Apparatus and method for equalization in distributed digital data transmission systems

A system comprising:

• a plurality of remote unit transmitters, each having a precode equalization digital filter;
• a transmission medium coupled to receive transmitted signals from said plurality of remote unit transmitters;
• a central unit receiver coupled to receive signals from said transmission medium transmitted by said plurality of remote unit transmitters, comprising:
• a demodulator coupled to receive transmitted signals modulated onto radio frequency carrier signals and generate demodulated signals;
• an analog-to-digital converter coupled to receive said demodulated signals from said demodulator and convert them to digital received symbol signals;
• a matched filter coupled to receive digital received symbol signals output by said analog-to-digital converter and filter said received symbol signals;
• an equalization circuit comprising:
• a demultiplexer circuit coupled to receive said filtered received symbol signals from said matched filter and demultiplex them to generate demultiplexed received symbol signals;
• and
• a rotational amplifier coupled to receive said demultiplexed received symbol signals from said demultiplexer circuit and to receive phase and amplitude error correction factors unique to the remote unit modem that transmitted the symbols being received and functioning to correct the phase and amplitude of said demultiplexed received symbol signals to generate corrected received symbols;
• a symbol equalization circuit including at least one digital filter circuit a slicer and a least means squares computation circuit coupled to said slicer and coupled to supply filter coefficients to said digital filter circuit, said symbol equalization circuit coupled to receive said corrected received symbol signals from said demultiplexer circuit, and having at least one digital filter therein for filtering said received symbol signals to provide upstream equalization and output decision data points derived from said corrected received symbols.

A system comprising:

• central unit transmitter;
• a transmission medium coupled to receive transmitted signals from said central unit transmitter;
• a plurality of remote unit receivers coupled to receive signals from said transmission medium transmitted by said central unit transmitter, each remote unit receiver including:
• a demodulator coupled to receive transmitted analog chip signals modulated onto a radio frequency carrier, and functioning to demodulate said radio frequency carrier and recover said transmitted analog chip signals;
• an analog-to-digital converter coupled to receive said recovered transmitted chip signals from said demodulator and convert them into one or more digitized transmitted chip signals;
• a matched filter coupled to receive said digitized transmitted chip signals output by said analog-to-digital converter, and functioning to filter said transmitted chip signals and output filtered chip signals;
• an equalization circuit comprising:
• a chip equalization circuit having at least one digital filter having programmable filter coefficients therein and coupled to receive and filter said filtered chip signals to provide equalization and having a coefficient input for receiving adjustments to said programmable filter coefficients of said at least one digital filter, and having an output at which faltered chip signals appear;
• a demultiplexer circuit coupled to receive said filtered chip signals output from said chip equalization circuit, and functioning to demultiplex said filtered chip signals into received symbols:
• and
• a symbol equalization circuit having at least one digital filter therein, and having an input coupled to receive said received symbols from said demultiplexer circuit, and having an output coupled to said chip equalization circuit, and functioning to filter said received symbols to provide a further measure of equalization.

A process for performing downstream or upstream equalization training to attempt to achieve convergence of equalizer filter tap coefficients of digital equalization filters in a modem having a symbol equalizer circuit coupled to receive signals from a demultiplexer circuit, and having a chip equalizer circuit, said symbol equalizer circuit having a digital food forward equalization filter having a main tap with a programmable coefficient and one or more side taps, each with a programmable coefficient, and a decision feedback equalization filter, said modem also having a slicer for determining which symbols wore transmitted to said modem and also having a rotational amplifier, said chip equalizer circuit in said modem comprised of a digital feed forward filter and a decision feedback equalizer filter coupled to output data to said demultiplexer circuit, comprising:

