With the rapidly increasing demand for large-scale fiber-to-the-home, the cost of transmission equipment is crucial. In this project, we develop communication theory for links consisting of a low-cost laser diode (e.g., a vertical-cavity surface-emitting laser, VCSEL) in the transmitter, whose intensity but not phase can be modulated. Similarly, the receiver is a photo diode that detects the intensity of the received lightwave. The resulting link is commonly referred to as the optical intensity channel.
Of the conventional digital modulation formats, only on-off-keying and its multilevel generalization, amplitude shift keying, can be used on an optical intensity channel, because they do not encode any information in the phase. These formats, however, suffer from weak power efficiency, which not only wastes energy but also makes them vulnerable to nonlinear impairments. We develop new theory for communications over the optical intensity channel and, based on this theory, new modulation and coding methods. We pay particular attention to subcarrier modulation methods, which means that the data to be transmitted is encoded onto a low-frequency sinusoid, using any coherent modulation format. This baseband signal is then biased to become positive and used to modulate the amplitude of the lightwave.
- Develop subcarrier modulation formats that allow larger bit rates than conventional formats, for a given link (bandwidth and noise) at a required target performance (power consumption and bit error rate)
- Jointly design modulation formats and channel codes for such links
- Find information-theoretic bounds for the performance of the optical intensity channel
- Extend all of the above to more realistic system models, including several types of noise, dispersion, and nonlinear distortion
Figure: An energy-efficient four-level modulation format for intensity-modulated links can be graphically represented as a tetrahedron.
Forskningen bedrivs i samarbete med forskningscentrum FORCE.