AU606241B2 - Apparatus and method for converting a frequency division multiplex to a time division multiplex - Google Patents

Description

f Declara nt) To THE COMMISSIONER OF PATENTS.

SHELSTON WATERS, PATENT ATTORNEYS, 55 CLARENCE STREET, SYDNEY, AJSTRALIA SW100

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j 'OPI DATE 14/06/89 APPLN. ID 27154 88 4 IwoF AOJP DATE 20/07/89 PCT NUMBER PCT/GB88/01039 INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 International Publication Number: WO 89/ 05071 H04J 15/00 Al (43) International Publication Date: I June 1989 (01.06.89) (21) International Application Number: PCT/GB88/01039 (74) Agent: GREENWOOD, John, David; British Telecommunications public limited company Intellectual (22) International Filing Date: 28 November 1988 (28.11.88) Property Unit, 151 Gower Street, London WCIE 6BA (GB).

(31) Priority Application Number: 8727847 (81) Designated States AU, JP, US.

(32) Priority Date: 27 November 1987 (27.11.87) (33) Priority Country: GB Published With international search report.

(71) Applicant (for all designated States except US): BRIT- ISH TELECOMMUNICATIONS PUBLIC LIMIT- ED COMPANY [GB/GB]; 81 Newgate Street, London ECIA 7AJ (GB).

(72) Inventors; and Inventors/Applicants (for US only) SMITH, David, William [GB/GB]; Braeside, Mill Lane, Campsea Ashe, Woodbridge. Suffolk HILL, Godfrey, Richard [GB/GB]; 34 Borrowdale Avenue, Ipswich, Suffolk IP4 2TJ LOBBETT, Roy, Andrew [GB/ GB]; 112 Chelsworth Road, Cavendish Road, Felixstowe IPI 8UG (GB).

(54)Title: APPARATUS AND METHOD FOR CONVERTING A FREQUENCY DIVISION MULTIPLEX TO A TIME DIVISION MULTIPLEX 19 22 12 q f REGEN f OPT SR\ n- CHANNEL 24 23 VC 21 fn TIME MULTIPLEXED SIGNAL n STEPS

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STAIRCASE DRIVE TO VCO OR SWITCHED SYNTHESISER (57) Abstract Apparatus for converting a frequency division multiplex (fdm) signal to a time domain multiplex (tdm) signal in which a voltage controlled oscillator (21) whose output is a staircase of reference frequencies mixed by a mixer (20) with a received fdm signal. Each received frequency is output in turn at a common intermediate frequency from a bandpass filter (22) thereby forming an fdm signal after passing through a demodulator (23) and low pass filter The conversion may take place in the optical domain by mixing optical reference frequencies with an optical fdm signal in, for example, a coherent heterodyne optical receiver.

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wo89/71 1. PCT/GB88/01039 WO 89/05071 1 APPARATUS AND METHOD FOR CONVERTING A FREQUENCY DIVISION MULTIPLEX TO A TIME DIVISION MULTIPLEX The invention relates to apparatus and methods for converting a frequency division multiplex (fdm) signal into a time division multiplex (tdr) signal. It finds particular but not exclusive application in a communication system comprising a primary station and a plurality of secondary stations, each of the stations having signal transmitting means and signal receiving means, a first communication link extending from the primary station to signal splitting means for supplying signals from the primary station to each of the secondary stations, and a second communication link extending from signal combining means, which receives signals from each of the secondary stations, to the primary station.

Such communication systems find application in a variety cf fields including local telephone networks and the like. One method of oDeratinc such a communication syste: is to distribute sicnals fro: the primary szation (or exchange) as a multiple., for example a common time division multiplex signal, to all the secondary staticns.

Each station them selects the time slots appropriate to it. Although this is relatively straight forward, difficulties arise if the secondary stations are to transmit information back to the primary station by this method. Ir. this case, the signals transmitted from 0 _l L, i i :I I- i~ PCT/GB88/01039 WO 89/05071 -2the secondary stations must be accurately timed to avoid signal "collisions" in the common path back to the primary station. The timing will depend principally on the path length and, in the case of optical communication systems using optical fibres, this path length can vary slightly with temperature effects. Thus a ranging system must be built into each terminal.

These difficulties can be obviated by combining the return signals from the secondary stations into a frequency division multiplex signal instead of attempting to combine these signals into a time division multiplex signal. This avoids the problems of timing due to variations in temperature and the like.

Typically, the signals transmitted between the stations will comprise optical signals and in this case the stations may be coupled by optical waveguides such as optical fibres. However other media, including air, could define the communication links. Furthermore, the signals could comprise electrical or radio frequency signals.

In some cases, separate paths may be provided between the primary station and the secondary stations rc carry signals in respective directions. Conveniently, however, a commor tra7smission path is used, for example a cormor optical waveguide in the case of optical signals. in the latter case the signal combining and splitting means are provided by a common element. In the case of a common transmission path, the signals could be transmitted with the same or different wavelengths in the opposite directions.

