DEVICE-CENTRIC ARCHITECTURES
Cellular designs have historically relied on the axiomatic role of “cells” as fundamental units
within the radio access network. Under such a design postulate, a device obtains service by
establishing a downlink and an uplink connection,carrying both control and data traffic, with
the base station commanding the cell where the device is located. Over the last few years, different trends have been pointing to a disruption of this cell-centric structure:
•The base station density is increasing rapidly, driven by the rise of heterogeneous networks.
While heterogeneous networks were already standardized in 4G, the architecture was not natively designed to support them. Network densification could require some major changes in 5G. The deployment of base stations with vastly different transmit powers and coverage areas, for instance, calls for a decoupling of downlink and uplink in a way that allows the corresponding
information to flow through different sets of nodes [5].
•The need for additional spectrum will inevitably lead to the coexistence of frequency bands with radically different propagation characteristics within the same system. In this context,[6] proposes the concept of a phantom cell where the data and control planes are separated: the control information is sent by high-power nodes at microwave frequencies, whereas the payload data is conveyed by low-power nodes at mmWave frequencies.
•A new concept called centralized baseband related to the concept of cloud radio access networks
is emerging ([7]), where virtualization leads to a decoupling between a node and the hardware allocated to handle the processing associated with this node. Hardware resources in a pool, for instance, could be dynamically allocated to different nodes depending on metrics defined by the network operator.
•Emerging service classes, described later in this article, could require a complete redefinition of the architecture. Current works are looking at architectural designs ranging from centralization or partial centralization (e.g., via aggregators) to full distribution (e.g., via compressed sensing and/or multihop).
•Cooperative communications paradigms such as cooperative multipoint (CoMP) or relaying,
which despite falling short of their initial hype are nonetheless beneficial [8], could require
a redefinition of the functions of the different nodes. In the context of relaying, for instance,recent developments in wireless network coding [9] suggest transmission principles that would allow recovering some of the losses associated with half-duplex relays. Moreover, recent
research points to the plausibility of full-duplex nodes for short-range communication in the notso-distant future.
•The use of smarter devices could impact the radio access network. In particular, both D2D
and smart caching call for an architectural redefinition where the center of gravity moves from
the network core to the periphery (devices, local wireless proxies, relays).
Based on these trends, our vision is that the cell-centric architecture should evolve into a
device-centric one: a given device (human or machine) should be able to communicate by
exchanging multiple information flows through several possible sets of heterogeneous nodes. In
other words, the set of network nodes providing connectivity to a given device and the functions
of these nodes in a particular communication session should be tailored to that specific device
and session. Under this vision, the concepts of uplink/downlink and control/data channel should
be rethought (Fig. 2). While the need for a disruptive change in architectural design appears clear, major research efforts are still needed to transform the resulting vision into a coherent and realistic proposition. Since the history of innovations [1] indicates that architectural changes are often the drivers of major technological discontinuities, we believe that the trends above might have a major influence on the development of 5G