  Shadowing from vehicles can significantly degrade the performance of
vehicle-to-vehicle (V2V) communication in multilink systems, e.g., vehicular
ad-hoc networks (VANETs). It is thus important to characterize and model the
influence of common shadowing objects like cars properly when designing these
VANETs. Despite the fact that for multilink systems it is essential to model
the joint effects on the different links, the multilink shadowing effects of
V2V channels on VANET simulations are not yet well understood. In this paper we
present a measurement based analysis of multilink shadowing effects in a V2V
communication system with cars as blocking objects. In particular we analyze,
characterize and model the large scale fading, both regarding the
autocorrelation and the joint multilink cross-correlation process, for
communication at 5.9 GHz between four cars in a highway convoy scenario. The
results show that it is essential to separate the instantaneous propagation
condition into line-of-sight (LOS) and obstructed LOS (OLOS), by other cars,
and then apply an appropriate pathloss model for each of the two cases. The
choice of the pathloss model not only influences the autocorrelation but also
changes the cross-correlation of the large scale fading process between
different links. By this, we conclude that it is important that VANET
simulators should use geometry based models, that distinguish between LOS and
OLOS communication. Otherwise, the VANET simulators need to consider the
cross-correlation between different communication links to achieve results
close to reality.
