Bandwidth Demand and Transceiver Options
Optical fiber in cellular infrastructure was first deployed in the 80s -- much the same time that fiber was also creeping into wireline networks for the first time. Then as now, the primary driver for fiber deployment was bandwidth demand, which will soon take a great leap forward as 5G cellular replaces older generations of cellular technology. What is being proposed in the 5G project is a cellular platform that can support 10 Gbps to mobile devices (needed for 4K and 8K video) and a two-order of magnitude increase in the number of devices connected to the cellular network (needed for the Internet of Things).
Linked to this expansive 5G vision (especially the video applications) are demanding latency requirements – not exceeding 1 ms. To top it all, 5G networks are expected to be considerably more energy efficient than the cellular generations that came before.
Optical Solutions for 5G Backhaul and Midhaul: No Surprises
By common consent this kind of network upgrade can only be achieved with extensive fiberization. In the past, fiber in cellular infrastructure was used only occasionally and mostly to connect cell sites to Mobile Switching Centers (MSCs) over a mobile backhaul network. Indeed, before 3G cellular, fiber wasn’t used much at all – copper-based TDM sufficed in the backhaul.
With 5G, the backhaul network will be strategically transformed. For the backhaul, cellular carriers will now be using packet-based transport over fiber increasingly, although there are going still be some copper and radio. Fiber may now be the technology of choice in the backhaul, but in some cases, environmental, regulatory, and time-to-market considerations work against fiber. More importantly, the fiberization of the fronthaul and midhaul segments of the mobile infrastructure is proceeding apace. Midhaul is the part of the mobile infrastructure that connects remote radio heads directly to the backhaul network.
Optical backhaul and midhaul invariably use SMF transport and generally backhaul infrastructure otherwise just reflects the technology being used in metro and regional wireline networks, which are quite similar in various ways to backhaul networks:
- Some backhaul infrastructure stretches hundreds of kilometers and today most longhaul backhaul links uses 100G in CFP modules either with or without an EDFA. More typically, 5G backhaul extends over a few tens of kilometers and the choice of transceiver technology is a little broader
- That said, 100G QSFP28 is very common with the ER variant being chosen for above 20 km to 40 km. For the backhaul/midhaul application, QSFP is chosen over CFP because of the massive difference in power consumption – QSFP consumes under 4w, while CFP is almost 30w
- Optical backhaul is already taking baby steps to the next generation, with 200G QSFP56 modules being offered specifically for backhaul/midhaul applications. No doubt 400G will follow 200G into this market.
Fronthaul Optics: The Rise of 25G
Fronthaul is generally the link between the core controller and the radio head or small cell. It also needs optical networking to fulfill its functional mission. Here, the fiber connects up the remote radio heads (RRUs) and the baseband units (BBUs). The growing interest in the fronthaul is also due in part to the use of the relatively new C-RAN architectures for 5G infrastructure, which has heavy and costly bandwidth requirements, that are creating significant new opportunities for fiber optic deployments. These opportunities are not just at 10G and 100G, but also at the relatively new 25G data rate. The fronthaul network, of course, represent significant costs for the 5G service providers, but these transceiver/optical networking costs can be justified since C-RANs can reduce cell site civil engineering costs, power consumption and maintenance.
The first inclination for a 5G service provider is to try out 10G transceivers for front haul because of their very low cost. Typically, the 10G module that is deployed in this market DWDM SFP, which can extend up to 40 km in its ER variant and up to 70 km in its ZR variant. When this isn’t enough data rate, 100G transceivers may offer a solution. However, what seems to be emerging as an optimal fronthaul solution is a 25G infrastructure.
There are several reasons why 25G optics is on the rise in 5G. One is that for some fronthaul applications 100G is overkill, while 10G doesn’t offer enough bandwidth. The advantages of 25G in mobile infrastructure is notably that 25G is lower cost because it uses an SFP format (SFP28), unlike the inherently more costly QSFP28 transceivers used for 100G.
The 25G transceivers that we are talking about in the previous paragraph consist of of the garden variety of SR, ER and LR transceivers. However, there is another potential – and more interesting use of 25G technology for 5G. About a year ago, the 3rd Generation Partnership Project (3GPP) released the first version of the specification on the Ethernet Common Public Radio Interface (eCPRI) used for 5G fronthaul, which will be used for 5G fronthaul interface. Based on the current literature, eCPRI may well be implemented over 25G WDM-PON, but a standard 25G transceiver can also handle eCPRI. This networking type is likely to be used in this application because it provides important benefits including low latency, fiber savings, plug-and-play Optical Network Units (ONUs), and simplified Operation and Maintenance (O&M).
Aspects of Third Party Providers
While one could perhaps make a similar comment about other kinds of service providers, we think that 5G carriers may be especially drawn to third-party suppliers for various reasons, but there are two reasons that seem especially powerful.
- First, despite all the ballyhoo, 5G is a somewhat risky prospect, so it makes sense to keep infrastructure costs very low. Third-party transceiver suppliers can help make that so. Specifically, with 5G being risky and service providers wanting to reduce cost, they can leverage their existing infrastructure by using WDM (possibly CWDM) solutions that third party suppliers can offer. No one really knows if all the additional services that 5G promises are services that the customer really wants. We can, for example, already get HD movies on our 4G phones, so can a huge upgrade of cell phone infrastructure be cost justified so that we can get ultra-HD movies?
- Also, with 5G at such an early stage of deployment, no one really knows for sure what the infrastructure “should” look like in any given geography. This is another reason to keep costs low; some transceivers may actually end up being thrown away. Conversely, some parts of the infrastructure may have to be expanded rapidly to meet unexpected demand and third-party transceiver firms have a good reputation for getting transceivers to customers rapidly.
All of the transceivers that we discuss above are available from third-party sources and we suspect that 5G service providers will use the third-party channel for at least part of their transceiver needs going forward.
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