Serial search acquisition system with adaptive threshold and optimal decision for spread spectrum systems
First Claim
1. A method for PN code acquisition in a PN code signal transmitted in a Code Division Multiple Access (CDMA) Direct Sequence Spread Spectrum (DSSS) system, comprising the steps of:
- receiving a transmitted PN code signal and multiplying said signal with a PN code replica signal to produce an input signal for use in determining the phase alignment of said PN code signals;
integrating the signal energy of said input signal over a time interval to obtain a first result;
establishing a noise estimate;
establishing a signal detection threshold having a value greater than that of said noise estimate;
establishing a signal classification threshold, having a value between that of said signal detection threshold and said noise estimate;
establishing a first number CN of PN phases i to be searched and a total number q of PN phases for said PN code replica signal;
maintaining a count cn of said PN phases i; and
searching for said alignment by detecting the signal energy levels and the noise levels of said input signal by the steps of;
evaluating whether said first result is equal to or exceeds the value of said signal detection threshold;
if NO, evaluating whether said first result exceeds the value of said signal classification threshold; and
if NO, updating the value of said noise estimate with the value of said first result and updating said count cn by 1, and determining whether said updated count is equal to or exceeds CN ;
if YES, ending the search;
if NO, determining whether the PN phase i of said PN code replica signal equals q;
if YES, ending the search;
if NO, updating the PN phase i of said PN code replica signal by 1, and continuing the search by repeating the foregoing steps using the phase updated PN code replica signal to multiply said transmitted PN code signal;
if YES, resetting said count cn to 0, and determining whether the PN phase i of said PN code replica equals q;
if YES, ending the search;
if NO, updating the PN phase i of said PN code replica signal by 1, and continuing the search by repeating the foregoing steps using the phase updated PN code replica signal to multiply said transmitted PN code signal.
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Abstract
A PN code acquisition system for Code Division Multiple Access (CDMA) Direct Sequence Spread Spectrum (DSSS) systems, with automatic decision threshold, is based on the combination of the Maximum Likelihood (ML) and Serial Search (SS) acquisition approaches. In contrast to the conventional SS acquisition system, the disclosed system adaptively estimates optimal threshold by exploiting the statistics of the signal and noise, and makes an optimal decision based on the threshold. The system estimates the threshold by employing ML estimation and applies the threshold as in SS acquisition and makes a decision by comparing the updated threshold with the current signal strength. Together with post detection verification logic, this approach will increase detection probability and reduce false alarm probability significantly. An optimum system parameter design approach and the advantages of this approach as compared to conventional approaches (ML or SS acquisition) are demonstrated.
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Citations
20 Claims
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1. A method for PN code acquisition in a PN code signal transmitted in a Code Division Multiple Access (CDMA) Direct Sequence Spread Spectrum (DSSS) system, comprising the steps of:
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receiving a transmitted PN code signal and multiplying said signal with a PN code replica signal to produce an input signal for use in determining the phase alignment of said PN code signals; integrating the signal energy of said input signal over a time interval to obtain a first result; establishing a noise estimate; establishing a signal detection threshold having a value greater than that of said noise estimate; establishing a signal classification threshold, having a value between that of said signal detection threshold and said noise estimate; establishing a first number CN of PN phases i to be searched and a total number q of PN phases for said PN code replica signal; maintaining a count cn of said PN phases i; and searching for said alignment by detecting the signal energy levels and the noise levels of said input signal by the steps of; evaluating whether said first result is equal to or exceeds the value of said signal detection threshold; if NO, evaluating whether said first result exceeds the value of said signal classification threshold; and if NO, updating the value of said noise estimate with the value of said first result and updating said count cn by 1, and determining whether said updated count is equal to or exceeds CN ; if YES, ending the search; if NO, determining whether the PN phase i of said PN code replica signal equals q; if YES, ending the search; if NO, updating the PN phase i of said PN code replica signal by 1, and continuing the search by repeating the foregoing steps using the phase updated PN code replica signal to multiply said transmitted PN code signal; if YES, resetting said count cn to 0, and determining whether the PN phase i of said PN code replica equals q; if YES, ending the search; if NO, updating the PN phase i of said PN code replica signal by 1, and continuing the search by repeating the foregoing steps using the phase updated PN code replica signal to multiply said transmitted PN code signal. - View Dependent Claims (2, 3, 4, 5)
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6. In an apparatus for PN code acquisition for CDMA-DSSS systems comprising:
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means for receiving a transmitted PN code signal and multiplying said signal with a PN code replica signal to produce an input signal for use in determining the phase alignment of said PN code signals; means for integrating the signal energy of said input signal over a time interval and producing an energy level output signal; signal energy detector means for using said energy level output signal to determine a signal level threshold and producing a signal indicative of said threshold; the improvement comprising; a noise energy level estimator for producing a noise estimate signal; signal classification means for using said signal level threshold signal and said noise estimate signal to produce a signal classification threshold; comparator means for comparing said energy level output signal to said signal classification threshold and for producing a first signal when said energy level output signal is smaller and a second signal when it is not; means, responsive to said first signal, for making said noise estimate signal equal to said energy level output signal; and means, responsive to said second signal, for changing the phase of said PN code replica signal. - View Dependent Claims (7, 8)
