  In this paper, a two-stage channel estimation scheme for two-way MIMO relay
systems with a single relay antenna is proposed. The backward channel is
estimated by using linear minimum mean square estimator (LMMSE) at the first
stage, where the optimal training signal is designed. We then mainly focus on
the forward channel estimation by using singular value decomposition (SVD)
based maximum likelihood method, and the related training signal is proposed.
We note that the forward channel estimator is nonlinear and by analyzing the
asymptotic Bayesian Cramer-rao Lower Bound (BCRLB), we seek BCRLB as the
criterion for training signal design. Finally, the numerical results show that
the proposed training signal can improve the MSE performance.
