Signaling for Optical Intensity Channels
Doktorsavhandling, 2013

With the growing popularity of social media services, e-commerce, and many other internet-based services, we are witnessing a rapid growth in the deployment of data centers and cloud computing platforms. As a result, the telecommunications industry has to continue providing additional network capacity to meet the increasing demand for bandwidth. The use of fiber-optic communications plays a key role in meeting this demand. Coherent optical transceivers improve spectral efficiency by allowing the use of multilevel in-phase and quadrature (I/Q) modulation formats, which encode information onto the optical carrier’s amplitude and phase. However, for short-haul optical links, using noncoherent optical transceivers, also known as intensity-modulated direct-detection (IM/DD) systems, is a more attractive low-cost approach. Since only the intensity of light can carry information, designing power- and spectrally-efficient modulation formats becomes challenging. Subcarrier modulation, a concept studied in wireless infrared communications, allows the use of I/Q modulation formats with IM/DD systems at the expense of power and spectral efficiency. This thesis addresses the problem of optimizing single-subcarrier modulation formats for noncoherent fiber and wireless optical communication systems in order to achieve a good trade-off between spectral efficiency, power efficiency, and cost/complexity. For the single-subcarrier three-dimensional signal space, denoted as raised-QAM in the literature, we propose a set of 4-, 8-, and 16-level modulation formats which are numerically optimized for average electrical, average optical, and peak power. In the absence of error-correcting codes, the optimized formats offer gains ranging from 0.6 to 3 dB compared to the best known formats. However, when error-correcting codes with performance near capacity are present, the obtained modulation formats offer gains ranging from 0.3 to 1 dB compared to previously known formats. In addition, laboratory experiments using the obtained 4- and 8-ary modulation formats were carried out. The performance improvement over the previously known formats conforms with the theoretical results. To address transceiver complexity, a two-dimensional signal space for optical IM/DD systems is proposed. The resulting modulation formats have simpler modulator and demodulator structures than the three-dimensional formats. Their spectra have in general narrower main lobes but slower roll-off, which make them a good choice for single-wavelength optical systems. The three-dimensional formats are more suitable for wavelength-division multiplexing systems, where crosstalk between adjacent channels is important.

lattice codes

mutual information

intensity modulation

noncoherent communications

Direct detection

sphere packing.

infrared communications

free-space optical communications

fiber-optical communications

VH, Sven Hultins gata 6
Opponent: Associate Professor Steve Hranilovic


Johnny Karout

Signaler och system, Kommunikationssystem, informationsteori och antenner, Kommunikationssystem

Experimental comparison of modulation formats in IM/DD links

Optics Express,; Vol. 19(2011)p. 9881-9889

Artikel i vetenskaplig tidskrift

A Two-Dimensional Signal Space for Intensity-Modulated Channels

IEEE Communications Letters,; Vol. 16(2012)p. 1361-1364

Artikel i vetenskaplig tidskrift

Demonstration of 8-level subcarrier modulation sensitivity improvement in an IM/DD system

Optics InfoBase Conference Papers,; (2011)

Paper i proceeding

Optimizing Constellations for Single-Subcarrier Intensity-Modulated Optical Systems

IEEE Transactions on Information Theory,; Vol. 58(2012)p. 4645-4659

Artikel i vetenskaplig tidskrift

Power efficient subcarrier modulation for intensity modulated channels

Optics Express,; Vol. 18(2010)p. 17913-17921

Artikel i vetenskaplig tidskrift


Informations- och kommunikationsteknik


Elektroteknik och elektronik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 3546

VH, Sven Hultins gata 6

Opponent: Associate Professor Steve Hranilovic