Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B3/00—Line transmission systems
  • H04B3/02—Details
  • H04B3/32—Reducing cross-talk, e.g. by compensating
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B3/00—Line transmission systems
  • H04B3/02—Details
  • H04B3/46—Monitoring; Testing
  • H04B3/487—Testing crosstalk effects

Definitions

• the invention relates to systems for, and methods of, reducing the noise present in the signals received and processed by devices within a communications system and to systems for, and methods of, reducing such noise in communications systems having high throughputs.
• the invention also relates to systems for, and methods of, reducing the power dissipation in devices within a communications system and to systems for, and methods of, reducing such power dissipation in communications systems having high throughputs.
• the invention further relates to a startup protocol for initiating normal transmission between transceivers within a high throughput communications system.
• a "high throughput" as used within the context of this disclosure may include, but is not limited to, one gigabit (GB) per second.
• GB gigabit
• the system includes a hub and a plurality of computers serviced by the hub in a local area network (LAN).
• LAN local area network
• Each of the computers is usually displaced from the hub by a distance which may be as great as approximately one hundred meters (100 m.).
• the computers are also displaced from each other.
• the hub is connected to each of the computers by a communications line.
• Each communication line includes unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper.
• Four unshielded twisted pairs of wires are provided in each communication line between each computer and the hub.
• the system shown in FIG. 1 is operative with several categories of unshielded twisted pairs of cables designated as categories 3, 4, 6 and 7 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 6 and 7 cables are the best quality (and highest cost).
• the throughput of a system is the rate at which the system processes data and is usually expressed in bits/second.
• a rapidly evolving area of communications system technology enables 1 Gb/second full-duplex communication over existing category-5 unshielded twisted pair cables.
• Such a system is commonly referred to as "Gigabit Ethernet.”
• a portion of a typical Gigabit Ethernet is shown in FIG. 2.
• the Gigabit Ethernet provides for transmission of digital signals between one of the computers and the hub and the reception of such signals at the other of the computer and the hub.
• a similar system can be provided for each of the computers.
• the system includes a gigabit medium independent interface (GMII) block which receives data in byte-wide format at a specified rate, for example 125 MHz , and passes the data onto the physical coding sublayer (PCS) which performs scrambling, coding, and a variety of control functions.
• GMII gigabit medium independent interface
• the PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals.
• the five symbol levels are -2, -1, 0, +1, and +2.
• Communication between the computer and hub is achieved using four unshielded twisted pairs of wires or cables, each operating at 250 Mb/second, and eight transceivers, one positioned at each end of a unshielded twisted pair.
• the full-duplex bidirectional operation provides for the use of hybrid circuits at the two ends of each unshielded twisted pair.
• the hybrid controls access to the communication line, thereby allowing for full-duplex bidirectional operation between the transceivers at each end of the communications line.
• Impairment signals include echo, near- end crosstalk (NEXT), and far-end crosstalk (FEXT) signals.
• NXT near- end crosstalk
• FXT far-end crosstalk
• FIG. 6 illustrates Four transceivers at one end of a communications line.
• the components of the transceivers are shown as overlapping blocks, with each layer corresponding to one of the transceivers.
• the GMII, PCS, and hybrid of FIG. 6 correspond to the GMH, PCS, and hybrid of FIG. 2 and are considered to be separate from the transceiver.
• the combination of the transceiver and hybrid forms one "channel" of the communications system. Accordingly, FIG. 6 illustrates four channels, each of which operates in a similar manner.
• the transmitter portion of each transceiver includes a pulse-shaping filter and a digital-to-analog
• each transceiver includes an analog-to-digital (A/D) converter, a first-in first-out (FIFO) buffer, a digital adaptive equalizer system including a feedforward equalizer (FFE) and a detector.
• the receiver portion also includes a timing recovery system and a near-end noise reduction system including a NEXT cancellation system and an echo canceller.
• the NEXT cancellation system and the echo canceller typically include numerous adaptive filters.
• Characteristics of the communication line may impact the ability of the NEXT cancellation system and echo cancellers to effectively cancel NEXT and echo noise. Measurements of typical cable responses, as well as simulation, show that in order to provide an adequate level of cancellation of these sources of interference, "long" echo and NEXT cancellers are required.
• the term “long” is used to describe a canceller having a large number of taps as necessitated by the characteristics of the cable. For example, FIG. 7 shows the echo impulse response for a 100m cable with a characteristic impedance of 85 ohm and 100 ohm terminations.
• the nominal characteristic impedance is 100 ohm, manufacturing standards allow for a 15% tolerance.
• the mismatch in impedance may result in a reflection at the far-end of the cable, which causes a secondary pulse with a delay of about one microsecond. Because of the long delay, cancelling this pulse requires an echo canceller with about 140 taps (125 taps to cover the one microsecond delay, plus approximately 15 additional taps to cancel the secondary pulse).
• the echo impulse response has additional reflections at intermediate values of delay.
• structural return loss of the cable may cause continuous variations of the characteristic impedance along the cable, which results in a large number of smaller reflections at intermediate points.
• These intermediate reflections mean that the echo canceller should not be configured to cancel only the initial impulse and the end reflection but instead should be configured to cover the full span of the impulse response. Varying cable characteristics result in a wide variability of cable impulse responses.
• the response of a particular cable may change as a result of its operating environment. For example, a change in operating temperature may change the impulse response of the cable. Accordingly, it is difficult to precompute the locations at which taps are required and to build these locations into the design of the echo and NEXT cancellers.
• phase adaptive filters contained within the transceiver converge, the timing recovery subsystem acquires frequency and phase synchronization, the differences in delay among the four wire pairs are compensated, and pair identity and polarity is acquired. Successful completion of the startup allows normal operation of the transceiver to begin.
• blind start In one startup protocol, known as "blind start", the transceivers converge their adaptive filters and timing recovery systems simultaneously while also acquiring timing synchronization.
