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Machine learning (ML) is playing an increasingly important role in processing large amounts of data generated by communication networks, since it can efficiently solve the problems of non-linearity and unstructured data. Recently, ML has been widely used in the processing of wireless channel data, as the noisy channel in real propagation environment is usually non-linear and unstructured. In this paper, a denoising method based on ML is presented. Two ML algorithms are used to classify and remove noise in channel impulse responses. Then, the results of the traditional noise threshold denoising are compared with ML denoising, and it is found that the denoising classifier using the bidirectional recurrent neural network has the better denoising performance. Finally, some channel parameters such as RMS delay spread are estimated based on measured channel data using different denoising methods. The results are evaluated and compared to explore the impact of denoising method on the extracted channel parameters.

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.

An important indicator commonly used to evaluate the quality of a communication link is signal-to-interference ratio (SIR). However, studies in the literature dealing with SIR for orthogonal frequency division multiplexing (OFDM) systems are relatively few. This paper analytically investigates OFDM system performance by including major channel impairments such as excess delay spread, synchronization error, sample timing error, frequency offset, and time-varying fading. We derive a closed-form expression for SIR, accounting for both variations of inter-carrier interference (ICI) and inter-symbol interference (ISI) with the number of subcarriers, and the susceptibility to channel impairments. A condition relating peak ISI to peak ICI is derived. An expression quantifying the impact of synchronization error in ISI is obtained. SIR behavior with respect to the tradeoff among numbers of subcarriers and other system parameters are investigated. A numerical method, as well as a polynomial equation for an analytical solution, is formulated to find the optimal number of subcarriers for a given set of constraints. We demonstrate the variations of SIR with various parameters and analyze the leakage of power (unwanted power distribution across subcarriers).

In a frequency division complementary coded CDMA (FD-CC-CDMA) system, multiple tuners have to be used for multi-carrier modulation, which has a very high complexity and power consumption. In this paper, we propose a CP-free OFDM scheme to implement multi-carrier modulation in an FD-CC-CDMA system, which is named as CP-free OFDM-CC-CDMA system. The transmitter in the CP-free OFDM-CC-CDMA system offers a unique flexibility to adjust the amount of data processed at the same time to ensure that the OFDM block size exceeds the delay spread of a multipath channel, which attributes to effective mitigation of multipath induced inter-symbol interference (ISI). At the receiver of a CP-free OFDM-CC-CDMA system, minimum mean square error (MMSE) equalization and MMSE combining are used to mitigate multi-user interference (MUI) and inter-carrier interference (ICI). The obtained results show that a CP-free OFDM-CC-CDMA system offers a much higher transmission efficiency than FD-CC-CDMA and OFDM-DS-CDMA systems. In addition, the proposed CP-free OFDM-CC-CDMA system outperforms OFDM-DS-CDMA system in terms of bit error rate due to its inherent powerful diversity gain.

Wireless channels generally exhibit dispersion in both time and frequency domain, known as doubly selective or doubly dispersive channels. To combat the delay spread effect, multicarrier modulation (MCM) such as orthogonal frequency division multiplexing (OFDM) and its universal filtered variant (UF-OFDM) can be employed, leading to a simple per-subcarrier one tap equalization. The time-varying nature of the channel, in particular, the intra-multicarrier-symbol channel variation induces spectral broadening and thus inter-carrier interference (ICI). Most existing works address both effects separately, focusing on the one effect while ignoring the respective other. This paper considers both effects simultaneously for cyclic prefix (CP)-, zero padded (ZP)- and universal filtered (UF)-based OFDM with a simple one tap equalization, assuming doubly dispersive wireless channel model. For this general channel model, we show that the independent (wide sense stationary uncorrelated scatter, WSSUS) selectivity in time and frequency domain starts to intertwine in contrast to the ideal cases with single selectivity. We derive signal-to-interference-plus-noise ratio (SINR) in closed form for arbitrary system settings and channel parameters, e.g., bandwidth, delay- and Doppler-spread. With the SINR analysis, we compare the three MCM schemes under different channel scenarios.

This paper proposes a multi-feature fusion based propagation scene recognition model for high-speed railway (HSR) channels and presents the channel relevance analysis of HSR scenes. Extensive field measurement data in typical HSR scenes, including rural, station, suburban and multi-link scenes, are collected with the assist of railway long-term evolution (LTE) networks. The datasets of space-time-frequency channel features, involving Ricean K-factor, root mean square delay spread, Doppler spread, and angle spread, are generated for the model training and testing as well as the relevance analysis. The proposed model merges a weighted score fusion scheme into the deep neural network (DNN) in order to adaptively determine the optimal weights for each feature stream. This weighted score fusion based DNN model is implemented and evaluated in terms of accuracy, confusion matrix, F-score, and receiver operating characteristic (ROC) curve, which exhibits better performance than other machine learning models like random forest, support vector machine (SVM), $k$ -nearest neighbor (KNN), and weighted KNN. In addition, the channel relevance of HSR scenes is analyzed from perspectives of high-dimensional distribution distance and joint correlation of multiple features. Two metrics, Wasserstein distance and correlation matrix collinearity, are used in the analysis. Statistical results are provided, which reveals the relatively strong channel relevance between the multi-link and suburban scenes.

In this work we present the results of a measurement campaign carried out in the Mediterranean sea aimed at characterizing the underwater acoustic channel in a wideband at ultrasonic frequencies centered at 80 kHz with a width of 96 kHz, covering two octaves from 32 to 128 kHz. So far, these type of wideband measurements are not found in the literature. Periodic orthogonal frequency division multiplexing (OFMD) sounding signals using Zadoff-Chu sequences have been specially designed for this purpose. The collected data has been post-processed to estimate the time-variant impulse and frequency responses and relevant parameters for system design like the time coherence, bandwidth coherence, delay spread and Doppler bandwidth. The statistical behavior of the channel gain random fluctuation has also been analyzed. This information has been extracted for both the global channel and each path separately. The wide bandwidth of the measurements have allowed the characterization of the channel in a scarcely explored ultrasonic band with an accuracy that is far beyond what is reported in previous works.

