I. INTRODUCTION
Mobile-to-mobile (M-to-M) communication channels, where both the transmitter and the receiver are in motion and equipped with low elevation antennas, find application in mobile ad-hoc wireless networks, intelligent transportation systems, and relay-based cellular networks. M-to-M channels differ from conventional fixed-to-mobile (F-to-M) cellular radio channels, where the base-station is stationary, elevated, and relatively free of local scattering. Akki and Haber [1], [2] showed that the received signal envelope of M-to-M channels is Rayleigh faded under non line-of-sight conditions, but the statistical properties differ from F-to-M channels. They were the first to propose a mathematical reference model for M-to-M Rayleigh fading channels. Vatalaro and Forcella [3] extended their models to account for scattering in three dimensions (3-D). Channel measurements for outdoor-to-indoor narrow-band mobile-to-mobile communications have been presented in [4] while those for outdoor-to-outdoor communications are given in [5]. More recently, measurements for wide-band mobile-to-mobile communications have been reported in [6].