For understanding Diversity techniques First we need to know about Fading in wirless communication.
The uncertainty or randomness is the main characteristic of wirelesscommunication which appears in user’s transmission channel and user’s location.
Fading means the loss of propagation experienced by the radio signal on forwardand reverse links. The signals which is received by mobile terminals come from severalpropagation paths those are called multi-path propagation.
To improve the performance of those fading channels, diversity techniques areused.
In diversity technique, communication channel is supplied with multipleTransmitting and Receiving antennas. The signal is transmitted and received throughmultiple paths. As a result, the probability that all replicas of signals will fadesimultaneously is reduced considerably.
The diversity techniques are classified under three domains namely, Temporaldiversity, frequency diversity and spatial diversity. Among these, due to its efficiency interms of system resource usage (no extra power and bandwidth utilization necessary)spatial diversity with multiple transmitting and receiving antennas are most popular.Spatial diversity provides a more robust and reliable communication, and by introducingadditional degrees of freedom to the system, it provides a higher system capacity withoutrequiring any additional power or bandwidth.