Error correction coding in optical communication systems
Today, most of the data globally is transmitted using optical fibers. It is therefore of great interest to study the capacity of fiber optical channels and also techniques, required to achieve it. The main focus of this project is to study information theoretic techniques for achieving the capacity limits of fiber optical channels.
In an optical communication system that operates at very high data rate, the challenge is to find codes with low overhead that are able to correct the errors due to linear and non-linear noise and due to dispersion and inter-channel cross talk with emphasis on low complexity and cost. It has become evident that forward error correction (FEC) coding is an integral part for the next generation of optical communication channels.
Our research is focused on designing novel FEC strategies, supporting very high speed optical communications, with a target of 1 Terabit per second throughput. The codes considered include
- Turbo codes,
- Low-Density Parity-Check Convolutional Codes (LDPC-CC),
- Polar codes, and
- Product/Staircase codes.
FEC is usually applied to the fiber optical systems as a binary error-correcting systems, independent of the other subsystems, such as modulation, Digital Signal Processing (DSP) and hardware. On the other hand, a near-capacity achieving communication system must be designed considering coding schemes, modulation, channel equalization and decoding algorithms jointly. As a part of this work, we study the various components of FEC and its interaction with other parts of the system to approach the fiber capacity. Together with the high speeds, high reliability is also a goal. We consider bit-error-rates (BER) better than 10-15 or 10-17 depending upon the application and therefore also study and analyze the error floor of some of the codes.
We study these issues within SPOC, where we in Flagship C focus on activities of Information theory, Capacity and Advanced Coding.
The research here is focused within the following main activities:
- Error-correcting decoding algorithms - design and implementation.
- Receiver design.
- Advanced coding, modulation and transmission systems using iterative decoding/reception.
- Achievable rate and capacity estimation for communication systems with a focus on optical communication channels.
- Application of high-speed forward error-correcting coding (FEC) in optical communication.
Advanced modulation and digital signal processing
The activities in this project are mainly focused around information theoretic estimation of achievable information rate (AIRs) and capacity for fiber optic channels. We study constructive approaches to estimate and/or lower bound the capacity and the AIR, which consequently lead to improved and novel transceiver architectures. Advanced modulation techniques are of particular interest, including constellation shaping and modulation optimization, along with their respective receiver design. Transceiver designs also include linear and nonlinear impairment mitigation and compensation through DSP.
The main activities in this project are:
- Probabilistic shaping for nonlinear fiber optic transmission.
- Error-correcting codes for near-capacity probabilistically shaped modulation.
- Digital signal processing for fiber nonlinearity compensation, particularly nonlinear phase noise.
- Information theoretic studies into the ultimate channel capacity.
Illustrations show received constellations: Uniform 256QAM (left), 256QAM with probabilistic shaping (right)