  Following fast growth of cellular networks, more users have drawn attention
to the contradiction between dynamic user data traffic and static data plans.
To address this important but largely unexplored issue, in this paper, we
design a new data plan sharing system named Prometheus, which is based on the
scenario that some smartphone users have surplus data traffic and are willing
to help others download data. To realize this system, we first propose a
mechanism that incorporates LT codes into UDP. It is robust to transmission
errors and encourages more concurrent transmissions and forwardings. It also
can be implemented easily with low implementation complexity. Then we design an
incentive mechanism using a Stackelberg game to choose assistant users ($AUs$),
all participants will gain credits in return, which can be used to ask for
future help when they need to download something. Finally real environment
experiments are conducted and the results show that users in our Prometheus not
only can manage their surplus data plan more efficiently, but also achieve a
higher speed download rate.
