WO2005032084A1

WO2005032084A1 - Adaptive equalisation for communication systems at transmitter with feedback from receiver

Info

Publication number: WO2005032084A1
Application number: PCT/GB2004/004048
Authority: WO
Inventors: David Srodzinski
Original assignee: Elonics Limited
Priority date: 2003-09-26
Filing date: 2004-09-22
Publication date: 2005-04-07
Also published as: GB0322599D0

Abstract

A method and apparatus for self adapting equalisation within communication systems is described. The method and apparatus employs adaptable equalisation elements located within one or more transmitters that are continuously adjusted based on decision feedback information relayed to it from one or more receivers. The equalisation elements within the transmitters Continuously receive adapting information from the receivers as part of a decision feedback loop via other transmitters normally present within a multi-way communications system. The adapting information is transmitted at a low frequency so as to be more robust and less sensitive to degradation than higher frequency signals transmitted within the system.

Description

ADAPTIVE EQUALISATION FOR COMMUNICATION SYSTEMS AT TRANSMITTER WITH FEEDBACK FROM RECEIVER

This invention relates to the field of communication systems. More particularly, this invention relates to communication systems that employ equalisation in order to overcome signal degradation mechanism within the system and in so doing can improve some performance metric of the system. State-of-the-art electronic self-adapting equalisation schemes for communications systems rely on automatically adapting some equalisation element in or near to the receiver using decision feedback equalisation (DFE) . Such a scheme is described in US Patent No. US 2003/123585, "Receiver with DFE and viterbi algorithm for block code transmission". However, in very high data-rate systems such a scheme would require a very fast and precise analog to digital convert and complex signal processing engine within the receiver in order to implement. As a result this approach quickly becomes expensive and impractical and therefore not suited for low cost applications. In addition, a major problem of this scheme is that the equalisation is done after the sensitive input stage of the electronics and, in the presence of noise, weak signals may have already been irreversibly corrupted such that equalisation would be of limited further benefit. This method requires that the data is sufficiently trellis encoded, and this places additional overhead on the traffic.

US Patent No. US 4,969,162 entitled "Polled data network auto-equalizer system and method" suggests an alternative arrangement whereby the equalisation can be done in the transmitter, based on a signal quality measuring function in an adjacent receiver in a two-way communications system. This transmitter based equalisation scheme is signal aware and, by learning about the signal degradation mechanisms through training, more cost effective equalisation can be done without requiring analog to digital conversion or complex simpler signal processing within the receiver. Also the equalisation is placed before the sensitive receiver input stage, allowing higher performance to be attained. However, this scheme has two major drawbacks. In the first instance it requires a training sequence to periodically train the link and in order to do this the normally transmitted data must be periodically interrupted. In "always on" services this is not acceptable. Secondly, this scheme adapts solely by relying on channel reciprocity. Reciprocity can be employed when the distortion in the bi-directional signals are assumed to be substantially identical for signals travelling in either direction. Reciprocity then allows the near-end receiver to adjust the near-end transmitter and likewise ■ the far-end receiver and transmitter. However in full- duplex schemes reciprocity cannot be solely relied upon, as the transmission and reception paths are not necessarily the same.

US Patent No. US 3,593,142 entitled "Digital Transmission system employing band limited analog medium with adaptive equalizer at transmitter" discloses a scheme wherein the near end receiver trains the far end transmitter and the far end receiver trains the near end transmitter, and in doing so no reciprocity is assumed or exploited. However, this requires an additional reverse link for the signalling channels to relay the adapting information, which may not always be cost-effective or practical to add into an existing system.

A second problem with the teachings of US Patent No. US 3,593,142 is that it needs a second encoder which puts a test signal into the side bands which therefore increases the data rate required on the transmit channel.

It is an object of the present invention to improve the transmission of data signals between a transmitter and a receiver.

