How does MIMO spatial multiplexing increase the capacity of a wireless link?

MIMO (Multiple-Input Multiple-Output) spatial multiplexing increases the capacity of a wireless link by transmitting independent data streams simultaneously on the same frequency from multiple transmit antennas to multiple receive antennas, using the spatial dimension (different propagation paths) to multiply the data rate without requiring additional bandwidth. The fundamental principle is: in a rich scattering environment (such as an urban or indoor channel), the signals from different transmit antennas travel along different paths (due to reflections, diffractions, and scattering from objects in the environment) and arrive at the receive antennas with different amplitude and phase combinations. If the propagation paths are sufficiently different (spatially uncorrelated), the receiver can mathematically separate the overlapping signals and decode each independent data stream. The capacity increase is: C_MIMO = min(N_tx, N_rx) x C_SISO, where C_SISO is the single-antenna capacity, N_tx is the number of transmit antennas, and N_rx is the number of receive antennas. For a 4x4 MIMO system (4 transmit, 4 receive antennas): the theoretical capacity is 4x that of a single-antenna system, effectively quadrupling the data rate in the same bandwidth. In practice: the actual multiplexing gain depends on the channel rank (the number of independent spatial paths available), which depends on the scattering environment. In a rich-scattering indoor environment: the channel rank is typically min(N_tx, N_rx) (full rank). In a line-of-sight rooftop-to-rooftop link: the channel rank may be 1 (no multiplexing gain) because all paths are correlated. The RF front-end requirements for spatial multiplexing include: independent and calibrated transmit and receive chains for each antenna (each chain has its own PA, LNA, mixer, and ADC/DAC), antenna spacing of at least lambda/2 (half-wavelength) to ensure spatial decorrelation between antenna elements, and sufficient linearity and dynamic range to handle the combined signal power from multiple streams.