Apparatus and method for equalization in distributed digital data transmission systems

• US 6,665,308 B1
• Filed: 09/17/1998
• Issued: 12/16/2003
• Est. Priority Date: 08/25/1995
• Status: Expired due to Term

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Terayon Communications Systems Incorporated (Motorola Solutions, Inc.)
• Inventors: Azenkot, Yehuda, Rakib, Selim Shlomo
• Primary Examiner(s): Patel, Ajit
• Application Number:
  US09/156,560
• Time in Patent Office: 1,916 Days
• Field of Search: 370/422, 370/423, 370/442, 370/441, 370/478, 370/479, 370/480, 370/485, 370/486, 370/487, 370/489, 370/490, 370/498, 370/503, 375/229, 375/222, 375/230, 375/232, 375/233, 375/234, 375/235, 375/240, 375/240.01, 375/240.03, 375/240.04, 375/240.05, 375/219, 375/220, 348/384.1, 348/385.1, 348/388.1
• US Class Current: 370/441
• CPC Class Codes: H03M 13/256 with trellis coding, e.g. with convolutional codes and TCM
• CPC Class Codes: H03M 13/41 using the Viterbi algorithm or Viterbi processors
• CPC Class Codes: H03M 13/4169 using traceback H03M13/4192 takes precedence
• CPC Class Codes: H03M 13/6331 Error control coding in combination with equalisation
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 2201/709709 Methods of preventing interference
• CPC Class Codes: H04B 7/2668 Arrangements for Wireless Code-Division Multiple Access [CDMA] System Synchronisation, for code acquisition H04B1/7075, for code tracking H04B1/7085
• CPC Class Codes: H04H 20/78 CATV [Community Antenna Television] systems
• CPC Class Codes: H04J 13/0074 Code shifting or hopping
• CPC Class Codes: H04J 14/007 Orthogonal Optical Code Multiplex
• CPC Class Codes: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
• CPC Class Codes: H04L 1/006 Trellis-coded modulation
• CPC Class Codes: H04L 1/16 in which the return channel carries supervisory signals, e.g. repetition request signals
• CPC Class Codes: H04L 12/2801 Broadband local area networks
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 25/03343 Arrangements at the transmitter end
• CPC Class Codes: H04L 27/362 Modulation using more than one carrier, e.g. with quadrature carriers, separately amplitude modulated H04L27/366 takes precedence
• CPC Class Codes: H04L 5/023 Multiplexing of multicarrier modulation signals multicarrier modulation H04L27/2601
• CPC Class Codes: H04L 5/026 using code division
• CPC Class Codes: H04L 5/06 the signals being represented by different frequencies combined with time-division multiplexing H04L5/26
• CPC Class Codes: H04L 7/04 : Speed or phase control by synchronisation signals H04L7/0075 takes precedence
• CPC Class Codes: H04L 7/041 : using special codes as synchronising signal
• CPC Class Codes: H04N 7/10 : Adaptations for transmission by electrical cable H04N7/12 takes precedence
• CPC Class Codes: H04N 7/17309 : Transmission or handling of upstream communications
• CPC Class Codes: H04W 48/16 : Discovering, processing access restriction or access information
• CPC Class Codes: H04W 74/0866 : using a dedicated channel for access

Abstract

A system for bidirectional communication of digital data between a central unit and a remote unit wherein the need for tracking loops in the central unit has been eliminated. The central unit transmitter generates a master carrier and a master clock signal which are used to transmit downstream data to the remote units. The remote units recover the master carrier and master clock and synchronize local oscillators in each remote unit to these master carrier and master clock signals to generate reference carrier and clock signals for use by the remote unit receiver. These reference carrier and clock signals are also used by the remote unit transmitters to transmit upstream data to the central unit. The central unit receiver detects the phase difference between the reference carrier and clock signals from the remote units periodically and adjusts the phase of the master carrier and master clock signals for use by the central unit receiver to receive the upstream data.

