Greening the Airwaves With Collaborating Mobile Network Operators. - George Koutitas; George Iosifidis; Bart Lannoo; Mathieu Tahon; Sofie Verbrugge; Pavlos Ziridis; Lukasz Budzisz; Michela Meo; Marco Ajmone Marsan; Leandros Tassiulas (IEEE Trans. Wirel. Commun., 2016)