Summary of Invention

According to the first aspect of the present invention, there is provided a method of continuously adjusting equalisation of a transmitted signal comprising the step of equalising the transmitted signal in a transmitter, wherein the equalisation is responsive to feedback provided via a duplex channel. Preferably the method further comprises iteratively improving a quality measurement of said transmitted signal.

Preferably the method comprises the steps: • start continuous transmission of a signal; • receiving said signal; • making a measurement of the quality of said received signal; • transmitting a control signal responsive to said measurement; • receiving said control signal; • equalising the transmitted signal responsive to said control signal.

Preferably the steps of making a measurement, transmitting a control signal, receiving said control signal, equalising said signal, transmitting said equalised signal and receiving said equalised signal are repeated continuously.

Optionally the method comprises the steps: • transmitting a first signal; • receiving said first signal; • making a first measurement of the quality of said received first signal; • transmitting a first control signal responsive to said first measurement; • receiving said first control signal; • equalising a second signal responsive to said first control signal; • transmitting said second signal; • receiving said second signal; • making a second measurement of the quality of said received second signal; • transmitting a second control signal responsive to said second measurement; • receiving said second control signal; and • equalising a third signal responsive to said second control signal.

Preferably said first second and third signals are data signals.

Preferably said first, second and third signals are contiguous in time.

Preferably said steps of transmitting a control signal further comprise the step of calculating and transmitting decision feedback perimeters.

Preferably the steps of transmitting a control signal further comprise the step of applying error correction to the control signal and the steps of receiving a control signal further comprise the step of decoding said error correction.

According to a second aspect of the present invention there is provided a communication apparatus comprising: one or more transmitters that provide means for transmitting one or more signals into one or more duplex channels; • one or more receivers that provides means for receiving said one or more transmitted signals, the one or more receivers comprising a measurement means for measuring the quality of said one or more received signal; • one or more means for generating feedback in response to the measured quality of the said one or more received signal; • one or more feedback duplex channel for carrying said feedback from one or more means for generating feedback to one or more transmitters; wherein said one or more transmitter comprises an adjustable equaliser means for equalising said one or more signals transmitted into said one or more duplex channels.

Preferably the feedback means further comprises a decision feedback means for calculating and transmitting decision feedback parameters.

Preferably the feedback means further comprises an error correction means for applying error correction to the control signal and the feedback receiver means further comprises a decoder means for decoding said error correction.

Aspects of the present invention describe a method and apparatus for self-adapting communications whereby equalisation within the transmitter source is continuously adapted using the reverse duplex channel of a two-way communications system.

This transmitter adaptation is based on measurement of the received signal quality made at the destination receiver and continuous decision feedback employed between, without interruption of the normally transmitted signal. To achieve this adapting information is continuously fed-back from elements within the receiver to elements within the transmitting source by multiplexing the adapting information along with the normally transmitted data on the reverse link. It does this without increasing the traffic or data rate requirements of the system.

Furthermore, because the adapting information is suitably low bandwidth, its integrity can be protected using an appropriate error coding and modulation scheme. In so multiplexing the data this invention suitably exploits the two-way nature of the communications channel to provide a continuous decision feedback path.

Detailed Description

In the following detailed description of the preferred embodiments or mode, reference is made to the accompanying drawings, which form part hereof, and in which are shown, by way of illustration, specific embodiments in which the invention may be practised. It is to be understood that other embodiments may be utilised and structural changes may be made without departing from the scope of the present invention.

FIGURE 1 shows a system block diagram of a two-way, full- duplex communications system, with separate transmit and receive paths on separate media onto which an adapting scheme is to be added;

FIGURE 2 shows the inclusion of a near-end adjustable transmitter source and far-end decision feedback element at the destination receiver and feedback from the receiver to the transmitter; and

FIGURE 3 shows an alternative embodiment of the communication system of Figure 2 with the addition of a far-end adjustable transmitter source and a near-end decision feedback at the destination receiver and feedback from the receiver to the transmitter.

The task of equalisation is to modify the physical characteristics of a transmitted signal in order to correct, accommodate or rectify some degradation mechanism that occurs. Such degradation mechanisms are normally frequency, and hence data content, dependent.

