Analysis of Laser Induced Distortions in Ultra Wide Band MB-OFDM over Fiber

Abstract

Performances of a UWB OFDM system over fiber based on laser direct modulation are found to be mainly limited by nonlinearities in the laser response. The OFDM signal quasi Gaussian level distribution leads to a redefinition of the optical modulation index.There is an increasing interest in ultra wideband (UWB) systems for indoor networks. UWB systems operate in the frequency range from 3.1 to 10.6GHz according to the U.S. Federal Communication Commission (FCC) [1] and are limited to distances of a few meters, with a negligible integration with the existing wired or wireless local area network. As a consequence, UWB-over-fiber technology may provide an effective solution to offer service availability across different networks and to achieve high-rate data access at any time and from any place [2]. Whereas recent papers have dealt with impulse based UWB [2], here we focus on the MB-OFDM over fiber technique [3] using laser direct modulation. We analyze the different parameters that come into play in order to reach the system optimum operating point keeping in mind the need for simplicity and economical solutions required by home networks. OFDM transmission over fiber may be critical because OFDM signals are characterized by a very large peak to average power ratio causing high sensitivity to nonlinearities in the electro-optic (e/o) conversion [4]. These distortions may be evaluated by the optical modulation index (OMI) of the signal [5], but this measure is somewhat approximate as will be demonstrated in this paper. Our goal is to define an easily measurable parameter capable of quantifying the distortions induced in the e/o conversion stage by taking into account the signal level distribution in order to exploit the advantages of OFDM modulation (e.g. high spectral efficiency and robustness to dispersion). The MB-OFDM parameters are described in the Standard ECMA-368 [3]. The maximum effective isotropic radiated power allowed by the FCC is equal to -41.3dBm/MHz corresponding to a power of -14dBm for a single OFDM bandwidth of 528MHz. Additionally, considering a directly modulated DFB laser with η Tx =0.102W/A, a single mode fiber of 1km, and a PIN photodiode with η Rx =0.8A/W, the corresponding gain of the optical link is found to be:out dB opt Rx Tx optRF 2 . 22 50 50 log 10 2 . 0 2 8 . 0 102 . 0 log 20 log 10 2 log 20 ,Given the low UWB power level and the amount of optical link loss, it is reasonable to realize the UWB transmission over fiber by means of the scheme represented in figure 1, i.e. by inserting a low noise RF amplifier before the laser. Thus, the parameters that come into play are the RF power corresponding to the modulation current, P RFin , the laser point of polarization, I bias , and the number of transmitted OFDM bands. We perform numerical simulations to explore the dependence of the error vector magnitude (EVM) [6] on these parameters. For each OFDM band, we generate a pseudo random bit sequence at 640Mb/s, we apply a QPSK modulation and obtain the OFDM signal according to [3] with Matlab™....

Integrated Optical Wireless Access: Advanced topologies for future access networks

“…As P RF is increases, the EVM correspondingly improves to a maximum value. After that, the degradation is observed due to non-linear effects, such as laser clipping [6]. Figure 5 right illustrates EVM against optical attenuation.…”

Ultra wideband over fibre transparent architecture for high-bit-rate home networks

“…Their values must be chosen to avoid laser clipping and third order inter-modulation effects [10]. VPI Transmission Maker simulations and the characterization of a RoF link showed that the RF power injected into the RoF link can be up to +18 dBm without strong degradation due to the clipping effect of the laser.…”