Analysis of capacity and probability of outage for free-space optical channels with fading due to pointing and tracking error

Free-space laser communications systems experience fading due to quasi-static pointing error and tracking error that impairs communications performance. Under a block fading model we show that using forward error correcting codes and interleaving - a simple non-mechanical data processing technique - much of the harmful effect of tracking error on communications performance can be removed. Using the concepts of fading capacity and outage capacity, we provide analytical tools that quantify the effects of fading on communications performance with and without interleaving. A link budget based on this analysis includes three loss terms due to pointing and tracking error: 1) static loss, which is primarily a function of pointing error; 2) capacity loss, which is the power difference between fading capacity and fade-free capacity; and 3) finite interleaver loss, which is the power difference between the probability of outage curve and fading capacity. Assuming pulse-position modulation and a Poisson channel, we derive closed-form solutions for the probability of outage of interleaved systems using a Gaussian beam with circularly symmetric Gaussian tracking error.