The 3.5 GHz and 28 GHz bands are both very important for the fifth-generation (5 G) wireless communication system. To model their propagation characteristics and study their channel properties, we conduct measurements at these two bands in indoor and outdoor scenarios. The measured bandwidths at 3.5 and 28 GHz are 100 and 400 MHz, respectively. To make a fair comparison, the measurement results at 28 GHz are divided into four groups with bandwidths from 100 to 400 MHz. This is also helpful to study the effect of bandwidth on the channel properties, especially numbers of clusters and their growth rate. We choose the Saleh-Valenzuela (S-V) model to analyze the wireless channel. Its cluster-based property is very suitable to describe the power delay profiles (PDPs) in our measurement. To maintain the time continuity of multipath components (MPCs) during the clustering, we propose a new heuristic cluster algorithm. Based on the clustering results, we obtain the channel parameters, i.e., the number of clusters, inter-cluster interval, intra-cluster root mean square (RMS) delay spread, cluster decay factor, and ray decay factor. From the comparative study, we can find that the rays suffer a larger attenuation at 28 GHz than at 3.5 GHz. Increasing bandwidth can increase the ray decay factor. But the effects of frequency on the cluster decay rate are different in indoor and outdoor scenarios. The cluster decay rate is smaller at 28 GHz in the indoor scenario.

Contemporary receiver windowed orthogonal frequency division multiplexing (RW-OFDM) algorithms have limited adjacent channel interference (ACI) rejection capability under high delay spread and small Fast Fourier Transform (FFT) sizes. Cyclic prefix (CP) is designed to be longer than the maximum excess delay (MED) of the channel to accommodate such algorithms in current standards. The robustness of these algorithms can only be improved against these conditions by adopting additional extensions in a new backward incompatible standard. Such extensions would deteriorate the performance of high mobility vehicular communication systems in particular. In this paper, we present a low-complexity Hann RW-OFDM scheme that provides resistance against ACI without requiring any intersymbol interference (ISI)-free redundancies. While this scheme is backward compatible with current and legacy standards and requires no changes to the conventionally transmitted signals, it also paves the way towards future spectrotemporally localized and efficient schemes suitable for higher mobility vehicular communications. A Hann window effectively rejects unstructured ACI at the expense of structured and limited intercarrier interference (ICI) across data carriers. A simple maximum ratio combining successive interference cancellation (MRC-SIC) receiver is therefore proposed to resolve this induced ICI and receive symbols transmitted by standard transmitters currently in use. The computational complexity of the proposed scheme is comparable to that of contemporary RW-OFDM algorithms, while ACI rejection and bit-error rate performance is superior in both long and short delay spreads. Channel estimation using Hann RW-OFDM symbols is also discussed., Comment: 10 pages, 5 figures comprising 3,2,1,2,2 subfigures and 4 author biographies. Final version accepted for publication in IEEE Transactions on Vehicular Technology

This paper introduces the design of a 41 GHz mmWave system dedicated for high-speed train communications between train and trackside. A novel frame structure is provided for the acquirement of channel state information as well as for the transmission of user data. Based on the proposed design principles, a system prototype has been established and employed in extensive measurement campaigns. The channel measurement results obtained from a recent measurement campaign under high-speed mobile conditions where the relative velocity between the transmitter and the receiver reaches 170 km/h are presented in this paper. Channel characteristics, including the power delay profile, RMS delay spread, fade depth, small-scale fading statistics, Ricean K factor, autocovariance function, coherence time, level crossing rate, and average fading duration are analyzed in detail. Moreover, the bit-error-rate performance for the proposed system under the channel whose characteristics are obtained from the measurements is simulated. Based on the simulation results, the key parameters for the proposed frame structure are determined to improve the system's robustness in fast varying channels and in the mean time achieve high spectrum efficiency. Besides, the throughput of the system prototype recorded during the transmission of three high-definition video streams is presented. Finally, the outage probability as a function of transmission distance and path loss exponent is estimated based on the channel measurement results.

In this paper, the intra-wagon channels at 60 and 300 GHz bands are characterized through measurement-validated ray-tracing (RT) simulations. To begin with, an in-house-developed three-dimensional RT simulator is calibrated and validated by a series of millimeter-wave and Terahertz channel measurements inside a high-speed train wagon. Then, the validated RT simulator is used to conduct extensive simulations with different transmitter (Tx) and receiver deployments. At low frequencies, the channel is strongly influenced by the line of sight (LOS), and therefore, is usually classified into LOS and non-LOS (NLOS) regions. However, the simulation results at 60 and 300 GHz bands show that the first-order reflection also imposes a significant impact on the channel characteristics. This motivates us to further classify the NLOS region into light-NLOS (L-NLOS) and deep-NLOS (D-NLOS) according to the existence of the first-order reflection. Through analyzing the area ratios of LOS, L-NLOS, and D-NLOS regions, we evaluate the Tx deployment strategies and suggest the optimum one. Based on RT simulation results, totally 12 cases (three propagation regions with two Tx deployments at two frequencies) are characterized in terms of path loss, shadow fading, root-mean-square delay spread, Rician $K$ -factor, azimuth/elevation angular spread of arrival/departure, cross-polarization ratio, and their cross correlations. All these parameters are fed into the 3GPP-like quasi-deterministic radio channel generator (QuaDRiGa). The good agreement between QuaDRiGa and RT proves that the 13 tables provided in this paper effectively parameterize the intra-wagon scenario for the standard channel model family. These results provide valuable insights into the system design and evaluation for intra-wagon communications.