Claims

1. A process for performing upstream equalization in a distributed digital data communication system having a central unit modem coupled to at least one remote unit modem by a transmission media, comprising:

sending training data upstream from a remote unit modem transmitter to said central unit modem, said transmission occurring at any time said remote unit modem transmitter needs to perform training but times so as to arrive at said central unit modem during an interval reserved for ranging and training transmissions;

receiving a downstream message from said central unit modem which is addressed to said remote unit modem which sent said training data upstream and which includes the actual equalization filter coefficients which have been set for tap coefficients of a digital upstream equalization filter in said central unit modem and which were derived by said central unit modem using an iterative adaption process to adapt said tap coefficients of said digital upstream equalization filter so as to equalize the data path from said remote unit modem which sent said training data to said central unit modem through said transmission media and convolving said equalization filter coefficients received from said central unit modem with existing tap coefficients of a digital precode filter in said remote unit modem transmitter and which was used to filter said upstream training data transmission said convolution resulting in new tap coefficients for said digital precode filter and setting said tap coefficients of said digital precode filter to the tap coefficients derived by said convolution calculation and using said digital precode filter for filtering further upstream payload data transmissions from said remote unit modem.

2. The process of claim 1 wherein said training data is sent upstream in a plurality of bursts, each burst being used for one interation in adapting said tap coefficients of said digital upstream equalization filter toward convergence.

3. The process of claim 1 wherein said upstream equalization process is performed initially on power-up of each said remote unit modem and is performed from time to time thereafter for each said remote unit modem.

4. The process of claim 1 further comprising a step of iteratively adapting filter tap coefficients of at least one digital upstream equalization filter in said central unit modem to equalize a data path through said transmission media between said remote unit modem and said central unit modem, and wherein said step of adapting iteratively adapting said tap coefficients of said digital upstream equalization filter in said central unit modem comprises the steps of adapting the tap coefficients of both a feed forward equalization filter and a decision feedback equalization filter.

5. The process of claim 2 wherein said upstream equalization process is performed initially on power-up of said remote unit modem and is performed from time to time thereafter.

6. The process of claim 1 wherein said remote unit modem and remote unit modem communicates with said central unit modem via code division multiplexed transmissions using orthogonal, cyclic spreading codes, and wherein said training data is transmitted upstream simultaneously on a plurality of neighboring, orthogonal, cyclic spreading codes.

7. The process of claim 6 further comprising the step of adapting digital filter tap coefficients of at least one digital upstream equalization filter in said central unit modem during reception of said training data so as to at least partially compensate for channel impairments in the transmission medium between said remote unit modem and said central unit modem so as to reduce errors in detecting which constellation points were sent by said remote unit modem to said central unit modem, and wherein said adaptation of tap coefficients is performed in the receiver of said central unit modem on training data that has been mixed down to baseband, digitized and simultaneously despread by a code division demultiplexer which uses the transpose of a spreading code matrix used to spread the spectrum of said training data in said remote unit modem or which has been depressed by simultaneously correlating the training data using a bank of correlators having a correlator dedicated to each spreading code used to code division multiplex said training data or by storing said training data in a memory and, in successive iterations of reading said training data out of memory, correlating said training data stored in said memory against one of the spreading codes used for training using a single correlator whose coefficients are changed to coefficients suitable to correlate one of said training codes on that particular iteration, and changing said correlator coefficients on each successive iteration of reading said training data out of memory until all training codes have been correlated.

8. The process of claim 1 further comprising the step of adapting tap coefficients of at least one digital upstream equalization filter in said central unit modem during reception of said training data so as to at least partially compensate for channel impairments in the transmission medium between said remote unit modem and said central unit modem so as to reduce errors in detecting which constellation points were sent by said remote unit modem to said central unit modem, and wherein the step of adapting said tap coefficients of said digital upstream equalization filter comprises the steps of adjusting the tap coefficients of both a feed forward equalizer to reduce precursor intersymbol interface and a decision feedback equalizer to reduce post cursor interference, and wherein the step of adjusting the tap coefficients is performed initially upon power up of said remote unit modem and from time to time thereafter.

