Spread spectrum signal processing

• US 20060013287A1
• Filed: 10/03/2003
• Published: 01/19/2006
• Est. Priority Date: 10/15/2002
• Status: Abandoned Application

• Current Assignee: Nordnav Technologies AB (Qualcomm, Inc.)
• Original Assignee: Nordnav Technologies AB (Qualcomm, Inc.)
• Inventors: Normark, Per-Ludvig, Stahlberg, Christian
• Application Number:
  US10/531,132
• Publication Number:
  US 20060013287A1
• Time in Patent Office: Days
• US Class Current: 375/142
• 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: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

The present invention relates to processing of spread spectrum signals, where a continuous signal of a comparatively high frequency is received. This signal is sampled at a basic sampling rate whereby a resulting sequence of time discrete signal samples is produced, which are in turn quantized into a corresponding level-discrete sample value. A plurality of data words are formed, which each includes one or more consecutive sample values. Information obtained from these data words is correlated with at least one representation of a signal source specific code sequence, which has been pre-generated in the form of a code vector. The correlation step specifically involves correlating at least each vector in a sub-group of the code vectors with at least one vector that has been derived from the data word. Thereby resulting data is produced.

Claims

1. A method of processing spread spectrum signals comprising:

receiving a continuous signal (S_IF) of a comparatively high frequency;

sampling the continuous signal (S_IF) at a basic sampling rate (r_s), whereby a resulting sequence of time discrete signal samples (S[s_i]) is produced;

quantizing each signal sample into a corresponding level-discrete sample value;

forming of a plurality of data words (d(1), . . . , d(N)) which each includes one or more consecutive sample values (s₁, . . . , S_n); and

correlating between information in the data words (d(k)) and at least one representation (CS(i)) of a signal source specific code sequence (CS), the method comprising preparing for the correlation step, wherein, before receiving the continuous signal (S_IF), a multitude of code vectors (CV;

CV_m) are pre-generated, each code vector representing a particular code sequence (CS(i)) of the at least one signal source specific code sequence (CS), and

the correlating step involving multiplying at least each vector in a sub-group (CV_m-E, CV_m-P;

CV_m-L) of the code vectors (CV;

CV_m) with at least one vector (S_IF-I(k);

S_IF-Q(k)) derived from the data word (d(k)).

2. A method according to claim 1, wherein each code vector (CV;

CV_m) represents a particular signal source specific code sequence (CS(i)) being sampled at the basic sampling rate (r_s) and quantized with the quantizing process being used to produce the level-discrete sample values (S₁, . . . , S_n).

3. A method according to claim 1, wherein the at least one signal source specific code sequence (CS) is pseudo random noise.

4. A method according to claim 1, wherein the receiving step involving down conversion of an incoming high-frequency signal (S_HF) to an intermediate frequency signal (S_IF), the high-frequency signal (S_HF) having a spectrum which is symmetric around a first frequency (f_HF) and the intermediate frequency signal (S_IF) having a spectrum which is symmetric around a second frequency (f_IF) being considerably lower than the first frequency (f_HF).

5. A method according to claim 4, further comprising the steps of:

determining a maximum frequency variation (f_D) of the second frequency (f_IF) due to Doppler-effects;

defining a Doppler frequency interval (f_IF-min−

f_IF-max) around the second frequency (f_IF), the Doppler frequency interval (f_IF-min, f_IF-max) having a lowest frequency limit (f_IF-min) equal to the difference between the second frequency (f_IF) and the maximum frequency variation (f_D), and a highest frequency limit (f_IF-max) equal to the sum of the second frequency (f_IF) and the maximum frequency variation (f_D);

dividing the Doppler frequency interval (f_IF-min−

f_IF-max) into an integer number of equidistant (Δ

f) frequency steps (f_IF-min, f_IF-min+Δ

f, . . . , f_IF-max); and

defining a frequency candidate vector (f_IF-C) for each frequency step (f_IF-min, f_IF-min+Δ

f, . . . , f_IF-max).

6. A method according to claim 5 further comprising the steps of:

determining an integer number of initial phase positions (φ

₀, . . . , φ

₇) for the frequency candidate vector (f_IF-C), and

defining a carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) for each combination of carrier frequency candidate vector (f_IF-C) and initial phase position (φ

₀, . . . , φ

₇).

7. A method according to claim 6 further comprising the steps of:

determining the number of elements (s_n) in each carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)); and

storing the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) according to a data format being adapted to performing multiplication operations between the data word (d(k)) and a segment of a carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)).

8. A method according to claim 7, wherein adapting of the data format involves adding at least one element to each segment of the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) such that the segment attains a number of elements which is equal to the number of elements in the data word (d(k)), thus enabling a segment of the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) and one of the at least one vector (S_IF-I(k);

S_IF-Q(k)) to be processed jointly by at least one of a SIMD-operation (multiple-bits multiplications) and an XOR-operation (1-bit multiplications).