This paper presents the analysis of short-term fading behavior in high-speed railway (HSR) communication networks, based on passive channel measurements. A long-term evolution (LTE)-based channel sounding system is used to measure the propagation characteristics in a dedicated HSR LTE network with multiple scenarios, such as rural, station, and suburban, and with the echo channel effect (ECE) caused by the same signaling from different remote radio units. The measurement data are partitioned into four cases, involving rural, rural with the ECE, station, and suburban, aiming to investigate the impact of two factors, e.g., the scenario and the ECE, on the propagation characteristics. Based on the processed data, the short-term fading behavior is characterized in terms of the fading severity and the time-frequency-space dispersion in the four cases. The distance-dependent models and statistical models of Ricean K-factor, delay spread, Doppler spread, and angular spread are proposed and compared with previous paper. The presented results will be useful in time-frequency-space channel modeling for future HSR mobile communication systems.

This correspondence paper addresses linear precoding design for sum rate maximization in low-latency orthogonal frequency-division multiplexing systems. In order to mitigate the overhead of cyclic prefix (CP) originating from shortened symbol duration for low-latency communications, 5G wireless systems need to adopt short CP lengths. As channel delay spread must be less than the CP length, we first derive the effective root-mean-square (RMS) delay spread and the achievable rate using the zero-forcing assumption. We construct a sum rate optimization problem for each user subject to delay spread constraints and then convert the problem into a solvable convex problem along with a semi-definite relaxation technique. The precoding matrix is finally obtained by solving optimization problems for all users. Numerical results reveal that the proposed scheme attains superior performance to the conventional sum rate optimization, as well as small RMS delay spread.

In this paper, a recently conducted measurement campaign for vehicle-to-vehicle (V2V) propagation channel characterization is introduced. Two vehicles carrying a transmitter and a receiver, respectively, have been driven along an eight-lane road with heavy traffic. The measurement was conducted with 100 MHz signal bandwidth at a carrier frequency of 5.9 GHz. Channels are observed consisting of two kinds of channel components, i.e., time-evolving clusters and clutter paths. A novel approach based on Hough transform is proposed to identify the clusters. Based on the cluster identification results, channel characteristics in composite, intracluster, and time-variant levels are analyzed. The parameters investigated include the composite root-mean-square (RMS) delay spreads and power decay versus delay behaviors of clusters and clutter paths, cluster RMS delay spread, cluster RMS Doppler frequency spread, correlations of cluster parameters, and coherence time of parameters of interest. The statistics constitute an empirical stochastic clustered-delay-line channel model focusing on the wideband characteristics observed in the realistic time-variant V2V propagation scenario.

This paper proposes a new geometry-based shallow underwater acoustic (UWA) channel model allowing for nonisovelocity ocean conditions. The fact that the isovelocity assumption does not hold in many real-world scenarios motivates the need for developing channel models for nonisovelocity UWA propagation environments. Starting from a geometrical model, we develop a stochastic channel model for a single-input single-output (SISO) vehicle-to-vehicle UWA channel assuming that the ocean surface and bottom are rough and that the speed of sound varies with depth. The effect of the nonisovelocity condition has been assessed regarding its influence on the temporal autocorrelation function, the frequency correlation function (FCF), and the power delay profile of the UWA channel model. The UWA channel model has also been validated by matching its FCF as well as the therefrom derived main characteristic quantities, such as the average delay, delay spread, and coherence bandwidth against measurement data. The proposed UWA channel model is very useful for the design and performance analysis of UWA communication systems under realistic propagation conditions.

As one of the most important parts of intelligent transportation systems, vehicle-to-vehicle (V2V) communications have been widely studied because of the potential improvements it can provide in terms of traffic efficiency and traffic safety through various applications. The research on propagation characteristics for V2V channels is of primary importance, and one particular significant topic is shadowing effects induced by obstructing vehicles between transmitter (Tx) and receiver (Rx). In this paper, we present shadowing measurement results for V2V channels in both urban and highway scenarios, in which a truck and a van were used as the obstacles. Channel measurements in the 5-GHz band were performed in both line-of-sight (LOS) and non-LOS (NLOS) conditions, and different relative positions of vehicles on the road were considered. Based on the measured results, statistics for the shadowing loss and the root mean square delay spread (RMS-DS) are provided, and shadowing effects in each case are compared. By utilizing the temporal power delay profile correlation coefficient, the stationarity distance for each case is estimated, and the nonstationarity phenomenon and its relation to the shadowing effect are investigated. The mean value of shadowing loss was found to range from 3.38 to 9.33 dB with standard deviation ranging from 3.20 to 6.98 dB, and a normal distribution can well describe the statistics of the shadowing loss. With smaller RMS-DS values, the highway shows larger shadowing losses than the urban street. Moreover, in any specific scenario (e.g., highway or urban), the shadowing effect is generally closely related to the RMS-DS and stationarity distance: Larger shadowing loss always occurs when RMS-DS is large and average stationarity distance is small.

We propose a low-dimensional parameter estimation scheme for satellite-based ranging in the presence of an attenuated line-of-sight (LOS) signal and a time-varying number of multipath signals. This is of interest for landmobile users of global navigation satellite systems in diverse scattering environments with limited satellite visibility (e.g., a car moving through an urban canyon). Conventional ranging techniques, which rely solely on estimation of LOS parameters, ignore or suppress multipath and thus sacrifice available information; on the other hand, exhaustive approaches involving detection and estimation of each time-variant echo are too cumbersome in terms of parameter dimension. We suggest an unconditional maximum likelihood estimator (U-MLE) for joint estimation of LOS synchronization parameters and a small set of multipath distribution parameters. Our key assumptions are Gaussian wide-sense stationary uncorrelated scattering (WSSUS) and a known structure of the power delay profile. The proposed estimator uses as input data only a few complex-valued correlator outputs, which are provided by conventional navigation receivers, and its complexity is independent of the actual number of present echoes. Simulations demonstrate that the U-MLE improves accuracy and robustness of LOS synchronization for various model-matched and -mismatched multipath settings, including standardized and approved urban scenarios.

