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The world of mobile communications is a rapidly moving one. Apps, smartphones, “4G,” small cells, heterogeneous networks and many more drivers have helped shape the mobile communications market in recent years and this innovation shows no sign of abating. The mobile backhaul portion of the network saw a huge revolution a few years ago with the move to high capacity Ethernet based backhaul with Synchronous Ethernet and 1588v2 Timing over Packet replacing previous synchronization schemes. But now there is a new kid on the block in the arena of transport networks that support mobile networks – mobile fronthaul. I get a mixed reaction when I discuss this with people in the industry, some know exactly what I’m talking about and others just look at me blankly as if I’ve just made the up the name. Mobile fronthaul is real and is happening now in mobile networks. It is also a key enabler in the move to Cloud-RAN, or C-RAN, architectures. This article will explain the importance of the C-RAN architecture and the drivers behind it, mobile fronthaul and some of the recent advances in mobile fronthaul solutions.
Mobile operators across the globe are evaluating, or even already migrating to, Cloud-RAN architectures, which move some of the wireless equipment traditionally located at the cell site deeper into the network at local central office sites. This new architecture has a big impact on power costs and space requirements at cell sites and the network operator’s ability to optimize the network. The power and space savings are substantial and form a large part of the business case for the migration to C-RAN. Studies published by China Mobile have shown savings of up to 30% in capital expenditures and 53% in operating expenditures with a C-RAN architecture.
Specifically with the new architecture, the baseband unit is moved back from the cell site and a number are co-located at a common location deeper in the network, sometimes referred to as a BBU hotel. The BBU is the device that converts the digital data steam for the wireless network into an RF signal. The BBU was previously located at the cell site and connected to the remote radio head over a copper connection and this is now migrating to a fiber connection to reduce power and as an enabler to C-RAN. While the copper connection was always limited to connections within the cell site, an optical connection allows longer distances to be spanned and a mobile fronthaul network to be created that connects the RRH to a remote BBU in the BBU hotel.
The previously mentioned optimization benefits are achieved as the co-located BBUs make LTE migration simpler as the X2 interface is now between co-located BBUs rather than a potentially complicated interface directly between a cell and its neighbors. Furthermore, operators can now start to load-balance across different antennas and move capacity between cells to better match the differing traffic demands at different times of the day within the local cells, creating a cloud of capacity that is deployed where and when it is needed.
This new mobile fronthaul network uses the common public radio interface protocol, which is a digitized RF signal and can be carried over a suitably enabled optical network. The protocol supports a number of line rates from approximately 600 megabits per second up to 10 gigabits per second and the majority of them line up well with today’s multi-rate wavelength division multiplexing-based optical systems. As the protocol is a digitized RF signal, it creates a higher data rate of traffic than would be seen in the corresponding backhaul network, making the economics of the fronthaul network particularly difficult. This protocol is also very sensitive to latency and requires transparent synchronization support. Both of which can be challenging for some optical networks; and therefore, special consideration has to be taken when selecting the underlying platform for a mobile fronthaul network.
Some vendors now offer passive WDM solutions to address mobile fronthaul and a small number now also offer active WDM solutions that can meet the tough latency and sync transparency requirements.
The passive solutions use WDM filters that typically support deployment in non-telecom standard environments, and can therefore be deployed in harsher environments. They offer the advantage of being completely transparent to the transmission protocol and synchronization and they don’t introduce any additional latency beyond that of the fiber. However, passive solutions put additional requirements on the optics in both the BBU and RRH as these now need to also be WDM specific wavelengths in addition to the existing CPRI and extended temperature range requirements. The network design can also be limited by the optical power available in these optics. This can be particularly challenging for ring based networks where the insertion loss of the nodes in the ring can exceed the available power budget of the optics making the design unfeasible.
Active WDM solutions can address the limitations of a passive design since they remove the need for WDM specific optics in the RRH and BBU and can extend the supportable distance by using WDM transponders or muxponders. These systems also have the advantage of being able to offer ring or point-to-point with additional protection schemes to protect the new network. But as mentioned earlier, special attention must be taken during equipment selection to key performance parameters such as transparent synchronization support and latency to ensure the CPRI protocol can be transported correctly.
Mobile fronthaul is not only beneficial to wireless operators; the move to fronthaul also opens up opportunities to fiber-based wholesalers to add fronthaul services to their product portfolio. This is especially true for those who already offer mobile backhaul services. In a realistic deployment scenario a mobile fronthaul network will overlap with mobile backhaul traffic from cell sites even further away from the core creating a hybrid domain where both traffic types will require transport over the same fiber based infrastructure.
So, mobile fronthaul is here and it is happening now. Many mobile operators are already considering its use, and as networks continue to evolve and support higher and higher traffic levels, the drivers behind these networks will continue to grow. Mobile operators and wholesalers who support them should all look at fronthaul and decide if the time is right for them to start to evaluate the architecture and fronthaul networks. Those that do should pay special attention to the capabilities of the optical platforms they plan to use to ensure that the high performance specifications can be met.
