A 5Gb/s 7-channel current-mode imaging receiver front-end for free-space optical MIMO

A 7-channel diversity receiver front-end based on current-summing for broadband free-space optical (FSO) MIMO communication is presented in this paper. This diversity receiver is designed for flip-chip bonding to a custom InGaAs metal-semiconductor-metal (MSM) photodetector array. Each channel employs a low input-impedance current mirror (CM) as the input stage, which allows the implementation of direct current-summing for equal-gain combining (EGC). The summed-up current signal drives a second stage transimpedance amplifier (TIA) to generate the output voltage. Implemented in an 180 nm CMOS technology, a total gain of 58.5 dBOmega, and -3 dB bandwidth of 3.7 GHz for 0.25 pF photodiode capacitance is achieved. The power consumption for a single front-end amplifier circuit is 4.2 mW, and for the second stage TIA is 10.3 mW with a single 1.8 V supply.