Hybrid frequency offset estimator

• US 20050058229A1
• Filed: 09/10/2004
• Published: 03/17/2005
• Est. Priority Date: 09/12/2003
• Status: Active Grant

• Current Assignee: Advantech Satellite Networks Inc.
• Original Assignee: Advantech Satellite Networks Inc.
• Inventors: Alagha, Nader S.
• Granted Patent: US 7,590,199 B2
• Time in Patent Office: Days
• US Class Current: 375/346
• CPC Class Codes: H04B 7/18523 Satellite systems for providing broadcast service to terrestrial stations, i.e. broadcast satellite service arrangements specially adapted for satellite broadcast receiving H04H40/90; picture transmission via satellite H04N1/00103; television transmission via satellite H04N7/20
• CPC Class Codes: H04L 2027/0034 using hypothesis testing
• CPC Class Codes: H04L 2027/0046 Open loops
• CPC Class Codes: H04L 2027/0065 Frequency error detectors H04L2027/0067 takes precedence
• CPC Class Codes: H04L 27/22 Demodulator circuits; Receiver circuits
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2679 Decision-aided
• CPC Class Codes: H04L 27/2695 with channel estimation, e.g. determination of delay spread, derivative or peak tracking channel estimation H04L25/0202

Abstract

A hybrid carrier frequency offset estimator that uses data-aided and non-data-aided signal processing techniques to produce multiple candidates for the carrier frequency offset within a return channel receiver in a DVB-RCS system using turbo coding and quadrature phase shift keying (QPSK) data modulation. In this system, the invention is employed to estimate signal distortion caused by carrier frequency offset so that this particular source of signal distortion can be removed to improve the ability of the receiver to maintain synchronization in low signal-to-noise conditions. This, in turn, allows the receiver to meet the DVB-RCS performance target, measured in terms of packet loss ratio, in low signal-to-noise ratio conditions and in particular for burst-mode data transmission with a short data packet size.

Claims

1. A signal processor, comprising:

a channel observer operable for receiving an input signal propagating at an expected carrier frequency experiencing a carrier frequency offset, the input signal comprising a distorted data signal in which modulating data has been influenced by the carrier frequency offset, the channel observer further operable for processing the input signal to obtain channel observations for the data signal;

a frequency estimator operable for removing modulation from the channel observations to obtain a modulation-free resultant signal comprising a representation of the carrier frequency offset, obtaining a frequency spectrum representation of the modulation-free resultant signal, processing the frequency spectrum representation of the modulation-free resultant signal to identify multiple carrier frequency offset candidates, and passing the carrier frequency offset candidates to a decoder for use in decoding the data signal.

2. The signal processor of claim 1, wherein the input signal comprises burst-mode multi-frequency time division multiplexed (MF-TDMA) data in which the data signal is encoded using a quadrature phase shift key (QPSK) modulation technique in accordance with the digital video broadcast return channel via satellite (DVB-RCS) standard.

3. The signal processor of claim 1, wherein the channel observer further comprises a front-end analog processor operable for amplifying the input signal, down-converting the amplified input signal to obtain an intermediate frequency signal, and filtering the intermediate frequency signal.

4. The signal processor of claim 1, wherein the channel observer further comprises a digital preliminary processor operable for digitizing a representation of the input signal to obtain a digital MF-TDMA signal, demultiplexing the MF-TDMA signal to obtain a concatenated single-channel TDMA signal, and detecting burst-mode data packets within the single-channel TDMA signal to obtain the channel observations.

5. The signal processor of claim 1, wherein the frequency estimator identifies the carrier frequency offset candidates by identifying multiple local maxima values in the frequency spectrum representation of the modulation-free resultant signal.

6. The signal processor of claim 1, wherein the channel observations comprise symbols having phase components carrying the modulating data.

7. The signal processor of claim 6, wherein the frequency estimator comprises a data-aided processor including a modulation remover that removes data modulation from known symbols within the channel observations by multiplying the known symbols in the channel observations with conjugate representations of the known symbols to cancel phase modulation due to the known symbols.

8. The signal processor of claim 6, wherein the frequency estimator comprises a non-data-aided processor including a modulation remover that removes data modulation from unknown symbols within the channel observations by scaling the phase components of the unknown symbols by a modulation index to cancel the phase components of the unknown symbols.

9. The signal processor of claim 1, wherein the frequency estimator obtains the frequency spectrum representation of the modulation-free resultant signal using a Fast Fourier Transform (FFT) technique.

