CMOS Integrated Remote Antenna Unit for Fiber-Fed Distributed MIMO Systems

A fully integrated remote antenna unit (RAU) intended for fiber-fed distributed multiple-input multiple-output systems is presented. The circuit is designed for narrowband (60 MHz) time-division duplex systems, where an IF over fiber approach is chosen to facilitate the use of low-cost optical components and integrated photodetectors. A novel antenna switch control scheme is introduced, which enables the use of an integrated antenna switch instead of a bulky off chip circulator. The reference frequency signal is distributed in the fiber together with user data and used by a phase-locked-loop-based frequency synthesizer to generate the local oscillator signal inside the RAU, hence synchronizing all RAUs of the distributed antenna system. At an operating frequency of 2.1 GHz, the measured optical-to-electrical conversion gain of the downlink is 71.7 dB, the error vector magnitude is 3.2%, and the adjacent channel leakage ratio is 39.2 dBc at an output power of +3 dBm for a 16-quadrature amplitude modulation (16-QAM) long-term evolution downlink signal. The uplink has a gain of 32.5 dB, a noise figure of 3.5 dB, and an in-band third-order intercept point of -12 dBm. Implemented in a standard 65-nm CMOS process, the complete RAU occupies just 2 mm
²
of die area and consumes 146 mW during downlink signal transmission and 122 mW during uplink signal reception.