US 5,920,589 A · Direct sequence spread spectrum DSP system

Abstract

A direct sequence spread spectrum (DSSS) DSP implementation technique utilizing a unique combination of multiple filters, transforms, despreading algorithm and spectral compression, all in a single coefficient set, providing a highly efficient system requiring relatively nominal computational hardware and software requirements. Reallocatable computational resources allow for frequency uncertainty in the transmitter, without the typical excessive noise bandwidth penalties. The present system further contemplates a unique energy detection algorithm for discerning spread spectrum while in the search mode, utilizing several discrete sampling data sets to increase signal strength, reducing noise, while changing the relative phase of the received chipping code with respect to the received signal. The spectral compression of the spreading sequence is achieved by representing the PRC as an array of two, complimenting fractional coefficients. The PRC code is filtered to achieve the required spectral compression characteristics, resulting in minimal despreading loss, while providing interference rejection of CW signals greater than 1.5 MHz from the center IF frequency.

Claims14

Dependent claims

A direct sequence spread spectrum system for receiving packetized data bursts of aggregate data bandwidth of less than 120 Kbit/second, comprising:

a receiving means for receiving an RF signal;
a converting means for converting said RF signal to an IF signal;
an attenuation means for selectively attenuating one of said RF signal prior to conversion by said converting means and said IF signal;
a band-limiting means to band-limit said IF signal;
a digitizing means to digitize said band-limited IF signal using harmonic sampling techniques to produce a digital signal whose center frequency is greater than a chipping frequency of a spreading code of said IF signal;
a downconversion means to quadrature downconvert said digital signal to a baseband digital signal;
a despreading means to despread said baseband digital signal;
a filtering means to filter said despread baseband digital signal using plural filters each having a bandwidth of one of greater than and equal to a bandwidth of a signal of interest;
a detection means to detect candidate signals in at least one of the plural filters;
a selection means to select a filter for demodulation from the plural filters based on a candidate signal detected therein;
and
a demodulation means to demodulate data from the candidate signal in the selected filter.

9 more elements

A method to enhance coarse search sensitivity in a direct sequence spread spectrum system utilizing dynamic thresholding techniques, comprising the steps of:

slipping one relative code phase of a pseudo-random code (PRC) used in despreading a received signal;
incrementing a PRC period counter;
collecting a data sample of said despread received signal for a duration of a period of said PRC;
determining a correlation power value in each of multiple channels of said data sample collected;
calculating a mean correlation power value in each of said multiple channels over a predetermined number of PRC periods;
applying a first trip algorithm based on a current maximum channel correlation value, the mean correlation power value, and a first threshold value to determine whether a first trip condition exists;
applying a second trip algorithm based on an average correlation power of the current and a previous maximum channel correlation value, the mean correlation power value, and a second threshold value to determine whether a second trip condition exists;
selecting, if both said first and second trip conditions exist, the trip algorithm having a largest peak-to-noise power value (SNR);
verifying the trip corresponding to the selected trip algorithm by collecting an additional data sample, recalculating correlation and mean correlation power values, and reapplying the selected trip algorithm;
performing message demodulation if the trip corresponding to the selected trip algorithm was verified as valid;
and
modifying the trip threshold value corresponding to the selected trip algorithm, if the trip corresponding to the selected trip algorithm was not verified as valid, by performing the substeps of,
incrementing a counter that counts false positive trips for the selected trip algorithm,
calculating a ratio of false positive trips against the a count of said PRC period counter;
increasing the threshold value corresponding to the selected trip algorithm if said false trip ratio is greater than a maximum target value, and
decreasing the threshold value corresponding to the selected trip algorithm if said false trip ratio is less than a minimum target value.

13 more elements

Dependencies are inferred from the claim text.

Direct sequence spread spectrum DSP system

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Direct sequence spread spectrum DSP system

Life of this patent

Abstract

Claims14