Applications of unmanned aircraft systems (UAS) are expected to expand dramatically in the coming decades. In order to provide safe and reliable links for UAS, NASA Glenn Research Center sponsored a measurement campaign in 2013 to characterize air-ground (AG) channels. In the first three of our AG channel characterization series papers, path loss, small scale fading, stationarity distance, root mean square (RMS) delay spread, spatial correlations, and tapped delay line models were presented for a range of ground site environment. In this paper, we expand the results to a unique and little-studied characteristic of the AG channels—airframe shadowing—for a medium-sized aircraft. Airframe shadowing occurs when the line-of-sight signal is obstructed by the airplane itself in some specific maneuvers. Based on over 200 aircraft wing/engine shadowing events in both C-band (5060 MHz) and L-band (968 MHz) collected in the measurement campaign, shadowing depth, duration, multiple antenna diversity gain, and small scale fading results and their statistics are reported. Algorithms are proposed to simulate a random shadowing event based upon shadowing depth and duration cumulative density functions. Airframe shadowing characteristics are independent of the local ground site conditions and link distance. The shadowing loss is modeled as a function of aircraft roll angle, but shadowing loss and duration are essentially uncorrelated. The maximum shadowing loss measured was over 35 dB and the duration was up to 74 s. As expected, deployment of multiple aircraft antennas is shown helpful to mitigate this shadowing.

It is essential to capture the cross-correlation properties of large-scale parameters (LSPs) among different base station links in cooperative multilink systems in order to make realistic performance assessments. In this work, propagation measurements are used to study the cross-correlation properties of different LSPs, namely large-scale fading, delay spread, azimuth spread, and elevation spread of four links. The interlink cross-correlation coefficients of these LSPs are assessed based on two different measurement approaches: 1) synchronous measurements, where the values of the LSPs of the considered links are estimated from the same measurement run; and 2) repeated measurements, where the values of the LSPs of the considered links are estimated from different measurement runs. Repeated measurements are attractive because they are simpler and less expensive. In this paper, we address the following question: Can repeated measurements be used instead of synchronous measurements in order to estimate the LSPs' cross-correlation properties of different links? Based on analysis of wideband synchronous and repeated multilink measurements in a suburban microcell environment at 2.6 GHz, we found that: 1) the mean values of the cross-correlation coefficients are preserved with repeated measurements, and 2) the estimates of the cross-correlation coefficients from repeated measurements are less spread around the mean value than those from synchronous measurements. These findings are explained based on detailed investigation of specific measured cases and further supported by results obtained from Monte Carlo simulations.

Applications for unmanned aircraft systems (UAS), or “drones,” are increasing rapidly. In order to provide safe and reliable links to integrate UAS into the National Airspace System (NAS), control and nonpayload communication (CNPC) system requirements are being specified. A comprehensive knowledge of the air-to-ground (AG) channels in the bands of interest (C-band and L-band) plays an essential role. The NASA Glenn Research Center has sponsored an AG channel measurement campaign for most of the typical ground site (GS) local environments, including over water [8] , hilly/mountainous [9] , suburban, and near-urban. As a continuation of our prior study, this paper addresses the suburban and near-urban scenarios. Our developed AG channel models include path loss, small-scale fading Ricean K factors, spatial and interfrequency correlations for multiple aircraft antennas, root-mean-square (RMS) delay spread, and wideband tapped delay line (TDL) models. The path loss is described by either log-distance or two-ray models, with small corrections for flight direction. The K factors were 12 (14) dB in L-band and 27.4 (28.5) dB in C-band in near-urban (suburban) environments. The interband signals were uncorrelated, but the intra-band signals were highly correlated, with the median correlation coefficient greater than 0.85. The C-band RMS delay spread was on average 10 to 60 ns, with maximum of approximately 4 μs. The TDL models are composed of the line-of-sight (LOS) component, a ground reflection, and up to seven intermittent multipath components (MPCs). Relative power, phase, occurrence probability, duration, and excess delays for these intermittent MPCs are quantified. An algorithm to simulate the AG channel impulse response (CIR) via the TDL models is provided.

Terahertz (0.1–10 THz) band communication is envisioned as a key technology to satisfy the increasing demand for ultra-high-speed wireless links. In this paper, a 3-D end-to-end model in the THz band is developed that includes the graphene-based reflectarray antenna response and the 3-D multipath propagation phenomena. In particular, the architecture of a graphene-based reflectarray antenna is investigated, and the 3-D radiation pattern is modeled. Moreover, a 3-D THz channel model based on ray tracing techniques is developed as a superposition of the line-of-sight (LoS), reflected, and scattered paths. By using the developed end-to-end model, an in-depth analysis on the 3-D channel characteristics is carried out. Specifically, the gain at the main beam of the graphene-based reflectarray antenna is 18 dB, and the 3-dB beamwidths in the elevation and the azimuth planes are $7^\circ$ and $10^\circ$ , respectively. The use of the reflectarray leads to a decrease of the delay spread from 1.23 to 0.099 ns, which suggests that the resulting coherence bandwidth reaches 2 GHz. Moreover, the root mean square (rms) angular spread in the elevation plane is less than $0.12^\circ$ , which is one tenth of that without beamforming. Furthermore, the wideband channel capacity at THz frequencies is characterized, which can be enhanced with a larger transmit power, a lower operating frequency, a larger bandwidth, and a higher beamforming gain. Finally, the beamforming gain enabled by the reflectarray antenna is compromised at the cost of the strict beam alignment, and the deviation needs to be smaller than $11^\circ$ . The provided analysis and the channel physical parameters lay out the foundation and are particularly useful for realizing reliable and efficient ultra-high-speed wireless communications in the THz band.

