Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in mulicarrier modems

• US 5,206,886 A
• Filed: 04/16/1990
• Issued: 04/27/1993
• Est. Priority Date: 04/16/1990
• Status: Expired due to Term

• Current Assignee: Telebit Corp. (Cisco Systems, Inc.)
• Original Assignee: Telebit Corp. (Cisco Systems, Inc.)
• Inventors: Bingham, John A. C.
• Primary Examiner(s): Chin, Stephen
• Application Number:
  US06/509,462
• Time in Patent Office: 1,107 Days
• Field of Search: 375/97, 375/98, 375/118, 375/119, 375/120, 331/1 R, 331/23, 455/258, 455/260, 455/265
• US Class Current: 375/344
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2679 Decision-aided

Abstract

A method of correcting for the impairments of phase jitter and frequency offset imposed by a transmission medium and of a clock frequency difference between a transmitting and a receiving multicarrier modem is disclosed. The method comprises two modes: a training mode that uses pilot tones, and a data mode that uses random data modulated onto many carriers. During the training mode the frequencies of the phase jitter are estimated, and the parameters of a correcting signal are initialized; during the data mode the correcting signal is used to compensate for the impairments, and then is updated from an estimate of the phase error.

Claims

1. A method for estimating, and compensating for phase jitter imposed on a received multicarrier signal by imperfect components in a transmission medium, said method comprising the steps of:

(a) multiplying said multicarrier signal received during a time interval by a locally generated correcting signal to produce a corrected received multicarrier signal, in which said locally generated correcting signal is an estimate of the phase shift required to correct for the effects of said phase jitter;

(b) generating a reference signal by demodulating and then decoding said corrected multicarrier signal to produce an estimate of the transmitted data, and then remodulating said data to produce a reference signal;

(c) calculating a phase-error signal which is the difference between the phase of said correcting multicarrier signal and that of said reference signal;

(d) filtering the calculated phase-error signal of step (b) to generate an estimate of the changes required to the parameters of said locally generated correcting signal, said parameters being the amplitudes, frequencies, and phases of a plurality of components of said correcting signal, or any equivalent set which contains the same information;

(e) updating the parameters of step (d) by said changes to generate new parameters of said correcting signal; and

(f) repeating steps (a) to (e) in each subsequent time interval.

2. The method of claim 1 further including the steps of:

(g) sampling said received multicarrier signal in synchronism with a locally generated clock signal;

(h) assembling the samples of step (g) into a block;

(i) performing the updating of step (e) after the reception of each block; and

(j) multiplying the samples in a subsequent block of said received multicarrier signal by said correcting signal, which is defined by said new parameters.

3. The method of claim 2 wherein said reference signal is calculated by the steps of:

(k) demodulating and decoding said corrected received multicarrier signal to estimate what data was modulated onto each of the many carriers;

(l) filtering the results of said demodulating and decoding in order to estimate the amplitude and corrected phase responses of said transmission medium at each of the many carrier frequencies;

(m) remodulating said carriers with said estimated data, using said amplitude and phase responses, to generate, as a reference, an ideal received multicarrier signal which has no phase jitter; and

(n) comparing said corrected received multicarrier signal with said remodulated, reference signal to generate said phase-error signal.

4. The method of claim 3 wherein said correcting signal comprises components at several frequencies, called jitter frequencies, at which it has been determined that there is phase jitter, further including the steps of:

(o) correlating said phase-error signal with complex sinusoidal signals at said jitter frequencies; and

(p) using the results of said correlations to define the changes that should be made to the parameters of several sinewave generators, which generate components, at the identified jitter frequencies, of said correcting signal.

5. The method of claim 4 wherein said parameters are the addresses of look-up tables and the multipliers that are applied to the output signals of said tables in order to generate the components of said correcting signal.

6. The method of claim 1 further comprising the step of:

(q) dividing said received multicarrier blocks using Fast Fourier Transform (FFT) and Inverse FFT (IFFT) algorithms before performing steps a. through f.

7. A method for calculating, in a modem that receives a multicarrier data signal which may be impaired by phase jitter, frequency offset and a difference between the frequencies of a locally generated sampling clock and the clock in a remote transmitting modem, the initial parameters of a locally generated correcting signal which corrects for said phase jitter, frequency offset and frequency difference, said method comprising the steps of:

(a) calculating the initial values of the parameters of said correcting signal during the reception of a training signal comprising a plurality of pilot tones;

(b) separating said pilot tones by bandpass filters, and calculating their phases at successive sampling times;

(c) analyzing said phases to identify linear and sinusoidal functions of time;

(d) estimating the frequencies, amplitudes and phases of said sinusoidal functions of time;

(e) using the estimates of step (d) to initialize the parameters of sinewave generators;

(f) comparing the slopes of said linear functions of step (c), and generating an estimate of both said frequency offset and said clock frequency difference;

(g) using said estimate of said frequency offset to initialize the zero-frequency component of said correcting signal;

(h) using said estimate of the clock frequency difference to initialize the state of a low-pass filter; and

(i) using the output signal of said low-pass filter to control an interpolator which interpolates between samples taken in synchronism with a locally generated clock signal to generate samples at the frequency of the clock in a remote transmitting modem.

