An ultra-broadband compact mm-wave butler matrix in CMOS for array-based MIMO systems

This paper presents the design scheme of an ultra-broadband compact mm-wave Butler Matrix. The design employs new transformer-based swapped-port couplers and lumped LC-based π-network phase shifters to achieve an extremely compact chip size and ultra-broad bandwidth. This scheme is implemented as a 4×4 Butler Matrix in a standard 65nm CMOS technology with a core area of 0.335×0.215 mm² and a broad bandwidth of 9.8 GHz with the center frequency of 63 GHz. This Butler Matrix design also achieves small insertion loss of 2.77 dB and high-quality matching among the ports with less than 0.6 dB amplitude mismatch and less than 5° phase imbalance at 63 GHz. Based on the measured S-parameters, the four concurrent electrical array patterns of the Butler Matrix achieve its array peak-to-null ratio of better than 17dB between 57 GHz and 67 GHz.