In this paper, we propose an iterative structure for channel estimation and data detection for multiple-input–multiple-output orthogonal frequency-division multiplexing systems (MIMO-OFDM) with insufficient cyclic prefix (CP) and a limited number of pilot subcarriers. Insufficient CP leads to increased intersymbol interference (ISI) and intercarrier interference (ICI). The interference corrupts pilot subcarriers used for channel estimation and affects the detection process. We propose a multistep channel estimation process, which is assisted by a small number of pilot subcarriers. First, the channel covariance matrix and the number of channel paths are acquired from the channel least square estimates at the pilot subcarriers. We then formulate a maximum likelihood process to approximate the time-domain channel. For data detection, a high-performance bidirectional M-algorithm (BDMA) is proposed for trellis-based equalization. Simulation results show that the channel estimate mean square error converges to the Cramer–Rao bound (CRB) after a few iterations. In addition, the achieved bit error rate (BER) can reach that of the sufficient CP case, even when the delay spread is much longer than the CP.1

The paper provides real-word wireless measurement data of the intravehicular channel for both the $\text{3}\hbox{--}\text{11 GHz}$ and the $\text{55}\hbox{--}\text{65 GHz}$ frequency bands under similar conditions. By spatially averaging channel impulse response realizations within a $10\times 10$ grid, we obtain the power-delay profile (PDP). The data measured at $\text{3}\hbox{--}\text{11 GHz}$ and $\text{55}\hbox{--}\text{65 GHz}$ exhibit significant differences in terms of root mean square (RMS) delay spread, number of resolvable clusters, and variance of the maximal excess delay. Moreover, we evaluate the spatial stationarity via the Pearson correlation coefficient and via the PDP collinearity, depending on the distance in the grid. The measured and calculated results indicate that a strong reverberation inside the vehicle produces similar PDPs within the range of approximately ten wavelengths. We also provide a linear piecewise model of the PDP in logarithmic scale and a generalized extreme value model of a small-scale signal fading. Our channel model is validated utilizing the Kolmogorov–Smirnov test.

The use of unmanned aircraft systems (UAS) is expanding rapidly. For safe and reliable integration of UAS into the National Airspace System (NAS), control and nonpayload communication (CNPC) system requirements are being developed. The air-to-ground (AG) channel characteristics essentially determine CNPC link performance. The National Aeronautics and Space Administration (NASA) Glenn Research Center has sponsored a comprehensive AG channel measurement campaign in 2013 in both the L- and C-bands recently allocated for UAS. These measurements covered nearly all typical ground site (GS) local environments, including over water, hilly, mountainous, suburb, and near urban. As a continuation of results reported for the over-water scenarios, a description of the measurement campaign, measured results, and complete AG channel models for hilly and mountainous terrains are provided in this paper. The path loss is modeled by a modified log-distance path loss model with a correction for flight direction; path loss results are close to those of free space. A strong ground reflection was observed only for some small portions of some of the flight paths. The small-scale fading is well modeled by the Ricean distribution with a $K$ -factor of 29.4 dB in the C-band and 12.8 dB in the L-band. The interband signals are uncorrelated. The spatial correlation for the two intraband channels with an aircraft antenna separation of 1.8 m is greater than 0.85. The C-band root-mean-square delay spread (RMS-DS) is 10 ns most of the time, with a maximum of 1 $\mu\text{s}$ in hilly terrain and 180 ns in mountainous terrain. Tapped delay line (TDL) models accounting for the line of sight (LOS) and up to seven intermittent multipath components (MPCs) were developed, including the MPCs' probability of occurrence, relative power, phase, duration, and excess delay.

The use of unmanned aerial systems (UASs), which are also known as unmanned aerial vehicles, and by the term “drones” in the popular press, is growing rapidly. To ensure safety, UAS control and nonpayload communication (CNPC) links must operate very reliably in a variety of conditions. This requires an accurate quantitative characterization of the air–ground (AG) channel, and this channel characterization is the focus of this paper. After providing motivation and background, we describe our methods and modeling approach, followed by a description of our simultaneous dual-band (L-band ∼970 MHz, C-band ∼5 GHz) measurement campaign and the over-water (OW) measurement sites. Example results for path loss and root-mean-square delay spread are provided, as well as the results for channel stationarity distance (SD), used in calculating small-scale Rician $K$ - factor and correlations between the two receiver antennas that we employed in each frequency band. Two distinct SD measures—the power delay profile (PDP) correlation coefficient and the spatial autocorrelation matrix collinearity—were used and found to be of the same order. Path-loss exponents are near that of free space, but significant two-ray cancelation effects for these OW settings warrant more accurate models, which we provide. Delay spreads in the OW channels are also dominated by the two-ray components and are hence typically very small (∼10 ns) but can exceed 350 ns. A third intermittent multipath component (MPC) is also present a nonnegligible fraction of time; hence, we provide statistical wideband AG channel models to represent this. Future papers in this series will report results for the AG channel with ground sites in other types of environments.

With the aim of providing an overview of the propagation channels in a vehicle engine compartment, narrow-band (1–2 MHz) and broadband (500 MHz) channels in the frequency range from 2 GHz to 7.75 GHz were analyzed. To secure as much generality of analysis as possible, more than 500 propagation channels of seven types of automobiles were measured. The path loss and delay spread were analyzed in terms of distance and frequency. Models that take into account the interdependence of distance and frequency were used, considering that the object sizes in an engine compartment are comparable to the measurement wavelengths. The path-loss exponent for distance was about one below a break point of 450 mm and about three above the break point. The exponent for frequency was around one. The root-mean-square (RMS) delay spread increased as distance increased. Based on the obtained delay spread model, the propagation channel model was finally provided, which is a function of frequency and distance. Then, the channel model was validated by comparing delay spreads calculated from the generated channels and the measured channels. It is also noted that the channel dependence on the types of vehicle was not significant.

