The quality of any wireless link is characterized by speed (or spectrum), range (coverage), and reliability (or security). All these parameters are improved by the Multiple Input – Multiple Output (MIMO), a multiple antenna system or a multidimensional wireless communication system. The MIMO technology exploits the use of multiple signals. Each signal is transmitted by different antennas into the wireless medium. It enables many signal paths and all these signals are received through multiple antennas.

Greater spectral efficiency, higher data rate, wider coverage, increased number of users and enhanced reliability can be achieved by MIMO technology without using additional frequency spectrum. Wi-Fi, Long Term Evolution (LTE), and many other radio, wireless and RF technologies are using the new MIMO wireless technology. Even now, there are many MIMO wireless routers in the market, and as this RF technology is becoming more widespread, more MIMO routers and other items of wireless MIMO equipment will be seen.

This chapter begins with the design trade off of high speed wireless link and need of MIMO system. MIMO system is explained with diversity concept and then types of diversity are discussed. Further the systems based on space diversity and their capacity is investigated. Finally, the MIMO system architecture, benefits and applications are presented.