AU714603B2

AU714603B2 - Hybrid radio and optical communication system

Info

Publication number: AU714603B2
Application number: AU58352/96A
Authority: AU
Inventors: Rolf Hofstetter; Harald Schmuck; Gustav Veith
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1995-07-06
Filing date: 1996-07-05
Publication date: 2000-01-06
Anticipated expiration: 2016-07-05
Also published as: DE19524650A1; AU5835296A

Description

P/00/0iU 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE

SPECIFICATION

STANDARD

PATENT

a.

a a a Invention Title: "HYBRID RADIO ANT) OPTICAL COMMUNICATION

SYSTEM"

The following statement is a fuill description of this invention, including the best method of perfon-ming it known to us:- This invention relates to a communication system in which at least one optical receiver is connected to an optical transmitter by a fibre-optic network.

Such a communication system is known, e.g. from R. Heidemann et al, "Transmission with ultra-high bit rate for the years following 2000", Elektrisches Nachrichtenwesen (Alcatel), 3rd quarter 1994, pages 241 to 251. Figure 3, in particular, shows the schematics of two system architectures: a point-to-point system and a point-to-multipoint system (distribution system). With the point-to-point system an optical receiver is connected to an optical transmitter by an optical wave guide.

With the point-to-multipoint system, a multiple of optical receivers are connected to an optical transmitter by a fibre-optic network. Optical power dividers divide light emitted by the optical transmitter into several branches. The emitted light is the carrier for an intelligence signal.

In addition to the above described communication system, where transmission is guided right up to the subscriber, a radio system is known, e.g. from R. Heidemann et al, "RACE 2005: microwave optical duplex antenna", lEE PROCEEDINGS-J, Vol.

140, No. 6, December 1993, pages 385 to 391 wherein an optical signal is S: transmitted by an optical wave guide to a base station. The base station transmits a 2 radio signal, which can be received by mobile-radio or fixed location subscribers (Figure 1).

An optical transmitter (dual frequency optical source) shown therein in Figure 2, comprises a DFB laser, an optical modulator, a signal source and an optical filter.

The light emitted by the DFB laser is a continuous wave signal (cw-signal) and has an optical frequency v. (carrier frequency). This light is directed to an optical modulator 0000 input, said modulator being controlled by a control voltage (cw-drive signal by the signal source. The light emerging from the optical modulator has a frequency spectrum with two distinct frequency components (spectral lines) which are arranged at the distance of the determined frequency w of the control voltage symmetrically around the suppressed optical carrier frequency v i.e. the two frequency components have a frequency spacing of 2w.

The optical filter connected downstream of the output of the first optical modulator separates the two frequency components: the optical filter therefore has two outputs O/P1 and O/P2.

To be able to transmit a light modulated by an intelligence signal with such an optical transmitter, a (first) output of the optical filter is connected with an optical modulator and the other (second) output with a coupler. The optical modulator modulates the light component emitted at the first output of the optical filter with an intelligence signal. In this case it is an external modulation of the light component emerging at the first output of the optical filter. The frequency spectrum of the modulated light component therefore has only one of the two frequency components. This modulated light component, which emerges at the output of the optical modulator, and the light component emerging at the second output of the optical filter is bunched into an optical wave guide by the coupler.

Light consisting of two light components therefore propagates in the optical wave guide: one light component modulated with an intelligence signal with the first frequency and an unmodulated light component with the second frequency. This component light hits the surface of a PIN photo diode in the optical receiver, where a desired microwave signal (Ghz range) is generated by coherent overlaying of the two light components.

The result conclusion from the above is that a different optical transmitter must be used .9 for the radio system than for the communication system.

.:2o It is an object of the present invention to specify an economical communication system, where a group of subscribers are connected to the optical transmitter by cables (optical wave guides, co-axial cables) and where a further group of subscribers can receive radio signals.

According to a first aspect of the invention, there is provided a communication system adopted to distribute intelligence signals over an optical network to cover us more with wire 9999 25 bind subscribers and to one or more radio communication subscribers in which at least one o optical receiver is connected to an optical transmitter by a fibre-optic network, wherein at least one base station is also connected to the fibre-optic network, the base station and the optical receiver receiving light transmitted by the optical transmitter, wherein said light comprises two spectral components, and wherein said base station converts the light into a radio transmission signal and said optical receiver simultaneously converts said light into an electric signal for transmission to subscribers.

