For a band-limited channel, there is only one way to significantly increase the spectral efficiency, and that is to apply multilevel modulation. In this project, we analyze multilevel modulation formats suitable for coherent, optical transmission. Modulation formats are optimized under ideal conditions as well as considering realistic optical impairments, such as self-phase modulation and transmitter/receiver saturation.
An electromagnetic carrier offers essentially four degrees of freedom in which data can be modulated: the amplitude and phase in two polarizations each. In order to maximize the data rate in a fiber-optical link, all four degrees of freedom should be exploited, which can be interpreted as four-dimensional modulation. Moreover, the transmitted power can be significantly reduced by transmitting dependent data in the four dimensions, instead of regarding them as independent, parallel subchannels. If the extra complexity can be afforded, coded modulation schemes can further improve the power efficiency with several dB at the same data rate and bandwidth. In the other extreme, there are short-link applications in which coherent transmission is too costly. E.g., developing modulation techniques for the so-called optical intensity channel has recently emerged as a promising spinoff from this project.
- Derive fundamental digital communication theory for transmission over fiber-optical links
- Design spectrally efficient modulation formats, which admit a higher bit rate in the same bandwidth
- Develop coded modulation schemes for optical channels come as close as possible to the fundamental limits, without unreasonably high decoding complexity
- Cooptimize coding, modulation, and detection algorithms over both polarizations
Forskningen sker i samarbete med forskningscentrum FORCE.