EP0898817A1

EP0898817A1 - Power line communication employing cordless telephone standard

Info

Publication number: EP0898817A1
Application number: EP97923172A
Authority: EP
Inventors: Benedict Russell Freeman
Original assignee: Northern Telecom Ltd; Nortel Networks Corp
Current assignee: Nortel Networks Corp
Priority date: 1996-05-16
Filing date: 1997-05-16
Publication date: 1999-03-03
Also published as: GB2313273A; GB9610266D0; AU2904397A; WO1997043836A1

Abstract

Telecommunications signals are carried on a cable (111) which carries mains electricity to a consumers' premises. A signal coupler (SC) and a subscribers' interface unit (SIU) are provided between the mains cable (111) and conventional subscribers' terminal equipment (25A, 25B, 25C) adapted for full duplex communication with a local telephone exchange. The subscribers' interface unit SIU converts telecommunications signals between this duplex form (typically analogue at audio frequencies) and time division duplex (TDD) form at a carrier frequency in the range 1 MHz to 20 MHz for coupling to the mains cable (111). In the example, TDD interface means (40) provides cordless telephony digital standard formatting/deformatting and modulating/demodulating at a carrier in the band 864 MHz to 868 MHz, and a frequency converter (50) converts between this band and, typically, 2 MHz to 6 MHz.

Description

POWER LINE COMMUNICATION EMPLOYING CORDLESS TELEPHONE STANDARD

FIELD OF THE INVENTION

This invention relates to a telecommunications method and system in which telecommunications signals are carried on a cable which carries mains electricity supply to consumers' premises.

BACKGROUND OF THE INVENTION

From published patent application GB-A-2,272,350 (Norweb) there is known a telecommunications system, in which system ; a telecommunications subscribers' apparatus is located at each of a plurality of low voltage a.c. mains electricity supply consumers' premises; a telecommunications base station for communicating with said subscribers' apparatus at said plurality of consumers' premises is located in the electricity supply distribution network; and a network conditioning unit is located at each said consumers' premises for coupling telecommunications signals at a carrier frequency of greater than approximately 1 MHz between the subscribers' apparatus and a cable carrying the mains supply to the premises, whereby the mains supply cable carries said telecommunications signals, and for providing electrical isolation between the subscribers' apparatus and the voltage of the mains supply.

One feature of the invention of GB-A-2,272,350 is the network conditioning unit which is disclosed in one example as being located at a consumers' premises as specified above and in other examples as enabling speech or data to be communicated via transceiver/modems on an 415V section of an electrical power distribution network. It is suggested that for a 415V network the carrier frequency may preferably be between 1-20 MHz; that voice and data signals may be transmitted over any or all of the sections of the power network by suitable detection, amplification and/or regeneration and reintroduction as and when necessary; that preferably full duplex facilities are provided; that transmission techniques including both frequency and time division multiplexing may be used; and that the spread spectrum method offers inherent security and good interference rejection characteristics using a large bandwidth. A detailed description is given of the electrical circuit and physical make-up of a network conditioning unit located at the premises of a user of 50Hz, 240V single phase or 415V three phase electricity supply including a port to which a user's communication equipment would be connected.

Inter alia, the disclosure of GB-A-2,272,350 (Norweb) does not provide a satisfactory arrangement in the case where telecommunications subscribers' apparatus are located at each of a plurality of low voltage (240V/415V) a.c. mains electricity supply consumers' premises. Using the spread spectrum large bandwidth as suggested by this disclosure it would be difficult to provide a telecommunications service from a single base station to a plurality of subscribers with carrier frequencies between 1 -10MHz as suggested by Norweb. Also the user's communication equipment, which is not considered by Norweb in any detail, must provide subscribers' apparatus which is compatible in use with the chosen transmission method while being convenient and reasonably affordable for the subscriber.

OBJECT OF THE INVENTION

The present invention seeks to provide a solution to these problems. The present invention also seeks to provide an improved telecommunications system operating over mains electricity supply cables and a method of operating the same. SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a method of conveying telecommunications signals between a base station and subscribers' apparatus located at each of a plurality of low voltage a.c. mains electricity supply consumers' premises; said base station being coupled to the electricity supply distribution network and said subscriber apparatus being coupled to the mains supply by a signal coupler wherein telecommunications signals are transmitted in full duplex form between that terminal equipment and a public switched telephone network local exchange; and in each said subscribers' apparatus signals are converted in a subscribers' interface unit between said full duplex form and time division duplex form. Preferably the said carrier frequency lies in the range 1 MHz to 20MHz.

