Radio over Fiber for Wireless Communications: From Fundamentals to Advanced Topics

Chapter 8DSP Modeling of the ROF Link Nonlinearity

8.1 Introduction

As we have seen in previous chapters, a single-mode optical fiber has enough bandwidth to support wideband wireless channels up to several GHz of frequency. The RF-optical modulators and receivers of the ROF link can also be designed to suit the bandwidth of interest. However, the nonlinear distortion (NLD) of the ROF link is a major concern in Fi-Wi applications. The NLD comes mainly from the RF-optical modulator, either the laser diode in a directly modulated link or the external optical modulator. The RF amplifier after the photodetector sometimes contributes to this nonlinearity as well. The focus of this chapter is to review the effects of the NLD of the ROF link and discuss various compensation efforts to overcome the NLD.

The NLD due to electro-optic modulators and its compensation have been widely researched with respect to HFC-based CATV networks [1992–1996]. However, there are a few main differences between a Fi-Wi system and an HFC system:

1. There is no time-dispersive wireless link in series with the nonlinear optical link in HFC CATV networks.
2. The HFC networks focus on broadcasting many television channels. Hence, the traffic is predominantly in the downlink direction. The reverse (up) links in HFC CATV networks usually carry very low bit rate data, usually from the cable modem. However, in Fi-Wi networks, the reverse links require the same bit rate capacity as the forward link.
3. The phase distortion ...