Arbitrary Analog/RF Spatial Filtering for Digital MIMO Receiver Arrays

摘要

Traditional digital multiple-input multiple-output (MIMO) receivers that feature element-level digitization face high instantaneous dynamic range challenges in the analog/RF domain due to the absence of analog/RF spatial filtering. Existing analog/RF spatial notch filtering techniques are limited in their noise, linearity, and spatial filtering bandwidth performance. More importantly, only single spatial notches have been demonstrated, providing insufficient filtering in practical scenarios. We propose a frequency-translational arbitrary spatial filtering technique that features not only arbitrary spatial filtering response generation at baseband for the protection of the following analog-to-digital converters, but modulated baseband input impedance that can be translated by passive mixers to achieve arbitrary spatial filtering at RF as well. This technique allows the synthesis and independent steering of an arbitrary number of spatial notches, and the independent adjustment of notch depths. Current-mode operation leads to superior linearity performance and ultra-wideband rejection. A four-element 65-nm CMOS 0.1-3.1 GHz prototype MIMO receiver array shows arbitrary spatial response formation, more than 50-dB spatial rejection across all measured directions, and an ultra-wide 320-MHz 20-dB rejection bandwidth for a single-notch setting at 500-MHz local oscillator (LO) frequency. Formation of a single spatial notch only moderately degrades the equivalent single-element double-sideband noise figure from 2.1-3.7 dB to 3.4-5.8 dB. In the notch direction, +34 dBV in-band output-referred IP3 is measured, an improvement of 33 dB compared with outside-notch directions. A wireless demonstration shows the receiver array demodulating a weak spatial signal in the presence of two strong in-band spatial signals, verifying the arbitrary spatial filtering functionality.