We know that we normally have several independent paths from TX to Rx of different lengths, these are affected and delayed differently, Signals that start out together at the transmitter will be separated in time when they reach the receiver
Diversity doesn’t work with Line of Sight, This is exploiting Multipath rather than fighting it to gain big benefits.
Diversity gain: The SNR gain given by extracting the same information from two or more independently fading channels.
Space diversity: Multiple antennas are used to receive the signal. Antenna spacing must be such that the fading at each antenna is independent (coherence distance).
Time diversity: Signal is transmitted in different time slots (coherence time). Channel coding plus interleaving is used to provide time diversity. Not effective over slow fading channels. Space-time processing: Smart antennas with antenna combining in space AND time.
Spatial multiplexing (SM): Transmission approaches where several transmit antennas carry multiple signals independent from each other to increase data rates.
Frequency diversity: Signal is transmitted in several frequency bands (coherence BW). Not effective on flat (non-frequency selective) channels. Techniques that exploit frequency diversity include: RAKE Receivers, OFDM and equalization.
Polarization diversity: use two antennas with different polarization for reception and/or transmission. Using Cross-polarized antennas for reception of a signal that, due to multiple reflections, arrives having different polarizations.