  Antennas of transmitters and receivers have been manipulated to increase the
capacity of transmission and reception of signals. Using many elements in
antennas to shape beams and direct nulls in a particular point for optimum
signal transmission and reception has over decades, had tremendous positive
influence in received power and signal to noise ratio (SNR). However, since the
antenna elements manipulation can be done both at base station and device
terminal, it gives rise to an important method of using several antennas to put
and obtain signals to and from space with increased capacity. This principle is
termed Multiple-input and Multiple-output (MIMO). This paper discusses
application of MIMO in the state of the art and next generation of wireless
systems (5G). It also discusses four models of MIMO, SISO, SIMO, MISO and MIMO,
considering three method of combing the signals from multipath propagations,
Selection combining (SC), Equal gain combing (EGC) and maximum ratio combining
(MRC). Spatial diversity and spatial multiplexing are also discussed as form of
MIMO. Finally, Massive or Hyper MIMO which is a new method of increasing
transmission capacity by very large scale for fifth generation of wireless
system is discussed with its challenges and opportunities. Key terms-Diversity
combining techniques, spatial multiplexing, channel state information (CSI).
Massive MIMO
