CA2315869A1 - Method of transmitting an information signal - Google Patents

Abstract

In a communications network with a center and a number of intermediate facilities, optical signals generated at the center are transmitted to the intermediate facilities unmodulated, where they are fed to modulators (M1, M2, M3, M4) to modulate them with information signals to be transmitted. The modulated signals are then transmitted from the intermediate facilities to the center without any separate light sources being necessary in the intermediate facilities.
The unmodulated optical signals contain CW light, for example. Alternatively, they contain carrier frequencies, which are additionally used for optical heterodyne detection. The introduction of WDM permits the transmission rate to be increased both in the downstream channel and the upstream channel and enables different intermediate facilities to transmit different types of information at the same frequencies.

Description

Method of Transmitting an Information Signal This invention relates to a method of transmitting an information signal over the upstream channel of a communications network with a downstream and an upstream channel, and to a transmitting/receiving facility of the communications network.
Communications networks are designed as point-to-multipoint networks, for example. Information such as voice, data, and video is transmitted from a center over a downstream channel to a plurality of customer premises, partly via intermediate facilities. From the customer premises, information such as voice, data, and video is transmitted over an upstream channel to the center and from there, if necessary, to other customer premises or another center. Applications are, for example, telephony, Internet, video-on-demand, video telephony, cable television, mobile radio. The topology of the communications network is, for example, a hybrid fiber/coax (HFC) or hybrid-fiber/radio (HFR) network.
In HFC and HFR, transmission from the center to at least one intermediate facility and to directly connected customer premises takes place over optical lines. The intermediate facilities in HFC are called optical network units (ONUS) or broadband optical network terminations (BONTs), and those in HFR are called base stations (BSs). To each ONU or each BONT
and to each BS, a plurality of customer premises are usually connected, e.g., by coaxial cable and radio, respectively.
To transmit information from the center to the intermediate facilities, use is made of one wavelength, for example. This necessitates a light source, e.g., a laser or a laser diode, at the center. From the intermediate facilities to the center, information is transmitted over the upstream channel using a further wavelength. Each intermediate facility therefore requires a light source. Particularly if dense wavelength-division multiplexing (DWDM) is used, high wavelength stability is needed, because the individual wavelengths have a small frequency spacing. The intermediate facilities are generally located outdoors, i.e., not in a building that is kept at a moderate temperature. Hence, they are subject to temperature variations. Particularly the wavelengths of laser sources, however, are highly temperature-sensitive, e.g., 12 GHz/°C, so that the stringent requirements placed on wavelength stability frequently cannot be met. In addition, because of the large number of intermediate facilities in a communications network, a correspondingly large number of laser sources are needed. Laser sources. are comparatively expensive components, so that the implementation of a communications network is very costly.
Approaches that have been followed to solve this problem include removing the laser sources from the w CA 02315869 2000-08-14 intermediate facilities in order to save costs and eliminate the temperature sensitivity.
EP 0 461 380 discloses a radiocommunications system in which an intermediate facility is equipped with a reflective optical modulator that modulates an unmodulated carrier frequency, which is additionally transmitted by the center, with the information signal to be transmitted, thus transmitting information over the downstream channel to the center without having a separate light source. For the transmission of information from the center to the intermediate facilities, use is made of a spectrum of carrier frequencies that are transmitted at a single wavelength. One carrier frequency is transmitted unmodulated. The other carrier frequencies are modulated with the respective information signals. At the receiving end, the output frequency for the radio signals is generated by mixing the modulated carrier frequencies with the unmodulated carrier frequency.
Various intermediate facilities, such as base stations, which must transmit different information on the same frequency, cannot be served directly by this method.
Each corresponding intermediate facility would have to be supplied over a separate optical line, which is costly.
In EP 0 809 372, a part of a modulated wavelength transmitted by the center is fed in an intermediate 3p facility to a modulator in which individual time slots are modulated in a TDM mode with the information signal to be transmitted, which is encoded in a subsequent CDMA encoder and then transmitted over the upstream channel to the center (TDM = time-division multiplexing, CDMA = code division multiple access).

