Partial band reconstruction of frequency channelized filters

Details Partial band reconstruction of frequency channelized filters Partial band reconstruction of frequency channelized filters

2002-11-25

2004-09-14

Chin, Stephen (Department: 2634)

Pulse or digital communications

Receivers

Interference or noise reduction

C375S240000, C704S231000

Reexamination Certificate

active

06792057

ABSTRACT:

FIELD OF THE INVENTION
This present invention relates to signal processing and more particularly, to signal reconstruction and for filter design for partial band reconstruction of a wideband channelizer.
BACKGROUND OF THE INVENTION
Signal detection and reconstruction are areas of importance for military and commercial applications. In most signal intelligence schemes, the system attempts to analyze a wide bandwidth detected signal and then subdivide the wide bandwidth into smaller bands. The smaller bands are investigated for the signals of interest by examining the energy response.
The subchannels or bins contain regions of signal energy and the processing measures the power in the various bins to locate regions of interest that have significant detectable power levels. In one implementation the wideband detection bandwidth is converted into a frequency domain snapshot using Fast Fourier Transform (FFT) processing many times per second. The snapshots are aligned in time with the ability to revisit various stored frequency bins, and are in time with a time delay between each snapshot.
To focus on a particular narrow band signal of interest, a certain number of adjacent channels are recombined into a narrowband time domain stream of data that can be further processed. There have been various attempts to employ an inverse transform to the time domain within the window of bins that does not affect the output stream. Ideally, the signal intelligence community would like to perform the perfect partial reconstruction of the desired narrowband window with no distortion.
Cosine filter banks are used in the prior art, but generally these filters are used with real data and not complex data. The cosine filter banks generally employ the same filter on the analysis and synthesis side.
There are many digital receiver systems used in the vast telecommunications area, and the main purpose of these receivers is to extract information signals from the many other interfering signals and noise interference. One example of a receiver is the channelized receiver. A channelized receiver comprises an antenna and a radio frequency front end that intercept radio frequency energy and performs signal conditioning and down conversion to a convenient intermediate frequency (IF). There are a number of characteristics that increase the ability to intercept a radio frequency signal of interest. Namely, a broadband instantaneous frequency coverage, good sensitivity, large dynamic range, simultaneous signal detection, arbitration and parameter encoding, and fine frequency measurement.
One problem with high sensitivity, narrow band intercept receivers is tuning to receive a signal having an unknown frequency. Reducing the bandwidth of a receiver generally increases its sensitivity, but results in tuning difficulties because the narrow bandwidth must be more precisely centered with respect to the incoming signal. One way conventional radar intercept receiving systems have tried to eliminate this problem is to search for the unknown signal with a less sensitive wide band receiver, and, once having detected a signal, tune a narrow band receiver to the detected signal. As the signal-to-noise ratio of the unknown signal becomes lower, the more difficult it is to utilize this method. In addition, it is usually desirable to rapidly identify the unknown signal in order to quickly tune the narrow band receiver to that frequency. Accordingly, channelized receivers having a plurality of filters each defining a contiguous passband portion of a search bandwidth have been utilized to quickly identify a channel in which an unknown signal resides, this channel then being used to identify a tuning frequency for a narrow band receiver. However, as the dynamic range of an unknown input signal increases, it becomes more difficult to determine the frequency of the signal without the use of complicated and complex redundancy comparison circuitry which is required when strong input signals provide output signals of substantially equal magnitude at two or more of the channelizer filters.
In order to widen RF bandwidth and improve the probability of intercept, the channelized receiver uses a number of contiguous filters, called a filter bank, to sort the input signal into segments of predetermined frequency. An input signal with a certain frequency will fall into a certain filter, and by measuring the output of the filters, the input signal frequency is estimated. Channelization generally refers to the filtering, decimation, interpolation and frequency conversion of received signals. A channelizer divides a wide receiver frequency band into many narrow frequency “bins” or channels, so that the receiver can and digitally process each individual channel separately. The channelizer can be used in conjunction with a parameter encoder. The parameter encoder characterizes each received RF signal in accordance with a predetermined set of parameters, such as frequency, pulse width, amplitude, time of arrival, type of modulation.
The analog channelized receiver is relatively expensive to fabricate because of the large number of filters required. In addition, the analog receiver size is bulky and the maintenance is difficult because it requires a large number of components. The digital channelized receiver requires a contiguous set of digital band pass filters with linear phase that cover the IF bandwidth. This coverage can be accomplished with a set of discrete digital filters, or the digital filter bank can also be effectively implemented by performing the short time Fourier transform which in effect performs the discrete Fourier transform on weighted and overlapped partitions of a collection of discrete time signals.
The short time Fourier transform complex modulates a low pass filter h(n) to form a uniform filter bank having one filter centered at each frequency bin of the fast Fourier transform. The low pass filter h(n) is, in effect, used to window the data. The established window slides across the data and then the discrete Fourier transform is calculated to give a frequency versus time output. Between successive fast Fourier transform calculations, M points are skipped which results in the output being decimated in time by M. It is also possible to generate a fine frequency digital channelized receiver by using an instantaneous frequency measurement algorithm. Such an instantaneous frequency measurement receiver uses the phase data generated by the short time Fourier transform filter bank to generate the fine frequency selection capability of the digital channelized receiver.
A prior art analog receiver system receives a radio frequency (RF) signal that is received by the antenna and then downconverted to an intermediate frequency (IF) by a RF front end. The RF front end typically comprises low noise amplifiers (LNAs) to boost the signal from the low reception power, filters to remove some of the noise, and mixers to downconvert to IF using a local oscillator signal. The receiver channelizer then extracts the desired channel. The channelizer generally has LNAs, mixers and filters. The selected channel is then processed at baseband by the receiver baseband unit to produce the received digital data stream.
In more state of the art receivers, there are more digital implementations than analog. Baseband processing generally has analog-to-digital conversion, digital filtering, decimation, equalization, demodulation, channel decoding, de-interleaving, data decoding, and timing extraction. In the case of multiple channels, the processing is performed in a similar fashion but the path is split to form multiple paths for each channel being processed with the digital interface being somewhere between the RF front end/back end and channelizer/de-channelizer blocks. This digitized implementation includes multistandard radio, wideband digital tuners, wideband radio or software defined radio.
Efficient digital channelizer/de-channelizer structures, that perform filtering, decimation/interpolation and frequency conversion, are important in terms of po

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