A Modulation Dependent Channel Coherence Metric for VANET Simulation Using IEEE 802.11p

The most common physical layer models for network simulations are the bit error rate and the SNR threshold model. In time-varying channels such as those experienced in vehicular networks, these models are assumed valid as long as the packet duration is less than the coherence time of the channel. The coherence time is a statistical measure of the channel invariance. It is independent of the signal parameters or the receiver structure. While it is convenient to decouple the system performance from the channel, this paper shows that this simplification may lead to inaccurate performance assessments and erroneous conclusions. This paper suggests a new metric, the normalized empirical coherence time (NETC), based on results from an extensive simulation campaign of a typical IEEE 802.11p system. The NETC delineates the minimum time (as a percentage of signal duration) over which the system achieves some performance threshold. The metric is explicitly a function of modulation, packet duration, and the traditional coherence time. This new metric could be used in place of the traditional coherence time as a constraint on the packet duration necessary to assume channel variation has negligible impact on performance.