' CA 02315869 2000-08-14 The invention provides three coherent solutions which permit temperature-independent operation of a communications network.
The method according to claim 1 is characterized in that in the downstream channel, optical signals are transmitted using wavelength-division multiplexing, the optical signals consisting of pairs of optical signals to permit optical heterodyne detection, each pair containing a modulated and an unmodulated optical signal, that a portion of the unmodulated optical signal of a given pair is extracted from the optical signals received over the downstream channel, and that the extracted optical signal is modulated with the information signal to be transmitted, and sent out over the upstream channel. In this method, individual wavelengths are utilized doubly. An unmodulated optical signal of a given wavelength, on the one hand, is mixed with a given modulated optical signal to generate the desired transmit frequency (for use in HFR), and, on the other hand, is modulated with an information signal to be transmitted. This represents an optimum utilization of existing wavelength capacities and allows more information signals to be transmitted with unchanged transmission capacity, particularly if optical heterodyne detection is used.
The transmitting/receiving facility for carrying out this method, which is claimed in claim 2, is characterized in that a receiving unit is provided for receiving optical signals transmitted in the downstream channel using wavelength-division multiplexing and containing at least one modulated and at least one unmodulated optical signal, and for extracting a given ' CA 02315869 2000-08-14 unmodulated optical signal from the optical signals, that an extraction unit is provided for extracting a portion of the extracted optical signal and for feeding said portion to a modulator, and that the modulator is adapted to modulate the extracted optical signal with an information signal to be transmitted, and to transmit the modulated signal over the upstream channel of the communications network.
The.method according to claim 3 is characterized in that in the downstream channel, optical signals are transmitted using wavelength-division multiplexing, the optical signals containing at least one modulated and at least one unmodulated optical signal, that. the unmodulated signal is transmitted as Cw light, that a given unmodulated optical signal is extracted from the optical signals received over the downstream channel, and that the extracted optical signal is modulated with the information signal to be transmitted, and subsequently sent out over the upstream channel. This method features the exclusive reservation of individual wavelengths for the transmission of CW light. Through the use of CW light, arbitrary types of modulation of the information signal to be transmitted can be carried out in the upstream channel. For instance, time division multiple access (TDMA), CDMA, or frequency division multiple access (FDMA) can be used. The price to be paid for this advantage is that individual wavelengths on which no information can be transmitted from the center to the intermediate facilities have to be reserved. Furthermore, if CW light is used, no synchronization problems will arise.
The method according to claim 4 is characterized in that in the downstream channel, optical signals are ~

transmitted using wavelength-division multiplexing, the optical signals containing at least one modulated and at least two unmodulated optical signals on different wavelengths, that at least two given unmodulated optical signals are extracted from the optical signals received over the downstream channel, and that the extracted optical signals are each modulated with an information signal to be transmitted, and then combined onto the upstream channel. The key feature of this method is that for the first time, wavelength-division multiplexing is performed in an intermediate facility.
The introduction of wavelength-division multiplexing allows more information to be transmitted over the same optical line. The network can be extended simply by inserting modulators and multiplexers in the intermediate facilities instead of providing new intermediate facilities, a new network topology, etc.
This particularly saves manufacturing, installation, and maintenance costs.
A transmitting/receiving facility for carrying out this method, which is claimed in claim 5, is characterized in that that a receiving unit is provided for receiving optical signals transmitted in the downstream channel using wavelength-division multiplexing, the optical signals containing at least one modulated and at least two unmodulated optical signals on different wavelengths, and for extracting at least two given unmodulated optical signals from the optical signals, that at least two modulators are provided for modulating the two extracted optical signals with a respective information signal to be transmitted, and that a multiplexer is provided for combining the modulated signals onto the upstream channel of the communications network using wavelength-division multiplexing.
The method according to claim 5, in particular, can be combined with either of the methods claimed in claims 1 and 3, as will be apparent from the following description.
Three embodiments of the invention will now be explained- with reference to the accompanying drawings, in which:
Fig. 1 is a schematic block diagram of a first communications network in accordance with the invention;
Fig. 2 is a schematic block diagram o.f a second communications network in accordance with. the invention; and Fig. 3 is a schematic block diagram of a third communications network in accordance with the invention.
The first embodiment will now be explained with reference to Fig. 1. Fig. 1 shows a first communications network in accordance with the invention. The communications network is designed as a point-to-multipoint network comprising a center that is connected by optical lines to a number of intermediate facilities, of which one is shown: For the sake of clarity, the plurality of customer premises that are connected to the intermediate facilities or directly to the center are not shown. The communications network is designed as an HFC or HFR network, for example.