• a disadvantage of such a startup is that there is a high level of interaction among the various adaptation and acquisition algorithms within the transceiver. This high level of interaction reduces the reliability of the convergence and synchronization operations which occur during startup.
• the invention relates to systems for, and methods of, reducing the noise and the power dissipation within communications systems.
• the invention also relates to a startup protocol for use in commumcations system.
• the invention relates to a communications system having a prespecified threshold error.
• the communications system includes a communication line having a plurality of twisted wire pairs, a plurality of transmitters, one transmitter at each end of each twisted wire pair and a plurality of receivers, one receiver at each end of each twisted wire pair, each receiver receiving a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of noise signals.
• the system also includes a plurality of adaptive filters responsive to the combination signal, each adaptive filter having a plurality of taps each having a coefficient, each tap switchable between an active and an inactive state.
• the system further includes a control device for periodically adjusting the transfer function of at least one of the adaptive filters by selectively deactivating the taps while ensuring that the error of the communications system does not exceed the threshold error.
• the communications system further includes a plurality of noise reduction systems each comprising at least one of the adaptive filters.
• One noise reduction system is associated with each receiver and provides at least one replica noise impairment signal.
• the system also includes a plurality of devices, one associated with each receiver. Each device is responsive to the combination signal received by such receiver and the replica noise impairment signal provided by the noise reduction system associated with such receiver for substantially removing at least one of the noise signals from the combination signal.
• the noise signals include a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the transmitters at the opposite end of the communications line, except for the transmitter at the opposite end of the twisted wire pair with which the receiver is associated, and the noise reduction system includes a FEXT cancellation system for providing, as one of the replica noise impairment signals, a replica FEXT impairment signal.
• the noise signals include a plurality of near-end crosstalk (NEXT) impairment signals, one from each of the transmitters at the same end of the communications line, except for the transmitter at the
• the noise reduction system includes a NEXT cancellation system for providing, as one of the replica noise impairment signals, a replica NEXT impairment signal.
• the noise signals include an echo impairment signal received from the transmitter at the same end of the twisted wire pair with which the receiver is associated, and the noise reduction system includes an echo canceller for providing, as one of the replica noise impairment signals, a replica echo impairment signal.
• the control device includes means for setting the state of each tap, means for calculating a present error for the system and means for comparing the present error to the threshold error.
• the means for setting the state of each tap includes means for specifying a tap threshold for each tap, means for comparing for each tap the absolute value of the tap coefficient with the tap threshold, and means for deactivating those taps having a coefficient with an absolute value less than the tap threshold.
• the invention is a method of operating a communications system having a communication line having a plurality of twisted wire pairs and a plurality of transceivers, one at each end of each of the twisted wire pairs.
• Each transceiver has a receiver and a transmitter. Each receiver receives a combination signal that includes a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of noise signals.
• Each transceiver further includes a plurality of adaptive filters responsive to the combination signal.
• Each adaptive filter has a plurality of taps each having a coefficient. Each tap is switchable between an active and an inactive state.
• the method includes the steps of specifying a threshold error for the system and periodically adjusting the transfer function of at least one of the adaptive filters by selectively deactivating the taps while ensuring that the error of the system does not exceed the threshold error.
• the method further includes, for each receiver, the steps of generating at least one replica noise impairment signal and combining the at least one replica noise impairment signal with the combination signal to produce an output signal substantially devoid of at least one of the noise signals.
• the step of adjusting the transfer function includes the steps of setting the state of each tap, calculating a present error for the system and comparing the present error to the threshold error.
• one transceiver of the communications system acts as a master and another transceiver acts as slave. Each transceiver has a noise reduction system, a timing recovery system and at least one equalizer.
• the method further includes the steps of executing a first stage during which the timing recovery system and the equalizer of the slave are trained and the noise reduction system of the master is trained, executing a second stage during which the timing recovery system and the equalizer of the master are trained and the noise reduction system of the slave is trained and executing a third stage during which the noise reduction system of the master is retrained.
• one transceiver of the communications system acts as a master and another transceiver acts as slave.
• Each transceiver having a noise reduction system, a timing recovery system and at least
• the method further includes the steps of executing a first stage during which the timing recovery system and the equalizer of the slave are trained and the noise reduction system of the master is trained, executing a second stage during which the timing recovery system of the master is trained in both frequency and phase, the equalizer of the master is trained and the noise reduction system of the slave is trained and executing a third stage during which the noise reduction system of the master is retrained, the timing recovery system of the master is retrained in phase and the timing recovery system of the slave is retrained in both frequency and phase.
• the invention is a communications system including a communication line having a plurality of twisted wire pairs, a plurality of transmitters, one transmitter at each end of each twisted wire pair and a plurality of receivers .
• One receiver is at each end of each twisted wire pair.
• Each receiver receives a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the remaining transmitters at the opposite end of the communications line.
• the system also includes a plurality of FEXT cancellation systems, one associated with each receiver. Each FEXT cancellation system provides a replica FEXT impairment signal.
• the system further includes a plurality of delay devices, one associated with each receiver. Each delay device is responsive to the combination signal received by such receiver for delaying the combination signal. Also includes are a plurality of first devices, one associated with each receiver. Each first device is responsive to the output of the delay device associated with such receiver and the replica FEXT impairment signal provided by the FEXT cancellation system associated with such receiver for substantially removing the FEXT impairment signals from the combination signal.
• the FEXT cancellation system includes means for receiving a signal from each of the receivers at the same end of the communications line except for the receiver with which the FEXT canceller is associated.
• the FEXT cancellation system also includes means for generating an individual replica FEXT impairment signal for each received signal and means for combining the individual replica FEXT impairment signals to generate the replica FEXT impairment signal.
• the delay device delays the combination signal by an amount substantially equal to the time delay between the arrival, at the receiver, of the FEXT impairment signals and the direct signal.