,hills Patent Attorneys SYDCE\99307006.9 5 November 1999 According to a second aspect of the invention there is provided a communication system adapted to distribute intelligence signals over an optical network to one or more wire line subscribers and to one or more radio communication subscribers, the system including an optical transmitter and modulation means to modulate the light output of the optical transmitter so that themodulated light output includes first and second spectral components and is suitable for a first opt-electric conversion process for transmission over one or more wire lines, and is also suitable for a second opto-electric conversion process for transmission over one or more radio links; at least one first opto-electric converter connected to the optical network to perform the first opto-electric conversion; and at least one second opto-electric converter connected to the optical network to perform the second opto-electric conversion.

woo.: 0* 9 9 0 0 0 0 9 November 1999 4 One advantage of the invention is, that only one optical transmitter is required, whose optical signals are suitable for guided transmission and for radio transmission.

In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows a principal diagram of the structure of a communication system Figure 2 shows a practical example of an optical transmitter, Figure 3 shows a diagram of the spectrum of light transmitted by the optical transmitter and a diagram of the spectrum of the electrical signal Figure 4 shows a table of possible combinations of control signals, and Figure 5 shows a further practical example of an optical transmitter Figure 1 shows a principal diagram of the structure of a communication system. An optical transmitter 1 is connected by an optic wave guide L1 to an optical power divider V1 which here, for example, has three outputs; each output is 0: connected with one end of an optical wave guide L2, L3, L4. The other end of optical wave guide L2 is connected to an optical receiver 2 and the other end of optical wave guide L3 is connected to an optical receiver 3. By means of an :ot optoelectrical converter (photo diode) each optical receiver 2, 3 converts the received light into an electric signal which is transmitted to connected subscribers 8, 9, e.g. by means of an electrical distribution network El, E2.

i The other end of optical wave guide L4 is connected with an optical power too divider V2 which, for example, has two outputs: each output is connected to a base station 4, 5 by an optical wave guide L5, L6. Each base station 4, 5 determines a radio cell, wherein, for example, fixed location radio subscriber terminals 6,7 are t: located. Each base station 4, 5 comprises an optoelectric converter (photo diode) which converts the received light into an electric signal which is directed to an antenna and transmitted as a radio signal. The optical wave guides L1, L6 and the optical power dividers V1, V2, form a fibre optic network.

Figure 2 shows a practical example of the optical transmitter 1 shown in Figure 1 This practical example is the subject matter of an older German patent application with the file number P 44 44 218.1. As an alternative to this, the optical transmitter known from R. Heidemann et al, "RACE 2005: microwave optical duplex antenna", lEE-Proceedings J, Vol. 140, No. 6, December 1993, can be used, whose design has already been described.

The optical transmitter comprises a light source 21, an optical modulator 22 with an input 25, an output 31 and a connection 29, and a control unit 23. One output 24 of light source 21 is connected to input 25 of the optical modulator 22 by means of optical wave guide 26, so that light emitted by light source 21 enters optical modulator 22. The optical wave guide L1 connects output 31 with the optical power divider V1 shown in Figure 1.

Control unit 23 has two outputs 28, 30 for electric control signals: output 28 is connected with connection 27 of light source 21, and output 30 with connection 29 of the optical modulator 22.

Usually, light source 21 is a laser module in which, e.g. a DFB laser, a temperature control circuit and a power control circuit are integrated. The DFB laser S" emits laser light which, e.g. has a wave length of 1550 nm, or to express it in another way, an optical frequency v. of approx. 200 THz. In the following, the general term "light"is used for such a laser light, irrespective of it being visible or invisible.

:A DFB laser is a monomode laser, i.e. it emits laser light which has only one wavelength ideally only one spectral line). Instead of the DFB laser, a BH-laser can be used, i.e. a multimode laser which emits laser light consisting of several, e.g. spectral lines.

It is possible to manufacture the DFB-laser, the optical wave guide 26 and the optical modulator 22 in the integrated optics technique.

