Free-space optical communication is an attractive alternative to radio communication for low power, long-range communication between small devices, primarily because utilization of shorter radiation wavelengths allows for more directional transceivers. At the transmitter, increased directionality allows for reduced transmission power because a given receiver will collect a larger fraction of the radiated energy. At the receiver, increased directionality allows for greater rejection of interfering signals. In free-space optical communication, this is especially important for rejecting interference from ambient daylight. However, in many applications the receiver does not know its own position, the transmitter position, or both. In such cases, it is desirable for the receiver to detect signals from a wide field of view to allow a communication link to be established with little a priori geometric information. Thus, when designing the directionality of a receiver, a tradeoff exists between the ability to reject interfering signals and the ability to allow communication over a broad set of geometric configurations.
Imaging receivers alleviate this tradeoff by arranging an array of highly directional receiver elements to form a compound receiver with a wider total field of view. In addition, imaging receivers provide other benefits, such as the ability to simultaneously receive transmissions from different directions without any other provisions taken to prevent the signals from interfering with one another.
Most of the benefits of imaging receivers improve as the resolution of the receiver array is increased. To implement high resolution optical imaging receivers, this dissertation proposes leveraging CMOS integrated circuit technology to fabricate the photodetectors and all signal processing circuits necessary for an imaging receiver on a single microchip. Theoretical calculations relevant to free-space optical communication channels, optical imaging, and electrical architectures and circuits suitable for array integration are discussed. A prototype receiver is fabricated in a commercial CMOS process and tested in a 1.5 km daytime free-space link to demonstrate the feasibility of CMOS imaging receivers for free-space optical communication.
December 31, 2004
Leibowitz, B. S. (2004). CMOS Imaging Receivers for Free-space Optical Communication. United States: University of California, Berkeley.