Multipath fading has for many years been a problem for mobile communication engineers, to be overcome by designing systems that have sufficient signal-to-noise ratio (SNR) or signal-to-interference ratio (SIR) margins. The fading is caused by multiple reflections in walls and ceilings (indoors) or cars and buildings (outdoors) that add constructively or destructively at the location of the terminal or base station antennas.

Base stations and/or terminals with multi-antenna configurations are expected to significantly contribute to much higher bit rates in mobile broadband systems. Higher bit rates using multi-antenna systems can, when there is rich scattering in the communication channel, be achieved 1) by increasing the modulation rate and/or reducing the coding rate as the Signal to Noise Ratio (SNR) is increased through the use of diversity combining, or 2) through the use of simultaneous partially uncorrelated communication channels (Multiple Input Multiple Output, MIMO).