The optical modulator 22 is, e.g. a Mach-Zehnder modulator, which is widely Sknown, e.g. from R.G. Walker, "High-Speed Ill-V Semiconductor Intensity Modulators", IEEE Journal of Quantum Electronics, Vol. 27. No. 3. March 1991, pages 654 to 667. The optical modulator 22 can however also be any one of the other modulators mentioned therein.

In the following it is explained how the control unit 23 controls light source 21 and optical modulator 22, so that the light emerging at its output 31 is modulated with an intelligence signal. Control unit 23 supplies a first control signal for light 6 source 21 and a second control signal for the optical modulator 22. Light source 21 is controlled by a current and the optical modulator 22 by a voltage. Therefore, control unit 23 comprises appropriate means, not shown in Figure 2, for example, a power source and a voltage source. The first control signal for light source 21 is a laser operating current (laser current) and the second control signal is a control voltage. When using a Mach-Zehnder modulator, the control voltage is made up of a bias voltage used to set an operating point, and a sinusoidal voltage. In the following, the control voltage is equated to the sinusoidal voltage without explicitly mentioning the bias voltage, for simplification purposes.

Figure 3 shows examples of two spectra: one spectrum of the optical power P opt of the light which propagates within the fibre optic network and a spectrum of the electrical power P el of the electrical signal, as it is available after conversion by a photo diode. The optical power Pop, is shown as function of the optical frequency v and electrical power Pei as function of the electrical frequency f.

The spectrum of the optical power Pop has two spectral components SA1, SA2, which are arranged symmetrically around the optical frequency v 200 THz) and which have a frequency spacing of 2w 60 GHz): one spectral component is at v and the other spectral components is at v w. An intelligence signal contained in the light (here: with ASK modulation) is indicated in the usual way.

The spectrum of electrical power Pe is given by the mixture of all spectral components SA1, SA2 of the light by a photo diode: it generates a baseband signal (0 f f i) and an intermediate frequency signal at 2w. An intelligence signal contained in the spectrum of electrical power PeI is received by optical receivers 2, 3 and by base stations 4, 5. The optical receivers 2, 3 to which subscribers are connected by an electrical cross-connection network, only use the baseband signal.

The base station 4,5 each use the intermediate frequency band signal to generate a radio signal.

So that the described optical transmitter emits light, whose two spectral components SA1, SA2 are modulated, it is necessary to design the control unit 23 (Figure 2) so that it generates two control signals.

Figure 4 shows in tabular form a selection of several combinations for the first and second control signal (columns A, B) to obtain a desired output signal (column A third control signal, also entered in this table (Column C) is explained in connection with Figure 5. Rows 0 9 of the Table show combinations of how to achieve light at output 31, which is modulated with a digital intelligence signal, and rows 10 17 of the Table show combinations of how to achieve light at output 31, which is modulated with an analog intelligence signal. An intelligence signal may be a data signal as well as a voice signal.

To achieve an intensity-modulated light (ASK; Amplitude Shift Keying,) at output 31 (row 1) the first control signal, i.e. the laser current, is a digital intelligence signal, so that light emitted from light source 21 is intensity-modulated corresponding to the intelligence contained in the laser current (ASK); light source 21 can be controlled, for example, so that the light intensity has two discreet values. The second control signal, i.e. the control voltage, is an amplitude-constant sinus signal with a determined constant frequency w The optical modulator 22 controlled therewith effects a splitting of the modulated light into two such modulated light components of different optical frequencies v. w, v. w.

9 Alternatively, it is possible (row 3) that the first control signal, i.e the laser current, is a direct current (DC) carrying no intelligence signal, so that light emitted 2.6 by the light source 21 has a constant intensity and the second control signal is a S digital intelligence signal, which is a variable-amplitude sinus signal with determined constant frequency (ASK). This means that the second control signal (sinus signal) is modulated corresponding to the digital intelligence signal to be transmitted, e.g.

switched on and off; its amplitude then has two discreet values.

