Method of processing a digital signal derived from an analog input signal of a GNSS receiver, a GNSS receiver base band circuit for carrying out the method and a GNSS receiver

• US 8,059,698 B2
• Filed: 03/30/2007
• Issued: 11/15/2011
• Est. Priority Date: 03/21/2007
• Status: Active Grant

• Current Assignee: U-Blox AG (u-blox Holding AG)
• Original Assignee: U-Blox AG (u-blox Holding AG)
• Inventors: Mester, Roland, Thiel, Andreas
• Primary Examiner(s): Joseph; Jaison
• Application Number:
  US11/729,820
• Publication Number:
  US 20080232441A1
• Time in Patent Office: 1,691 Days
• Field of Search: 375/150, 375/136, 375/147
• US Class Current: 375/150
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/36 relating to the receiver frond end
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 1/708 Parallel implementation
• CPC Class Codes: H04B 1/7095 Sliding correlator type
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

An acquisition unit of a GNSS receiver base band circuit comprises a correlator (40) with a correlator shift register (43) to which a correlation sequence derived from a basic sequence characteristic for a satellite is fed by a code generator (41). Each of the N=32 or more memory cells of the correlator shift register (43) is connected to two correlator cells (44a, 44b) for multiplying digital values of the correlator sequence from the memory cell with data values of a data sequence, adding up the products and storing the sum in a register as a correlation value pertaining to one of N relative phase positions of the correlation sequence relative to the data sequence during a correlation phase. During a subsequent read-out phase the registers of each row of correlator cells (44a; 44b) are connected to form a daisy chain and the correlation values are fed through the same to appear sequentially at the output of the last register in the row while the correlator shift register (43) and the rows of registers are sequentially filled with initial values for the next correlation phase.

Claims

1. A method of processing a digital signal derived from an analog input signal of a GNSS receiver containing at least one encoded binary sequence characteristic of at least one GNSS satellite and consisting of repetitions of a characteristic basic sequence which extends over a basic interval, the method comprising the following consecutive steps:

deriving at least one finite length data sequence consisting of digital data values from the digital signal,

sequentially feeding digital values of a correlation sequence derived from the basic sequence to a correlator shift register comprising a plurality of sequentially connected memory cells in order to sequentially fill the memory cells of the correlator shift register with consecutive values of the correlation sequence, the consecutive values forming an initial subsequence,

during a correlation phase;

feeding the correlation sequence further through the correlator shift register and correlating the data sequence with a plurality of copies of the correlation sequence having different phase positions with respect to the data sequence in parallel by in each case sequentially multiplying the data values each with the value from one of the plurality of memory cells of the correlator shift register and adding up the products to form a correlation result, and storing each one of the plurality of correlation results in one of a plurality of registers,

during a read-out phase which follows upon the correlation phase;

shifting the plurality of correlation results through the plurality of registers in such a way that the correlation results appear sequentially at the output of a last one of the plurality of registers.

2. Method according to claim 1, where the correlation sequence consists of fixed multiples of basic values which make up the basic sequence.

3. Method according to claim 1, where the length of the data sequence differs from a multiple of a length of the correlation sequence by between 1% and 5%.

4. Method according to claim 1, where during the read-out phase each of the registers is filled with an initial value after the correlation result has been shifted to a subsequent register.

5. Method according to claim 1, where several correlation phases, each followed by a read-out phase, are carried out and during further read-out phases the correlator shift register is in each case filled with values of the correlation sequence forming a further initial subsequence not overlapping with the previous initial subsequence and preferably immediately following upon the same.

6. Method according to claim 1, where deriving the data values from the digital sequence involves a step of integration where consecutive digital values of the digital sequence separated by a length of the basic interval are summed up.

7. Method according to claim 6, where previous to integration the digital signal is mixed with a plurality of different frequency signals in that every sample is multiplied sequentially with numbers each representing one of the frequency signals.

8. Method according to claim 1, where an evaluation is carried out determining a correlation indicator from the correlation results corresponding to a phase position and selecting a set of largest correlation indicators from the correlation indicators corresponding to diverse phase positions, each together with the corresponding phase position.

9. Method according to claim 8, where in a normal mode the correlation values are used as correlation indicators in the evaluation.

