalmatrix gelingt. Die numerischen und messdatenbasierten Ergebnisse zeigen die Überlegenheit der beiden eigens entwickelten Verfahren und diskutieren für welche Systemparameter ihr Einsatz von Vorteil ist. Zum Schluss werden die entwickelten Kanalschätzungsverfahren an ein eigens entwickeltes SBHB-Empfangssystem angepasst und mit Hilfe dessen messtechnisch untersucht. Um dies zu ermöglichen, liegt ein Schwerpunkt auf der praktischen Implementierung und Kalibrierung des SBHB-Systems. Die messtechnischen Untersuchungen verifizieren erstmals die präsentierten Kanalschätzungsverfahren und beweisen damit, dass ein Einsatz hybrider Beamformingsysteme niedriger Komplexität mit den entwickelten Verfahren bei 28 GHz möglich ist.

We present a technique to jointly estimate the channel taps and the frequency offset due to the Doppler effect of a special class of doubly dispersive aeronautical channels. The algorithm includes the use of pulse repetition techniques at the transmitter and the “power method” subspace iteration at the receiver. We show that transmitting constant-amplitude zero-autocorrelation sequences for channel sounding yields estimators with low computational complexity. Numerical simulations indicate performance comparable to the estimation-theoretic lower bound.

This paper presents an evaluation of multicarrier channel sounding techniques using different random and pseudo-random sequences to modulate the OFDM sounding signal. The Random (Rand), Pseudo-Noise (PN) and Zadoff-Chu (ZC) were tested, both in laboratory simulations and in field measurements. For the laboratory simulations Matlab routines were used to generate OFDM signals modulated with each of the three sounding signals, that were then convoluted with a synthesized transfer function of a test channel with six multipath components and added Gaussian noise and Doppler fading. The resulting signals are then correlated with a copy of the original signal to provide the multipath power delay profiles. The root mean square deviation (RMSD) and the relationship between peak power and mean power (PAPR) were used as metrics for the comparison between the test transfer function and the simulation detected multipath delay profiles, showing slight advantages of the ZC sequence. The three sequences were then used in field measurements to characterize an urban channel at 700 MHz. In the field measurements, the ZC sequence showed the lowest detection threshold, allowing for the detection of a larger number of multipath components.

While millimeter-wave (mmWave) wireless has recently gained tremendous attention with the transition to 5G, developing a broadly accessible experimental infrastructure will allow the research community to make significant progress in this area. Hence, in this paper, we present the design and implementation of various programmable and open-access 28/60 GHz software-defined radios (SDRs), deployed in the PAWR COSMOS advanced wireless testbed. These programmable mmWave radios are based on the IBM 28 GHz 64-element dual-polarized phased array antenna module (PAAM) subsystem board and the Sivers IMA 60 GHz WiGig transceiver. These front ends are integrated with USRP SDRs or Xilinx RF-SoC boards, which provide baseband signal processing capabilities. Moreover, we present measurements of the TX/RX beamforming performance and example experiments (e.g., real-time channel sounding and RFNoC-based 802.11ad preamble detection), using the mmWave radios. Finally, we discuss ongoing enhancement and development efforts focusing on these radios.

Orthogonal Frequency-Division Multiplexing (OFDM) is a popular modulation scheme, which requires a linear channel. Unfortunately, the most sensitive receivers for visible light communications, silicon photomultipliers (SiPMs), have a non-linear response. Despite this incompatibility, results from an easily implemented method of combining OFDM and SiPMs are shown to successfully limit the bit error rate to below the limit required by forward error correction.

This paper presents the first vector network analyzer (VNA)-based sub-Terahertz (sub-THz) phase-compensated channel sounder at 220 330 GHz using radio-over-fiber (RoF) 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 paper 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 multi-channel/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_ in 220-330 GHz, compared to 15_ at 220-288 GHz and 37_ 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 300m (limited by the optical cable length in our system and subject to over-the-air signal transmission loss in practical environment).

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.

A channel sounding and communications experiment was performed in the Oslofjord in April/May 2019, where 8 bottom units were deployed in a network configuration. Five units were equipped with a software-defined acoustic modem operating in the 4–8 kHz band, programmed to transmit probe signals and communication packets. All transmitted signals were recorded by all units over 35 unique horizontal links and 5 vertical links. Measured power delay profiles are used to predict receiver output SNR, examining the effect of ambient noise, reverberation of previously transmitted packets, and the packet's own reverberation. It is shown that modem performance is limited by the own reverberation, and that the mean prediction error is below 3 dB when this reverberation is taken into account.

Tremendous research funding and efforts have been invested in millimeter-wave (mmWave) communication systems for fifth-generation (5G) and beyond wireless communications. Understanding the influence of the surrounding environment on the propagation channel is vital to mmWave system design and deployment. In this paper, principal multipath components (MPCs) are studied in in an outdoor microcell scenario at 39 GHz. The direction-scan channel sounding measurement is conducted in a typical urban living district. By adapting the 3D environment model and material parameters, the ray-tracing simulator is tuned to match the simulated MPCs with the measurement. Based on this, the main propagation mechanisms and influential objects are identified. The method, observations, and analysis in this work are useful for understanding mmWave channel and developing communication systems.