• Each receiver receives a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the remaining transmitters at the opposite end of the communications line.
• the method includes, for each receiver, the steps of generating a replica FEXT impairment signal and combining the replica FEXT impairment signal with the combination signal to produce an output signal substantially devoid of FEXT impairment signals.
• the invention involves a method for reducing power dissipation within a communications system having at least one adaptive filter with a plurality of taps , each tap switchable between an active and an inactive state, each tap having a coefficient.
• the method includes the steps of a) computing an initial system error; b) for each active tap, setting a tap error threshold; c) for each active tap, deactivating those taps having a coefficient with an absolute value less than the tap error threshold set for the active tap; d) computing a subsequent system error; e) if the difference between the subsequent system error and the initial system error is less than a prespecified value, increasing the tap error threshold for each active tap; and f) repeating steps c) through e) until the difference between the subsequent system error and the initial system error exceeds the prespecified value.
• the invention is a startup protocol for use in a communications system having a master transceiver at one end of a twisted wire pair and a slave transceiver at the opposite end of the twisted wire pair.
• Each transceiver has a near-end noise reduction system
• the protocol includes the step of, during a first phase, maintaining the master in a half-duplex mode during which it transmits a signal but does not receive any signals, maintaining the slave in a half-duplex mode during which it receives the signal from the master but does not transmit any signals, converging the master near-end noise reduction system, adjusting the frequency and phase of the signal received by the slave such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the master, and converging the equalizer of the slave.
• the protocol further includes the step of, during a second phase, maintaining the slave in a half-duplex mode during which it transmits a signal using a free-running clock but does not receive any signals, maintaining the master in a half-duplex mode during which it receives the signal from the slave but does not transmit any signals, converging the slave near- end noise reduction system, adjusting the frequency and phase of the signal received by the master such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the slave, and converging the equalizer of the master.
• FIGURE 1 is a schematic block diagram of a communications system providing a plurality of computers connected to a hub by communications lines to form a local area network (LAN);
• LAN local area network
• FIG. 2 is a schematic block diagram of a communications system providing a gigabit medium independent interface (GMII), a physical coding sublayer (PCS) and a plurality of unshielded twisted pairs of wires, each with a transceiver at each end;
• GMII gigabit medium independent interface
• PCS physical coding sublayer
• FIG. 4 is a schematic block diagram of a portion of the communications system of FIG. 2 depicting the echo impairment signal received by receiver A from transmitter A;
• FIG. 5 is a schematic block diagram of a portion of the communications system of FIG. 2 depicting the FEXT impairment signals received by receiver A from opposite transmitters F, G, and H;
• FIG. 6 is a schematic block diagram of a communications system including a plurality of transceivers, each having a NEXT cancellation system, an echo canceller, a feed forward equalizer, digital adaptive filter system including one detector, and a timing recovery circuit;
• FIG. 7 depicts an impulse response for an echo signal passing through a 100m communications line;
• FIG. 13 is a schematic block diagram of the FEXT cancellation systems of FIG. 9, each including a plurality of ATFs and an adder and receiving as input transmitted signals from opposite transmitters;
• FIG. 14 depicts a direct impulse response arriving at the receiver after a FEXT impulse response
• FIG. 15 depicts a direct impulse response and FEXT impulse response arriving at the receiver at substantially the same time
• FIG. 16 depicts a direct impulse response arriving at the receiver before a FEXT impulse response
• FIG. 17 is a schematic block diagram of a communications system in including a plurality of transceivers, each having a NEXT cancellation system, an echo canceller, and a FEXT cancellation system, digital adaptive filter system including one detector, and a timing recovery circuit;
• FIG.20 depicts the mean squared error (MSE) to signal ratio as a function of time during the initial convergence of a communications system
• FIG. 21 depicts the taps of an echo canceller which remain active after convergence of a communications system having an error threshold of 24 dB
• FIG. 22 depicts the taps of an echo canceller which remain active after convergence of a communications system having an error threshold of 26 dB;
• FIG. 23 is a flow chart illustrating another embodiment of a power dissipation reduction method
• FIG.24 is a schematic block diagram depicting the master-slave relationship between the transceivers of each of the transceiver channels of FIG. 2;
• FIG.25 is a timing diagram depicting the stages of one embodiment of a startup protocol.
• FIG. 26 is a timing diagram depicting the stages of another embodiment of a startup protocol.
• Ethernet for the purposes of explanation and understanding of the invention. However, it will be understood that the concepts of this invention and the scope of the claims apply to other types of communications systems than a Gigabit Ethernet.
• a communications system incorporating the features of this invention is generally indicated at 10 in FIG. 1.
• the system 10 includes a hub 12 and a plurality of computers serviced by the hub in a local area network (LAN).
• LAN local area network
• computers 14 are shown by way of illustration but a different number of computers may be used without departing from the scope of the invention.
• Each of the computers 14 may be displaced from the hub 12 by a distance as great as approximately one hundred meters (100 m.).
• the computers 14 are also displaced from each other.
• the hub 12 is connected to each of the computers 14 by a communications line 16.
• the communication line 16 comprises a plurality of unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in the system 10 between each computer and the hub 12.
• the system shown in FIG. 1 is operative with several categories of twisted pairs of cables designated as categories 3, 4, 5, 6 and 7 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 6 and 7 cables are the best quality (and highest cost). Gigabit Ethernet uses category 5 cables.
• the communications system includes a standard connector designated as a gigabit media independent interface (GMII) 28.
• GMH 28 may be an eight bit wide data path in both the transmit and receive directions. Clocked at a suitable frequency, such as 125 MHz, the GMII results in a net throughput in both directions of data at a suitable rate such as 250 Mb/second per
• the GMII provides a symmetrical interface in both the transmit and receive directions.
• a physical coding sublayer (PCS) 30 receives and transmits data between the GMII 28 and the transceivers 20.
