Modulation and coding techniques for optical communication systems

This dissertation investigates the applications of communication and information theories, especially coding and modulation techniques, to optical communication systems. Specifically, we demonstrate with three cases where fundamental limits can be examined and system performance can be improved through systematic methods.; In the first case, we investigate spectrally efficient coded multilevel modulation for direct-detection optical fiber communication systems with optical amplifiers. The dominant noise in the received photocurrent is the beat noise of amplified spontaneous emission of the signal with itself. Although the decision variables are higher-order Chi-square distributed, we approximate them by first-order Chi-square random variables. This allows us to work with the electric field magnitudes of the transmitted optical signals and to approximate the amplifier noise as additive white Gaussian noise. Using this approximation, we develop a pairwise error probability bound, which shows that the appropriate code design criterion is to maximize the minimum Euclidean distance between the electric field amplitudes of transmitted signal sequences.; In the second case, we study channel capacities of optical IM/DD systems using multiple-subcarrier modulation (MSM) with fixed bias, where we address the more fundamental question of spectral efficiency limits for such systems. The main finding comes from the fact that, due to input waveforms during each symbol period being non-negative with fixed means, their Fourier coefficients must lie inside certain trigonometric moment spaces, thus the channel capacities of MSM systems can be upper-bounded by sphere-packing Gaussian noise in these moment spaces.; In the third case, we propose some novel techniques for average power reduction for optical IM/DD systems. In this research, we study two classes of techniques for reducing the average optical power requirement in intensity-modulated optical systems using multiple BPSK or QPSK subcarriers. The first class of techniques involves block coding between the information bits to be transmitted and the symbol amplitudes modulated onto the subcarriers, in order to increase the minimum value of the multiple-sub carrier electrical waveform. The second class of techniques involves replacing the fixed d.c. bias by a bias signal that varies on a symbol-by-symbol basis. These techniques are applicable as long as all subcarriers originate from the same transmitter and are symbol-synchronized. Applied separately or in tandem, they provide simple and effective means for average power reduction. (Abstract shortened by UMI.)...