Spread spectrum receiver performance improvement

• US 6,606,349 B1
• Filed: 02/04/1999
• Issued: 08/12/2003
• Est. Priority Date: 02/04/1999
• Status: Expired due to Term

• Current Assignee: CSR Technology Incorporated (Qualcomm, Inc.), Conexant Systems Incorporated (Synaptics Incorporated)
• Original Assignee: SIRF Technology Incorporated (Qualcomm, Inc.)
• Inventors: Kudhrethaya, Shridhara A., Underbrink, Paul A.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Fan, Chieh M.
• Application Number:
  US09/244,696
• Time in Patent Office: 1,650 Days
• Field of Search: 375/150, 375/130, 375/140, 375/141, 375/142, 375/143, 375/145, 375/147, 375/149, 375/152, 375/343, 370/203, 370/208, 370/209, 370/320, 370/335, 370/342, 370/441, 370/479, 701/200, 701/213
• US Class Current: 375/150
• CPC Class Codes: H04B 1/70755 Setting of lock conditions, e.g. threshold

Abstract

Embodiments of the invention are used to improve performance of spread spectrum communications equipment. The improvement may take the form of improved sensitivity to weak signals, quicker acquisition time of weak signals, or the reduction of multipath interference. To acquire a spread spectrum signal PN codes are commonly compared with the spread spectrum signal. This comparison is often in reality a successive series of comparisons with the same code. Successive comparisons are often carried out in a process called hypothesis testing by which the correlated output of the spread spectrum signal and the PN code must pass a series of thresholds. This method is adopted to prevent identifying a PN code as being present based on a spurious correlation value. This method of successive comparisons also prevents a low correlated value from being rejected immediately thereby helping assure that signals with low signal strength can be identified also. By adjusting the series of comparison levels within the hypothesis tester depending on the signal strength weak signals can be identified quicker and easier, multipath signals can be rejected better, and amplification devises such as active antennas can be switched on and off.

Claims

1. A method for improving reception of spread spectrum communications, the method comprising:

receiving a spread spectrum signal;

determining the strength of the spread spectrum signal;

correlating the spread spectrum signal with a particular PN code to produce a correlated output;

adjusting a threshold value based on the signal strength of the spread spectrum signal to a first predetermined threshold level;

comparing the correlated output to the threshold value to determine whether the particular PN code is present in the spread spectrum signal;

adjusting the threshold value of the spread spectrum signal to a next predetermined threshold level if a final threshold level is not equal to the next predetermined threshold level; and

switching between active and passive antenna operation mode depending on the signal strength.

2. The method of claim 1 wherein the comparison is performed by using a hypothesis testing mechanism.

3. The method of claim 1 wherein the signal strength is determined by:

correlating the spread spectrum signal with a PN code guaranteed not to present; and

using the correlation value determined by correlating the spread spectrum signal with the PN code guaranteed not to be present in the system to determine the signal strength.

4. The method of claim 1 wherein the signal strength is determined by:

correlating the spread spectrum signal with a PN code not being received by the system; and

using the correlation value determined by correlating the spread spectrum signal with a correlation code not being received in the system to determine the signal strength.

5. An apparatus for improving a spread spectrum communications system comprising:

a spread spectrum signal receiver;

means for determining the signal strength of a spread spectrum signal received by the receiver;

a first correlator for correlating the spread spectrum signal with a PN code;

a comparison unit for comparing an output of the first correlator to a first threshold level for the purpose of deciding if a particular PN code is present in the spread spectrum signal;

means for adjusting the first predetermined threshold level to a second predetermined threshold value based on the signal strength of the spread spectrum signal; and

switching between active and passive antenna operation depending on the signal strength.

6. An apparatus as in claim 5 wherein the comparison unit further comprises a mechanism for performing mathematical operations on the values output from the first correlator.

7. An apparatus as in claim 5 wherein the means for determining signal strength further comprises:

a PN code not currently being received by the receiver;

a comparator for comparing the output of the second correlator to the second predetermined threshold value.

8. An apparatus as in claim 5 wherein the comparator contains a hypothesis testing mechanism.

9. An apparatus as in claim 5 wherein the means for determining signal strength further comprises:

a plurality of PN codes within the spread spectrum signal received by the receiver;

a correlator for correlating the plurality of PN codes with the spread spectrum signal;

a device for retaining the correlated values; and

a comparison circuit for comparing the correlated values with a predetermined maximum value.

10. An apparatus as in claim 7 wherein the code not currently being received is the 25th GPS code.

11. A method for improving reception of spread spectrum communications, the method comprising:

(a) sampling a spread spectrum signal spread by a first PN code;

(b) selecting an initial predetermined value for a current threshold value;

(c) correlating the spread spectrum signal with a selected PN code to produce a correlated output;

(d) determining the signal strength of the spread spectrum signal;

(e) switching between an active and passive antenna mode depending upon the signal strength;

(f) comparing the correlated output to the current threshold value;

(g) upon the correlated output exceeding the current threshold value and the current threshold value exceeding a desired predetermined final threshold value, indicating the selected PN code is present in the spread spectrum signal;

(h) upon the correlated output exceeding the current threshold value and the current threshold value is below the desired final predetermined threshold value adjusting the current threshold value to a next predetermined threshold level;

(i) when the correlated output is below the current threshold value;

incrementing a failure counter and determining whether the failure counter exceeds a desired failure total;

(j) when the correlated output is below the current threshold value and the failure counter exceeds the desired failure total, indicating the selected PN code is not present in the spread spectrum signal;

(k) repeating (a) through (j) until there is an indication that the spread spectrum signal is either present or not present.

12. The method according to claim 11 wherein the initial value of the current threshold value is based on signal strength of the spread spectrum signal, the method further comprising determining the signal strength of the spread spectrum signal.

13. The method according to claim 12 wherein adjusting the current threshold value is dependent upon the signal strength of the spread spectrum signal.

14. An apparatus for improving reception of a spread spectrum signal comprising:

a spread spectrum signal receiver;

means for sampling a spread spectrum signal received by the receiver;

means for selecting an initial predetermined value for a current threshold value;

a correlator operably receiving a sampled spread spectrum signal and a selected PN code;

means for determining the signal strength of the spread spectrum signal;

means for switching between an active and passive antenna mode depending upon the signal strength;

a comparator operably receiving an output of the correlator and comparing the output of the correlator to the current threshold value;

means for determining reception of the spread spectrum signal, the reception determining means;

(i) adjusting the current threshold value upon (a) the correlated output the current threshold value and (b) the current threshold value is less than a desired final predetermined threshold value (ii) indicating the presence of the selected PN code in the spread spectrum signal upon (a) the correlated output exceeding the current threshold value and (b) the current threshold value exceeding the desired final predetermined threshold value, and (iii) incrementing a failure counter when the correlated output is below the current threshold value; and

means for indicating failure to detect the selected PN ode n the spread spectrum signal upon the failure counter exceeding a desired failure rate.

15. An apparatus as in claim 14 wherein the means for selecting an initial value for a current threshold value includes means for determining the signal strength of the spread spectrum signal.

16. An apparatus as in claim 14 further comprising a plurality of additional correlators each working in substantially parallel with the correlator to correlate the spread spectrum signal with time shifted copies of the selected PN code to facilitate quicker acquisition.