Vehicular Channel Analyses
Although vehicular channels have been studied extensively at the DSRC band (5.9 GHz), few studies exist on millimeter wave (mmWave) vehicular channels. Understanding of the mmWave propagation in vehicular channels is still very limited. Early work on channel measurements and modeling done in the 1990s and early 2000s reported interesting effects that become more pronounced at mmWave frequencies such as road surface curvature, road undulation, and road surface roughness. Investigations into other important parameters for communication systems such as the delay spread and Doppler spread are close to non-existent probably due to the limitation of the equipment at that time. Motivated by this lack knowledge on mmWave vehicular channels, we are developing analytical models to investigates important characteristics of mmWave vehicular channels incorporating important mmWave features, e.g., the use of narrow beams or the effect of blockage.
The increase in Doppler spread, which leads to small channel coherence time, is one of the main concern when moving to mmWave frequencies. Classical result assuming omnidirectional beam suggests that the Doppler spread will increase proportional to the increase in carrier frequency. This, however, does not hold for mmWave systems that use narrow beams. There are results characterizing the channel coherence time with directional beams but the effect of beam pointing error due to the mobility have not been investigated. We introduced a model using a ring of scatters to capture the effect of pointing error due to the mobility. Our result incorporates both the effect of beamwidth in reducing the Doppler spread and increasing the susceptibility to pointing error. Unlike existing results that suggest that the coherence time goes to infinity as the beamwidth goes to zero, our result shows that there is a non-zero beamwidth that maximizes the coherence time. If the beam is too narrow, then pointing error dominates performance. If the beam is too wide then Doppler due to multipaths is the performance limiting factor.
Most popular pathloss models use only the Euclidean distance between the transmitter and the receiver, but in urban environments where blockage due to buildings is often encountered, Euclidean distance does not provide enough information to characterize the pathloss. Instead of using a pathloss model based on the Euclidean distance of the link, we introduced a new pathloss model mainly featuring the reflections in urban networks. The new pathloss model is based on line-of-sight (LOS) distance, non-line-of-sight (NLOS) distance and a certain corner loss at the intersection. Using stochastic geometry, we are able to get tractable results of some important metrics like coverage probability and capacity and analyze the effects of LOS and NLOS interferers. We can then give insights about the scaling law of these metrics with intensity of the streets and transmitters and optimize over the deployment of transmitters and flow control. Further work will be extended to the more practical case considering the street width and the mobility of the vehicles.
This research is supported in part by the National Science Foundation under Grant No. NSF-CCF-1319556, the U.S. Department of Transportation through the Data-Supported Transportation Operations and Planning (D-STOP) Tier 1 University Transportation Center, the Texas Department of Transportation under Project 0-6877 entitled “Communications and Radar-Supported Transportation Operations and Planning (CAR-STOP)”, and by a gift from TOYOTA InfoTechnology Center, U.S.A., Inc.
V. Va and R. W. Heath, Jr., “Basic Relationship between Channel Coherence Time and Beamwidth in Vehicular Channels,” in Proc. of the IEEE 82nd Vehicular Technology Conference (VTC2015-Fall), Boston, USA, pp. 1-5, Sep. 2015. Video of presentation.
V. Va, J. Choi, and R. W. Heath, Jr., “The Impact of Beamwidth on Temporal Channel Variation in Vehicular Channels and its Implications“, submitted to IEEE Transactions on Vehicular Technology, 2015, ArXiv preprint, arXiv:1511.02937
Y.Wang, K. Venugopal, A. F. Molisch, R. W. Heath Jr., “Analysis of Urban Millimeter Wave Microcellular Networks“, 84th IEEE Vehicular Technology Conference (VTC2016-Fall), Montreal, Canada, Sep. 2016. Video of presentation
V. Va, T. Shimizu, G. Bansal and R. W. Heath, Jr., “Millimeter Wave Vehicular Communications: A Survey,” Foundations and Trends in Networking, vol. 10, no. 1, pp. 1-113, Jun. 2016.
Data-Supported Transportation Operations & Planning Center A Tier 1 USDOT University Transportation Center at The University of Texas at Austin