9. The process of claim 1 further comprising the step of adapting filter tap coefficients of at least one digital upstream equalization filter in said central unit modem during reception of said training data so as to at least partially compensate for channel impairments in the transmission medium between said remote unit modem and said central unit modem so as to reduce errors in detecting which constellation points were sent by said remote unit modem to said central unit modem, and wherein the step of adapting the tap coefficients of said at least one digital upstream equalization filter comprises the steps of convolving tap coefficients of a chip equalization filter with newly calculated tap coefficients of a symbol equalization filter generated from error signals derived from analysis of data going into and coming out of a slicer regardless of the type of multiplexing used to send data from different sources.

10. The process of claim 1 further comprising the step of adapting filter tap coefficients of at least one digital upstream equalization filter in said central unit modem during reception of said training data so as to at least partially compensate for channel impairments in the transmission medium between said remote unit modem and said central unit modem so as to reduce errors in detecting which constellation points were sent by said remote unit modem to said central unit modem, and

wherein the step of adapting the tap coefficients of at least one digital upstream equalization filter comprises the steps of iteratively calculating new tap weights or coefficients by multiplexing error signals derived from a slicer times corresponding tap input signals used to calculate a convolution sum between said tap input signals and the tap coefficients of said at least one equalization filter times a predetermined step size controlling the rate of convergence of said equalization process, with the result being added to the old tap coefficients to derive new tap coefficients for use in adjusting the constellation point data entering said slicer on the next iteration, where said error signals are calculated by comparing received constellation point data going into a slicer to detected constellation data coming out of the slicer.

11. The process of claim 10 where said step of adapting tap coefficients comprises the steps of adapting the tap coefficients of both a feed forward equalization filter as well as a decision feedback equalization filter.

12. The process of claim 1 further comprising the step of adapting filter tap coefficients of at least one digital upstream equalization filter in said central unit modem during reception of said training data so as to at least partially compensate for channel impairments in the transmission medium between said remote unit modem and said central unit modem so as to reduce errors in detecting which constellation points were sent by said remote unit modem to said central unit modem, and wherein the step of adapting said tap coefficients of said digital upstream equalization filter comprises the steps of adjusting the tap coefficients of a feed forward equalizer to reduce precursor intersymbol interference and a decision feedback equalizer to substantially reduce post cursor intersymbol interference using a least mean square calculation circuit which calculates new tap weight coefficient signals for the feed forward equalizer and the decision feedback equalizer using error signals generated by a difference calculation circuit which calculates the differences between the desired constellation points output by a slicer circuit in response to received constellation points and the actual received constellation points, said calculation of new tap weight coefficient signals being done iteratively using a step size value and multiple bursts of training data.

13. The process of claim 1 wherein the step of transmitting training data comprises the steps of initially setting the tap coefficients of said digital precode filter in said remote unit transmitter to initial values such that said digital precode filter does not affect the training data during transmissions thereof prior to convergence of the tap coefficients of said at least one digital upstream equalization filter in said central unit modem followed by transfer of said tap coefficients of said digital upstream equalization filter in said central unit modem after convergence on values which tend to equalize the data path between said remote unit modem which sent said training data and said central unit modem, said transfer being by said downstream message addressed to said remote unit modem which transmitted said training data for convolving with said initial values of precode filter tap coefficients in said remote unit modem precode filter so as to derive new tap coefficients for said precode filter.

14. The process of claim 1 wherein the step of transmitting training data comprises the steps of initially setting the tap coefficients of said precode filter in said remote unit transmitter to previously obtained convergence values obtained during a prior equalization process such that said precode filter affects the training data during transmissions thereof in a way to at least partially compensate said signals for prior characteristics of the channel, said precode filter tap coefficients being updated to new values after convergence of the coefficients of said at least one digital upstream equalization filter in said central unit modem during the current equalization process.

15. 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.