• the PCS 30 performs such functions as scrambling and encoding/decoding data before forwarding the data to either the transceiver or the GMII.
• the PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five symbol levels are -2, -1,
• the PCS also controls several functions of the transceivers, such as skew control as explained below.
• Transceiver Circuitry Four of the transceivers 20 are illustrated in detail in FIG. 9. The components of the transceivers 20 are shown as overlapping blocks, with each layer corresponding to one of the transceivers.
• the GMII 28, PCS 30, and hybrid 26 of FIG. 9 correspond to the GMII, PCS, and hybrid of FIG. 2 and are considered to be separate from the transceiver.
• the combination of the transceiver 20 and hybrid 26 forms one "channel" of the communications system. Accordingly, FIG. 9 illustrates four channels, each of which operate in a similar manner.
• each transceiver 20 includes a pulse shaping filter 32 and a digital to analog (D/A) converter 34.
• the D/A converter 34 operates at 125 MHz.
• the pulse shaping filter 32 receives one one-dimensional (1- D) symbol from the PCS. This symbol is referred to as a TXDatax symbol 36, where x is 1 through 4 corresponding to each of the four channels.
• the TXDatax symbol 36 represents 2 bits of data.
• the PCS generates one 1-D symbol for each of the channels.
• the symbol for each channel goes through a spectrum shaping filter of the form 0.75 + 0.25z " ' at the pulse shaping filter 32 to limit emissions within FCC requirements.
• This simple filter shapes the spectrum at the output of the transmitter so that its power spectral density falls under that of communications systems operating at 100 Mb/second on two pairs of category -5 twisted pair wires.
• the symbol is then converted into an analog signal by the D/A converter 34 which also acts as a lowpass filter.
• the analog signal gains access to the unshielded twisted pair wire 18 through the hybrid circuitry 26.
• the receiver portion of each transceiver includes a signal detector 41, an A/D converter 42, a FIFO 44, a digital adaptive equalizer system, a timing recovery circuit and noise reduction circuitry.
• the digital adaptive equalizer system includes a feed-forward equalizer (FFE) 46, two devices 50, 56, a skew adjuster 54 and two detectors 58, 60. The functions of these components, as related to the present invention, are explained below.
• the noise reduction circuitry includes a NEXT cancellation system 38, an echo canceller 40 , and a FEXT cancellation system 70.
• the A/D converter 42 provides digital conversions of the signals received from the hybrid
• the A D converter 42 samples the analog signals in accordance with an analog sample clock signal 78 provided by the decision-directed timing recovery circuit 64.
• the FIFO 44 receives the
• the FIFO 44 forwards individual signals to the FFE 46 in accordance with a digital sample clock signal 80 provided by the timing recovery circuit 64.
• the FFE 46 receives digital signals from the FIFO 44 and filters these signals.
• the FFE 46 is a least mean squares (LMS) type adaptive filter which performs channel equalization and precursor inter symbol interference (ISI) cancellation to correct for distortions in the signal.
• LMS least mean squares
• the signal introduced into the A D converter 42 and subsequently into the FIFO 44 and FFE 46 has several components. These components include the direct signal received directly from the transmitter 22 at the opposite end of the unshielded twisted pair wire 18 with which the receiver 24 is associated. Also included are one or more of the NEXT, echo, and FEXT impairment signals from other transmitters 22 as previously described. The signal including the direct signal and one or more of the impairment signals is referred to as a "combination signal.”
• the skew adjuster 54 performs two functions. First, it compensates for the difference in length of the unshielded twisted pairs 18 by delaying the first soft decision 52 so that the second soft decisions 66 from all of the receivers in the system are in sync. Second, it adjusts the delay of the first soft decision 52 so that the second soft decision 66 arrives at the first device 56 at substantially the same time as the output of the FEXT cancellation system 70.
• the skew adjuster 54 receives skew control signals 82 from the PCS 30.
• the skew adjuster 54 forwards the second soft decision 66 to a first device 56, typically a summing device. At the first device 56 the second soft decision 66 is combined with the output of the FEXT cancellation system 70 to produce a signal which is substantially devoid of FEXT impairment signals. This signal is referred to as a "third soft decision" 68.
• the first detector 58 receives the third soft decision 68 from the first device 56.
• the first detector 58 provides an output signal, i. e., a "final decision" 72.
• the first detector 58 may be a sheer which produces a final decision 72 corresponding to the analog signal level closest in magnitude to the level of the third soft decision 68.
• the first detector 58 may also be either a symbol-by-symbol detector or a sequential detector which operates on sequences of signals across all four channels simultaneously, such as a Viterbi decoder.
• the first detector 58 is a symbol-by-symbol detector.
• Each first detector 58 includes a sheer 98, adaptive feedback filter 100 and an adder 102.
• the adder 102 combines the third soft decision 68 with the output of the adaptive feedback filter 100 to provide an output which is introduced to the sheer 98.
• the output of the sheer 98 in introduced to the adaptive feedback filter 100.
• the first detector 58 provides an output signal 72 which corresponds to the discrete level from the set [-2, -1, 0, 1, 2] which is closest to the difference between the third soft decision 68 and the output of the feedback filter 100.
• the adaptive feedback filter 100 corrects for distortion in the third soft decision 68.
• This filter 100 uses past slicer 98 decisions to estimate postcursor ISI caused by the channel. This ISI is canceled from the third soft decision 68 to form the final decision signal 72.
• the first detector 58 is a combination of a sequential decoder with a decision feedback equalizer (DFE) using the architecture usually known as multiple DFE architecture (MDFE) sequential detector.
• the sequential decoder 58 looks at all signals from all four channels at the same time and at successive samples from each channel over several periods of unit time.
• a sequential decoder receives as input at least one signal from each of the first devices 56.
