What are the different diversity techniques used in wireless communication?

Diversity techniques combat fading by providing multiple independent signal copies. Spatial diversity: Multiple antennas separated by λ/2 or more receive independent fading signals; MIMO exploits this. Time diversity: Same data transmitted at different times separated by more than coherence time; uses interleaving and coding. Frequency diversity: Same data on multiple frequencies separated by more than coherence bandwidth; spread spectrum and OFDM use this. Polarization diversity: Antennas with different polarizations (horizontal/vertical) provide independent channels. Combining methods: Selection combining (choose best), equal gain combining, and maximum ratio combining (MRC - optimal, weights by SNR).