Multichannel, multimode DOCSIS headend receiver

A process for multichannel, mixed-mode reception of digital data transmissions at a cable modem termination system (hereafter CMTS) receiver coupled via a plurality of hybrid fiber coaxial (hereafter HFC) cable systems to a plurality of DOCSIS compliant, asynchronous time division multiple access and synchronous code division multiple access cable modems, said CMTS receiver comprising a plurality of channel receivers coupled to a plurality of HFC systems, each channel receiver comprising a plurality of analog front end circuits, each coupled to one of said plurality of HFC systems and each coupled through multiplexers to any one of a plurality of digital front end receivers, each said digital front end receiver coupled via a front end buffer and an arbiter to a shared back end demodulator and coupled to control circuitry, said process comprising the steps:

• (1) using upstream channel descriptor message data and MAP message data to determine when a burst on a channel will be transmitted and whether the channel is or is not mixed mode;
• (2) selecting an available digital front end receiver to receive said burst and sending control data to circuitry in said digital front end receiver to configure it to be coupled to the analog front end circuit coupled to the HFC system on which the burst will be transmitted and to configure said digital front end receiver to receive each sub-channel burst in said channel (hereafter sub-channel burst refers to the channel burst if the channel is not mixed-mode);
• (3) loading burst parameter data and channel parameter data pertaining to each expected sub-channel burst in a front end buffer of said selected digital front end receiver in a position to prepend the data of said sub-channel burst when that data is received;
• (4) filtering out unwanted radio frequency signals from the HFC system on which the burst is arriving and wide band sampling said burst or bursts from each sub-channel;
• (5) mixing each sub-channel burst down to baseband during the time the burst from that sub-channel is being received;
• (6) resampling each sub-channel burst at a multiple of a symbol clock rate of the burst;
• (7) filtering and decimating each sub-channel burst down to a sample rate of some predetermined number of samples per symbol based upon the symbol rate of each sub-channel burst;
• (8) performing a narrow band excision to remove or suppress narrow band noise;
• (9) writing burst parameter data and carrier phase and amplitude error correction factors pertaining to a particular sub-channel burst into a front end buffer to prepend the burst data of the burst;
• (10) writing the burst data of each sub-channel burst into a front end buffer that has the burst parameter data and error correction factors pertaining to the burst;
• (11) providing multichannel capability by repeating steps 1 through 10 for bursts scheduled on any of the HFC cable systems to which said CMTS receiver is connected using other available digital front end receivers of said CMTS receiver;
• (12) arbitrating among the collection of data from bursts already received and stored in the front end buffers of said digital front end receivers, and supplying said burst data and burst parameter data and error correction factors to said shared back end demodulator according to a priority scheme established by said arbitration;
• and
• (13) recovering the data of every burst using said shared back end demodulator.

A head end receiver apparatus for multichannel, mixed mode reception of digital data in a distributed system, comprising:

• a plurality of analog front end means, each coupled to one of a plurality of hybrid fiber coaxial (hereafter HFC) cable systems, each said system coupled to a plurality of cable modems that transmit DOCSIS 1.x, ATDMA or SCDMA bursts, for filtering out unwanted radio frequency signals and wideband sampling of burst signals on a channel which may have two or more sub-channels transmitting bursts of different center frequency, symbol rate and/or multiplexing type;
• a plurality digital front end receiver means, each capable of being selectively coupled to any one of said analog front end means, each digital front end receiver circuit for receiving a select control signal and coupling itself to one of said analog front end means and receiving samples therefrom, mixing the samples of each channel or sub-channel burst received from said selected analog front end means down to baseband, resampling the sample data of each burst at a multiple of the symbol rate of said burst, filtering and decimating each sub-channel burst down to a predetermined number of samples per symbol based upon the symbol rate of the burst, performing narrow band excision to remove or suppress narrow band noise, writing burst parameter data pertaining to the burst being received and phase and amplitude error correction factors pertinent to the burst being received into a front end buffer to prepend the burst data or act as a header for said burst data, and writing the preamble and data symbols of the burst being received into said front end buffer;
• a control means coupled to at least said plurality of digital front end receiver means for controlling said digital front end receiver means to receive multiple channels of data from one or more of said HFC systems, each channel being either mixed mode or single mode;
• arbitrator means coupled to the front end buffer of each of said plurality of digital front end receiver means, for selecting burst data to be further processed according to a priority scheme, and providing said burst data at an output;
• a shared demodulator coupled to receive each burst's preamble and data symbols when selected by said arbitrator means and for recovering the data encoded in said data symbols and making predetermined measurements to support ranging by the cable modem and developing phase and amplitude error correction factors for each cable modem from preamble symbols transmitted by said cable modem and using said phase and amplitude error factors developed for each cable modem to correct phase and amplitude errors in bursts transmitted from said cable modem, and developing upstream equalization coefficients from the preamble symbols of at least some bursts for sending to the cable modem which transmitted said burst;
• and further comprising equalizer means for processing both time division multiplexed data bursts and code division multiplexed data bursts after despreading to reduce echoes and other noise and use phase and amplitude error correction factors developed for the cable modem which transmitted said data burst to correct for phase and amplitude offset errors in said burst.

