Diversity technique can be used to improve system performance in fading channels. Instead of transmitting and receiving the desired signal through one channels, we obtain L copies of the desired signal through M different channels. The idea is that while some copies may undergo deep fades, others may not. We might still be able to obtain enough energy to make the correct decision on the transmitted symbol. There are several different kinds of diversity which are commonly employed in wireless communication systems that are shown below:

Space Diversity: This method is used to achieve diversity is to use M antennas to receive M copies of the transmitted Signal. The antennae should be spaced far enough apart so that different received copies of the signal undergo independent fading. In this method, no additional work is required on the transmission end and no additional bandwidth or transmission time is required. This special diversity can be employed to combat both frequency selective fading and time selective fading.

Frequency Diversity:  This method is used to achieve diversity is to modulate the information signal through M different carriers. Each carrier should be separated from the others by at least the coherence bandwidth (Δf)c so that different copies of the signal undergo independent fading. At the receiver, the L independently faded copies are optimally combined to give a statistic for decision. Frequency diversity can be used to combat frequency selective fading.

Time Diversity: This method is used to achieve diversity is to transmit the desired signal in M different periods of time, i.e., each symbol is transmitted M times. The intervals between transmissions of the same symbol should be at least the coherence time (Δt)c so that different copies of the same symbol undergo independent fading. Error control coding, together with interleaving can be an effective way to combat time selective (fast) fading.