10. A signal processor, comprising:

a channel observer operable for receiving an input signal propagating at an expected carrier frequency experiencing a carrier frequency offset, the input signal comprising a distorted data signal in which modulating data has been influenced by the carrier frequency offset, the channel observer further operable for processing the input signal to obtain channel observations for the data signal;

a frequency estimator operable for removing modulation from the channel observations to obtain a modulation-free resultant signal comprising a representation of the carrier frequency offset, obtaining a frequency spectrum representation of the modulation-free resultant signal, processing the frequency spectrum representation of the modulation-free resultant signal to identify at least one carrier frequency offset candidate, and passing the carrier frequency offset candidates to a decoder for use in decoding the data signal; and

the frequency estimator comprising a data-aided processor operable for processing known data elements in the channel observations to obtain a data-aided frequency spectrum representation comprising a modulation-free representation of the carrier frequency offset associated with the known data elements, a non-data-aided processor operable for processing unknown data elements in the channel observations to obtain a non-data-aided frequency spectrum representation comprising a modulation-free representation of the carrier frequency offset associated with the unknown data elements, and a combiner for combining the data-aided and non-data-aided frequency spectrum representations.

11. The signal processor of claim 10, wherein the data-aided and non-data-aided frequency spectrum representations are converted to a LOG domain, independently scaled, and summed to obtain the frequency spectrum representation of the modulation-free resultant signal.

12. The signal processor of claim 11, wherein the data-aided and non-data-aided frequency spectrum representations are independently scaled to allow adjustment within the frequency estimator.

13. The signal processor of claim 12, wherein a scaling factor for either the data-aided or the non-data-aided processor may be independently set to zero or a non-zero value and thereby allow the signal processor to switch between purely data-aided, purely non-data-aided, and hybrid processing modes.

14. The signal processor of claim 10, wherein the data-aided and non-data-aided processors are frequency aligned to facilitate combining the data-aided and non-data-aided frequency spectrum representations.

15. The signal processor of claim 14, wherein the channel observations include symbols comprising data modulated within phase components of the symbols, and wherein

the data-aided processor comprises a modulation remover that removes data modulation from known symbols within the channel observations by multiplying the known symbols in the channel observations with conjugate representations of the known symbols to cancel the phase components of the known symbols;

the non-data-aided processor comprises a modulation remover that removes data modulation from the unknown symbols within the channel observations by scaling the phase components of the unknown symbols by a modulation index to cancel the phase components of the unknown symbols;

the data-aided processor comprises a data-aided pre-transform processing filter;

the non-data-aided processor comprises a non-data-aided pre-transform processing filter; and

the data-aided pre-transform processing filter and the non-data-aided pre-transform processing filter are cooperatively configured to remove the effect of the modulation index and thereby align the data-aided and non-data-aided frequency spectrum representations.

16. The signal processor of claim 15, wherein:

the data-aided pre-transform processing filter comprises a data-aided decimation filter operating at a data-aided decimation rate;

the non-data-aided pre-transform processing filter comprises a non-data-aided decimation filter operating at a non-data-aided decimation rate; and

the non-data-aided decimation rate is substantially equal to the data-aided decimation rate multiplied by the modulation index to align the data-aided and non-data-aided frequency spectrum representations.

17. The signal processor of claim 10, wherein the frequency estimator identifies the carrier frequency offset candidate for the channel observations by identifying a maximum value in the frequency spectrum representation of the modulation-free resultant signal.

18. The signal processor of claim 10, wherein:

the data-aided processor comprises a data-aided modulation removal module, a data-aided frequency spectrum module, and a data-aided scaling module; and

the non-data-aided processor comprises a non-data-aided modulation removal module, a non-data-aided frequency spectrum module, and a non-data-aided scaling module.

19. The signal processor of claim 10, wherein the input signal comprises burst-mode multi-frequency time division multiplexed (MF-TDMA) data in which the data signal is encoded using a quadrature phase shift key (QPSK) modulation technique in accordance with the digital video broadcast return channel via satellite (DVB-RCS) standard.

20. The signal processor of claim 19, wherein the channel observer further comprises a front-end analog processor operable for amplifying the input signal, down-converting the amplified input signal to obtain an intermediate frequency signal, and filtering the intermediate frequency signal.

