Channel Sounder Research Articles

Experimental Investigation of Frequency Domain Channel Extrapolation in Massive MIMO Systems for Zero-Feedback FDD

Estimating downlink (DL) channel state information (CSI) in frequency division duplex (FDD) massive multi-input multi-output (MIMO) systems generally requires downlink pilots and feedback overheads. Accordingly, this paper investigates the feasibility of zero-feedback FDD massive MIMO systems based on channel extrapolation. We use the high-resolution parameter estimation (HRPE), specifically the space-alternating generalized expectation-maximization (SAGE) algorithm, to extrapolate the DL CSI based on the extracted parameters of multipath components in the uplink channel. We apply the HRPE to two different channel models: the vector spatial signature (VSS) model and the direction of arrival (DOA) model. We verify these methods through real-world channel data acquired from channel measurement campaigns with two different types of channel sounders: a) a switched array-based, real-time, time-domain, outdoors setup at 3.5 GHz, and b) a virtual array-based, high-accuracy, frequency-domain, indoors setup at 2.4 and 5-7 GHz. The performance metrics of the extrapolated channels that we evaluate include the mean squared error, beamforming efficiency, and spectral efficiency in multiuser MIMO scenarios. The results show that the HRPE-based channel extrapolation performs best under the simple VSS model, which does not require array calibration, and if the BS is in an open outdoor environment having line-of-sight (LOS) paths to well-separated users.

Methodology for Benchmarking Radio-Frequency Channel Sounders Through a System Model

Development of a comprehensive channel propagation model for high-fidelity design and deployment of wireless communication networks necessitates an exhaustive measurement campaign in a variety of operating environments and with different configuration settings. As the campaign is time-consuming and expensive, the effort is typically shared by multiple organizations, inevitably with their own channel-sounder architectures and processing methods. Without proper benchmarking, it cannot be discerned whether observed differences in the measurements are actually due to the varying environments or to discrepancies between the channel sounders themselves. The simplest approach for benchmarking is to transport participant channel sounders to a common environment, collect data, and compare results. Because this is rarely feasible, this paper proposes an alternative methodology - which is both practical and reliable - based on a mathematical system model to represent the channel sounder. The model parameters correspond to the hardware features specific to each system, characterized through precision, in situ calibration to ensure accurate representation; to ensure fair comparison, the model is applied to a ground-truth channel response that is identical for all systems. Five worldwide organizations participated in the cross-validation of their systems through the proposed methodology. Channel sounder descriptions, calibration procedures, and processing methods are provided for each organization as well as results and comparisons for 20 ground-truth channel responses.

Beamformer Design for Physical Layer Security in Dual-Polarized Millimeter Wave Channels

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.

A Novelty Massive MIMO Channel Model Based on the Birth‐and‐Death Process of Spatial Multipath Cluster

AbstractIn this paper a novelty Massive Multiple Input Multiple Output (MIMO) channel model based on the birth‐and‐death process of multipath clusters is proposed. First, Massive MIMO measurements are conducted in typical outdoSor scenarios with the bandwidth of 100 MHz at frequencies of 1.4725 and 6 GHz, respectively, following with the investigation of radio propagation characteristics over antenna array. Then we research on the subchannel correlation and nonstationary based on the channel sounding experiments. Furthermore, we build up a Markov chain to characterize the birth‐and‐death process of the multipath clusters over the array, which is helpful to bring deeply insight into the cluster evolution over the antenna array axis. And the analysis results are consistent with the theory of the cluster's visible region. At last, the paper systematically demonstrates how does the spatial correlation influence the capacity of the communication system. The whole research work aims to contribute the radio channel modeling of the fifth‐generation (5G) Massive MIMO communication system.

Multipoint intermittent fault detection of electronic interconnections using channel sounding techniques

Intermittent faults (IFs) in electrical interconnection are short duration transients. These faults are random, non-predictable and non-stationary in nature making them hard to troubleshoot using traditional equipment. They can be detected only if the test equipment has test coverage both in time and area at that instant. This paper proposes a novel approach of intermittent fault detection by injecting a wideband signal and using channel-sounding techniques. A novel Direct Sequence Spread Spectrum (DS-SS) technique is presented for multipoint IF detection and health monitoring. Digital communication sounding technology and its methods devised for detection and classification of an IF, and channel characterisation by its transfer function is discussed in detail. A combination of transmitter and receiver provide an effective tool for continuously monitoring of the multi-point interconnect. The proposed method has been tested using a functional prototype having pre-induced IFs. Intermittent signal is generated by applying external vibration on the connector and intermittency is detected by acquiring and processing the propagated signal. The results demonstrate the effectiveness of the method in detection and classification of intermittent interconnections.

