Compensating pre-filter for an OFDM transmitter

• US 8,160,176 B2
• Filed: 09/18/2009
• Issued: 04/17/2012
• Est. Priority Date: 03/05/2009
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Dent, Paul Wilkinson, Khayrallah, Ali S.
• Primary Examiner(s): Tran, Khai
• Application Number:
  US12/562,225
• Publication Number:
  US 20100226416A1
• Time in Patent Office: 942 Days
• Field of Search: 375/295, 375/260, 375/219, 375/298, 375/308, 375/350, 375/296, 455/114.3, 455/126
• US Class Current: 375/295
• CPC Class Codes: H04L 25/0202 Channel estimation
• CPC Class Codes: H04L 25/0242 using matrix methods
• CPC Class Codes: H04L 27/2634 Inverse fast Fourier transform [IFFT] or inverse discrete Fourier transform [IDFT] modulators in combination with other circuits for modulation
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

A method and apparatus for reducing group delay and/or amplitude errors applied to a transmission signal by one or more transmission filters is described herein. The present invention characterizes the errors introduced by one or more transmission filters relative to a desired frequency response, e.g., the group delay and/or amplitude errors relative to a flat group delay and flat amplitude, respectively. Based on the errors, the present invention pre-compensates the digital frequency domain samples used to generate the transmission signal. In so doing, the present invention reduces the errors in the filtered transmission signal without placing limits on the design of the transmission filters.

Claims

1. A method of generating a filtered analog transmission signal for transmission to a remote device, wherein the filtered analog transmission signal comprises multiple frequency components, the method comprising:

generating a modulated multi-frequency signal stream from digital frequency-domain samples input to a modulator;

filtering the modulated multi-frequency signal stream using one or more filters to suppress out-of-band spectral components from the filtered analog transmission signal, said one or more filters collectively having a frequency-domain response over the multiple frequency components comprising a first maximum error relative to a desired frequency response; and

pre-compensating one or more of the digital frequency-domain samples using a pre-compensation unit such that a collective frequency-domain response of the pre-compensation unit and the one or more filters comprises a second maximum error relative to the desired frequency response less than the first maximum error.

2. The method of claim 1 wherein the first maximum error comprises at least one of a first maximum group delay error relative to a flat group delay and a first maximum amplitude error relative to a flat amplitude, and wherein the second maximum error comprises at least one of a second maximum group delay error relative to a flat group delay less than the first maximum group delay error and a second maximum amplitude error relative to a flat amplitude less than the first maximum amplitude error.

3. The method of claim 2 wherein pre-compensating one or more of the digital frequency-domain samples comprises pre-compensating an amplitude and a phase of one or more of the digital frequency-domain samples such that the collective frequency-domain response of the pre-compensation unit and the one or more filters comprises the second maximum group delay error and the second maximum amplitude error.

4. The method of claim 1 further comprising determining frequency-specific complex factors based on frequency-specific errors relative to the desired frequency response, wherein pre-compensating one or more of the digital frequency-domain samples comprises multiplying one or more of the digital frequency-domain samples by the corresponding complex factor.

5. The method of claim 4 further comprising storing the complex factors in a look-up table, wherein multiplying one or more of the digital frequency-domain samples by the corresponding complex factor comprises retrieving complex factors from the look-up table and multiplying each of the retrieved complex factors by the corresponding digital frequency-domain sample.

6. The method of claim 1 further comprising upconverting the modulated multi-frequency signal stream to a predetermined radio frequency, wherein filtering the modulated multi-frequency signal stream comprises filtering the modulated multi-frequency signal stream with one or more baseband filters to suppress out-of-band spectral components at baseband and filtering the upconverted modulated multi-frequency signal stream with one or more RF filters to suppress out-of-band spectral components at RF, and wherein pre-compensating one or more of the digital frequency-domain samples comprises:

multiplying one or more of the digital frequency-domain samples by a corresponding baseband complex factor determined based on frequency-specific errors associated with the collective frequency response of the one or more baseband filters relative to the desired frequency response; and

multiplying one or more of the digital frequency-domain samples by a corresponding RF complex factor determined based on frequency-specific errors associated with the collective frequency response of the one or more RF filters relative to the desired frequency response.

7. The method of claim 1 wherein generating the analog transmission signal comprises:

selecting a number of the digital frequency-domain samples based on a desired transmission data rate;

generating an input data block for the selected number of frequency-domain samples for input to a modulator, wherein a size of the modulator exceeds the selected number of digital frequency-domain samples;

padding the input data block to generate an expanded data block having a size matched to the size of the modulator;

modulating the expanded data block to generate a modulated multi-frequency signal stream; and

configuring one or more properties of at least one of the filters based on the selected number of frequency components.

8. The method of claim 7 wherein pre-compensating one or more of the digital frequency-domain samples comprises pre-compensating one or more of the digital frequency-domain samples of the input data block such that the collective frequency-domain response of the pre-compensation unit and the one or more filters comprises the second maximum error, and wherein padding the input data block comprises padding the pre-compensated input data block to generate the expanded data block.

9. The method of claim 1 further comprising:

receiving a signal from a remote device, wherein the received signal comprises multiple frequency components;

determining a preliminary set of scatterer parameters for a reception channel based on the received signal, wherein the preliminary set of scatterer parameters comprises a set of scattering coefficients for each of a plurality of path delays, and wherein each scattering coefficient corresponds to a scattering object;

determining a modified set of scatterer parameters for the future transmission channel by calculating extrapolated scattering coefficients for the modified set of scatterer parameters based on the preliminary set of scatterer parameters, and frequency and time properties associated with the transmission and reception channels; and

estimating a future transmission channel response based on the modified set of scatterer parameters.

