Modeling of optical fiber dispersion on QAM protocol in affordable RF-over-fiber channels

Abstract

We investigate the effect of optical chromatic dispersion on ordinary double-sided modulation (DSM) quadrature amplitude modulation (QAM-M) protocol in RF-over-fiber (RFOF) channels. QAM-M protocol is common in RFOF communications channels. An analytical expression was derived for the channel’s response, the standard deviation of the QAM’s clusters, and for the estimations of the channel’s bit error rate (BER). The main findings are as follows: (A) unlike single-sided modulation (SSM) QAM or coherent QAM, ordinary DS QAM is governed by three length scales, and therefore it is much more susceptible to dispersion; (B) the QAM’s clusters cannot be bounded, even for weak dispersion, and therefore, zero-error detection is unattainable without dispersion compensation; and (C) a linear dispersion compensation filter was derived to mitigate the dispersion effects post-detection. These results suggest that it is not essential to operate the channel in a SSM QAM mode. A simpler and affordable option may be to operate the channel in an ordinary DSM QAM method and to apply a dispersion compensating filter in the electrical or even in the digital domain. This method is a low-cost solution to improve substantially the RFOF channels performances.