In the evolution of communication networks, there has always been a need to increase the capacity to cope with the continuous growing demand for data transmission. However, with the arrival of the Internet-of-Things and the commoditization of broadband access through smartphones, tablets, smart-watches, and all types of connecting devices, future networks must be capable of providing higher bandwidth and Quality of Experience, as wellas operating in dense networks with a massive number of simultaneous connections. This high number of connections will be very heterogeneous, spanning from highly-demanding data rate applications to low-complexity and high energy-efficient Machine-to-Machine communications. In such a dense and complex scenario, a more flexible use of spectrum resources is deemed to be the way to meet the growing requirements for data transmission. In particular, this article focuses on Device-to-Device communications and small cell deployments as emerging facilitators of such a demanding and heterogeneous scenario. The pros and cons of both complementary strategies are identified from both a technical and a business point of view, and main standardization activities are discussed. The aim of this article is to identify and describe open challenges and to inspire new areas for research that make viable the next generation of dense networks.Peer ReviewedPostprint (published version

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the deployment of low cost and low power base stations has been recognized in recent years as a promising cost-efficient solution and energy-efficient strategy. In this paper the femtocell and macrocell deployment options have been compared in the context of indoor mobile broadband deployment, with focus on the effect of different backhauling solutions in power consumption and the total deployment cost. This study has been based on the deployment of mobile broadband services within an area of one square kilometers in a new densely populated business district where the different user demands, spectrum bandwidths, backhaul technologies and radio access technologies are taken into account. Moreover, various deployment scenarios reflecting the business perspectives of mobile operators have been looked into as well.The main findings reached indicate that backhaul solutions contribute differently to cost and power consumption depending on the employed deployment strategy. However, contributions to the total power consumption and to the CapEx and OpEx elements of the total deployment cost elements turned to be more significant in the case of femtocell deployment scenarios than in the case of macrocell ones. It is worthy of notice that the femtocell deployment is more costefficient, especially in high demand situation when new macro sites are needed to be deployed.