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Dec 06 2011

Coherent Technology: Making 100 Gb/s Viable

A high-performance electro-optics engine enables performance and cost benefits that make 100-Gb/s transmission commercially viable.

The need for coherent technology

Moving from 10-Gb/s to 40-Gb/s and 100-Gb/s line speeds comes with technical challenges. As bit rates per channel increase beyond 10 Gb/s, fiber impairments that severely degrade transmission performance and signal quality also increase. Chromatic dispersion, polarization mode dispersion (PMD), optical noise and non-linear effects become significant problems. Coherent technology combines coherent detection and multi-level signal formats. The Alcatel-Lucent advanced electro-optics engine leverages the concepts of optimized signal modulation with coherent detection, allowing communication service providers (CSPs) to cost-effectively overcome fiber impairments that occur at high line speeds and making 100-Gb/s transmission commercially viable. Benefits of this solution include:

  • Electronic dispersion compensation at the receiver end
  • No need for dispersion compensation modules (DCMs) on the line
  • Accelerated electronic dispersion compensation for faster wavelength restoration
  • Extended transmission reach
  • Possibility of operating new signals over old and new fiber plants without interfering with existing signals
  • Performance and reach increase in both existing and new fiber networks

The design of the electro-optics engine also provides other advantages. Because the chipset is developed in-house, attention could be paid to design details such as minimizing the number of components. This streamlined design reduces complexity, power consumption and operating expenses as well as enhances manufacturability and reliability.

Coherent detection and modulation formats

When coherent detection is applied to optical transmission, a phase-modulated signal is sent to an optical mixer. A local oscillator, or continuous wave laser, tuned to a desired frequency on the ITU-T 50-GHz grid also sends a signal to the mixer. Beating ensures that only the optical signal operating at the same frequency as the oscillator is amplified. All other frequencies are considered not “coherent” with the oscillator and are attenuated (Figure 1). Because the local oscillator acts as a low-noise phase reference, coherent detection provides an extra 2-dB improvement in noise sensitivity compared to differential detection.

The methods used in coherent detection allow the information contained in the phase, amplitude and polarization of the optical signal to be extracted from the interference signal that is converted to the electrical domain by a bank of photodiodes for further processing. When ultra-high-speed silicon is used, coherent detection allows for more advanced Digital Signal Processing (DSP). Different phase-domain and polarization-domain modulation formats can be combined with coherent detection. However, two have emerged as the primary candidates for 40-Gb/s and 100-Gb/s optical systems:

  • PDM-QPSK: Here the signal is a combination of Polarization-Division Multiplexing (PDM) and single-carrier quaternary phase-shift keying (QPSK). The signal carries two bits per symbol per polarization. PDM-QPSK can produce 40-Gb/s signals using a symbol rate of 10 Gbaud or 100-Gb/s signals using a symbol rate of 25 Gbaud, not counting the bandwidth required for optical transport network (OTN) overhead including forward error correction (FEC).
  • PDM-BPSK: Here the signal is a combination of PDM and single-carrier binary phase-shift keying (BPSK). The signal carries one bit per symbol per polarization. PDM-BPSK can produce 40-Gb/s signals using a symbol rate of 20 Gbaud, not counting the bandwidth required for OTN overhead including forward error correction (FEC).

Architectural advantages

Although the coherent receiver architecture is more complex than in conventional solutions, it offers two key advantages:

  • The full polarization, amplitude and phase signal information transported by the optical field is retrieved. Advanced DSP can then be applied to this information to compensate for linear distortions such as chromatic dispersion and Polarization Mode Dispersion (PMD).
  • Both orthogonal polarizations of the optical signal can be used to encode multiple information bits. PDM doubles transport capacity and increases spectral efficiency. At the same time, it maintains compatibility with the 50-GHz channel spacing supported by the installed base of reconfigurable optical add-drop multiplexers (ROADMs).

Performance and reach advantages

Coherent technology improves performance and reach in three main ways:

  • Eliminates dispersion compensation modules: In traditional wavelength division multiplexing (WDM) networks, DCMs or tunable dispersion compensation modules (TDCMs) are placed at amplification sites along the line to compensate for chromatic dispersion. These modules introduce loss (meaning additional amplification is required) and non-linear distortions. Amplification and non-linear effects increase the addition of optical noise on the line, thereby reducing transmission reach.

In contrast, electro-optics with coherent technology enables electronic-based dispersion compensation at the receiver end. This removes the need for DCMs and improves total transmission reach.

  • Accelerates restoration: Conventional non-coherent technology requires an optical TDCM on the receiver side to finely tune residual dispersion. This is because in-line dispersion management techniques cannot provide enough compensation to support all network applications. However, TDCM tuning typically takes minutes, preventing fast wavelength reconfiguration when restoration is required.

In contrast, electro-optics with coherent technology uses advanced DSP algorithms, providing changes in electronic dispersion compensation within milliseconds. Faster dispersion compensation gives CSPs much greater flexibility in network reconfiguration and much faster wavelength restoration capabilities.

  • Improves signal propagation: In older fiber infrastructures based on non-coherent technology, PMD contributes significantly to signal propagation impairments. These signal distortion “penalties” prevent high-performance transmission of 40-Gb/s or 100-Gb/s signals.

