A four-channel multi-input multi-output (MIMO) complex baseband receiver for spectrum and spaceaware cognitive radio applications is presented. The MIMO baseband receiver comprises a capacitive harmonic-rejection downconverting mixer (HRM) receiver and a signal-separation multi-input multi-output analog core (MAC) on a single integrated circuit. The HRM receiver performs frequency selection of the incoming RF signals by programmable spectral downconversion and filtering with minimal harmonic folding. The subsequent MAC separates the spectrally overlapping but spatially diverse signals by weighted complex matrix multiplication. The entire signal path is implemented using energy-efficient g(m)-C analog circuits with digitally controlled capacitive weighting for configurable baseband down-/upconversion ranging from -24 to +24 MHz in the HRM, and programmable spatial filtering with 4x4 complex (8x8 real) 14-bit coefficients in the MAC. Measurements demonstrate greater than 65 dB harmonic-folding rejection by the HRM, and greater than 48.5 dB spatial signal separation by the MAC. The 65 nm CMOS IC occupies 3.27 mm(2) active area, and consumes 480 mu W digital power at 45 MHz LO and 840 mu W analog power at 3 MHz baseband from a 1.2 V supply.