  In this paper, we deal with the problem of maximizing the profit of Network
Operators (NOs) of green cellular networks in situations where
Quality-of-Service (QoS) guarantees must be ensured to users, and Base Stations
(BSs) can be shared among different operators. We show that if NOs cooperate
among them, by mutually sharing their users and BSs, then each one of them can
improve its net profit. By using a game-theoretic framework, we study the
problem of forming stable coalitions among NOs. Furthermore, we propose a
mathematical optimization model to allocate users to a set of BSs, in order to
reduce costs and, at the same time, to meet user QoS for NOs inside the same
coalition. Based on this, we propose an algorithm, based on cooperative game
theory, that enables each operator to decide with whom to cooperate in order to
maximize its profit. This algorithms adopts a distributed approach in which
each NO autonomously makes its own decisions, and where the best solution
arises without the need to synchronize them or to resort to a trusted third
party. The effectiveness of the proposed algorithm is demonstrated through a
thorough experimental evaluation considering real-world traffic traces, and a
set of realistic scenarios. The results we obtain indicate that our algorithm
allows a population of NOs to significantly improve their profits thanks to the
combination of energy reduction and satisfaction of QoS requirements.