According to another aspect of the present invention there is also provided a telecomm unications system , in which system telecommunications subscribers' apparatus is located at each of a plurality of low voltage a.c. mains electricity supply consumers' premises said subscribers' apparatus being coupled to the mains supply by a signal coupler; a telecommunications station for communicating with said subscribers' apparatus at said plurality of consumers' premises is located in the electricity supply distribution network; and wherein the subscribers' apparatus includes; subscribers' terminal equipment adapted for transmission of telecommunications signals in full duplex form between that terminal equipment and a telecommunications base station; and a subscribers' interface unit for conversion of telecommunications signals between said full duplex form and time division duplex form for coupling to said mains supply cable by said signal coupler.

In the method and system according to the invention, use of time division duplex transmission technique on the mains supply cable reduces the bandwidth requirement for a plurality of subscribers, and use of the interface unit enables each subscriber to use conventional public switched telephone network terminal equipment such as a voice telephone, facsimile machine or computer modem.

Preferably, in the method and system according to the invention said base station is located at a local substation supplying said low voltage mains supply to said plurality of consumers' premises, and said telecommunications signals are communicated between said base station and at least most of said consumers' premises without amplification or regeneration. We have found by experiment that using carrier frequencies up to 20MHz, signal amplification or regeneration is not necessary for the usual distance of up to approximately 300m between a local substation supplying low voltage mains electricity and the consumers' premises.

In accordance with a further aspect of the present invention there is further provided a subscribers' interface unit for a telecommunications system operating over mains electricity supply, which interface unit includes; first conversion means for conversion of telecommunications signals between said full duplex form and a time division duplex form at a carrier frequency above 20MHz, and second conversion means for conversion of telecommunications signals between said time division duplex form at said carrier frequency above 20MHz and the same time division duplex form at said carrier frequency in the range 1 MHz to 20MHz.

In the method according to the invention there may be, in said interface unit, conversion of telecommunications signals between said full duplex form and a time division duplex form at a carrier frequency above 20MHz; and conversion of telecommunications signals between said time division duplex form at said carrier frequency above 20MHz and at the same time division duplex form at said carrier frequency in the range 1 MHz to 20MHz. In this method there may be conversion, or correspondingly in the interface unit there may be adaptation of the first conversion means for conversion, of telecommunications signals between said full duplex form as analogue signals and said time division duplex form at said carrier frequency above 20MHz. Thus conventional telephone subscribers' terminal equipment may be used transmitting and receiving voice or data information at audio frequencies. Conversion in the method, or correspondingly adaptation of the first conversion means, could otherwise be of said full duplex signals as digital signals such that more recent integrated systems digital network telephone subscribers' terminal equipment could be used transmitting and receiving voice and data information at a line rate of 192kbit/s.

In accordance with a further aspect of the invention, said conversion of telecommunications signals between said full duplex form and said time division duplex form at a carrier frequency above 20MHz preferably includes; conversion between telecommunications signals in said duplex form and comprising user information and signalling information on a single path and telecommunications signals in digital form and comprising said user information and said signalling information on separate paths; and conversation between telecommunications signals in said digital form with user information and signalling information on separate paths and telecommunications signals on a single path in said time division duplex form having a predetermined digital format at said carrier frequency above 20MHz.

The conversion between telecommunications signals in said digital form with user information and signalling information on separate paths and the telecommunications signals on a single path in said time division duplex form having said predetermined digital format at said carrier frequency above 20MHz specified in the preceding paragraph may include; converting said telecommunications signals in said digital form with user information and signalling information on separate paths into digital signals in said predetermined time division duplex format before modulation onto said carrier frequency above 20MHz. Converting digital signals in said predetermined time division duplex format after demodulation from said carrier frequency above 20MHz into said telecommunications signals in said digital form with user information and signalling information on separate paths, and generating a time division duplex burst timing signal; modulating said digital signals in said predetermined time division duplex format onto said carrier frequency above 20MHz in a transmission path under control of said burst timing signal; conveying telecommunications signals from said transmission path to said single path in said predetermined time division duplex format under control of said burst timing signal; conveying telecommunications signals from said single path in said predetermined time division duplex format to a reception path under control of said burst timing signal; and demodulating digital signals from said carrier frequency above 20MHz in said predetermined time division duplex format in said reception path under control of said burst timing signal.

In the method or in the subscribers' interface unit, said predetermined time division duplex format and said carrier frequency above 20MHz advantageously correspond with the digital cordless telephony CT2 standard. Advantages of using the CT2 standard are as follows. Existing radio handset circuitry can be comparatively easily and economically adapted and used to provide time division duplex interface means. The CT2 4MHz bandwidth having 40 channels of 100KHz, on carrier frequencies between 864MHz and 868MHz, conveniently occupies only a small portion of the bandwidth available on the electricity supply cable when converted by the above specified second conversion means to a carrier frequency for each channel in the range 1 MHz to 20MHz.

