  Facing the challenge of meeting ever-increasing demand for wireless data, the
industry is striving to exploit large swaths of spectrum which anyone can use
for free without having to obtain a license. Major standards bodies are
currently considering a proposal to retool and deploy Long Term Evolution (LTE)
technologies in unlicensed bands below 6 GHz. This paper studies the
fundamental questions of whether and how the unlicensed spectrum can be shared
by intrinsically strategic operators without suffering from the tragedy of the
commons. A class of general utility functions is considered. The spectrum
sharing problem is formulated as a repeated game over a sequence of time slots.
It is first shown that a simple static sharing scheme allows a given set of
operators to reach a subgame perfect Nash equilibrium for mutually beneficial
sharing. The question of how many operators will choose to enter the market is
also addressed by studying an entry game. A sharing scheme which allows dynamic
spectrum borrowing and lending between operators is then proposed to address
time-varying traffic and proved to achieve perfect Bayesian equilibrium.
Numerical results show that the proposed dynamic sharing scheme outperforms
static sharing, which in turn achieves much higher revenue than uncoordinated
full-spectrum sharing. Implications of the results to the standardization and
deployment of LTE in unlicensed bands (LTE-U) are also discussed.
