Systems utilizing multiple transmit and multiple receive antennas are commonly known as Multiple Input Multiple Output (MIMO) systems. A seminal information theory paper by Foschini and Gans of Lucent Technologies, shows that the capacity of these systems increases at least linearly with the number of transmit antennas as long as the number of receive antennas is greater than or equal to the number of transmit antennas. This unmatched capacity improvement over regular SISO systems has fuelled a huge interest in MIMO techniques, leading to the development of many forms of MIMO systems including improved diversity techniques, Space-Time Coding schemes, and spatial multiplexing approaches.

A general model of a MIMO system is shown in the figure below:

MIMO systems create multiple independent spatial signatures for transmitted data streams by virtue of different transmission paths to the receiver. These independent spatial signatures can either be exploited to provide redundancy of the transmitted data, thus improving the reliability of transmission or these can be exploited to increase the number of data streams, increasing the data rate of the system. The multiple spatial signatures can also be used for combating interference in the system. Some of the common MIMO schemes are  

• Beamforming

• Space Time Coding

• Spatial Multiplexing

Detailed overviews of MIMO techniques can be found in [1] and [2].

References 

1. A. J. Paulraj, D. A. Gore, R. U. Nabar, and H. Bölcskei, “ An Overview of MIMO Communications - A Key to Gigabit Wireless”, Proceedings of the IEEE, Vol. 92, No. 2, Feb. 2004

2. Gesbert, D.; Shafi, M.; Da-shan Shiu; Smith, P.J.; Naguib, A., “From theory to practice: an overview of MIMO space-time coded wireless systems”, IEEE Journal on Selected Areas in Communications, Volume: 21 , Issue: 3 , April 2003, Pages:281 - 302 

3. Shafi, M.; Gesbert, D.; Da-shan Shiu; Smith, P.J.; Tranter, W.H., “Guest editorial: MIMO systems and applications. 1”, IEEE Journal on Selected Areas in Communications, Volume: 21, Issue: 3, April 2003, Pages: 277 – 280

4. Shafi, M.; Gesbert, D.; Da-shan Shiu; Smith, P.J.; Tranter, W.H., “Guest editorial MIMO systems and applications. II”, IEEE Journal on Selected Areas in Communications, Volume: 21, Issue: 5, June 2003, Pages: 681 - 683.  

5. R.A. Soni, R.M. Buehrer, and R.D. Benning, An Intelligent Antenna System for cdma2000,” IEEE Signal Processing Magazine, vol. 19, no. 4, pp 54-67, July 2002.