In the subscribers' interface unit according to the invention, said second conversion means preferably includes; a frequency converter including one mixing means and one local oscillator means which serve for frequency down conversion and for frequency up conversion in alternate time division duplex burst periods; one amplifier means having an amplifier input and an amplifier output; and switching means responsive to a or respectively the time division duplex burst timing signal generated within and derived from said first conversion means, such that for frequency down conversion by said frequency converter the amplifier input is coupled to said mixing means and the amplifier output is coupled to said signal coupler, and such that for frequency up conversion by said frequency converter the amplifier input is coupled to said signal coupler and the amplifier output is coupled to said mixing means. BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of a telecommunications method and system in accordance with the present invention, and in particular of the subscribers' interface unit of this system, will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a substation and low voltage cables for supplying electricity to a plurality of consumers' premises, with a telecommunications base station located in the substation for communication with the consumers' premises via the electricity cables;

Figure 2 shows one of the consumers' premises of Figure 1 with a subscribers' interface unit which couples telephone subscribers' terminal equipment to the electricity supply cable via a signal coupler; Figure 3 shows circuit details of a signal coupler of Figure

2, and also shows a block schematic circuit of the subscribers' interface unit of Figure 2 comprising first conversion means connected to the subscribers' terminal equipment and second conversion means connected to the signal coupler; Figure 3a shows a signal coupler;

Figure 4 shows schematic circuit details of time division duplex interface means which is part of the first conversion means of Figure 3; and

Figure 5 shows schematic circuit details of the second conversion means of Figure 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring.now to Figure 1 , a plurality of low voltage a.c. mains electricity supply consumers' premises 10 are shown connected by a distribution network of 415V three phase/240V single phase 50Hz mains supply cables 1 1 to a local substation SS in which the 415V is derived from an incoming 11 KV cable 12 via a transformer 13. A telecommunications base station BS is located at the substation SS and telecommunications signals are carried on the mains supply cables 1 1 between the base station BS and subscribers' apparatus in at least some of the consumers' premises 10. The base station BS is connected between an injection point 14 on the 415V side of the transformer 13 and a transmission path 1 5 which is connected further into a telecommunications system.

Communication of telecommunications signals between the base station BS and a consumers' premises 10 is each time at a selected carrier frequency in the range 1 MHz to 20MHz. We have found by experiment that using carrier frequencies up to 20MHz signal amplification or regeneration is not necessary for the usual distance of up to approximately 300m between a local substation SS supplying low voltage mains electricity and a consumers' premises 10. If it is necessary for longer distances between the substation SS and a consumers' premises 10, amplification or regeneration may be provided at a suitable point along that distance. We consider it to be most convenient and practical for the telecommunications base station BS to be located at the local substation SS as shown in Figure 1. However the base station BS could be located elsewhere in the electricity supply distribution network, for example it could be located at a 33kV to 11kV transformer station which distributes 11 kV electricity supply to a number of local substations BS with amplification and regeneration being provided at each substation BS.

Referring now to Figure 2, a 240V single phase 50Hz a.c. mains electricity supply cable 1 1 is shown entering a consumers' premises 10, and this mains supply is connected via the usual fuse 21 to the usual meter box 22 which connects to power circuits 23 and lighting circuits 24 in the consumers' premises 10. A signal coupler SC, which will be described in detail later with respect to Figure 3, is connected between the fuse 21 and the meter box 22. The signal coupler is primarily for coupling telecommunications signals at the selected frequency between telecommunications subscribers' apparatus in the consumers' premises

10 and the mains supply cable 11 , and for providing electrical isolation between this subscribers' apparatus and the voltage of the mains supply. The subscribers' apparatus in the consumers' premises includes conventional public switched telephone network terminal equipment such as a voice telephone 25A, facsimile machine 25B or computer modem 25C which is adapted for transmission of telecommunications signals in full duplex form between that terminal equipment and a public switched telephone network local exchange. Figure 2 shows a conventional telephone terminal point 28 connected to a conventional two-wire line 29 leading to such a local exchange. When used in the telecommunications system of this invention, the terminal equipment 25A and/or 25B, 25C is instead connected by the subscribers' telephone cable 26 to another, similar, telephone terminal point 27. The subscribers' apparatus in the consumers' premises also includes a subscribers' interface unit SIU, connected between the terminal point 27 and the signal coupler. The subscribers' interface unit SIU is for conversion of telecommunications signals between the full duplex form and time division form at the carrier frequency in the range 1 MHz to 20MHz for coupling to the mains supply cable 1 1 by the signal coupler.