• 1) loading a default value into a register in said symbol equalizer circuit that stores a main tap coefficient or last tap coefficient value of said feed forward equalization filter of said symbol equalizer circuit;
• 2) enabling main tap updating and disabling side tap adaptation in said feed forward equalization filter of said symbol equalizer circuit for a plurality of bursts of training data transmissions;
• 3) storing in a memory a predetermined value for an adaptation coefficient;
• 4) receiving said plurality of bursts of training data and adapting said main tap coefficient of said food forward equalization filter of said symbol equalizer circuit using said adaptation coefficient during successive iterations of training data transmissions so as to minimize slicer errors;
• 5) enabling tap adaptation for all main and side taps of said feed forward and said decision feedback equalization filters of said symbol equalizer circuit for a predetermined number of bursts of training data transmission;
• 6) setting a predetermined value for said adaptation coefficient;
• 7) receiving training data transmissions for a predetermined number of bursts and adapting all tap coefficients of said feed forward and said decision feedback equalization filters of said symbol equalizer circuit using said adaptation coefficient during successive iterations of training data transmissions so as to minimize slicer errors;
• 8) determining if said equalization training has resulted in a stable convergence of said symbol equalizer circuit digital equalization filter tap coefficients by checking a value for said main tap coefficient of said feed forward equalization filter of said symbol equalizer circuit after the most recent interation of filter tap coefficient updating to determine how close to an expected value of one it is and checking values for side tap coefficients of said feed forward equalization filter of said symbol equalizer circuit after the most recent interation of tap updating to see how close to their expected values of zero said side tap coefficients are and concluding that the equalization process has not stabilized if the main and side taps coefficient values of the symbol equalizer feed forward equalization filter after the latest iteration of filter tap coefficient updating are not sufficiently close to expected values of said main and side tap coefficients;
• 9) if said equalization process has not yet stabilized, incrementing a counter that counts the number of attempts to achieve equalization, and repeating steps 5) through 9) until either the equalization process has stabilized or a predetermined number of iterations through these steps have been completed without stabilization having been achieved;
• 10) if stabilization has not been achieved after a predetermined number of attempts, concluding that equalization training has failed;
• 11) if the equalization process has stabilized, normalizing the symbol equalizer circuit filter tap coefficients;
• 12) calculating new tap coefficients for said feed forward and decision feedback equalizer filters En said chip equalizer circuit by convolving current filter tap coefficients of said digital feed forward and a decision feedback equalizer filters of said chip equalizer with the normalized symbol equalizer filter tap coefficients calculated in step 11 divided by two to derive new chip equalizer filter coefficients for downstream transmission equalization filtering in an RU receiver or new precoder filter coefficients for upstream transmissions from an RU transmitter, and determining if convergence has occurred;
• 13) if convergence has occurred, setting chip equalizer filter tap coefficients to the newly calculated main tap coefficients calculated in step 12.

A process for performing upstream equalization training of equalizer tap coefficients in a digital precode equalization filter in a remote unit modem transmitter said precode equalization filter having a digital feed forward equalization filter having a programmable main tap coefficient and a plurality of programmable side tap coefficients said process for performing upstream equalization training being performed using a central unit modem receiver coupled to said remote unit modem transmitter by a transmission medium, said central unit modem receiver having a symbol equalizer circuit coupled to receive demultiplexed symbols from a demultiplexer circuit and having a digital filter coupled to receive decision data from a slicer within said symbol equalizer circuit, said symbol equalizer circuit having a digital teed forward equalization filter having a programmable main tap coefficient and a plurality of programmable side tap coefficients, comprising:

