Alexander Paier

A geometry-based stochastic MIMO model for vehicle-to-vehicle communications

Vehicle-to-vehicle (VTV) wireless communications have many envisioned applications in traffic safety and congestion avoidance, but the development of suitable communications systems and standards requires accurate models for the VTV propagation channel. In this paper, we present a new wideband multiple-input-multiple-output (MIMO) model for VTV channels based on extensive MIMO channel measurements performed at 5.2 GHz in highway and rural environments in Lund, Sweden. The measured channel characteristics, in particular the nonstationarity of the channel statistics, motivate the use of a geometry-based stochastic channel model (GSCM) instead of the classical tapped-delay line model. We introduce generalizations of the generic GSCM approach and techniques for parameterizing it from measurements and find it suitable to distinguish between diffuse and discrete scattering contributions. The time-variant contribution from discrete scatterers is tracked over time and delay using a high resolution algorithm, and our observations motivate their power being modeled as a combination of a (deterministic) distance decay and a slowly varying stochastic process. The paper gives a full parameterization of the channel model and supplies an implementation recipe for simulations. The model is verified by comparison of MIMO antenna correlations derived from the channel model to those obtained directly from the measurements.

Vehicular Channel Characterization and Its Implications for Wireless System Design and Performance

To make transportation safer, more efficient, and less harmful to the environment, traffic telematics services are currently being intensely investigated and developed. Such services require dependable wireless vehicle-to-infrastructure and vehicle-to-vehicle communications providing robust connectivity at moderate data rates. The development of such dependable vehicular communication systems and standards requires accurate models of the propagation channel in all relevant environments and scenarios. Key characteristics of vehicular channels are shadowing by other vehicles, high Doppler shifts, and inherent nonstationarity. All have major impact on the data packet transmission reliability and latency. This paper provides an overview of the existing vehicular channel measurements in a variety of important environments, and the observed channel characteristics (such as delay spreads and Doppler spreads) therein. We briefly discuss the available vehicular channel models and their respective merits and deficiencies. Finally, we discuss the implications for wireless system design with a strong focus on IEEE 802.11p. On the road towards a dependable vehicular network, room for improvements in coverage, reliability, scalability, and delay are highlighted, calling for evolutionary improvements in the IEEE 802.11p standard. Multiple antennas at the onboard units and roadside units are recommended to exploit spatial diversity for increased diversity and reliability. Evolutionary improvements in the physical (PHY) and medium access control (MAC) layers are required to yield dependable systems. Extensive references are provided.

Path Loss Modeling for Vehicle-to-Vehicle Communications

Vehicle-to-vehicle (V2V) communications have received increasing attention lately, but there is a lack of reported results regarding important quantities such as path loss. This paper presents parameterized path loss models for V2V communications based on extensive sets of measurement data collected mainly under line-of-sight conditions in four different propagation environments: highway, rural, urban, and suburban. The results show that the path loss exponent is low for V2V communications, i.e., path loss slowly increases with increasing distance. We compare our results to those previously reported and find that, while they confirm some of the earlier work, there are also differences that motivate the need for further studies.

Car-to-car radio channel measurements at 5 GHz: Pathloss, power-delay profile, and delay-Doppler spectrum

We carried out a car-to-infrastructure (C2I) and car-to-car (C2C) 4x4 multiple-input multiple-output (MIMO) radio channel measurement campaign at 5.2 GHz in Lund, Sweden. This paper presents first results on pathloss, power-delay profiles, and delay-Doppler spectra in a C2C highway scenario, where both cars were traveling in opposite directions. A pathloss coefficient of 1.8 yields the best fit with our measurement results in the mean square sense. The measured Doppler shift of the line of sight path matches exactly with theoretical calculations. Selected paths are investigated in the delay and Doppler domain. The avererage delay spread is 250 ns; Doppler shifts of more than 1000 Hz are observed.

On Wireless Links for Vehicle-to-Infrastructure Communications

Future intelligent transportation systems (ITS) will necessitate wireless vehicle-to-infrastructure (V2I) communications. This wireless link can be implemented by several technologies, such as digital broadcasting, cellular communication, or dedicated short-range communication (DSRC) systems. Analyses of the coverage and capacity requirements are presented when each of the three systems is used to implement the V2I link. We show that digital broadcasting systems are inherently capacity limited and do not appropriately scale. Furthermore, we show that the Universal Mobile Telecommunications System (UMTS) can implement the V2I link using either a dedicated channel (DCH) or a multimedia broadcast/multicast service (MBMS), as well as a hybrid approach. In every case, such V2I systems scale well and are capacity limited. We also show that wireless access in vehicular environment (WAVE) systems scale well, provide ample capacity, and are coverage limited. Finally, a direct quantitative comparison of the presented systems is given to show their scaling behavior with the number of users and the geographical coverage.

