Today, the majority of optical networks are operating at 10Gbit/s transmission speeds, and service providers are struggling with more than a few challenges. Service providers need to manage and support rapidly increasing traffic demands driven by factors such as mobile traffic, high-speed Internet, and new high-bandwidth applications. However, a 10G network is limited in the capacity that it can support and cannot scale to meet the demands that service providers are seeing.

In many cases, operators are simply transporting the same rate that they are receiving -- 10G traffic -- so the network is not efficient, and it takes longer to turn up each service. These providers are limited in services they can offer, and cannot support new higher speed services such as 40GE, 100GE, and OTU-4. The service providers also worry about the quality of their existing older fiber, and whether it will be able to support higher capacity systems moving forward.

Operators with these problems are currently turning to 40G or 100G coherent systems to solve these challenges, to cost effectively scale while reducing capital expenses, and to simplify operations of their network.

Coherent technology is able to provide the following benefits:
• Reduce the cost per transported bit and leverage existing fiber plant and line system to carry 4 or 10 time more traffic.
• Improve operational costs by reducing space and power consumption.
• Support new higher speed OTN and Ethernet services.
• Speed up time to revenue with the ability to more quickly activate new services.

Why the Need for a New Technology?
Moving to higher speed 40G/100G technologies based on the equivalent technology implementation used at 10G (TDM) results in significant technical challenges in realizing a 40G or 100G solution that is amenable to practical deployment.

First, if using traditional TDM methods of increasing transmission rates to reach 100G, one would need components that operate 10 times faster than those who have 10G. From the beginning, there would be a dependency on these higher-speed, higher-cost, limited-supply components that have questionable reliability.

When moving from 10G to 40G via traditional TDM methods, and because the decision interval is shorter, the receiver has to process the information much faster. There is also a 6dB loss in noise margin, resulting in 75 percent reduced reach performance from a 10G system, and a resulting 90 percent reduced reach at 100G. What does this mean? To compensate for the reduced reach, operators would need to add expensive, high-powered Raman amplifiers or regens to their network. They would also require separate photonic systems for the transport of the higher line rates.

Tolerance to fiber linear impairments, in particular to chromatic and polarization mode dispersion, also significantly increases. For example, system sensitivity to chromatic dispersion (CD) is 16 times worse at 40G and 100 times worse at 100G, and sensitivity to polarization mode dispersion (PMD) is 4 times worse at 40G and 10 times worse at 100G. The result: A lot more compensators are needed in the network as well as significant engineering to properly equalize for dispersion, and in some cases of high PMD fiber the need to remove and replace fiber.

The Move to Coherent Technology
Coherent detection is a fundamental change in how optical networks operate and will perform moving forward. Because of its recognized benefits, the industry as a whole is now moving toward implementing coherent technologies for high-capacity networking.

Comparing a coherent receiver with traditional receivers is analogous to comparing a Morse code telegraph with a satellite digital radio. (See Figure 1.) Prior to coherent detection, optical receivers would simply look for and detect the presence or absence of light in the transmitted signal. A coherent receiver is a much more intelligent receiver as it operates with a local oscillator to tune into the exact frequency for which it is looking. In addition to the amplitude of the signal, this receiver also has access to phase and polarization information. Therefore, one can apply advanced modulation techniques to add more bits of information per symbol that is being received instead of having to force equipment to work 4 or 10 times faster. This is 1 characteristic of the coherent receiver that allows it to scale cost effectively for higher transmission rates.

  
Figure 1. Coherent by Analogy: Morse Code is an example of non-coherent optics as it is a dumb receiver while the digital radio is a tunable receiver with digital enhancements.

Another very important characteristic of a coherent receiver is that it has access to and is proportional to the optical electrical field (E-field). Thus, one can more easily apply advanced digital signal processing techniques to electronically compensate for linear impairments such as CD and PMD. The ensuing benefit is that the engineering, cost, and added equipment associated with trying to manually correct for signal degradations due to fiber impairments is eliminated since it is all handled internally with the equipment via software. What is even more impressive is that now links that could handle only 2.5G of traffic because of high PMD or bad quality fiber can now be re-purposed for 40G and even 100G wavelengths.