16. The apparatus of claim 15 wherein said central unit receiver equalization circuit further comprises a first computer programmed for controlling upstream equalization training by performing the following functions:

sending a message to one remote unit transmitter instructing it to send training data to said central unit receiver,

enabling convergence of coefficients of said symbol equalization circuit to compensate for channel impairments of said transmission media detected during transmission of said training data,

after convergence, transmitting said symbol equalization circuit coefficients to said remote unit transmitter that transmitted said training data, and

setting a main tap coefficients of said symbolization equalization circuit to one and

setting side tap coefficients of said symbol equalization circuit to all zones after

transmitting the converged tap coefficients to said remote unit transmitter, and wherein said remote unit transmitter which transmitted said training data further comprises a second computer programmed to assist in the equalization process by performing the following functions;

initially setting coefficients of said precode equalization filter to values that are such as to cause said precode equalization filter to not predistort the transmitted signal during transmission of said training data,

after receiving said coefficients from said symbol equalization circuit in said central unit receiver after convergence, calculating new tap coefficients for said precode equalization filter of said remote unit transmitter by convolving the old tap coefficients of said precode equalization filter with the new tap coefficients received from said symbol equalization circuit and for setting the coefficients of said precode equalization filter to the tap coefficient values so calculated.

17. The apparatus of claim 16 wherein said first and said second computer in each said remote unit transmitters are each programmed to repeat the upstream equalization process for the corresponding remote unit transmitter upon power up and from time to time thereafter.

18. The apparatus of claim 15 further comprising a chip equalization filter circuit coupled to filter received signals output by said demodulator and supply the filtered signals to said demultiplexer, and wherein said symbol equalization circuit and said chip equalization circuit each comprises at least a feed forward equalization filter and a decision feedback equalization filter.

19. The apparatus of claim 16 further comprising a chip equalization filter circuit coupled to filter received signals output by said demodulator and supply the filtered signals to said demultiplexer, and wherein said symbol equalization circuit and said chip equalization circuit each comprises at least a feed forward equalization filter and a decision feedback equalization filter.

20. The apparatus of claim 17 further comprising a chip equalization filter circuit coupled to filter received signals output by said demodulator and supply the filtered signals to said demultiplexer, and wherein said symbol equalization circuit and said chip equalization circuit each comprises at least a feed forward equalization filter and a decision feedback equalization filter.

21. The apparatus of claim 18 wherein said equalization circuit in said central unit receiver further comprises a first computer programmed to control converging said coefficients of said symbol equalization circuit feed forward and decision feedback equalization filters in said central unit receiver and transferring them to said precede equalization filter of the remote unit transmitter that transmitted said training data to achieve upstream equalization by performing the following functions:

setting a main tap coefficient of said feed forward equalization filter to a default value and sending a message to a single remote unit transmitter instructing it to start sending a burst of training data and to set tap coefficients of said precode equalization filter of said remote unit transmitter to values that cause said precede equalization filter to not predistort the transmitted training data signal;

enabling adaptation of a main tap coefficient of said symbol equalizer circuit feed forward equalization filter over a first plurality of transmitted symbols of training data transmitted from said single remote unit transmitter while disabling adaptation of side tap coefficients of said symbol equalizer circuit feed forward and decision feedback equalization filters;

enabling adaptation of all tap coefficients of said symbol equalizer circuit feed forward and decision feedback equalization filters over a second plurality of transmitted symbols of training data transmitted from said single remote unit transmitter;

testing for stability of said symbol equalizer main and side tap coefficients relative to expected values, and continuing to adapt said coefficients until stability of said coefficients has been achieved or until a predetermined number of attempts at achieving stability have failed to achieve stability,

when stability has been achieved, normalizing the coefficients of said symbol equalization circuit by dividing them by the value of the main tap of said feed forward equalization filter and sending the normalized coefficients of said symbol equalization circuit to said remote unit transmitter which sent said burst of training data;

calculating new tap weights for said precode equalization filter of said remote unit transmitter that transmitted said training data by convolving the precode equalizer filter coefficients used when said training data burst was sent with the normalized symbol equalizer circuit coefficients and setting the resultingcoefficients as the new tap weight coefficients of said precode equalization filter.

