Vehicle to Vehicle RF Propagation Measurement

Contributed by John S. Davis

This page describes vehicle to vehicle RF propagation in the 900 MHz band focusing on determining delay spread, probability distribution parameters (in particular, the Rician K factor) and path loss rates. The parameters and results are discussed with implications for the use of RF communication between automobiles in Intelligent Vehicle/Highway Systems (IVHS), in particular for Autonomous Vehicle Control Systems (AVCS).

The results of RF wireless vehicle to vehicle measurements at close range are:

path loss with an exponential drop off rate on the order of "20 log d" to "30 log d"
RMS delay spread of a few nanoseconds, say up to 20 ns. Of note is the RMS delay spreads that had an "oscillating" behavior that can be explained by considering wave interference of the line-of-sight and a ground-reflected wave or other n-ray channel characterization models.
Additionally, the Rician K factor results mostly range from 5 to 11 with extreme values close to 20 and as low as about 1.

The channel is not strictly Rician. Unlike the traditional view of a Rician channel, it is inferred that the channel consists of two dominant components: a LOS signal and a strong reflection off of the roadway. Over distance the two dominant components sum up in a oscillatory manner.

Most literature on RF mobile propagation focuses on transmission between users in automobiles to and from a shared fixed base station. In such scenarios, transmission distances can be quite large, often measured in terms of city blocks. Furthermore, a line of sight (LOS) path between the transmitter and the receiver is not always likely as the base station antenna is often blocked by buildings and other obstacles. The channel is quite different when considering vehicle to vehicle communication. Based on the proposed platoon structures of IVHS designs, it can be assumed that the transmission distances will be quite small compared to the RF transmission cases cited above. In addition, a LOS signal path is virtually guaranteed since any obstacles in the roadway would not only be harmful to RF transmission, but also to driver safety.

Vehicle to Vehicle Channel Model

Vehicle-to-vehicle data communication will mainly consist of the continuous (routine) exchange of telemetric data such as vehicle status, speed, and acceleration. Interfering signals will be present from vehicles within the platoon and from outside the platoon (from vehicles in other lanes). Vehicles with bumper mounted directional antennas are considered.

The vehicle-to-vehicle radio link can be modelled statistically as a Rician fading channel. The dominant component in the Rician fading channel is likely to be relatively strong compared to the reflected signal (large Rician -factor), and the delay spread is likely to be relatively small because reflections occur in the immediate vicinity of the transmitter and receiver antenna. The propagation channel is modelled as a dominant component consisting of a direct-line-of-sight wave and a ground reflected wave, a set of early reflected waves, and intersymbol interference caused by excessively delayed waves.

Models proposed for micro-cellular propagation consider path loss with a transition from free-space propagation to groun wave propagation.

Measurement Setup and Results

The experiments were primarily concerned with measurement of delay spread, Rician K factor and path loss versus distance; under various scenarios of automobile and antenna positions. The measurements were taken under stationary conditions on a roadway environment with parked cars. The antennas were supported by tripods at a height approximating an automobile's front hood. Two automobiles were parked in tandem at particular separation distance. As this distance was varied (from 6 to 40 feet), the parameters of interest were measured. At each distance, six measurements were taken at slightly different positions (by one half of the carrier wavelength to become uncorrelated) to average out fading.

Photo: A network analyzer is a useful tool to characterize a wideband short-range radio channel. Photo taken at the PATH research lab, Richmond Field station, U.C. Berkeley.

The measurement setup consisted of four components. The components are

omnidirectional discone (receive and transmit) antennas,
coaxial cables,
a micr wave network analyzer and
a portable PC.

The micr wave network analyzer served as a signal generator and could cover a frequency range of 50 MHz to 13.5 GHz with a maximum frequency resolution of 100 kHz. The network analyzer automatically calibrated out the coaxial cable loss. All tests had a transmit power level of +10 dBm. The network analyzer provided frequency response as well as the time domain instantaneous impulse response data corresponding to a given frequency response sweep. This data was ported to the PC for analysis via a GPIB connection.

Figure: Propagation Measurement Setup

Path Loss Attenuation

Figure: Path Loss

Multipath Reception

Figure: Frequency Transfer function (Amplitude in dB versus Carrier Frequency in GHz)

Delay Spread

There are two interesting issues to note with the delay spread measurements in this experiment.

First, the RMS delay spreads are, quite reasonably, very small. Compared to similar indoor wireless measurements that were have taken at 2.4 GHz with the same antenna separation distance, the vehicle to vehicle measurements have significantly smaller delay spreads (as much as a factor of 2). It is inferred that this difference is not due to the difference in carrier frequency but rather a difference in the two types of channel. One case being a relatively tame channel between two closely spaced automobiles, and the other being a relatively cluttered office cubicle environment. Given this difference, it seems quite reasonable that the vehicle to vehicle channel provides few obstacles that can lead to significant reflections, which is the primary cause of delay spreads.
A second interesting point is that the delay spread values increase slightly with distance but in a consistent oscillating pattern. The RMS delay spread has an oscillating pattern and no four consecutive points can be connected by a monotonic curve.

One explanation is the effect of a LOS path plus a ground reflection as the dominant components of the Rician channel model. One can view the channel as a phasor sum of:

LOS,
strong ground reflection, and
many random scatters.

The RMS delay spread tends to be inversely correlated with the amplitude of the two strong components. The oscillations diminish as distance increases. Beyond a certain turnover distance, the phasor sum of the two dominant phasor components no longer oscillate.

Figure RMS Delay Spread vs. Antenna Separation Distance

Rician K Factor

The Rician K factor is very important in completing the knowledge about the associated probability density function of a wireless channel. Indeed, to know the distribution of a wireless channel without knowing the parameters about which the distribution occurs is useless. Knowledge of the Rician K factor can be useful in determining the bit error rate of a channel among other useful metrics. The Rician K- factor can be measured and computed using a set of channel samples at different frequencies.

The largest measured K factor was 17.6. This value occurred several times. More typical values were in the range of 5.0 to 11.0. Large K factors were anticipated due to the obvious strong LOS signal that is present between the two vehicles. Contrary to anticipation, however, were occasional measured K factors around 1.5 (the lowest was 1.38).

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Chapter: Wireless Channels