  To exploit users' heterogeneous data demands, several mobile network
operators worldwide have launched the mobile data trading markets, where users
can trade mobile data quota with each other. In this paper, we aim to
understand the importance of data trading market (DTM) by studying the users'
operator selection and trading decisions, and analyzing the operator's profit
maximizing strategy. We model the interactions between the mobile operator and
the users as a three-stage Stackelberg game. In Stage I, the operator chooses
the operation fee imposed on sellers to maximize its profit. In Stage II, each
user chooses his operator. In Stage III, each DTM user chooses his trading
decisions. We derive the closed-form expression of the unique Nash equilibrium
(NE) in Stages II and III, where every user proposes the same price such that
the total demand matches with the total supply. We further show that the Stage
I's problem is convex and compute the optimal operation fee. Our analysis and
numerical results show that an operator with a small initial market share can
increase its profit by proposing a DTM, which is in line with the real-world
situation in Hong Kong.