22. The apparatus of claim 15 wherein said first computer is also programmed to determine if convergence of the taps of said symbol equalizer circuit toward expected values has occurred, and, if so, loading the symbol equalizer main tap coefficient into a rotational amplifier for use in developing phase and amplitude error correction factors, and then setting the main tap of said symbol equalizer feed forward equalization filter to one and setting the side taps of said feed forward and decision feedback equalization filters to zero, and wherein each said remote unit transmitter transmits payload data using QAM-16 modulation, and further comprising a rotational amplifier coupled to receive output signals from said demultiplexer circuit of said central unit receiver, and a computer coupled to said rotational amplifier and programmed for controlling operation thereof to make sure said rotational amplifier has not falsely locked onto false minima in the error surface of the QAM-16 constellation of transmitted data by checking each of the rotational amplifier'"'"'s amplitude and phase correction factors for each remote unit transmitter against the amplitude and phase correction factors, respectively, of the symbol equalizer circuit'"'"'s main tap coefficient after convergence, and, if a difference is found in either tho amplitude correction factor or the phase correction factor which is larger than a threshold amount, setting said amplitude and phase correction factors of said rotational amplifier equal to said symbol equalizer circuits main tap coefficient amplitude and phase correction factors, respectively, and wherein said equalization circuit in said central unit receiver further comprises a computer programmed for controlling the process for training of said coefficients of said symbol equalization circuit and using them to calculate new tap weight coefficients for said precode equalization filter of the remote unit transmitter that transmitted said training data by performing the following functions:

loading default coefficients into said symbol equalizer circuit and adapting said coefficients over a first plurality transmitted symbols of training data transmitted from a single remote unit transmitter;

testing for stability of said coefficients relative to expected values, and continuing to adapt said coefficients until stability of said coefficients has been achieved;

when stability has been achieved, normalizing the coefficients of said symbol equalization circuit by dividing them by the value of the main tap, and then sending said coefficients to said remote unit transmitter and calculating new tap weights for said precode equalization filter of said remote unit transmitter that transmitted said training data by convolving the old precode equalizer filter coefficients and the quantity equal to the normalized symbol equalizer circuit coefficients divided by two, and, after sending said symbol equalizer filter coefficients to said remote unit transmitter, setting the coefficients of said symbol equalizer circuit to transparency values.

23. The apparatus of claim 20 wherein said first computer for controlling the convergence of said coefficients of said symbol equalization circuit is programmed to check when convergence has occurred by determining if a main coefficient of said symbol equalization circuit has reached one and the remaining side coefficients have reached zero by calculating the amplitude correction factor of the side coefficients of the feed forward and decision feedback equalization filters as the summation for all side taps of the sum of the squares of the real and imaginary components of each side tap of said feed forward equalization filter plus the summation for all side taps of the sum of the squares of the real and imaginary components of each side tap of said decision feedback equalization filter, and then calculating an amplitude correction factor of the symbol equalization circuit main coefficient by calculating the sum of the squares of the real and imaginary parts of the symbol equalizer circuit main coefficient, and then calculating the ratio of the amplitude correction factor of the side coefficients to the amplitude correction factor of the main coefficient and comparing said ratio to a threshold of convergence and concluding that convergence has occurred if said ratio is loss than said threshold of convergence.

24. The apparatus of claim 20 wherein said first computer for controlling the convergence of said coefficients of said symbol equalization circuit functions to determine when the convergence process has achieved stability by comparing the values of each of the said coefficients of said feed forward equalization filter to a coefficient threshold value, and if any one of the side coefficients is not smaller than the threshold value, restarting the training process to achieve coefficient convergence, and, if all side coefficients of said feed forward equalization filter arc smaller than said coefficient threshold value, comparing the values of each of the side coefficients of said decision feedback equalization filter to a coefficient threshold value, and if any one of the side coefficients is not smaller than the threshold value, restarting the training process to achieve coefficient convergence, and, if all side coefficients of said decision feedback equalization filter are smaller than said coefficient threshold value, calculating the composite amplitude of the side coefficients of both said feed forward and decision feedback equalization filters as the summation of the sum of the squares of the real and imaginary components of all side coefficients of the feed forward equalization filter plus the summation of the sum of the squares of the real and imaginary components of the side coefficients of said decision feedback equalization filter, and then calculating the amplitude of the main coefficient as the sum of the squares of the main coefficient real and imaginary parts, and then calculating the ratio of the composite amplitude of tho side coefficients divided by the amplitude of the main coefficient and concluding that the equalization convergence process is stable if said ratio is less than a stability threshold, and, if not, restarting the convergence process.