The signal coupler will in practice be installed at each consumers' premises 10 connected by electricity supply cable 1 1 to a common injection point 14 leading to a telecommunications system base station BS (as shown in Figure 1 ), whether or not that particular consumers' premises also has a subscribers' interface unit SIU enabling subscribers' terminal equipment 25A, 25B, 25C to be connected in the telecommunications system of this invention. This is because the signal coupler also serves for preventing noise sources from the internal low voltage premises wiring 23, 24 at each premises 10 contaminating or corrupting the telecommunications signals being transmitted between any of the premises 10 and the injection point 14.

Referring now to Figure 3a, exemplary circuit details are shown of the signal coupler SC with circuit connections as detailed in Table 1 ; the equipment must work in a number of countries, some of which have IT power systems and some of which have TT power systems (and some, like the UK, have both). It is hence necessary to provide an interface that can couple signals from phase to ground, and from phase to neutral. Other design options include sending the RF signal in both balanced and unbalanced configurations. For these reasons the invention provides a high degree of flexibility in configuration.

Table 1

* Connecting one side of the RF output to earth when the input connections are live and neutral is necessary to ensure that and common mode transients coupled by C2 are returned safely to the local ground.

The input 1 1 1 connects to a capacitor 301 which reduces the current available from the incoming mains supply. A Y2 rated capacitor has been found to provide suitable levels of current reduction.

Two principal methods for reducing the output current capacity are possible; the use of a fuse or a capacitor. It is necessary to use a high rupture capacity fuse because of the high prospective fault current available at this point in the supply, even though the fuse rating would be relatively low (in the range 2 - 5A). Such fuses and their associated fuse-holders are physically large and this can preclude their use at the consumer end. It would also be possible for consumers to 'tap in' and obtain a supply of 'free' electricity, up to a limit governed by the fuse rating and for these reasons it is unlikely that a fuse will be implemented for current reduction in practice. Nevertheless fuse 302 is provided for overload protection.

It is also necessary to reduce the level of transient signals down to acceptable levels. This is achieved by the voltage breakdown components, a metal oxide varistor (MOV) 303 and a gas discharge tube 304. The gas discharge tube has two functions, (1 ) to act as a means of 'backing off the voltage that appears across the MOV to prolong the life of the MOV and ensure transient co-ordination between the capacitors 301 and 305 and (2) to provide a low capacitance under normal conditions so that the RF signal is not shorted out.

Capacitor 301 is considered to provide basic insulation within standards such as European Standard (EN 60950). Hence the output from the mains interconnect box is not touch safe, but the available output current ('free' electricity) is reduced to a low level. A fuse is also provided at the output; this both provides protection in the event that the capacitor goes short circuit and also enables the 'live' lead going to the RF Injection Box 309 to be isolated without disturbing the connection to the mains electricity supply.

Capacitors classified as Y2 have a maximum transient voltage of 4 to 5kV. Hence they are not specified to withstand the 6 kV transients that are possible on the incoming supply. This is catered for by 'transient co- ordination' whereby the incoming transient is applied across the two capacitors, 301 and 307 since the two capacitors are of the same nominal value, the voltage across each component would be 3kV (except for the operation of the voltage breakdown components).

It is known that Metal Oxide Varistors degrade with the number of pulses 'absorbed'. They can also get hot with time and offer an increased leakage current due to this degradation. Their voltage breakdown characteristic is such that they clamp the incoming supply. Such devices can therefore be used directly across the supply. However, unless a means of protection from thermal disintegration is provided (such as a thermal fuse) then the device should be provided with a means of ensuring that the full mains voltage is not applied to the device. Reducing the number of transients to be absorbed by the device also increases its expected life. Gas discharge tubes do not have this memory effect but do have a characteristic such that, once turned on, they go to a low resistance state. It is therefore not recommended to use such devices directly across the mains.

~5

Using a gas discharge tube and a MOV in series provides the necessary transient reduction capacity while ensuring that both devices operate safely. For this application a 275 V MOV and a 800 V gas discharge tube have been found to provide satisfactory results. Tests using a

10 transient generator showed that the transient voltage is limited to 2.5kV with input transients of 6.5 kV.

In practice therefore the voltage across 307 should not exceed 2.5 kV and the voltage across 301 should not exceed 3.5 kV - both components

15 are therefore working within their specified parameters. The voltage applied to the input of the transformer is a maximum of 2.5 kV, which is a typical voltage. Signals are coupled across transformer 306 and are output at 32, to be processed in conversion means 50 (figure 3). Fuse 302 is arranged to blow in the event of failure or fault in the capacitor

20 301. The capacitors 301 and 307 are low impedance elements over the required telecommunications frequency spectrum of 1 MHz to 20MHz and high impedance isolating elements at the 50Hz frequency of the mains electricity supply.

