  In this paper, while considering the impact of antenna correlation and the
interference from neighboring users, we analyze channel estimation and training
sequence design for multi-input multi-output (MIMO) two-way relay (TWR)
systems. To this end, we propose to decompose the bidirectional transmission
links into two phases, i.e., the multiple access (MAC) phase and the
broadcasting (BC) phase. By considering the Kronecker-structured channel model,
we derive the optimal linear minimum mean-square-error (LMMSE) channel
estimators. The corresponding training designs for the MAC and BC phases are
then formulated and solved to improve channel estimation accuracy. For the
general scenario of training sequence design for both phases, two iterative
training design algorithms are proposed that are verified to produce training
sequences that result in near optimal channel estimation performance.
Furthermore, for specific practical scenarios, where the covariance matrices of
the channel or disturbances are of particular structures, the optimal training
sequence design guidelines are derived. In order to reduce training overhead,
the minimum required training length for channel estimation in both the MAC and
BC phases are also derived. Comprehensive simulations are carried out to
demonstrate the effectiveness of the proposed training designs.