• 1) loading a default value into a register in said central unit modem receiver symbol equalizer circuit that stores a main tap value of said symbol equalizer's digital feed forward equalization filter;
• 2) enabling main tap updating and disabling side tap adaptation in said feed forward equalization filter of said symbol equalizer circuit for a plurality of bursts of training data transmissions;
• 3) storing in a memory a predetermined value for an adaptation coefficient;
• 4) receiving a training data burst from said remote unit transmitter and adapting said main tap coefficient using said adaptation coefficient to adapt tap weight coefficients of said symbol equalizer digital filter during successive iterations of passing symbols from said training burst through said slicer and said symbol equalizer, said adaptation of tap weight coefficients being made in steps so as to minimize slicer errors;
• 5) enabling tap adaptation for all main taps and side taps of said feed forward equalization filter and said decision feedback equalization filter in said central unit modem receiver symbol equalization circuit for a plurality of bursts of training data transmission:
• 6) setting a predetermined value for said adaptation coefficient;
• 7) receiving said training data burst from said remote unit transmitter and adapting all main and side tap weight coefficients of said digital feed forward organization filter and said digital decision feedback equalization filter in said central unit modem receiver symbol equalization circuit using said adaptation coefficient during successive iterations of passing symbols from said training data burst through said slicer and said symbol equalizer, said adaptation of said tap weight coefficients being made in steps so as to minimize slicer errors;
• 8) after adapting said main and side tap coefficient values of said symbol equalization circuit of said central unit modem receiver during each iteration, determining if said main and side tap coefficients of said symbol equalization circuit of said central unit modem receiver have stabilized on predetermined expected values by comparing the value for said main tap coefficient in said central unit modem receiver symbol equalization circuit feed forward equalization filter to an expected value of one and comparing the values for said side tap coefficients in said central unit modem receiver symbol equalization circuit to expected values of zero and concluding that the equalization process has not stabilized if said main tap coefficient and said side tap coefficients of said symbol equalization circuit feed forward equalization filter and decision feedback equalization filter are not sufficiently close to their expected values;
• 9) if said equalization process has not yet stabilized, incrementing a counter and repeating steps 5) through 9) until either the equalization process has stabilized or said counter indicates a predetermined number of passes through these steps 5 through 9 have been completed without stabilization having been achieved;
• 10) if stabilization has not been achieved after a predetermined number of attempts to achieve stabilization, concluding that equalization training has failed;
• 11) if the equalization training process has stabilized, normalizing all the symbol equalizer filter tap coefficients by dividing them by the symbol equalizer feed forward equalization filter main tap coefficient and transmitting said symbol equalizer coefficients to said remote unit modem which transmitted said training burst;
• 12) calculating new main tap and side tap coefficients for said precode equalization filter of said remote unit transmitter that transmitted said iterations of training data by convolving existing filter tap coefficients of said precode filter of said remote unit transmitter when said training burst was transmitted with said normalized symbol equalizer filter tap coefficients as calculated in step 11 and dividing the result by two, and sifting the new precode equalization filter tap coefficients to the resulting newly calculated tap coefficients calculated in this step 12, and setting the tap coefficients of said symbol equalizer circuit filter tap coefficients of said central unit modem receiver to values which render said symbol equalizer circuit transparent in that said symbol equalizer circuit will not alter the amplitude or phase of signals passing therethrough.

A process of downstream equalization and synchronous upstream time division multiplexed centrally controlled transmission in a distributed digital data communication system having at least one central unit modem transmitting digital data to a plurality of remote unit modems via a transmission medium comprising a hybrid fiber coaxial cable signal distribution system, said remote unit modem having a receiver which includes a demultiplexer circuit and a digital equalization filter which filters symbols output by said demultiplexer circuit, said digital equalization filter having filter tap coefficients which define the filter's characteristics, comprising:

• 1) transmitting training data downstream from said central unit modem to a remote unit modem over a hybrid fiber coaxial cable system having a plurality of remote unit modems coupled thereto;
• 2) adapting filter tap coefficients of a digital equalization filter in a downstream receiver in said remote unit modem so as to at least partially compensate for channel impairments in the downstream transmission medium between said central unit modem and said remote unit modem so as to reduce reception errors;
• 3) synchronizing a local dock in each said remote unit modem to a master clock in said central unit modem using information transmitted downstream from said central unit modem, and requesting upstream bandwidth in messages sent upstream to said central unit modem from a remote unit modem, and receiving upstream bandwidth grant messages which assign timeslots for upstream transmission to said remote unit modem which sent said upstream bandwidth request;
• and
• 4) performing any ranging process to ascertain to correct ranging offset or time delay to use in upstream transmissions to achieve synchronization with upstream timeslot boundaries of upstream timeslots assigned in said upstream bandwidth grant messages to said remote unit modem for upstream transmissions and using said local clock synchronized to said master clock to time transmissions of known preamble data and payload data upstream from said remote unit modem to said central unit modem, and using said ranging offset in such a way that upstream data transmissions from said remote unit modem are properly aligned in time upon arrival at said central unit modem with the boundaries of timeslots assigned for upstream transmissions by said remote unit modem.

A process for performing upstream equalization for a remote unit modem in a code division multiplexed distributed digital data communication system having a central unit modem having a symbol equalization circuit and at least one remote unit modem having a precode filter coupled to said central unit modem by a transmission media, comprising:

