  Security is an important issue in wireless sensor networks (WSNs), which are
often deployed in hostile environments. The q-composite key predistribution
scheme has been recognized as a suitable approach to secure WSNs. Although the
q-composite scheme has received much attention in the literature, there is
still a lack of rigorous analysis for secure WSNs operating under the
q-composite scheme in consideration of the unreliability of links. One main
difficulty lies in analyzing the network topology, whose links are not
independent. Wireless links can be unreliable in practice due to the presence
of physical barriers between sensors or because of harsh environmental
conditions severely impairing communications. In this paper, we resolve the
difficult challenge and investigate topological properties related to node
degree in WSNs operating under the q-composite scheme with unreliable
communication links modeled as independent ON/OFF channels. Specifically, we
derive the asymptotically exact probability for the property of minimum degree
being at least k, present the asymptotic probability distribution for the
minimum degree, and demonstrate that the number of nodes with a fixed degree is
in distribution asymptotically equivalent to a Poisson random variable. We
further use the theoretical results to provide useful design guidelines for
secure WSNs. Experimental results also confirm the validity of our analytical
findings.
