查看文章

This article presents an efficient error-control code (ECC) and modulation for an optical communications channel. The coded modulation is designed to support NASA’s Mars Laser Communications Demonstration (MLCD)—an optical communications link from Mars to Earth that will operate at data rates of up to 49 mega-bits per second (Mbps). The MLCD laser terminal will be carried aboard the Mars Telecommunications Orbiter and is scheduled to launch in 2009.

For our purposes, the optical communications channel may be reduced to the block diagram in Fig. 1. User information u is encoded, modulated, and transmitted over an optical channel that is modeled as a Poisson point process. At the receiver, the signal is demodulated and decoded (shown as an iterative procedure), yielding estimates of the user data. Wyner [1] showed that under peak and average power constraints, negligible capacity is lost when restricting the modulation to a binary, slotted scheme. Furthermore, for peak and average power constraints typical of a deep-space link, restricting the modulation to pulse-position modulation (PPM)[2] is near-capacity achieving as well [3]. The efficiency of PPM for an optical channel has been noted elsewhere, eg,[4].