The overpass is a special over-road structure for vehicular travel, constituting one type of roadway intersection. Real-time communications between on- and under-overpass vehicles can contribute to effective vehicle-to-vehicle (V2V) communications, including, for example, optimal route selection and overpass accident warning messaging. Ensuring effective communication requires a quantitative characterization of the overpass propagation channel. In this paper, we provide measurement and analytical results for V2V propagation path loss and root-mean-square delay spread, and from these results, we develop tapped-delay line channel models that are applicable to the 5-GHz band for two example overpasses. These two example overpasses are termed i) one-lane metal-bottom overpass and ii) two-lane metal-bottom overpass. Due to the unique structure of the overpasses, we divide the radio propagation space around the overpass into four different areas: a two-ray area, a short-term partial-shadowing area, a (full) shadowing area, and a long-term partial-shadowing area. In the two-ray area, a line-of-sight (LOS) path and a ground-reflected path are the dominant propagation mechanisms, whereas in the other areas, the overpass body (floor, walls, and columns) attenuates the LOS signal. The accuracy of our measurement results and the developed channel model are verified by a geometry-based stochastic channel modeling approach, in which the height dimension is first introduced to characterize the diffraction phenomenon in V2V communication. The actual measurement and simulation results show good consistency.

This paper aims to present a simple but robust model characterizing the frequency-dependent transfer function of an in-vehicle ultrawideband (UWB) channel. A large number of transfer functions spanning the UWB (3–11 GHz) are recorded inside the passenger compartment of a four-seated sedan. It is found that the complex transfer function can be decomposed into two terms, the first term being a real-valued long-term trend that characterizes frequency dependence with a power law and the second term forming a complex correlative discrete series that may be represented via an autoregressive (AR) model. An exhaustive simulation framework is laid out based on empirical equations characterizing trend parameters and AR process coefficients. The simulation of the transfer function is straightforward as it involves only a handful of variables; however, it is in good agreement with the actual measured data. The proposed model is further validated by comparing different channel parameters, such as coherence bandwidth, power delay profile, and root-mean-square delay spread, obtained from raw and synthetic data sets. It is also shown how the model can be compared with existing time-domain Saleh–Valenzuela-influenced models and related IEEE standards.

We investigate the performance of phase compensation (PC) and time reversal pre-equalizers in multipath indoor channels when a quantized version of the channel state information (CSI) is available at the transmitter side. We conduct a comprehensive experimental study to assess their spatial and temporal focusing as a function of the number of quantization levels over the frequency band of 2–12 GHz. To characterize multipath compression performance, root-mean-square (RMS) delay spread and peak-to-average power ratio (PAPR) are calculated for different numbers of quantization levels. Bit error rate (BER) curves have been simulated for data rates in the range of 125–4000 Mb/s based on the measured channel responses. To assess spatial focusing, we investigate the decay of the received response peak power as we move away from the intended receiver. Our study suggests that for the same feedback rate, PC has considerably superior performance in suppressing multipath dispersions and focusing the transmitted energy at the intended receiver.

The results of a wideband channel measurement campaign carried at Ku-band and X-band are presented and discussed in this paper. The propagation effects in the satellite-to-mobile radio link due to the environment surrounding the terrestrial terminal have been characterized using a sweep time delay cross-correlation (STDCC) sounder. The campaign was carried out in four different environments: wooded, rural, suburb and urban. From these results, we obtained the root-mean-square delay spread and coherence bandwidth of the radio channel. The low coherence bandwidth values found for urban and suburb environments suggest that channel equalization would be needed for high-data-rate services. The delay spread values for these two environments are also larger than for wooded and rural environments. A detailed analysis of the results shows that three-state semi-Markov narrow-band channel models for satellite communications could be easily extended to wideband.

In this paper, we present a wideband multiple-input multiple-output (MIMO) car-to-car (C2C) channel model based on a geometrical semi-circular tunnel (SCT) scattering model. From the geometrical SCT scattering model, a reference channel model is derived under the assumption of single-bounce scattering in line-of-sight (LOS) and non-LOS (NLOS) propagation environments. In the proposed reference channel model, it is assumed that an infinite number of scatterers are randomly distributed on the tunnel wall. Starting from the geometrical scattering model, the time-variant transfer function (TVTF) is derived, and its correlation properties in time, frequency, and space are studied. Expressions are presented for the space–time–frequency cross-correlation function (STF-CCF), the two-dimensional (2D) space CCF, the 2D time–frequency CCF (TF-CCF), the temporal autocorrelation function (ACF), and the frequency correlation function (FCF). Owing to the semi-circular geometry, we reduced the originally threefold integrals to double integrals in the computations of the correlation functions, which simplifies the numerical analysis considerably. From the TVTF characterizing the reference model, an efficient sum-of-cisoid (SOC) channel simulator is derived. Numerical results show that both the temporal ACF and the FCF of the SOC channel simulator match very well with those of the reference model. A validation of the proposed model has been done by fitting the delay spread of the reference model to that of the measured channel, which demonstrates an excellent agreement. The proposed channel simulator allows us to evaluate the performance of C2C communication systems in tunnel environments.