8. The method of claim 7 wherein the output signal of said low-pass filter is used to control a phase-locked loop which adjusts the frequency and phase of said locally generated clock to match that of the clock in a remote transmitting modem.

9. The method of claim 7 wherein the samples of the receive signal are processed in blocks using FFT and IFFT algorithms.

10. The method of claim 7 wherein the frequencies, amplitudes and phases of said sinsoidal functions of time are estimated by performing an FFT on the sets of phases of a plurality of pilot tones.

11. The apparatus of claim 10 further including means for estimating the frequencies of said components of phase jitter by performing an FFT on the sets of phases of said pilot tones.

12. Apparatus for estimating, and compensating for phase jitter imposed on a received multicarrier signal by imperfect components in a transmission medium, said apparatus comprising:

means for multiplying said multicarrier signal received during a time interval by a locally generated correcting signal to produce a corrected received multicarrier signal, wherein said locally generated correcting signal is an estimate of the phase shift required to correct for the effects of said phase jitter;

means for demodulating and then decoding said corrected multicarrier signal, estimating the transmitted data, and then remodulating said data to form said reference signal;

means for calculating a phase-error signal which is the difference between the phase of said corrected multicarier signal and that of said reference signal;

means for filtering said phase-error signal to generate an estimate of the changes required to the parameters of said locally generated correcting signal;

means for updating said parameters by said changes to generate new parameters of said correcting signal; and

means for repeating the operation of the multiplying, calculating, filtering, and updating means in each subsequent time interval.

13. The apparatus of claim 12 further including:

means for sampling said received multicarrier signal in synchronism with a locally generated clock signal;

buffer means for assembling the samples of the sampling means into a block;

means for performing the updating of said parameters after the reception of each block; and

means for multiplying the samples in a subsequent block of said received multicarrier signal by said correcting signal, defined by said new parameters.

17. The apparatus of claim 12 further including means for processing samples of said received multicarrier signal in blocks using Fast Fourier Transform (FFT) and Inverse FFT (IFFT) algorithms.

14. The apparatus of 13 further including:

means for calculating said correcting signal by demodulating and then decoding said corrected multicarrier signal to estimate what data was modulated onto each of the many carriers;

means for filtering the results from said demodulating and decoding means in order to estimate the amplitude and corrected phase responses of said transmission medium at each of the carrier frequencies;

means for remodulating said carriers with said estimated data, using said amplitude and phase responses, to generate, as a reference, an ideal received multicarrier signal which has no phase jitter; and

means for comparing said corrected received multicarrier signal with said remodulated, reference signal to generate said phase-error signal.

15. The apparatus of claim 14 adapted to the situation in which said correcting signal comprises components at several frequencies, called jitter frequencies, at which it has been determined that there is significant phase jitter, said apparatus further including:

means for correlating said phase-error signal with complex sinusoidal signals at said jitter frequencies;

means for generating samples of cosine and sine waves of defined amplitudes and phases at each of said jitter frequencies;

means for using the results of said correlation operations to define the changes that should be made to said amplitudes and phases; and

means for summing said samples of cosine and sine waves to generate said correcting signal.

16. The apparatus of claim 15 wherein said cosine and sine wave generators are implemented by look-up tables and multipliers, wherein said look-up tables generate components of said correcting signal with the required frequency and phase, and said multipliers adjust the amplitudes of said components.

18. Apparatus for calculating, in a modem that receives a multicarrier data signal which may be impaired by phase jitter, frequency offset and a difference between the frequencies of a locally generated sampling clock and the clock in a remote transmitting modem, the initial parameters of a locally generated correcting signal which corrects for said impairments, said apparatus comprising:

means for calculating the initial values of the parameters of said correcting signal during the reception of a training signal comprising a plurality of pilot tones;

means for separating said pilot tones by bandpass filters, and calculating their phases at successive sampling times;

means for analyzing said phase to identify linear and sinusoidal functions of time;

means for estimating the frequencies, amplitudes and phases of said sinusoidal functions of time;

means for using the estimates produced by said estimating means to initialize the parameters of sinewave generators;

means for comparing the slopes of said linear functions calculated by said analyzing means, and for generating an estimate of both said frequency offset and said clock frequency difference;

means for using said estimate of said frequency offset to initialize the zero-frequency component of said correcting signal;

means for using said estimate of the clock frequency difference to initialize the state of a low-pass filter; and

means for using the output signal of said low-pass filter to control an interpolator which interpolates between samples taken in synchronism with a locally generated clock signal to generate samples at the frequency of the clock in a remote transmitting modem.

19. The apparatus of claim 18 wherein the output signal of said low-pass filter is used to control a phase-locked loop which adjusts the frequency and phase of said locally generated clock to match that of the clock in a remote transmitting modem.

20. The apparatus of claim 18 further including means for processing samples of said received multicarrier signal in blocks using FFT and IFF algorithms.