On the Analogy Between Vehicle and Vehicle-Like Cavities With Reverberation Chambers

Abstract

Deploying wireless systems in vehicles is an area of current interest. Often, it is implicitly assumed that the electromagnetic environment in vehicle cavities is analogous to that in reverberation chambers, it is therefore important to assess to what extent this analogy is valid. Specifically, the cavity time constant, electromagnetic isolation and electric field uniformity are investigated for typical vehicle and vehicle-like cavities. It is found that the time constant is a global property of the cavity (i.e., it is the same for all links). This is important, as it means that the root-mean-square delay spread for any link is also a property of the cavity, and thus so is the coherence bandwidth. These properties could be exploited by wireless systems deployed in vehicles. It is also found that the field distribution is not homogeneous (and is therefore not uniform), but can be isotropic. For situations where the field distribution is isotropic, the spatial coherence is well defined, and therefore multiple-input-multiple-output antenna arrays can be used to improve performance of wireless systems. For situations where the field distribution is not isotropic, the angular spread is not uniform, and therefore beamforming can be used to improve performance of wireless systems.

Characterizing the Spectral Properties and Time Variation of the In-Vehicle Wireless Communication Channel

“…First, we use the reverberation chamber to verify an important assumption, specifically that the time variation can be considered to be a WSSUS process. Secondly, in a metal cavity which has been verified to have vehicle-like properties [3], [11], this metal cavity was placed in a fully anechoic chamber that allowed us to perform tightly controlled measurements, having a very low noise floor (i.e., the anechoic chamber is shielded from external radiation, so only thermal noise is present). Finally, measurements were performed in an actual vehicle, which enabled us to measure the ground truth.…”

“…Before enumerating these causes we note that it has been shown that external objects have little influence on the propagation process in the vehicle cavity [3]. The primary causes of time variation are:…”

“…To understand the significance of these four channel time variation causes (i.e., 1-4 above), consider the propagation process in the vehicle cavity which has been shown to be analogous to that in the reverberation chamber [2], [3]. At sufficiently high frequencies (i.e., greater than 1.7 GHz [3]), the cavity exhibits a standing wave pattern.…”

“…At sufficiently high frequencies (i.e., greater than 1.7 GHz [3]), the cavity exhibits a standing wave pattern. Variations caused by absorptive and reflective stirring can be considered to be a change in the boundary conditions of this standing wave pattern.…”

“…Specifically, it would be possible to evaluate the information capacity of the channel, and to use the statistical properties of the time variation model to improve channel estimation/ prediction. Throughout this paper, the property that the electromagnetic wave propagation in vehicle cavities is similar to that in a reverberation chamber is used [2], [3].…”