Beyond the Ultra-Dense Barrier: Paradigm Shifts on the Road Beyond 1000x Wireless Capacity

It has become increasingly clear that the current design paradigm for mobile broadband systems is not a scalable and economically feasible way to solve the expected future "capacity crunch," in particular in indoor locations with large user densities. "Moore's law," for example, state-of-the art signal processing and advanced antenna techniques now being researched, as well as more millimeter wave spectrum, indeed provide more capacity, but are not the answer to the three to four orders of magnitude more capacity at today's cost, that is needed. We argue that solving the engineering problem of providing high data rates alone is not sufficient. Instead, we need to solve the techno-economic problem to find both business models and scalable technical solutions that provide extreme area capacity for a given cost and energy consumption. In this article we will show that achieving very high capacities is indeed feasible in indoor environments. However, to become economically viable, approaches with radically different fundamental cost factors compared to those used in today's cellular systems are needed. To reach very high capacity we must venture beyond the ultra-dense barrier, that is, networks where the number of access points in an area is (considerably) larger than the active number of mobile terminals. In such networks area capacities of more than 1 Gb/s/m2 are perfectly feasible. The problem set encountered in such UDN is very different from conventional cellular systems and their solution requires conceptually new tools. We will address some of the fundamental aspects and performance limits, modeling of propagation, deployment and user traffic, and discuss the techno-economics of various network architectures. Finally we will summarize some of the most significant unsolved research questions in the field.