Signal acquisition and timing for a free space laser communications receiver

Abstract

NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a novel free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We will focus here on the receiver design which consists of a distributed array of telescopes, each with a Geiger-mode Avalanche Photo Diode (APD) array capable of detecting and timing individual photon arrivals to within a fraction of a nanosecond. Using an array of telescopes has the advantage of providing a large collection area without the cost of constructing a very large monolithic aperture. A key challenge of using a distributed array receiver is combining the detected photons from each of the telescopes so that the combined system appears as a single large collector. This paper will focus on the techniques employed by the receiver to spatially acquire a deep-space downlink laser signal, synchronize the timing of all the photon arrivals at each telescope, and combine the photon detections from each telescope into a single data stream. Results from a hardware testbed utilizing this receiver concept will be shown that demonstrate an efficiency of less than one incident photon per bit at data rates up to 14 Mbps, while operating within 1 dB of the channel capacity.