Indoor positioning using mobile radio signals at different frequency bands has gained remarkable attention. Thus, evaluating the influence of the radio propagation channel at different carrier frequencies with respect to the ranging performance is of interest. This paper presents a measurement-based comparison of the outdoor-to-indoor wideband propagation channel at S-band and C-band. We evaluate channel parameters that have an impact on the ranging and, in turn, on the positioning performance. Our comparison is threefold and is based on large-scale channel parameters, ranging performance, and spatial characteristics of the channel. In the first set of comparisons, we consider received power, power delay profile (PDP), delay spread, mean delay, and a number of multipath components (MPCs). The second set of comparisons considers ranging performance: Both the correlator-based and recursive-Bayesian-filter-based ranging estimators are evaluated using the measurement data. In the third set of comparisons, spatial characteristics of the propagation channel at S-band and C-band are compared and analyzed for range estimates to provide an insight for positioning applications.

We present an optimal decoder for physical-layer network coding (PNC) in a multipath fading channels. Previous studies on PNC have largely focused on the single path case. For PNC, multipath not only introduces inter-symbol interference (ISI), but also cross-symbol interference (Cross-SI) between signals simultaneously transmitted by multiple users. In this paper, we assume the transmitters do not have channel state information (CSI). The relay in the PNC system, however, has CSI. The relay makes use of a belief propagation (BP) algorithm to decode the multipath-distorted signals received from multiple users into a network-coded packet. We refer to our multipath decoding algorithm as MP-PNC. Our simulation results show that, benchmarked against synchronous PNC over a one-path channel, the bit error rate (BER) performance penalty of MP-PNC under a two-tap ITU channel model can be kept within 0.5dB. Moreover, it outperforms a MUD-XOR algorithm by 3dB -- MUD-XOR decodes the individual information from both users explicitly before performing the XOR network-coding mapping. Although the framework of fading-channel PNC presented in this paper is demonstrated based on two-path and three-path channel models, our algorithm can be easily extended to cases with more than three paths., Comment: Submitted to IEEE Transactions on Vehicular Technology

This paper presents measurements and statistical characterization of 300- to 320-GHz desktop channels. The measurements are performed in line-of-sight (LoS) and non-LoS (NLoS) environments. From the large set of LoS measured data, the parameters for the single-slope path-loss model with shadowing are devised. The results show that the path-loss exponent is around 1.9 and that the variations due to shadowing are similar across different frequencies and different bandwidths. Furthermore, the impact of different materials on the path loss is studied in the NLoS environment. The results show that metal objects in the propagation path cause multiple strong reflections, leading to a higher path loss. Furthermore, the statistical analysis of multipath propagation is performed. The root-mean-square (RMS) delay spread, the mean excess delay, the maximum excess delay, and the coherence bandwidth for LoS and NLoS environments are calculated. The results show that the mean excess delay and RMS delay spread increase with distance and that the RMS delay spread in the desktop terahertz channel is much smaller than that in typical indoor ultrawideband channels. In addition, the power delay profiles for LoS and NLoS environments are analyzed. The results show that strong reflections from the transmitter and receiver electronics are present in both LoS and NLoS environments. Finally, the statistical analysis of the measured signal amplitude in LoS and NLoS environments is performed. For both LoS and NLoS propagation environments, it is found that the lognormal distribution provides the best fit.

Linear polarization (LP) has been the traditional transmission strategy employed in current mobile wireless communication systems. Recently, circular polarization (CP) has regained much interest as CP proves to be a potential candidate to maximize the polarization efficiency component of the link budget. In this paper, the essential concepts of CP are first reinforced by a tutorial description in terms of mathematical insight, propagation characteristics, as well as related applications. Most reported works on CP chiefly focus on the theoretical description of its propagation benefits, such as significant amplitude fade and delay spread (DS) reduction, without simultaneously providing convincing proofs. In other words, CP propagation has not been investigated in sufficient depth. Therefore, we then propose a method for synthesizing CP that is implementation friendly to practical antennas. The engineering validity is subsequently verified by a series of chamber measurements. Intuitively, the polarization trajectory of a synthetical circularly polarized electromagnetic wave is illustrated to further prove the point. Finally, by conducting channel measurement campaigns in a typical urban macrocell scenario, radio propagation characteristics with a dipole as the receive apparatus are analytically extracted for both LP and CP. To facilitate fair competition, the radiation field patterns of two relevant transmitting antennas are intentionally designed to be identical. Numerical results quantitatively demonstrate that, compared with LP, CP can significantly alleviate branch power imbalance between vertical and horizontal polarizations and decrease DS. These advantages are in line with theoretical anticipation and can be fully exploited to upgrade the performance limits of current mobile wireless communication systems.

Channel-width adaptation can optimize multiple performance metrics of a wireless communication link, including transmission rate, communication range, resilience to delay spread, and power consumption. Supporting variable channel width has been considered one of the most critical features of a communication radio. How to leverage a channel-width adaptive radio to improve throughput of a wireless network is a challenging problem in the medium access control (MAC) layer. So far, there exist research results on either theoretical analysis or protocol implementation for a point-to-multipoint (PMP) infrastructure network. However, the impact of channel width to a multihop wireless network has not been fully investigated yet. More specifically, how to exploit variable channel width to enhance throughput of a multihop wireless network remains an unresolved research issue. This paper addresses this issue in wireless mesh networks (WMNs) considering orthogonal frequency-division multiple-access (OFDMA)-based channel-width adaptation. Theoretical analysis is first carried out to identify appropriate algorithms for channel width adaptation. To this end, resource allocation with OFDMA-based channel-width adaptation is formulated as an optimization problem, which is proved to be NP-complete. To reduce the computational complexity, a greedy algorithm is derived to obtain a suboptimal solution. Based on such a greedy algorithm, a distributed MAC protocol is designed for channel-width adaptation for OFDMA-based WMNs. It takes advantage of variable channel width in different time slots to achieve highly efficient resource allocation. Simulation results illustrate that the distributed MAC protocol significantly outperforms MAC protocols based on traditional channel-width adaptation.

