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

Claim 3

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.

Claim 4

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; (I) 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 multicarriersignal with said remodulated, reference signal to generate said phase-error signal.

Claim 5

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.

Claim 6

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.

Claim 11

The method of claim 7 in which said filtered slope is used as the control for a phase-locked loop which adjusts the frequency of said locally generated clock to match that of the remote transmitter dock.

Claim 14

The method of claim 9 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.

Claim 15

The method of claim 1 wherein the samples of the receive signal are processed in blocks using Fast Fourier Transform (FFT) and Inverse FFT (IFFT) algorithms.

Claim 16

The method of claim 6 wherein the samples of the receive signal are processed in blocks using FFT and IFFT algorithms.

Claim 17

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

Claim 18

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

Claim 19

The method of claim 9 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.

Claim 20

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.

Claim 21

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.

Claim 22

The apparatus of claim 16 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.

Claim 25

The apparatus of claim 18 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.

Claim 26

The apparatus of claim 19 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.

Claim 27

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.

Claim 28

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.

Claim 29

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.

Claim 30

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.

Claim 31

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 dock.

Claim 32

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 dock.

Claim 33

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 dock 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 dock in a remote transmitting modem.

Claim 34

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 dock 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 dock in a remote transmitting modem.

Claim 35

The apparatus of claim 24 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.

Claim 36

The apparatus of claim 16 further including means for processing samples of said received multicarrier signal in blocks using Fast Fourier Transform (FFT) and Inverse FFT (IFFT) algorithms.

Claim 37

The apparatus of claim 21 further including means for processing samples of said received multicarrier signal in blocks using FFT and IFFT algorithms.

Claim 38

The apparatus of claim 22 further including means for processing samples of said received multicarrier signal in blocks using FFT and IFFT algorithms.

Claim 39

The apparatus of claim 24 further including means for processing samples of said received multicarrier signal in blocks using FFT and IFFT algorithms.

Claim 40

The apparatus of claim 15 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.