Digital receiver device

• US 7,656,932 B2
• Filed: 05/04/2006
• Issued: 02/02/2010
• Est. Priority Date: 05/04/2005
• Status: Active Grant

• Current Assignee: STMicroelectronics SAS (STMicroelectronics NV), Universite De Provence (Aix-Marseille I)
• Original Assignee: STMicroelectronics SAS (STMicroelectronics NV), Universite De Provence (Aix-Marseille I)
• Inventors: Meillere, Stephane, Durand, Beno t, Collard Bovy, Anne, Fraschini, Christophe, Courmontagne, Philippe
• Primary Examiner(s): Bayard; Emmanuel
• Application Number:
  US11/429,452
• Publication Number:
  US 20060262833A1
• Time in Patent Office: 1,370 Days
• Field of Search: 375/147, 375/136, 375/316, 375/324, 375/340, 375143-144
• US Class Current: 375/143
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22
• CPC Class Codes: H04B 1/7093 Matched filter type

Abstract

A digital processing device for a modulated signal, arranged at the input of a radio frequency receiver chain, suited in particular to a transmission system a direct sequence spread spectrum operation, comprising an analog-to-digital converter performing undersampling of the signal received, leading to an overlapping of the frequency range of the undersampled wanted signal by the frequency range of an interfering signal, demodulation means connected at the output of the analog-to-digital converter in order to bring the undersampled wanted signal back to baseband, a low pass filter connected at the output of the demodulation means and a filter matched to the spreading code used, and an additional filtering unit arranged between the low pass filter and the matched filter, for implementing a stochastic matched filtering operation to improve the signal-to-noise ratio at the input of the matched filter.

Claims

1. Digital processing device for a modulated signal, arranged at the input of a radio frequency receiver chain, suited in particular to a transmission system using binary carrier phase modulation by means of a binary message on which a direct sequence spread spectrum operation has been carried out, this device comprising an analog-to-digital converter performing undersampling of the signal received, leading to an at least partial overlapping of the frequency range of the undersampled wanted signal by the frequency range of an interfering signal, demodulation means connected at the output of the analog-to-digital converter in order to bring the undersampled wanted signal back to baseband, a low pass filter connected at the output of the demodulation means and a filter matched to a spreading code used, wherein said device includes an additional filtering unit arranged between the low pass filter and the matched filter, said filtering unit implementing a stochastic matched filtering operation to improve the signal-to-noise ratio at the input of the filter matched to the spreading code.

2. Processing device as claimed in claim 1 wherein the additional filtering unit includes a plurality Q of finite response base filters mounted in parallel, each of which receives the undersampled signal supplied at the output of the low pass filter, each filter being characterized by a set of N coefficients, this number N being determined such that it corresponds to the minimum number of samples for describing one bit of the spread message, the coefficients of each of the Q filters corresponding respectively to components of Q eigen vectors associated with at least Q eigen values greater than 1 of a matrix B⁻

1A, where B is the variance-covariance matrix of the interfering signal and A the variance-covariance matrix of the wanted signal.

3. Processing device as claimed in claim 2 wherein for each filter of the plurality Q of finite response filters, the additional filtering unit includes means for multiplying the signal obtained at the output of said filter, with, respectively, the central coefficient of the vector resulting from the product between the variance-covariance matrix of the interfering signal and the eigen vector defining the coefficients of said filter, said unit further comprising means of summing up the vectors resulting from all of these operations, supplying a signal corresponding to the output signal of the reformatted low pass filter having an improved signal-to-noise ratio.

4. Processing device as claimed in claim 3, further comprising a comparator installed at the output of the additional filtering unit, capable of comparing the amplitude of the output signal supplied by the summation means to a reference value and of delivering a binary signal at the output of the filtering unit based on said comparison.

5. Processing device as claimed in claim 4 wherein the comparator has an adjustable reference value.

6. Processing device as claimed in claim 2, further comprising inserted between the analog-to-digital converter and the demodulation means, it includes an estimation unit provided for estimating the center frequency of the signal after undersampling, the signal present at the output of the estimation unit being filtered by a band-pass filter before being applied to the demodulation means, so as to retain only a single spectral motif from amongst the plurality of spectral motifs representative of the signal after undersampling.

7. Processing device as claimed in claim 6 wherein the estimation unit includes means for determining the parameter N defining the order of the filters of the plurality Q of finite response filters of the additional filtering unit, and for configuring the additional filtering unit with said parameter N.

8. Processing device as claimed in claim 1 wherein the interfering signal corresponds to the transmission channel noise.

9. Processing device as claimed in claim 1 wherein the sampling frequency corresponds to at least twice the bandwidth of the signal transmitted.

10. Processing device as claimed in claim 1 wherein the filter matched to the spreading code is a digital finite impulse response filter.

