Optimizing the operator’s economy in multi-tier cellular networks with underlaid D2D communications

In this paper, we consider a three-tier cellular network, consisting of a Macrocell base station, several Femtocell base stations, cellular users, and Device-to-Device (D2D) users. Each user is attached to the Macrocell or one of the Femtocells while D2D users communicate directly. The Macrocell divides the spectrum into several subchannels. The Femtocell users are allowed to communicate over the common spectrum which is allocated to one of the Macrocell users. We consider the scenario in which the Femtocells do not interfere each other while the Macrocell users experience interference from Femtocells and vice versa. Likewise, the Macrocell users would suffer from the interferences caused by underlaid Femtocells and D2D communications in the same spectrum. The average outage probability over the shared sub-channel is investigated assuming the effect of D2D users, and the closed form expressions are obtained in an interference limited scheme. Then, the object function is defined to minimize the operator’s cost function for establishing Femtocell network. The problem is generally non-convex and hence, it is solved through separation to subproblems to derive the near-optimal solutions for the intensity of Femtocells and power allocation. The analytical results are verified by the simulation results, which show that although the D2D and Femtocells improve the network performance by handling the data traffic, increasing the D2D users’ transmission power and the intensity of physical distribution of Femtocells increase the network cost function.