The Fusion of Fronthaul and Backhaul

- Overview

What are fronthaul and backhaul in networking? Backhaul connects the mobile network to the wired network, while fronthaul describes the network architecture that connects remote base stations to the BBU. More specifically, wireless backhaul is the wireless communication system that gets data from remote locations to the main network. A baseband unit (BBU) is a telecommunications network device used to process baseband signals.

The underlying transport network provides all critical connectivity from the radio access network (RAN) to the packet core, which then connects to the public Internet. Changes in transport network architectures tend to receive less attention than developments in radio access technologies, but they are equally important. 

Distributed antenna systems (DAS), remote radio heads (RRH) and small cells, as well as the functional split between radio and data centers being discussed by standards bodies, will all be part of 5G deployments and all place different requirements on the transport networks.

- Converging Fronthaul and Backhaul into an Integrated 5G Transport Network

The term fronthaul is used to describe the connection between the cell tower radio itself (radio head or RH) and the mobile network control backbone (baseband unit or BBU). CPRI (Common Public Radio Interface) is a well-known standard for this interconnection. The backhaul is the link between the base station and the core wired network, usually fiber or coax, and in some cases a broadband, proprietary wireless link. 

Fronthaul, backhaul, and various hybrid architectures will be required to accommodate the cost-effective, backward-compatible, dense network infrastructure deployments necessary to deliver the broadband, low-latency demands of 5G systems. 

Converging fronthaul and backhaul into an integrated 5G transport network is a forward-looking concept that targets a flexible, reconfigurable, software-defined transport architecture. It envisions a single network that can support various functional divisions between antennas and packet cores. This view is consistent with the development of network functions virtualization (NFV) and cloud RAN (CRAN), which points to a data center that can be configured to support any functional split deployed in the network. At one extreme, legacy base stations and backhaul can be accommodated. At the other extreme, a densely distributed network of radio heads configured for massive MIMO can exchange compressed digitized radio samples for cloud-based processing.

- Mobile Fronthaul

Fronthaul is associated with a new and different type of radio access network (RAN) architecture consisting of a centralized baseband controller and independent radio heads installed at remote cell sites at distances ranging from several kilometers to tens of kilometers. These BBU and RU functional blocks, and the equipment that performs these functions, are located at greater distances from each other than in the mobile backhaul model.

In the fronthaul model, the RU device is now called the Remote Radio Head (RRH), but is still located at the cell site. The BBU is now relocated to a centralized and protected location serving multiple RRHs. The optical link interconnecting the new centralized BBU and multiple RRHs is called fronthaul.

Radio equipment, including Remote Radio Units (RU) and Remote Radio Heads (RRH), is fundamental in wireless infrastructure. With mobile traffic demand projected to grow exponentially over next few years, networks are being designed to deliver a greater number of connections, as well as higher bandwidth per connection.  

To support these objectives, 5G is expected to drive radio bandwidth >10x. The number of radio units deployed is also expected to grow substantially. In terms of the antenna configuration, from current 2x2 MIMO format, 5G will see all the way to 64 x 64 MIMO for millimeter wave radios.

Due to the diversity of spectrum/frequency and capacity, even LTE radios are built in a wide range of form factors, interfaces, frequency bands and power levels. With 5G, we expect to see even more diverse deployment scenarios for radio units. Some of these variations will re-format Remote Radio Head / Unit (RRH /RRU) designs from the LTE era, while others may take a form of an Active Antennae Unit / System (AAU / AAS).

- Mobile Backhaul

Mobile backhaul refers to the transport network that connects the core network and the RAN (Radio Access Network) of the mobile network.

In its simplest form, backhaul connects mobile networks to wired networks by backhauling traffic from geographically dispersed cell sites to a mobile switching office (MTSO). These links interconnecting macrocell sites (such as those where you can easily see large towers from a distance) with MTSOs are rapidly migrating from slower TDM-based T1/E1 connections to packet-based Fiber Ethernet The network link is usually connected to the macro base station through a 1Gbps physical interface.

In a typical macrocell site, a baseband unit (BBU) connects to a radio unit (RU). The former handles user and control data, while the latter generates radio signals that are transmitted over the airwaves via antennas mounted on towers.

Recently, the introduction of small cells has given rise to the concept of fronthaul, which is a transport network that connects the macrocell to the small cells. Whilst mobile backhaul and fronthaul are different concept, the term mobile backhaul is generally used to encompass both concepts. Furthermore, innovations to reduce demand on mobile backhaul sometimes involve architectural changes in the antenna and the controller. 

5G is introducing several new features such as enhanced coordinated multipoint (CoMP), dual connectivity (X2 and eX2 interfaces) and carrier aggregation that will make radio access networks (RAN) more complex to construct. Together these changes will put greater demands on the transport network and have a disruptive impact on the optical network architecture to better accommodate these requirements.

[More to come ...]