One side benefit of coherent technology is its power to increase service agility and unlock more flexible photonic architectures. The spectral selectivity characteristic of the coherent receiver, and the fact that it can tune to a specific wavelength, provides these advantages and will likely impact the way photonic systems are designed moving forward.

Having a tunable receiver enables flexible ROADM architectures that are not locked to specific wavelengths, and also allows operators to remotely switch the receiver to whichever path and whichever wavelength that is desired. That is a significant change compared to current network deployments that use fixed filters, which allow for a fixed set of wavelengths to be carried equally on a 50GHz grid. In the future, one can imagine a more flexible type of optical filter where the coherent receiver can tune to a precise frequency, enabling the optimal spectrum to be used depending on the data rate and performance required. For example, a 40G wavelength may only need 25GHz of spectrum while a 1Tbps channel may require 180GHz of spectrum. Overall, coherent technology opens up new opportunities for more advanced, flexible optical architectures.

Once tunable receivers are deployed in the future, one can then complete the wavelength filtering at the receiver, and replace optical filters with simple, low-cost power splitters. For example, 1 node transmits information across multiple wavelengths, and the tunable receiver at the other end will select the appropriate wavelength of interest. (See Figure 2.)

Figure 2. Future, more flexible architectures enabled by the tunable receiver.

Moving Beyond 100G
Using coherent technology, there are 3 dimensions one can exploit to increase system capacity. (See Figure 3.) The dimensions, which provide both benefits and drawbacks, include the ability to INCREASE:

Figure 3. These are the 3 dimensions of capacity evolution.

1. The baud rate or symbol rate of the system. This is the speed at which components will need to operate. By increasing the baud rate, one is dependent on higher-speed components being available in the market. Ideally, components that have been available for some time should be used, so that they are qualified, their reliability is proven, and that they have an appropriate production cost point.

2. The number of bits per symbol. The more bits there are in the constellation, the more traffic and higher capacity that can then be transported. However, the more bits that are introduced, the closer they are to each other in the constellation, and the less tolerance to noise that exists. As the number of bits per symbol is increased, the performance and reach of the solution is impacted negatively.

3. The number of subcarriers. This option allows the increase of capacity using more components than by increasing the baud rate, but it enables the use of more reliable, easily available, and lower-cost components. It is important to note that in a multiple sub-carrier solution, the resulting "super channel" operates and is managed by the system as a single channel. Moreover, the use of super channels will permit the realization of next-generation systems at 400 Gb/s and 1Tb/s with current technologies. The spectral selectivity capability of the coherent receiver is used to properly extract information from each of the subcarriers.

In the design of a new high-capacity system, each of the 3 dimensions mentioned must be exploited appropriately in order to optimize for spectral efficiency, performance, cost and reliability.

A Promise Fulfilled
As a whole, 40G coherent systems have been widely deployed since mid-2008 and service providers are now moving from a trial phase to the deployment phase with 100G coherent systems. These deployments are rapidly proving the distinct benefits of coherent networking for service providers, including the following:
• A fourfold or tenfold increase in traffic carrying capacity, with the ability to leverage existing infrastructure and fiber.
• The ability to turn up services faster, resulting primarily from larger-sized wavelength trunks which allows operators to more quickly deploy 10G services as needed.
• The support for new higher bandwidth OTN and Ethernet services, allowing service providers to expand their service offering.
• A lower CapEx, simpler engineering, and reduced latency resulting from the electronic dispersion compensation integrated in the transponders -- eliminating the requirement for fixed compensators and their associated amplifiers in the network.

Coherent technology is a transformative technology for optical networks, and is essential in order to scale capacity to terabit levels and beyond while promising new advanced capabilities in network programmability and planning.

Helen Xenos is Senior Product and Solutions Marketing Manager at Ciena Corporation and is responsible for introducing Ciena optical and Carrier Ethernet solutions to market. She has more than 17 years of experience in the telecommunications industry. For more information, please email hxenos@ciena.com or visit www.ciena.com.

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