What is the difference between SU-MIMO and MU-MIMO in terms of RF design requirements?

The difference between SU-MIMO (Single-User MIMO) and MU-MIMO (Multi-User MIMO) in terms of RF design requirements primarily affects the base station (access point) antenna system, the calibration requirements, and the linearity demands on the RF front end. SU-MIMO transmits multiple data streams to a single user using multiple antennas at both ends. The RF requirements are: the transmitter and receiver each need N_t and N_r independent RF chains (PA, mixer, LO, ADC/DAC per antenna), antenna spacing of greater than or equal to lambda/2 at both ends for spatial decorrelation, moderate calibration requirements (phase and gain matching between chains within ±2 degrees and ±0.5 dB), and linearity requirements are determined by the signal PAPR (Peak-to-Average Power Ratio), which is 8-12 dB for OFDM-based systems regardless of MIMO configuration. MU-MIMO transmits different data streams to different users simultaneously on the same time-frequency resource, using spatial separation (beamforming) to direct each user's signal away from other users. The RF requirements are: the base station needs a large antenna array (8-64+ elements in massive MIMO) with precise calibration (phase matching within ±1 degree across all elements for effective beam nulling), very tight phase and amplitude calibration across the entire array (errors in calibration directly degrade the spatial separation between users and cause inter-user interference), higher linearity requirements than SU-MIMO (because the combined signal for multiple users has higher PAPR than a single-user signal; the PA must handle the combined peak power without creating intermodulation that would leak between user beams), and the LO distribution must be phase-coherent across all antenna elements (any LO phase drift between elements degrades the beamforming accuracy and inter-user isolation).