SYSTEM AND METHOD FOR ENHANCED DATA DECODE IN A GPS RECEIVER

• US 20100134349A1
• Filed: 12/02/2008
• Published: 06/03/2010
• Est. Priority Date: 12/02/2008
• Status: Active Grant

• Current Assignee: CSR Technology Incorporated (Qualcomm, Inc.)
• Original Assignee: CSR Technology Incorporated (Qualcomm, Inc.)
• Inventors: Lennen, Gary, Keal, William Kerry
• Granted Patent: US 8,151,170 B2
• Time in Patent Office: Days
• US Class Current: 342/357.770
• CPC Class Codes: G01S 19/243 Demodulation of navigation message
• CPC Class Codes: G01S 19/27 creating, predicting or correcting ephemeris or almanac data within the receiver

Abstract

Systems and methods are disclosed herein for improving the sensitivity of satellite data decode in a satellite navigation receiver. The low signal ephemeris data decoding system of the present disclosure achieves a 5 db improvement in decoding sensitivity over conventional system by operating down to a CN0 of 21 dB-Hz. The improved sensitivity is achieved through a combination of reducing the number of data bits to be decoded, overcoming the inherent differential decoding problem of an all data bit polarity inversion, improving the probability of seeing single bit decoding error in an ephemeris word, running the parity correction algorithm, and reducing the undetected word error rate. The improved sensitivity makes it possible to predict the orbit of the satellite and to determine the receiver'"'"'s location with higher accuracy even when operating in challenging signal conditions.

Claims

1. A data decoding system for a satellite-based navigation system comprising:

a data prediction subsystem adapted to provide predicted data bits for a satellite;

a differential data decoding subsystem adapted to provide decoded data bits from the satellite for data bits other than the predicted data bits;

a data parity check and correction subsystem adapted to perform parity check and to correct bit errors in the decoded data bits from the differential data decoding subsystem and the predicted data bits from the data prediction subsystem to provide equivalent data words for a plurality of frames; and

a frame combination subsystem adapted to compare the equivalent data words for the plurality of frames to provide a decoded data word.

2. The data decoding system of claim 1, wherein the data prediction subsystem provides the predicted data bits for GPS ephemeris data by using older ephemeris data and almanac data of a GPS satellite.

3. The data decoding system of claim 1, wherein the data prediction subsystem is a network transmitting the predicted data bits to assist data decoding systems within the network.

4. The data decoding system of claim 1, further comprising a CN0 measurement subsystem adapted to measure a signal strength of the satellite.

5. The data decoding system of claim 4, wherein if the signal strength of the satellite measured by the CN0 measurement subsystem is below a threshold the differential data decoding subsystem, the data parity check and correction subsystem, and the frame combination subsystem are disabled.

6. The data decoding system of claim 4, wherein the signal strength is measured over 32 bits of an extended GPS word comprising of 30 bits of a GPS word and last 2 bits of a previous GPS word.

7. The data decoding system of claim 1, wherein the differential data decoding subsystem comprises:

a plurality of differential phase decoders adapted to provide a plurality of hypothesized decoder outputs; and

a majority vote unit adapted to provide the decoded data bits from the satellite based on the plurality of hypothesized decoder outputs.

8. The data decoding system of claim 7, wherein each of the plurality of differential phase decoders further comprises:

a differential phase unit adapted to provide a phase difference between a phase of a data bit to be decoded and a phase of a surrounding bit separated by a bit distance from the data bit to be decoded;

a bit inversion decision unit adapted to provide a bit inversion decision based on the phase difference; and

an exclusive-or unit adapted to provide the hypothesized decoder output by inverting a previous decoder output separated from the hypothesized decoder output by the bit distance in the differential phase unit based on the bit inversion decision;

9. The data decoding system of claim 8, wherein for the differential phase decoder the data bit to be decoded is not one of the predicted data bits from the data prediction subsystem.

10. The data decoding system of claim 8, wherein for the differential phase decoder the previously decoded output is initialized with one of the predicted bits from the data prediction subsystem.

11. The data decoding system of claim 7, wherein the majority vote unit adds the plurality of hypothesized decoder outputs to generate a summation.