Compensation of Motion-Induced Phase Errors and Enhancement of Doppler Unambiguity in TDM–MIMO Systems by Model-Based Estimation

Utilization of multiple input multiple output (MIMO) systems in radar and channel sounding has gained increased attention in recent years. Quite often, time-division multiplexing (TDM) is employed to realize orthogonal waveforms at the transmitter. Apart from its advantages, TDM has two severe drawbacks. First, motion-induced phase variations become indistinguishable from phase migration due to the signal's arrival direction. This is termed angle-Doppler coupling, which causes ambiguities in angle, and Doppler estimation. Second, the unambiguously resolvable Doppler, i.e., the Doppler bandwidth, is reduced. In this letter, a model-based estimation approach will be proposed, which compensates for angle-Doppler coupling, and restores the Doppler bandwidth. A data model for the MIMO observations is derived, which is exploited by a maximum likelihood estimator to infer angle, delay, and Doppler from the observations. The performance of the proposed approach will be testified by simulations.

An Efficient Calibration of MIMO Channel Sounders With Internal Crosstalk

In this paper, a calibration of multiple-input multiple-output (MIMO) channel sounders with internal crosstalk is considered.The objective is to minimize the number of back-to-back (B2B) connections required to estimate the transmitter (Tx) and receiver (Rx) response matrices that convey the information about linear distortion and internal crosstalk. A signal and system model is developed for the B2B measurements, where only some pairs of the Tx and Rx ports of the sounder are utilized among all pairs of the ports. Using the measurement model, a least-square estimation problem is then formulated and converted in the frequency domain to weighted rank-one approximation problems. The notion of system identifiability of a MIMO channel sounder is introduced and some optimal sets of B2B connections are proposed. Given a proposed optimal set of B2B connections, the alternate convex search (ACS) algorithm with a proper initialization is also proposed to solve the weighted rank-one approximation problems. Finally, it is shown how to calibrate field measurement data by using the estimated response matrices. Numerical results show that, only after a couple of iterations, the ACS algorithm with the proposed initialization achieves a comparable identification and calibration performance to the conventional method that requires the B2B connections of all port pairs.

Determining the repetition period of transionospheric radio channel sounding to retrain a facility of correction for dispersion distortions

Correction for dispersion distortions in the transionospheric radio channel poses a topical problem of determining the appropriate repetition period of sounding. Research focused on studying the variations of the sounding repetition period to increase the efficiency of correction for dispersion distortions in the channel. There is proposed a method for experimental studying the diurnal variations of the coherence time (“lifetime”) of the transionospheric channel. The method is supported by the derived equations that establish relations between the channel lifetime and the time derivative of the total electron content of the ionosphere. Research findings allowed to determine the appropriate repetition period of channel sounding to retrain the facility of correction for dispersion distortions in the transionospheric radio channels.

Robust 60-GHz Beamforming for UAVs: Experimental Analysis of Hovering, Blockage, and Beam Selection

Unmanned aerial vehicle (UAV) mounted millimeter-wave (mmWave) base stations as well as aerial backhaul links will enable on-demand deployment of network resources. However, prior work has shown aerial links are prone to the frequent disruption caused by: 1) constant hovering due to GPS inaccuracies that impact narrow beamwidths; 2) blockages in the direct line of sight; and 3) suboptimal beam selection, especially if reduced angular sectors are searched in a highly dynamic environment. This article characterizes the impact of each of these phenomena for aerial mmWave links and proposes methods to distinctly identify when they occur in isolation or in combination during deployment. Furthermore, it also proposes corrective actions at the UAV, appropriate for the specific type(s) of impacting events: physical displacement from its earlier location, angular rotation around its vertical axis, or beamwidth adjustment. Our approach relies on exploiting the information contained in the angular domain of a large data set of experimentally collected beam-selection outcomes, under the above practical scenarios. We incorporate GPS accuracy models and antenna radiation patterns to create a robust model of potential outages. We then propose device-agnostic algorithms that jointly optimize UAVs' physical movement and the beamforming procedure. The experimental results obtained by mounting a pair of 60-GHz channel sounders on M600 DJI UAVs reveal loss reduction of up to 74.7%, translated into 260% physical layer bit-rate improvement compared to the classical 802.11ad standards-defined approach.