21. The signal processor of claim 20, wherein the channel observer further comprises a digital preliminary processor operable for digitizing the filtered intermediate frequency signal to obtain a digital MF-TDMA signal, demultiplexing MF-TDMA signal to obtain a concatenated single-channel TDMA signal, and detecting burst-mode data packets within the single-channel TDMA signal to obtain the channel observations.

22. A signal processor, comprising:

a channel observer operable for receiving an input signal propagating at an expected carrier frequency experiencing a carrier frequency offset, the input signal comprising a distorted data signal in which modulating data has been influenced by the carrier frequency offset, the channel observer further operable for processing the input signal to obtain channel observations for the data signal;

a frequency estimator operable for removing modulation from the channel observations to obtain a modulation-free resultant signal comprising a representation of the carrier frequency offset, obtaining a frequency spectrum representation of the modulation-free resultant signal, processing the frequency spectrum representation of the modulation-free resultant signal to identify multiple carrier frequency offset candidates, and passing the carrier frequency offset candidates to a decoder for use in decoding the data signal; and

the frequency estimator comprising a data-aided processor operable for processing known data elements in the channel observations to obtain a data-aided frequency spectrum representation comprising a modulation-free representation of the carrier frequency offset associated with the known data elements, a non-data-aided processor operable for processing unknown data elements in the channel observations to obtain a non-data-aided frequency spectrum representation comprising a modulation-free representation of the carrier frequency offset associated with the unknown data elements, and a combiner for combining the data-aided and non-data-aided frequency spectrum representations.

23. The signal processor of claim 22, wherein the data-aided and non-data-aided processors are frequency aligned to facilitate combining the data-aided and non-data-aided frequency spectrum representations.

24. The signal processor of claim 23, wherein the input signal comprises burst-mode multi-frequency time division multiplexed (MF-TDMA) data in which the data signal is encoded using a quadrature phase shift key (QPSK) modulation technique in accordance with the digital video broadcast return channel via satellite (DVB-RCS) standard.

25. The signal processor of claim 24, wherein the channel observer further comprises a front-end analog processor operable for amplifying the input signal, down-converting the amplified input signal to obtain an intermediate frequency signal, and filtering the intermediate frequency signal.

26. The signal processor of claim 25, wherein the channel observer further comprises a digital preliminary processor operable for digitizing the filtered intermediate frequency signal to obtain a digital MF-TDMA signal, demultiplexing MF-TDMA signal to obtain a concatenated single-channel TDMA signal, and detecting burst-mode data packets within the single-channel TDMA signal to obtain the channel observations.

27. The signal processor of claim 26, wherein the frequency estimator obtains the data-aided and non-data-aided frequency spectrum representations using a Fast Fourier Transform (FFT) technique.

28. A signal processor, comprising:

a channel observer operable for obtaining channel observations for an input signal comprising distorted data;

a data-aided signal distortion analyzer operable for processing known data elements in the channel observations to obtain a data-aided signal distortion indicator;

a non-data-aided signal distortion analyzer operable for processing unknown data elements in the channel observations to obtain a non-data-aided signal distortion indicator;

a combiner for combining the data-aided and non-data-aided signal distortion indicators to obtained a combined signal distortion indicator.

29. The signal processor of claim 28, further comprising a signal distortion candidate selector for processing the combined signal distortion indicator to identify multiple candidates for a parameter associated with the signal distortion.

30. A signal processor, comprising:

a channel observer operable for obtaining channel observations for an input signal comprising distorted data; and

a signal distortion candidate indicator for processing the channel observations to identify multiple candidates for a parameter associated with the signal distortion.

31. A method for operating a signal processor of a burst-mode data input signal exhibiting a low signal-to-noise ratio, comprising the steps of:

applying data-aided and non-data-aided signal processing techniques to the input signal to create a combined signature representing a signal distortion meter associated with the data signal; and

analyzing the combined signature to identify multiple candidate values for the signal distortion parameter.

32. In or for a signal processor of a burst-mode, modulating data input signal operating at a carrier frequency experiencing carrier frequency offset and exhibiting a low signal-to-noise ratio, an improvement comprising:

a carrier frequency offset estimator operable for applying data-aided and non-data-aided signal processing techniques to the input signal to obtain a combined frequency spectrum comprising a representation of a carrier frequency offset associated with the input signal; and

a multiple peak selector operable for identifying multiple carrier frequency offset candidates by identifying local maxima values in the combined frequency spectrum representation.