• sending training sequence data to said central unit modem from a remote unit modem using each of a plurality of different, neighboring orthogonal, cyclic spreading codes while filter tap coefficients of said precede filter are set so as to render said precode filter transparent in that twill not alter the amplitude or phase of signals passing therethrough:
• adjusting tap weight coefficients of one or more adaptive equalizer filters in said symbol equalization circuit in a receiver in said central unit modem to reduce detection errors by a slicer in said receiver, with one or more adaptive equalizer filters located in said receiver so as to receive signals from a spread spectrum demultiplexing circuit;
• and reiterating for a plurality of symbols from one or more bursts of training sequence data to said central unit modem using each of a plurality of different, neighboring orthogonal spreading codes to spread the spectrum of the training data and again adjusting the tap weight coefficients of said one or more adaptive equalizer filters in said symbol equalizer circuit of said receiver of said central unit modem to minimize errors, and
• when detection errors have been minimized after one or more iterations, sending the final tap weight coefficients of said adaptive filter or filters of said symbol equalizer circuits in said central unit modem receiver to said remote unit modem for use in calculating now tap weight coefficients for said precode filter by convolving the precode filter tap weight coefficients in use when said training data was transmitted with said new tap weight coefficients sent by said central unit modem and using said newly calculated tap weight coefficients as new tap weight coefficients for said precode filter, and setting tap weight coefficients of said one or more adaptive filters of said symbol equalizer in said receiver of said central unit modem to values which render said symbol equalizer transparent in that said symbol equalizer will not alter the amplitude or phase of signals passing therethrough.

A process for performing downstream equalization in a distributed data communication system having a central unit modem coupled to at least one downstream modem by a transmission medium, comprising:

• sending training sequence data to said remote unit modem from said central unit modem using each of a plurality of different, neighboring orthogonal cyclic spreading codes;
• adjusting tap weight coefficients of a first adaptive equalizer circuits in a receiver in said remote unit modem to minimize detection errors and reiterating for a plurality of bursts of said training sequence data from said central unit modem to said remote unit modem using each of a plurality of different, neighboring orthogonal spreading codes to spread the spectrum of the data and again adjusting the tap weight coefficients of said first adaptive equalizer in said remote unit modem to minimize errors, and when detection errors have been minimized after a plurality of bursts of training sequence data, sending the final tap weight coefficients for use as the tap weight coefficients for a second adaptive equalizer in said receiver of said remote unit modem and setting the tap weight coefficients of said first adaptive equalizer in said receiver of said remote unit modem to initialization values to render said first adaptive equalizer transparent and repeating the process for all remote unit modems initially on power up and from time to time thereafter so as to update the tap weight coefficients of said second adaptive equalizer without starting training from a completely untrained state.

A process of upstream equalization training in a distributed data communication system having at least two remote unit modems transmitting multiplexed digital data over a shared transmission media to a central unit modem having a symbol equalization circuit having an FEE filter and a DFE filter coupled to the output of a slicer circuit which receives received symbols from a demultiplexing circuit where each said remote unit modem has a precode filter which filters the output signal of said modem, said precode filter having and FEE and a DFE filter, comprising the steps of:

• 1) setting the tap coefficients of said FFE and DEE filters of said precode filter of a remote unit modem which is to transmit training data to values which render said precede filter transparent in that said precode filter does not affect the amplitude or phase of signals passing therethrough;
• 2) setting the main tap coefficient of said FFE filter in said symbol equalization circuit to an initial value:
• 3) enabling adaptation of said main tap coefficient of said FEE filter and disabling adaptation of side taps of said FEE and DFE filters and sending a message to one remote unit modem requesting said remote unit modem to send a first plurality of symbols of training data and adapting said main tap coefficient during processing by said slicer of multiple iterations of symbols from said training data;
• 4) enabling adaptation of all taps of said FFE and DFE filters of said symbol equalization circuit, and processing multiple iterations of symbols from said training burst through said slicer and adapting all said EFE and DEE tap weight coefficients to minimize slicer errors;
• 5) determining if the FFE and DEE tap coefficients of said symbol equalization circuit have reached values sufficiently close to their expected values to deem the equalization training process to have stabilized;
• 6) if the training process is deemed not to have stabilized, incrementing a first count and repeating step 4 and testing again as defined in step 5 and continuing to repeat steps 4 and 5 until either the training process is deemed to have stabilized or said first count exceeds a maximum number of attempts;
• 7) if the training process has stabilized, normalizing the tap coefficients of said symbol equalization circuit by dividing the tap weight coefficents of said symbol equalization circuit by the value of the main tap coefficient of said FEE filter of said symbol equalization circuit, and sending the normalized symbol equalization circuit coefficients to said remote unit modem which transmitted said training data:
• 8) computing new values for the precode FEE and DFE filter tap weight coefficients by convolving the precode filter tap weight coefficients used to filter said training data with the tap weight coefficients received from said symbol equalization circuit of said central unit modem, and setting the precode filter tap weight coefficients equal to the resulting tap weight coefficients of said convolution, and setting tap weight coefficients of said symbol equalization circuit to transparency values;
• 9) determining if the equalization training process has converged by determining if the main tap and side tap coefficients of said symbol equalization circuit are close to expected values;
• 10) if the equalization training process has not converged incrementing a second count and repeating steps 4 through 11 until either said second count has reached a predetermined number of allowable attempts at achieving convergence or convergence is reached;
• 12) if convergence has been reached, transmitting the new FEE and DFE tap coefficients from said symbol equalization circuit in said central unit modem to said remote unit modem that transmitted said training data and setting the precode filter EFE and DFE tap coefficients to the new values calculated in step 8;
• 13) repeating steps 1 through 12 for each remote unit modem upon initial powerup to correct for channel impairments of said shared data path which are peculiar to the particular path between each remote unit modem and said central unit modem;
• and
• 14) from time to time, repeating steps 2 through 12 for each remote unit modem so as to update said tap coefficients of said remote unit modem transmitter precede filter without starting again from completely untrained tap coefficients.

