5G mobile transport and the essential role of PON

Around the world, we are seeing accelerating demand for both fixed and mobile broadband -- gigabit connectivity both in the home and on the move. Next-generation fiber broadband technologies and 5G have evolved separately in response to this growing demand yet they are increasingly interdependent. In fact, when fiber and 5G join forces, they complement each other and accelerate gigabit connectivity to the benefit of all.

The complementarity of 5G and fiber creates a mutually beneficial, symbiotic partnership. 5G fixed wireless access can be used to complete the gaps in fiber-to-the-home (FTTH) deployments and increase the coverage where deploying fiber all the way to the home is difficult, costly, or takes too long. When in the home, consumers switch from the cellular network to Wi-Fi, offloading the 5G traffic to Wi-Fi and further on, to FTTH. This helps operators to better manage RAN capacity and costs, free 5G capacity for critical applications, and still provide an exceptional customer experience for consumers at home. Lastly, mobile users expect from 5G much higher speeds and universal coverage, and that needs a high-performance mobile transport network. Existing FTTH networks using passive optical network (PON) technology are perfectly placed to provide that transport. How and why is the focus of this article.

5G places new demands on mobile transport

5G is different from previous generations of mobile technology in ways that have a significant impact on mobile transport.

First, delivering flawless 5G mobile services demands a transport network that can support massive connectivity, super-high data rates, and ultra-low latency. Second, because of the shorter range of 5G, there will be much more new cell sites to ensure coverage, so new transport needs to be deployed to connect those cites. And third, 5G requires more efficient architecture, relying on centralized and distributed processing for some key functionality. That has a significant impact on the performance characteristics of the mobile transport layer. Let’s dive into that last point a little more.

With 5G densification (i.e., the increase in number of 5G cells compared to previous generations of mobile technology), it would be very cost-inefficient to keep the existing functionality in the radio network as it is. In 5G some functionality is moved from radio cells to more centralized locations, so that the resources can be shared among multiple cells. The centralized locations can further be split, so that latency-sensitive functionalities are closer to the radio unit (we call them distributed units), while less latency sensitive functionalities can be closer to the core (centralized units).

The result is a mobile transport network split into three domains, each with different bandwidth and latency requirements (Figure 1):

1. Backhaul domain is transport between centralized units and the core network. This transport has relaxed bandwidth and latency requirements.
2. Midhaul domain refers to transport between distributed units and centralized units. This domain needs more bandwidth, but latency is still pretty relaxed.
3. Finally, we have the fronthaul domain, which is the transport between radio cells and the distributed units. This domain needs very high bandwidth but also has extremely important low-latency requirements.

We refer to these three domains collectively as anyhaul.

Fiber access delivering cost efficiency to 5G

A recent survey by industry analysts IHS Markit shows that operators’ primary concern about 5G mobile transport is cost. Operators are looking at solutions that are cost-efficient, and PON is technology that delivers that efficiency. Leveraging an FTTH broadband network to provide mobile transport brings several benefits; this option is already being used for 4G and 5G anyhaul.

In most cases, FTTH networks already exist in the locations where 5G cells will be prevalent, i.e., urban and suburban areas, so they are readily available. By design, FTTH networks can connect a huge number of access points. Even in millimeter-wave 5G deployments, the number of small cells will be around 10% of the residential FTTH installed base, so it is not a stretch for FTTH to absorb these additional connection points. A Bell Labs study shows that using FTTH for mobile transport in 5G can deliver upwards of a 50% savings in total cost of ownership, compared to other transport technologies. Converged operators enjoy the biggest benefits; having their own FTTH network means no external lease costs, so they only need to fund the last few meters of connectivity from the fiber drop to the cell. Use of existing FTTH infrastructure delivers up to 60% more cost-efficiency compared with other transport solutions.

Another plus-point for FTTH is its ability to support the huge bandwidth demand that 5G creates. The capacity on PON-based FTTH networks can be easily upgraded without disruptive (and costly) infrastructure changes. In fact, fixed broadband providers are already investing in next-generation PON technologies to support demand for gigabit services to homes. Most are opting for XGS-PON, which provides 10-Gbps symmetrical speeds and which is expected to be the mainstream PON technology in just a few years. XGS-PON is already being used successfully for LTE and 5G midhaul and backhaul. However, in areas with a higher density of small cells per PON or higher throughput per cell, 25G PON will better fit the transport requirements. 25G PON relies on a mature ecosystem of optical technologies that is already being used in volumes in data centers. This factor helps 25G-PON to be cost-efficient and fast-to-deploy, making it the most attractive next step for PON.

There are other new technologies and approaches that enable FTTH to meet the stringent performance requirements of 5G anyhaul. First up is Cooperative Transport Interface (CTI), which ensures that FTTH meets the low-latency anyhaul demands. CTI enables the mobile traffic scheduler to notify the fiber access node that there is an upcoming request for mobile transport. This ability ensures that mobile traffic has priority and PON resources are allocated for the time period needed. The delay associated with the allocation process is shortened from 1500 µsec to 10 µsec. This technique has already been demonstrated in practice and is currently under standardization. The use of a custom chipset in such applications enables tailoring of the design to meet the stringent low-latency requirements of 5G anyhaul.

Network slicing, using software-defined access networking (SDAN) technology, is another enabler of efficient mobile anyhaul on FTTH networks. Slicing uses virtualization to partition a single physical infrastructure into discrete virtual networks, each with its own performance characteristics. Each slice can be allocated to a different operator or a different service. A slice dedicated to mobile anyhaul is an obvious use case and enables network operators to segregate residential and business broadband from mobile anyhaul traffic, thus maintaining the integrity of each service all while using a common infrastructure.

A perfect partnership

Mobile transport is the foundation of 5G success, and FTTH will play a key role, as it brings down both the cost and time to deploy. FTTH is ready to scale up to the density, capacity, and capabilities that the 5G era will require—even in the most dense, usage-heavy, and bandwidth-hungry locations—reliably, cost-effectively, and flexibly. As such, it takes the relationship between fixed and mobile technologies to a new level of partnership.

Ana Pesovic works within the Fixed Networks group at Nokia.