12. The data decoding system of claim 11, wherein the majority vote unit compares the summation against a majority threshold to provide a bit of the decoded data bits.

13. The data decoding system of claim 1, wherein the data parity check and correction subsystem operates on a (32, 26) Hamming code word of an extended GPS word where the 26 data bits of the (32, 26) Hamming code word comprises 24 information bearing bits and 2 last parity bits of a previous (32, 26) Hamming code word.

14. The data decoding system of claim 13, wherein each of the equivalent data words for the plurality of frames from the data parity check and correction subsystem is a GPS data word comprising the 24 information bearing bits and the 6 parity bits of the (32, 26) Hamming code word.

15. The data decoding system of claim 1, wherein the frame combination subsystem compares a data word from the equivalent data words for the plurality of frames only if the data word has a successful parity check from the data parity check and correction subsystem.

16. The data decoding system of claim 15, wherein the frame combination subsystem provides a decoded data word if the equivalent data words for the plurality of frames match.

17. The data decoding system of claim 4, wherein if the signal strength of the satellite measured by the CN0 measurement subsystem is below a threshold the frame combination subsystem compares the equivalent data words for more than two frames.

18. A method of decoding data bits from satellites of a satellite-based navigation system, comprising:

predicting data bits for the satellites to form predicted data bits;

reducing number of data bits to be decoded by using the predicted data bits;

performing a plurality of differential phase decoding between a data bit to be decoded and a plurality of data bits surrounding the data bit to be decoded to form a decoded data bit;

performing data parity checking and error correction on a data word comprising a plurality of predicted data bits and a plurality of decoded data bits to form equivalent parity checked and corrected data words for a plurality of frames;

comparing the equivalent parity checked and corrected data words for the plurality of frames to form a decoded data word.

19. The method of claim 18, wherein said predicting data bits for GPS satellites comprises predicting GPS ephemeris data by using older ephemeris data and almanac data.

20. The method of claim 18, wherein said predicting data bits for the satellites comprises receiving the predicted data bits from a network.

21. The method of claim 18, further comprising measuring signal strength to form a CN0 measurement.

22. The method of claim 21, further comprising disabling performing the plurality of differential phase decoding if the CN0 measurement is below a threshold.

23. The method of claim 18, wherein performing each of the plurality of differential phase decoding comprises:

calculating a phase difference between a phase of the data bit to be decoded and a phase of a surrounding data bit where the surrounding data bit is separated from the data bit to be decoded by a bit distance;

comparing the phase difference to decision boundaries to form a bit inversion decision; and

inverting a previous decoded data bit separated from the data bit to be decoded by the bit distance used in calculating the phase difference based on the bit inversion decision to form a hypothesized decoded data bit;

24. The method of claim 23, wherein performing the plurality of differential phase decoding further comprises:

adding the hypothesized decoded data bit from each of the plurality of differential phase decoding to form a sum; and

comparing the sum against a majority threshold to form the decoded data bit;

25. The method of claim 23, wherein said reducing the number of data bits to be decoded by using the predicted data bits comprises initializing the previous decoded data bit with one of the predicted data bits.

26. The method of claim 18, wherein said performing data parity checking and error correction comprises operating on a (32, 26) Hamming code word of an extended GPS word where the 26 data bits of the (32, 26) Hamming code word comprises 24 information bearing bits and 2 last parity bits of a previous (32, 26) Hamming code word.

27. The method of claim 21, wherein said comparing the equivalent parity checked and corrected data words for the plurality of frames comprises comparing equivalent parity checked and corrected data words for more than two frames if the CN₀ measurement is below a threshold.

28. A data decoding system for a satellite-based navigation system comprising:

a data prediction subsystem adapted to provide predicted data bits for a satellite;

a differential data decoding subsystem adapted to provide decoded data bits from the satellite for data bits other than the predicted data bits; and

a data parity check and correction subsystem adapted to perform parity check and to correct bit errors in the decoded data bits from the differential data decoding subsystem and the predicted data bits from the data prediction subsystem to provide a decoded data word for a frame;