A head end receiver apparatus for multichannel, mixed mode reception of digital data in a distributed system, comprising:

• a plurality of channel receivers, each capable of receiving single mode or mixed-mode channels, each having a front end buffer;
• an arbiter coupled to the front end buffers of all said channel receivers for supplying burst data of various bursts on the basis of a priority scheme;
• a back end demodulator coupled to receive burst data from said aribiter and recover payload data encoded into each burst;
• and
• a control circuit coupled to control said plurality of channel receivers to receive and process multiple single mode or mixed-mode channels simultaneously if necessary.

A process for receiving digital data in a central modem coupled to a plurality of remote modems via a shared transmission medium, comprising:

• using a counter to establish upstream minislot boundaries;
• assigning minislots for particular bursts and assigning burst parameters that define the type of burst to be transmitted during said assigned minislots and transmitting said assigned minislots and burst parameters to a remote modem which has requested bandwidth;
• receiving radio frequency carrier signals and filtering out unwanted radio frequency signals and digitally sampling one or more desired radio frequency carriers modulated with bursts of symbols that encode digital data to generate a sample stream representing a desired channel;
• mixing said sample stream down to baseband, and excising narrow band noise;
• filtering in a matched filter to improve the signal to noise ratio;
• despreading the spectrum of any bursts that are code division multiplexed;
• detecting the start of each burst by detecting a unique word preamble, and measuring a time offset between an actual start time and an anticipated start time and using said time offset to send a message to a remote modem which transmitted said burst to assist said remote modem in ranging to achieve minislot synchronization so that said remote modem's bursts can be timed to arrive at said central modem aligned in time with the minislot boundaries assigned to said transmission;
• recovering a carrier frequency of each burst;
• recovering the symbol clock of each time division multiplexed burst;
• processing a preamble portion of each burst to generate phase and amplitude correction factors unique to the remote modem which transmitted said burst and using said phase and amplitude correction factors developed for a particular remote modem to correct phase and amplitude offset errors of the data portion of a burst from said remote modem;
• processing a preamble portion of each burst in an equalizer to develop upstream equalization coefficients for the remote modem which transmitted said burst, and transmitting said upstream equalization coefficients to said remote modem for its use in adjusting the tap coefficients of a preemphasis filter therein used to filter upstream transmissions to said central modem;
• filtering time division multiplexed bursts using the same equalizer used to generate said upstream equalization coefficients to remove echoes and using a predictor to remove further noise, and outputting filtered symbols;
• filtering bursts of symbols that have been code division multiplexed using cyclic, orthogonal spreading codes using the same equalizer used to develop said upstream equalization coefficients to remove echoes and using a predictor digital filter to remove further noise, and setting state and tap coefficients of said equalizer and predictor digital filters to a new state at the beginning of each spreading interval, said new state determined according to a predetermined algorithm designed to make use of the property that echoes of chips within the same spreading interval will appear in different, predictable codes than the symbol data encoded in said chips in a code domain after despreading when the spectrum was spread using cyclic, orthogonal spreading codes, and outputting filtered symbols;
• detecting and error correcting the digital data encoded in said filtered symbols.

A process for filtering symbols of time division multiplexed bursts and symbols of code division multiplexed bursts after despreading using a same equalizer filter, comprising the steps:

• receiving noise corrupted symbols of time division multiplexed bursts and receiving noise corrupted chips of synchronous code division multiplexed (SCDMA) bursts which have had their spectra spread using cyclic, orthogonal spreading codes, and bypassing a despreading step for said time division multiplexed bursts while processing said received, noise corrupted chips of said SCDMA bursts with a despreader to recover noise corrupted symbols therefrom, each of said time division multiplexed and SCDMA bursts having been filtered in a transmitter which transmitted said burst using a pre-emphasis filter having its tap coefficients set to equalize the channel through which the burst was transmitted;
• filtering said noise corrupted symbols of said time division multiplexed bursts using a digital equalizer filter to remove echoes and summing the resulting filtered symbols with the output of a digital predictor filter to remove further noise, and outputting filtered symbols from said time division multiplexed bursts;
• filtering said noise corrupted symbols output by said despreading step using the same digital equalizer filter used to filter said noise corrupted symbols of said time division multiplexed bursts to remove echoes and summing the resulting filtered symbols with the output of the same digital predictor digital filter to remove further noise, and controlling the states and coefficients said digital equalizer filter and said digital predictor filter according to any algorithm designed to make use of the property that echoes of chips within the same spreading interval will appear in different, predictable codes than the symbol data encoded in said chips in the code domain after despreading if the spectrum of the spread spectrum burst being processed was spread using cyclic, orthogonal spreading codes and an echo delay is small enough that the echo of a chip is still within the same spreading interval time, and outputting filtered symbols.