• the sequential decoder 58 in general, is responsive to the sequences of the output signals from the first devices 56 for (1) passing acceptable sequences of such signals and (2) discarding unacceptable sequences of such signals in accordance with the constraints established by the code standard associated with the system. Acceptable sequences are those which obey the code constraints and unacceptable sequences are those which violate the code constraints.
• the second detector 60 receives the first soft decision 52 from the second device 50.
• the second detector 60 is a symbol-by-symbol detector similar to the first detector 58 (FIG. 10). It provides an output signal 74 which corresponds to the discrete level from the set [-2, -1,
• the second detector 60 produces output signals 74 without the benefit of FEXT cancellation, as a result, these decisions have a higher error rate than those made by the first detector 58, which enjoys the benefits of FEXT cancellation. Because of this fact, these decisions are called "tentative decisions". It is important to note that the postcursor ISI present in the input to the second detector 60 is canceled using the adaptive feedback filter 100, (FIG. 10) contained within the second detector, whose inputs are the tentative decisions 74. The coefficients of this adaptive feedback filter 100 are the same as those of the adaptive feedback filter associated with the first detector 58 (FIG. 9).
• a third device 62 receives the first soft decision signal 52 from the second device 50 and the tentative decision signals 74 from the second detector 60. At the third device 62 the first soft decision 52 is combined with the tentative decision signal 74 to produce an error signal 76 which is introduced into the timing recovery circuit 64.
• the -15- recovery circuit 64 receives the tentative decision 74 from the second detector 60 and the error signals 76 from the third device 62. Using these signals as inputs the timing recovery circuit 64 outputs an analog clock sync signal 78 which is introduced to the A/D converter 42 and a digital clock sync signal 80 which is introduced into the FIFO 44. As previously mentioned, these signals control the rate at which the A/D converter 42 samples the analog input it receives from the hybrid 26 and the rate at which the FIFO forwards digital signals to the FFE 46.
• NEXT cancellation is accomplished using three adaptive NEXT cancelling filters 84 as shown in the block diagram of FIG. 11.
• Each NEXT cancellation system 38 receives three TXDatax symbols 36 from each of the transmitters at the same end of the communications line 18 as the receiver with which the NEXT cancellation system is associated.
• Each NEXT cancellation system 38 includes three filters 84, one for each of the TXDatax symbols 36.
• filters 84 model the impulse responses of the NEXT noise from the transmitters and may be implemented as adaptive transversal filters (ATF) employing, for example, the LMS algorithm.
• the filters 84 produce a replica of the NEXT impairment signal for each TXDatax symbol 36.
• a summing device 86 combines the three individual replica NEXT impairment signals 92 to produce a replica of the NEXT impairment signal contained within the combination signal received by the receiver with which the NEXT cancellation system 38 is associated.
• the replica NEXT impairment signal 88 is introduced into the second device 50 (FIG. 9) where it is combined with the combination signal 48 to produce a first soft decision signal 52 which is substantially devoid of NEXT impairment signals.
• Echo cancellation is accomplished with an adaptive echo cancelling filter 85 as shown in the block diagram of FIG. 12.
• Each echo canceller 40 receives the TXDatax symbols 36 from the transmitter at the same end of the twisted wire pair 18 as that of the receiver with which the echo canceller is associated.
• each echo canceller 40 includes one filter 85.
• These filters 85 model the impulse responses of the echo noise from the transmitter and may be implemented as ATFs employing, for example, the LMS algorithm.
• the filter produces a replica of the echo impairment signal contained within the combination signal received by the receiver with which the echo canceller 40 is associated.
• the replica echo impairment signal 90 is introduced into the second device 50 (FIG. 9) where it is combined with the combination signal 48 to produce a first soft decision signal 52 which is substantially devoid of echo impairment signals.
• Each FEXT cancellation system 70 includes three filters 87, one for each of the tentative decision symbols 74. These filters 87 model the impulse responses of the FEXT noise from transmitters and may be implemented as ATFs employing, for example, the LMS algorithm.
• the filters 87 produce a replica of the FEXT impairment signal 96 for each individual tentative decision symbol 74.
• a summing device 108 combines the three individual replica FEXT impairment signals 96 to produce a replica of the FEXT impairment signal contained within the combination signal 48 received by the receiver with which the FEXT cancellation system is associated.
• the replica FEXT impairment signal 94 is introduced into the first device 56 (FIG.
• the third soft decision signal 68 which is substantially devoid of FEXT impairment signals.
• the higher error rate of the tentative decisions 74 does not degrade the performance of the FEXT cancellation system 70, because the decisions used to cancel FEXT are statistically independent from the final decisions 72 made by the receiver whose FEXT is being canceled.
• the symbols provided by the first detector 58 are decoded and descrambled by the receive section of the PCS 30 before being introduced to the GMH. Variations in the way the wire pairs are twisted may cause delays through the four channels by up to 50 nanoseconds. As a result, the symbols across the four channels may be out of sync.
• the PCS also determines the relative skew of the four streams of 1-D symbols and adjusts the symbol delay, through the skew adjuster 54, prior to their arrival at the first detector 58 so that sequential decoder can operate on properly composed four-dimensional (4-D) symbols. Additionally, since the cabling plant may introduce wire swaps within a pair and pair swaps among the four unshielded twisted pairs, the PCS 30 also determines and corrects for these conditions.
• the FEXT noise experienced by receiver A and originating from transmitter F can be modeled as the convolution of the data symbols transmitted by F with a certain impulse response that depends on the properties of the cable and models the coupling characteristics of the unshielded twisted pairs used by transmitter F and receiver A.
• a typical measured FEXT impulse response 104 is show in FIGS 14-16. A similar description can be given for all the other possible receiver-transmitter combinations. Therefore, there are a total of twelve FEXT impulse
• FEXT is an impairment for many communications systems other than Gigabit Ethernet
• a given receiver usually does not have access to the symbols detected by the other receivers, because these receivers may not be physically located in the same place, and/or because they operate at rates that are not synchronized to the data rate of the receiver suffering from FEXT.
