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Institut Informationstechnische Systeme


Signal Processing in Optical Fiber Transmission Systems

(in English only)


High capacity transmission over optical fibers is nowadays the standard technology that provides the backbone for our modern information and communication society. The increasing requirement for higher capacities with dense WDM (Wavelength Division Multiplex) and TDM (Time Division Multiplex) systems faces the demand for cheaper and more economic solutions.
Compared to all optical devices, electrical applications are strongly limited by speed. However, electronics can employ digital signal processing, enabling much more complex functions, and an electronic chipset is usually much more cheap than its optical counterpart.
During optical transmission several linear and non-linear effects distort the signal pulse. Especially in ultra long haul systems the non-linearities dominate due to the high launch power at the emitter.
Electrical equalizers already exist for mobile and radar communications. However, the distortions are merely linear with a Gaussian noise statistic. It is now desirable to adapt these methods to the optical channel with high non-linearities and a non-gaussian noise statistics.
The advantage of electrical equalizers after optical transmission lines is the ability to compensate linear effects and partly mitigate nonlinear impairments with a cheap electronic chipset. Such devices could decrease the number of DCFs (Dispersion Compensated Fibers), improve the BER (Bit Error Rate), compensate for multichannel effects or increase repeater distances.  
Recent research includes the development of digital signal processing algorithms for next-generation fiber optic coherent receivers for polarization-multiplexed transmission, including timing recovery, equalization, and carrier recovery.


Research topics - short listing

  • Development of DSP-algorithms for coherent polarization-multiplexed receiversfor long-haul systems
  • Development of next generation optical receivers for access networks
  • Simulation of optical transmission lines
  • Evaluation of different modulation formats such as NRZ/RZ-OOK, duobinary, CSRZ, DPSK, DQPSK and higher order DQAM
  • Adaptive electrical compensation mitigating linear and non-linear distortions
  • Maximum Likelihood Sequence Estimation (MLSE)

Special simulation tool developed at our institution

  • Linux PC cluster with 10 servers with 8 processors each for a total of 80 CPUs
  • Parallelization of simulation task yields near linear increase in computing speed with respect to number of processors
  • Single channel/ multi channel scenarios with multiple spans lead to reasonable simulation durations


Contributors: P. Leoni, A. Lobato, Z. Wu, B. Lankl


cluster 2

Linux-Cluster for simulation of optical transmission systems and electrical receivers including equalization and decoding. Due to sophisticated programming we nearly reach 30-fold processing speed with 10 standard PCs compared to the processing power of a single PC.


BER performance of combined chromatic dispersion (CD) and differential group delay (DGD) distortions with equalization by a 16-state Viterbi-Algortihm and 4-bit quantization. The plot contains results for duobinary (DB,solid colored), NRZ (solid white) and CSRZ (transparent) modulation.


Various detection and equalization schemes for DQPSK modulation have been evaluated at our institute including digital clock-phase recovery.