Vehicle-to-vehicle (V2V) wireless communication systems are crucial for intelligent transportation applications such as traffic load control, autonomous vehicles, and collision avoidance. Therefore, it is essential to thoroughly understand the realistic V2V channel propagation conditions to develop suitable V2V communication systems and standards. However, due to high costs, most existing V2V studies focus on characterizing the vehicular channel theoretically and comparing it with simulation data or studying the empirical V2V channel in a specific scenario. In this work, we investigate the time variability of the received signal based on experimental data collected in an extensive measurement campaign at 5.9 GHz in a large set of propagation environments: urban-high traffic density, urban-low traffic density, suburban, highway, and rural environments. The time-variability of the received signal is characterized by time and frequency. The relationship between coherence time and the terminal velocity, coherence time and transmitter-receiver separation distance, and the relationship between Doppler spread and both effective velocity and separation distance, have been investigated. Furthermore, the correlation between coherence time and separation distance and effective velocity was investigated, as well as the correlation between Doppler spread and separation distance and effective velocity. The results provide a deeper understanding of the temporal variability of the channel and how it is influenced by factors such as traffic density, unique/multiple driving directions, terminal speed, separation distance, characteristics of the propagation environment, and visibility conditions. This knowledge is crucial for developing vehicular communication systems and can be applied to design strategies that reduce temporal variability in received signals.

In this paper, we present a narrowband power spectral density (PSD) estimation technique using a 40 MHz bandwidth channel sounder based on a lightweight and portable software-defined radio (SDR) platform. This setup is intended for the characterization of communication channels in unmanned aerial vehicle (UAV). A post-processing step using a finite impulse response (FIR) digital filter is compared with laboratory setups to verify its calibration. Tests were performed with a 1 MHz signal on carriers at 850 MHz and 3.5 GHz. In addition, the challenges of transmitting, receiving, and storing samples at a rate of 40 MS/s are discussed.

Metro and railway systems are one of the most used transportation systems for people in almost all countries. Nevertheless, the access to high throughput wireless services is still very limited inside the wagons (cars). A deep analysis of the wireless channel inside wagons is needed to deploy new efficient and high throughput networks as the ones provided by fifth-generation (5G) systems. Although several works have analyzed the intra-wagon channel, some limitations are usually present: only certain user equipment-access point situations were considered, the number of studied propagation mechanisms was limited, and only some channel parameters were extracted. For these reasons, in this work the wireless channel in an intra-wagon environment is thoroughly analyzed using simulations performed with a ray-tracing tool calibrated and validated with wideband measurements. Thanks to the accurate ray-tracing tool the main replicas are identified in different typical user equipment-access point positions; the contribution of each propagation mechanism to the total power is extracted; and the angular spread in azimuth and elevation for the direction of arrival and departure are obtained. This analysis is performed in the frequency range from 25 to 40 GHz, where spectrum for several 5G bands has been already allocated.

In this paper, the maximum achievable throughput is analyzed in the intra-wagon channel when multiple-input single-output (MISO) and orthogonal frequency division multiplexing (OFDM), MISO-OFDM, techniques are used. This analysis is performed from real wideband propagation channel measurements at 28 and 37 GHz, two potential frequency bands to deploy the future fifth-generation (5G) wireless communications networks. Four different scenarios in terms of the access point (AP) and user equipment (UE) positions inside the wagon have been considered, using 4 and 8 antennas at the AP. The performance of both quasi-orthogonal space-time block code (QSTBC), combined with Hadamard matrices, and transmit beamforming techniques is studied and evaluated from simulation results. The simulation results take into account the signal-to-noise ratio (SNR) and the antenna correlation for each antenna array configuration at the AP. These results provide useful insight to better understand the intra-wagon channel properties and deploy the future 5G wireless networks in this particular scenario at mmWave frequencies, where high-data-rates are expected to support different types of digital applications.

A thorough millimeter-wave measurement campaign is carried out in an indoor environment with an aim at characterizing the short-term fading channel behavior. The measurements are conducted in a variety of scenarios, with frequencies ranging from 55 GHz to 65 GHz, in lineof-sight and non-line-of-sight conditions, and combinations of horizontal and vertical polarizations at both transmitter and receiver. A number of fading models are tested, namely Rayleigh, Rice, Nakagami-m, α-μ, κ-μ, η-μ, and α-η-κ-μ. The statistics under analysis are those characterizing the fading amplitude and the frequency selectivity. In particular, the probability density and cumulative distribution functions for the former and level crossing rate per bandwidth unit for the latter are the respective first- and second-order statistics used. To this end, from the experimental data, the parameters of the models are estimated and the corresponding theoretical curves are plotted and compared with the empirical ones. Whereas the required theoretical formulations of the first-order statistics of these models are already well known, those of the second-order statistics as well as these fitting process in such a band shown here are unprecedented in the literature.

We present a small-scale fading analysis of the vehicular-to-vehicular (V2V) propagation channel at 5.9 GHz when both the transmitter (Tx) and the receiver (Rx) vehicles are inside a tunnel and are driving in the same direction. This analysis is based on channel measurements carried out at different tunnels under real road traffic conditions. The Rice distribution has been adopted to fit the empirical cumulative distribution function (CDF). A comparison of the K factor values inside and outside the tunnels shows differences in the small-scale fading behavior, with the K values derived from the measurements being lower inside the tunnels. Since there are so far few published results for these confined environments, the results obtained can be useful for the deployment of V2V communication systems inside tunnels.

The spectral efficiency of MIMO (multiple input multiple output) channels is limited by both the spatial correlation and the rank deficiency of the channel. Earlier studies have focused less attention on how the frequency band affects these channel properties. In this paper, the results derived from an extensive measurement campaign performed at 4 frequency bands between 2 and 12 GHz in an office scenario, show a frequency band dependence on the MIMO channel capacity. Increasing the number of antenna elements under these correlated channels, we found a frequency band dependence on the ergodic, 10% and 1% outage capacities. Taking into account the path loss effect isolation, high SNR regime, equal electric separation between array elements, and stationary conditions in the measurements, remarkable changes on the spectral efficiency were observed for different frequency bands in line-of-sight (LOS) and non-line-of-sight (NLOS) propagation conditions.

This letter presents infinite series expressions for the outage probability, the probability density function, the average error probability for binary modulations, and the average signal-to-noise ratio of dual selection combiners over correlated fading with arbitrary fading parameters at each input of the combiner. The outage probability is calculated for both thermal noise and interference-limited scenarios. The results obtained for the outage probabilities specified for identical fading parameters at both branches of the combiner are contrasted with the results of other studies in the literature