• Aspects of the present invention takes advantage of the fact that in Gigabit Ethernet transceivers the decisions that correspond to all four channels are available to the four receivers and the decisions may be made synchronous.
• the replica noise impairment signals arrive at the summing devices at substantially the same time as the combination signal and/or soft decision signals.
• the FEXT impairment signal because the impairment is caused by the transmitters at the opposite end of the receiver there is likely to be a delay between the time that the second soft decision signal 66 arrives at the first device 56 and the time at which the replica FEXT impairment signal 94 arrives.
• the group delay of the FEXT signal 104 could be smaller than the group delay of the desired signal 106.
• the tentative decisions 74 provided by receivers B, C, and D of FIG. 5 arrive at the FEXT cancellation system 70 of receiver A too late to be effective in canceling the FEXT impairment.
• the invention employs a skew adjuster 54 which, as previously stated, delays the first soft decision signal 52 by a time substantially equal to or greater than the time delay between the arrival at the receiver of the direct signal and the FEXT impairment signals associated with such receiver. If the output is delayed by an amount greater than the time delay, which would result in the situation illustrated in FIG. 16, the adaptive feedback filter 87 (FIG. 13) within the FEXT cancellation system 70 compensates for the over delay by delaying the replica FEXT impairment signal 94 so that it arrives at the first device 56 (FIG. 9) at substantially the same time as the second soft decision signal 66.
• both the FEXT impairment signal 104 and the direct signal 106 arrive at the receiver at the
• the summing device 112 receives the replica NEXT, echo, and FEXT impairment signals 88, 90, 94 and the combination signal 48 and produces a first soft decision 52 substantially devoid of impairment signals.
• This first soft decision 52 is introduced into the detector 110 and the third device 62.
• the detector 110 may include either a single symbol-by-symbol detector or both a symbol-by-symbol detector and a sequential detector. In the case of a symbol-by-symbol detector the final decision 72 and second output 114 of the detector 110 are identical.
• the final output is provided by the sequential detector and is introduced to the PCS 30.
• the second output 114 is provided by the symbol-by-symbol detector and is introduced to the timing recovery circuitry 64 and the third device 62 for use in determining the error signal 76.
• the NEXT cancellation system, echo canceller and FEXT cancellation system use ATFs to effectively cancel the noise from the combination signal.
• An example of an ATF which may be employed is shown in FIG. 18.
• the ATF 120 includes a plurality of taps 122 each including a multiplier 124 and an adder 126. Associated with each tap 122 is a coefficient C n , where n is 0 though x-1 where x is the number of taps in the ATF.
• the circuitry associated with each tap 122 includes a 1-bit storage (not shown) that allows for activation and deactivation of the tap.
• the values of the coefficients C n are adjusted in accordance with an LMS algorithm as mentioned before.
• registers 128 Interposed between the taps 122 are registers 128. These registers 128 provide data to the taps 122 at timed intervals in accordance with a clock signal.
• the NEXT , echo, and FEXT cancellers 38, 40, 70 are configured with ATFs 120 which employ a sufficient number of taps 122 to provide adequate cancellation with the worst-case expected impulse responses. This may require 140 taps 122 as in the example of FIG. 7, or even more for longer cables.
• the taps 122 are examined after convergence, and those taps that are found not to contribute significantly to the performance of the system are deactivated. When the tap 122 is deactivated, it is removed from the NEXT, echo and FEXT replica computation and from the adaptation and its contribution to the overall power dissipation of the system is substantially eliminated.
• all taps 122 are active, so the NEXT, echo and
• FEXT cancellers 38, 40, 70 are converged along their entire length. After convergence, the taps 122 are examined to determine which ones can be deactivated using the tap scanning algorithm depicted in FIG. 19. At step S 1 , an acceptable level of error for the system S ea is specified. At step S2, a tap coefficient threshold T th is set for each active tap. While each individual tap may have a unique tap threshold T th , in a preferred embodiment of the invention the tap thresholds for all taps are substantially the same. The initial value of the tap coefficient threshold T ⁇ is sufficiently low such that only a few taps 122 are deactivated and the performance of the system is not significantly affected.
• the tap coefficient threshold T th is initially set equal to the tap coefficient C n having the minimum absolute value.
• a reasonable value can be determined by simulation. This initial value is not critical, as long as it is sufficiently low to avoid a large degradation of the performance of the system the first time the tap scanning procedure is applied.
• the absolute value of the tap coefficient C n for each active tap is compared to the tap coefficient threshold T ⁇ . If the tap coefficient C n is less than the tap coefficient threshold T ⁇ the tap 122 is deactivated at step S4. This process is repeated for each tap 122 in the filter 120. Preferably, the determination of whether to deactivate a tap 122 is done in a sequential manner starting at the input end of the filter 120.
• the error for the system S ec is computed. This error is computed by first computing the MSE for each active tap 122 by multiplying the absolute value of the tap coefficient C n by the average energy signal. The error of the filter 120 associated with the tap 122 is determined by summing the individual tap errors. The error of the system is then determined by summing the individual filter errors.
• a determination may be made based on the MSE of an individual filter.
• an acceptable level of error for a filter F ea is specified.
• Individual taps are deactivated, as previously described, and a computed filter error F ec is calculated. If the deactivation of a tap does not cause the computed filter error F ec to exceed the acceptable filter error F ea the tap remains inactive.
• the contribution of each deactivated tap to the MSE of the filter and, in turn, the system is calculated. If the MSE contribution of the tap is determined to be an acceptable amount the tap remains deactivated.
• the final result of the tap scanning algorithm is that, in typical channels of the communication system, a large number of taps 122 is deactivated, and power dissipation is reduced by a large factor.
