WO2002065671A2

WO2002065671A2 - Hybrid channel communication

Info

Publication number: WO2002065671A2
Application number: PCT/GB2002/000611
Authority: WO
Inventors: Michael Charles Parker; Stuart Douglas Walker
Original assignee: Fujitsu Telecommunications Europe Ltd; University Of Essex
Priority date: 2001-02-13
Filing date: 2002-02-12
Publication date: 2002-08-22
Also published as: WO2002065671A3; GB0103507D0; AU2002231963A1

Abstract

There is provided a method for facilitating communication over a communications link. The link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion (4) arranged for carrying electrical signals and a second portion (2) arranged for carrying optical signals. The method comprises the steps of: allowing application of electrical signals to the first portion (4) of the channel at a first of the two locations; receiving the electrical signals at an interface between the first and second portions of the channel; and applying signals to the second portion (2) of the channel using optical signal applying means (8). The step of applying signals to the second portion (2) includes the step of using the electrical signals received at the interface to modulate the output signal of the optical signal applying means (8).

Description

Hybrid Channel Communication

This invention relates to hybrid channel communications. In this sense, a hybrid channel is one in which there is at least one portion arranged for carrying electrical signals and at least one portion arranged for carrying optical signals.

The provision of high quality broadband network communication services to end users is becoming increasingly important. This is because of the use of the telephone network for telecommunications between computers for example when using the internet. In particular, users are continually desiring to use services which require higher and higher data rates and thus the telecommunications systems arranged to deliver that data to the end user and support the services must offer increasing bandwidth.

However, there is a problem that the existing telephone network in most countries largely consists of twisted pair copper cables at least in those regions where the network is most highly branched as it approaches the end user. As is well known, such copper cables have a very restrictive effect on the bandwidth which may be provided to a user. In general terms, the total bandwidth which the cables can supply is inversely proportional to the length of the cables themselves. Thus, relatively high data rates can be accommodated over relatively short lengths of copper cable but as length increases, so the available bandwidth falls. It is widely appreciated that the provision of optical fibres throughout the telephone network and to each end user would readily provide bandwidths in excess of those currently required by most users. However, replacing the whole of the access and metro network with fibres is extremely expensive and this cost becomes least tolerable when considering the most branched part of the network, i.e. that arriving at individual users.

Taking these above facts into account it has been proposed to use optical fibre for part, or indeed most, of the telephone network but, at least in the short or medium term, rely on the existing copper cabling locally to each user.

However, using such networks is problematic.

In one existing system, copper cabling between each end user and a local distribution unit, at a distance of, say, 60 to 300 metres from the end user's premises is relied on. VDSL (very high speed digital subscriber loop) signals are transmitted in both directions over this relatively short span of copper between the user and the distribution unit. VDSL modem equipment is provided at the user's premises and at the distribution point for the encoding and decoding of the VDSL signals. At the distribution point STM-1 SDH/Sonet equipment is also provided to take the information decoded from the VDSL signals and apply this to an optical fibre which leads away from the distribution point to a local exchange. (STM-1 is an acronym for synchronous transport module at level 1, SDH is an acronym for synchronous digital hierarchy, Sonet is an acronym for synchronous optical network and

SDH/Sonet is an industry standard for a complete transport system, in which TDM (time division multiplexing) and demultiplexing with optical line terminals are combined, using a TDM frame structure optimised for use by digital switches.)

However, the provision of VDSL modem equipment and STM-1 SDH/Sonet equipment at distribution points at the street level is highly undesirable. All of this equipment is relatively expensive, complex, susceptible to damage, as well as requiring high levels of power and a controlled environment for proper operation. The problems of power supply and controlled environment are particularly acute in respect of VDSL modems as these are relatively power hungry and intolerant of adverse conditions such as increased temperature. Thus, whilst the provision of a VDSL modem at a user's premises is not particularly problematic, as a suitable power source will normally be readily available and controlling its environment and protecting it from damage is realistic, at a distribution point all of these factors cause problems.

Thus, it is one object of the present invention to provide a communications link and system and a method of communication where the provision of complex and intolerant equipment at remote and exposed positions such as distribution points can be avoided.

It is a further object of at least some embodiments of this invention to provide a system in which the minimum demand in quality of the communications link as a whole is made.

According to a first aspect of the present invention there is provided a method for facilitating communication over a communications link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals and the method comprising the steps of: allowing application of electrical signals to the first portion of the channel at a first of the two locations; receiving the electrical signals at an interface between the first and second portions of the channel; and applying signals to the second portion of the channel using optical signal applying means; wherein the step of applying signals to the second portion includes the step of using the electrical signals received at the interface to modulate the output signal of the optical signal applying means.

According to a second aspect of the present invention there is provided a communications link comprising, a hybrid signal channel provided between two locations, which channel has a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals, a terminal arranged to allow application of electrical signals to the first portion of the channel at a first of the two locations, and an interface between the first and second portions of the channel comprising means for receiving the electrical signals and optical signal applying means arranged for applying signals to the second portion of the channel, wherein the interface is arranged to use the received electrical signals to modulate the output signal of the optical signal applying means.