10. The method of claim 9 wherein one or more of the multiple frequency components of the received signal were pre-compensated at the remote device such that a collective frequency-domain response of a remote device pre-compensation unit and one or more remote device filters comprises the second maximum error relative to the desired frequency response less than the first maximum error.

11. The method of claim 1 wherein the digital frequency-domain samples comprise OFDM samples, and wherein the filtered analog transmission signal comprises a filtered analog OFDM transmission signal.

12. A wireless transceiver configured to generate a filtered analog transmission signal for transmission to a remote device, wherein the filtered analog transmission signal comprises multiple frequency components, the wireless transceiver comprising:

a modulator to generate a modulated multi-frequency signal stream from digital frequency-domain samples input to the modulator;

one or more filters to filter the modulated multi-frequency signal stream to suppress out-of-band spectral components from the filtered analog transmission signal, said one or more filters collectively having a frequency-domain response over the multiple frequency components comprising a first maximum error relative to a desired frequency response; and

a pre-compensation unit configured to pre-compensate one or more of the digital frequency-domain samples such that a collective frequency-domain response of the pre-compensation unit and the one or more filters comprises a second maximum error relative to the desired frequency response less than the first maximum error.

13. The wireless transceiver of claim 12 wherein the first maximum error comprises at least one of a first maximum group delay error relative to a flat group delay and a first maximum amplitude error relative to a flat amplitude, and wherein the second maximum error comprises at least one of a second maximum group delay error relative to a flat group delay less than the first maximum group delay error and a second maximum amplitude error relative to a flat amplitude less than the first maximum amplitude error.

14. The wireless transceiver of claim 13 wherein the pre-compensation unit is configured to pre-compensate an amplitude and a phase of one or more of the digital frequency-domain samples such that the collective frequency-domain response of the pre-compensation unit and the one or more filters comprises the second maximum group delay error and the second maximum amplitude error.

15. The wireless transceiver of claim 12 further comprising a controller configured to determine frequency-specific complex factors based on frequency-specific errors relative to the desired frequency response, wherein the pre-compensation unit pre-compensates the digital frequency-domain samples by multiplying one or more of the digital frequency-domain samples by the corresponding complex factor.

16. The wireless transceiver of claim 15 further comprising memory configured to store the complex factors in a look-up table, wherein the controller is configured to retrieve one or more complex factors from the look-up table, and wherein the pre-compensation unit is configured to multiply each of the retrieved complex factors by the corresponding digital frequency-domain sample.

17. The wireless transceiver of claim 12 further comprising an upconverter to upconvert the modulated multi-frequency signal stream to a predetermined radio frequency, wherein the one or more filters comprises one or more baseband filters and one or more RF filters, wherein the one or more baseband filters filter the modulated multi-frequency signal stream to suppress out-of-band spectral components at baseband, and wherein the one or more RF filters filter the upconverted modulated multi-frequency signal stream to suppress out-of-band spectral components at RF, and wherein the pre-compensation unit is configured to pre-compensate one or more of the digital frequency-domain samples by:

multiplying one or more of the digital frequency-domain samples by a corresponding baseband complex factor determined based on frequency-specific errors associated with the collective frequency response of the one or more baseband filters relative to the desired frequency response; and

multiplying one or more of the digital frequency-domain samples by a corresponding RE complex factor determined based on frequency-specific errors associated with the collective frequency response of the one or more RF filters relative to the desired frequency response.

18. The wireless transceiver of claim 12 further comprising:

an encoder to encode transmission data to generate an input data block for a selected number of the digital frequency-domain samples,

wherein the selected number of digital frequency-domain samples is based on a desired transmission data rate,

wherein a size of the modulator exceeds the selected number of digital frequency-domain samples,

wherein the modulator is configured to pad the input data block to generate an expanded data block having a size matched to the size of the modulator and modulate the expanded data block to generate the modulated multi-frequency signal stream, and

wherein at least one of the filters is configured to filter the modulated multi-frequency signal stream based on the selected number of frequency components.

19. The wireless transceiver of claim 18 wherein the pre-compensation unit pre-compensates the digital frequency-domain samples by pre-compensating one or more of the digital frequency-domain samples of the input data block such that the collective frequency-domain response of the pre-compensation unit and the one or more filters comprises the second maximum error, and wherein the modulator pads the input data block by padding the pre-compensated input data block to generate the expanded data block.

20. The wireless transceiver of claim 12 further comprising:

a receiver unit to receive a signal from a remote device, wherein the received signal comprises multiple frequency components; and

a receive processor configured to estimate a future transmission channel response by;

determining a preliminary set of scatterer parameters for a reception channel based on the received signal, wherein the preliminary set of scatterer parameters comprises a set of scattering coefficients for each of a plurality of path delays, and wherein each scattering coefficient corresponds to a scattering object; and

determining a modified set of scatterer parameters for the future transmission channel by calculating extrapolated scattering coefficients for the modified set of scatterer parameters based on the preliminary set of scatterer parameters, and frequency and time properties associated with the transmission and reception channels.

21. The wireless transceiver of claim 20 wherein one or more of the multiple frequency components of the received signal were pre-compensated at the remote device such that a collective frequency-domain response of a remote device pre-compensation unit and one or more remote device filters comprises the second maximum error relative to the desired frequency response less than the first maximum error.

22. The wireless transceiver of claim 12 wherein the digital frequency-domain samples comprise digital OFDM samples, and wherein the filtered analog transmission signal comprises a filtered analog OFDM transmission signal.