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9. A method for searching a pseudorandom noise (PN) code space containing a total number q of PN chips or phases i, comprising the steps of:
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predetermining values for; a first number of PN phases to be searched, CN ; a first signal detection threshold, TSD1 ; a first noise estimate, EN ; and a first signal classification threshold, TSC1, having a value between said first signal detection threshold and said first noise estimate; maintaining a count cn of said PN phases; and (a) combining a received PN code signal with a first local PN code signal to obtain an input signal; (b) integrating the signal energy of said input signal over a first predetermined time period τ
1 to obtain a first result Z1 ;(c) comparing the value of said first result Z1 to said first signal detection threshold TSD1 value times (1-y), where y is between 1/16 and 1/8; and
,(i) if said first result Z1 value is less than said first signal detection threshold TSD1 value times (1-y), comparing said first result Z1 value to said first signal classification threshold TSC1 value; (1) if said first result Z1 value is less than said first signal classification threshold TSC1 value;
then,updating the values of said first noise estimate EN and said phase count cn ; comparing the updated phase count value with said first number of phases to be searched CN ; and if said updated phase count value is equal to or greater than said first number of phases CN, ending the search;
else,if the total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal, and then repeating said (a,b,c) combining, integrating, and comparing steps with said updated values; if said total number of phases q is passed, ending the search; (2) if the value of said first result Z1 is equal to or greater than said first signal classification threshold TSC1, value, then, setting said count cn of said PN phases to zero; and
,if said total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal; and
then repeating said (a,b,c) combining, integrating, and comparing steps with said updated values;if said total number of phases q is passed, ending the search; (ii) if the value of said first result Z1 is equal to or greater than said first signal detection threshold TSD1 value times (1-y), comparing said first result value to said first signal detection threshold TSD1 value; and
,if said first result value is greater than said first signal detection threshold value, then, changing said first signal detection threshold value to the value of said first result Z1, and updating said first signal classification threshold TSC1 value to a value equal to the average of said updated signal detection threshold value and said first noise estimate EN value;
else,retaining said first threshold TSD1 and TSC1 values as said updated threshold values; and (d) integrating the signal energy of said input signal over a second predetermined time period τ
2 to obtain a second result Z2 ;(e) comparing the value of said second result Z2 to a second signal detection threshold TSD2 value; and
,(i) if said second result Z2 value is less than said second signal detection threshold TSD2 value, then, comparing said second result Z2 value to a second signal classification threshold TSC2 value; and if said second result Z2 value is less than said second signal classification threshold TSC2 value, then, updating the value of said first noise estimate EN and said phase count cn ; comparing said updated phase count cn value with said first number of phases to be searched CN ; and if said updated phase count cn value is equal to or greater than said first number of phases CN, ending the search;
else,if the total number q of phases in said PN space has not been reached, updating the phase i of said local PN code signal, and then repeating said (a,d,e) combining, integrating, and comparing steps with the updated values; if said total number q is passed, ending the search; (ii) if the value of said second result Z2 is equal to or greater than said second signal detection threshold TSD2 value, then, changing said second signal detection threshold TSD2 value to the value of said second result Z2, and updating said second signal classification threshold TSC2 value to a value equal to the average of the values of said second result Z2 and said first noise estimate EN, and updating said first number of phases to be searched CN ; and then, setting said count cn of said PN phases to zero; and
,if said total number q of phases in said PN space has not been reached, updating the phase i of said local PN code signal, and then repeating said (a,d,e) combining, integrating, and comparing steps with updated values;
else,ending the search. - View Dependent Claims (10, 11, 12)
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13. An apparatus for PN code acquisition in CDMA-DSSS systems, with automatic decision threshold, comprising:
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means for receiving a transmitted PN code signal and multiplying said signal with a PN code replica signal to produce an input signal for use in determining the phase alignment of said PN code signals; means for integrating the signal energy of said input signal over a time interval to obtain a first result; means for storing a noise estimate value; means for storing a signal detection threshold value greater than said noise estimate value; means for storing a signal classification threshold value between said signal detection threshold and said noise estimate values; means for storing a first number CN of PN phases i to be searched and a total number q of PN phases; means for maintaining a count cn of said PN phases; and means for searching for said alignment by detecting the signal energy levels and the noise levels of said input signal, comprising; means for evaluating whether said first result exceeds said signal detection threshold and producing a first signal if YES and a second signal if NO; means, responsive to said first signal, for setting the value of said signal detection threshold equal to said first result and updating said signal classification threshold to the average value of said first result and said noise estimate; means, responsive to said second signal, for evaluating whether said first result exceeds said signal classification threshold and producing a third signal if NO and a fourth signal if YES; means, responsive to