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Alternatively, the communications network may consist of optical lines between the center and the intermediate facilities and of copper lines between the intermediate facilities and the customer premises, over which the information signals are transmitted in, e.g., the ADSL format or the like (ADSL = Asynchronous Digital Subscriber Line). The communications network may also consist of hybrids of the above network structures. For the implementation of the downstream channel and the upstream channel, two separate optical lines, as shown, or a common optical line can be used.
In the simplest case, the communications network can also be a point-to-point connection between two transmitting/receiving facilities.
Information signals are transmitted from the center to the intermediate facilities using dense wavelength-division multiplexing (DWDM). By assigning four wavelengths to one intermediate facility, a base station, for example, can be supplied over the downstream channel with the fourfold amount of data.
For each 90° sector of a base station, one wavelength is used. In DWDM networks, a channel spacing of 50, 100, 200 GHz is possible. High wavelength stability is necessary. Laser sources exhibit high temperature sensitivity: about 12 GHz/°C, as opposed to other DWDM
components, such as optical filters, about 700 MHz/°C, and arrayed waveguides (AWGs), about 1.5 GHz/°C.
Outside of buildings, temperatures may vary between -40°C and +85°C. Then, particularly with a 50-GHz channel spacing, wavelength stability is no longer ensured. Therefore, provision is made to move the laser sources from the intermediate facilities to the center, which is located inside a building and is not subject to the above temperature variations.
The center contains eight optical transmitters TX1, TX2, TX3, TX4, TX5, TX6, TX7, TX8, a multiplexer 10, a demultiplexer 12, and four optical receivers RX5, RX6, RX7, RX8.
The intermediate facility shown contains a receiving unit 11, four optical receivers RX1, RX2, RX3, RX4, four optical modulators M1, M2, M3, M4, and a multiplexer 13.
Optical transmitter TX1 contains a light source, e.g., a laser or a laser diode. The light source is operated in the CW mode, i.e., it emits CW light. This light is modulated directly or indirectly with the information signal to be transmitted, the indirect modulation being effected by an electro-optic modulator. The modulated output signal is applied to multiplexer 10. Optical transmitter TX1 generates modulated light of a first wavelength ~,1.
Optical transmitters TX2, TX3, TX4 operate on the same principle as optical transmitters TX1. They also generate an optically modulated signal that is applied to multiplexer 10. The difference is that firstly, information signals with other contents are used.
Secondly, each optical transmitter TX1, TX2, TX3, TX4 generates light of a different wavelength. Optical transmitter TX2 generates light of wavelength ~,2.
Optical transmitter TX3 generates light of wavelength ~,3. Optical transmitter TX4 generates light of wavelength ~,4.
Optical transmitter TX5 contains a light source, e.g., a laser or a laser diode. The light source is operated in the CW mode, i.e., it emits CW light. This light is transmitted directly to multiplexer 10. The generated light is not modulated. It is transmitted over the downstream channel to the intermediate facilities, modulated there with an information signal to be transmitted, and then transmitted over the upstream channel back to the center, where the transmitted information signals are evaluated.
Optical transmitters TX6, TX7, TX8 operate on the same principle as transmitter TX5. They also generate an unmodulated optical signal that is applied to multiplexer 10. Each of the optical transmitters TX1, TX2, TX3, TX4, TXS, TX6, TX7, TX8 generates light of a different wavelength.
Multiplexer 10 is designed as a wavelength-division multiplexer, for example. It contains, for example, DWDM components, such as optical splitters, optical combiners, or the like.
Receiving unit 11 receives the optical signals transmitted by the center over the downstream channel.
Receiving unit 11 contains, for example, a wavelength-division demultiplexer including DWDM components for separating the received optical signals into different-wavelength signals. At the output of receiving unit 11~, eight different signals with eight different wavelengths, for example, are then available. Four of these signals are applied to the four optical receivers RX1, RX2, RX3, RX4. In each receiver RX1, RX2, RX3, RX4, the received signals are demodulated, so that the transmitted information signals can be detected and analyzed or further processed. Alternatively, only a conversion of the received signals is performed, e.g., in order to transmit them in the correct format to customer premises by radio. The four signals at the output of receiving unit 11, with wavelengths ~,5, ~,6, ~ ~~~ ~,g~ are unmodulated. They are applied to modulators M1, M2, M3, M4. Modulators M1, M2, M3, M4 are electro-optic modulators that modulate the signals of wavelengths ~,5, ~,6, ~.7, ~,8 with respective information signals to be transmitted. The signals to be transmitted originate, for example, from customer premises that are connected to the intermediate facility. The information signals contain, for example, voice, data, and/or video for telephony, Internet, E-mail, video telephony, etc. Modulation is effected using TDMA, CDMA, or FDMA. The signals so modulated are applied to multiplexer 13. Multiplexer 13 is designed as a wavelength-division multiplexer, for example. It contains DWDM components, for example, such as optical splitters, optical combiners, optical couplers, or the like. The combined optical signals are then transmitted simultaneously over the upstream channel to the center.
Demultiplexer 12, constructed from DWDM components, receives the optical signals of wavelengths ~,5, ~,6, ~,7, ~,8, separates them into four signals with one wavelength each, and feeds these four signals to the four optical receivers RX5, RX6, RX7, RX8. In each optical receiver RX5, RX6, RX7, RX8, the received optical signal is demodulated and then fed to a processing device for analysis. Alternatively, only a conversion is performed in order to retransmit a corresponding information signal directly over the downstream channel. The center then serves as a relay station or an exchange, e.g. in the case of a telephone connection between two customer premises connected to the center.
The second embodiment will now be explained with reference to Fig. 2. Fig. 2 shows a second communications network in accordance with the invention. The communications network serves to optically transmit millimeter-wave signals using wavelength-division multiplexing. The carrier frequencies are in the range of 20 GHz to 60 GHz, for example. Applications are particularly in mobile radio systems with optical feeders to base stations that serve cells with a radius of 10 m to 2 km. This embodiment uses optical heterodyne detection, employing heterodyne sources. The heterodyne sources are light sources that generate two carrier frequencies, one of which is transmitted unmodulated, and the other is transmitted after being modulated with the information signal to be transmitted. At the receiving end, the desired output frequency for the transmission of radio signals containing the signal to be transmitted is generated by mixing the two carrier frequencies. At least two unmodulated carrier frequencies and at least two carrier frequencies modulated with information signals are transmitted on at least two different wavelengths. In this way, different intermediate facilities, such as base stations, can transmit different information on the same frequencies. A first pair of carrier frequencies consisting of an unmodulated carrier and a carrier modulated with a ~

