Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1998-09-08
2001-06-12
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S340000, C329S345000
Reexamination Certificate
active
06246729
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communications and more particularly to a method and apparatus for decoding a phase encoded data signal wherein a windowed data signal is transformed into real and imaginary frequency components. The real and imaginary frequency components are converted to phase and magnitude information. The magnitude information is used to calculate a threshold based upon a signal-to-noise ratio, and the phase of the data signal is corrected by identifying and sorting magnitude peaks. The data signal is differentially demodulated and the demodulated data signal is then multiplied by a correlation sequence so as to facilitate its being converted into a binary number.
BACKGROUND OF THE INVENTION
The use of phase encoding for modulating a digital data signal is well known. Such phase encoding comprises varying the phase of a digital signal so as to represent one or more data bits with each phase transition.
For example, if four different phases, e.g., 0°, 90°, 180°, and 270°, are used, then causing the phase to recede, to be reduced from 0° to 270°, reduced from 90° to 0°, reduced from 180° to 90°, or reduced from 270° to 180°, may be utilized to represent a the binary number 0; and similarly, advancing the phase, i.e., from 0° to 90°, 90° to 180°, 180° to 270°, 270° to 0°, may be utilized to represent the binary number 1.
The use of such phase encoding has the potential to offer a degree of noise immunity while facilitating comparatively high data rates.
However, such phase encoding is susceptible to noise interference when very high data rates are utilized, especially when the level of the transmitted signal is very low, such as when spread spectrum techniques are utilized.
In view of the foregoing, it would be beneficial to provide means for decoding phase encoded digital data communications which provides for a high degree of reliability, particularly when high data rates are utilized with low power signals, such as those common in spread spectrum communications, particularly in electrically noisy environments.
SUMMARY OF THE INVENTION
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method and apparatus for decoding a phase encoded data signal.
The method for decoding a phase encoded data signal comprises windowing the data signal in order to reduce side lobes in a frequency spectrum of the data signal, so as to mitigate interference with other signals; transforming the windowed data signal into real and imaginary frequency components thereof; converting the real and imaginary frequency components of the data signal into phase and magnitude information with respect thereto; and summing a plurality of frequencies which substantially comprise noise to provide a magnitude of a noise floor. The magnitude of the data signal is compared to the magnitude of the noise floor so as to form a signal-to-noise ratio. The signal-to-noise ratio is compared to a threshold value and further processing of the signal is facilitated when the signal-to-noise ratio exceeds the threshold value. A maximum magnitude of the data signal is identified and a phase correction is calculated and added to the data signal. The data signal is then differentially demodulated and the demodulated data signal is multiplied with a correlation sequence to form a product signal. The product signal is then hard-limited and converted to a binary number, thus completing the demodulation or decoding process.
The step of windowing the data signal comprises multiplying the data signal by a window function. According to the preferred embodiment of the present invention, the step of windowing the data signal comprises using a 4 sample Blackman-Harris window, a Hamming window, or a Blackman window.
According to the preferred embodiment of the present invention, the step of transforming the windowed data signal comprises performing a Fast Fourier Transform (FFT) upon the windowed data signal, preferably so as to transform the windowed data signal into approximately 16,384 real frequency components and approximately 16,384 imaginary frequency components.
According to the preferred embodiment of the present invention, the step of converting the real and imaginary frequency components of the data signal into phase and magnitude information with respect thereto comprises performing a polar to rectangular conversion upon only those frequencies which are required for decoding the data signal.
According to the preferred embodiment of the present invention, the step of summing a plurality of frequencies which substantially comprise noise comprises summing approximately 500 frequencies above a frequency of the data signal and approximately 500 frequencies below the frequency of the data signal.
According to the preferred embodiment of the present invention, the step of comparing the signal-to-noise ratio to a threshold value comprises comparing the signal-to-noise ratio to a threshold value of between approximately 6 dB and approximately 50 dB, preferably approximately 6 dB.
The step of identifying a maximum magnitude of the data signal preferably comprises reading frequency components and calculating a slope of the magnitudes and defining a center frequency of the data signal.
The frequency components are preferably read from left to right so as to calculate a slope of the magnitudes. A center frequency of the data signal is then defined as the frequency immediately to the left of the first frequency which results in a calculation of a negative slope.
According to the preferred embodiment of the present invention, those peaks which cannot be processed real time by the phase correction circuit are discarded and phase correction is thus not performed thereon.
The step of adding the phase correction to the data signal preferably comprises modifying a phase of a data signal when a peak of the data signal is not in the center of a transform bin.
According to the preferred embodiment of the present invention, the step of adding the phase correction to the data signal comprises modifying a phase of the data signal when a peak of the data signal is not in the center of a Fourier Transform bin by determining a peak magnitude of the data signal and a next-magnitude of the data signal and then forming a ratio thereof. Next, the ratio is multiplied by 90° and an even/odd phase range is added thereto. The even/odd phase range is defined such that an even numbered Fourier Transform bin is −270° at the left edge, 0° in the middle, and +270° on the right edge and such that an odd numbered Fourier Transform bin is +270° on the left edge, −180°/+180° in the middle, and −270° on the right edge, with the amount of correction being linear between the boundary edges.
The step of differentially demodulating the data signal comprises subtracting a phase of a current data signal from a phase of a previous data signal and providing a linearly scaled number between −1 and +1, where −1 is a phase of −90° is 0°, 180°, and −180°, and +1 is a phase of +90°. The transfer function that is used to convert phase to a linearly scaled number is shown in FIG.
6
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The step of multiplying the demodulated data signal with a correlation sequence preferably comprises multiplying the demodulated data signal with a 13-bit Willard sequence of 1111100101000 which is modified by replacing the 0's with negative 1's to provide a number between −13 and +13, where +13 is the maximum correlation value, and also comparing the number to a threshold equal to +10 and when the number exceeds the threshold, then a correlation detection is found. The correlation detection defines the start of a message. The message is preferably formatted into a block size of 127 bits, of which 16 bits are synchronization bits and 111 bits are data bits. The 16 synchronization bits preferably comprise 13 Willard bits, a start
Anderson Terry J.
Chin Stephen
Deppe Betsy L.
Hoch, Jr. Karl J.
Northrop Grumman Corporation
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