Abstract
This article presents a two-layer low-complexity fully connected (FC) RF/analog weighting multi-input–multi- output (MIMO) transceiver comprising an RF weight layer and an analog–baseband weight layer. The transceiver can serve as a small-scale MIMO system by itself and can serve as the building block in a larger hybrid MIMO system. The architecture mitigates the complexity versus spectral-efficiency tradeoffs of existing MIMO architectures and enables MIMO stream/user scalability, superior energy efficiency, and spatial- processing flexibility. A compact, reconfigurable bidirectional circuit architecture is introduced, including a new Cartesian- combining/splitting beamforming receiver/transmitter, dual-band bidirectional beamforming network, dual-band frequency trans- lation chains, and baseband Cartesian beamforming with an improved programmable gain amplifier design. A 28-/37-GHz band, two-layer, eight-element, four-stream hybrid MIMO trans- ceiver prototype is designed in 65-nm CMOS to demonstrate the above features. The prototype achieves accurate beam/null- steering capability, excellent area/power efficiency, and state-of- the-art TX/RX mode performance in two simultaneous bands while demonstrating multi-antenna (up to eight) multi-stream (up to four) over-the-air spatial multiplexing operation using the proposed energy-efficient two-layer hybrid beamforming scheme.