Modulation and Detection Techniques for （调制和检测技术）.pdf

Modulation and Detection Techniques for Optical Communication Systems Joseph M. Kahn Stanford University, Department of Electrical Engineering, 372 Packard Building, 350 Serra Mall, Stanford, CA 94305-9515 USA jmk@ee.stanford.edu Abstract: Performance and implementation complexity of various binary and nonbinary modulation methods with coherent, differentially coherent and noncoherent detection are compared. Nonbinary modulation with coherent detection maximizes spectral efficiency and improves tolerance to transmission impairments, while enabling effective, low-complexity electrical compensation of these impairments. 1. Introduction Currently deployed fiber and free-space optical communication systems use on-off keying (OOK) with direct detection, and some are beginning to use differential phase-shift keying (DPSK) with interferometric detection. The further evolution of modulation and detection techniques will increase spectral efficiency in wavelength-division- multiplexed (WDM) systems, improve robustness against transmission impairments, and facilitate electrical compensation of such impairments. As explained in this paper, nonbinary modulation with coherent detection yields the best performance in all of the above respects, but at the cost of increased implementation complexity. Throughout this paper, we consider fiber or free-space systems that use optical amplifiers and/or nonlinear optical wavelength converters, and assume that that amplified spontaneous emission (ASE) is the dominant noise source. We follow the notation used in [1], to which the reader is referred for further details and refere