Low Latency, Optical Back-and-Front-hauling for 5G Networks

Looking forward to the not-so-far future, wireless networks will comprise centralised processing, mixed macro and small cells deployments as well as new radio technologies in order to support very high data rates and traffic types that are characterised by connectionless and sporadic transmission of short packets such as those employed by machine to machine communication devices. In current radio access networks, there is a very clear difference between the fronthaul and backhaul. In comparison with the advent of centralised radio access networks, the difference between the fronthaul and backhaul networks has been shifted further away from the user. Subsequently, in the latest propositions for 5G architectures, they are being merged into an `xhaul' (crosshaul) network where the fronthaul and backhaul concepts no longer exist. In particular instead of using a dedicated centralised processing pool for a set of cellular access points, the idea of using centralised processing within the core network has emerged. With the use of network function virtualisation and centralised processing in data centres, cloud-RANs can be used to provide access to the distribution antenna, removing the need for backhauling and fronthauling. The cloud-RAN can perform the duties of the Mobility Management Entity and Serving Gateway and at the same time can also process each cellular access point's analogue processing using a flexible virtualised software environment. Assuming this is used along with split processing, what needs also to be addressed is the means of communication between the cloudRAN and the distribution antenna. Traditional solutions such as the common public radio interface might not be sufficient to meet all requirements. Largely, Ethernet is being proposed. When Ethernet is used, software-defined networking (SDN) can also be used to dynamically control data flows to and from the cloud-RAN, as well as providing additional benefits, such as network slicing, allowing multiple cellular operators to use the same xhaul infrastructure. This chapter, therefore, largely elaborates on xhaul networks by investigating the potential of SDN to provide an effective user experience for the services provided. The control of specific services such as billing, roaming and registration could then be sent via alternative links such as satellite links, as latency for these packets are not critical, resulting in reduced packet delay on the data plane. It is apparent that for Gbps wireless connectivity, targeted by 5G, the data rate requirements on the centralised cloud xhaul link will be in the range of several Gbps with a latency requirement close to 1 ms.