An error correction circuit for use in correcting symbol errors in received symbols of both time division multiplexed (TDMA) and synchronous code division multiplexed (SCDMA) bursts, comprising:

• an input for receiving symbols of said TDMA bursts, and despread symbols of said SCDMA bursts;
• a feed forward digital equalization filter (FFF) coupled to receive symbols at said input and having an output;
• a rotational amplifier having an input coupled to said output of said FFF and having an output, and having a first input for receiving an amplitude offset correction factor and a second input for receiving a phase offset correction factor, both offset correction factors being unique to the transmitter which transmitted said burst;
• a first summer having an output and having a first input coupled to said output of said rotational amplifier, and having a second input;
• a second summer having an output, and having a first input coupled to said output of said first summer, and having a second input;
• a symbol output coupled to said output of said second summer;
• a slicer having an input coupled to said output of said second summer, and having an output;
• a third summer having a first input coupled to said input of said slicer and a second input coupled coupled to said output of said slicer, and having an output at which an error signal appears;
• a fourth summer having a first input coupled to said output of said first summer, and having a second input coupled to said output of said slicer, and having an output at which a interference estimate signal appears;
• a first multiplier having an input coupled to receive said interference estimate signal, and having a multiplier input for receiving a 1 α
• signal which is a complex number having the real part of the denominator α equal to said amplitude offset correction factor input to said rotational amplifier and having the imaginary part of the denominator α equal to said phase offset correction factor input to said rotational amplifier, and having an output at which the product of said interference estimate signal and said 1 α
• signal appears;
• a digital predictor filter having a state input coupled to said output of said first multiplier and having an output coupled to said second input of said second summer, and having one or more filter coefficient inputs;
• a second multiplier having an input coupled to said output of said digital predictor filter and having an input at which a multiplier equal to α is applied, and having an output at which the product of said state data times α appears;
• a predictor buffer coupled to store the states of said predictor filter at the end of each spreading interval of an SCDMA burst, and coupled to store the tap weight coefficients of said predictor filter at predetermined times;
• a digital equalization feedback filter having a state input coupled to receive the output from said slicer, and having an output coupled to said second input of said first summer, and having filter coefficient inputs;
• a computer or other control circuit controlling said error correction circuit according to a predetermined algorithm to remove errors caused by echoes and other noise from said TDMA and SCDMA bursts.

An error correction circuit for use in correcting symbol errors in received symbols of both time division multiplexed (TDMA) and synchronous code division multiplexed (SCDMA) bursts, comprising:

• an input for receiving symbols of said TDMA bursts, and despread symbols of said SCDMA bursts;
• a feed forward digital equalization filter (FFF) coupled to receive symbols at said input and having an output;
• a first summer having an output and having a first input coupled to said output of said FFF, and having a second input;
• a second summer having an output, and having a first input coupled to said output of said first summer, and having a second input;
• a rotational amplifier having an input coupled to said output of said second summer and having an output, and having a first input for receiving an amplitude offset correction factor and a second input for receiving a phase offset correction factor, both offset correction factors being unique to the transmitter which transmitted said burst;
• a symbol output coupled to said output of said rotational amplifier;
• a slicer having an input coupled to said output of said rotational amplifier, and having an output;
• a third summer having a first input coupled to said input of said slicer and a second input coupled coupled to said output of said slicer, and having an output at which an error signal appears;
• a fourth summer having a first input coupled to said output of said first summer, and having a second input coupled to said output of said slicer, and having an output at which a interference estimate signal appears;
• a digital predictor filter having a state input coupled to said output of said fourth summer and having an output coupled to said second input of said second PATENT summer, and having one or more filter coefficient inputs;
• a predictor buffer coupled to store the states of said predictor filter at the end of each spreading interval of an SCDMA burst, and coupled to store the tap weight coefficients of said predictor filter at predetermined times;
• a digital equalization feedback filter having a state input coupled to receive the output from said slicer, and having an output coupled to said second input of said first summer, and having filter coefficient inputs;
• a computer or other control circuit controlling said error correction circuit according to a predetermined algorithm to remove errors caused by echoes and other noise from said TDMA and SCDMA bursts.