In contrast, Alcatel-Lucent advanced DSP techniques used in coherent technology compensate for the effects of PMD, enabling 40-Gb/s and 100-Gb/s transmissions over any fiber network. Coherent technology also does not require differential demodulation, which adds to detection noise. That means it requires a lower optical signal-to-noise ratio to achieve a given bit error rate (BER) and thereby increases reach.

Upgrade advantages

Compatibility with existing conventional 10-Gb/s and 40-Gb/s channels with different modulation formats is an important issue when considering photonic network upgrades. The ability of new photonic technology to work with existing fiber plants is another key consideration. Alcatel-Lucent advanced electro-optics with coherent technology enables seamless upgrades of 2.5-Gb/s or 10-Gb/s networks with the following benefits:

  • Automatic, electronic-based, chromatic dispersion compensation allows new 40-Gb/s and 100-Gb/s channels to adapt to existing dispersion managed links without changing existing DCMs.
  • Highly tolerant dispersion compensation allows 40-Gb/s and 100-Gb/s channels to be added to older fiber plants that were designed without DCMs for 2.5-Gb/s transport.
  • PMD compensation allows upgrades to 40 Gb/s and 100 Gb/s in older fiber networks with high PMD values where even transmission of 10-Gb/s channels would be a challenge.

Ideal for any fiber network

Next-generation electro-optics with coherent technology provides advantages in both existing and new network deployments. Table 1 summarizes the advantages provided in each case.

Market success for 100-Gb/s coherent solutions

The 100-Gb/s coherent technology solutions have enjoyed fast market acceptance. This may seem surprising given the gradual adoption of 40 Gb/s when they first appeared on the market. But there are several reasons for it:

  • First generation 40-Gb/s solutions offered by dense wavelength division multiplexing (DWDM) vendors did not meet all of the requirements for unregenerated reach, 50-GHz compatibility, PMD and chromatic dispersion resilience and compatibility with 10-Gb/s neighboring channels. Second generation solutions were more successful, spurring upgrades to 40 Gb/s and higher speeds.
  • A 100-Gb/s PDM-QPSK single-carrier solution can be used to upgrade the existing installed base without modifications in dispersion mapping or introduction of guard bands. It also compensates for the very large PMD values of the oldest fiber infrastructures.
  • A 100-Gb/s PDM-QPSK single-carrier solution is considered the best option for the future because it uses the most suitable modulation format for 100-Gb/s transmission. Future 100-Gb/s transponders will make use of the same single-carrier modulation format, enabling backward compatibility with present Alcatel-Lucent technology through electronic configuration and common spares with the installed base.
  • Coherent solutions at 40 Gb/s and 100 Gb/s are available today. These solutions offer high capacity and transmission performance, compatibility with any type of fiber and easy deployment in existing and new networks.
  • Coherent technology complements the Alcatel-Lucent Zero Touch Photonics approach by allowing multiple channels to drop to a single receiver. This enables remote channel selection simply by tuning the local oscillator.
  • Coherent technology is based on receiver algorithms that can measure nearly all fiber parameters without affecting traffic. In addition, coherent detection with DSP allows powerful interface-testing capabilities.

Best-in-class solutions

The company offers two single-carrier coherent solutions for 40-Gb/s and 100-Gb/s applications on its photonic platforms:

  • Single-carrier PDM-BPSK with coherent detection: PDM-BPSK offers high performance over ultra-long distances. This is due to its high resilience to non-linear effects and transmission compatibility with existing 10-Gb/s non-return-to-zero (NRZ) ASK signals. Many vendors now advocate PDM-BPSK as the best solution at 40 Gb/s, especially when compared to first generation 40-Gb/s coherent transponders based on PDM-QPSK.
  • Single-carrier PDM-QPSK with coherent detection: PDM-QPSK is recognized by vendors and standards bodies as the best solution for 100-Gb/s transport. It provides ultra-high capacity, long unregenerated reach, the best resilience to fiber impairments, and compatibility with the existing 10-Gb/s and 40-Gb/s channels base.

These solutions have been deployed in numerous network configurations, increasing regional and long haul reach over several different fiber types and operating with 10-Gb/s, 40-Gb/s and 100-Gb/s mixed signals.

Continual innovation

The new Alcatel-Lucent enhanced performance 100G coherent solution leverages extensive research efforts as well as ample field and manufacturing experience. The resulting improvements in the electro-optics engine enable operators to transmit 100G wavelengths on routes up to 30 percent greater in length without the need for regeneration. This not only translates to upfront and ongoing capital expenditure (CAPEX) savings, but also reduces the total cost of ownership (TCO) through the reduced need for power, space and truck rolls. Additionally, in Greenfield applications, the same enhanced performance allows for the engineering and implementation of a less costly and less complex photonic line with a fewer number of elements to be managed, thus lowering initial first cost and TCO through the reduced need for power and space. To contact the author or request additional information, please send an e-mail to networks.nokia_news@nokia.com.

About Sam Bucci

Sam Bucci heads the Optical Networking Business Unit for Nokia’s IP/Optical Networks Business Group. Sam spends an inordinate amount of time on airplanes working with customers around the world. It is the insight he gains on his travels combined with his deep knowledge of photonic networks which makes him a recognized expert and visionary in the optical field.

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