Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-01-19
2001-12-04
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C342S378000, C342S383000
Reexamination Certificate
active
06327298
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to nullification of interference signals, and more particularly to nullification of interference signals in a receiver system utilizing post-correlation temporal nulling.
2. Description of the Prior Art
A number of different techniques are used to decrease the effects of interference signals, particularly jamming signals, in receivers. Interference signals may be due to jamming signals, reflection of the transmitted signal or interference from other signals. Jamming signals are often narrowband such as continuous wave (CW-sinusoidal), modulated or narrowband noise like in nature and are designed to disturb the operation of the receiver. Countering the effects of jamming signals in global positioning system (GPS) receivers, which require the reception of numerous signals to derive a location from satellites at random locations, is particularly difficult.
Various techniques have been used to decrease the interference level in such receivers. One such technique for minimizing the effects of interference signals, particularly caused by jamming schemes, is adaptive antenna spatial nulling. Adaptive spatial nulling rejects interference signals by reducing the directional sensitivity of the receiver antenna in the direction of the interfering signal. Another well known adaptive nulling technique utilizes an adaptive transversal filter in the form of a tapped delay line for “pre-correlation” temporal nulling for canceling narrowband jammers.
Use of the above described temporal nulling technique has various drawbacks, particularly when used with a GPS receiver, such as Assignee's Anti-Jam GPS Receiver (AGR), which utilizes a “post-correlation” nulling approach for implementing adaptive spatial interference cancellation. In particular, to use the above described technique with the AGR receiver, a transversal filter must be positioned in front of each antenna processing channel. For example, for a receiver adapted to receive signals from a five-element antenna array, five independent transversal filters would have to be utilized, one filter being placed after each antenna element. The increased cost and space requirements associated with such an implementation renders the above approach impractical in most cases.
“Post-correlation” temporal nulling has not been successfully implemented in the past due to the fact that after pseudo-noise (PN) code spreading in a PN receiver, a narrowband jammer, e.g., continuous wave (CW) interference, is “spread out” such that its waveform is transformed into a Gaussian noise waveform in the receiver baseband.
What is needed therefore is a post-correlation temporal nulling system for use with the AGR receiver which is practical, economical and space efficient.
SUMMARY OF THE INVENTION
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a circuit for nulling a received interference signal from a received code modulated signal. The circuit includes a correlator for correlating the received interference signal and received code modulated signal with a replicate code to generate an interference noise signal and a demodulated signal, a first processor for generating actual orthogonal interference signals based upon the actual interference noise signal, a generator for producing a replicate interference signal having a frequency and waveform substantially similar to the received interference signal, a second processor for generating replicate orthogonal signals based upon the replicate interference signal and encoding the replicate orthogonal signals with the replicate code to generate replicate noise signals, a weighting network for weighting the replicate noise signals to generate weighted replicate noise signals, and a combiner for combining the weighted replicate noise signals with the actual orthogonal interference signals to produce a sum signal in which the actual orthogonal interference signals are nulled.
In another aspect, the present invention provides a method for nulling a received interference signal from a received code modulated signal, including the steps of correlating the received interference signal and received code modulated signal with a code which is a replica of the received code to generate an actual interference noise signal and a demodulated signal, generating actual orthogonal interference signals based upon the actual interference noise signal, generating a replicate interference signal having a frequency and waveform substantially similar to the received interference signal, generating replicate orthogonal signals based upon the replicate interference signal and encoding the replicate orthogonal signals with the replica code to generate replicate noise signals and weighting the replicate noise signals to generate weighted replicate noise signals, and combining the weighted replicate noise signals with the actual orthogonal interference signals to produce a sum signal in which the actual orthogonal interference signals are nulled.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures that follow. In the figures and the written description, numerals indicate the various features of the invention, like numerals referring to like features throughout for both the drawing figures and the written description.
REFERENCES:
patent: 4734701 (1988-03-01), Grobert
patent: 5317322 (1994-05-01), Grobert
patent: 6160841 (2000-12-01), Stansell, Jr. et al.
Alkov Leonard A.
Lenzen, Jr. Glen H.
Nguyen Dung X.
Pham Chi
Raufer Colin M.
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