25. 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.

26. The apparatus of claim 25 wherein said chip equalization circuit and said symbol equalization circuit each comprise a feed forward digital filter and a decision feedback equalization filter.

27. The apparatus of claim 26 wherein said equalization circuit of said remote unit receiver includes a computer programmed to control the downstream equalization training process by performing the following functions:

setting a main tap coefficient of said feed forward digital filter of said symbol equalization circuit to a default value prior to receipt of training data transmissions from said central unit transmitter;

enabling adaptation of said main tap coefficient and disabling adaptation of said side tap coefficients of said feed forward and decision feedback equalization filters of said symbol equalization filter during reception of a plurality of bursts of training data;

enabling adaptation of said main said tap coefficients of said feed forward and decision feedback equalization filters of said symbol equalization filter during reception of a plurality of bursts of training data;

determining if the downstream equalization process has achieved stability by comparing the magnitudes of side taps of said feed forward and decision feedback equalization filter to threshold values, and, if any side tap is greater than the threshold value against which it is compared, restarting the downstream equalization training process, and further testing for stability by computing a composite amplitude of the side tap coefficients of both said feed forward and decision feedback equalization filters as the summation of the sum of the squares of the real and imaginary components of all side tap coefficients of the feed forward equalization filter plus the summation of the sum of the squares of the real and imaginary components of the side tap coefficients of said decision feedback equalization filter, and then calculating the amplitude of the main tap coefficient as the sum of the squares of the main tap coefficient rail and imaginary parts, and then calculating the ratio of the composite amplitude of the side tap coefficients divided by the amplitude of the main tap coefficient and concluding that the equalization convergence process is stable if said ratio is less than a stability threshold, and, if not, restarting the convergence process;

if the convergence process has stabilized, normalizing all the tap coefficients of the symbol equalizer circuit to the main tap coefficient of the feed forward equalization filter; and

computing new tap coefficients for said chip equalization circuit by convolving the old tap coefficients of said chip equalization circuit with the tap coefficients said symbol equalization circuit converged upon;

determining if convergence has occurred by checking tap weight coefficients of said symbol equalizer circuit against expected values; and

if convergence has occurred, setting the tap coefficients of said chip equalizer circuit equal to the newly computed tap coefficients and setting the tap coefficients of said symbol equalizer circuit to values which render it transparent in that will not alter the amplitude or phase of signals passing therethrough.

28. 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.

29. The process of claim 28 further comprising the step of setting said main tap coefficient of said symbol equalizer circuit feed forward equalization filter to one and setting the side tap coefficients of said feed forward equalization filter and said decision feedback equalization filter to zeroes after convergence of the coefficients of said symbol equalizer circuit and calculation of new chip equalizer filter tap coefficients in step 12, and comparing a main tap coefficient of said symbol equalizer circuit after convergence has been received to amplitude and phase error correction factors of said rotational amplifier in said modem to ensure said rotational amplifier has net locked onto a false minima.

