CMOS "smart pixel" for free-space optical communication

Fully integrated imaging receivers present a method of low power free-space optical communication with advantages over radio frequency and single element optical communication for a variety of network scenarios. This paper discusses the theoretical performance of such receivers and the design of a single “smart pixel” for use in a 2.5 Mbps integrated CMOS imaging receiver. The receiver has a simulated input referred noise power of –51 dBm (optical) and a dynamic range greater than 30 dB (optical) when operating at 2.5 Mbps. Power consumption is less than 50 μW and the pixel fits on a 150 μm pitch in a standard 0.35 μm digital CMOS process, readily allowing fabrication of inexpensive imaging arrays as large as 64 × 64 pixels. The receiver presented covers all aspects of reception from optical detection through A/D conversion (thresholding). An asynchronous digital serial receiver could decode the resulting data stream from this receiver. A low overhead architecture for communicating decoded data packets from any pixel in the imaging array to the edge of the chip is briefly described. A prototype of this receiver has been fabricated and demonstrated to receive a –32.6 dBm optical signal at 875 kbps with a bit error rate of 74 × 10. Although this receiver demonstrates the feasibility of a smart pixel imaging receiver consuming less power than comparable RF receivers, the fundamental limits based on thermal noise suggest an opportunity to improve the minimum detectable signal by one order of magnitude over the current implementation.