  A distance bounding system guarantees an upper bound on the physical distance
between a verifier and a prover. However, in contrast to a conventional
wireless communication system, distance bounding systems introduce tight
requirements on the processing delay at the prover and require high distance
measurement precision making their practical realization challenging. Prior
proposals of distance bounding systems focused primarily on building provers
with minimal processing delays but did not consider the power limitations of
provers and verifiers. However, in a wide range of applications (e.g., physical
access control), provers are expected to be fully or semi-passive introducing
additional constraints on the design and implementation of distance bounding
systems.
  In this work, we propose a new physical layer scheme for distance bounding
and leverage this scheme to implement a distance bounding system with a
low-power prover. Our physical layer combines frequency modulated continuous
wave (FMCW) and backscatter communication. The use of backscatter communication
enables low power consumption at the prover which is critical for a number of
distance bounding applications. By using the FMCW-based physical layer, we
further decouple the physical distance estimation from the processing delay at
the prover, thereby enabling the realization of the majority of distance
bounding protocols developed in prior art. We evaluate our system under various
attack scenarios and show that it offers strong security guarantees against
distance, mafia and terrorist frauds. Additionally, we validate the
communication and distance measurement characteristics of our system through
simulations and experiments and show that it is well suited for short-range
physical access control and payment applications.