,5 It is also possible to control the light source 21 by a direct current carrying an intelligence signal and the optical modulator 22 by a control voltage carrying the intelligence signal, so that light emitted at output 31 is quadrature-amplitudemodulated (row Here, there are two possibilities: both intelligence signals are equal or the intelligences signals are different. With a QAM-modulated light (row 8), its intensity reproduces one intelligence signal and its phase the other intelligence signal. It is possible to regain both intelligence signals with the help of an appropriate demodulator. Even with a combination as shown in row 17, the intelligence signals can be retained in the intensity and phase of the light.

As the Table is self-explanatory, not all combinations will be shown here. In addition to the already explained abbreviations ASK, CW and DC, the Table contains the following abbreviations: FSK, Frequency Shift Keying PSK, Phase Shift Keying QAM, Quadrature Amplitude Modulation AM, Amplitude modulation FM, Frequency modulation The optical transmitter shown in Figure 2 can emit both amplitude-modulated intelligence signals, i.e. ASK, AM, QAM and angle-modulated intelligence signals, i.e.

FSK, FM, PSK, PM.

Figure 5 shows a further practical example of an optical transmitter. This practical example is an expanded version of the first practical example shown in Figure 2. The components shown in Figure 2, namely light source 21, optical modulator 22 and control unit 23 are shown with the same arrangement and with S the same designators in Figure 5. Therefore, only the expansion components are o explained herein.

.2 Output 31 of the first optical modulator 22 is connected by an optical wave guide 32 to input 36 of an optical modulator 26.

Control unit 23 is expanded such that it has an output 33 for a third control signal. The third control signal, like the second control signal, is also a control voltage. This output 33 is connected to a connection 34 of optical modulator 26. It also applies here, that the control voltages also have a bias voltage each, although these are not mentioned explicitly in the following.

Light emerges at an output 35 of the optical modulator 26, which also has a frequency spectrum with two spectral components. This output 35 is connected to the optical wave guide L1 (Figure 1).

To explain how control unit 33 controls light source 21 and the two optical modulators 22, 26, we again refer to Figure 4. Combinations of the three control signals (columns A, B, C) which are possible for the optical transmitter shown in Figure 5 are listed in rows 2, 4, 7, 9, 14, 15 16.

As an example it is explained how a light with two frequency components, which is ASK-modulated (row 2) by a digital intelligence signal, emerges at output the first control signal, i.e. the laser current is a direct current (DC) which is not modulated by an intelligence, so that the light source 21 emits a light of constant intensity. The second control signal, i.e. the (first) control voltage, is an amplitudeconstant sinus signal with determined constant frequency W. This effects that light emerging at output 31 of the optical modulator 22 is not modulated but contains two frequency components. The third control signal, i.e. the (second) control voltage, carries an intelligence signal and is a variable-amplitude sinus signal with determined, constant frequency. Therefore, the modulator 26 effects that both frequency components of the entering light are intelligence-modulated.

The optical transmitter shown in Figure 5 may also transmit amplitudemodulated, i.e. ASK, AM as well as angle-modulated, i.e. FSK, FM, PSK, PM intelligence signals.

g* Both practical examples of the optical transmitter are based on the common So idea that by suitable control of the individual components (see Table in Figure 4) light emerges from the optical transmitter, which contains two frequency components and which is modulated with an intelligence signal. Therefore, light propagates in the optical wave guide L1, L6 which consists of two light components which are both modulated by an intelligence signal. A coherent overlaying of these two light components takes place in the optical receivers 2, 3 and the base stations 4,5 o..

S:

S

Claims

1. A communication system adapted to distribute intelligence signals over an optical network to one or more wire line subscribers and to one ore more radio communication subscribers in which at least one optical receiver is connected to an optical transmitter by a fibre-optic network, wherein at least one base station is also connected to the fibre-optic network, the base station and the optical receiver receiving light transmitted by the optical transmitter, wherein said light comprises two spectral components, and wherein said base station converts the light into a radio transmission signal and said optical receiver simultaneously converts said light into an electric signal for transmission to subscribers.

2. A communication system adapted to distribute intelligence signals over an optical network to one or more wire line subscribers and to one or more radio communication subscribers, the system including: an optical transmitter and modulation means to modulate the light output of the optical transmitter so that the modulated light output includes first and second spectral components and is suitable for a first opto-electric conversion process for transmission over one or more wire lines, and is also suitable for a second opto-electric conversion process for transmission over one or more radio links; at least one first opto-electric converter connected to the optical network to perform the first opto-clectric conversion; and o• at least one second opto-electric converser connected to the optical network to perform i the second opto-electric conversion.