25 In figure 3 signal coupler SC has these inputs A,B,C, one of which, A, is connected to a mains electricity input 1 1 1 from the fuse 21 (Figure 2) carries 50Hz single phase 240V electrical power at a maximum current of 100 amps to the meter box 22 (Figure 2). The unit is in a metal box which prevents radiation of communications signals to externally located

30 appliances and provides a connection to earth for the signal coupler.

The subscribers' interface unit SIU consists of first conversion means 300 for conversion of telecommunications signals between full duplex form at the telephone terminal point 27 and time division duplex form at 35 a carrier frequency above 20MHz, and second conversion means 50 for conversion of telecommunications signals between this time division duplex form at this carrier frequency above 20MHz and the same time division duplex form at the signal coupler input/output port 32 but at the carrier frequency in the range 1 MHz to 20MHz. All the circuits of the subscribers' interface unit SIU may be powered by a suitable d.c. voltage derived from the consumers' mains supply.

The first conversion means 300 includes signal separation interface means 310 for conversion between telecommunications signals in duplex form at the terminal 27 and comprising user information and signalling information on a single path provided by the subscribers' telephone cable 26 and telecommunications signals in digital form and comprising the user information 341 and the signalling information 342 on separate paths as shown in Figure 3. The first conversion means 300 also includes time division duplex interface means 40 for conversion between telecommunications signals in the digital form with user information 341 and signalling information 342 on separate paths and telecommunications signals on a single path 401 in the time division duplex form having a predetermined digital format at the carrier frequency above 20MHz.

The signal separation interface means 310 will now be described in detail with reference to Figure 3. This will be followed by a description of the detail of the time division duplex interface means 40 with reference to Figure 4 and then a description of the detail of the second conversion means 50 with reference to Figure 5.

The signal separation interface means 310 consists of a line card unit 320 and a processing unit 330. The full duplex signals at the telephone point 27 are analogue signals at audio frequencies on a single path. The line card unit 320 is a commercially available circuit, similar in function to a conventional local exchange line circuit, which provides conversion between these analogue signals on a single path and the conventional components of these analogue signals on separate paths. These components are the three components of signalling information as the outgoing off-hook signal on path 331 , the incoming ringing signal on path 332, and the outgoing multifrequency address signals on path 333, and also the two-way user information signals (voice or data) on path 334. The processing unit 330 includes analogue to digital conversion, digital to analogue conversion and a micro-processor function which together effect conversion between the user information (voice or data) in analogue form on path 334 and that user information in digital form on path 341 , and also effect conversion between the off- hook, ringing and address signalling information in analogue form on paths 331 , 332 and 333 and that signalling information in digital form combined on path 342 separate from path 341.

It is envisaged that an alternative implementation of the signal separation interface means 310 could be provided for the case where the voice telephone 25A, facsimile machine 25B or computer modem 25C are provided with ISDN (integrated systems digital network) termination equipment for full duplex transmission and reception of user and signalling information in digital form, or where the subscribers' terminal equipment such as a voice telephone is integrally adapted for digital operation. In this case the full duplex signals at the telephone point 27 will be digital user and signalling information signals on a single path and the signal separation interface means 310 will provide conversion between these digital signals at the telephone point 27 and the user information 341 and signalling information 342 in digital form on separate paths.

Referring now to Figure 4, the time division duplex interface means 40 will be described in detail.

Formatting/deformatting means 41 , which may be implemented essentially by a microprocessor, interfaces with the digital user information and digital signalling information paths 341 , 342 and provides the following three functions. Firstly it converts the digital user information 341 and the digital signalling information 342 into digital logical signals 411 in a predetermined time division duplex format before modulation onto a carrier frequency. Secondly it converts digital logical signals 412 in this predetermined time division duplex format after demodulation from this carrier frequency into the digital user information 341 and the digital signalling information 342. Thirdly it generates a time division duplex burst timing signal 402.

A transmission path 42 has a modulator 421 , a frequency up converter 422 and a power amplifier 423 which are controlled by the burst timing signal 402 and provide the time division duplex formatted digital signals 411 modulated on to the carrier frequency above 20MHz in alternate time division duplex burst periods as telecommunications signals 424 to a switch 44.

