Investigation of Optical Modulators in Optimized Nonlinear Compensated LTE RoF System

Abstract

In this paper, we investigate a nonlinear compensation technique with two different architectures using direct modulation (DM) and external modulation (EM) techniques, termed as DM based frequency dithering (DMFD) and EM based frequency dithering (EMFD). We show that DMFD and EMFD methods operate substantially different in radio-over-fiber (RoF) system by optimizing the dithering technique relative to the LTE technology. The proposed techniques is only applicable if the condition of {f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> <; f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> <; f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RF</sub> } is met, where f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> represents the dithering boundary limit of 14 MHz, f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> is DMFD signal frequency and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RF</sub> is the RoF carrier frequency. Analysis of the optical launch power for DMFD and EMFD methods reveal that the stimulated Brillouin scattering threshold is above ~6 dBm for the LTE-RoF system. In addition, we also unveil that DMFD and EMFD methods do not introduce additional distortion for the linear and optimum optical launch power regions, which are frequency chirp driven regions. If the given condition is met, the proposed method improves the LTE-RoF system without any shortcoming. Finally, at 10 dBm launch power, DMFD and EMFD methods exhibits an average signal-to-noise ratio gain of ~5.95 and ~7.71 dB, respectively.