first information signal, whose frequency separation is equal to that of a second pair of carrier frequencies consisting of an unmodulated carrier and a carrier modulated with a second information signal, is transmitted at a first wavelength, while the second pair is transmitted at a second wavelength.
Alternatively, each carrier frequency can be transmitted at a separate wavelength, so that four wavelengths are needed to transmit the four carrier frequencies. It is also possible to transmit two different information signals using three carrier frequencies that are transmitted at at least two different wavelengths. To accomplish this, one carrier frequency is transmitted unmodulated and serves as a reference, and the other two carrier frequencies are modulated with the information signals and are separated from the unmodulated carrier frequency by the same distance. Alternatively, two or more unmodulated reference carrier frequencies can be used at one wavelength in order to be able to generate identical output frequencies for different information signals in different intermediate facilities. To do this, in an ascending frequency sequence, for example, one reference frequency, three modulated carrier frequencies, one reference frequency, three modulated carrier frequencies, etc. are transmitted.
The communications network is designed as a point-to-multipoint HFC or HFR network, for example. It comprises a center from which information is transmitted over a downstream channel to at least one intermediate facility, of which only which one is shown for simplicity. The link between the center and the intermediate facilities consists of optical lines, such as glass optical fibers. An intermediate facility is designed as a base station or ONU, for example. The base station serves a cell of a mobile radio system, and the ONU serves a number of customer premises in which a set-top box, for example, is used as a communications unit. Transmission from the intermediate facilities to the center takes place over the upstream channel, which consists of optical lines. The optical lines may correspond to those for the downstream channel, in which case they are operated bidirectionally. Alternatively, different types of unidirectional lines can be used.
The center contains four optical transmitters TX10, TX20, TX30, TX40, a multiplexer 14, a demultiplexer 16, and four optical receivers RX50, RX60, RX70, RX80.
The intermediate facility shown contains a receiving unit 15, four extraction units 18, 19, 20, 21, four coupling units 22, 23, 24, 25, four optical receivers RX10, RX20, RX30, RX40, four electro-optic modulators M10, M20, M30, M40, and a multiplexer 17. As a minimum configuration of an intermediate facility, which is designed only for unidirectional operation, for example, only the receiving unit 15, one coupling unit, e.g., 22, and one optical receiver, e.g., RX10, are necessary. Another intermediate facility could then contain, for example, receiving unit 15, coupling unit 23, and optical receiver RX20. To permit bidirectional operation, it suffices to insert one extraction unit, e.g., 18 or 19, and one electro-optic modulator, e.g., M10 or M20.
Optical transmitter TX10 contains two light sources, such as lasers or laser diodes, for generating two different wavelengths X10 and X20. On wavelength X10, an unmodulated carrier frequency is transmitted, and on wavelength X20, a carrier frequency modulated with an information signal is transmitted. Wavelengths X10 and X20 are applied to multiplexer 14. The multiplexer is designed as a wavelength-division multiplexes to transmit all wavelengths over a common optical line.
Optical transmitters TX20, TX30, TX40 operate on the 10 same principle as optical transmitter TX10 except that other information signals are transmitted.
All optical transmitters TX10, TX20, TX30, TX40 can be upgraded to transmit more than one carrier frequency on one wavelength. Then, two or more wavelengths are transmitted from the center to the intermediate facilities, with each wavelength transmitting one or more carrier frequencies. In this way, the capacity of the optical lines is utilized in an optimum manner. A
defined assignment of the carrier frequencies to intermediate facilities must then be provided. Each intermediate facility then includes, for example, filters, e.g., bandpass filters, for selecting the carrier frequencies assigned to it.
Receiving unit 15 contains a demultiplexer that separates the received optical signals according to wavelengths. Wavelengths X10 and X20 are applied through extraction unit 18 and coupling unit 22 to optical receiver RX10. Wavelengths X30 and X40 are applied through extraction unit l9 and coupling unit 23 to optical receiver RX20. Wavelengths X50 and X60 are applied through extraction unit 20 and coupling unit 24 to optical receiver RX30. Wavelengths X70 and X80 are applied through extraction unit 21 and coupling unit 25 to optical receiver RX40.
All extraction units 18, 19, 20, 21 are designed as asymmetrical optical couplers, for example: They extract a portion of the received optical signal of a given wavelength and feed it to one of electro-optic modulators M10, M20, M30, M40. The extracted portion, which amounts to, e.g., 30~ of the received signal and contains an unmodulated carrier frequency, is used to transmit an information signal over the upstream channel to the center. The unmodulated carrier frequency is modulated in the respective electro-optic modulator M10, M20, M30, M40 and then applied to multiplexes 17. Multiplexes 17 is designed as a wavelength-division multiplexes to combine the signals to be transmitted onto a single optical line. If necessary, multiplexes 17 is followed by an optical amplifier.
All coupling units 22, 23, 24, 25 are designed, for example, as optical couplers to combine the received optical signals that belong together into one. The outputs of the couplers are fed to the associated optical receivers RX10, RX20, RX30, RX40, in which processing or retransmission of the signal as, e.g., a radio signal takes place.
An extraction unit with associated coupling unit can also be designed as a single component by using an 3~ asymmetric, wavelength-dependent optical coupler with two input ports and two output ports.
The optical signals transmitted over the upstream channel are separated according to wavelengths at the center in demultiplexer 16, and each wavelength is applied to an optical receiver RX50, RX60, RX70, RX80 for evaluation.
Multiplexers 14 and 17, receiving unit 15, and demultiplexer 16 may also be constructed from arrayed waveguides (AWGs). This reduces the manufacturing costs.
The third embodiment will now be explained with reference to Fig. 3. Fig. 3 shows a third communications network in accordance with the invention. The communications network is designed as a point-to-multipoint network comprising a center that is connected by optical lines to a number of intermediate facilities, of which one is shown. For the sake of clarity, the plurality of customer premises that are connected to the intermediate facilities or directly to the center are not shown. The communications network is designed as an HFC or HFR network, for example.
Alternatively, the communications network may consist of optical lines between the center and the intermediate facilities and of copper lines between the intermediate facilities and the customer premises, over which the information signals are transmitted in, e.g., ADSL format or the like. The communications network may also consist of hybrids of the above network structures. For the implementation of the downstream channel and the upstream channel, two separate optical 00 lines, as shown, or a common optical line can be used.
In the simplest case, the communications network can also be a point-to-point connection between two transmitting/receiving facilities.

