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Coded Modulation Techniques for Direct-Detection Optical Fiber Communication Systems with Optical Amplifiers

In this paper, 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 with signal and 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 code design criterion is to maximize the minimum Euclidean distance between the electric field amplitudes of transmitted signal sequences. Hence, the coded modulation schemes developed for additive white Gaussian noise channels, such as trellis-coded modulation and multi-level coding, can also be applied in our case. We evaluate the performance of various coded multilevel modulation schemes and compare their performance to coded and uncoded on-off keying. Moreover, the error performance can be improved by using asymmetrical coded modulation techniques, which jointly optimize the coded modulation and the signal constellation.