  Sharing spectrum with a communicating incumbent user (IU) network requires
avoiding interference to IU receivers. But since receivers are passive when in
the receive mode and cannot be detected, the network topology can be used to
predict the potential receivers of a currently active transmitter. For this
purpose, this paper proposes a method to detect the directed links between IUs
of time multiplexing communication networks from their transmission start and
end times. It models the response mechanism of commonly used communication
protocols using Granger causality: the probability of an IU starting a
transmission after another IU's transmission ends increases if the former is a
receiver of the latter. This paper proposes a non-parametric test statistic for
detecting such behavior. To help differentiate between a response and the
opportunistic access of available spectrum, the same test statistic is used to
estimate the response time of each link. The causal structure of the response
is studied through a discrete time Markov chain that abstracts the IUs' medium
access protocol and focuses on the response time and response probability of 2
IUs. Through NS-3 simulations, it is shown that the proposed algorithm
outperforms existing methods in accurately learning the topologies of
infrastructure-based networks and that it can infer the directed data flow in
ad hoc networks with finer time resolution than an existing method.