Within a communications system typical signal degradation mechanisms include the rise time, fall time, bandwidth ordi"S"tOrt'i"on" of- the- receiver- or transmitter, dispersionr reflection and bandwidth limitations within the media and any other signal impairment or interference from other signals. The term degradation mechanism will be so used extensively throughout this document for any linear or non-linear, stationary or non-stationary or other non- ideal effect anywhere in the communication system.

The resultant effects of these degradation mechanisms on the signal are dependent on the inter-relationship of the signal being transmitted and the degradation mechanism itself. Within some bounds these are repeatable effects. These will be generally referred to as deterministic effects throughout this document. In digital communications systems the signal quality metric of system performance is generally the channel Bit Error Rate (BER) and the manifestation of the degradation mechanisms is commonly referred to as Inter-Symbol- Interference (ISI) . In analogue communications systems the signal quality metric of the system performance is generally the channel signal to noise ration (SNR) and the manifestation of the degradation mechanism commonly referred to as distortion.

A typical prior-art two-way non-adapting communications system is shown in Figure 1. This is a full duplex system in which data can be simultaneously transmitted and received to and from the near end 11 to the far end 12 over separate transmission media 3,8. The near end to far end transmission is referred to as Channel (A) 13. Channel (A) 13 transmits an input signal, in (A) 1, via the transmitter 2, through the media 3, to the receiver 4 and out in the form of out (A) 5. Channel (B) 8 transmits a second input signal, in(B) 10, via the transmitter 9, through the media 8, to the receiver 7 and out in the form of out (A) 6. Of note, this particular system shows no adapting or no equalisation elements.

A single channel of the self-optimising equalisation scheme is shown in Figure 2, for the purpose of explanation of aspects of the present invention. In this case Channel (A) 13 is arranged to be optimised with the simple transmitter 2 being replaced with a transmitter 15 that includes an adjustable equalisation element. Transmitter 15, that includes an adjustable equalisation element, will be referred to as an adaptable or an adjustable transmitter throughout. Adjustable equalisation transmitters are known in the art e.g. as described in US patent No. US 6,393,062 entitled "Methods and circuits for generating a pre-emphasis waveform" . The described transmitters are employed in electronic systems with cable/circuit board media or as described in Canadian Patent No. CA 2,238,449 entitled "Equalisation, pulse shaping and regeneration of optical signals" for use in optical systems with optical fibre media.

An output from the far-end receiver 4 is monitored and assessed by an added signal quality measurement and feedback algorithm element 19, hereafter referred as the decision feedback 19. The decision feedback 19 makes decisions on the quality of the signal received. Such decision feedback elements are well known in the art, such as described in US Patent No. US 2003/081668 entitled "Fast computation of decision feedback equaliser coefficients".

Following the monitoring of the received signal by the receiver 4 a low bandwidth decision information signal drx(A)_l 18 is produced by the decision feedback 19 and relayed to the far end transmitter 9. The signal drx(A)_l 18 is then multiplexed or combined with the in(B) signal 10 in the far end transmitter path and then this combined signal is transmitted through Channel(B), represented by the form of the signal in(B) + drx(A)_2 17.

One method of multiplexing the low bandwidth feedback information is to exploit the data carrying capacity of control or reserved fields that are present in all major communications systems, as described in detail below. Alternatively, a specifically designated field could be incorporated with the standard communication system protocol. In addition, because of its low bandwidth requirement the feedback signal can be made suitably impairment and error resilient by encoding using an appropriate error correcting code (ECC) scheme.

As a first example, the IEEE Ethernet 802.3 2002 standard Clause 37 for gigabit Ethernet has provision for a low bandwidth "Management Function" within the Auto- Negotiation definition. The IEEE Ethernet standard has provision to use unformatted code fields /U<10:0>/ as in section 37.2.4.3.10. These fields could be used and suitably error protected for this purpose.