The switch 44 is controlled by the burst timing signal 402 for conveying the telecommunications signals 424 from the modulating transmission path 42 via bandpass filter 45 to the single path 401. The switch 44 also conveys telecommunications signals 431 modulated on the same carrier frequency above 20MHz and in the same time division duplex format from the single path 401 and the bandpass filter 45 in alternate time division duplex burst periods to a reception path 43.

The reception path 43 has a low noise amplifier 432, a frequency down converter 433 and a demodulator 434 which are controlled by the burst timing signal 402 and provide the demodulated time division duplex formatted digital signals 412 to the formatting/deformatting means 41.

The above-mentioned predetermined time division duplex format and the above-mentioned carrier frequency above 20MHz in this exemplary embodiment of the invention are in accordance with the digital cordless telephony CT2 standard. The CT2 standard operates with a 4MHz bandwidth having 40 channels of 100KHz on carrier frequencies between 864MHz and 868MHz, and the time division duplex burst frequency is 500Hz. The modulator 421 and the demodulator 434 operate on a carrier frequency of 150MHz. Channel selection and conversion to and from a particular carrier frequency in the 864MHz to

868MHz band is effected in the up converter 422 and in the down converter 433. The circuitry of the time division duplex interface means 40 may be custom designed, or it may be adapted from existing CT2 radio handset circuitry in which similar circuits to the formatting/deformatting means 41 interface with a line unit including a keypad, microphone and receiver of the radio handset and the single path 401 connects to an antenna of the radio handset. If such a radio handset line unit is made use of for the present invention then it will include the analogue to digital conversion and digital to analogue conversion functions which have been described above as functions of the processing unit 330 of the signal separation interface means and may still be considered as functions of this processing unit 330 for the purpose of this invention.

For example, 40 channels in the range 2MHz to 6MHz would typically provide sufficient traffic capacity for the number of subscribers connected to a single telecommunications base station at a local electricity supply substation; and if additional capacity was required, then another 4MHz bandwidth could also be used, for example another 40 channels in the range 8MHz to 12MHz. Said predetermined time division duplex format and said carrier frequency above 20MHz could alternatively be in accordance with the digital cordless telephony DECT standard in which 10 carrier frequencies are provided separated by 1728KHz between 1881.792MHz and 1897.344MHz, i.e. occupying a bandwidth of 15.552MHz.

Referring now to Figure 5, the second conversion means 50 will be described in detail.

The second conversion means 50 includes a frequency converter 51 , one amplifier means 52 and switching means 53.

The frequency converter 51 includes one mixing means 51 1 and one local oscillator means 512 which serve for frequency down conversion and for frequency up conversion in alternate time division duplex burst periods. An attenuator 513 is provided between the mixing means 511 and the path 401 to ensure a suitably low power level for operation of the mixing means 51 1 during frequency down conversion and a low pass filter 514 is provided on the other side of the mixing means. The frequency converter provides conversion between telecommunications signals in CT2 time division duplex format at a carrier frequency within the 4MHz band from 864MHz to 868MHz on the path 401 and these telecommunications signals in the same CT2 time division duplex format at the signal coupler input/output port 32 where they are at a carrier frequency also within a 4MHz band but within the range 1 MHz to 20MHz, for example 2MHz to 6MHz.

The amplifier means 52 is a single amplifier chain having a low noise amplifier 521 and a high power amplifier 523 in series between an amplifier input 524 and an amplifier output 525.

The switching means 53 is responsive to the 500Hz time division duplex burst timing signal 402 generated within and derived from said first conversion means 300, such that for frequency down conversion by the frequency converter 51 , the amplifier input 524 is coupled to the mixing means 51 1 and the amplifier output 525 is coupled to the signal coupler input/output port 32 via a low pass filter 54, and such that for frequency up conversion by the frequency converter 51 the amplifier input 524 is coupled to the signal coupler input/output port 32 via the filter 54 and the amplifier output 525 is coupled to the mixing means 511.

The switching means 53 is made up of a two input combiner 531 having its sum port connected to the amplifier input 524, a two output splitter 532 having its sum port connected to the amplifier output 525, and two switches 534, 535 controlled by the time division duplex burst timing signal 402, such that for frequency down conversion the first switch 534 couples the mixing means 51 1 to one input of the combiner 531 via the filter 514, and the second switch 535 couples one output of the splitter 532 to th.e signal coupler port 32 via the filter 54, and such that for frequency up conversion the second switch 535 couples the signal coupler port 32 to the other input of the combiner 531 via the filter 54, and the first switch 534 couples the other output of the splitter 532 to the mixing means 511 via the filter 514.

