Interference mitigation for MIMO receivers: RF frond-end featuring adaptive analog spatial filtering

The ever increasing demand for wireless connectivity is leading to congested radio spectrum especially in the sub-6 GHz. This makes the interference problem a major challenge for wireless systems. This thesis targets hybrid analog digital beamforming receiver architectures to exploit the potential of spatial filtering for interference mitigation.
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). A 4-element 22-nm FD-SOI CMOS prototype MIMO Rx achieves widely tunable RF operating frequency range, covering much of the sub-6GHz bands (0.7-5.7 GHz). The Rx front-end also achieves a highly blocker tolerant performance with B1dB of -11.5 dBm, a high in-band linearity (in-notch IIP3 of +20 dBm), and a wide notch suppression bandwidth. A single-element noise figure of 5.5-7 dB is achieved on the vector modulator constellation corners. The chip of 0.52 mm2 active area consumes 77-139mW at an LO-frequency of 0.7-5.7 GHz from a 0.8V supply.
We propose to characterize the interference robustness of MIMO receivers featuring hybrid analog-digital beamforming using EVM. This thesis also demonstrates the effectiveness of hybrid beamforming in interference mitigation utilizing adaptive Minimum Mean Square Error (MMSE) and Error Vector Magnitude (EVM) as an optimization criterion. Extensive EVM measurements carried out with a conductive set-up using a 4-element 22nm FD-SOI CMOS prototype MIMO receiver chip verify the performance of the adaptive MMSE algorithm. Over-the-Air (OTA) measurements with a linear 4-element dipole antenna array with half-wavelength spacing in the 2.4 GHz ISM-band quantify the improvement for real world scenarios, e.g., having a multipath propagation channel and mutual coupling between the antenna array elements. OTA results show that a rejection of 22.5 dB and 24.5 dB can be achieved on average in an in-door laboratory environment utilizing Vector Modulator (VM) constellations with 16 and 64 points, respectively.