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Rightsizing Pluggable Optics for Mobile Fronthaul

Telecom companies like yours are preparing for the imminent arrival and rollout of 5G. A vital part of your modern 5G network will be through deploying stable, cost-effective Centralized Radio Access Network (C-RAN) infrastructure. Designing, building, and deploying a C-RAN is a significant undertaking, and will form a big part of your capital expenditure. This is particularly true for the “Fronthaul,” a part of the C-RAN telecoms architecture that links together your centralized radio controllers and the radio heads at the edge of your cellular network.

C-RAN and Fronthaul are changing how telecoms networks operate and driving the need to upgrade or replace existing infrastructure.

The Need to Upgrade Telecoms Transceivers

 

Traditional transceivers are not optimized for new C-RAN and Fronthaul requirements. As DWDM technology becomes more popular, older transceivers do not have enough optical power or efficiency to function well. This results in a significant waste of resources and increased OPEX costs.

Instead, telecoms network operators need newer, hybrid transceivers that have the right features, capacity, and efficiency to deal with the changes caused by upgrading C-RAN and Fronthaul technology.

 

Link Budget and Link Loss

On old network configurations, the most notable source of link and signal loss was distance attenuation. The efficiency and operating parameters of transceivers only considered distance attenuation when calculating how to optimize links.

Distance Attenuation is a reduction in optical signals as they travel along a fiber. It is caused by impurities in the fiber, bending of the cable, and other factors.

5G fronthaul relies heavily on DWDM multiplexing techniques. This technology enhances fiber efficiency but increases link loss. Even a powerful transceiver that operated over 40 km in an older network is unlikely to have the link budget to perform over the same distance in a 5G C-RAN network.

Chromatic Dispersion means that light spreads and pulses as it moves through a fiber. If these overlap, this can cause errors in the signal, leading to information loss.

Chromatic Dispersion Issues
In addition to link loss, telecom network operators must also monitor and manage chromatic dispersion levels. Previously, chromatic dispersion was only a concern over large distances.  As network demand and speed increases, the tolerance for chromatic dispersion decreases very quickly.

The higher bandwidths demanded by 5G makes chromatic dispersion a significant issue.

The Need to Replace or Upgrade Optical Transceivers

 

The combination of link loss and chromatic dispersion means that optical transceivers must be upgraded or replaced as part of a C-RAN and Fronthaul rollout for 5G. A smart upgrade policy will help you to reduce your CAPEX and OPEX costs while providing the network capacity and speed that your customers demand.

Fronthaul Solutions for Optical Transceivers

As an example, we’ll explore the use of:

  • A traditional DWDM transceiver.
  • A typical DWDM Fronthaul access network.
  • A span of 20 km.

For DWDM, transceivers at the lowest budget claim 40 km as a maximum operating distance. The transceiver has a link budget of approximately 14 dB and a chromatic dispersion tolerance of approximately 800ps/nm.

We expect a transceiver marketed at 40km to have more than enough power to feed a 20 km connection. Unfortunately, with the high link loss expected of modern Fronthaul, that is not the case, as the table below shows:

Figure 1: Fronthaul Link loss

5G Fronthaul 20 km DWDM Link Loss
Distance Attenuation 0.25 dB/km 5 dB
DWDM Mux/Demux Insertion Loss 6 dB per module 12 dB per link
Fiber Splicing Estimated 1 dB
Connectors and Patch Panels Estimated 1.5 dB
Margin of Error / Fiber Aging 3 dB
Total Loss 22.5 dB
40km Transceiver Link Budget 14 dB
Net Result -8.5 dB
20 km DWDM Chromatic Dispersion
10Gb/s chromatic dispersion 17 ps/nm/km 340 ps/nm
40 km Transceiver 800ps/nm
Net Result 460 ps/nm

In summary, 40km transceivers have no issues dealing with 20km chromatic dispersion levels at 10Gbps. Critically though, the signal loss introduced by WDM Mux/Demuxers is too high for the transceiver’s link budget, resulting in significant issues.

Adding a Transceiver with More Power and Link Budget

To overcome this challenge, a solution might be to upgrade transceivers to a higher budget option, such as 80km transceivers with a 24dB link budget.

Here are the results:

5G Fronthaul 20 km DWDM Link Loss
Distance Attenuation 0.25 dB/km 5 dB
DWDM Mux/Demux Insertion Loss 6 dB average for each input 12 dB
Fiber Splicing Estimated 1 dB
Connectors and Patch Panels Estimated 1.5 dB
Margin of Error / Fiber Aging 3 dB
Total Loss 22.5 dB
40km Transceiver Link Budget 24 dB
Net Result +1.5 dB
20 km DWDM Chromatic Dispersion
10Gb/s chromatic dispersion 17 ps/nm/km 340 ps/nm
40 km Transceiver 1400ps/nm
Net Result 1060 ps/nm

In summary, the 80km transceiver with a 24dB link budget will function without issues. However, it would also be an overkill in terms of chromatic dispersion. Deploying this type of transceiver means you are paying a premium for high dispersion tolerance levels that would go to waste.

Fronthaul DWDM access networks have up to 40 connections per macro cell site, up to 80 DWDM transceivers. The price of a transceiver has a significant impact on both CAPEX and OPEX when you’re buying 80 of them.

Alternative Options for Fronthaul-Ready Transceivers at a Reasonable Cost

Right-sizing your transceivers is vital for balancing network effectiveness with budgetary requirements and customer demands. Your infrastructure should be flexible enough to scale to the new needs of your network.

One option is “Hybrid” transceivers. These operate between the traditional 40km and 80km modules and have a 24dB link budget coupled with 800ps/nm chromatic dispersion tolerance. They are designed for modern Fronthaul, C-RAN deployment. They deliver considerable CAPEX savings to operators because they are right-sized to perfectly meet modern

Modern filters can also provide a significant advantage. The latest generation of passive AAWG Gaussian WDM filters produce so little insertion loss, that existing 14dB transceivers will have enough power to operate across 20km links. While Gaussian models are more expensive than regular models, they save on TCO through avoiding larger expenses incurred by upgrading or replacing transceivers.

Here at LamdaGain, we’re experts at designing, deploying, sourcing, and consulting on telecoms networks. Stay up to date on AAWG Gaussian WDM, Fronthaul, C-RAN and transceivers through our blog. We can help you save time, money, and energy with your next network upgrade or deployment. Reach out to us to get more information! Contact us

Leo

Technology Evangelist