A process of downstream equalization training in a distributed data communication system having at least two remote unit modems each having a receiver with a symbol equalization circuit having an FFE filter and a DE filter coupled to the output of a demultiplexing circuit and having a chip equalization circuit having FFE and DFE filters coupled to an input of said demultiplexing circuit, said remote unit modem coupled via a shared transmission media to a central unit modem having a transmitter therein having a precode filter and which is capable of sending training and other data to said remote unit modems, comprising the steps of:

• 1) sifting the tap coefficients of said precode filter in said central unit modem transmitter either to initial values which render it transparent or to initial values which are average equalization values for the paths to all remote unit modems;
• in each remote unit modem at its own pace:
• 2) setting the tap coefficients of said FFE and DEE filters of said symbol equalization circuit of a remote unit modem which is to receive said training data to values which render it transparent in that it does not affect the amplitude or phase of signals passing therethrough;
• 3) enabling adaptation of the main tap of said FFE filter and disabling adaptation of the side taps of said FFE and DEE filters in said remote unit modem receiver symbol equalization circuit and transmitting from said central unit modem a first plurality of bursts of training data and adapting said main tap coefficient during said bursts;
• 4) enabling adaptation of all taps of said FFE and DFE filters of said symbol equalization circuit of said remote unit modem receiver and sending a second plurality of bursts of training data from said central unit modem to said remote unit modem and adapting all said FFE and DFE taps of said remote unit modem symbol equalization circuit during said bursts of training data;
• 5) determining if the remote unit modem symbol equalization circuit FFE and DFE tap coefficients have reached values sufficiently close to their expected values to deem the equalization training process to have stabilized;
• 6) if the training process is deemed not to have stabilized, incrementing a first count and repeating step 4 and testing again as defined in step 5 and continuing to repeat steps 4 and 5 until either the training process is deemed to have stabilized or said first count exceeds a maximum number of attempts;
• 7) if the training process has stabilized, normalizing the remote unit modem receiver symbol equalization circuit tap coefficients by dividing them by the value of the main tap coefficient of said FFE filter of said symbol equalization circuit;
• 8) computing new values for the remote unit modem receiver chip equalization circuit FFE and DFE filter coefficients by convolving the old remote unit modem receiver chip equalization circuit FEE and DEE filter tap coefficients with the new FEE and DEE tap coefficients received from said symbol equalization circuit of said central unit modem;
• 9) determining if the equalization training process has converged in said remote unit modem receiver symbol equalization circuit by determining if the main tap and side tap coefficients of said symbol equalization circuit are close to expected values;
• 10) if the equalization training process has not converged, incrementing a second count and repeating steps 4 through 11 until either said second count has reached a maximum number of allowable iterations or convergence is reached;
• 12) if convergence has been reached, transmitting the new FFE and DFE tap coefficients from said symbol equalization circuit in said remote unit modem receiver to said remote unit modem receiver chip equalization circuit and setting the remote unit modem receiver chip equalization circuit FFE and UFE filter tap coefficients to the new values calculated in step 8;
• 13) repeating steps 1 through 12 for each remote unit modem upon initial powerup to correct for channel impairments of said shared data path which are peculiar to the particular path between each remote unit modem and said central unit modem;
• and
• 14) from time to time, repeating steps 2 through 12 for each remote unit modem receiver so as to update said tap coefficients of said remote unit modem receiver chip equalization circuit without starting again from completely untrained tap coefficients.