According to a third aspect of the present invention there is provided a communications system comprising a communications link as defined above and terminal signal applying means arranged for applying electrical signals to the first portion of the channel via the terminal.

In a development the method is for communicating over a hybrid channel and comprises the step of applying electrical signals to the first portion.

In any such a method, link, or system the signals received at the interface may be used directly to modulate the output optical signal. That is to say, there is no need to process or decode the electrical signals to determine their information content. It is sufficient to use the varying incoming signals to directly drive, or control driving of, the optical signal applying means. This has the advantage that simple, robust, and lower power usage devices may be provided at the interface.

The method may comprise the further step of/ the link or system may comprise means for, compensating for limitations in the quality of at least part of the link, preferably a plurality of elements of the link, and more preferably still, the link as a whole. Preferably the compensation is carried out either at one end of the link or both ends of the link.

Whilst the compensation for limitations in the quality of say a wireless transmission channel is a known process, the current method and system offer more than this when compensation is applied to a plurality of elements of the link or the link as a whole. The device performing the compensation for a plurality of elements of the link, or the whole link, and typically provided at one end of the link, just sees signals being received along the link, it does not, and in general, cannot, know where any impairments in quality arise. Thus, when appropriately chosen or specified, the compensation can provide compensation for limitations or defects in the link as a whole, be these caused by the first portion of the channel, the second portion of the channel or in any equipment provided in one of the two channel portions or at the interface between the two channel portions. The first portion, second portion etc can be considered to be distinct elements of the link. It is important to appreciate that some forms of compensation which can compensate for certain types of problems with certain types of transmission channel may have little or no effect on problems caused by other types of impairment in a link. Thus the compensation used can need careful choosing. It is particularly preferred if the compensation is such as to reduce the effect of any undesirable characteristics of the optical signal applying means.

Any one or a combination of many different compensation methods may be used.

Whole link equalisation is one preferred compensation method. By whole link equalisation we mean a process of the type in which, the effect of the whole link's characteristics on signals passed along the link is determined and used to calculate an inverse channel transfer function which can then be used to process received signals to remove the effect of the whole link as far as possible.

Another preferred form of compensation is a frequency selective method. Frequency selective methods may be such that a frequency range having good transmission characteristics in the channel is used to carry a higher density of information than a frequency range having less good transmission characteristics.

Communicating along the channel using discrete multitone modulated (DMT) signals is particularly preferred. Discrete multitone modulation includes a frequency selective method of compensation and it has been realised it is particularly suited for use with the systems and methods of this application. This is because it provides effective compensation for many of the impairments that can be present in the link.

Where discrete multitone modulation is used, discrete multitone modulated signals will be applied to the first portion of the channel and components in the system will be arranged to handle DMT signals.

Whilst a wide variety of different types of signal may be applied to the first portion of the channel, the use of xDSL (Digital Subscriber Loop) signals and, particularly VDSL (Very high speed Digital Subscriber Loop) signals, is preferred. Such signals offer high data rates to users and can accommodate DMT. Thus the terminal may comprise a VDSL modem or allow connection to a VDSL modem.

Typically the method will be used where there are, and the link and system will comprise, a plurality of channels each comprising respective first portions arranged for carrying electrical signals whilst sharing a common interface and a common second portion arranged for carrying optical signals.

Preferably the interface is arranged to receive a plurality of separate electrical signals and the method comprises the further step of, or the interface is arranged for, subcarrier multiplexing the plurality of electrical signals onto the optical signal applied to the second portion of the channel.

Although, the compensation techniques can provide a high quality channel, this need not be so high as to allow error free communication. Forward error correction may be provided at the location from which signals are transmitted.

Typically the optical signal applying means will comprise a laser diode. Preferably, the optical signal applying means comprise a vertical cavity surface emitting laser (VCSEL). Where a laser is used, the signal received at the interface can be used to modulate the amplitude and/or phase of the light output by the laser. In some cases, the first portion of the channel may comprise a significant length of cabling arranged for carrying electrical signals, in which case the terminal will be remote from the interface. This may reflect a case where the terminal is at an end user's premises and the interface is at a point within a network.

In other cases the first portion of the channel may comprise circuitry between the terminal and the interface, the terminal and interface being in substantially the same location. This may reflect a case where the terminal and interface are both at one point within a network and the second portion of the channel leads away from that point, typically towards end users.

Communication in both directions over the channel will generally be catered for. Where signals are passed in the opposite direction to that considered above, the method comprises following additional steps: applying an optical signal to the second portion of the channel at a position remote from the interface; receiving the optical signals at the interface between the first and second portions of the channel; and applying electrical signals to the first portion of the channel; wherein the step of applying signals to the first portion includes the step of demodulating the optical signals received at the interface to retrieve signals to be applied to the first portion of the channel.

