Examples

Impact of DAC Imperfections on OFDM

  • Description

    OFDM [1] enables spectral efficient transmission as it presents low signal bandwidth and offers a simple way to generate and detect multilevel modulation such as mQAM. OFDM is also capable of adapting to the transmission channel by making use of bit and power loading. These advantages rely on signal generation and detection in the digital domain, which, in turn, imposes stringent requirements in terms of performance for the digital-to-analog and analog-to-digital converters (DAC and ADC) [2].

    Even though a specific DAC resolution is present by design, imperfections of the DAC will cause noise in the generated signal. This noise can be quantified by idealizing a converter with a noninteger resolution that will yield the same performance. The resolution of this converter is known as the effective number of bits (ENOB). The system capacity of an otherwise ideal system is fundamentally limited by the ENOB.

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  • Typical Results

    This application example shows how one possible DAC imperfection, clock jitter, can affect an OFDM signal. Figure 1 shows the effect that different jitter values have on ENOB at different frequencies for a 6bit 10Gs/s DAC.

    In order to isolate the effect of jitter, the simulated system is assumed to be ideal. The DAC converts the OFDM signal from the digital to the analog domain, and an ideal OFDM decoder provides symbols to the Symbol Error Rate (SER) estimator.

    A 5Gbaud real-valued OFDM signal with 63 subcarriers (subcarrier at DC is discarded) is considered (Figure 2). All subcarriers have the same modulation. Figure 3 shows how a jitter-induced ENOB affects the OFDM signal for different subcarrier modulation.

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  • Further Information

    Keywords: OFDM, DAC, ADC, mQAM, ENOB

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

    [1] Lowery, A.J., et al., "Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems," in Journal of Lightwave Technology, vol.25, no.1, pp.131-138, Jan. 2007
    [2] Kester, W., "DAC Interface Fundamentals" tutorial, MT-019, Rev.A, 2008 Internet: http://www.analog.com/media/en/training-seminars/tutorials/MT-019.pdf

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