A second example exploits the Sonet (SDH) standards, as defined by the ITU-T. The SDH standard explicitly defines a transmission management function for use in a performance monitoring function. ITU-T recommendations G.783 and 1.432 offer two detailed examples of such sub- layers within the Sonet protocol.

The 1.432 recommendation states that the BIP-96 field inserted into the B2 field of 622kbps data can be examined using the far end block error (FEBE) , and the error count inserted into the Z3 field and sent back. These allow relaying of the error count from the far end to the near end section termination point. This error count may be used as the feedback path in the decision feedback equalisation system and the error count information further used to adjust the transmitter equaliser.

In practice the error count information is made available to an adaptable algorithm within the transmitter so as to complete the feedback path to complete the DFE system. However, a downside of this error field in existing Sonet protocol is that it is not very robustly error protected in order to make the integrity of the error data robust to channel impairments.

It would be preferable to transmit suitably encoded and ECC protected low bandwidth parameter or error feedback information. In an alternative embodiment of the present invention, this is achieved using the Sonet reserved octet fields "R" to transmit an encoded and ECC protected low bandwidth DFE parameter or error feedback information.

This combined signal 17 is received back at the near end 11 where the information is de-multiplexed by the receiver 7 so as to extract the decision information drx(A)_3 16. The decision feedback signal drx(A)_3 15 is then used to adjust equalisation characteristics of the near end adaptable transmitter 15.

Specific methods of multiplexing the signal drx(A)_l 18 with the in(B) signal 10 have been described. However, it will be appreciated by those skilled in the art that a number of alternative methods of multiplexing the information signals may be similarly employed. The transmitter 15 is continuously adjusted until the system performance of Channel (A) 13 is optimised based on the quality decision made in the receiver 4. Hence the system continuously self-adapts Channel (A) and optimises itself by adjusting the transmitter 15 characteristics, the feedback path being provided by using continuous multiplexing of the fed-back signal onto Channel (B) .

An alternative embodiment, as shown in Figure 3, shall now be described. Here, in addition to the near-end adjustable transmitter 15 and decision feedback 19, a far-end adjustable transmitter 24 and near end decision feedback 20 is added to Channel (B) in a similar manner to Channel (A) . An output from the near-end receiver 7 is monitored and assessed by the second decision feedback element 20. The decision feedback makes decisions on the quality of the signal received on Channel (B) . The information based on this decision outcome can then be used to adjust the far end adjustable transmitter 24.

In a similar manner to that described above a low frequency signal drx(B)_l 21 is relayed to the near end transmitter 15. The signal drx(B)_l 21 is then multiplexed with the normally transmitted signal in (A) 1 in the near end transmitter 15. The combined signal is thereafter transmitted through Channel (A) , represented by the form of the signal in (A) + drx(B)_2 22. Once this combined signal 22 is transmitted to the far end receiver 4 the information is de-multiplexed so as to extract the decision information drx(B)_3 23. The decision feedback signal drx(B)_3 23 is then used to adjust equalisation characteristics of the far end transmitter 24. The transmitter 24 is adjusted until the system performance of Channel (B) 14 is optimised based on the quality decision made in the receiver 7. Hence the described system self-adapts both Channel (A) 13 and Channel (B) 14 and optimises itself by adjusting the transmitter characteristics of each respective channel.

The performance metric of a communication system is normally the system bit error rate (BER) or signal to noise ratio (SNR) . However any other performance metrics of the system can be monitored for decision feedback purposes, such as eye height, distortion, jitter, symbol by symbol ISI degradation, deterministic jitter, distortion, interference, data-rate, bandwidth or capacity.

The above described method and apparatus is in relation to a two-way communications system with transmission from the near end to the far end and from the far end to the near end across Channel (A) 13 and Channel (B) 14. However, in alternative embodiments, more than two-way communications can be employed, such as three-way or any number of way communications.