Similarly, the system may comprise means for applying an optical signal to the second portion of the channel at a position remote from the interface, and the interface may comprise means for receiving the optical signals, means for demodulating the optical signals to retrieve signals to be applied to the first portion of the channel, and means for applying electrical signals to the first portion of the channel.

In some embodiments, communication between two terminals may be facilitated where the signal channel comprises three portions, the first and second portions as defined above and a third portion, which is arranged for carrying electrical signals. In such a case the second portion is disposed between the first and third portions and two interfaces are provided. When signals are transmitted in one direction a first of the interfaces receives electrical signals from the first portion and applies optical signals to the second portion, whilst a second of the interfaces receives the optical signals from the second portion and applies electrical signals to the third portion. When signals are transmitted in the opposite direction the roles of the interfaces are reversed.

In one case, the first portion may comprise a significant length of cabling, say to a terminal at a user's premises, the second portion may comprise an optical fibre, and the third portion may comprise circuitry between the respective interface and the second terminal which may be disposed at substantially the same location within a network, say at a local exchange.

In another case, the first portion may comprise a significant length of cabling, say to a terminal at a user's premises, the second portion may comprise a passive optical network, and the third portion may comprise a significant length of cabling, say to a second terminal at another user's premises.

In all such cases, any and all signal processing, encoding, decoding, compensation and error correction can occur at one of, or both of the terminals. Whilst compensation of the whole link is mentioned above, a particularly important form of compensation is that for impairments in the link caused by optoelectronic components provided at the interface(s). These components typically include one or more laser diode and one or more optoelectronic receiver.

According to a fourth aspect of the present invention there is provided a method for communicating over a communications link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals and the method comprising the steps of: applying an optical signal to the second portion of the channel at a position remote from an interface between the first and second portions of the channel; receiving the optical signals at the interface; and applying electrical signals to the first portion of the channel; wherein the step of applying signals to the first portion includes the step of demodulating the optical signals received at the interface to retrieve signals to be applied to the first portion of the channel.

According to a fifth aspect of the present invention there is provided an interface for use in a communications link comprising a hybrid signal channel having a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals, the interface comprising: i) means for passing signals in a first direction comprising, means for receiving the electrical signals from the first portion, and optical signal applying means arranged for applying signals to the second portion of the channel, wherein the interface is arranged to use the received electrical signals to modulate the output signal of the optical signal applying means; and ii) means for passing signals in a second, opposite, direction comprising, means for receiving optical signals from the second portion, means for demodulating the optical signals to retrieve signals to be applied to the first portion of the channel, and means for applying electrical signals to the first portion of the channel.

Many of the additional features defined above in relation to the first, second and third aspects of the invention also apply in relation to the fourth and fifth aspects of the invention. In particular, compensation techniques can be used to mitigate impairments to quality in the link.

It is envisaged that all of the above methods, apparatus and systems will typically be used in relation to a broadband network, particularly a public access or metro network or a LAN (local area network) or MAN (metropolitan area network). In such a case, the first portion of the channel, which is arranged for carrying electrical signals, can comprise the copper twisted pair wires which currently make up most, if not all, of the public telephone network in the majority of countries. The second portion of the channel, in such cases, can comprise an optical fibre and the interface might typically be at a distribution point which is relatively local to the end user. Equally the first portion of the channel might comprise circuitry within a local exchange and the second portion an optical fibre leading away from the exchange. Thus the provision of this method encourages the introduction of fibre networks as far as distribution points.

Whilst telephone networks are referred to it will be appreciated that the system, method and apparatus described in this application are particularly applicable for use with the transmission of high volume data from computers and perhaps video links rather than simple voice data.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:-

Figure 1 schematically shows a communications system;

Figure 2 shows part of the communications system shown in Figure 1, but including more detail of a distribution point in that system;

Figure 3 shows part of the communications system shown in Figure 1 but showing more detail of the components at the local exchange; and

Figure 4 shows a second communications system.

Figure 1 schematically shows a communications system embodying the invention both in terms of a communications system as a whole and a communications link within the system. The communications system shown in Figure 1 can be used in carrying out methods according to the present invention.

The communications system generally comprises a local exchange 1 connected by optical fibre 2 to a distribution point 3 which in turn is connected by electrical cable 4 to a customer premises 5.

Whilst Figure 1 only shows a single distribution point 3 and a single customer premises 5, it will be appreciated that in practice there are many optical fibres 2 leaving the local exchange 1 and ending at respective distribution points 3 and further that there are a plurality of customer premises 5 which are connected to each distribution point 3. Typically in existing networks, each distribution point 3 will be arranged to accept connections from 24 separate copper cables which therefore may supply up to 24 separate premises.

The arrangement of components and functioning of those components in respect of each distribution point 3 and each customer premises 5 will be substantially identical and therefore for the sake of clarity most of the remaining description will be in terms of only a single distribution point 3 and a single customer premises 5.