said third signal, for updating the value of said noise estimate by the value of said first result and updating said count cn by 1, and determining whether said updated count is equal to or greater than CN and producing a fifth signal if YES and a sixth signal if NO; means, responsive to said fifth signal, for ending the search; means, responsive to said sixth signal, for updating the PN phase i of said PN code replica signal by 1, and determining whether said updated phase equals q, and producing a seventh signal if YES and an eighth signal if NO; means, responsive to said seventh signal, for ending the search; means, responsive to said eighth signal, for continuing the search by actuating said foregoing means to repeat the foregoing functional steps using the phase updated first result; means, responsive to said fourth signal, for setting said count cn to zero; and means, responsive to the setting of said count cn to zero, for checking the PN phase i with respect to the total number q, and producing a ninth signal if the PN phase i is not equal to q, and a tenth signal if the PN phase i is equal to q; means, responsive to said ninth signal, for updating the PN phase i of said PN code replica signal by 1, and continuing the search by actuating said searching means to repeat the foregoing functional operations using the phase updated first result; and means, responsive to said tenth signal, for ending the search. - View Dependent Claims (14, 15)
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16. A CDMA-DSSS system for acquiring a transmitted PN code signal, comprising:
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means for receiving a transmitted PN code signal; means for producing a cross correlation between said received PN code signal and a locally generated PN code, to achieve phase alignment thereof, and outputting a cross correlation signal indicative thereof; first comparator means for comparing said cross correlation signal with a signal detection threshold and producing a first signal if said cross correlation signal exceeds the signal detection threshold, and a second signal if it does not; means, responsive to said first signal from said first comparator means, for updating said signal detection threshold to the value of said cross correlation signal; second comparator means, responsive to said second signal from said first comparator means, for comparing said cross correlation signal with a signal classification threshold, having a value between said signal detection threshold and a noise estimate, and producing a first signal if said cross correlation signal exceeds the signal classification threshold, and a second signal if it does not; means, responsive to said first signal from said second comparator means, for updating the phase of said locally generated PN code unless a first predetermined phase limit has been reached; counter means for maintaining a count of the updated phases of said locally generated PN code; means, responsive to said second signal from said second comparator means, for updating said noise estimate and comparing the count of the updated phases of said locally generated PN code in said counter means and a second predetermined phase limit, and updating the phase of said locally generated PN code unless said second predetermined phase limit has been reached; and means for ending the updating of the phase of said locally generated PN code when either said first or second predetermined phase limit is reached. - View Dependent Claims (17, 18)
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19. A method for searching a pseudorandom noise (PN) code space containing a total number of PN chips or phases, q, comprising the steps of:
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predetermining a first number of PN phases to be searched CN ; predetermining a first signal detection threshold TSD having a value comprising TSD1 (1-y), y being between 1/16 and 1/8 and TSD1 being a threshold value based on signal energy; predetermining a first noise estimate EN having a value less than that of said threshold value TSD1 ; predetermining a first signal classification threshold TSC, having a value TSC1 between that of said threshold value TSD1 and that of said first noise estimate EN ; maintaining a count cn of said PN phases including a first phase count; (a) combining a received PN code signal with a first local PN code signal, having a phase i, to obtain an input signal; (b) integrating the signal energy of said input signal over a first predetermined time period to obtain a first result Z1 ; (c) comparing the value of said first result Z1 to said first signal detection threshold TSD value, and, (i) if said first result Z1 value is less than said first signal detection threshold TSD value, comparing said first result Z1 value to said first signal classification threshold value TSC1 ; and if said first result Z1 value is less than said first signal classification threshold TSC value, then, updating said first noise estimate EN and said first phase count, and comparing said updated phase count with said first number of phases to be searched CN, and if said updated phase count is equal to or greater than said first number CN, ending the search;
else,if the total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal, and then repeating said combining, integrating, and comparing steps with updated values; when said total number of phases q is reached, ending the search; (ii) if said first result Z1 value is equal to or greater than said first signal detection threshold TSD value, comparing said first result Z1 value to said threshold value TSD1 and if said first result Z1 value exceeds said threshold value TSD1, setting said threshold value TSD1 equal to said first result Z1 and updating said first signal detection threshold value TSD1 value to the average value of said first result Z1 and said first noise estimate EN ; and (d) integrating the signal energy of said input signal over a second predetermined time period to obtain a second result Z2 ; (e) comparing the value of said second result Z2 to a second signal detection threshold value TSD2, and, if said second result Z2 is equal to or greater than said second signal detection threshold value TSD2, setting said second signal detection threshold value TSD2 equal to said second result Z2, resetting said PN phase count cn to zero, and if said total number of phases q has not been reached, updating the phase i of said local PN code signal and combining it with said received PN code signal, and repeating said integrating, and comparing steps for said second predetermined time period; and when said first number CN, or said total number of phases q, has been reached, ending the search.
