wireless transmission

help me wit the basic question...

what is diversity? what r different techniques ? why we need it?

Diversity is a process where two or more receiving antennas and receivers are used, mutually located far apart. By doing so, short-wave signals that suffer by fading arrive to different antennas/receivers with different levels, and the system selects the best at a given moment.
Diversity systems are also used in satellite communications where local loss is caused by rain, etc. High loss always affects one but not the other station at a given time.

Try google and Wikipedia for more details.