Explain MIMO system capacity and how spatial multiplexing achieves it.

MIMO (Multiple-Input Multiple-Output) uses multiple antennas at transmitter and receiver. Channel capacity scales with min(Nt, Nr) in rich scattering: C = log2(det(I + (SNR/Nt)*H*H^H)) where H is channel matrix. Spatial multiplexing transmits independent data streams on different spatial modes (eigenmodes of channel), achieving multiplexing gain. Requires channel knowledge for precoding (SVD decomposition) or spatial separation (V-BLAST detection). Trade-off between multiplexing gain (rate increase) and diversity gain (reliability). Massive MIMO scales to hundreds of antennas for 5G capacity improvement.