In a distributed digital data communication system having a plurality of remote unit modems coupled by a shared data path to a central unit modem, each said remote unit modem having a transmitter with a precode filter, and said central unit modem having a symbol equalization circuit and coupled to receive output signals from a demultiplexing circuit, a process to implement upstream equalization training, comprising the steps:

• 1) receiving an invitation from said central unit modem to begin training, and transmitting a ranging transmission which identifies said remote unit modem for purposes of fine tuning a ranging offset, and setting the tap coefficients of said precode filter of a remote unit modem which is to transmit training data to default values;
• 2) receiving a ranging offset measurement from said central unit modem, and adjusting a ranging offset value in accordance with said ranging offset measurement made at said central unit modem based upon said ranging transmission;
• 3) transmitting a burst of known training data symbols from said remote unit modem to said central unit modem, and filtering said training data using said precode filter;
• 4) receiving upstream equalization filter coefficients from said central unit modem, and convolving said filter coefficients received from said central unit modem with said precoder filter coefficients used when said training data was transmitted, and sifting the precoder filter coefficients to the resulting coefficients from said convolution calculation;
• 5) transmitting a ranging transmission which identifies said remote unit modem for purposes of fine tuning a ranging offset, and receiving another ranging offset measurement from said central unit modem made at said central unit modem based upon said ranging transmission, and adjusting a ranging offset value in accordance with said ranging offset measurement.

In a distributed digital data communication system having a plurality of remote unit modems coupled by a shared data path to said central unit modem a plurality of pairs comprised of one remote unit modem coupled by said shared data path to said central unit modem, each remote unit modem having a receiver with a symbol equalization circuit coupled to receive signals from the output of a demultiplexing circuit and having a chip equalization filter coupled to supply signals to an input of said demultiplexing circuit, each of said symbol and chip equalization circuits having an FFE and DFE filter therein, and said central unit modem having a transmitter capable of sending training data to all remote unit modems, at least two computers in the remote unit and central unit modems of each pair, said at least two computers programmed to cooperate to control the performance of the following functions to implement downstream equalization training:

• 1) setting the tap coefficients of said precede filter in said central unit modem transmitter either to initial values which render it transparent or to initial values which are average equalization values for the paths to all remote unit modems;
• in each remote unit modem at its own pace:
• 2) setting the tap coefficients of said FFE and DFE filters of said symbol equalization of a remote unit modem which is to receive said training data to values which render it transparent in that it does not affect the amplitude or phase of signals passing therethrough;
• 3) enabling adaptation of the main tap of said FFE filter and disabling adaptation of the side taps of said FFE and DFE filters in said remote unit modem receiver symbol equalization circuit and transmitting from said central unit modem a first plurality of bursts of training data and adapting said main tap coefficient during said bursts:
• 4) enabling adaptation of all taps of said FEE and DFE filters of said symbol equalization circuit of said remote unit modem receiver and sending a second plurality of bursts of training data from said central unit modem to said remote unit modem and adapting all said FFE and DFE taps of said remote unit modem symbol equalization circuit during said bursts of training data;
• 5) determining if the remote unit modem symbol equalization circuit FFE and DFE tap coefficients have reached values sufficiently close to their expected values to doom the equalization training process to have stabilized;
• 6) if the training process is deemed not to have stabilized, incrementing a first count and repeating step 4 and testing again as defined in step 5 and continuing to repeat steps 4 and 5 until either the training process is deemed to have stabilized or said first count exceeds a maximum number of attempts;
• 7) if the training process has stabilized, normalizing the remote unit modem receiver symbol equalization circuit tap coefficients by dividing them by the value of the main tap coefficient of said FEE filter of said symbol equalization circuit;
• 8) computing new values for the remote unit modem receiver chip equalization circuit FEE and DFE filter coefficients by convolving the old remote unit modem receiver chip equalization circuit FFE and DFE filter tap coefficients with the new FFE and DFE tap coefficients received from said symbol equalization circuit of said central unit modem;
• 9) determining if the equalization training process has converged in said remote unit modem receiver symbol equalization circuit by determining if the main tap and side tap coefficients of said symbol equalization circuit are close to expected values;
• 10) if the equalization training process has not converged, incrementing a second count and repeating steps 4 through 11 until either said second count has reached a maximum number of allowable iterations or convergence is reached;
• 12) if convergence has been reached, transmitting the new FEE and DEE tap coefficients from said symbol equalization circuit in said remote unit modem receiver to said remote unit modem receiver chip equalization circuit and setting the remote unit modem receiver chip equalization circuit FFE and DEE filter tap coefficients to the new values calculated in step 8;
• 13) repeating steps 1 through 12 for each remote unit modem upon initial powerup to correct for channel impairments of said shared data path which are peculiar to the particular path between each remote unit modem and said central unit modem;
• and
• 14) from time to time, repeating steps 2 through 12 for each remote unit modem receiver so as to update said tap coefficients of said remote unit modem receiver chip equalization circuit without starting again from completely untrained tap coefficients.

