Access networks everywhere are scaling. 5G and IoT promise to interconnect exponentially more devices than before, with higher speed and latencies. This puts more pressure than ever on fixed and mobile access networks.
In fixed access, there will be a significant expansion in the deployment of fiber-to-the-home (FTTH) links and passive optical networks (PONs). Meanwhile, power-hungry business customers require capacity expansions to 100G and beyond. In the mobile sector, carriers must deliver on their 5G promises without the expensive deployment of new fiber infrastructure. The ever-increasing Internet traffic combined with the flat or declining revenue margins makes scaling up more difficult.
Coherent solutions can help cope with some of these requirements, but their size and expense made them impractical to implement in access networks. In the last few years, the miniaturization of coherent transceivers enables cost and size reductions that made this technology more accessible.
Coherent solutions are still a decade away from becoming mainstream in mobile access networks, but at least they will soon have an impact in cable networks, business services, and edge data centers. Given the continuing advances in standardization and the focus on more affordable components for shorter links, a future with coherent optics in the access network domain is upon us.
Mobile x-haul is not moving into Coherent anytime soon
The initial rollout of 5G has already happened in most developed countries, with many operators upgrading their 1G SFP to 10G SFP+ devices and deploying more wavelength division multiplexing (WDM). Mobile networks must now move to the next phase of 5G deployments: this will require installing more and smaller base stations to increase the number of devices connected to the network exponentially.
These more mature phases of 5G deployment will require operators to scale fiber capacity cost-effectively. The 25G tunable transceivers used in this new deployment phase support a typical reach of 10 km, reaching up to 15 or even 20 km with extra amplification and compensation. For now, that capacity of 25G and 10-20km distances seems to be the sweet spot for radio access network transport.As more and more 5G antennas are deployed and more 5G users are connected, traffic in radio access networks will keep growing. This increase in traffic demand will translate up through successive network layers to the core, passing through backhaul and sometimes mid-haul stages. This will require an increase in transport capacity beyond 25G, but for now 50G and 100G transceivers are being used in limited quantities by operators in support of massive MIMO and packet fronthaul. Both will remain niche products with volumes far smaller than the mainstream 25G devices.
Fixed Access is Moving into Coherent PON NetworksAs data demands continue to grow in cable networks, direct detect optical technology used in prior passive optical networks (PON) will not be enough. PON technology needs to move into the domain of 50G and 100G link capacity, and such progress will require coherent technology. Operators want to upgrade their 10G PON networks, and the industry seems to be converging into a consensus pick of 50G over lower-capacity 25G PON, as reported by LightCounting research. The moves into 50G and a potential 100G later will benefit from the broader adoption of coherent technology. 50G PON already hits the limits of direct detect technology, and even at those speeds, the devices will require additional complexity compared to typical direct detect devices. For reasons like these, Nokia predicted in a recent white paper that 50G-PON would be “more of a quantum leap than an evolution”. Anticipating such needs, the non-profit R&D organization CableLabs is pushing to develop a 100G Coherent PON (C-PON) standard. According to CableLabs, several applications justify the development of 100G PON standards and technology:
- Aggregation of 10G PON and DOCSIS 4.0
- 5G back- and mid-haul for some macro-cell sites
- Long-reach rural scenarios
- High density/high split ratio urban scenarios, such as distributed access networks (DAA)