Scalable and Compact Fully-Connected Network based on Gm-Cell Grid for 28-GHz Multi-stream MIMO Receiver

This paper presents a scalable and compact 28-GHz 4-element 4-stream CMOS fully-connected receiver designed for multi-beam multiple-input multiple-output (MB-MIMO) operation, where high beam-to-beam (B2B) isolation enables concurrent in-band communication. To overcome the scalability and crosstalk issues of conventional architectures, a 2D Gm-cell grid-based network is implemented. This design employs variable-gain phase shifters with a current-bleeding technique to maintain constant output impedance, while differential input/output lines are utilized to suppress capacitive crosstalk, achieving over 25 dB of B2B isolation. The fabricated chip occupies 5.33 mm² and demonstrates a total data rate of 6.4 Gb/s in a 4-stream MB-MIMO over-the-air test using four different modulation schemes. Notably, the fully connected network occupies only 3.49% of the total core area, highlighting exceptional area efficiency. Overall, the proposed architecture provides a compact yet high-performance solution for scalable MB-MIMO receivers, achieving superior B2B isolation and robust multi-stream operation.