Information signals are transmitted from the center to the intermediate facilities using dense wavelength-division multiplexing (DWDM). By assigning four wavelengths to one intermediate facility, a base station, for example, can be supplied over the downstream channel with the fourfold amount of data.
For each 90° sector of a base station, one wavelength is used. In DWDM networks, a channel spacing of 50, 100, 200 GHz is possible. High wavelength stability is necessary. Laser sources exhibit high temperature sensitivity: about 12 GHz/°C, as opposed to other DWDM
components, such as optical filters, about 700 MHz/°C, and arrayed waveguides (AWGs), about 1.5 GHz/°C.
Outside of buildings, temperatures may vary between -40°C and +85°C. Then, particularly with a 50-GHz channel spacing, wavelength stability is no longer ensured. Therefore, according to the invention, the laser sources are moved from the intermediate facilities to the center, which is located inside a building and is not subject to the above temperature variations.
The center contains four optical transmitters TX100, TX200, TX300, TX400, a multiplexer 28, a demultiplexer 26, and four optical receivers RX500, RX600, RX700, RX800.
The intermediate facility shown contains a receiving unit 29, four optical receivers RX100, RX200, RX300, RX400, four extraction units 30, 31, 32, 33, four optical modulators M100, M200, M300, M400, and a multiplexer 27.