9. A method according claim 1 further comprising the steps of:

determining a maximum variation a code rate due to Doppler-effects, defining a Doppler rate interval (CR_C-min−

CR_c-max) around a center code rate (CR_c), the Doppler frequency interval having a lowest code rate limit (CR_c-min) equal to the difference between the center code rate (CR_c) and the maximum code rate variation (CR_D), and a highest frequency limit (CR_c-max) equal to the sum of the center code rate (CR_c) and the maximum code rate variation (CR_D);

dividing the Doppler rate interval (CR_C-min−

CR_C-max) into an integer number of equidistant (Δ

CR) code rate steps (CR_C-min, CR_C-min+Δ

CR, . . . , CR_c-max); and

defining a code rate candidate (CR_c-c) for each code rate step (CR_C-min, CR_C-min+Δ

CR, . . . , CR_c-max).

10. A method according to claim 9 further comprising determining an integer number of possible initial code phase positions (0.0, . . . , 0.9) for each code rate candidate (CR_c-c).

11. A method according to claim 10 further comprising defining, for each signal source specific code sequence (CS(i)), a set of combinations between code rate candidate (CR_c-c) and code phase position (0.0, . . . , 0.9), each combination representing a code rate-phase candidate vector.

12. A method according to claim 11 further comprising generating, for each signal source specific code sequence (CS(i)), a set of code vectors (CV) by:

sampling each code rate-phase candidate vector with the basic sampling rate (r_s) whereby a corresponding code vector (CV) is produced.

13. A method according to claim 1 further comprising generating a modified code vector (CV_m) on basis of each code vector (CV) by:

copying a particular number of elements (E_e) from the end of an original code vector (CV) to the beginning of the modified code vector (CV_m); and

copying the particular number of elements (E_b) from the beginning of the original code vector (CV) to the end of the code vector (CV_m).

14. A method according to claim 13 further comprising storing, for each signal source specific code sequence (CS(i)), a set of modified code vectors ({CV_m(CR_c-c, Cph)}), where

each modified code vector (CV_m) contains a number of elements (s₁, . . . , s_m) representing a sampled version of at least one full code sequence (CS),

a particular modified code vector (CV_m) is defined for each combination of code rate candidate (CR_c-c) and code phase position (Cph).

15. A method according to claim 14 further comprising adapting the data format of the modified code vectors (CV_m) with respect to the data format of the at least one vector (S_IF-I(k);

S_IF-Q(k)) derived from the data word (d(k)), such that a modified code vector (CV_m) and one of the at least one vector (S_IF-I(k);

S_IF-Q(k)) may be processed jointly by at least one of a SIMD-operation (multiple-bits multiplications) and an XOR-operation (1-bit multiplications).

16. A method according to claim 14 further comprising involving an initial acquisition phase and a subsequent tracking phase, wherein during the acquisition phase a set of preliminary parameters are established which are required for initiating a decoding of signals received during the tracking phase, a successful acquisition phase resulting in at least one signal source specific code sequence (CS) being identified and a transmitted signal being rendered possible to track, each of the at least one signal source specific code sequence (CS) being associated with tracking characteristics in the form of:

a modified code vector (CV_m),

a carrier frequency candidate vector (f_IF-C),

an initial phase position (φ

_c),

a code phase position (Cph), and

a code index (CI) denoting a starting sample value for the modified code vector (CV_m).

17. A method according to claim 16 wherein said tracking involves:

calculating, based on the tracking characteristics, a prompt pointer (P_p) for each modified code vector (CV_m), the prompt pointer (P_p) indicating a code sequence start position, and an initial prompt pointer (P_p) being equal to the code index (CI); and

assigning, around each prompt pointer (P_p), at least one pair of early- and late pointers (P_E, P_L;

P_E1, P_L1 P_E2, P_L2), where the early pointer (P_E) specifies a sample value being positioned at least one element before the prompt pointer'"'"'s (P_p) position, and the late pointer (P_L) specifies a sample value being positioned at least one element after the prompt pointer'"'"'s (P_p) position.

18. A method according to claim 17, wherein said tracking involves:

receiving a sequence of incoming level-discrete sample values (1210),

forming data words (d(1), . . . , d(N)) of the sample values (1210) such that each data word (d(k)) contains a number of elements which is equal to the number of elements (S_n) in each carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C));

calculating a relevant set of carrier frequency-phase candidate vectors (V_fφ

(f_IF-C, φ

_C)) for the data word (d(k)); and

acquiring, for each carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) in the relevant set, a pre-generated in-phase representation (f_IFI) and a quadrature-phase representation (f_IFQ) of the vector (V_fφ

(f_IF-C, φ

_C)) respectively.

19. A method according to claim 18, wherein said tracking involves:

multiplying each data word (d(k)) with the in-phase representation (f_IFI) of the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)), in the relevant set to produce a first intermediate-frequency-reduced information word (S_IF-I(k)), multiplying each data word (d(k)) with the quadrature-phase representation (f_IFQ) of the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) in the relevant set to produce a second intermediate-frequency-reduced information word (S_IF-Q(k)).