The communications system shown in Figure 1 comprises a hybrid communications channel consisting of portions arranged to carry optical signals, in particular the optical fibre 2, and portions arranged to carry electrical signals, in particular the copper cable 4. In this embodiment we are considering how the signals are transferred from the local exchange 1 to the customer premises 5 and vice versa. How the signals pass to and from the remainder of the network is of little concern but, for example, as shown in Figure 1 and Figure 3, the local exchange 1 may be connected into an SDH/Sonet ring via a respective node 6.

The communications system is arranged to allow communication in both directions between the local exchange 1 and customer premises 5. The transmission in each direction is achieved in substantially the same way. Although systems of the present invention may be carried out using a wide variety of different signals, in this embodiment VDSL signals are used for communication between the local exchange 1 and the customer premises 5. Thus, the local exchange comprises a VDSL modem 7 which is arranged to receive signals from and output signals to the node 6. The VDSL modem 7 is also connected to a photodiode receiver and laser diode transmitter module 8. The photodiode receiver and laser diode transmitter module 8 is arranged to receive the electrical signals from the VDSL modem 7 and output optical signals onto the optical fibre 2 when transmission is occurring in one direction and to receive optical signals from the optical fibre 2 and output electrical signals to the VDSL modem 7 when transmission is occurring in the other direction.

The distribution point 3 similarly comprises a photodiode receiver and laser diode transmitter module 8, in this case arranged to receive and transmit optical signals to the opposite end of the optical fibre 2 and to receive and output electrical signals to the copper cabling 4 between the distribution point 3 and the customer premises 5. A VDSL modem 7 is provided at the customer premises and connected to the copper cable 4. At the customer premises 5, appropriate data may be transferred between a computer (or whatever device the customer is using) and the VDSL modem 7. As indicated in Figure 1 the present system may also make use of error correction and in particular forward error correction, such that when data is being transmitted from the customer towards the local exchange 1, forward error correction 9 may be carried out by the user's equipment.

Within the communications system, a communications link can be considered to exist, between the local exchange 1 and the customer premises 5. This communications link comprises all of the components between respective

VDSL modems 7 provided at the local exchange 1 and customer premises 5. Thus, this communications link includes a hybrid communications channel comprising a first portion embodied by the electrical cable 4 for carrying electrical signals, a second portion embodied by the optical fibre 2 for carrying optical signals, and a third portion embodied by the circuitry between the photodiode receiver and laserdiode transmitter module 8 and the VDSL modem 7 at the local exchange 1.

Further, it will be seen from the above description that the two photodiode receiver and laser diode transmitter modules 8 are part of the communications link and be considered to be acting as interfaces between the portions of the hybrid channel carrying optical and electrical signals.

Figure 2 schematically shows components provided at the distribution point 3 in more detail. In Figure 2, whilst only a single distribution point 3 is shown, a number of separate customer premises 5 are shown to aid understanding of the operation of the components provided at the distribution point 3.

As can be seen in Figure 2, the photodiode receiver laser diode transmitter module 8 at the distribution point 3, in fact comprises a laser transmitter 81, an optoelectronic receiver 82 and a splitter 83. In an alternative the splitter 83 may be replaced by an appropriate optical circulator.

The distribution point 3 comprises an array of upconvert mixers 10, each of which is arranged to receive signals from a respective VDSL modem 7 provided at a customer premises 5 via respective copper cabling 4. Thus, the number of upconvert mixers 10 in the array is determined by the number of separate cables 4 which the distribution point 3 can accept. Thus, if we keep to the simple case where a single line is provided to each customer premises 5, the number of upconvert mixers 10 will match the number of the premises 5 which are supplied by the distribution point 10. Thus, for example, in a typical UK distribution point supporting 24 output cables, 24 upconvert mixers will be provided in the array. It should of course be appreciated however, that in some practical implementations there may not in fact be 24 separate mixers, but rather a module able to deal with 24 incoming signals. The output of each of the mixers 10, is connected to the laser transmitter 81.

A similar array of, say 24, downconvert mixers 11 is also provided at the distribution point 3. In this case, an output from the optoelectronic receiver 82 is fed into each of the downconvert mixers 11, and an output from each of the downconvert mixers 11 is supplied to a respective cable 4 for provision to the respective customer premises 5.

A local oscillator 12, fed with a reference signal via the optical cable 2 from the local exchange 1, is provided and connected to each upconvert mixer 10 and each downconvert mixer 11. This oscillator is provided for use in subcarrier multiplexing and demultiplexing of signals.

In transmission from the customer premises 5, VDSL signals are output by the respective modems 7 and received at the distribution point 3, where the signals are subcarrier multiplexed by the upconvert mixers 10. That is to say, each of the received VDSL signals is used to modulate a carrier having a distinct frequency based on that supplied by the local oscillator 12. Thus, the first VDSL signal may modulate a carrier having a frequency f₀, the second having a frequency of f₀ + Δf and so on. The resulting signal is applied to the laser transmitter 81 and used to modulate the output of the laser 81 as applied to the optical fibre 2 through the splitter 83.