Optical transmitter TX100 contains a light source, e.g., a laser or a laser diode. The light source is operated in a CW mode, i.e_, it emits CW light. This light is modulated directly or indirectly with the information signal to be transmitted, the indirect modulation being effected by an electro-optic modulator. The modulated output signal is applied to multiplexes 28. Optical transmitter TX100 generates modulated light of a first wavelength x,100. The modulation technique used is TDM, for example. Not all existing time slots are used for transmitting information from the center to the intermediate facilities. The time slots not used for transmission are used to transmit information from the intermediate facilities to the center without employing optical transmitters in the intermediate facilities. In the assigned time slots, the intermediate facilities can use TDMA, FDMA, or CDMA. Instead of TDM, frequency-division multiplexing (FDM) or code-division multiplexing (CDM) can be used as the modulation technique. In FDM, part of the available frequencies are used for the transmission of information from the center to the intermediate facilities, and the remainder are used for the transmission of information from the intermediate facilities to the center. In CDM, part of the available codes are used for the transmission of information from the center to intermediate facilities, and the remainder are used for the transmission of information from the intermediate facilities to the center.
Optical transmitters TX200, TX300, TX400 operate on the same principle as optical transmitters TX100. They also generate an optically modulated signal that is applied to multiplexes 28. The difference is that firstly, information signals with other contents are used.
Secondly, each optical transmitter TX100, TX200, TX300, TX400 generates light of a different wavelength.
Optical transmitter TX200 generates light of wavelength X200. Optical transmitter TX300 generates light of wavelength X300. Optical transmitter TX400 generates light of wavelength X400.
10 - Multiplexer 28 is designed as a wavelength-division multiplexer, for example. It contains, for example, DWDM components, such as optical splitters, optical combiners, or the like.
Receiving unit 29 receives the optical signals transmitted by the center over the downstream channel.
Receiving unit 29 contains, for example, a wavelength-division demultiplexer including DWDM components for separating the received optical signals into different-wavelength signals. At the output of receiving unit 29, eight different signals with eight different wavelengths, for example, are then available. Four of these signals are applied to the four optical receivers RX100, RX200, RX300, RX400. In each receiver RX100, RX200, RX300, RX400, the received signals are demodulated, so that the transmitted information signals can be detected and analyzed or further processed. Alternatively, only a conversion of the received signals is performed, e.g., in order to transmit them in the correct format to customer premises by radio.
All extraction units 30, 31, 32, 33 are designed as asymmetrical optical couplers, for example. They extract a portion of the received optical signal of a given wavelength and feed it to one of electro-optic modulators M100, M200, M300, M400. The extracted portion, e.g., 30~ of the received signal, is used to transmit an information signal over the upstream channel to the center.
Modulators M100, M200, M300, M400 are electro-optic modulators that modulate the signals of wavelengths 7.100, x,200, x,300, 7,400 with respective information signals to be transmitted. The signals to be transmitted originate, for example, from customer premises that are connected to the intermediate facility. The information signals contain, for example, voice, data, and/or video for telephony, Internet, E-mail, video telephony, etc. Modulation is effected using TDMA, CDMA, or FDMA. The signals so modulated are applied to multiplexer 27. Multiplexer 27 is designed as a wavelength-division multiplexer, for example. It contains DWDM components, for example, such as.optical splatters, optical combiners, optical couplers, or the like. The combined optical signals are then transmitted simultaneously over the upstream channel to the center.
Demultiplexer 26, constructed from DWDM components, receives the optical signals of wavelengths x,100, x,200, x,300, x,400, separates them into four signals with one wavelength each, and feeds these four signals to the four optical receivers RX500, RX600, RX700, RX800. In each optical receiver RX500, RX600, RX700, RX800, the received optical signal is demodulated and then fed to a processing device for analysis. Alternatively, only a conversion is performed in order to retransmit a corresponding information signal directly over the downstream channel. The center then serves as a relay station or an exchange, e.g. in the case of a telephone connection between two customer premises connected to the center.

