Chair of Information and Coding Theory

Optical Superposition Modulation


Optical superposition modulation (optical SM) is a family of pulsed modulation schemes matched to digitally-controlled LED arrays [1]. Each individual LED is operated in "on/off" mode. The information is encoded in the sum of intensities. This summation inherently takes place at the photodetector(s) without extra complexity, known as spatial summing architecture. In contrast to modulation schemes delivering a continuous-valued waveform (like multi-carrier modulation schemes schemes), the signal space after superposition is quantized and hence not fully exploited. But there are good reasons to superimpose two-level waveforms. The main limitations of LEDs are limited peak power, limited bandwidth, and their nonlinear characteristic. Two-level current sources prevent losses due to nonlinear effects. The driver circuitry is simple, yet efficient, because current control management is sufficient. Many hardware platforms offer a digital output interface, avoiding a digital-to-analog converter at the transmitter side.

At our chair, recently a refinement of optical SM has been invented [2,3], dubbed constrained superposition intensity modulation (CSIM). A key feature is that the "on" and "off" times are adapted to the rise and fall times of the light sources. Hence, given an arbitrary solid-state light source and hence arbitrary bandwidth limitation, the modulation scheme is matched to this imperfection. In CSIM, the data rate is boosted by time-shifting the individual waveforms. The array dimension is arbitrary. Mature concepts from magnetic storage devices are borrowed in order to encode the data streams subject to a minimization of the average number of switching operations per information bit. A minimization of the number of switching operations per information bit increases the overall power efficiency including the driver circuit.


Selected References:

[1] P.A. Hoeher, Visible Light Communications: Theoretical and Practical Foundations. Munich: Hanser Fachbuch, 2019, ISBN: 978-3-446-46206-9, E-Book: ISBN: 978-3-446-46172-7.

[2] G.J.M. Forkel and P.A. Hoeher, "Constrained intensity superposition: A hardware-friendly modulation method," IEEE/OSA Journal of Lightwave Technology, vol. 36, no. 3, pp. 658-665, Feb. 2018.

[3] G.J.M. Forkel, T. Wettlin, and P.A. Hoeher, "Constrained coding for hardware-friendly intensity modulation," in Proc. 6th Int. Conf. on Photonics, Optics and Laser Technology, Funchal, Portugal, Jan. 2018.

[4] G.J.M. Forkel, Binäre Superpositionsmodulation beschränkter Sequenzen und ihre Anwendung in der optischen Unterwasserkommunikation, Christian-Albrechts-Universität zu Kiel, 2020.


The projects have been sponsored by: