  In this correspondence the cumulants of the mutual information of the flat
Rayleigh fading amplify-and-forward MIMO relay channel without direct link
between source and destination are derived in the large array limit. The
analysis is based on the replica trick and covers both spatially independent
and correlated fading in the first and the second hop, while beamforming at all
terminals is restricted to deterministic weight matrices. Expressions for mean
and variance of the mutual information are obtained. Their parameters are
determined by a nonlinear equation system. All higher cumulants are shown to
vanish as the number of antennas n goes to infinity. In conclusion the
distribution of the mutual information I becomes Gaussian in the large n limit
and is completely characterized by the expressions obtained for mean and
variance of I. Comparisons with simulation results show that the asymptotic
results serve as excellent approximations for systems with only few antennas at
each node. The derivation of the results follows the technique formalized by
Moustakas et al. in [1]. Although the evaluations are more involved for the
MIMO relay channel compared to point-to-point MIMO channels, the structure of
the results is surprisingly simple again. In particular an elegant formula for
the mean of the mutual information is obtained, i.e., the ergodic capacity of
the two-hop amplify-and-forward MIMO relay channel without direct link.
