  LTE in the unlicensed band (LTE-U) is a promising solution to overcome the
scarcity of the wireless spectrum. However, to reap the benefits of LTE-U, it
is essential to maintain its effective coexistence with WiFi systems. Such a
coexistence, hence, constitutes a major challenge for LTE-U deployment. In this
paper, the problem of unlicensed spectrum sharing among WiFi and LTE-U system
is studied. In particular, a fair time sharing model based on \emph{ruin
theory} is proposed to share redundant spectral resources from the unlicensed
band with LTE-U without jeopardizing the performance of the WiFi system.
Fairness among both WiFi and LTE-U is maintained by applying the concept of the
probability of ruin. In particular, the probability of ruin is used to perform
efficient duty-cycle allocation in LTE-U, so as to provide fairness to the WiFi
system and maintain certain WiFi performance. Simulation results show that the
proposed ruin-based algorithm provides better fairness to the WiFi system as
compared to equal duty-cycle sharing among WiFi and LTE-U.
