  In the past few decades, the world has witnessed a rapid growth in mobile
communication and reaped great benefits from it. Even though the fourth
generation (4G) mobile communication system is just being deployed worldwide,
proliferating mobile demands call for newer wireless communication technologies
with even better performance. Consequently, the fifth generation (5G) system is
already emerging in the research field. However, simply evolving the current
mobile networks can hardly meet such great expectations, because over the years
the infrastructures have generally become ossified, closed, and vertically
constructed. Aiming to establish a new paradigm for 5G mobile networks, in this
article, we propose a cross-layer software-defined 5G network architecture. By
jointly considering both the network layer and the physical layer together, we
establish the two software-defined programmable components, the control plane
and the cloud computing pool, which enable an effective control of the mobile
network from the global perspective and benefit technological innovations.
Specifically, by the cross-layer design for software-defining, the logically
centralized and programmable control plane abstracts the control functions from
the network layer down to the physical layer, through which we achieve the
fine-grained controlling of mobile network, while the cloud computing pool
provides powerful computing capability to implement the baseband data
processing of multiple heterogeneous networks. We discuss the main challenges
of our architecture, including the fine-grained control strategies, network
virtualization, and programmability. The architecture significantly benefits
the convergence towards heterogeneous networks and it enables much more
controllable, programmable and evolvable mobile networks.