11. A receiver, comprising:

an analog-to-digital converter operable to convert a modulated analog signal into an under-sampled digital modulated signal, the modulated analog signal including a first component having a frequency spectrum spread to a first-component bandwidth according to a spreading code and including a second component, the converter operable to sample the modulated analog signal at a sampling frequency at least twice the first-component bandwidth;

a demodulator coupled to the analog-to-digital converter and operable to recover from the under-sampled signal a demodulated digital signal including the first and second components having respective strengths;

an emphasizer coupled to the demodulator and operable to generate a modified demodulated digital signal from the demodulated digital signal by increasing the strength of the first component of the demodulated digital signal relative to the strength of the second component of the demodulated digital signal; and

a de-spreader coupled to the emphasizer and operable to generate a digital baseband signal from the modified demodulated digital signal and the spreading code.

12. The receiver of claim 11 wherein the second component of the modulated analog signal comprises a noise component.

13. The receiver of claim 11, further comprising:

an estimator coupled between the converter and the demodulator and operable to determine a center frequency of the under-sampled signal;

a band-pass filter coupled between the estimator and the demodulator, having substantially twice the first-component bandwidth substantially centered about the center frequency, and operable to generate a filtered under-sampled signal; and

wherein the demodulator includes,

an oscillator operable to generate a demodulation signal having a frequency substantially equal to the center frequency; and

a mixer coupled to the oscillator, operable to receive the filtered under-sampled signal from the band-pass filter, and operable to generate the demodulated digital signal as a product of the filter under-sampled signal and the demodulation signal.

14. The receiver of claim 13, further comprising a low-pass filter coupled between the demodulator and the emphasizer and having substantially the first-component bandwidth.

15. The receiver of claim 11, further comprising:

wherein the modified demodulated digital signal comprises an amplitude; and

a comparator coupled to the emphasizer and operable to generate a binary signal having a first level if the amplitude of the modified demodulated digital signal is greater than a threshold and having a second level if the amplitude is less than the threshold.

16. The receiver of claim 11 wherein the emphasizer comprises:

a finite-impulse-response filter operable to generate an intermediate signal from the demodulated digital signal; and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal from a product of the intermediate signal and a predetermined value.

17. The receiver of claim 11 wherein the emphasizer comprises:

finite-impulse-response filters each operable to generate a respective first intermediate signal from the demodulated digital signal;

multipliers each coupled to a respective filter and each operable to generate a respective second intermediate signal equal to a product of a respective first intermediate signal and a respective predetermined value; and

an adder circuit operable to generate the modified demodulated digital signal from a sum of the second intermediate signals.

18. The receiver of claim 11 wherein:

the first component of the demodulated digital signal has a symbol rate; and

the emphasizer comprises,

a finite-impulse-response filter having an order related to a quotient of the sampling frequency divided by the symbol rate and operable to generate an intermediate signal from the demodulated digital signal, and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal from a product of the intermediate signal and a predetermined value.

19. The receiver of claim 11 wherein the emphasizer comprises:

a finite-impulse-response filter having one or more coefficients related to an autocorrelation of the spreading code and operable to generate an intermediate signal from the demodulated digital signal; and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal from a product of the intermediate signal and a predetermined value.

20. The receiver of claim 11 wherein the emphasizer comprises:

a finite-impulse-response filter having one or more coefficients related to an autocorrelation of the second component of the modulated analog signal and operable to generate an intermediate signal from the demodulated digital signal; and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal from a product of the intermediate signal and a predetermined value.

21. The receiver of claim 11 wherein the emphasizer comprises:

a finite-impulse-response filter having coefficients related to elements of an eigen vector of a product of a variance-covariance matrix of the spreading code and a transpose of a variance-covariance matrix of the second component of the modulated analog signal, the eigen vector being associated with an eigen value of the product greater than one, the filter operable to generate an intermediate signal from the demodulated digital signal; and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal from a product of the intermediate signal and a vector value related to a product of the variance-covariance matrix of the second component of the demodulated analog signal and the eigen vector.

22. The receiver of claim 11 wherein the emphasizer comprises:

a finite-impulse-response filter having coefficients respectively equal to elements of an eigen vector of a product of a variance-covariance matrix of the spreading code and a transpose of a variance-covariance matrix of the second component of the modulated analog signal, the eigen vector being associated with an eigen value of the product greater than one, the filter operable to generate an intermediate signal from the demodulated digital signal; and

a multiplier coupled to the filter and operable to generate the modified demodulated digital signal as a vector equal to a product of the intermediate signal and a vector value equal to a product of the variance-covariance matrix of the second component of the demodulated analog signal and the eigen vector.

