  In this paper, we study a broker-based TV white space market, where
unlicensed white space devices (WSDs) purchase white space spectrum from TV
licensees via a third-party geo-location database (DB), which serves as a
spectrum broker, reserving spectrum from TV licensees and then reselling the
reserved spectrum to WSDs. We propose a contract-theoretic framework for the
database's spectrum reservation under demand stochasticity and information
asymmetry. In such a framework, the database offers a set of contract items in
the form of reservation amount and the corresponding payment, and each WSD
chooses the best contract item based on its private information. We
systematically study the optimal reservation contract design (that maximizes
the database's expected profit) under two different risk-bearing schemes:
DB-bearing-risk and WSD-bearing-risk, depending on who (the database or the
WSDs) will bear the risk of over reservation. Counter-intuitively, we show that
the optimal contract under DB-bearing-risk leads to a higher profit for the
database and a higher total network profit.
