Adaptive ingress filtering system

Details Adaptive ingress filtering system Adaptive ingress filtering system

1998-06-02

2001-05-08

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Reexamination Certificate

active

06229857

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a system for reducing ingress and more particularly to a method and apparatus for filtering an input signal for reducing the level of an undesired ingress signal.
2. Description of the Prior Art
Signal ingress is a common problem that degrades performance in systems that use signals for communicating information. One solution to this problem is to move the carrier frequency of the desired signal away from the frequency of the ingress signal. However, this is usually not possible due to the constraints or the regulations on the system. Another solution is to filter the ingress signal with an analog notch filter in the front end or an intermediate frequency (IF) section of the receiver in the system. However, the frequency of the ingress signal is typically not known before hand so the notch filter must be capable of being tuned if it is to be effective. Unfortunately, such tuned analog notch filters tend to be complex and difficult to integrate into the digital integrated circuits used in a receiver and therefore add to the expense of a system. Another solution is to use an equalizer of the type commonly used for multipath suppression for digitally notch filtering the ingress signal. However, unless the equalizer is very large, and therefore expensive, an ingress signal may use up the equalizer's entire dynamic range, or the equalizer may filter a notch band that is too wide to allow enough of the desired signal to pass. Another solution is to spread the spectrum of the desired signal when it is transmitted and then de-spread the signal when it is received. The spreading may be done either by frequency hopping or phase sequence modulation. However, the circuitry for such spread spectrum systems is much more complicated and correspondingly more expensive than the circuitry for systems that do not use spread spectrum. There is a need for an ingress filter that is simple, inexpensive, and easy to integrate into the digital integrated circuits that are used in a receiver.
FIG. 1
is a diagram in the complex IQ plane showing an exemplary quadrature phase shift key (QPSK) signal where correct modulation symbols states are shown at locations
1
,
2
,
3
, and
4
. The correct symbol states
1
,
2
,
3
, and
4
are modulated by an undesired ingress signal as shown as patterns
5
,
6
,
7
, and
8
, respectively. An I axis and a Q axis divide the IQ plane horizontally and vertically, respectively, into quadrants. When the QPSK signal is detected anywhere in the upper right quadrant the signal is demodulated as the symbol corresponding to the location
1
, when the input signal is detected anywhere in the upper left quadrant the signal is demodulated as the symbol corresponding to the location
2
, when the input signal is detected anywhere in the lower left quadrant the signal is demodulated as the symbol corresponding to the location
3
, and when the input signal is detected anywhere in the lower right quadrant the signal is demodulated as the symbol corresponding to the location
4
. However, the undesired ingress signal causes undesired ingress modulation causing the actual received modulation symbol state locations in the patterns
5
-
8
to occur within a roughly circular range about the locations
1
-
4
, respectively. The patterns
5
-
8
are broadened by a spiraling effect of amplitude modulation of the ingress signal and by noise. As shown in the diagram the undesired modulations
5
-
8
will cause the signal to be detected in a different quadrant than the associated respective locations
1
-
4
and will therefore result in incorrect determinations of the modulation symbol states.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and apparatus that is simple, inexpensive, and suitable for integration in a digital integrated circuit for filtering an undesired ingress signal from an input signal in order to increase the likelihood that the correct modulation symbol states are determined.
Briefly, in a preferred embodiment, the present invention includes an adaptive ingress filter for receiving an input signal having at least two possible input signal symbols having undesired ingress modulation due to an undesired ingress signal. The ingress filter includes an ingress synthesizer for separating and recreating an ingress signal and an ingress subtractor for subtracting the recreated ingress signal from the input signal for providing a filtered input signal where the level of the undesired modulation caused by the ingress signal has been substantially reduced. The ingress synthesizer includes a replica generator for generating replicas corresponding respectively to the correct input signal symbols, a first summer for subtracting the replicas from the input signal for providing respective intermediate signals, a second summer for subtracting a recreated ingress signal from the intermediate signals for providing respective error signals, a decision circuit for selecting the intermediate signal corresponding to the error signal having the smallest magnitude, and an ingress generator for processing the selected intermediate signal for providing the recreated ingress signal to the second summer and the ingress subtractor. The ingress generator processes the selected intermediate signal by converting the signal to component signals indicative of the magnitude, phase, and frequency of the selected intermediate signal, individually filtering the component signals, and using the filtered component signals for synthesizing the recreated ingress signal. Optionally, the index of the particular replica that corresponds to the selected intermediate signal may be used for identifying the correct symbols without further processing. However, in a preferred embodiment the ingress subtractor subtracts the recreated ingress signal from the input signal for providing the filtered input signal. The filtered input signal is then used for determining the correct input signal symbols.
A receiver of the present invention includes a first demodulator subsection for carrier synchronization and frequency spectrum filtering, a sequence detector/generator for recognizing known sequences in the input signal and providing information for the sequences to the ingress filter, an interburst ingress estimator for using the input signal during an interburst gap for providing an initial ingress estimate signal for aligning the ingress synthesizer to the ingress signal at the start of a signal burst, the ingress filter for filtering an undesired ingress signal from the input signal, a second demodulator subsection for equalizing the filtered input signal and determining the correct input signal symbols, and a processor for deriving communication information from the signal symbols.
An advantage of the present invention for filtering an undesired ingress signal from an input signal is that it is simple, inexpensive, and suitable for integration in a digital integrated circuit.


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