The primary station receiving means may include a splitting means to split the incoming frequency division multiplex signal into a plurality of subsidiary signals, one for each secondary station, and a pluralizy of

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WO 89/05071 PCT/GB88/01039 3demodulating circuits for receiving respective ones of the subsidiary signals and for regenerating the information associated with the signal from the corresponding secondary station. However, this arrangement needs separate demodulating circuits for each channel or secondary station.

It is an object of the present invention to overcome the need for such separate demodulating circuits.

Accordingly there is provided apparatus for converting a frequency division multiplex into a time division multiplex comprising a reference frequency generator for repeatedly generating a series of reference frequencies in steps at a rate synchronised to the data rate of the frequency division multiplex, the number of reference frequencies being equal to the number of carrier frequencies in the frequency division multiplex signal; mixing means for mixing the frequency division multiplex signal and the reference frequencies to generate intermediate frequencies, the combination of respective pairs of carrier and reference frequencies generating a common intermediate frequency; a bandpass filter for passing only the common intermediate frequency from the output of the mixing means; and demodulating means for generating a data signal from the common intermediate frequency signal as a time division multiplex signal.

Typically, the reference frequency generator will comprise a voltage controlled oscillator. The series of frequencies may be generated in steps such that the sweep frequency of the full staircase of frequencies is 3o preferably substantially twice the data rate. Other sweep frequencies are also possible.

The generation of the common intermediate frequency may be carried out in the electrical domain or in the optical domain, for example using a coherent cctica! heterodyne de-ection scheme in which the local ctical t PCT/G B88/01039 I WO 89/05071 -4oscillator is stepped through a staircase of optical frequencies to be mixed with a received optical fdm signal.

According to a second aspect of the invention there is provided a method for converting a frequency division multiplex to a time division multiplex comprising repeatedly generating a series of reference frequencies in steps at a rate synchronised to the data rate of the frequency division multiplex, the number of reference frequencies being equal to the number of carrier io0 frequencies in the common frequency division multiplex signals; mixing the common frequency division multiplex signal and the reference frequencies to generate intermediate frequencies, the combination of respective pairs of carrier and reference frequencies generating a common intermediate frequency; and demodulating the common intermediate frequency to generate a data signal in the form of a time division multiplex signal.

According to a further aspect of the invention there is provided a communications system comprising a primary station; and a plurality of secondary stations, each of the stations having signal transmitting means and signal receiving means, a first communication link extending from the primary station to signal splitting means for supplying signals from the primary station to each of the secondary stations, and a second communication link extending from signal combining means, which receives signals from each of the secondary stations, to the primary station wherein the signal transmitting means of the primary station and the signal receiving means of the secondary stations are adapted to transmit and receive respectively multiplex signals, the signal transmitting P means of the secondary stations are adapted to transmit signals at respective carrier frequencies, the signal il WO 89/05071 PCT/GB88/01039 combining means combining these signals into a common frequency division multiplex signal and the receiving means of the primary station is adapted to receive the common frequency division multiplex signal, the primary s station including apparatus for converting the common frequency division multiplex signal into a time division multiplex according to the first aspect of the invention.

Examples of such an apparatus, method and communication systems in accordance with the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic block diagram of one example of a communication system; Figure 2 is a block diagram of a fdm to tdm converter according to the present invention; and Figure 3 is a schematic block diagram of an optical domain fdm to tdm converter according to the present invention.

The communication system which is shown schematically in Figure 1 comprises a primary station or exchange lhaving an optical transmitter 2 and a receiver 3. The transmitter 2 is connected to an electrical signal multiplexer 4 which receives data signals on a plurality of channels 5, these signals being destined for respective ones of n terminals or secondary stations 6. The data signals received by the signal multiplexer 4 are multiplexed onto a common electrical signal in a time division manner and the time division multiplex signal is fed to an optical transmitter 2. The transmitter 7 is modulated in accordance with the incoming electrical signal to general a time division multiplex optical signal which is fed along an optical fibre 9 to an optical fibre 1C. The optical fibre 10 extends to an optical s itier .~ral- i i WO 89/05071 PCT/GB88/01039 -6- 11 which splits the incoming signal into a number of subsidiary signals, one for each secondary station 6.

Each station 6 includes conventional demultiplexing circuitry to enable it to read the correct data contained within the time slots associated with that station.

When a secondary station 6 wishes to transmit information back to the exchange 1, it generates an optical carrier signal with a frequency (f unique to that station which is modulated with the data. The modulated signal is fed along the same optical fibre as the incoming signal to the optical splitter 11 which acts as a combiner to combine the signals from the stations 6 to form a frequency division multiplex signal which is fed back along the optical fibre 10 to an optical receiver 12 in the exchange 1. The receiver 12 converts the incoming optical signal into an electrical signal which is then fed to a fdm to tdm converter 19 according to the present invention which will be now be described with reference to Figure 2.

Figure 2 illustrates the fdm to tdm convertor 19 of a receiver shown in Figure 1. The frequency divisionmultiplex signal is received by the receiver 12, and fed to a mixer circuit 20. The other of the mixer circuit 20 is connected to a voltage controlled oscillator 21 which is driven in a staircase manner, as shown in the drawing, so as to supply a series of different frequencies at stepped intervals to the mixer circuit In practice, the transmitting circuit 2, 4 of the primary station will respond to a clock signal generated within the primary station. To assist in demodulating the time division multiplex signal, this clock signal will be transmitted also to each of the secondary stations 6.