Channel Sounding and Ray Tracing for Intrawagon Scenario at mmWave and Sub-mmWave Bands

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.

Time-variance of 60 GHz vehicular infrastructure-to-infrastructure (I2I) channel

In this paper, we study the time-variance of a roadside infrastructure to infrastructure (I2I) channel operating at 60 GHz millimetre wave (mmWave) band, where the time-variance is caused by moving vehicles acting as scatterers. At first, measurement data is obtained by placing the transmitter (TX) and the receiver (RX) at different heights to emulate a link between two nonidentical roadside units (RSUs), and time-domain channel sounding is performed by sending complementary Golay sequences from the TX to the RX. A linear piece-wise interpolation of the corresponding temporal auto-correlation function (ACF) is used to find the Doppler spread of the I2I channel, where our interpolation method compensates for a slower sampling rate. Next, a framework is presented for time-variant channel impulse response (CIR) simulation which focuses on moving scatterers only and validates the linear piece-wise ACF model. The framework is useful for time-variant vehicular I2I channel simulation and in speed estimation related vehicular applications. Finally, a double-slope curve-fitted analytical model for ACF is proposed as a generalisation to the linear piece-wise model. The proposed model and its Doppler spectrum are found to be in agreement with the analytical results for fixed-to-fixed (F2F) channels with moving scatterers and matches perfectly with the measured data.

Performance evaluation of OFDMA and MU-MIMO in 802.11ax networks

IEEE 802.11ax defines a new access method called Orthogonal Frequency Division Multiple Access (OFDMA) which can be used in both downlink (DL) and uplink (UL) directions. OFDMA divides the bandwidth into several Resource Units (RUs) and allows multiple stations to transmit or to receive simultaneously. UL OFDMA supports Scheduled Access (SA) RUs and Random Access (RA) RUs. Only scheduled stations are allowed to send on SA RUs, while the other stations should contend for the RA RUs. Moreover, 802.11ax defines UL MU-MIMO and enhances DL MU-MIMO. In this paper, we introduce and evaluate the efficiency of DL and UL multi-user transmissions using OFDMA and MIMO. Our results show that UL OFDMA is particularly useful when multiple stations regularly need to transmit few MPDUs. In this case, UL OFDMA may outperform full bandwidth access with up to 400%. Moreover, we find that increasing the number of RA RUs does not effectively reduce the collision rate but significantly decreases the throughput. Furthermore, we show that UL MU-MIMO significantly improves the network throughput and that the performance of DL MU-MIMO is clearly enhanced thanks to the decreased duration of the channel sounding procedure.

Measurement‐based determination of parameters for non‐stationary TDL models with reduced number of taps

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.

Channel modelling of a cooker hob environment for smart kitchens

This study presents a study on the radio channel modelling for realising reliable wireless communication in kitchen environments. The study allows optimising the wireless communication between a cooking pot and the kitchen hood in a smart home's kitchen. The delay, angular and path loss characteristics of the wireless channel are obtained from channel sounding measurements. The authors compared path loss at frequencies 868 MHz, 2.4 and 5.25 GHz. The influence of the metallic pot on the antenna characteristics is assessed by measuring the radiation pattern of the antenna. The antenna gain increases when the antenna is placed next to the metallic pot. Based on the channel sounding measurements and antenna model, a two-ray channel model is proposed consisting of one line-of-sight component and a component caused by reflections on the hood and hob. For the validation of the proposed model, a narrowband patch antenna is designed and used for measurements in a real kitchen. The validation confirms the general applicability of the proposed method. The results of the joint channel and antenna model are used in link budget calculations. Via the link budget, the minimum required TX power to allow reliable wireless communication is calculated.

