SPECTRAL ANALYSIS OF NONLINEAR PHASE NOISE ARISING IN COHERENT FIBER-OPTIC TRANSMISSION SYSTEMS OF OFDM SIGNALS
V. A. Vardanyan
Siberian State University of Telecommunications and Information Science, Novosibirsk, Russia
Keywords: fiber optic transmission systems, OFDM, QPSK, M-QAM, self-phase modulation (SPM), cross-phase modulation (XPM), Q-factor
Abstract
A coherent fiber-optic system for transmitting OFDM signals is considered. The spectrum of nonlinear phase noise arising in an optical fiber under nonlinear operating conditions of the optical path is investigated. It is shown that the spectrum of nonlinear phase noise consists of frequency components both at the fundamental frequency of the channel and at the combination frequencies falling within the band of OFDM channel signals. Estimation formulas are given for determining the noise immunity of channel signals by calculating the Q-factor depending on the transmission system parameters, in particular, on the number, modulation format (QPSK, M-QAM), and power of channel signals. An example of calculating the Q-factor for channel signals of the QPSK format is given. The need to limit the optical power level in the fiber is shown in order to achieve the required signal quality indicator; for example, when transmitting a multichannel signal over a 100-km fiber, it is necessary to limit the power level to 6.5, 7.4, and 8.8 dBm when using error correction methods FEC, HD-FEC, and SD-FEC, respectively
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