Multiple Input Multiple Output - spatial multiplexing and diversity
Spatial Multiplexing: Send multiple data streams simultaneously using different spatial paths. Capacity scales with min(Nt, Nr)!
Diversity Gain: Combat fading by receiving signal through multiple independent paths. Diversity order = Nt × Nr.
Beamforming: Focus energy in specific directions for increased range or reduced interference.
Capacity Formula: C = log₂|I + (SNR/Nt)·H·H†| bits/s/Hz where H is the Nr×Nt channel matrix.
Applications: Wi-Fi 4/5/6 (up to 8×8), LTE-Advanced (up to 8×8), 5G NR (up to 256×256 massive MIMO).