Recent advances in analog/RF interference mitigation for massive MIMO receivers

Digital MIMO receivers featuring digitization at each element are critical for (massive) MIMO applications since they support complex space-time signal processing. However, the lack of spatial selectivity in the analog/RF domain necessitates high-dynamic-range analog-to-digital converters (A/Ds) to accommodate the uneven spatial power distribution, limiting the scale of such MIMO systems. This paper reviews analog/RF spatial filtering techniques that have been proposed in recent years to address this problem. A scalable spatial notch suppression technique allows the synthesis of a steerable spatial notch, which eliminates the strongest in-band spatial block in the analog/RF domain. The spatial notch is synthesized in the baseband, and the impedance transparency of passive mixers is used to translate the notch to the antenna interface. In a second prototype, a more general arbitrary spatial filter (ASF) adaptively filters the spatial domain signals to equalize the power levels across all directions. Current mode operation ensures superior linearity and ultra-wideband spatial suppression. Measurements from CMOS prototypes have been provided to verify the claims.