Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in multicarrier modems
AssigneeTelebit Corporation
InventorJohn A. C. Bingham
Life of this patent
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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.
Claims10
A method for estimating, and compensating for frequency offset imposed on a received multicarrier signal by imperfect components in a transmission medium, said method comprising the steps of:
- (a) multiplying the 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 frequency offset;
- (b) demodulating and then decoding said corrected received multicarrier signal in order to estimate what data was modulated onto each of the carriers;
- (c) filtering the results of step (b) in order to estimate the corrected phase response of said transmission medium at each of the carrier frequencies;
- (d) fitting said corrected phase response of the medium to a linear function of frequency;
- and
- (e) interpreting the zero-frequency intercept of said linear function as being caused by a difference between the phase of said correcting signal and the phase that is common to all carriers of said multicarrier received signal, low-pass filtering and integrating said zero-frequency intercept, and using the result of said filtering to update the low-frequency component of said correcting signal.
A method for correcting, in a modem that receives, via a transmission medium, data modulated onto many carriers, for a difference between the frequencies of a locally generated sampling clock and the clock in a remote, transmitting modem, said method comprising the steps of;
- (a) sampling said received multicarrier signal in synchronism with a locally generated clock signal;
- (b) interpolating between said samples according to an interpolation parameter in order to convert samples taken at the frequency of said locally generated clock into interpolated samples at the frequency of the clock in said remote transmitting modem;
- (c) demodulating and then decoding said interpolated samples in order to estimate what data was modulated onto each of the many carriers;
- (d) filtering the results of said demodulating and decoding in order to estimate the phase response of said transmission medium at each of the many carrier frequencies;
- (e) fitting said phase response of said transmission medium to a linear function of frequency;
- (f) interpreting the slope of said linear function of frequency as being caused by a difference between the sampling frequencies of transmitter and receiver;
- and
- (g) low-pass filtering said slope, and using the result as said interpolation parameter.
Apparatus for estimating, and compensating for frequency offset imposed on a received multicarrier signal by imperfect components in a transmission medium, said apparatus comprising:
- means for multiplying the received 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 frequency offset;
- means for demodulating and then decoding said corrected received multicarrier signal in order 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 corrected phase response of said transmission medium at each of the carrier frequencies;
- means for fitting and correcting phase response of said transmission medium to a linear function of frequency;
- means for interpreting the zero-frequency intercept of said linear function as being caused by a difference between the phase of said correcting signal and the phase that is common to all carriers of said received multicarrier signal, low-pass filtering said zero-frequency intercept, and using the result of said filtering to update the zero-frequency component of said correcting signal.
Apparatus for correcting, in a modem that receives, via a transmission medium, data modulated onto many separate carriers, for a difference between the frequencies of a locally generated sampling clock and the clock in a remote, transmitting modem, said apparatus comprising:
- means for sampling said received multicarrier signal in synchronism with a locally generated clock signal;
- means for interpolating between the resultant samples of said sampling means according to an interpolation parameter, in order to convert samples taken at the frequency of said locally generated clock into interpolated samples at the frequency of the clock in said remote transmitting modem;
- means for demodulating and then decoding said interpolated samples in order to estimate what data was modulated onto each of the many carriers;
- means for filtering the results of said demodulating and decoding means in order to estimate the phase response of said transmission medium at each of the carrier frequencies;
- means for fitting said phase response of said transmission medium to a linear function of frequency;
- means for interpreting the slope of said linear function of frequency as being caused by a difference between the sampling frequencies of transmitter and receiver;
- and
- means for low-pass filtering said slope, and using the result as said interpolation parameter.
Apparatus for correcting, in a modem that receives, via a transmission medium, data modulated onto many separate carriers, for a difference between the frequencies of a locally generated sampling clock and the clock in a remote, transmitting modem, said apparatus comprising:
- means for sampling said received multicarrier signal in synchronism with a locally generated clock signal;
- means for controlling the phase of said locally generated clock signal in order to synchronize it with the clock in said remote transmitting modem:
- means for demodulating and then decoding said samples in order to estimate what data was modulated onto each of the many carriers;
- means for filtering the results of said demodulating and decoding means in order to estimate the phase response of said transmission medium at each of the carrier frequencies;
- means for fitting said phase response of said transmission medium to a linear function of frequency;
- means for interpreting the slope of said linear function of frequency as being caused by a difference between sampling frequencies of transmitter and receiver;
- and
- means for low-pass filtering said slope, and using the result to control the phase of said locally generated clock.
Dependencies are inferred from the claim text.