Characterization of Vehicle-to-Vehicle Radio Channels from Measurements at 5.2 GHz

The development of efficient vehicle-to-vehicle (V2V) communications systems requires an understanding of the underlying propagation channels. In this paper, we present results on pathloss, power-delay profiles (PDPs), and delay-Doppler spectra from a high speed measurement campaign on a highway in Lund, Sweden. Measurements were performed at a carrier frequency of 5.2xa0GHz with the communicating vehicles traveling on the highway in opposite directions. A pathloss coefficient of 1.8 shows the best fit in the mean square sense with our measurement. The average root mean square (RMS) delay spread is between 263xa0ns and 376xa0ns, depending on the noise threshold. We investigate and describe selected paths in the delay-Doppler domain, where we observe Doppler shifts of more than 1,000xa0Hz.

Non-WSSUS vehicular channel characterization in highway and urban scenarios at 5.2GHz using the local scattering function

The fading process in high speed vehicular traffic telematic applications at 5 GHz is expected to fulfill the wide-sense stationarity uncorrelated scattering (WSSUS) assumption for very short time-intervals only. In order to test this assumption we apply the concept of a local time- and frequency-variant scattering function, which we estimate from measurements of vehicle-to-vehicle wave propagation channels by means of a multi-window spectrogram. The obtained temporal sequence of local scattering functions (LSF) is used to calculate a collinearity measure. We define the stationarity time as the support of the region where the collinearity exceeds a certain threshold. The stationarity time is the maximum time duration over which the WSSUS assumption is valid. Measurements from an highway with vehicles driving in opposite directions show stationarity times as small as 23 ms whereas vehicles driving in the same direction show stationarity times of 1479 ms.

First Results from Car-to-Car and Car-to-Infrastructure Radio Channel Measurements at 5.2GHZ

Car-to-car and car-to-infrastructure (henceforth called C2X) communications are constantly gaining importance for road- safety and other applications. In order to design efficient C2X systems, an understanding of realistic C2X propagation channels is required, but currently, only few measurements have been published. This paper presents a description of an extensive measurement campaign recently conducted in an urban scenario, a rural scenario, and on a highway. We focused on 4 x 4 multiple-input multiple-output (MHVIO) measurements at a center frequency of 5.2 GHz with high Doppler resolution. As first results from evaluating the measurement data we present the power-delay profile and the delay-Doppler spectrum from a selected, especially interesting measurement run from an urban measurement route. We observe dispersed Doppler contributions between zero and the Doppler shift corresponding to the relative speed of the cars, and very concentrated (in the Doppler domain), contributions from double reflections. Surprisingly, we also found paths with larger delays and zero Doppler shifts.

Radio Channel Measurements at Street Intersections for Vehicle-to-Vehicle Safety Applications

This paper presents the results of an empirical study of wireless propagation channels for vehicle-to-vehicle communications in street intersections, a scenario especially important for collision avoidance applications. The results are derived from a channel measurement campaign performed at 5.6 GHz in four different types of urban intersections. We present results on typical power delay profiles, pathloss and delay spreads and discuss important propagation mechanisms. By comparing the results of the different intersections, we find that absence of line-of-sight is problematic for system coverage, especially when there are few other significant scattering objects in and around the intersection. Roadside buildings can create important propagation paths that account for a considerable part of the total received power.

Average Downstream Performance of Measured IEEE 802.11p Infrastructure-to-Vehicle Links

For the launch of intelligent transport systems (ITS), it is necessary to have detailed understanding of their performance. The draft standard IEEE 802.11p is the physical and medium access control layer (PHY/MAC) standard extension for wireless access in vehicular communications to IEEE 802.11. In order to evaluate its performance, we carried out an infrastructure-to-vehicle trial on a highway using an IEEE 802.11p prototype. This paper presents evaluation results of the average downstream performance of the PHY. Shadowing effects mainly caused by trucks lead to a strongly fluctuating performance of the link quality, especially for settings with long packet lengths and high vehicle speeds. The maximum achievable range, where the frame-success-ratio is continuously larger than 0.25, is about 700m. The maximum data volume that can be transmitted when a vehicle drives by a roadside unit is achieved at low data rates of 6 and 9Mbit/s.