In a distributed digital data communication system having a plurality of remote unit modems coupled by a shared data path to said central unit modem, a plurality of pairs comprised of one remote unit modem coupled by said shared data path to said central unit modem, each remote unit modem having a receiver with a symbol equalization circuit coupled to receive signals from the output of a demultiplexing circuit and having a chip equalization filter coupled to supply signals to an input of said demultiplexing circuit, each of said symbol and chip equalization circuits having an FFE and DFE filter therein, and said central unit modem having a transmitter capable of sending training data to all remote unit modems, at least two computers in the remote unit and central unit modems of each pair, said at east two computers programmed to cooperate to control the performance of the following functions to implement downstream equalization training:

• 1) setting the tap coefficients of said precode filter in said central unit modem transmitter either to initial values which render it transparent or to initial values which are average equalization values for the paths to all remote unit modems;
• in each remote unit modem at its own pace:
• 2) setting the tap coefficients of said FFE and DFE filters of said symbol equalization of a remote unit modem which is to receive said training data to values which render it transparent in that it does not affect the amplitude or phase of signals passing therethrough;
• 3) enabling adaptation of the main tap of said FFE filter and disabling adaptation of the side taps of said FFE and DFE filters in said remote unit modem receiver symbol equalization circuit and transmitting from said central unit modem a first plurality of bursts of training data and adapting said main tap coefficient during said bursts;
• 4) enabling adaptation of all taps of said FFE and DFE filters of said symbol equalization circuit of said remote unit modem receiver and sending a second plurality of bursts of training data from said central unit modem to said remote unit modem and adapting all said EFE and DFE taps of said remote unit modem symbol equalization circuit during said bursts of training data;
• 5) determining if the remote unit modem symbol equalization circuit FFE and DFE tap coefficients have reached values sufficiently close to their expected values to deem the equalization training process to have stabilized;
• 6) if the training process is deemed not to have stabilized, incrementing a first count and repeating step 4 and testing again as defined in step 6 and continuing to repeat steps 4 and 5 until either the training process is deemed to have stabilized or said first count exceeds a maximum number of attempts;
• 7) if the training process has stabilized, normalizing the remote unit modem receiver symbol equalization circuit tap coefficients by dividing them by the value of the main tap coefficient of said FEE filter of said symbol equalization circuit;
• 8) computing new values for the remote unit modem receiver chip equalization circuit FFE and DFE filter coefficients by convolving the old remote unit modem receiver chip equalization circuit FFE and DFE filter tap coefficients with the new FEE and DFE tap coefficients received from said symbol equalization circuit of said central unit modem;
• 9) determining if the equalization training process has converged in said remote unit modem receiver symbol equalization circuit by determining if the main tap and side tap coefficients of said symbol equalization circuit are close to expected values;
• 10) if the equalization training process has not converged, incrementing a second count and repeating steps 4 through 11 until either said second count has reached a maximum number of allowable iterations or convergence is reached;
• 12) if convergence has been reached, transmitting the new FFE and DFE tap coefficients from said symbol equalization circuit in said remote unit modem receiver to said remote unit modem receiver chip equalization circuit and setting the remote unit modem receiver chip equalization circuit FFE and DEE filter tap coefficients to the new values calculated in step 8;
• 13) repeating steps 1 through 12 for each remote unit modem upon initial powerup to correct for channel impairments of said shared data path which are peculiar to the particular path between each remote unit modem and said central un modem;
• and
• 14) from time to time, repeating steps 2 through 12 for each remote unit modem receiver so as to update said tap coefficients of said remote unit modem receiver chip equalization circuit without starting again from completely untrained tap coefficients.