Thus, at the distribution point 3, all of the VDSL signals are subcarrier multiplexed onto an optical signal leaving the distribution point 3. All of the information content in the VDSL signals is retained and no decoding or re- encoding is required whatsoever at the distribution point 3. The optical signal can be processed upon reception elsewhere as will be described below.

When signals are travelling in the opposite direction along the optical fibre 2, these are received by the optoelectronic receiver 82 via the splitter 83 and supplied to the downconvert mixers 11. Here the original VDSL signals are recovered in the reverse process to that mentioned above and can then be output along the appropriate copper cables 4 to the respective customers.

Figure 3 shows the local exchange 1 of the present embodiment in more detail. The structure and operation of the part of the local exchange 1 shown is similar to that of the distribution point 3. When signals are received at the local exchange 1 from the optical fibre 2, they are passed through an optical circulator 83 and on to an optoelectronic receiver 82. The output of the receiver 82 is supplied to an array of downconvert mixers 11 which recover the original VDSL signals and supply them to a respective VDSL modem 7. Then the signals may be further processed as required and applied to the SDH/Sonet ring via the local exchange node 6.

When signals are to be output by the local exchange 1, the appropriate electrical signals are output by the respective VDSL modems 7, passed through an array of upconvert mixers 10 whose outputs are multiplexed and used to modulate a laser transmitter 81 to apply signals to the optical fibre 2 in the same way as described above with reference to the distribution point 3.

Figure 3 shows only a single optical fibre 2 being received at the local exchange 1, and only considers the components necessary for handling the signals carried by that fibre 2. However, the local exchange 1 will in fact receive many fibres and contain the necessary components to handle the respective signals.

It will be appreciated that the above system allows bi-directional communication over a hybrid communication channel including portions arranged to carry optical signals 2, and portions arranged to carry electrical signals 4, and more particularly allows the provision of high bandwidth communication channels to consumers whilst retaining use of part of the existing copper wire telephone network 4 and whilst eliminating the need for complex and expensive equipment such as VDSL modems and SDH/Sonet equipment at distribution points 3.

Figure 4 shows an alternative communications system which also embodies systems and links of the present invention, and which can be used for carrying out methods according to the invention.

In this case there are a plurality of customer premises 5 (only two of which are shown in Figure 4) which are connected to one another via copper cable 4, appropriate distribution points 3, optical fibre links 2 and a central passive optical network 13.

Here the components and operation of the system are substantially the same as that described above, except that, rather than providing an interface at the local exchange 1, both of the interfaces in the communications link are provided at distribution points 3. Thus, the whole communications link still comprises a first portion for carrying electrical signals, a second portion for carrying optical signals and a third portion for carrying electrical signals but these portions are embodied slightly differently. Thus, in the present case, both the first and third portions consist of copper cabling 4 provided between the respective distribution points 3 and the respective customer premises 5, and the second portion comprises the optical fibres 2 leading between the passive optical network 13 and the distribution points 3 as well as the passive optical network 13 itself.

The functions provided by the local exchange 1 in the previous embodiment are replaced by those provided at a customer's premises 5 and its respective distribution point 3. In this second embodiment, both of the terminals of the communication link are provided at respective customer premises 5 and there is a physical separation between each terminal and its respective interface as the interfaces are provided at the distribution points 3.

This is in contrast to the embodiment shown in Figures 1 to 3, where although a first terminal is provided at the customer premises 5, and this is separated from its respective interface at the distribution point 3, a second terminal at the other end of the communications link is provided at the local exchange 1 and thus is at substantially the same physical location as the respective interface. However, these differences are largely theoretical, and as far as the system as a whole is concerned, and importantly as far as the customers are concerned, the operation of the two embodiments is substantially identical.

It is important to note that in some cases, the two customer premises 5 at respective ends of the communications link in the second embodiment may be served by the same distribution point 3. In such a case, there is still conceptually the same form of link as shown in Figure 4 but the signals can be considered to be "reflected" at the passive optical network 13 and pass twice along the same optical fibre 2 and twice through the same distribution point 3, which thus acts as two separate interfaces.

Similarly, of course, in the first embodiment signals may ultimately travel between two customer premises 5 served by the same distribution point 3.

Further features which are applicable to both of the embodiments described above are now discussed below.

An important aspect of the above systems and methods which is yet to be discussed is the provision of compensation for impairments present in the complete communications link.