Intravehicular wireless sensor network (IVWSN) is a cutting edge research topic that delivers cost reduction, assembly, and maintenance efficiency by removing the wiring harnesses within the vehicle and enables the integration of new sensors into the locations inside a vehicle where cable connection is not possible. Providing energy efficiency through the low-duty-cycle operation and high reliability by exploiting the large bandwidth, ultrawideband (UWB) has been determined to be the most appropriate technology for IVWSNs. We investigate the UWB channel model for IVWSNs within the engine compartment of a vehicle by collecting an extensive amount of data for 19 × 19 links for different types and conditions of the vehicle. These include a Fiat Linea with engine off, Fiat Linea with engine on, and Peugeot Bipper with engine off. The path-loss exponent is estimated to be around 3.5 without exhibiting much variation when the engine is turned on and for different types of vehicles. The power variation around the expected path loss has lognormal distribution with zero mean and standard deviation in the range of [5.5, 6.3] dB for different types of vehicles with almost no variation when the engine of the same vehicle is turned on. The clustering phenomenon in the power delay profile (PDP) is well represented by a modified Saleh-Valenzuela (SV) model. The interarrival times of the clusters are modeled using a Weibull distribution. The cluster-amplitude and ray-amplitude decay functions are represented with a dual-slope linear model with breakpoint around 26.6 and 5.5 ns, respectively. The parameters of the Weibull distribution and these dual-slope linear models do not vary significantly for different types and conditions of the vehicle. The variations of the observed PDPs around the SV model is well modeled by independent normal random variables with zero mean and with a variance independent of the delay bin, and the type and condition of the vehicle. We propose a simulation model for the UWB channel within the engine compartment based on these findings and validate it by comparing the received energy and root mean square (RMS) delay spread of the generated and observed PDPs.

Through the implementation of a simple linearly scalable 1-W infrared (IR) transmitter, which is centrally located on the ceiling of a sports utility vehicle (SUV), and for 15 passenger configurations, an analysis into the received power, power deviation, minimum bandwidth, and maximum root-mean-square (RMS) delay spread is provided for the regions of the vehicle most likely to benefit from the deployment of intravehicle optical wireless (OW) communication systems. Several specific regions, including the areas around a passenger's legs, arms, necks, and shoulders, are shown to have beneficial channel characteristics for the use of personal electronics equipment such as laptops, tablet PCs, or wireless headphones. Similarly, a region around the headrest of the front seat is shown to have potential for the deployment of in-car entertainment solutions independent of the passenger configuration. This analysis, which is the first to introduce the concept of channel variation from multiple passenger configurations, aims to show that OW is a potential candidate for future intravehicular communication systems.

The problem of localizing a transmitting source by observing the signal at several base stations (BSs) is considered. In this paper, a direct position estimation (DPE) algorithm for a dense multipath channel is formulated and analyzed, based on a Gaussian approximation. The algorithm provides robust and accurate position estimation in a multipath environment, which is a challenging problem, particularly when the signal-to-noise ratio (SNR) is low and when the multipath is dense. It is shown that the DPE is superior to the common indirect approach. In the indirect approach, the time of arrival (TOA) is estimated independently at each BS, whereas in the DPE, all the received signals from all BSs are processed jointly. We show that the proposed algorithm approximates the maximum-likelihood (ML) solution. We treat TOA based on synchronized source and receivers and the differential TOA (DTOA) where only the receivers are synchronized. Analytic results for the performance of the algorithm are provided. Finally, simulations demonstrate the advantage of DPE over the indirect method at low SNR for various multipath channel models and that the analysis closely approximates the simulation results.

In this paper, a novel receiver for Hexagonal Multicarrier Transmission (HMT) system based on the maximizing Signal-to-Interference-plus-Noise Ratio (Max-SINR) criterion is proposed. Theoretical analyses show that there is a timing offset between the prototype pulses of the proposed Max-SINR receiver and the traditional projection receiver. Meanwhile, the timing offset should be matched to the channel scattering factor of the doubly dispersive (DD) channel. The closed form timing offset expressions of the prototype pulse for Max-SINR HMT receiver over DD channel with different channel scattering functions are derived. Simulation results show that the proposed Max-SINR receiver outperforms traditional projection scheme and obtains an approximation to the theoretical upper bound SINR performance. Consistent with the SINR performance improvement, the bit error rate (BER) performance of HMT system has also been further improved by using the proposed Max-SINR receiver. Meanwhile, the SINR performance of the proposed Max-SINR receiver is robust to the channel delay spread estimation errors., 7 pages. The paper has been accepted by IEEE Transactions on Vehicular Technology 2013. Copyright transferred to IEEE. arXiv admin note: substantial text overlap with arXiv:1212.5792

This paper deals with channel estimation for orthogonal frequency-division multiplexing (OFDM) in time-variant wireless propagation channels. We particularly consider the challenges of the IEEE 802.11p standard, which is the worldwide dominant system for vehicular communications. For historic reasons, 802.11p uses a pilot pattern that is identical to the pattern used in 802.11a, which was initially designed for the estimation of indoor channels with little or no time variations. Therefore, this pilot pattern violates the sampling theorem for channels with both large delay spread and large Doppler spread, as often occurs in vehicular communications. To remedy this problem, we design a robust iterative channel estimator based on a 2-D subspace spanned by generalized discrete prolate spheroidal sequences. Due to the tight subspace design, the iterative receiver is able to converge to the same bit error rate (BER) as a receiver with perfect channel knowledge. Furthermore, we propose a backward compatible modification of the 802.11p pilot pattern such that the number of iterations sufficient for convergence can be reduced by a factor of 2-3, strongly reducing implementation complexity.