  The wireless medium contains domain-specific information that can be used to
complement and enhance traditional security mechanisms. In this paper we
propose ways to exploit the spatial variability of the radio channel response
in a rich scattering environment, as is typical of indoor environments.
Specifically, we describe a physical-layer authentication algorithm that
utilizes channel probing and hypothesis testing to determine whether current
and prior communication attempts are made by the same transmit terminal. In
this way, legitimate users can be reliably authenticated and false users can be
reliably detected. We analyze the ability of a receiver to discriminate between
transmitters (users) according to their channel frequency responses. This work
is based on a generalized channel response with both spatial and temporal
variability, and considers correlations among the time, frequency and spatial
domains. Simulation results, using the ray-tracing tool WiSE to generate the
time-averaged response, verify the efficacy of the approach under realistic
channel conditions, as well as its capability to work under unknown channel
variations.
