Patent ID: 6360369
Filing Date: 2002-03-19
Classification: H04B,H04L,H04N

Abstract:
An interference tolerant modem method for removing ingress at the head end only of a cable modem termination system, the method comprising:a complex input signal from the cable system; a variable band stop filter bank for accepting the signal from the cable system and filtering out ingress at specified frequencies; a non-linear equalizer comprising an inner equalizer and an outer equalizer for removing distortion created by the band stop filter; the inner equalizer having: a first finite impulse response filter accepting the output of the band stop filter and minimizing the inter symbol interference due to symbols that have yet to be detected; a second finite impulse response filter for removing inter symbol interference from symbols that have been detected; an inner equalizer summer for combining the output of the finite impulse response filters; and an inner equalizer quadrature amplitude modulation symbol detector for making a hard decision on the output of the inner equalizer summer, whereby the inter symbol interference, that occurs before the symbol decision, is shifted to occur after the symbol decision, thereby increasing the inter symbol interference that occurs after the symbol decision which can be removed by the second finite impulse response filter; an outer equalizer having: an outer equalizer Nyquist filter for accepting the quadrature amplitude modulation symbol detected by the inner equalizer to form an estimate of the part of the designed signal that was removed by the band stop filter bank; a band pass filter bank, it being a complement of the band stop filter bank such that the output of the band pass filter will be exactly equal to the part of the desired signal that was removed by the band stop filter at the time the ingress was removed, thereby cancelling exactly the inter symbol interference, whereby the outer equalizer will not have any residual inter symbol interference; an outer equalizer summer for combining the signal output from the band pass filter bank and the band stop filter bank, the sum being equal to the original signal with ingress removed; and an outer equalizer quadrature amplitude modulation symbol detector for passing the output signal from the outer equalizer summer to a quadrature amplitude modulation to binary converter wherein an ingress free binary signal is produced without reducing the data rate or changing the frequency of the signal.