Simplified narrowband excision
View Patent ↗An improved ingress cancellation filter comprising a Fast Fourier Transform circuit which replaces the analysis filter bank of the prior art to break the incoming signal down into sub bands, an ingress cancellation filter that weights each sub band based upon the probability that the sub band is corrupted by noise, and an inverse Fast Fourier Transform circuit to put the weighted sub bands back together into an output signal and replacing the synthesis filter bank of the prior art. Also, an improved predictor filter which can be used with CDMA circuitry by initializing the predictor filter at the beginning of each spreading interval using the samples received on the first L unused codes of the spreading interval.
1. A method for weighting samples in a sub band of frequencies in a narrowband excision system for transmissions from a plurality of cable modems in a DOCSIS compatible digital data transmission system to a cable modem termination system, comprising the steps:
1) decomposing the signal received over a hybrid fiber coaxial cable system from a cable modem into a plurality of sub bands using a bank of analysis filters or a fast fourier transform calculation circuit;
2) estimating the transmitted signal power in each of a plurality of sub bands of the transmitted signal from the cable modem whose transmissions are being processed;
3) calculating the average received signal power in each sub band of the signal received at said cable modem termination system from the cable modem;
4) for each said sub band, divide the estimated transmitted signal power in said sub band calculated in step 2 by the average received signal power calculated in step 3 to derive the weighting factor for each sub band, wherein deriving the weighting factor comprises calculating a running average of the product of each sample's complex number times its conjugate; and
5) multiply the samples in each sub band by the weighting factor calculated in step 4 for said sub band,
wherein step 2 is done for each sub band in the transmission from the cable modem by performing the following steps:
A) averaging the received signal power in each sub band;
B) computing the sum of the power of all N sub bands;
C) subtracting the sum of the power of a predetermined number L of the strongest sub bands from the total power calculated in step B;
D) scale the total power calculated by step C by the factor N/(N-L) so as to account for the signal lost by eliminating the signal power of said L strongest sub bands to derive a new total power; and
E) for each sub band, scale the new total power by a predetermined factor corresponding to the spectral power of the transmit filter in the Kth sub band to derive the transmitted signal power in the Kth sub band.
2. The process of claim 1 wherein step 1 is done using a bank of analysis filters.
3. The process of claim 1 wherein step 1 is done using a finite impulse response filter feeding the inputs of a Fast Fourier Transform calculation circuit.
4. The process of claim 1 wherein step 3 comprises calculating the average signal power of the signal output from each analysis filter.
5. The process of claim 1 wherein step 3 comprises calculating the average power of the signal defined by the samples output in each bin output by a Fast Fourier Transform calculation circuit.
6. The process of claim 1 further comprising the step of recombining the weighted samples calculated step 5 to generate a narrowband interference excised output signal.
7. The process of claim 1 wherein step 2 is done for each sub band in the transmission from a cable modem using data transmitted from said cable modem termination system to said cable modem which sets its transmit power.
8. The process of claim 1 wherein step 2 is done as a continuous operation so as to adaptively track variation in received signal power.