23. A system, comprising:

a receiver, comprising,

an analog-to-digital converter operable to convert a modulated analog signal into an under-sampled digital modulated signal, the modulated analog signal including a first component having a frequency spectrum spread to a first-component bandwidth according to a spreading code and including a second component, the converter operable to sample the modulated analog signal at a sampling frequency at least twice the first-component bandwidth;

a demodulator coupled to the analog-to-digital converter and operable to recover from the under-sampled signal a demodulated digital signal including the first and second components having respective strengths;

an emphasizer coupled to the demodulator and operable to generate a modified demodulated digital signal from the demodulated digital signal by increasing the strength of the first component of the demodulated digital signal relative to the strength of the second component of the demodulated digital signal; and

a de-spreader coupled to the emphasizer and operable to generate a digital baseband signal from the modified demodulated digital signal and the spreading code.

24. A method, comprising:

receiving a modulated analog signal at a receiver

undersampling the modulated analog signal at a sampling frequency to generate an under-sampled digital modulated signal, the modulated analog signal including a first component having a frequency spectrum spread to a first-component bandwidth according to a spreading code and including a second component, the sampling frequency being at least twice the first-component bandwidth;

recovering from the under-sampled signal a demodulated digital signal including the first and second components having respective strengths;

generating a modified demodulated digital signal from the demodulated digital signal by reducing the strength of the second component of the demodulated digital signal relative to the strength of the first component of the demodulated digital signal;

generating a digital baseband signal from the modified demodulated digital signal and the spreading code; and

outputting the digital baseband signal at an output of the receiver.

25. The method of claim 24, further comprising:

determining a center frequency of the under-sampled signal;

generating a filtered under-sampled signal having substantially twice the first-component bandwidth substantially centered about the center frequency; and

wherein recovering includes,

generating a demodulation signal having a frequency substantially equal to the center frequency, and

generating the demodulated digital signal as a product of the filtered under-sampled signal and the demodulation signal.

26. The method of claim 24, further comprising:

limiting a bandwidth of the demodulated digital signal to substantially the first-component bandwidth; and

generating the modified demodulated digital signal from the bandwidth-limited demodulated digital signal.

27. The method of claim 24, further comprising:

generating a binary signal having a first level if an amplitude of the modified demodulated digital signal is greater than a threshold and having a second level if the amplitude is less than the threshold; and

wherein generating the digital baseband signal comprises generating the digital baseband signal from the binary signal.

28. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal with a finite-impulse-response filter; and

generating the modified demodulated digital signal from a product of the intermediate signal and a predetermined value.

29. The method of claim 24, further comprising:

receiving the modulated analog signal from a propagation channel; and

wherein the second component of the modulated analog signal comprises noise from the channel.

30. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a predetermined value.

31. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating first intermediate signals from the demodulated signal using respective finite-impulse-response filters;

generating second intermediate signals by multiplying each of the first intermediate signals by a respective predetermined value; and

generating the modified demodulated digital signal by summing together the second intermediate signals.

32. The method of claim 24 wherein:

the first component of the demodulated digital signal has a symbol rate; and

modulating the demodulated digital signal comprises,

generating an intermediate signal from the demodulated digital signal with a finite-impulse-response filter having an order related to a quotient of the sampling frequency divided by the symbol rate, and

generating the modified demodulated digital signal by multiplying the intermediate signal by a predetermined value.

33. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having one or more coefficients related to an autocorrelation of the spreading code; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a predetermined value.

34. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having one or more coefficients related to an autocorrelation of the second component of the modulated analog signal; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a predetermined value.

35. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having one or more coefficients related to an autocorrelation of the second component of the demodulated digital signal; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a predetermined value.

36. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having coefficients related to elements of an eigen vector of a product of a variance-covariance matrix of the spreading code and a transpose of a variance-covariance matrix of the second component of the modulated analog signal, the eigen vector being associated with an eigen value of the product greater than one; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a vector value related to a product of the variance-covariance matrix of the second component of the modulated analog signal and the eigen vector.

37. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having coefficients related to elements of an eigen vector of a product of a variance-covariance matrix of the spreading code and a transpose of a variance-covariance matrix of the second component of the demodulated digital signal, the eigen vector being associated with an eigen value of the product greater than one; and

generating the modified demodulated digital signal by multiplying the intermediate signal by a vector value related to a product of the variance-covariance matrix of the second component of the demodulated digital signal and the eigen vector.

38. The method of claim 24 wherein generating the modified demodulated digital signal comprises:

generating an intermediate signal from the demodulated digital signal using a finite-impulse-response filter having coefficients respectively equal to elements of an eigen vector of a product of a variance-covariance matrix of the spreading code and a transpose of a variance-covariance matrix of the second component of the modulated analog signal, the eigen vector being associated with an eigen value of the product greater than one; and

generating the modified demodulated digital signal as a vector by multiplying the intermediate signal by a vector value equal to a product of the variance-covariance matrix of the second component of the modulated analog signal and the eigen vector.