Impact of Terrain Features for Tactical Network Connectivity

Abstract : This document focuses on performance prediction for a mobile ad hoc network (MANET) in actual terrain, where line-of-sight (LOS) between nodes is used as a proxy for connectivity. We have analyzed three regions that span three different types of terrain and have produced connectivity maps at a variety of ranges. For each of the three sites, we also computed the average probability of link closure as a function of the distance between nodes. The three terrain types chosen differ significantly in how well a single value characterizes the probability of link closure over an extended range of separations. However, we find that even for the most challenging case, many aspects of network behavior can be characterized in terms of a single characteristic probability. The changes in individual link state will occur over a characteristic (average) length scale, and we discovered that this varies slowly as the distance between nodes increases. These results, based on extensive sampling of real terrain, suggest that many features of MANET performance can be accurately described in terms of average parameters, independent of the node separation. In particular, we modeled link closure as a birth-death process (i.e., a Markov process where the state is the number of closed links and the transitions are integer increments (births) or decrements (death)). This type of model lets us estimate the probability that an initial closed path in a network will continue to be closed under mobility, using average properties of the terrain. We found that low pair-wise connectivity does not necessarily impair overall network connectivity when nodes are static but that networks suffer dramatic degradation when nodes become mobile, even of lost LOS is the only cause of lost connectivity. This will occur for ordinary terrain of a type found across much of the world. We have also estimated the length scale over which this degradation will occur based on average features of the terrain.