To simultaneously mitigate the linear and nonlinear channel impairments in high-speed optical communications, we
propose the use of non-binary low-density-parity-check-coded modulation in combination with a coarse backpropagation
method. By employing backpropagation, we reduce the memory in the channel and in return obtain significant
reductions in the complexity of the channel equalizer which is exponentially proportional to the channel memory. We
then compensate for the remaining channel distortions using forward error correction based on non-binary LDPC codes.
We propose non-binary-LDPC-coded modulation scheme because, compared to bit-interleaved binary-LDPC-coded
modulation scheme employing turbo equalization, the proposed scheme lowers the computational complexity and
latency of the overall system while providing impressively larger coding gains.
A key challenge set by carriers for 40Gb/s deployments was that the 40Gb/s wavelengths should be deployable over
existing 10Gb/s DWDM systems, using 10Gb/s link engineering design rules. Typical 10Gb/s link engineering rules are:
1. Polarization Mode Dispersion (PMD) tolerance of 10ps (mean);
2. Chromatic Dispersion (CD) tolerance of ±700ps/nm;
3. Operation at 50GHz channel spacing, including transit through multiple cascaded [R]OADMs;
4. Optical reach up to 2,000km.
By using a combination of advanced modulation formats and adaptive dispersion compensation (technologies rarely seen
at 10Gb/s outside of the submarine systems space), vendors did respond to the challenge and broadly met this
As we now start to explore feasible technologies for 100Gb/s optical transport, driven by 100GE port availability on core
IP routers, the carrier challenge remains the same. 100Gb/s links should be deployable over existing 10Gb/s DWDM
systems using 10Gb/s link engineering rules (as listed above).
To meet this challenge, optical transport technology must evolve to yet another level of complexity/maturity in both
modulation formats and adaptive compensation techniques. Many clues as to how this might be achieved can be gained
by first studying sister telecommunications industries, e.g. satellite (QPSK, QAM, LDCP FEC codes), wireless
(advanced DSP, MSK), HDTV (TCM), etc.
The optical industry is not a pioneer of new ideas in modulation schemes and coding theory, we will always be followers.
However, we do have the responsibility of developing the highest capacity "modems" on the planet to carry the core
backbone traffic of the Internet. As such, the key to our success will be to analyze the pros and cons of advanced
modulation/coding techniques and balance this with the practical limitations of high speed electronics processing speed
and the challenges of real world optical layer impairments.
This invited paper will present a view on what advanced technologies are likely candidates to support 100GE optical IP
transport over existing 10Gb/s DWDM systems, using 10Gb/s link engineering rules.