Optical Access Technologies

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Overview

Impairments in Radio-over-Fiber Transmissions

Description

Upcoming wireless communication will be based on a combination of different technologies as WiFi, WiMax, UMTS, GSM or Bluetooth. This evolution implies more complexity at the base stations, and thus, a higher cost for the network. In Radio-over-Fiber (RoF) systems this complexity is removed in a single central station and the transmission of the radio signals between the central and base stations is realized with optical fibers. In this configuration, the cost of the base stations (acting as passive antenna) can be drastically reduced. Another advantage of RoF systems is the possible seamless integration of different wireless technologies with CATV and broadband internet (e.g. Ethernet).

Typical Results

The transmission of 3x (16-QAM 100Mb/s) channels over 100km SSMF is simulated using the schematic illustrated in Figure 1. The 3 channels are multiplexed in the electrical domain and the resulting RF signal is used to drive a Mach-Zehnder modulator (MZM).
In unidirectional transmissions, the system performances are limited by noise (from the laser, the optical and electrical amplifiers and the photo-detector) and by 2nd and 3rd order intermodulation distortions (IMD2, IMD3) resulting from the nonlinear nature of the transmission line.
In this example, the magnitude of the driving signal is varied: At low modulation index, the transmission is limited by noise. For higher modulation index, the SNR is improved but the transfer function of the MZM becomes nonlinear and nonlinear distortions are generated. The signal spectra for low and high modulation index are displayed in Figure 2. The system BER is reported against the magnitude of the driving RF signal in Figure 3. It is estimated using three methods (Gaussian approximation, PDF fitting [1] and Monte-Carlo). At low modulation powers the optical noise dominates and the signal statistics is Gaussian. All approaches give the same results. However, at high modulation powers nonlinear distortions arise and the Gaussian approach underestimates the BER.

Keywords

Radio-over-Fiber (RoF), Sub-Carrier Modulation (SCM), Quadrature Amplitude Modulation (QAM), Intermodulation distortions, Second Order Spur (IMD2), Third Order Spur (IMD3), Bit Error Rate (BER)