Channel sounders are used to measure channel characteristics for radio systems. There are several types of channel sounders used today: continuous-wave (CW), direct pulse, frequency domain using a vector network analyzer (VNA), correlation-based, and swept-time delay cross-correlator. Each of these has unique advantages and disadvantages. CW systems have a larger dynamic range than other systems with a signal that can propagate further into the environment. As the audio sampling rates allow smaller file sizes than other systems, data collection can be continuous and last for several hours. This article discusses a CW-channel sounder system, which has been used to make numerous propagation loss measurements in various cities in the United States of America. Such propagation measurements should be accurate, reproducible, and free of artifacts or biases. This article shows how to set up the measurement, how to validate and verify that the system is making reliable measurements, and finally, it shows results from some of the measurement campaigns such as repeatability measurements, clutter loss measurements (where clutter loss is defined as the excess loss from free-space transmission loss), and reciprocity measurements.

This paper reports the development of a compact and highly programmable 28-GHz phased array subsystem that is currently being integrated in a city-scale testbed for advanced wireless research and experimentation. The subsystem hardware consists of a 28-GHz 64-element state-of-the-art dual-polarized phased array antenna module (PAAM), low-noise DC-DC power regulators with a single 12-V input, current sensors and an ADC for real-time supply monitoring, a PLL for LO (5 GHz) generation, IF (3 GHz)/LO splitters, IF/LO baluns, and programmable switches. All of these components are integrated in a compact (10” x5.75”) PCB which also supports the direct plug-in of a commercial-off-the-shelf FPGA system-on-module (SoM). The IF switches and splitters couple the PAAM to 20 IF ports that enable a rich set of MIMO configurations, including up to 8 simultaneous 16-element independent beams in each TX/RX mode. The FPGA and associated software enable the configuration of all subsystem features from a high-level application program interface (API), facilitating MIMO millimeter-wave wireless experiments. 28-GHz channel sounding measurements using the phased array subsystem coupled with software-defined radios is presented as a testbed-level experiment example.

The large swaths of bandwidth available in the THz band (0.1-10 THz) make it an ideal candidate to meet the ever-increasing data rate demands for upcoming applications. However, detailed channel sounding measurements are required before an eventual deployment of THz band communication can be considered. Keeping this goal in mind, in the current paper we present double-directional channel measurements in the 140-141 GHz range in an urban environment on a linear route for distances up to 15 m. Using our results, we analyze how key channel parameters change as we move from short to longer distances. These measurements allow us to take another step towards creation of detailed THz channel models.

Millimeter wave technology is an essential component of most solutions that address the coverage and throughput demands of next-generation cellular networks. To overcome the high propagation losses however, it is necessary to deploy large antenna arrays for spatial localization of energy by beamforming. This imposes a significant communication overhead, especially when channel reciprocity does not hold. In this work, we study the problem of successive one-bit feedback-assisted beam alignment. We exploit the sparse nature of the millimeter wave channel to pose the beamforming problem as a questioning strategy. We consider both adaptive ( closed-loop ) and non-adaptive ( open-loop ) channel sounding techniques which are robust to erroneous feedback signals caused by noisy quantization. In the adaptive case, we formulate new sounding signals by drawing a parallel with the well known Ulam's problem. In the non-adaptive case, the beams are designed in accordance to an open-loop code. We demonstrate that multiple paths can also be resolved by using ideas from group testing. Finally, we show the efficacy of our proposed techniques via simulations.

This article presents the design of a complete radio system receiver to detect, in real time, the direction of arrival (DOA) of an incoming industrial, scientific, and medical (ISM)-band signal at 5.8 GHz. When a transmitter continuously sends a binary phase-shift keying (BPSK), modulated pseudo-noise (PN) code, the receiver estimates the DOA based on the received signal strength (RSS) and performs the channel sounding. The device that we describe includes a pattern-reconfigurable monopole antenna array, a front end, and a systemon-module (SOM). The SOM controls the antenna's main lobe direction by positive-intrinsic-negative (p-i-n) diode switching, configures the front-end modules, completes the data acquisition, and performs the digital signal processing (DSP) for the DOA estimation. The system has an average DOA resolution of 90d in the horizontal plane, with a success rate higher than 90%. It is presented as an educational platform for electrical engineering undergraduate and M.S. degree students.

Time-division multiplexed (TDM) channel sounders, in which a single RF chain is connected sequentially via an electronic switch to different elements of an array, are widely used for the measurement of double-directional/MIMO propagation channels. This paper investigates the impact of array switching patterns on the accuracy of parameter estimation of multipath components (MPC) for a time-division multiplexed (TDM) channel sounder. The commonly-used sequential (uniform) switching pattern poses a fundamental limit on the number of antennas that a TDM channel sounder can employ in fast time-varying channels. We thus aim to design improved patterns that relax these constraints. To characterize the performance, we introduce a novel spatio-temporal ambiguity function, which can handle the non-idealities of real-word arrays. We formulate the sequence design problem as an optimization problem and propose an algorithm based on simulated annealing to obtain the optimal sequence. As a result we can extend the estimation range of Doppler shifts by eliminating ambiguities in parameter estimation. We show through Monte Carlo simulations that the root mean square errors of both direction of departure and Doppler are reduced significantly with the new switching sequence. Results are also verified with actual vehicle-to-vehicle (V2V) channel measurements., 11 pages, submitted to IEEE Transaction on Wireless Communications. arXiv admin note: text overlap with arXiv:1805.06611