Dear Mohammed El-Absi

U can read this Article 

i hope will be it's helpful

Article Compact Multiband MIMO Antenna for Future Wireless Applications

Although unrealistic, you may try to optimize the pilot sequence based on the knowledge of correlation matrix.

What you observe is correct for independent Rayleigh fading channels. The channel to each BS antenna is estimated independently when there is no correlation between the channels, thus the MSE is not affected by the number of antennas.

If you would instead consider correlated Rayleigh fading, then the MSE per antenna will reduce as you add more antennas.

Here are some papers of potential interest to you:

J. Kotecha and A. Sayeed, “Transmit signal design for optimal estima- tion of correlated MIMO channels,” IEEE Trans. Signal Process., vol. 52, no. 2, pp. 546–557, 2004.

Y. Liu, T. Wong, and W. Hager, “Training signal design for estimation of correlated MIMO channels with colored interference,” IEEE Trans. Signal Process., vol. 55, no. 4, pp. 1486–1497, 2007.

E. Björnson, B. Ottersten, “A Framework for Training-Based Estimation in Arbitrarily Correlated Rician MIMO Channels with Rician Disturbance,” IEEE Transactions on Signal Processing, vol. 58, no. 3, pp. 1807-1820, 2010.

Thanks a lot! That is helpful.