Impairments in Radio-over-Fiber Transmissions

  • Description

    Upcoming wireless communication will be based on a combination of various technologies such 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 (16QAM 100 Mb/s) channels over 100 km SSMF is simulated using the schematic illustrated in Figure 1. The three 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 photodetector) and by second- and third-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 the Monte Carlo method. 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.

  • Further Information

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

    Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.

    [1] G. Mouil Sil, H. Louchet, A. Richter, OFC/NFOEC 2007, paper JThA82, 2007.

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