  Recently, the mobile network operators (MNOs) are exploring more time
flexibility with the rollover data plan, which allows the unused data from the
previous month to be used in the current month. Motivated by this industry
trend, we propose a general framework for designing and optimizing the mobile
data plan with time flexibility. Such a framework includes the traditional data
plan, two existing rollover data plans, and a new credit data plan as special
cases. Under this framework, we formulate a monopoly MNO's optimal data plan
design as a three-stage Stackelberg game: In Stage I, the MNO decides the data
mechanism; In Stage II, the MNO further decides the corresponding data cap,
subscription fee, and the per-unit fee; Finally in Stage III, users make
subscription decisions based on their own characteristics. Through backward
induction, we analytically characterize the MNO's profit-maximizing data plan
and the corresponding users' subscriptions. Furthermore, we conduct a market
survey to estimate the distribution of users' two-dimensional characteristics,
and evaluate the performance of different data mechanisms using the real data.
We find that a more time-flexible data mechanism increases MNO's profit and
users' payoffs, hence improves the social welfare.
