Editor's Note: In part three of our expert lesson on 100G DWDM optical network transport, Eve Griliches, managing partner of ACG Research looks at the enabling technologies and modulation strategies required to promote higher-speed optical channel rates.
Read the rest of this expert lesson on 100G DWDM optical network transport:
Table of Contents
- Telecom industry prepares for 100G DWDM optical network transport
- A short history of 100G DWDM optical network transport development
- Enabling technologies for 100G DWDM network transmission and future market projections
Enabling technologies for 100G DWDM network transmission
Previous long-haul transmission systems supported internal modulation and used non-return-to-zero (NRZ) modulation formats. Both 40G and 100G optical channel rates required external modulation formats, of which the most common are amplitude, frequency and phase modulation, which is the most likely to be deployed.
Depending on the modulation format, a provider can modulate intensity, phase, frequency and polarization or any combination thereof.
A multilevel phase modulation is most interesting since it has a small sensitivity penalty, provides constant intensity and is relatively simple to generate. Examples of multilevel phase modulation are differential phase-shift keying (DPSK) and quadrature phase-shift keying (QPSK), which has four phases, two bits per symbol at about 25 GBaud rate per symbol.
Depending on the modulation format, a provider can modulate intensity, phase, frequency and polarization or any combination thereof. Each modulation scheme affects the capability to support 10G, 40G and 100G on the same fiber and same infrastructure, making decisions on modulation format for support critical. Each format has its own complexity and cost; optical signal-to-noise ratio (OSNR) sensitivity; spectral efficiency; tolerance to fiber nonlinearity; and tolerance to chromatic dispersion and polarization mode dispersion (PMD).
The Optical Internetworking Forum (OIF) is working on standardizing the modulation format for 100G at dual-polarization quadrature phase-shift keying (DP-QPSK). Dual polarization means there are two 50G optical signals; one goes up and down, the other side by side. Both signals are on the same frequency, but one is horizontal, while the other is vertical. The signals are thus polarized at 90-degree angles so that they never interact with each other. QPSK provides four phase states in the receiver, which drops the signals down to 25 GBaud signals. While DPSK is certainly a fine modulation technique, if PMD is an issue on the fiber, providers will probably deploy DQPSK where the PMD tolerance is better.
In addition, 100G performance will be improved by adding a coherent receiver, which eliminates the need for dispersion compensation at each amp site and development comes within a digital signal processor ASIC. With coherent detection, the chromatic dispersion and PMD mitigation are done electronically within the chip at the receiver end, cleaning up the signal. This requires high-speed analog-to-digital conversion followed by advanced digital signal processing (DSP).
To date, this improvement has been exciting to the optical community, as the packaging is typical CMOS, which should be a better and more cost-optimized technology that separates dynamic dispersion compensation methods. Polarization demultiplexing can be combined and included in the DSP to improve tolerance against distortion without affecting noise or performance.
Phase modulation, coherent receivers, polarization multiplexing, electronic dispersion compensation and enhanced forward error correction (FEC) all provide promising solutions to deliver 100G transmission. But there will always be the inherent trade-offs of cost, complexity and maturity of the technology and how they affect the performance of the entire transmission system and margin.
What slowed the 40G market were the multiple modulation formats proposed, which led to proprietary deployments at much higher cost points and large packaging. Each vendor chose a different format to deliver 40G, resulting in higher costs across the board for any implementation of 40G. This is not the case today. Inexpensive cost packaging with standard modulation formatting will enable lower-cost components to come to market in volume within smaller packages and with lower power generation.
100G DWDM technology constraints
Typical transmission issues and impairments arise in the gray area of operations and margin. Linear issues typically are crosstalk or filtering on the line, amplification, OSNR, PMD and dispersion compensation. These often depend on the chosen fiber type, bit rate, modulation scheme deployed and channel spacing. Increasing channel spacing to 50 GHz enables nonlinear impairments such as intrachannel effects where the colors travel at different speeds, resulting in forward mixing frequency (FWM) or pulses overlapping and causing intrachannel effects.
Research is focusing on coherent detection and electronic compensation to correct or reduce the impairments of higher bit rate transmission. However, there are different approaches: Other phases and dispersion compensation may be done in the time domain or the frequency domain. There are trade-offs with these directions, with some providing better performance but lower margin; some offering a better system margin but at higher cost points.
Read the rest of our expert lesson on 100G DWDM optical network transport development, including articles on how the telecom industry is preparing for 100G DWDM optical network transport and a history of 100G DWDM development.
About the author: Eve Griliches, managing partner of ACG Research, has extensive experience in technology product management and the telecommunications industry. She was IDC program director for the Telecommunications Equipment group, where she provided in-depth analysis on many key technologies in the telecom market. Her product management experience at network equipment vendors include Marconi (Ericsson), PhotonEx, Nortel Networks, Bay Networks and Welfleet. She can be reached at firstname.lastname@example.org.
This was first published in April 2010