• FIG. 20 computer simulations of the tap scanning algorithm when operating on the channel whose echo response is shown in FIG. 7 are presented in FIG. 20.
• This figure shows both the master and slave MSE to signal ratios as a function of time during the initial convergence of the system.
• the tap scanning algorithm begins, and as a result the MSE to signal ratio increases to the prespecified target of 24dB.
• FIG. 21 shows the taps of the echo canceller after convergence with a threshold of 24 dB
• FIG. 22 shows the taps with a threshold of 26 dB.
• the deactivated taps are shown as zeros.
• the total number of active taps for the echo canceller is twenty-two.
• the number of active taps 122 for the three NEXT cancellers (not shown) forming the NEXT cancellation system is six, two, and zero, respectively.
• the total number of active taps for the echo canceller is forty-seven.
• the number of active taps 122 for the three NEXT cancellers (not shown) is six, two, and zero, respectively.
• step SI 3 the absolute value of the tap coefficient C n , for each active tap is compared to the tap coefficient threshold T th . If the tap coefficient C n is less than the tap coefficient threshold T th the tap 122 is deactivated at step SI 4. This process is repeated for each tap 122 in the filter 120. Preferably, the determination of whether to deactivate a tap 122 is done in a sequential manner starting at the input end of the filter 120. At step S15, the subsequent error for the system S es is computed.
• the impulse responses of NEXT, echo and FEXT may change during normal operation, for example as a result of temperature changes. It is therefore desirable to periodically activate previously deactivated taps 122, preferably in a sequential manner, and recheck if the absolute value of the tap coefficient C n is below the tap threshold T ⁇ . If a tap coefficient C n has grown to a value above the tap threshold T th , the tap 122 remains active, otherwise it is deactivated. Similarly, those taps 122 that were active may fall below the tap threshold T ⁇ , in which case they are deactivated. All this can be accomplished with a periodic reapplication of the sequential tap scanning algorithm during normal operation.
• the power dissipation could be large during the initial convergence transient before the tap scanning algorithm has had a chance to deactivate the taps. Although the average power dissipation of the system is still greatly reduced, the peak power is not.
• a preferred embodiment of the invention compensates for this by converging the NEXT, echo and FEXT cancellers in stages. For example, a block of 20 taps is converged at a time, and the tap scanning algorithm is then applied to these taps on a per-block basis.
• the MSE during the initial convergence is large, for example as a result of the fact that the initial block of 20 taps may not be large enough to provide a lower MSE, it may be better to monitor the sum of the squared values of the coefficients of deactivated taps 122 as a measure of whether the algorithm can be terminated.
• Start-Up Protocols One of the most critical phases of the operation of the communications system is the transceiver startup.
• the adaptive filters contained within the FFE 46 (FIG. 9), echo canceller 40, NEXT cancellation system 38, FEXT cancellation system 70, timing recovery system 64 and detector 58 of the receiver portion of each transceiver converge.
• the actual output of the adaptive filters are compared to expected output of the filters to determine the error.
• the error is reduced to substantially zero by adjusting the coefficients of the algorithm which defines the transfer function of the filter.
• the timing recovery system is converged by adjusting the frequency and phase of the phase lock loop and the local oscillator contained within the timing recovery system so that the signal-to-noise ratio of the channel is optimized.
• the differences in delay among the four wire pairs are compensated, and pair identity and polarity, are acquired. Successful completion of the startup ensures that the transceiver can begin normal operation.
• the slave 132 synchronizes both the frequency and phase of its receive and transmit clocks to the signal received from the master 130, using the timing recovery system 64 (FIG. 9) located in the receiver 24.
• the slave 132 transmit clock maintains a fixed phase relationship with the slave receive clock at all times.
• the receive clock at the master 130 synchronizes, in phase but not in frequency, with the signal received from the slave transmitter 22.
• the master 130 receive clock follows the master transmit clock with a phase difference determined by the round trip delay of the loop. This phase relationship may vary dynamically as a result of the need of the master 130 receive clock to track jitter present in the signal received from the slave 132.
• the protocol consists of three phases 134, 136, 138 during which the receivers are trained, e. g., adaptive filters are converged, timing synchronization is acquired, etc., followed by normal operation which begins during phase four 140.
• the master begins transmitting to the slave using a transmit clock signal that is fixed in both frequency and phase.
• the master trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E).
• the slave trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D).
• DFE DFE
• the transition from the first phase 134 to the second phase 136 occurs after a fixed and prespecified period of time.
• the slave transitions from the first phase 134 to second phase 136 when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (T).
• the master receiver includes a signal detector 41 (FIG. 9) which detects energy in the line coming from the slave.
• the master transitions from the first phase 134 to the second phase 136 when it detects this energy from the slave. Therefore, the slave takes the initiative in transitioning from the first phase 134 to the second phase 136, and the master follows when it detects the signal from the slave.
• the convergence of the echo canceller and NEXT cancellation system during the first phase 134 at the master is done with the objective of allowing the signal detector at the master to detect the signal from the slave. Without proper echo and NEXT cancellation, the signal detector would be triggered by the echo and NEXT noise present in the receiver.
• the master discards the echo canceller and NEXT cancellation system coefficients which result from the converging in the first phase 134. This may be done by resetting the adaptive filters in the echo canceller and NEXT cancellation system.
• the echo canceller and NEXT cancellation system coefficients obtained during the first phase 134 may differ from the final values to be reacquired in the third phase 138.
• the slave trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E).
• the master trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D).
• the master simultaneously acquires timing synchronization in phase only (P). The master may also at this time compensate for the differential delay among the four twisted wire pairs, identify the four pairs, and correct the polarity of the pairs.
• the slave saves the timing recovery state variables that had been acquired during the first phase 134, and freezes its frequency and phase.
• the slave is guaranteed to sample with the correct phase, the signal coming to it from the master when the master resumes transmission at the beginning of the third phase 138.