Claims (7)

1. A method of transmitting an information signal over the upstream channel of a communications network with a downstream and an upstream channel, characterized in that in the downstream channel, optical signals are transmitted using wavelength-division multiplexing, the optical signals consisting of pairs of optical signals to permit optical heterodyne detection, each pair containing a modulated and an unmodulated optical signal, that a portion of the unmodulated optical signal of a given pair is extracted from the optical signals received over the downstream channel, and that the extracted optical signal is modulated with the information signal to be transmitted, and sent out over the upstream channel.

2. A transmitting/receiving facility of a communications network with a downstream and an upstream channel, characterized in that a receiving unit (15) is provided for receiving optical signals transmitted in the downstream channel using wavelength-division multiplexing and containing at least one modulated and at least one unmodulated optical signal, and for extracting a given unmodulated optical signal from the optical signals, that an extraction unit (18, 19, 20, 21) is provided for extracting a portion of the extracted optical signal and for feeding said portion to a modulator (M10, M20, M30, M40), and that the modulator (M10, M20, M30, M40) is adapted to modulate the extracted optical signal with an information signal to be transmitted, and to transmit the modulated signal over the upstream channel of the communications network.

3. A method of transmitting an information signal over the upstream channel of a communications network with a downstream and an upstream channel, characterized in that in the downstream channel, optical signals are transmitted using wavelength-division multiplexing, the optical signals containing at least one modulated and at least one unmodulated optical signal, that the unmodulated signal is transmitted as CW light, that a given unmodulated optical signal is extracted from the optical signals received over the downstream channel, and that the extracted optical signal is modulated with the information signal to be transmitted, and subsequently sent out over the upstream channel.

4. A method of transmitting an information signal over the upstream channel of a communications network with a downstream and an upstream channel, characterized in that in the downstream channel, optical signals are transmitted using wavelength-division multiplexing, the optical signals containing at least one modulated and at least two unmodulated optical signals on different wavelengths, that at least two given unmodulated optical signals are extracted from the optical signals received over the downstream channel, and that the extracted optical signals are each modulated with an information signal to be transmitted, and then combined onto the upstream channel.

5. A transmitting/receiving facility of a communications network with a downstream and an upstream channel, characterized in that a receiving unit (11) is provided for receiving optical signals transmitted in the downstream channel using wavelength-division multiplexing, the optical signals containing at least one modulated and at least two unmodulated optical signals on different wavelengths, and for extracting at least two given unmodulated optical signals from the optical signals, that at least two modulators (M1, M2, M3) are provided for modulating the two extracted optical signals with a respective information signal to be transmitted, and that a multiplexer (13) is provided for combining the modulated signals onto the upstream channel of the communications network using wavelength-division multiplexing.

6. A transmitting/receiving facility as claimed in claim 2 or 5, characterized in that the receiving unit (11, 15) is designed as a wavelength-division multiplexer.

7. A method of transmitting information signals using optical heterodyne detection, characterized in that at least one unmodulated carrier frequency and at least two carrier frequencies modulated with information signals are transmitted on at least two different wavelengths, or that at least two unmodulated carrier frequencies and at least two carrier frequencies modulated with information signals are transmitted on at least one wavelength.