May be these articles will are useful to you:

1) C. Fragouli, N. Al-Dhahir, and W. Turin, “Training-Based Channel Estimation for Multiple-Antenna Broadband Transmission,” IEEE Transactions on Wireless Communications, vol. 2, no. 2, pp. 384–391, March 2003

2) Oomke Weikert and Udo Zolzer, “Efficient MIMO Channel Estimation With Optimal Training Sequences”