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    Last Modified: February 6, 2018

    The stream of symbols, which are obtained after mapping on bits to symbols on the transmitter, are rectangular pulses. Because these pulses include sharp transition, the bandwidth of the signal to be transmitted must be limited. To smoothen the symbol transitions, these pulses are shaped using a pulse-shaping filter. Filters such as root-raised cosine (RRC) introduce intersymbol interference (ISI). During demodulation, a receiver filter restricts the demodulation signal bandwidth, maximizes the signal-to-noise ratio, and removes ISI in the signal. This receiver filter is also known as the matched filter.

    The demodulator estimates for symbol timing, carrier frequency offset, and phase offset and corrects the acquired signal accordingly. This signal is then passed through a measurement filter and used for measurement. In most modulation schemes, the matched filter is used as the measurement filter. If you set the measurement filter type property to Auto, use the following table to determine the measurement filter.

    Modulation Type Pulse-Shaping Filter Measurement Filter Reference Filter
    ASK, FSK, PSK, QAM, MSK Rectangular Rectangular Rectangular ⊗ Rectangular
    Raised Cosine None Raised Cosine
    Root-Raised Cosine Root-Raised Cosine Root-Raised Cosine
    FSK, MSK Gaussian None Gaussian

    The bits recovered from the demodulation process are re-modulated using a reference filter to obtain a reference waveform. The measurement waveform is compared against this reference waveform to measure error vector and waveform correlation factor (Rho).

    A reference filter is defined as shown in the following equation:

    You can provide pulse shaping and measurement filters by specifying the filter type as Custom.

    Refer to the Custom Filters topic for more information about how to specify the filter coefficients and the coefficient spacing.

    Measurement Filter Information

    Number of Coefficients Size for FSK Size for MSK Size for PSK/QAM/ASK
    Rectangular (samples/symbol) (samples/symbol) (samples/symbol)
    Raised Cosine 1 1 1
    Root-Raised Cosine

    (samples/symbol) * filter length + 1

    (samples/symbol) * filter length + 1

    (samples/symbol) * filter length + 1

    Gaussian 14 * (samples/symbol) + 1 3 * (samples/symbol) - 1 N/A

    The RRC filter length depends on the alpha value. Filter length = 20 if alpha ≥ 0.2 Filter length = 40 if alpha < 0.2

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