The loss provided by the attenuator 513 may be typically 10dB and by the mixing means 51 1 may be typically 7dB. The loss provided by the other components of the second conversion means 50 may typically amount to another 10dB. Since the mains electricity supply cable will have a high loss for the telecommunications signals along its length between the consumers' premises and the local substation, typically up to 40dB, it is desirable for the amplifiers 521 , 523 to provide a gain which compensates for the losses in the second conversion means 50, say 27dB gain in the case of this aggregate of typical losses just indicated.

Using one mixing means 51 1 and one local oscillator means 512 for frequency down conversion in transmission from the subscriber and for frequency up conversion in reception by the subscriber, together with one amplifier means 52 operated bi-directionally, is particularly advantageous for component costs, power consumption and stability compared with using a separate frequency converter and amplifier for transmission and reception. There is also an advantage in the time division duplex burst timing signal 402 used for controlling this bi¬ directional operation being substantially cost-free in being already generated in the first conversion means 300 which provides conversion of the telecommunications signals between full duplex form and time division duplex form.

The present invention thus provides a system which is particularly advantageous for reduced component costs, power consumption and stability.

Claims

1. A method of conveying telecommunications signals between a base station BS and subscribers' apparatus 25A,B,C located at each of a plurality of low voltage a.c. mains electricity supply consumers' premises 10; said base station being coupled to the electricity supply distribution network and said subscriber apparatus being coupled to the mains supply by a signal coupler wherein telecommunications signals are transmitted in full duplex form between that terminal equipment and a public switched telephone network local exchange; and in each said subscribers' apparatus signals are converted in a subscribers' interface unit between said full duplex form and time division duplex form.

2. A method as according to claim 1 wherein the signal couples telecommunications signals at a carrier frequency of greater than approximately 1 MHz.

3. A method according to claim 1 wherein the signals are converted between the said full duplex form and time division duplex form at a carrier frequency in the range 1 MHz to 20MHz for coupling to said mains supply cable by said signal coupler.

4. A method according to claims 1 to 3 wherein the signal coupler is located at each said consumers' premises for coupling telecommunications signals between the subscribers' apparatus and a cable 11 carrying the mains supply and the telecommunications signals and for providing electrical isolation between the subscribers' apparatus and the mains supply.

5. A method according to any one of claims 1 to 4 wherein, with said base station being located at a local substation supplying said low voltage mains supply to said plurality of consumers' premises, said signals are communicated between said base station and at least most of said consumers' premises without amplification or regeneration.

6. A method according to claims 1 to 5 wherein, in said interface unit, there is conversion of signals between said full duplex form and a time division duplex form at a carrier frequency above 20MHz; and there is conversion of signals between said time division duplex form at said carrier frequency above 20MHz and the same time division duplex form at said carrier frequency in the range 1 MHz to 20MHz.

7. A method according to claim 6, wherein signals are converted between said full duplex form as analogue signals and said time division duplex form at said carrier frequency above 20MHz

8. A method according to any one of claims 1 to 7 wherein said conversion of signals between said full duplex form and said time division duplex form at a carrier frequency above 20MHz includes; conversion between signals in said duplex form and comprising user information and signalling information on a single path and telecommunications signals in digital form and comprising said user information and said signalling information on separate paths, and conversion between signals in said digital form with user information and signalling information on separate paths and signals on a single path in said time division duplex form having a predetermined digital format at said carrier frequency above 20MHz

9. A method according to claim 8 where said conversation between signals in said digital form with user information and signalling information on separate paths and telecommunications signals on a single path in said time division duplex form having said predetermined digital format at said carrier frequency above 20MHz includes, converting said signals in said digital form with user information and signalling information on separate paths into digital signals in said predetermined time division duplex format before modulation onto said carrier frequency above 20MHz, converting digital signals in said predetermined time division duplex format after demodulation from said carrier frequency above 20MHz into said signals in said digital form with user information and signalling information on separate paths, and generating a time division duplex burst timing signal; modulating said digital signals in said predetermined time division duplex format onto said carrier frequency above 20MHz in a transmission path under control of said burst timing signal; conveying signals from said transmission path to said single path in said predetermined time division duplex format under control of said burst timing signal, conveying telecommunications signals from said single path in said predetermined time division duplex format to a reception path under control of said burst timing signals, and demodulating digital signals from said carrier frequency above 20MHz in said predetermined time division duplex format in said reception path under control of said burst timing signal.

