Communications: directive radio wave systems and devices (e.g. – With particular circuit – Digital processing
Reexamination Certificate
1999-12-08
2001-02-20
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
Digital processing
C342S196000, C342S013000
Reexamination Certificate
active
06191727
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to radio frequency (RF) energy analysis systems and methods. More particularly, the present invention analyzes wideband RF in realtime to extract potential signals of interest while removing noise. The present invention performs the extraction and analysis function in all bands, Hf to microwave, for radar and communications signals.
BACKGROUND OF THE INVENTION
Typical signal collection and processing systems detect the presence of signals of interest in an RF environment by determining whether signal power within a certain frequency range exceeds a predefined threshold level for a sufficient duration of time. “Channelized” systems, for example, tune a receiver having a known bandwidth to a certain frequency and collect all the RF energy present in the environment in that frequency range. These systems determine whether the signal power exceeds the predefined threshold and, if so, conclude that a pulse exists in that range. The channelizers then define a pulse start time as the time at which the signal energy first exceeded threshold, and a pulse end time as the time that signal energy falls below threshold. A known deficiency of such channelized systems is that they require significant resources to monitor a large number of frequency bands. Another deficiency of these systems is that each channelizer is tuned to a fixed bandwidth that may or may not be consistent with the bandwidths of the signals of interest. Consequently, these systems do not provide optimal sensitivity.
“Compressive receivers” continually sweep a broad bandwidth with a narrowband filter. These systems can detect narrowband pulses in a broadband environment, but suffer from an inability to detect the presence of signal energy that is present in the environment during periods in which the narrowband filter is not tuned to the frequency band in which that signal energy is present. Additionally, the bandwidth of the narrowband filter is tuned to a fixed bandwidth that may or may not be consistent with the bandwidths of the signals of interest. Consequently, these systems do not provide optimal sensitivity, do not necessarily capture the signal of interest, and are analog systems.
Instantaneous Frequency Measurement (IFM) receivers minimize the sweep time limitations of the compressive receiver by providing a broadband frequency discriminator that rapidly responds to a signal's presence. The IFM receiver, however, is unable to provide accurate frequency measurements in the presence of multiple simultaneous input pulses, as are typically encountered in crowded RF environments.
Broadband signal processing systems are often required to detect the presence of narrowband signal energy in a wideband RF environment that includes, for example, radar pulses and/or communications pulses. It is desirable that such systems are able to detect all that RF energy that is present in a wide frequency range for a certain period of time. It is also desirable to minimize the resources required to detect these signals. Designers of such signal processing systems, therefore, would benefit from methods and apparatus that analyze wideband radio frequency spectra that include both radar and communications signals to extract potential signals of interest while removing noise and other unwanted RF energy.
SUMMARY OF THE INVENTION
The present invention satisfies these needs in the art by providing apparatus and methods for processing RF signals. The inventive method comprises generating an energy map of collected radio frequency (RF) energy as a function of time and frequency for a predefined dwell period and dwell bandwidth. The collected RF energy can include energy from communications signals as well as radar signals, transient signals as well as continuous signals. From the energy map, it can be determined whether a pulse is present in the RF spectrum. If a pulse is present in the RF spectrum, then a pulse bandwidth and pulse duration can be determined from the energy map.
The energy map can be generated by dividing the dwell period into a set of k time windows and dividing the dwell bandwidth into a set of n frequency bins. An energy grid comprising n×k frequency-time cells can then be generated. Each frequency-time cell corresponds to one of the frequency bins and to one of the time windows and has a value based on the collected RF energy present in the corresponding frequency bin during the corresponding time window. A binary value can be assigned to each of the frequency-time cells based on whether the collected RF energy present in the corresponding frequency bin during the corresponding time window exceeds a predefined energy presence threshold. If noise is present in the RF spectrum, the noise can be filtered from the energy map.
If a pulse is present in the RF spectrum, a tag can be generated for the pulse that includes a pulse characterization parameter that characterizes the pulse. The pulse characterization parameter can be based, for example, on pulse width, center frequency, angle of arrival, or time of arrival.
A method according to the present invention can also include “pulse healing.” That is, for a first pulse and a second pulse, a combined pulse duration can be defined that extends from a start time of the first pulse to an end time of the second pulse. It is then determined whether the start time of the second pulse exceeds the end time of the first pulse by less than a predefined threshold, which can be based, for example, on the combined pulse duration. If the start time of the second pulse exceeds the end time of the first pulse by less than the predefined threshold, then the first pulse and the second pulse are combined into a single pulse (i.e., the single pulse is “healed”).
Similarly, pulses can be “healed” in frequency. That is, for a first pulse and a second pulse, a combined pulse bandwidth can be defined that extends from a lower frequency of the first pulse to an upper frequency of the second pulse. It is then determined whether the lower frequency of the second pulse exceeds the upper frequency of the first pulse by less than a predefined threshold, which can be based, for example, on the combined pulse bandwidth. If the lower frequency of the second pulse exceeds the upper frequency of the first pulse by less than the predefined threshold, then the first pulse and the second pulse are combined into a single pulse.
Another method for processing RF signals according to the invention comprises receiving a set of time domain energy samples representing signal energy present in an RF spectrum. The set of time domain energy samples can be transformed into a set of frequency domain power samples. Transforming the set of time domain samples into a set of frequency domain samples can include dividing the set of time domain energy samples into a plurality of N windows, each of which is associated with a predefined window period. For each of the N windows, an FFT is performed to generate a set of K frequency bins. Each of the frequency bins has a value based on energy present in a predefined frequency band during the corresponding window period.
From the set of frequency domain power samples, it can be determined whether a signal of interest is present in the RF spectrum. A subset of the set of frequency domain power samples can be forwarded to a follow on system, where the subset corresponds to the signal of interest. To determine whether a signal of interest is present can include generating an energy map that represents energy present in the RF spectrum as a function of frequency and time. The energy map can be a bitmap comprising N×K frequency cells, wherein each frequency cell has a binary value based on the value of a corresponding frequency bin. The binary value can be based, for example, on whether the value of the corresponding frequency bin exceeds a predefined threshold.
REFERENCES:
patent: 3665512 (1972-05-01), Hall et al.
patent: 3876946 (1975-04-01), LaClair et al.
patent: 4166980 (1979-09-01), Apostolos et al.
paten
Haber Conrad H.
Springer Joseph F.
L-3 Communications Corporation
Sotomayor John B.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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