20. A method according to claim 19, wherein said tracking involves:

multiplying the first intermediate-frequency-reduced information word (S_IF-I(k)) with a modified code vector (CV_m-p(k)) starting at a position indicated by the prompt pointer (P_p) to produce a first prompt-despread symbol string (Λ

_Ip(k)), multiplying the first intermediate-frequency-reduced information word (S_IF-I(k)) with a modified code vector (CV_m-E(k)) starting at a position indicated by an early pointer (PE) to produce a first early-despread symbol string (Λ

_IE(k));

multiplying the first intermediate-frequency-reduced information word (S_IF-I(k)) with a modified code vector (CV_m-L(k)) starting at a position indicated by a late pointer (P_L) to produce a first late-despread symbol string (Λ

_IL(k));

multiplying the second intermediate-frequency-reduced information word (S_IF-Q(k)) with a modified code vector (CV_m-p(k)) starting at a position indicated by the prompt pointer (P_p) to produce a second prompt-despread symbol string (Λ

_Qp(k));

multiplying the second intermediate-frequency-reduced information word (S_IF-Q(k)) with a modified code vector (CV_M-E(k)) starting at a position indicated by the early pointer (P_E) to produce a second early-despread symbol string (Λ

_QE(k)); and

multiplying the second intermediate-frequency-reduced information word (S_IF-Q(k)) with a modified code vector (CV_M-L(k)) starting at a position indicated by the late pointer (P_L) to produce a second late-despread symbol string (Λ

_QL(k)).

21. A method according to claim 20, wherein said tracking involves deriving, for each despread symbol string, a resulting data word (D_R-Ip(k), D_R-IE(k), D_R-IL(k), D_R-Qp(k), D_R-QE(k);

D_R-QL(k)).

22. A method according to claim 21 wherein said deriving comprises deriving the resulting data words (D_R-Ip(k), D_R-IE(k), D_R-IL(k), D_R-Qp(k), D_R-QE(k);

D_R-QL(k)) by looking up a respective pre-generated value in a table.

23. A method according to claim 19, wherein multiplying comprises performing the multiplication between the data word (d(k)) and the in-phase representation (f_IFI) of the carrier-frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) respective between the data work (d(k)) and the quadrature-phase representation (f_IFQ) of the carrier frequency-phase candidate vector (V_fφ

(f_IF-C, φ

_C)) by means of at least one of a SIMD-operation (multiple-bits multiplications) and an XOR-operation (1-bit multiplications).

24. A method according to claim 19, wherein multiplying comprises performing the multiplication between the intermediate-frequency-reduced information words (S_IF-I(k);

S_IF-Q(k)) and the modified code vectors (CV_m-p, CV_m-E;

CV_m-L) by means of at least one of a SIMD-operation (multiple-bits multiplications) and an XOR-operation (1-bit multiplications).

25. A method according to claim 19 further comprising propagating, in connection with completing the processing of a current data word (d(k)) and initiating the processing of a subsequent data word (d(k+1)):

a pointer (P_d) indicating a first sample value of the subsequent data word (d(k+1));

a group of parameters describing the relevant set of carrier frequency-phase candidate vectors (V_fφ

(f_IF-C, φ

_C));

the relevant set of code vectors (CV_m); and

prompt-, early-, and late pointers (P_P, P_E, P_L).

26. A computer program directly loadable into the internal memory of a computer, comprising software for controlling the steps of claim 1 when said program is run on the computer.

27. A computer readable medium, having a program recorded thereon, where the program is to make a computer control the steps of claim 1.

28. A signal receiver for receiving navigation data signals transmitted in a navigation satellite system comprising:

a radio front end unit adapted to receive a continuous radio signal (S_HF) and in response thereto produce a corresponding electrical signal (S_IF) comparatively high frequency;

an interface unit adapted to receive the electrical signal (S_IF) and in response thereto produce a sequence of sample values being divided into data words (d(k));

a digital processor unit adapted to receive the data words (d(k)) and in response thereto demodulate a data signal and including a memory means; and

a computer program according to claim 16 is loaded in the memory means.

29. A software receiver comprising:

a receiver capable of receiving a radio signal;

means for digitizing the radio signal; and

a software correlator capable of mixing the digitized radio signal to form a baseband signal using bit-wise parallelism.

30. The software receiver of claim 29 wherein said software correlator comprises:

means for computing correlations between the baseband signal and at least one pseudo-random number (PRN) code using the bit-wise parallelism.

31. The software receiver of claim 30 wherein said software correlator further comprises:

means for computing accumulations from the correlations using the bit-wise parallelism.

32. The software receiver of claim 31 further comprising:

application-specific code capable of computing navigation data using the accumulations.

33. The software receiver of claim 29 wherein said means for digitizing comprises:

means for down-converting the radio signal to an intermediate frequency; and

a digitizer capable of digitizing the intermediate frequency.

34. The software receiver of claim 33 wherein said digitizer produces at least one bit/sample.

35. The software receiver of claim 33 wherein said digitizer is an analog to digital converter.