chapter 8, Fade Statistics for Lasercom Systems

Excerpt

8.1 Introduction

The application areas for laser communication, or lasercom, systems are numerous. In particular, lasercom links are being strongly considered by various branches of the military for supporting all tactical operations. The reason for this military interest mostly concerns the shortfalls of conventional radio frequency (RF) communication systems. In addition to mobility and size problems, these RF shortfalls include (i) limited bandwidth, (ii) easily targeted, (iii) jamming is relatively easy, and (iv) transmission can be intercepted. Short-range applications for nonmilitary uses include data links that are not well suited for fiber optics, such as transmitting between buildings or across streets. Lasercom links are also being considered as backup to fibers. Another area of great interest for high-data-rate capability concerns satellite lasercom links, either crosslinks between satellites in various orbits or simply downlink/uplink paths to and from the earth's surface.

The performance of a lasercom system can be significantly diminished by turbulence-induced scintillation resulting from beam propagation through the atmosphere. Specifically, scintillation can lead to power losses at the receiver and eventually to fading of the received signal below a prescribed threshold. The reliability of a lasercom system operating in such an environment can be deduced from a mathematical model for the probability density function (PDF) of the randomly fading irradiance signal. In terms of this PDF model, we define the probability of fade, also called the probability of miss, as the cumulative distribution of probability below the prescribed threshold. Hence, the probability of fade is simply one minus the probability of detection. Other fade issues involve the expected number of fades below threshold and their mean fade time.

Our analysis of fade probabilities associated with a lasercom link involves (i) line-of-sight terrestrial links, (ii) uplink paths to an aircraft/satellite, and (iii) downlink paths from a satellite/aircraft. Because the optical links may include strong irradiance fluctuation cases as well as weak fluctuation cases, we need a model that is valid in both weak and strong irradiance fluctuations. The primary PDF model we use is based on the modulation process described in Chapters 2–5.