There are a number of approaches for providing a workable communications system in accordance with that described above, for example, by taking great concern to provide the highest quality components throughout the system or trying to compensate for impairments in different stages within the communications link. However, it has been realised by the applicants that it is possible to provide compensation for the communications link as a whole and that this is by far the most efficient way of providing a workable communications link . This has the advantages that any processing required in connection with the compensation and any equipment or circuitry required to support the compensation can be provided at one or both ends of the communication link i.e. at customer premises 5 or the local exchange 1. That is to say it may be provided in equipment provided in the region of one or both of the terminals. Some forms of compensation may be dealt with within the VDSL modems 7 themselves or in equipment associated with those modems. The important point in this strategy is that, whilst compensation techniques are known for dealing with the impairments in a channel, say a wireless transmission channel, here more is being gained. Since the compensation is carried out from either one or both ends of the communications link, the compensation technique can be chosen to take into account impairments in quality not just in the elongate signal channels provided by the copper cable 4 or the optical fibre 2 but also in any equipment provided in the communications link. Thus, in particular, the compensation techniques can be selected to mitigate impairments produced in the communication link by the laser transmitter. For example, where the laser transmitter has non-linearities in its operation these may be tolerated and compensated for.

Compensation processing and equipment may be provided at points within the link, say at the distribution point 3 and may provide compensation for only part of the link in addition to or in alternative to "whole link" compensation, but this is less preferred. For one thing it is less efficient and does not give all of the benefits mentioned above and for another, it will tend to increase the complexity and expense of the equipment necessary at the distribution point 3 which is undesirable.

One form of compensation which is used in the above embodiments is "whole link" equalisation. Equalisation of a signal channel is a familiar concept to those skilled in the art. In the present embodiments however, the effect of the communication link's characteristics as a whole, on signals passed through the link is determined and used to calculate an inverse channel transfer function which can then be used to process received signals to remove the effect of the communications link as far as possible.

A further form of compensation which is provided in the above embodiments is obtained by use of discrete multitone (DMT) modulation in the VDSL modems 7. In discrete multitone modulation, a range of frequencies is available to a modem when outputting signals. This range is split into a number of discrete sub-bands which may be used by the modem to send information. In operation, the modem is able to detect which of those sub-bands are experiencing a high quality transmission path and which of them are experiencing high levels of degradation. The modem then assigns a higher density of information to those sub-bands which are experiencing high quality transmission and reduces the density of information sent in frequency bands which are suffering a high degree of degradation. DMT is particularly effective for mitigating impairments in the link as a whole, including those caused by the optoelectronic components.

When the VDSL modem 7 is performing this frequency selecting operation, all it sees is the quality of signal path available for each frequency and reduces the amount of information put into frequencies which are experiencing poor transmission. The VDSL modem 7 will not, and in general cannot, know what it is that is causing the poor quality transmission in any particular sub-band. Thus, if for whatever reason frequencies in a certain sub-band are not passing well through the communications link then the VDSL modem 7 will reduce the amount which it uses these frequencies. This will occur whether the reduction in quality is due to impairment in the copper cable signal channel, the linearity of the laser transmitter or any other factor.

The use of compensation of the whole communications link in general, and frequency selective compensation in particular, and DMT more particularly, leads to great advantages in the present system and method which would not be immediately apparent to a person considering setting up a communications system or method based on a hybrid communications channel. It is important to realise that even if the use of compensation is thought of for part of a communications link, it is not a normal practice to apply this across the whole of the link.

It should be stressed that whilst the above embodiments make use of VDSL signalling this is not an absolute requirement. In general, any form of signalling can be used and the performance of the system will be all the better if this includes at least some of form of compensation and preferably a frequency selective form of compensation applied at one or more ends of the communications link. For example, other digital subscriber loop signals might be used.

Although it is not essential, it is preferred that each of the laser transmitters 81 provided within a link or communications system of the type described above is a vertical cavity surface emitting laser (VCSEL). Further, it is preferred if "off the shelf" components can be used in the production of the interfaces, be they provided at a distribution point 3 or a local exchange 1. As such, existing components used, for example in GSM mobile radio technology can be appropriate. For example, standard triband GSM mobile radio subcarrier multiplexing up conversion and down conversion integrated circuits may be used. Similarly, the VDSL modem and any other associated equipment can be that which is standardly available.

Vertical cavity surface emitting lasers are particularly attractive for use in systems and methods of the present application since modulation bandwidth can exceed 3 GHz and the wavelengths at which they transmit (850 nm to 1300 nm now, and in the future 1550 nm) are particularly suited to this application.

It is envisaged that a system implemented in accordance with either of the above embodiments would be able to provide 51 Mb/s channels to each customer premises 5 upon the assumption that the copper span 4 would not exceed 300m.

Different forms of modulation may be used to modulate the laser signals so as to carry the VDSL signals. Possibilities include bi-polar phase shift key (BPSK), 4-QAM (Quadrature Amplitude Modulation) signalling, and indeed more complex constellations.

In implementing the system, drivers will be required for driving the VSCEL lasers. Such a driver might operate at a low voltage of say, 3.3 volts, have a small feature size (of the order of less than 0.8 μm), include parallel CMOS buffers and offer a drive current of 10's mA at frequencies above 1GHz. Commercially available devices can offer such features.

