25G has become the mainstream in 5G fronthaul

Besides ultra-low latency, 5G applications demand high-bandwidth connections over the wireless link and through to the radio access network (RAN). On top of that, when air interfaces employ a higher number of multi-input and multi-output (MIMO) antennas or a broader spectral bandwidth, the associated fronthaul traffic grows exponentially. As a consequence, the adoption of 25Gbit/s fronthaul interfaces is quickly ramping up and will soon become a universal requirement for most 5G fronthaul deployments. And the lack of fiber resources in highly populated areas is having an impact on the optical solutions used for 5G fronthaul networks, which means operators must get the most out of each fiber.

Many operators are now worried about how to meet 5G fronthaul requirements and upgrade existing 10Gbit/s links to 25Gbit/s in the most efficient way – both for cost and fiber usage. Moving to passive DWDM 5G fronthaul with ADVA’s new 25G DWDM tunable pluggable optics can provide an answer. Let’s find out why.

The benefits of passive DWDM for fronthaul networks

Densely-populate areas have both a lack of optical fiber resources and an exorbitant cost for building or leasing fiber. That drives the adoption of DWDM technology in fronthaul networks to efficiently use existing fibers. DWDM enables low-latency transport of a high number of services over a single fiber or a fiber pair. Services are multiplexed and transmitted on different wavelengths using DWDM equipment at both link ends. It’s as if you had numerous virtual fibers.

With DWDM, multiple radio units (RU) and distributed units (DU) can be interconnected with just a single fiber or fiber pair. Furthermore, other services not related to 5G, such as enterprise and residential services, could be transported over the same fiber and with the same DWDM equipment, without contention between services.

There are different 5G fronthaul DWDM options, including passive DWDM as shown in Figure 1. They feature passive optical line systems and pluggable optical transceivers that can be plugged directly into the application device, such as the RU. Operators prefer these since they have lower cost and operational complexity than traditional active DWDM solutions. How is that possible?

Passive DWDM fronthaul solutions only use network components that are indispensable. That means optical transceivers and DWDM multiplexers/demultiplexers. They don’t use powered elements, such as transponders, optical amplifiers or ROADMs, or other elements, such as dispersion compensating units. This allows users to develop quite simple DWDM links with minimum capital and operational expenses. In return, the maximum optical link length is lower compared to traditional active DWDM solutions. The maximum length, and therefore the maximum distance between RU and DU, is determined by the optical specifications of the DWDM optical transceivers and optical transmission impairments such as fiber attenuation and chromatic dispersion.

The fiber’s chromatic dispersion is the main reach limiting factor. This limitation is much stronger for data rates higher than 10Gbit/s. Most high-end DWDM optical transceivers with line rates higher than 100Gbit/s use coherent detection technology with sophisticated digital signal processing (DSP) to handle strong dispersion distortion. However, coherent detection is not a cost-efficient solution for line rates lower than 100Gbit/s.

So what happens when upgrading links to 25Gbit/s? Does it mean passive DWDM is not an option for 25Gbit/s fronthaul networks? 

New 25G DWDM SFP28 with 40km reach 

Indeed, most commercial full C-band tunable 25Gbit/s optical transceivers are limited to a typical reach of about 10km over standard G.652 fiber. While this reach is enough for many fronthaul links, they require the use of forward error correction (FEC) on the host equipment, which was not needed for 10Gbit/s links. This not only adds a new feature request on the host, but it also adds unnecessary latency or latency asymmetries to the fronthaul. 

This has changed with ADVA’s new AccessWave25™ optical transceiver. With ADVA’s patent-pending distance-optimization innovation, this new 25Gbit/s DWDM pluggable transceiver with SFP28 form factor can operate at the full rate even without any FEC for distances below 20km. When the  FEC on the host is enabled, AccessWave25™ extends reach to 40km over standard G.652 single-mode fiber without the need for dispersion compensation or optical amplification. Moreover, AccessWave25™ optical performance enables deployments over today’s existing 10Gbit/s-based fronthaul even longer than 10km. This means that the AccessWave25™ enables a smooth and cost-efficient upgrade to 25Gbit/s DWDM fronthaul with a minimum impact on latency and existing infrastructure. Furthermore, with its extended reach, it allows a larger DU consolidation. 

AccessWave25TM innovation also provides relevant operational benefits. Let’s highlight three significant features: 

• A fully C-Band tunable interface with G.metro wavelength auto-tuning technology that eliminates cumbersome installation and commissioning procedures as well as large inventory pools
• A hardened design offering a broad range of deployment options, including outdoor operation in radio units
• An out-of-band communication channel, independent from the traffic, to monitor the status and major parameters of the remote plug

DWDM pluggable optical transceivers are a tiny part of 5G fronthaul networks. However, they can have a significant impact on their performance and cost-efficiency. That’s because they not only determine the link bandwidth but also the maximum distance between DU and RU in passive DWDM deployments. And they also play a key role in the installation and commissioning process.  With its advanced features, ADVA’s new AccessWave25™ increases the transport capacity and the maximum distance between DU and RU at minimum cost and maximum operational simplicity. Please let us know if you’d like to know more about our AccessWave25™ optical pluggable transceivers and our passive DWDM solutions for 5G fronthaul networks. We’d be glad to help you.