3. A communication system as claimed in claim 1 or 2, wherein the optical transmitter comprises a light source controlled by an operating current, an optical modulator for modulating light from the light source and controlled by a control voltage, and a control unit which modulates the operating current for the light source or the control voltage for optical modulator with the intelligence signal, so that light emerging at an output of the optical modulator is modulated by the intelligence signal.

4. A communication system as claimed in claim 2, wherein the control unit controls the light source with a direct current carrying the intelligence signal, so that light emitted by the light source is intensity-modulated, and wherein the control unit controls the R IVUIL~ IIUWIi IC -IIIIUl CnOS ll hills Patent Attorneys SYDCE\99307006.9 5 November 1999 11 optical modulator with a control voltage of constant amplitude and constant frequency, so that light emerging at the output of the optical modulator is intensity modulated.

5. A communication system as claimed in claim 2, wherein the control unit controls the light source with a direct current carrying no intelligence signal, so that light emitted by the light source has a constant intensity, and wherein the control unit controls the optical modulator with a variable-amplitude and constant-frequency control voltage carrying the intelligence signal, so that light emerging at the output of the optical modulator is intensity-modulated.

6. A communication system as claimed in claim 2, wherein the control unit controls the light source with a direct current carrying no intelligence signal, so that light emitted by the light source has a constant intensity, and wherein the control unit controls the optical modulator with a constant-amplitude and variable-frequency control voltage carrying the intelligence signal, so that light emerging at the output of the optical modulator is angle-modulated.

7. A communication systems as claimed in claim 1, wherein the optical transmitter comprises a light source controlled by an operating current, an optical modulator for modulating light from the light source and controlled by a control voltage, and a control unit which modulates the operating current for the light source and the control voltage for the optical modulator either with a single intelligence signal or with two different intelligence signals, so that light emerging at an output of the optical modulator is modulated by one or two intelligence o signals.

8. A communication system as claimed in claim 1, wherein the optical transmitter comprises a light source controlled by an operating current, a first optical modulator for modulating light from the light source and controlled by a first control voltage, a second optical 2* 25 modulator controlled by a second control voltage and having an input for light coming from the *op first optical modulator, and a control unit which controls the light source and the two optical p modulators in such a way that light emerging at an output of the second optical modulator is modulated by an intelligence signal.

9. A communication system as claimed in claim 8, wherein the control unit controls the light source with a direct current carrying no intelligence signal, so that light emitted by the g ight source has a constant intensity, the control unit controlling the first optical modulator with Patent Attorneys SYDCE\99307006.9 5 November 1999 i i 12 the first control voltage of constant amplitude and constant frequency, so that light emerging at an output of the first optical modulator carries no intelligence, and wherein the control unit controls the second optical modulator with the second control voltage carrying the intelligence signal, so that light emerging at the output of the second optical modulator is intensity-or angle- modulated.

A communication system as claimed in claim 9, wherein the intelligence signal is an amplitude-modulated signal, so that light emerging at the output of the second optical modulator is intensity-modulated.

11. A communication system as claimed in claim 9, wherein the intelligence signal is an angle-modulated signal, so that light emerging at the output of the second optical modulator is angle-modulated.

12. A communication system as claimed in claim 9, wherein the control unit controls the light source with a direct current carrying no intelligence signal, so that light emitted by the light source has a constant intensity, the control unit controlling the first optical modulator with a variable-amplitude and constant-frequency first control voltage carrying the intelligence signal, so that light at an output of the first optical modulator is intensity-modulated, and wherein the control unit controls the second optical modulator with the second constant- amplitude and variable-frequency control voltage carrying the intelligence signal, so that light emerging at the output of the second modulator is intensity-modulated. S

13. A communication system substantially as herein described with reference to Figs. 1 5 of the accompanying drawings. Dated this 5th day of November 1999 2 ALCATEL N.V. by its attorneys Freehills Patent Attorneys ills Patent Attorneys SYDCE\99307006.9 5 November 1999