Why C-RAN fronthaul is a big challenge to existing network infrastructure technologies

Building C-RAN fronthaul on dark fibre will not be a practical solution as rollout reaches the next level. Technologies such as MPLS and OTN won't match requirements either. What's required is active fibre monitoring, writes Michael Ritter.

Cloud or Centralised Radio Access Network (C-RAN) is a new cellular network architecture promising to boost efficiency and lower costs when deploying public access small cell RANs. The C-RAN architecture separates the Base Band Unit (BBU) from the Remote Radio Head (RRH) using the Common Public Radio Interface (CPRI). Centralising BBUs at macro cell sites or central office locations enables more precise spectrum control and interference coordination while reducing power consumption, footprint and complexity of small cells deployed at street level or in indoor locations.

The clear advantage of C-RAN has resulted in first live network implementations in dense environments such as stadiums, malls, tall buildings and city centres. The increased flexibility and efficiency on the radio access side however comes at the expense of fronthaul connectivity challenges. CPRI is a digitised, serial radio interface with capacities in the range of multiple Gigabits per second, well beyond the data rate of the user traffic it actually carries. In addition, CPRI has very strict requirements on transmission latency and jitter in order to guarantee faultless system operation and therefore superior user experience.

Challenges of dark fibre
The requirement for Gigabits per second transmission capacity with strict low-latency performance between pooled BBUs and distributed RRHs has driven mobile network operators to build their first C-RAN deployments using dedicated, direct fibre connectivity – often referred to as dark fibre. Building on dark fiber is a viable method, but is not practical in the future when the public access small cell rollout reaches the next level.

As opposed to network infrastructure technologies such as Optical Transport Network (OTN) and Multi-Protocol Label Switching (MPLS), dark fibre is difficult to manage and troubleshoot. The missing monitoring capability of dark fibre networks does not support detecting faults and service impairments. In many cases, a fibre break cannot be isolated from a failure caused by the RRH. Contrary, infrastructure technologies such as OTN and MPLS cannot be applied due to their system latency, jitter performance and phase shift characteristics that do not support the CPRI specification. The significantly increased connectivity cost compared to dark fibre would be prohibitive due to the cost-sensitivity of CPRI-based fronthaul.

Mobile network operators now have complete visibility of their leased or self-provided fibre access infrastructure and can isolate fibre breaks from failures caused by network elements

Active fibre monitoring
However, there are solutions coming to the market that meet the simplicity and cost challenge when connecting RRHs to centralised BBUs. Active fibre monitoring based on an advancement of Optical Time-Domain Reflectometer (OTDR) technology enables network operators to actively monitor their fibre plant during provisioning and while in service. Mobile network operators now have complete visibility of their leased or self-provided fibre access infrastructure and can isolate fibre breaks from failures caused by network elements.

In the same way, dark fibre providers can now offer managed dark fibre or wavelength services to their mobile network operator customers, assigning strict Service Level Agreements (SLA) to their offering. Active fibre monitoring is a straightforward solution that solves the operational problems and meets the cost challenges of volume deployment in access networks.

Michael Ritter is VP of Technical Marketing for ADVA Optical Networking.

For More on C-RAN on The Mobile Network:
Small cells or Cloud RAN? Do you understand the trade-offs?
Getting the Best Out of Cloud RAN in LTE
NTTDoCoMo proposes new LTE-A based C-RAN
Cloud RAN pros, cons and disruptive potential