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February 9, 2016

Staying in Sync with Mobile Fronthaul and the Migration to Cloud-RAN

Jon_Baldry_Web_By Jon Baldry

Director Metro Marketing

For a few years now, cloud-radio access networks (C-RAN) have been considered the next step in mobile networks. C-RAN will provide network operators with significant economic and operational advantages, including the centralization and virtualization of key network functions. In order to migrate to this new architecture, while at the same time support the massive growth of mobile data, operators need to utilize a fiber-deep access network with mobile fronthaul.

Those of us who deal with the optical transmission networks that provide the underlying plumbing to support these networks have been discussing mobile fronthaul for an equally long period. As with many new technologies, implementing these often takes quite a bit of time to come to fruition. At Infinera we first trialed mobile fronthaul in live operator networks back in 2013, which at the time was one of the first trials using active wavelength division multiplexing (WDM) technology. As pioneers in this market, we knew it would take a long time for mobile fronthaul technologies to become mainstream due to the complexity of the overall C-RAN architecture that mobile fronthaul supports.

Since 2013, we’ve been enhancing this technology, executing trials such as the joint live field trial we announced in late 2015 with EBlink in Orange France’s network, and planning for initial deployments in 2016. Those initial deployments are now starting, and we expect even larger scale deployments beginning in 2017.

As a quick recap, C-RAN involves splitting two main functions that traditionally are located within the cell site and moving one of these to a centralized location a little deeper in the network. These two functions are the remote radio head (RRH) that remains in the cell site and the base band unit (BBU) that moves to the more centralized location, a “BBU hotel” in a central office location.

Mobile Fronthaul
Fiber Connected Antennas Allow the Base Band Unit (BBU) to be Moved from the Cell Site to a BBU-Hotel, Bringing Advantages Such as Less Power and Space Consumption at Cell Sites and Migration to a Cloud-RAN (C-RAN) Architecture.

The BBU converts the desired digital signal into an analog signal ready for radio frequency (RF) transmission over the wireless network, and the RRH connects to the antenna to transmit this RF signal to end users over the air. By splitting these functions across two locations, operators can greatly reduce costs in the network through simpler and smaller cells sites and optimize the network for LTE-Advanced and 5G functions where complex multi-cell operations can be performed from a single location, the BBU hotel.

Now here’s the catch. The simplified cell site and the BBU hotel site contain multiple BBU instances that now need to communicate using an analog RF signal rather than the previous digital backhaul connection from the cell site. To enable transmission over a reasonable distance, this RF signal is digitized and transported using a protocol called common public radio interface (CPRI), creating a digitized RF domain within the transmission network that is termed mobile fronthaul, as opposed to mobile backhaul, which remains as the network from the BBU back to the mobile core.

The CPRI protocol is very sensitive to latency and transparent synchronization transfer by the underlying transmission system, so in most instances is limited to a distance of approximately 20 kilometers. This is a sufficient distance to build clusters of cell sites connected to a centralized BBU hotel location, but due to these sensitivities, the performance characteristics of the transport network become critically important. Anything other than excellent latency and sync performance quickly eats into the available distance budget or renders transmission of the CPRI protocol impossible. While mobile fronthaul might sound relatively easy to do, it is actually one of the most demanding applications within WDM networks.

Mobile Fronthaul

Mobile fronthaul networks vary hugely and as a consequence require a range of deployment scenarios, covering simple point-to-point passive WDM using WDM filters only to support colored optics in the RRH and BBU, semi-passive options that add further management functions, and active WDM options using transponders and muxponders to support systems that can’t take colored optics or that require more advanced networking functions such as ring architectures.

Over the last couple of years, Infinera has created a mobile fronthaul solution that supports the full range of deployment options. In addition, we’ve been involved in live mobile fronthaul field trials across the globe as mobile operators evaluate the technology and build their plans towards C-RAN architectures.

2016 looks to be an exciting year in the mobile fronthaul market, with initial commercial deployments and a number of trials continuing ahead of an expected larger ramp of deployments from 2017 onward. If you will be at MWC in Barcelona and want to talk to Infinera about mobile fronthaul, then contact us and we can make sure you are in sync with the latest thinking on how picking the right solution is critical to the success of a C-RAN architecture!

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