Interference Mitigation for MIMO Receivers: RF Front-end Featuring Adaptive Analog Spatial Filtering

Abstract

Multiple-Input Multiple-Output (MIMO) and beamforming systems have recently attracted increasing attention, because they allow for spatial multiplexing and diversity gains, improving link throughput and link reliability. As the spectrum becomes heavily utilized in sub-GHz bands that are favored over millimeter-wave frequencies for their propagation characteristics as well as energy-efficiency, strong mutual interference can occur. MIMO systems offer a solution for efficient use of scarce radio spectrum.In analog beamforming phased array receivers, i.e., Multiple-Input Single-Output (MISO) receiver systems, phase-shifting and possibly amplitude weighting is applied to multiple antenna signals before summing into a single output that only requires one ADC per array. Analog beamforming is a favorable method because of SNR improvement, its ability to cancel out interferers before entering the ADC, and its ability to improve the interference robustness of the receiver. However, many analog beamforming receivers lack flexibility to realize adaptive beamforming. On the other hand, digital MIMO systems are capable of advanced signal processing, achieving an improved data capacity and link reliability for realistic channel models that include propagation reflections and multi-path fading. In a digital MIMO receiver, however, element-level analog-to-digital conversion exposes the RF front-ends and following ADCs to strong in-band interferers, limiting the dynamic range of the receiver. Hybrid analog-digital approach, e.g., adding analog spatial filtering before entering the ADCs can relax its dynamic range and hence the number of required ADC bits. Chapter 2 presents a survey of recently published interference mitigation techniques for MISO and MIMO beamforming receivers. Two MIMO receiver architectures exist for interference mitigation in the analog domain: 1) a MIMO receiver featuring analog spatial notch filtering; 2) a multi-beam MIMO receiver architecture implemented using multiple parallel phased arrays.In chapter 3, we propose a new reconfigurable multi-beam architecture supporting both digital MIMO and analog spatial filtering. Spatial interference rejection is achieved using a set of flexible orthogonal beams with a programmable spatial-direction. RF/analog beamforming is realized by an improved constant-Gm vector modulator (VM). In addition to significant linearity enhancement, RF frequency range is extended in a compact and power efficient way, so that the MIMO receiver covers sub-6 GHz vii viii ABSTRACT bands in a software-defined radio fashion. A 0.7-5.7 GHz 22 nm FD-SOI prototype chip achieves >29 dB spatial filtering for a single notch and an ultra-wideband 20 dB notch suppression bandwidth of 2.3 GHz at broadside excitation at an LO frequency of 2.5 GHz. In the notches, an IIP3 of +16 dBm and B1dB of -11.5 dBm at 41 dB gain is achieved, improving IIP3 and B1dB by 35dB and 27dB, respectively, by spatial filtering. A single-element noise figure of 5.5-7 dB is achieved on the vector modulator constellation cor...