  In this paper we investigate the optimal relay location in the sense of
maximizing suitably defined coverage region for MIMO relay channel. We consider
the general Rayleigh fading case and assume that the channel state information
is only available at the receivers (CSIR), which is an important practical case
in applications such as cooperative vehicular communications. In order to
overcome the mathematical difficulty regarding determination of the optimal
relay location, we provide two analytical solutions, and show that it is
possible to determine the optimal relay location (for a desired transmission
rate) at which the coverage region is maximum. Monte Carlo simulations confirm
the validity of the analytical results. Numerical results indicate that using
multiple antennas increases coverage region for a fixed transmission rate, and
also increases the transmission rate linearly for a fixed coverage.