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20. A method for searching a pseudorandom noise (PN) code space containing a total number q of PN chips or phases i, comprising the steps of:
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predetermining values for; a first number of PN phases to be searched CN ; a first signal detection threshold TSD ; a first noise estimate EN ; and a first signal classification threshold TSC, having a value between that of said first signal detection threshold and that of said first noise estimate; and maintaining a count cn of said PN phases including a first phase count cn1 ; (a) combining a received PN code signal with a first local PN code signal, having a phase i, to obtain an input signal; (b) integrating the signal energy of said input signal over a first predetermined time period to obtain a first result Z1 ; (c) comparing the value of said first result Z1 to said first signal detection threshold value TSD1 times (1-y), where y is between 1/16 and 1/8, and, (i) if said first result Z1 is less than said first signal detection threshold value TSD1 times (1-y), comparing said first result to said first signal classification threshold value TSC1 ; (1) if said first result Z1 is less than said first signal classification threshold value TSC1, then, updating said first noise estimate value EN1 to EN2 and said first phase count cn1 to cn2, and comparing said updated phase count cn2 with said first number of phases CN to be searched, and if said count cn2 is equal to or greater than said number CN, ending the search, else, if the total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal to i+1, and then repeating said (a,b,c) combining, integrating, and comparing steps with updated values; if said total number q is reached, ending the search; (2) if said first result Z1 is equal to or greater than said first signal classification threshold value TSC1, then, setting said count cn of said PN phases to zero and, if said total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal to i+1, and then repeating said (a,b,c) combining, integrating, and comparing steps with updated values; if said total number q is reached, ending the search; (ii) if said first result Z1 is equal to or greater than said first signal detection threshold value TSD1 times (1-y), comparing the value of said first result Z1 to the value of said first signal detection threshold value TSD1, and, if said first result Z1 is greater than said first signal detection threshold value TSD1, changing said first signal detection threshold value TSD1 to said first result Z1, and updating said first signal classification threshold value TSC1 to a value equal to the average of the value of said first result Z1 and said first noise estimate value EN1, else, retaining said first threshold values TSD1 and TSC1 ; and (d) integrating the signal energy of said input signal over a second predetermined time period to obtain a second result Z2 ; (e) comparing said second result Z2 to a second signal detection threshold value TSD2, and, (i) if said second result Z2 is less than said second signal detection threshold value TSD2, comparing said second result Z2 to a second signal classification threshold value TSC2, and if said second result Z2 is less than said second signal classification threshold value TSC2, then, updating said first noise estimate EN1 to EN2 and said first phase count cn1 to cn2, and comparing said updated phase count cn2 with said first number of phases CN to be searched, and if said updated phase count cn2 is equal to or greater than said number CN, ending the search, else, if the total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal to i+1, and then repeating said (a,b,c,d,e) combining, integrating, and comparing steps with updated values; when said total number q or said number CN is reached, ending the search; (ii) if said second result Z2 is equal to or greater than said second signal detection threshold value TSD2, changing said second signal detection threshold value TSD2 to the value of said second result Z2, and updating said second signal classification threshold value TSC2 to a value equal to the average of said second result Z1 and said first noise estimate value EN1, and updating said number of phases CN to be searched, and then, setting said count cn of said PN phases to zero and, if said total number of phases q in said PN space has not been reached, updating the phase i of said local PN code signal to i+1, and then repeating said (a,b,c,d,e) combining, integrating, and comparing steps with updated values; when said total number q or said number CN is reached, ending the search.
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Specification