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WO 89/05071 PCT/GB88/01039 7 Furthermore, the secondary stations 6 will use the same clock signal when modulating their respective frequencies to generate return signals and this enables the VCO 21 to be synchronised to the incoming signal.

The time period of each step in the staircase of frequencies may be selected as shown in Figure 2 selected so that the sweep frequency of the full staircase of frequencies is twice or a higher intergral of the data rate of the incoming signal and this defines the time period of each time division of the resultant time division multiplex signal. It may be necessary to adjust the phase of the individual return channels at the secondary stations 6.

The frequencies generated by the VCO 21 are chosen such that the result of mixing each of these frequencies with a respective one only of the incoming frequencies f-fn is a common intermediate frequency. The mixing of any of the other frequencies within the staircase with any of the other frequencies on the incoming signal will result in a different intermediate frequency.

A bandpass filter 22 is positioned downstream of the mixer 20 to eliminate all but the common intermediate frequency which is then fed to a demodulator 23 whoseoutput is fed to a lowpass filter 24. The output 25 from the lowpass filter 24 is an n-channel time division multiplex signal which can then be analysed in a conventional manner.

The primary station 1 may alternatively transmit frequency multiplexed signals to the secondary stations 6, 30 the stations 6 being provided with appropriate demultiplexers to enable them to select the appropriate frequency associated with the station.

i WO89/05071 PCT/GB88/01039 WO 89/05071 -8- 8 It will be appreciated that apparatus for converting a fdm to a tdm according to the present invention can be employed with communications systems other than in the exemplary system shown in Figure 1.

The present invention is also applicable to fdm to tdm conversion in the optical domain, for example in an optical heterodyne coherent receiver, in which case the mixer comprises an optical detector which mixes a number of optical reference frequencies from a variable local oscillator 24, a laser for example, with the received optical fdm signal from the secondary stations 6 as shown in Figure 3.

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Claims (9)

1. Apparatus for converting a frequency division multiplex into a time division multiplex comprising a reference frequency generator for repeatedly generating a series of reference frequencies in steps at a rate synchronised to the data rate of the frequency division multiplex, the number of reference frequencies being equal to the number of carrier frequencies in the frequency division multiplex signal; mixing means for mixing the frequency division multiplex signal and the reference frequencies to generate intermediate frequencies, the combination of respective pairs of carrier and reference frequencies generating a common intermediate frequency; a bandpass filter for passing only the common intermediate frequency from the output of the mixing means; and demodulating means for generating a data signal from the common intermediate frequency signal as a time division multiplex signal.

2. Apparatus according to either of claims 1 and 2, 2: wherein the repeat frequency of the series of frequencies generated by the reference frequency generator is substantially twice the data rate cf the incoming frequency division multiplex signal.

3. Apparatus according to either of claims 1 and 2 in which the reference frequency generator generates optical reference frequencies.

4. Apparatus system according to either of claims 1 and 2, wherein the reference frequency generator comprisos a voltage controlled oscillator.

5. A method of converting a frequency division multiplex into a time division multiplex comprising repeatedly Sgenerating a series of reference frequencies in steps at a WO 89/05071 Pcr/GB88/01039~ 10 rate synchronised to the data rate, the number ofreference frequencies being equal to the number of carrier frequencies in the common frequency division multiplex signal; mixing the common frequency division multiplex signal and the reference frequencies to generate intermediate frequencies, the combination of respective pairs of carrier and reference frequencies generating a common intermediate frequency; and demodulating the common intermediate frequency to generate a data signal in the form of a time division multiplex signal.

6. A method according to claim 5, wherein the repeat frequency of the series of frequencies is substantially twice the data rate.

7. A method according to either of claims 5 and 6 in which the reference frequency generator generates optical reference frequencies and the mixing takes place in the optical domain.

8. A communication system comprising a primary station; and a plurality of secondary stations, each of the stations having signal transmitting means and signal receiving means, a first communication link extending from the primary station to signal splitting means for supplying signals from the primary station to each of the secondary stations, and a second communicat.rn link extending from signal combining means, which receives signals from each of the secondary stations, to the primary station wherein the signal transmitting means of the primary station and the signal receiving means of the secondary stations are adapted to transmit and receive respectively multiplex signals, the signal transmitting means of the secondary stations are adapted to transmit signals at respective carrier frequencies, the signal combining means combining these signals into a common frequency division multiplex signal and the receivinc k -I WO 89/05071 PCT/GB88/01039 11 means of the primary station is adapted to receive the common frequency division multiplex signal, the primarystation including apparatus for converting the coiwuon frequency division multiplex signal into a time division multiplex as claimed in any one of claims 1 to 4.

9. A system according to claim 8, in which the communication links comprise optical waveguides. A system according to either of claims 8 and 9 wherein the signal splitting means and the signal combining means are provided by a common element, a common communication link being provided between the common element and the primary station. I 1 I I