Correction of code drift in a non-coherent memory

• US 6,643,678 B2
• Filed: 07/26/2002
• Issued: 11/04/2003
• Est. Priority Date: 09/12/2000
• Status: Active Grant

• Current Assignee: L-3 Communications Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Interstate Electronics Corp. (L3Harris Technologies, Inc.)
• Inventors: McKenney, Michael F., Van Wechel, Robert J.
• Primary Examiner(s): Mai, Tan V.
• Application Number:
  US10/207,425
• Publication Number:
  US 20030076910A1
• Time in Patent Office: 466 Days
• Field of Search: 708/530, 708/490, 375/147, 375/150
• US Class Current: 708/530
• CPC Class Codes: G01S 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: G06F 7/4812 Complex multiplication
• CPC Class Codes: G06F 7/4818 using coordinate rotation digital computer [CORDIC]

Abstract

An apparatus and method allow receivers to quickly acquire a pseudorandom noise signal. A receiver can include a Doppler correction circuit, which permits correlation data with frequency shift in the code to be non-coherently integrated among relatively fewer addresses or tap positions in memory. One receiver includes a Doppler offset generator that can advantageously offset a time index used to address a tap position in a non-coherent memory to compensate for code drift in a code with a frequency offset. The amount of offset is computed by accumulating clock cycles of a clock signal that is related to the frequency offset computed by the DFT or FFT frequency bin. The offset aligns a correlation peak in the received code such that the correlation peak can be accumulated in relatively fewer tap positions or addresses.

Claims

1. A method of correcting for code drift in a non-coherent memory of a pseudorandom noise receiver, where the non-coherent memory stores non-coherent accumulations of correlation in elements of the non-coherent memory, the method comprising:

receiving a time index signal, where the time index signal relates to addresses of elements of the non-coherent memory;

receiving an offset signal, where the offset signal is approximately related to an inverse of a product of a frequency computation associated with the non-coherent memory and a number of elements per code chip;

applying the time index signal to the time offset signal to generate a compensated signal;

applying the compensated signal to the non-coherent memory to retrieve a first accumulation;

summing the first accumulation with a sample to generate a second accumulation; and

storing the second accumulation in the non-coherent memory.

2. The method as defined in claim 1, wherein the applying the time index signal to the time offset signal comprises subtracting the time offset signal from the time index signal.

3. The method as defined in claim 1, further comprising:

receiving a clock signal, where the clock signal has a period of approximately the inverse of the product of the frequency computation associated with the non-coherent memory and the number of elements per code chip; and

counting in response to the clock signal to generate the time offset signal.

4. The method as defined in claim 1, wherein the compensated signal indicates an element that is earlier in time relative to the time index signal when the frequency computation associated with the non-coherent memory is positive.

5. The method as defined in claim 1, wherein the pseudorandom noise receiver computes a frequency offset in a Fast Fourier Transform (FFT) manner.

6. The method as defined in claim 1, wherein the pseudorandom noise receiver computes a frequency offset in a Discrete Fourier Transform (DFT) manner.

7. The method as defined in claim 1, wherein the number of tap positions per code chip is approximately 2.

8. A Doppler correction circuit that corrects for code drift in a non-coherent memory of pseudorandom noise receiver, where the non-coherent memory stores non-coherent accumulations of correlation in elements of the non-coherent memory, the Doppler correction circuit comprising:

means for receiving a time index signal, where the time index signal relates to addresses of elements of the non-coherent memory;

means for receiving an offset signal, where the offset signal is approximately related to an inverse of a product of a frequency computation associated with the non-coherent memory and a number of elements per code chip;

means for applying the time index signal to the time offset signal to generate a compensated signal;

means for applying the compensated signal to the non-coherent memory to retrieve a first accumulation;

means for summing the first accumulation with a sample to generate a second accumulation; and

means for storing the second accumulation in the non-coherent memory.

9. A synchronizing circuit that compensates for code drift over time in at least a portion of a non-coherent integration memory, the synchronizing circuit comprising:

an offset occurrence circuit adapted to receive a clock signal and to provide an indication with a period related to an inverse of a product of a frequency computation associated with the at least portion of the non-coherent integration memory;

a counter circuit adapted to accumulate indications provided by the offset occurrence circuit, where an output of the counter circuit is termed a time offset signal; and

an adder circuit adapted to sum a time index signal with the time offset signal, where the time index signal relates to a memory address with no code drift, where an output of the adder circuit is applied to the address of the non-coherent integration memory such that a memory location indicated by the output of the adder circuit is synchronized with the code.

10. The synchronizing circuit as defined in claim 9, wherein the counter circuit further includes an inverter circuit adapted to compute a 2'"'"'s complement inversion of the accumulation of indications such that the time offset signal is negative.

11. The synchronizing circuit as defined in claim 9, wherein the offset occurrence circuit comprises a plurality of offset occurrence circuits and a multiplexer, where the multiplexer is adapted to select an output of the offset occurrence circuit that corresponds to the portion of the non-coherent integration memory.

12. The synchronizing circuit as defined in claim 9, wherein the offset occurrence circuit comprises a clock divider circuit.