In alternatives, systems and links may be provided that make use of WDM (Wave Division Multiplexing) in the optical part of the signal channel. This may be implemented by the provision of optical signal applying means at the interface(s) which are capable of applying signals at two or more distinct wavelengths. Typically this will mean the provision of a plurality of laserdiode transmitters each arranged to transmit a different wavelength of light. The output of each of the laserdiodes can be modulated with a respective subcarrier multiplexed signal as described above. The use of wave division multiplexing can significantly increase the capacity of the link and system, since each laser can be used to transmit signals received from say 24 distinct sources and all of these optical signals may be transmitted along a common optical fibre. Moreover, the same set of carrier frequencies may be used in subcarrier multiplexing each set of input signals. This is because the different wavelengths of light produced by each laser ensure an appropriate spacing of all of the signals when considered in the frequency domain. There are commercially available units which include a plurality of independently controllable laserdiodes each operating at a distinct wavelength which can be used in such implementations.

Tunable lasers, in particular, tunable laserdiodes may be used which allow flexible allocation of wavelengths to and at the distribution points. This is particularly applicable where WDM is used as discussed above. In at least some cases, a single tunable laser may be used for transmitting at two or more different wavelengths at distinct times.

The above embodiments, and communications links, methods and systems of the invention in general, can give various advantages.

For example, there is a common downstream upstream optical path between the distribution point 3 and the local exchange 1 and common optoelectronic components are used for a plurality of end users. Further, since there is an optical connection between the local exchange 1 and the distribution point 3, there is no need to provide a stable local oscillator at each distribution point 3 since a suitable reference signal may be provided along the optical fibre 2. Thermal management problems can be minimised because only relatively low powered components need be provided at the distribution point 3; high power equipment being sited at the local exchange 1 or at customer premises 5.

The link, system, and method facilitate a graceful evolution from the current copper infrastructure to a complete optical system by providing a useful and workable partly optical system.

In the first embodiment described above separate components, for example VDSL modems, are shown at the local exchange 1 for dealing with the signals from each end user. However, this is not essential. It is possible for equipment to be shared at the local exchange. In some cases a common module capable of handling more than one signal may be provided. Equipment may be timeshared. For example, it is most unlikely that every user will be using their system permanently, thus an "insufficient" number of lines may be catered for, knowing that at anyone time, say only 60% of users will be using. In another case, the processors may be able to work faster than realtime and thus time slots can be allocated. Such principles might also be applied at distribution points, but the scope is much greater at the local exchange where increased numbers of lines and handling capabilities improves the statistics for having "insufficient" equipment.

These principles of timesharing and providing "insufficient" equipment may also be used, for example, in a case where a bank of tunable lasers are provided at the exchange which are capable of transmitting at different wavelengths.

In some cases existing copper cabling between the local exchange 1 and the distribution point 3 may be used to supply power to the distribution point.

Claims

1. A method for facilitating communication over a communications link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals and the method comprising the steps of: allowing application of electrical signals to the first portion of the channel at a first of the two locations; receiving the electrical signals at an interface between the first and second portions of the channel; and applying signals to the second portion of the channel using optical signal applying means; wherein the step of applying signals to the second portion includes the step of using the electrical signals received at the interface to modulate the output signal of the optical signal applying means.

2. A method according to claim 1 in which the signals received at the interface are used directly to modulate the output optical signal.

3. A method according to any preceding claim comprising the further step of compensating for limitations in the quality of a plurality of elements in the link.

4. A method according to claim 3 in which the further step is one of compensating for limitations in the quality of the link as a whole.

5. A method according to claim 3 or claim 4 in which the compensation is carried out at at least one end of the link.

6. A method according to any one of claims 3 to 5 in which the compensation is such as to reduce the effect of any undesirable characteristics of the optical signal applying means.

7. A method according to any one of claims 3 to 6 in which the compensating step comprises the step of performing whole link equalisation.

8. A method according to any one of claims 3 to 7 wherein the method of compensating is a frequency selective method.

9. A method according to any preceding claim comprising the step of communicating along the channel using discrete multitone modulated (DMT) signals.

10. A method according to any preceding claim in which the interface is arranged to receive a plurality of separate electrical signals and the method comprises the further step of subcarrier multiplexing the plurality of electrical signals onto the optical signal applied to the second portion of the channel.

11. A method according to any preceding claim in which the optical signal applying means comprises a laser diode arranged so that the signal received at the interface can be used to modulate the amplitude and/or phase of the light output by the laser.

12. A method according to claim 11 in which the laser is a vertical cavity surface emitting laser.

13. A method according to any preceding claim comprising the further steps of applying an optical signal to the second portion of the channel at a position remote from the interface; receiving the optical signals at the interface between the first and second portions of the channel; and applying electrical signals to the first portion of the channel; wherein the step of applying signals to the first portion includes the step of demodulating the optical signals received at the interface to retrieve signals to be applied to the first portion of the channel.