• the slave also freezes the coefficients of the DFE, FFE and FEXT cancellation system acquired during the first phase 134.
• the transition from the second phase 136 to the third phase 138 may occur after a fixed and prespecified period of time. While the duration of the first, second, and third phases 134, 136, 138 is fixed, the duration is not necessarily equal for all phases. In a preferred embodiment, however, the master transitions from the second phase 136 to third phase 138 when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (P). Like the master, the slave receiver includes a signal detector 41 (FIG. 9) which detects energy in the line coming from the master. The slave transitions from the second phase 136 to the third phase 138 when it detects this energy from the master. Therefore the master takes the initiative in transitioning from
• the slave follows when it detects the signal from the master.
• the slave freezes the coefficients of the echo cancellers and NEXT cancellation system and maintains a steady state condition during which the operating characteristics of the slave are not adjusted.
• the master freezes the coefficients of the DFE, FFE and FEXT cancellation system and the phase of its clock signal.
• the master also retrains its near-end noise reduction system by reconverging its echo canceller and NEXT cancellation system (E) during the third phase 138. It is important to note that in the third phase 138 the slave resumes transmission using the clock recovered from the signal transmitted by the master, and therefore the master already knows the correct frequency with which to operate its receiver.
• phase four 140 all coefficients of the adaptive filters previously frozen are unfrozen and the transmission of data is ready to take place.
• the protocol consists of three phases 144, 146, 148 during which the receivers are trained, e. g., adaptive filters are converged, timing synchronization is acquired, etc., followed by normal operation which begins during phase four 150.
• the startup protocol is initiated by the master when it starts transmitting a signal to the slave using a transmit clock signal which is fixed in both frequency and phase.
• the master trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E).
• the slave receiver includes a signal detector 41 (FIG. 9) which detects energy in the line coming from the master.
• the slave Upon detection of the signal from the master, the slave trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D). While training its equalizers and far-end noise reduction system the slave simultaneously acquires timing synchronization in both frequency and phase (T). It may also at this time compensate for the differential
• the transition from the first phase 144 to the second phase 146 occurs after a fixed and prespecified period of time.
• the slave transitions from the first phase 144 to second phase 146 when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (T).
• the master receiver also includes a signal detector 41 (FIG. 9) which detects energy in the line coming from the slave.
• the master transitions from the first phase 144 to the second phase 146 when it detects this energy from the slave. Therefore, the slave takes the initiative in transitioning from the first phase 144 to the second phase 146, and the master follows when it detects the signal from the slave.
• the convergence of the echo canceller and NEXT cancellation system during the first phase 144 at the master is done with the objective of allowing the signal detector at the master to detect the signal from the slave. Without proper echo and NEXT cancellation, the signal detector would be triggered by the echo and NEXT noise present in the receiver.
• the master discards the echo canceller and NEXT cancellation system coefficients which result from the converging in the first phase 144. This may be done by resetting the adaptive filters in the echo canceller and NEXT cancellation system.
• the echo canceller and NEXT cancellation system coefficients obtained during the first phase 144 may differ from the final values to be reacquired in the third phase 148.
• the slave trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E).
• the slave also freezes the coefficients of the DFE, FFE and FEXT cancellation system acquired during the first phase 144.
• the master trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D).
• D adaptive filters contained within its DFE, FFE and FEXT cancellation system
• the master simultaneously acquires timing synchronization in both frequency and phase (T).
• the master may also, at this time, compensate for the differential delay among the four twisted wire pairs, identify the four pairs, and correct the polarity of the pairs.
• the slave may run off of any stable clock that has a frequency offset of less than a prespecified limit, e. g., for example 200ppm, when compared to the master transmit
• phase synchronization is a normal function that the master performs in any form of startup protocol, however, frequency synchronization is not a usual master function, and it is only performed during the second phase 146 of the startup with the objective of interoperating properly with slave transceivers that do not save the timing recovery state variables at the transition from the first phase 144 to the second phase 146.
• the transition from the second phase 146 to the third phase 148 may occur after a fixed and prespecified period of time. While the duration of the first, second, and third phases 144, 146, 148 is fixed, the duration is not necessarily equal for all phases. In a preferred embodiment, however, the master transitions from the second phase 146 to third phase 148 when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (T). The master begins transmitting a signal to the slave. The slave transitions from the second phase 146 to the third phase 148 when it detects this signal from the master. Therefore the master takes the initiative in transitioning from the second phase 144 to the third phase 148, and the slave follows when it detects the signal from the master.
• D DFE, FFE and FEXT cancellation system
• T timing synchronization
• the slave freezes the coefficients of the echo canceller and the NEXT cancellation system and maintains its near-end noise reduction system in a steady state condition. Because the timing recovery state variables had not been saved at the transition to the second phase 146, the slave also reacquires timing synchronization in both frequency and phase (T) during the third phase 148.
• the master freezes the coefficients of its DFE and FEXT cancellation system and the frequency of its clock signal.
• the master also retrains its near-end noise reduction system by reconverging its echo canceller and NEXT cancellation system (E).
• the master also reacquires timing synchronization in phase only (P). It is important to note that in the third phase 148 the slave resumes transmission using the clock recovered from the signal transmitted by the master, and therefore the master already knows the correct frequency with which to operate its receiver.
• the "relative sampling phases” of the four receivers i. e., the differences in sampling phases of three of the receivers versus one of them arbitrarily used as reference, are also known, because they were acquired during the second phase 146.
• the "overall sampling phase” of the receivers i. e., the sampling phase of the receiver arbitrarily chosen as reference, is not yet known and has to be acquired
• phase 148 When both master and slave have completed their training operations, they exchange messages indicating that they are ready to transmit valid data.
• phase four 150 all coefficients of the adaptive filters previously frozen are unfrozen and the transmission of data is ready to take place.

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• 1999
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