A number of alternative embodiments of the described method and apparatus shall now be discussed. For example the described method and apparatus is particularly suited for employment with optical fibre media 3, 8 however it may also be employed with many other types of transmission media 3, 8 including, but not limited to, printed circuit board, cable and over-air. Similarly, the described- method and apparatus is particularly suited for employment with optical transmission techniques, but may also be employed with many other types of transmission technique including, but not limited to, electrical, electro-magnetic or magnetic.

Although separate media channels 3, 8 have been described it will be readily apparent to those skilled in the art that the transmissions can be across one or more shared media channels such as, but not limited to, optical wave division multiplexing schemes (DWDM, CWDM) .

The described method and apparatus is suited for employment with a range of transmission signal formats including, but not limited to, analogue, digital, serial, pulse amplitude modulated (PAM encoded) , modulated, un- modulated, return to zero coding, non return to zero coding, encoded data, non encoded data, multi-level, binary, continuous or discontinuous, framed, burst or packet based or any combination of these.

A range of transmitter equalisation schemes 15, 24 for control and correction of the signals and the correction mechanism itself in any combination may be employed including, but not limited to, electronic, mechanical, optical, magnetic, thermal, chemical or other physical techniques.

The above described method and apparatus relates to a simultaneous full-duplex communication system with simultaneous transmission and reception between the near 11 and far ends 12. However, in alternative embodiments, transmissions can be non-simultaneous as employed in half duplex communications systems.

The described apparatus is for a communication system where only one adjustable transmitter 15, 24 is used for each media channel 3, 8. However, in an alternative embodiment, transmissions are made from more than one transmitter sharing one or more media channels. Similarly instead of only one receiver 4, 7 being used for each media channel 3, 8 alternative embodiments employ more than one receiver, sharing one or more media channels, to carry out the receiving process. Furthermore, the transmitter and receiver elements can be joined or part joined within the same combined element or component of the system.

In a further alternative embodiment the transmitters 15, 24 instead of being distinct and separate elements. Are arranged as a combination of separate, not necessarily distinct elements in any combination or form. Similar alternatives can also be employed in relation to the receivers 4, 7 that are also described above as distinct and separate elements.

The described communication system employs only the destination receiver 4, 7 to relay the adapting parameter or coefficient information to their respective adjustable source transmitter 15, 24. However, the system could be easily adapted so that the receiver near or adjacent to an adaptable transmitter can provide similar, additional or all parameter or coefficient information for adapting its near or adjacent transmitter. i.e. information determined and relayed by receiver 4 maybe used to control the equalisation of adjustable transmitter 24 and likewise receiver 7 to transmitter 15. This embodiment can be used when reciprocity in the channel is to be exploited to provide some, part or all of the equalisation information and so split the equalisation decision and control making between near and far ends.

Within the described communication system only one set of receivers 7, 4 is employed to decide the adapting parameters or coefficient information to relay to the source transmitters 15, 24. However, the decision used to adapt parameters or coefficient information could alternatively be based on one or more receivers and relayed via one or more channels to one or more of the transmitters.

In a further alternative embodiment the signal quality or decision making element is placed within, adjacent to, or near to the adaptable transmitter 9, 15 instead of as described above where both the signal quality and decision making elements 19, 20 are within or close the receivers.

The described apparatus shows single transmitters 15, 24, receivers 4, 7 and media 3, 8 elements per channel 13, 14. However, in an alternative embodiment, the communications system contains additional filters, transducers, amplifiers, sensors or other elements or components between the transmitter and receiver.

As an alternative to the described continuous media 3, 8 between the transmitters 15, 24 and receivers 4, 7 the communication system can contain separate sections of media, separated by filters, transducers, sensors, transponders, transceivers, transmitters, receivers or other elements. This results in the media being separated into one or more sections of not necessarily the same type of media.

The described method shows the relay of the adapting parameter or coefficient information to the source transmitters 15, 24 via continuous multiplexing with the normally transmitted signal 17, 22. It will be appreciated by those skilled in the art that any alternative multiplexing techniques such as, but not limited to, frequency division, time division, code division or phase division multiplexing can be employed.

