Coding Strategies for Robust Link Blockage Mitigation in SATCOM

An important problem in satellite communication systems which operate in the Ka and EHF frequency bands consists of the degradation in link performance of mobile terminals due to blockage of the link to the satellite, especially in urban environments. In this paper, we focus on the hard and soft link blockage problem, and we provide a very efficient and cost effective solution via the use of fountain codes. We first introduce a hard-blockage channel model based on a composite Poisson process, which yields an excellent match to actual measured data. The soft-blockage is essentially due to signal shadowing, and we model this channel by means of the generalized Nakagami-m distribution with the parameter m in the range m = 4, or 5. This model approximates well the measured data, and it can be used as a more tractable alternative to the traditional lognormal distribution, and it has been used extensively in terrestrial wireless systems. The advantage of this model is that it covers a wide range of channel conditions that lead to shadowing, ranging from shallow to relatively deep shadowing/fading. We then consider the use of fountain codes, with focus on Raptor codes, as our main mitigation technique to combat the degradation in link performance due to hard- or soft-blockage. The performance of the receiver is obtained in terms of average probability of bit and message error as a function of bit energy-to-noise density ratio, Eb=No, and other parameters of interest, via a combination of analysis and computer simulations, and we show that the use of fountain codes is extremely effective in overcoming the effects of hard- and soft-link blockage.