Patent EA-002528-B1 (EA)
Title: Echo phase offset correction in a multi-carrier demodulation system
Inventor: EBERLEIN ERNST
IPC: H04J1/02
Filed: 1998-04-14
Abstract: 1. A method of performing an echo phase offset correction in a multi-carrier demodulation system, comprising the steps of: differential phase decoding (142) phase shifts based on a phase difference between simultaneous carriers having different frequencies; determining an echo phase offset for each decoded phase shift by eliminating (500) phase shift uncertainties corresponding to codeable phase shifts from said decoded phase shift; averaging (520) said echo phase offsets in order to generate an averaged offset; and correcting (524) each decoded phase shift based on said averaged offset. 2. The method according to claim 1, wherein said step of differential phase decoding comprises the step of differential phase decoding phase shifts based on a phase difference between simultaneous carriers which are adjacent in the frequency axis direction. 3. The method according to claim 1 or 2, wherein said step of differential phase decoding comprises the step of differential phase decoding phase shifts based on phase differences between at least three simultaneous carriers which are equally spaced in the frequency axis direction. 4. The method according to one of claims 1 to 3, further comprising a step of comparing (516) an absolute value of a symbol associated with a respective decoded phase shift with a threshold, wherein only phase shifts having associated therewith symbols having an absolute value exceeding said threshold are used in said step of averaging said echo phase offsets. 5. A method of performing an echo phase offset correction in a multi-carrier demodulation system, comprising the steps of: differential phase decoding phase shifts based on a phase difference between simultaneous carriers having different frequencies, said phase shifts defining signal points in a complex plane; pre-rotating said signal points into the sector of said complex plane between -45 degree  and +45 degree ; determining parameters (a, b) of a straight line approximating the location of said pre-rotated signal points in said complex plane; determining a phase offset based on said parameters (a, b); and correcting each decoded phase shift based on said phase offset. 6. The method according to claim 5, wherein said simultaneous carriers are equally spaced in the frequency axis direction. 7. The method according to claim 5 or 6, wherein said step of determining said parameters (a, b) comprises at least squares method for selecting those parameters which minimize the deviations of said pre-rotated signal points from said straight line. 8. The method according to claim 27, wherein said parameters (a, b) are determined as follows:     wherein x and y designate the coordinates of the signal points in the complex plane, i is an index from 1 to N, and K is the number of signal points. 9. The method according to claim 8, wherein said phase offset (φk) is determined as follows:   wherein vk is a given decision variable. 10. An echo phase offset correction device for a multi carrier demodulation system, comprising: a differential phase decoder (142) for decoding phase shifts based on a phase difference between simultaneous carriers having different frequencies; means for determining an echo phase offset for each decoded phase shift comprising means (500) for eliminating phase shift uncertainties corresponding to codeable phase shifts from said decoded phase shift; means (520) for averaging said echo phase offsets in order to generate an averaged offset; and means (524) for correcting each decoded phase shift based on said averaged offset. 11. The device according to claim 10, wherein said differential phase decoder is adapted for decoding said phase shifts based on a phase difference between simultaneous carriers which are adjacent in the frequency axis direction. 12. The device according to claim 10 or 11, further comprising means (516) for comparing an absolute value of a symbol associated with a respective decoded phase shift with a threshold, wherein said means for averaging said phase offsets only uses phase shifts having associated therewith symbols having an absolute value exceeding said threshold. 13. The device according to one of claims 10 to 12, wherein said differential phase decoder is adapted for decoding said phase shifts based on phase differences between at least three simultaneous carriers which are equally spaced in the frequency axis direction. 14. An echo phase offset correction device for a multicarrier demodulation system, comprising: a differential phase decoder for decoding phase shifts based on a phase difference between simultaneous carriers having different frequencies, said phase shifts defining signal points in a complex plane; means for pre-rotating said signal points into the sector of said complex plane between -45 degree  and +45 degree ; means for determining parameters (a, b) of a straight line approximating the location of said pre-rotated signal points in said complex plane; means for determining a phase offset based on said parameters (a, b); and means for correcting each decoded phase shift based on said phase offset. 15. The device according to claim 14, wherein said differential phase decoder comprises means for decoding phase shifts of at least three simultaneous carriers which are equally spaced in the frequency axis direction. 16. The device according to claim 14 or 15, wherein said means for determining said parameters (a, b) comprises means for performing a least squares method for selecting those parameters which minimize the deviations of said pre-rotated signal points from said straight line. 17. The device according to claim 16, wherein said means for determining said parameters (a, b) calculates said parameters (a, b) as follows:     wherein x and y designate the coordinates of the signal points in the complex plane, i is an index from 1 to N, and K is the number of signal points. 18. The device according to claim 17, wherein said means for determining said phase offset (φk) calculates said phase offset (φk) as follows:   wherein vk is a given decision variable.