A further alternative method is for the communication system to periodically, or non-periodically, interrupt the normally transmitted signal in order to relay or insert the adapting parameters or coefficients without simultaneous multiplexing required. In a yet further alternative method the communications system relays the adapting parameters or coefficients by exploiting periods or other events where the normally transmitted signal is not present or not made to be not present, e.g. at additional predefined or other times such as at power on of the system or components or start up of the system or a component.

The described method shows that the normally transmitted signals 10, 1 are not modified in order to accommodate the adapting parameter or coefficient information 18, 21. However, in an alternative embodiment, the normally transmitted signal is modified in order to accommodate the adapting parameter or coefficient information.

The above described communication system shows the relay of the adapting parameter or coefficient information to the source via existing or non-dedicated channels 13, 8. However, in an alternative embodiment, the parameter or coefficient information is relayed by an additional or dedicated channel or channels.

Whereas the described feedback decision signals 18, 16, 21, 23 between receivers 4, 7 and transmitters 15, 24 are shown to be singular signals in an alternative embodiment they comprise multiple or parallel signals. Furthermore, the described feedback decision signals between receivers and transmitters 21, 33, 23 and 18, 17, 16 are essentially the same signals throughout the feedback. However, in alternative embodiments the feedback signals are altered en route, such as, but not limited to, coding, removing or compaction of the information to reduce the amount of data relayed.

The described apparatus employs an adjustable transmitter equalising filter elements within the transmitters 15, 24 to adjust the transmitter characteristics. However, in alternative embodiments, this function is achieved by employing other adjustable techniques that act to adjust the signal wave-shape, amplitude, time duration, zero crossing, edge speed, pulse width or other characteristic of the waveform which has the effect of providing equalisation. The method employs information contained within the feedback parameters to adapt the transmitters 15, 24 and hence optimise the system. However, in an alternative embodiment, other additional information is used to adapt the transmitters e.g. knowledge of the past, future and present signal stream content or any other system information made available such as temperature, time or system usage not within or not within the system drawn.

Aspects of the present invention describes a self- adaptive communications system whereby an adaptable equalisation element within a transmitter is continuously adjusted based on decision feedback information relayed to it from a receiver. The equalisation element within the transmitter continuously receives adapting information from a receiver as part of a decision feedback loop via another transmitter normally present within a two-way communications system. An essential feature is that it employs the communication of low bandwidth adapting information, that is inherently less sensitive to degradation, from the signal quality measurement in the receiver back to the sending adaptable transmitter. This is done continuously by multiplexing the adapting information using onto existing return path of the two-way or duplex communications channel. Thus a passive monitoring is achieved that requires no data rate increase in order to function.

One application particular suited to the described method and apparatus is in high data rate digital optical communications systems of 1 Giga bits per second and beyond . The described method and apparatus has several inherent advantages. In the first instance it can be employed to improve a communication system's performance by employing equalisation to counteract the signal degradation mechanisms within a channel. By enclosing a configurable equalisation element within a signal quality measuring and decision feedback loop a self-adapting equalisation scheme is realised.

A further advantage of this self-adapting scheme is that it can be employed to improve some desired metric of the communication system's performance, such as the BER. By improving the system's performance an adaptable system allows a greater volume of traffic to be delivered, over longer distances with improved system reliability thus providing a more cost effective system.

Furthermore the described continuously self-adapting schemes can additionally self-compensate for impairments to the transmitted signal that change over time, such as the effects of temperature and component ageing, and so keeps the system running at best performance with minimum external maintenance requirements.

One or more of the described embodiments exhibit the further advantages in that they do not require the normally transmitted data to be interrupted in order to train, do not to rely on channel reciprocity and do not to require an additional reverse link. Such improvements assist in making the system more cost-effective and practically increase the traffic carrying capacity of the communication system when compared to other known schemes. Three essential elements of any communications system are the transmitter, used to transmit the information, a receiver, used to receive the information, and the transmission media over which the information is conveyed. The described self-adapting equalisation systems can be used to correct or compensate for any or all signal impairment or degradation mechanism in the communications channel including those in the transmitter, media or receiver portions.

