Modulation Optimization for Noncoherent Optical Systems
There has been a significant improvement in high-speed electronics and optical components over the past few years, and this is one of the key enabling factors for high data rate transmission. Moreover, multi- level modulation formats, which encode information onto the carrier’s amplitude and phase, are necessary to increase the spectral efficiency of communication systems. For short-haul low cost optical links, as the ones used in data centers and access networks for example, non- coherent optical communication with directly modulated lasers and direct detection receivers is more cost-effective than using coherent communication systems. Such types of noncoherent communications use the intensity of the optical carrier to carry information. These systems can be modeled as an additive white Gaussian noise channel with input constrained to being nonnegative. Subcarrier modulation, a concept studied in the wireless infrared communications context, allows the use of in-phase and quadrature phase (I/Q) modulation formats for such types of noncoherent optical systems.
We propose a novel quaternary subcarrier modulation format in [Paper A] which is a hybrid between on-off keying (OOK) and ternary phase-shift keying. At asymptotically high signal-to-noise ratios, this hybrid format offers a 1.2 dB average electrical power gain and 0.6 dB average optical power gain compared to OOK, making it the most power-efficient format for noncoherent optical systems with a spectral efficiency of 1 bit/s/Hz. However, for systems that are limited by laser nonlinearities, we show that OOK is the best choice at this spectral efficiency.
In [Paper B], we complement the theoretical results obtained in [Paper A] by carrying out an experimental link analysis, thus com- paring the different modulation formats with spectral efficiency of 1 bit/s/Hz. In agreement with the theory, the proposed modulation format is more power efficient in terms of average optical power com- pared to the other modulation formats under study. However, the gain comes at the cost of higher transceiver complexity. In addition, we study the impact of propagation in multimode fiber for different fiber lengths, and the analysis shows that all the studied modula- tion formats have a similar performance penalty due to the modal dispersion present in multimode fibers.
The agreement of theory with the obtained experimental results motivated us to optimize higher-level modulation formats for different power measures. In [Paper C], we propose a set of 4-, 8-, and 16-level single-subcarrier modulation formats for noncoherent optical systems which are optimized for average electrical, average optical, and peak power. In the absence of error-correcting codes, the optimized mod- ulation formats offer a gain ranging from 0.6 dB to 3 dB compared to the best known formats. It was also noticed that modulation for- mats optimized for peak power perform well in average-power limited systems. When capacity-achieving error-correcting codes are present, the obtained modulation formats offer a gain ranging from 0.3 dB to 1 dB compared to previously known formats. The modulation for- mats optimized for average optical power have a better performance, except in peak-power limited systems. We also analyze and design modulation formats which are optimized for the low signal-to-noise ratio regime when capacity-achieving codes are used.
intensity modula- tion
free-space optical communications