  In an attempt to utilize spectrum resources more efficiently, protocols
sharing licensed spectrum with unlicensed users are receiving increased
attention. From the perspective of cellular networks, spectrum underutilization
makes spatial reuse a feasible complement to existing standards. Interference
management is a major component in designing these schemes as it is critical
that licensed users maintain their expected quality of service. We develop a
distributed dynamic spectrum protocol in which ad-hoc device-to-device users
opportunistically access the spectrum actively in use by cellular users.
  First, channel gain estimates are used to set feasible transmit powers for
device-to-device users that keeps the interference they cause within the
allowed interference temperature. Then network information is distributed by
route discovery packets in a random access manner to help establish either a
single-hop or multi-hop route between two device-to-device users. We show that
network information in the discovery packet can decrease the failure rate of
the route discovery and reduce the number of necessary transmissions to find a
route. Using the found route, we show that two device-to-device users can
communicate with a low probability of outage while only minimally affecting the
cellular network, and can achieve significant power savings when communicating
directly with each other instead of utilizing the cellular base station.