10. Method according to claim 8, where in a deep search mode correlation values determined with a plurality of different data sequences but corresponding to the same basic sequence, Doppler frequency and phase position are added up to form correlation sums which are then used as correlation indicators in the evaluation.

11. Method according to claim 10, where in deep search mode a minimum of correlation values is determined and subtracted each time a correlation value is added.

12. Method according to claim 9, where a deep search mode with correlation values being determined with a plurality of different data sequences but corresponding to the same basic sequence, Doppler frequency and phase position are added up to form correlation sums which are then used as correlation indicators in the evaluation is employed whenever at least one previous normal mode evaluation has not yielded a sufficient number of largest correlation indicators each exceeding an average of correlation indicators by a given threshold.

13. GNSS receiver base band circuit, with an acquisition unit for processing the digital values of the digital signal, the acquisition unit comprising a correlator with a code generator for generating a correlation sequence derived from a characteristic basic sequence, a correlator shift register having a plurality of sequentially connected memory cells and being configured to be fed by the code generator, a plurality of correlator cells, each connected to an input of the correlator and to one of the memory cells of the correlator shift register and comprising a multiplier for multiplying a data value present at an input of the correlator with a value of the correlation sequence read from the memory cell to form a product, a register for storing a value and an adder configured to add the product to the said value, and a switch configured to connect an input of the register either to the output of the adder or to the register of a previous correlator cell where applicable.

14. GNSS receiver base band circuit according to claim 13, where in a first correlator cell the switch is configured to connect the input of the register either to the output of the adder or to the output of a register holding an initial value.

15. GNSS receiver base band circuit according to claim 13, the registers of the correlator cells forming at least one row of registers with consecutive registers connected via one of the switches.

16. GNSS receiver base band circuit according to claim 15, comprising a semiconductor chip with a chip surface carrying the correlator cells implemented as identical structures, with correlator cells whose registers form a row of registers being arranged in a straight line.

17. GNSS receiver base band circuit according to claim 16, where the chip surface of the semiconductor chip also carries the correlator shift register, with the memory cells of the same being implemented as identical structures and arranged in a row parallel to the at least one row of correlator cells in such a way that each memory cell is situated beside the at least one correlator cell which is connected to the same.

18. GNSS receiver base band circuit according to claim 15, the number of correlator cells whose registers form a row of registers being at least 64 and preferably at least 128.

19. GNSS receiver base band circuit according to claim 13, the acquisition unit further comprising an integrator preceding the correlator, the integrator comprising at least one adder and at least one first memory unit and at least one second memory unit and switching means configured to either connect the at least one first memory unit to the adder for feeding an intermediate value stored in a memory cell of the first memory unit to the same and writing the result of an addition of a digital value to the intermediate value to the same memory cell and to connect the at least one second memory unit to an output of the integrator or connect the at least one second memory unit to the adder and the at least one first memory unit to the output of the integrator.

20. GNSS receiver base band circuit according to claim 13, the acquisition unit further comprising at least one register configured for intermediate storage of a digital value pertaining to a digital signal fed to the input of the acquisition unit, a plurality of oscillators configured to sequentially provide frequency signal values pertaining to different frequencies, a mixer configured to multiply the digital value with a frequency signal value and a multiplexer configured to connect each of the oscillators to the mixer in turn.

21. GNSS receiver base band circuit according to claim 13, the acquisition unit further comprising an evaluator with a processing unit connected to the output of the correlator and configured to determine correlation values and an arbitration unit configured to select a set of largest correlation indicators each together with its phase position from correlation indicators determined from the correlation values.

22. GNSS receiver base band circuit according to claim 21, the evaluator comprising an adder with one input connected to the output of the processing unit and a memory with a plurality of memory cells, each corresponding to a basic sequence, a Doppler frequency and a phase position, the memory being configured to feed a value from a memory cell to a second input of the adder for adding a correlation value corresponding to the same basic sequence, Doppler frequency and phase position to it and deposing the sum in the same memory cell.

23. GNSS receiver comprising a base band circuit according to claim 13 and further comprising a radio frequency circuit with at least one mixer for providing an analog signal and with at least one A/D converter for converting the analog signal to a digital signal.