comms.py 文件源码

def rcosfir(beta, sps, span=None):
    """Generates a raised cosine FIR filter.

    :param beta: shape of the raised cosine filter (0-1)
    :param sps: number of samples per symbol
    :param span: length of the filter in symbols (None => automatic selection)

    >>> import arlpy
    >>> rc = arlpy.comms.rcosfir(0.25, 6)
    >>> bb = arlpy.comms.modulate(arlpy.comms.random_data(100), arlpy.comms.psk())
    >>> pb = arlpy.comms.upconvert(bb, 6, 27000, 18000, rc)
    """
    if beta < 0 or beta > 1:
        raise ValueError('Beta must be between 0 and 1')
    if span is None:
        # from http://www.commsys.isy.liu.se/TSKS04/lectures/3/MichaelZoltowski_SquareRootRaisedCosine.pdf
        # since this recommendation is for root raised cosine filter, it is conservative for a raised cosine filter
        span = 33-int(44*beta) if beta < 0.68 else 4
    delay = int(span*sps/2)
    t = _np.arange(-delay, delay+1, dtype=_np.float)/sps
    denom = 1 - (2*beta*t)**2
    eps = _np.finfo(float).eps
    idx1 = _np.nonzero(_np.abs(denom) > _sqrt(eps))
    b = _np.full_like(t, beta*_sin(_pi/(2*beta))/(2*sps))
    b[idx1] = _np.sinc(t[idx1]) * _cos(_pi*beta*t[idx1])/denom[idx1] / sps
    b /= _sqrt(_np.sum(b**2))
    return b