  We discuss the influence of random- and periodic impulse noise on narrowband
(< 500 kHz frequency band) Power Line Communications. We start with random
impulse noise and compare the properties of the measured impulse noise with the
common theoretical models like Middleton Class-A and Mixed Gaussian. The main
difference is the fact that the measured impulse noise is noise with memory for
the narrowband communication, whereas the theoretical models are memoryless.
Since the FFT can be seen as a randomizing, operation, the impulse noise is
assumed to appear as Gaussian noise after the FFT operation with a variance
that is determined by the energy of the impulses. We investigate the problem of
capacity loss due to this FFT operation. Another topic is that of periodical
noise. Since periodic in the time domain means periodic in the frequency
domain, this type of noise directly influences the output of the FFT for an
OFDM based transmission. Randomization is necessary to avoid bursty- or
dependent errors.