Multi-Frequency Multi-Scenario Millimeter Wave MIMO Channel Measurements and Modeling for B5G Wireless Communication Systems

Millimeter wave (mmWave) bands have been utilized for the fifth generation (5G) communication systems and will no doubt continue to be deployed for beyond 5G (B5G). However, the underlying channels are not fully investigated at multi-frequency bands and in multi-scenarios by using the same channel sounder, especially for the outdoor, multiple-input multiple-output (MIMO), and vehicle-to-vehicle (V2V) conditions. In this paper, we conduct multi-frequency multi-scenario mmWave MIMO channel measurements with 4 × 4 antennas at 28, 32, and 39 GHz bands for three cases, i.e., the human body and vehicle blockage measurements, outdoor path loss measurements, and V2V measurements. The channel characteristics, including blockage effect, path loss and coverage range, and non-stationarity and spatial consistency, are thoroughly studied. The blockage model, path loss model, and time-varying channel model are proposed for mmWave MIMO channels. The channel measurement and modeling results will be of great importance for further mmWave communication system deployments in indoor hotspot, outdoor, and vehicular network scenarios for B5G.

Industrial wireless channel measurements in a 2.4 GHz ISM radio band using a low-cost SDR-based channel sounder

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.

Calibration of Vector Network Analyzer for Measurements in Radio Frequency Propagation Channels.

In situ measurements of radio frequency (RF) spectrum activity provide insight into the physics of radio frequency wave propagation and validate existing and new spectrum propagation models. Both of these parameters are essential to supporting and preserving interference-free spectrum sharing, as spectrum use continues to increase. It is vital that such propagation measurements are accurate, reproducible, and free of artifacts and bias. Characterizing the gains and losses of components used in these measurements is vital to their accuracy. A vector network analyzer (VNA) is a well-established, highly accurate, and versatile piece of equipment that measures both magnitude and phase of signals, if properly calibrated. This article details the best practices for calibrating a VNA. Once calibrated, it can be used to accurately measure components of a correctly configured propagation measurement (or channel sounding) system or can be used as a measurement system itself.

Dynamic Channel Modeling for Indoor Millimeter-Wave Propagation Channels Based on Measurements

In this contribution, a recently conducted measurement campaign for indoor millimeter-wave (mm-wave) propagation channels is introduced. A vector network analyzer (VNA)-based channel sounder was exploited to record the channel characteristics at the frequency band from 28-30 GHz. A virtual uniform circular array (UCA) with a radius of 0.25 m was formed using a rotator with 360 steps. Moreover, by taking advantage of fiber-optic technique applied in the channel sounder, measurements at 50 positions were performed from an indoor hall to an indoor corridor along a long pre-defined route. A low-complexity high-resolution propagation estimation (HRPE) algorithm is exploited to estimate the propagation parameters of multipath components (MPCs). Based on the HRPE estimation results, a novel clustering identification and tracking algorithm is proposed to trace clusters. Composite channel characteristics, cluster-level characteristics and dynamic (or birth-death) behaviours of the clusters are investigated, which constitute a dynamic model for the indoor mm-wave channel.

Path Loss and Directional Gain Measurements at 28 GHz for Non-Line-of-Sight Coverage of Indoors With Corridors

Adequate coverage with high-gain antennas is a key to realizing the full promise of the bandwidth available at mm/cm wave bands. We report extensive indoor measurements at 28 GHz (1000 links, 9.9 million individual power measurements, 10 offices, 2 buildings), with/without line of sight (LOS) using a continuous wave channel sounder, with a 10° spinning horn, capable of capturing a full azimuth scan every 200 ms, in up to 171 dB path loss to characterize coverage with 90% confidence level. The environment had prominent corridors and rooms, as opposed to open/mixed offices in latest 3GPP standards. Guiding in corridors leads to much lower RMS azimuth spread (7° median in corridor non-LOS (NLOS) versus 42° in 3GPP) and higher penetration loss into rooms and around corners (30–32 dB, some 12 dB more loss than 3GPP at 20 m NLOS). Measured path gain in NLOS is predicted by a mode-diffusion model with 3.9 dB RMS error. Scattering degraded azimuth gain by up to 4 dB in the corridor and 7 dB in rooms with 90% probability. Link simulations in a canonical building indicate every corridor needs an access point to provide 1 Gb/s rate to adjoining rooms within 50 m using 400 MHz of bandwidth.

A Perturbation Approach to the Beam Tracing in the Line-of-Sight Massive MIMO Systems

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.