30. The process of claim 28 wherein said training data is transmitted on a plurality of neighboring, orthogonal, cyclic spreading codes in a code division multiplexed distributed digital data transmission system, and wherein said modem is a remote unit cable modem which includes a code division demultiplexing circuit and a rotational amplifier coupled to receive signals from said code division demultiplexer circuit, and further comprising the step of determining whether the equalization process has resulted in convergence by calculating the amplitude correction factor of the side tap coefficients of the feed forward and decision feedback equalization filters as the summation for all side tap coefficients of the sum of the squares of the real and imaginary components of each side tap coefficient of said feed forward equalization filter plus the summation for all side tap coefficients of the sum of the squares of the real and imaginary components of each side tap coefficient of said decision feedback equalization filter, and then calculating an amplitude correction factor of the symbol equalization circuit feed forward equalization filter main tap coefficient by calculating the sum of the squares of the real and imaginary parts of the symbol equalizer feed forward equalization filter main tap coefficient, and then calculating the ratio of the amplitude correction factor of the side tap coefficients to the amplitude correction factor of the symbol equalizer main tap coefficient and comparing said ratio to a threshold of convergence and concluding that convergence has occurred if said ratio is less than said threshold of convergence, and, if convergence has occurred, loading the real and imaginary tap values of said main tap of said symbol equalizer feed forward equalization filter into said rotational amplifier.

31. 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.

32. 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.

33. The process of claim 32 wherein said remote unit modem also has second equalization filter coupled to filter received data and supply the filtered data to said demultiplexer, and wherein the step of sending training data downstream comprises the step of said central unit modem sending training data to said remote unit modem which is code division multiplexed by sending the same pseudonoise sequence having its spectrum spread by a plurality of adjacent orthogonal, cyclic spreading codes using BPSK modulation, further comprising the steps of using the coefficients derived in step 2 to derive new coefficient values for said second equalization filter by convolving said coefficients derived in step 2 with the coefficients of said second equalization filter which were used to filter said training data while said filter coefficients derived in step 2 were being derived, and setting the coefficients of said equalization filter coupled to receive symbols output by said demultiplexer circuit to initialization values.

34. The process of claim 32 further comprising the steps of:

1) sending a known training data burst upstream to said central unit modem and filtering said training data burst using a precode filter having its tap weights set to transparency values; and

2) receiving tap weight coefficients from said central unit modem, and convolving said tap weight coefficients with said precode filter coefficients used to filter said training data burst, and setting the tap weight coefficients of said precode filter to the tap weights resulting from said convolution.

35. The process of claim 32 wherein the downstream equalization process is conducted for every remote unit modem upon power up and is repeated for every remote unit modem from time to time thereafter.

36. 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.

37. The process of claim 36 wherein the steps of sending training sequence data to said central unit modem comprise the steps of sending a burst of known training sequence data.

38. The process of claim 37 wherein the steps of sending training sequence data to said central unit modem comprise the steps of sending bursts of training sequence data using BPSK moderation during gaps between frames of data being sent upstream from said remote unit modems to said central unit modem.

39. The process of claim 37 wherein a plurality of remote unit modems exist, and said upstream equalization process is performed individually for each remote unit modem by said central unit modem such that each remote unit modem receives tap coefficients for its transmitter precede filter which at least partially compensate for channel impairments of the channel between that particular remote unit modem and said central unit modem.

40. The process of claim 37 wherein said upstream equalization process is performed at initial powerup of said remote unit modem and is performed again from time to time thereafter.

41. 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.

42. 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.

43. The process of claim 42 wherein step 9 comprises the steps of:

calculating a composite amplitude correction factor of the side taps of said symbol equalization circuit FFE and DFE filters;

computing the ratio of the side tap composite amplitude correction factor divided by the main tap amplitude correction factor of the symbol equalization circuit FFE filter;

comparing said ratio to a convergence threshold, and deeming the training process to have converged if the ratio is less than the convergence threshold.

45. The process of claim 43 wherein step 9 comprises the steps of:

calculating a composite amplitude correction factor of the side taps of said symbol equalization circuit FFE and DFE filters;

computing an amplitude correction factor of the symbol equalization circuit FFE filter;

computing the ratio of the side tap composite amplitude correction factor divided by the main tap amplitude correction factor of the symbol equalization circuit FFE filter;

comparing said ratio to a convergence threshold, and deeming the training process to have converged if the ratio is less than the convergence threshold.

44. 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.

46. 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.

47. 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.

48. 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.