OFDM for High-Speed Access Systems

  • Description

    OFDM is a novel modulation format for optical communications where data is transmitted over many orthogonal subcarriers using the Discrete Fourier Transform. Since carriers are encoded independently, different modulation formats and spectral efficiencies can be used for each of them, thus adapting modulation to channel characteristics (bit loading). Furthermore, digital signal processing can be performed at the transmitter side to account for characteristics of the transmission channel, including electro-optical conversion and fiber characteristics. At the receiver side, digital equalization can be performed and each subcarrier processed independently, enabling powerful mitigation of transmission impairments.

  • Typical Results

    This application example compares simulations with published experimental results. The setup is intended to emulate a cost-effective OFDM-based optical access system scenario [1]. The laser is directly modulated using DMT (real-valued OFDM), while direct detection is performed at the receiver. The transmitted signal is composed of 16 subcarriers, each modulated with 64QAM, and includes a 25% cyclic prefix resulting in a net signal rate of 11.25 Gbit/s. The total signal bandwidth is only 2.5 GHz. After transmission through a 25-km-long SMF, an attenuator is used in front of the receiver to control the received power. As direct modulation is used, the resulting chirp interacts with the fiber chromatic dispersion (CD) and degrades the received signal. In order to reduce chirp, and thus degradations, a 23% modulation index is used.
    Simulation and experimental results [1] are displayed in Figure 1. The flexibility of the simulation environment allows to perform many types of analysis. In Figure 2 the constellation diagrams for different subcarriers are shown demonstrating how each subcarrier is affected differently by the transmission. In order to reach a larger number of users, the modulation index can be increased. However, this would increase the degrading impact of chirp-CD interactions along the link. Figure3 shows the BER depending on the fiber length for each subcarrier, when the modulation index is increased to 67%. Apparently, the performance of carriers at higher frequencies suffers from this increase in modulation index.

  • Further Information

    Keywords: OFDM, access networks, Discrete Fourier Transform

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

    [1] J.M. Tang, P.M. Lane and K.A. Shore, "High-Speed Transmission of Adaptively Modulated Optical OFDM Signals Over Multimode Fibers Using Directly Modulated DFBs", J. Lightwave Technol. 24 (1), 429-441, 2006.

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