10 A method according to any one of claims 1 to 9, wherein said predetermined time division duplex format and said carrier frequency above 20MHz are in accordance with the digital cordless telephony CT2 standard

1 1. A telecommunications system, in which system, telecommunications subscribers' apparatus is located at each of a plurality of low voltage a c mams electricity supply consumers' premises said subscribers' apparatus being coupled to the mains supply by a signal coupler SC, a telecommunications station for communicating with said subscribers' apparatus at said plurality of consumers' premises is located in the electricity supply distribution network, and wherein the subscribers' apparatus includes; subscribers' terminal equipment adapted for transmission of telecommunications signals in full duplex form between that terminal equipment and a telecommunications base station, and a subscribers' interface unit for conversion of telecommunications signals between said full duplex form and time division duplex form for coupling to said mains supply cable by said signal coupler.

12. A system according to claim 11 wherein the signal coupler is located at each said consumers' premises for coupling signals between the subscribers' apparatus and a cable carrying the mains supply and said signals to the premises, and for providing electrical isolation between the subscribers' apparatus and the voltage of the mains supply.

13. A system according to claims 1 1 or 12 wherein the signal coupler couples telecommunications signals at a carrier frequency of greater than approximately 1 MHz.

14. A telecommunications system according to any one of claims 1 1-13, wherein said base station is located at a local substation 55 supplying said low voltage mains supply to said plurality of consumers' premises, and wherein said telecommunications signals are communicated between said base station and at least most of said consumers' premises without amplification or regeneration.

15. The subscribers' interface unit of the telecommunications system according to any one of claims 11 -14, wherein said interface unit includes; first conversion mea ns for conversion of telecommunications signals between said full duplex form and a time division duplex form at a carrier frequency above 20MHz; and second conversion means for conversion of telecommunications signals between said time division duplex form at said carrier frequency above 20MHz and the same time division duplex form at said carrier frequency in the range 1 MHz to 20MHz.

16. A subscribers' interface unit according to claim 15, wherein said first conversion means is adapted for conversion of telecommunications signals between said full duplex form as analogue signals and said time division duplex form at said carrier frequency above 20MHz.

17. A subscribers' interface unit according to claims 15 or 16, wherein said first conversion means includes; signal separation interface means for conversion between telecommunications signals in said duplex form and comprising user information and signalling information on a single path and telecommunications signals in digital form and comprising said user information and said signalling information on separate paths; and time division duplex interface means for conversion between telecommunications signals in said digital form with user information and signalling information on separate paths and telecommunications signals on a single path in said time division duplex form having a predetermined digital format at said carrier frequency above 20MHz.

18. A subscribers' interface unit according to claim 17 wherein said time division duplex interface means includes; formatting/deformatting means for converting said telecommunications signals in said digital form with user information and signalling information on separate paths into digital signals in said predetermined time division duplex format before modulation onto said carrier frequency above 20MHz, for converting digital signals in said predetermined time division duplex format after demodulation from said carrier frequency above 20MHz into said telecommunications signals in said digital form with user information and signalling information on separate paths, and for generating a time division duplex burst timing signal; a transmission path having means controlled by said burst timing signal for modulating digital signals provided by said formatting/deformatting means in said predetermined time division duplex format onto said carrier frequency above 20MHz; a reception path having means controlled by said burst timing signal for demodulating digital signals from said carrier frequency above 20MHz and providing demodulated digital signals to said formatting/deformatting means in said predetermined time division duplex format; and a switch controlled by said burst timing signal for conveying telecommunications signals from said modulating transmission path to said single path in said predetermined time division duplex format and for conveying telecommunications signals from said single path in said predetermined time division duplex format to said demodulating reception path.

19. A subscribers' interface unit according to claim 17 or 18, wherein said predetermined time division duplex format and said carrier frequency above 20MHz are in accordance with the digital cordless telephony CT2 standard.

20. A subscribers' interface unit according to any one of claims

15 to 19, wherein said second conversion means includes; a frequency converter including one mixing means and one local oscillator means which serve for frequency down conversion and for frequency up conversion in alternate time division duplex burst periods; one amplifier means having an amplifier input and an amplifier output; and switching means responsive to a or respectively the time division duplex burst timing signal generated within and derived from said first conversion means, such that for frequency down conversion by said frequency converter the amplifier input is coupled to said mixing means and the amplifier output is coupled to said signal coupler, and such that for frequency up conversion by said frequency converter the amplifier. input is coupled to said signal coupler and the amplifier output is coupled to said mixing means.

21. A subscribers' interface unit according to claim 20, wherein said switching means comprises a two input combiner having its output connected to the amplifier input, a two output splitter having its input connected to the amplifier output, and two switches controlled by said time division duplex burst timing signal, such that for frequency down conversion the first switch couples the mixing means to one input of the combiner and the second switch couples one output of the splitter to the signal coupler, and such that for frequency up conversion the second switch couples the signal coupler to the other input of the combiner and the first switch couples the other output of the splitter to the mixing means.