14. A communications link comprising, a hybrid signal channel provided between two locations, which channel has a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals, a terminal arranged to allow application of electrical signals to the first portion of the channel at a first of the two locations, and an interface between the first and second portions of the channel comprising means for receiving the electrical signals and optical signal applying means arranged for applying signals to the second portion of the channel, wherein the interface is arranged to use the received electrical signals to modulate the output signal of the optical signal applying means.

15. A communications link according to claim 14 in which the signals are used to directly modulate the output optical signal.

16. A communications link according to claim 14 or claim 15 comprising means for compensating for limitations in the quality of a plurality of elements in the link.

17. A communications link according to claim 16 in which the means for compensating for limitations is a means for compensating for limitations in the quality of the whole link.

18. A communications link according to claim 16 or 17 in which the compensation means is provided at at least one end of the link.

19. A communications link according to any one of claims 16 to 18 in which the means for compensating for limitations is arranged such as to reduce the effect of any undesirable characteristics of the optical signal applying means.

20. A communications link according to any one of claims 16 to 19 in which the compensating means is arranged for performing whole link equalisation.

21. A communications link according to any one of claims 14 to 20 arranged for communication along the channel using discrete multitone modulated signals (DMT).

22. A communications link according to any one of claims 14 to 21 in which the interface is arranged to receive a plurality of separate electrical signals and the interface is arranged for subcarrier multiplexing the plurality of electrical signals onto the optical signal applied to the second portion of the channel.

23. A communications link according to any one of claims 14 to 22 in which the optical signal applying means comprises a laser diode arranged so that the signal received at the interface can be used to modulate the amplitude and/or phase of the light output by the laser.

24. A communications link according to claim 23 in which the laser is a vertical cavity surface emitting laser.

25. A communications link according to any one of claims 14 to 24 in which the communications link is arranged for communication in both directions over the channel.

26. A communications link according to claim 25 which comprises means for applying an optical signal to the second portion of the channel at a position remote from the interface, and wherein the interface comprises means for receiving the optical signals, means for demodulating the optical signals to retrieve signals to be applied to the first portion of the channel, and means for applying electrical signals to the first portion of the channel.

27. A communications system comprising a communications link according to any one of claims 14 to 26 and terminal signal applying means arranged for applying electrical signals to the first portion of the channel via the terminal.

28. A method for communicating over a communications link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals and the method comprising the steps of: applying an optical signal to the second portion of the channel at a position remote from an interface between the first and second portions of the channel; receiving the optical signals at the interface; and applying electrical signals to the first portion of the channel; wherein the step of applying signals to the first portion includes the step of demodulating the optical signals received at the interface to retrieve signals to be applied to the first portion of the channel.

29. An interface for use in a communications link comprising a hybrid signal channel having a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals, the interface comprising: i) means for passing signals in a first direction comprising, means for receiving the electrical signals from the first portion, and optical signal applying means arranged for applying signals to the second portion of the channel, wherein the interface is arranged to use the received electrical signals to modulate the output signal of the optical signal applying means; and ii) means for passing signals in a second, opposite, direction comprising, means for receiving optical signals from the second portion, means for demodulating the optical signals to retrieve signals to be applied to the first portion of the channel, and means for applying electrical signals to the first portion of the channel.

30. A method for facilitating communication over a communications link comprising a hybrid signal channel provided between two locations, the channel comprising a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals and the method comprising the steps of: allowing application of electrical signals to the first portion of the channel at a first of the two locations; receiving the electrical signals at an interface between the first and second portions of the channel; and applying signals to the second portion of the channel using an optical signal transmitter; wherein the step of applying signals to the second portion includes the step of using the electrical signals received at the interface to modulate the output signal of the optical signal transmitter, the method comprising the further steps of communicating along the channel using discrete multitone modulated signals (DMT) and compensating for limitations in the quality of a plurality of elements in the link by performing whole link equalisation.

31. A method according to claim 30 in which the step of compensating for limitations in quality is such as to reduce the effect of undesirable characteristics in the optical transmitter.

32. A communications link comprising, a hybrid signal channel provided between two locations, which channel has a first portion arranged for carrying electrical signals and a second portion arranged for carrying optical signals, a terminal arranged to allow application of electrical signals to the first portion of the channel at a first of the two locations, and an interface between the first and second portions of the channel comprising a receiver for receiving the electrical signals and optical signal transmitter arranged for applying signals to the second portion of the channel, wherein the interface is arranged to use the received electrical signals to modulate the output signal of the optical signal transmitter and the communications link is arranged for communication along the channel using discrete multitone modulated signals (DMT) and comprises a compensator arranged for compensating for limitations in the quality of a plurality of elements in the link by performing whole link equalisation.

33. A communications link according to claim 32 in which the compensator is arranged such as to reduce the effect of any undesirable characteristics of the optical signal transmitter.