Since the equalisation scheme can be incorporated before the receiver's sensor electronics it allows for the signal to be kept above the noise floor avoiding unwanted corruption. In addition since a transmitter knows what it is trying to send and given feedback can determine what the receiver thinks it is receiving, thus a more intelligent decision feedback schemes has been produced.

An adaptable transmitter equalisation scheme is also less complex than an equivalent adapting receiver's, requiring lower performance building blocks, less power and can be more easily designed for low cost and high volume manufacturing. In so doing the described apparatus can contribute to higher performance systems and/or to less expensive or complex or power demanding or sized ones to be developed and manufactured.

Further modifications and improvements may be added without departing from the scope of the invention as defined by the appended claims.

Claims

1) A method of continuously adjusting equalisation of a transmitted signal comprising the step of equalising the transmitted signal in a transmitter, wherein the equalisation is responsive to feedback provided via a duplex channel .

2) A method as claimed in Claim 1 wherein the method further comprises iteratively improving a quality measurement of said transmitted signal.

3) A method as claimed in Claim 1 or Claim 2 wherein the method comprises the steps: a) start continuous transmission of a signal; b) receiving said signal; c) making a measurement of the quality of said received signal; d) transmitting a control signal responsive to said measurement ; e) receiving said control signal; f) equalising the transmitted signal responsive to said control signal.

4) A method as claimed in Claim 3 wherein the steps are repeated indefinitely.

5) A method as claimed in Claim 3 or Claim 4 wherein the control signal is transmitted at a low frequency.

6) A method as claimed in any of Claims 3 to 5 wherein the method further comprises the steps of: a) transmitting a first signal; b) receiving said first signal; c) making a first measurement of the quality of said received first signal; d) transmitting a first control signal responsive to said first measurement; e) receiving said first control signal; f) equalising a second signal responsive to said first control signal; g) transmitting said second signal; h) receiving said second signal; i) making a second measurement of the quality of said received second signal; j) transmitting a second control signal responsive to said second measurement; k) receiving said second control signal; and 1) equalising a third signal responsive to said second control signal.

7) A method as claimed in Claim 6 wherein said first second and third signals are data signals.

8) A method as claimed in Claim 6 or Claim 7 wherein said first, second and third signals are contiguous in time.

9) A method as claimed in any of Claims 3 to Claim 8 wherein said step of transmitting a control signal further comprises calculating and transmitting decision feedback perimeters.

10) A method as claimed in any of Claims 3 to Claim 9 wherein said step of transmitting a control, signal further comprises applying an error correction to the control signal and the steps of receiving a control signal further comprise the step of decoding said error correction.

11) A communication apparatus comprising: one or more transmitters that provides means for transmitting one or more signals into one or more duplex channels; one or more receivers that provides means for receiving said one or more transmitted signals, the one or more receivers comprising a measurement means for measuring the quality of said one or more received signal; one or more means for generating feedback in response to the measured quality of the said one or more received signal; one or more feedback duplex channel for carrying said feedback from one or more means for generating feedback to one or more transmitters; wherein said one or more transmitter comprises an adjustable equaliser means for equalising said one or more signals transmitted into said one or more duplex channels.

12) A communication apparatus as claimed in Claim 11 wherein the one or more means for generating feedback further comprises a decision feedback means for calculating and transmitting decision feedback parameters.

13 ) A communication apparatus as claimed in Claim 11 or Claim 12 wherein the one or more means for generating feedback further comprises an error correction means for applying error correction to the feedback.

14) A communication apparatus as claimed in any of Claims 11 to 13 that further comprises one or more feedback receivers that provides a means for extracting the feedback from the one or more feedback duplex channels and relaying the feedback to the adjustable equaliser means.

15) A communication apparatus as claimed in Claim 14 wherein the one or more feedback receivers further comprises a decoder means for decoding said error correction.