Code-Domain Multiplexing for Shared IF/LO Interfaces in Millimeter-Wave MIMO Arrays

Millimeter-wave (mm-wave) multi-input-multi-output (MIMO) systems with the digitization of every element enable spatial multiplexing, virtual arrays for radar, and digital beamforming for high mobility scenarios. However, per-element digitization results in a formidable I/O challenge in large-scale tiled MIMO mm-wave arrays. This article analyzes a code-domain approach to multiplex signals from different elements as well as the LO on a single interface. System considerations are presented, and the approach is demonstrated through a 28 GHz four-element MIMO receiver in 65-nm CMOS. Measurements show the feasibility of digital beamforming after de-multiplexing of the baseband signals from the IF/LO interface. Each element in the array achieves 16-dB conversion gain while consuming 60 mA from 1.2 V. The IC occupies 5.75 mm2in 65-nm CMOS, achieving small size and compatibility with λ/2 × λ/2 antenna spacing due to the absence of phase shifters/combiners required in phased arrays.