The invention relates to a telecommunication system comprising a transmission medium simultaneously accessible to at least three terminals, each terminal comprising a send oscillator coupled to the transmission medium via send means and, means for modulating the output signal of the send means. The transmission medium is also coupled to an input of receive means in each terminal, the output of the receive means being coupled to demodulation means for obtaining a demodulated signal from an input signal to the receive means.
Such a telecommunication system is known from the conference lecture "TBOSS--A transparent optical subscriber system with full duplex multi-access provided by tunable coherent single laser transceivers", Proc. OFC., Feb. 18-22, 1991, San Diego, p. 183, paper ThN3.
For realizing telecommunication systems which make communication among a plurality of terminals possible and which require little or no switching equipment, a transmission medium can be utilized that is simultaneously accessible to all the terminals.
Examples of such a transmission medium are a coaxial cable, a radio channel and a so-called reflective optical star network. Such a transmission medium has a given number of ports. If a signal generated by a send oscillator is sent through one of these ports, this signal is distributed over all the ports of the transmission medium. A signal supplied to the transmission medium by a terminal can thus be received by all the other terminals. The send oscillator may be an electrical oscillator or an optical oscillator such as, for example, a LED or a laser.
For transporting a baseband signal through such a transmission medium, the signal from a send oscillator is amplitude, frequency or phase modulated by the baseband signal with the aid of the modulation means before the oscillator signal is fed to the transmission medium.
In order to permit more than two terminals to use the transmission medium simultaneously, measures are necessary for avoiding mutual interference. A method of achieving this is to define time slots in which, alternately, only one of the terminals supplies a signal to the network.
A disadvantage of this method is that the network is not always used as efficiently as possible. For example, if only one terminal is supplying a message to the network, the channel is not utilized during the time slots reserved to the other terminals.
In such a case it is better to make the transmission medium fully available to that single terminal. A problem is then that with the known telecommunication system a terminal receiving information from another terminal cannot receive any information from a further terminal. However, this is often desired for sending urgent messages to the receiving terminal. Neither is it possible in the known telecommunication system for a terminal to send simultaneously two or more different messages to one or more terminals.
It is an object of the invention to provide a telecommunication system as described above, in which simultaneous communication between any particular terminal and one or more other terminals is possible.
For this purpose, the invention is characterized in that an input of the aforesaid modulation means is coupled to an output of further modulation means for modulating at least one further baseband signal to be transmitted on an associated subcarrier, and an output of the aforesaid demodulation means is coupled to further demodulation means for deriving at least one further baseband signal from the demodulated signal.
By way of these measures it is possible for an additional terminal to communicate with a first and/or a second terminal by supplying a further signal to the transmission medium while concurrently modulating on a subcarrier the baseband signal intended for the first and/or second terminal.
In the demodulated signal obtained at the first and/or second terminal there are now two (or more) signals coming from different terminals. Since at least one of the signals is modulated on a subcarrier, the frequency spectrums of the signals do not overlap and the signals coming from different terminals may be separated in the receiver simply by means of filters.
It is possible for the baseband signal from one of the terminals not to be modulated on a subcarrier, but only the baseband signal(s) from the further terminal(s) to have been modulated on a different subcarrier (on different subcarriers). Alternatively, it is possible for the baseband signals from all the terminals to have been modulated on individual subcarriers.
An embodiment of the invention is characterized in that the send oscillator is tunable, the receive means comprise mixer means, the input of the receive means is coupled to a first input of the mixer means, each of the terminals comprise a receive oscillator whose output is coupled to a second input of the mixer means, and the output of the mixer means is coupled to the output of the receive means.
These measures achieve that the maximum amount of information that can be transported through the transmission medium may be much larger than the amount of information that can be supplied to the transmission medium per unit of time by a single terminal. The latter amount is restricted by the limited speed of the electronic circuits in the sending terminal.
If the bandwidth of the transmission medium is such that the transmission capacity is much larger than the amount of information that can be supplied per unit of time by a single transmitter, different carriers for different simultaneously transmitting stations can be utilized to advantage if the send oscillator is tunable. Consequently, the total amount of information transported through the transmission medium may be much larger than the amount of information transmitted by a single terminal.
Certainly with known optical reflective star networks the use of different optical carriers may be highly advantageous, because the available bandwidth of such networks may be several hundred GHz.
Heterodyne receivers are used for a receiving terminal to receive, without utilizing complex tunable optical filters, from one of a plurality of terminals transmitting at different frequencies. In these receivers the received signal is converted into an intermediate frequency signal having a much lower intermediate frequency, so that the intermediate-frequency signal may be processed with the aid of conventional electronic components. For this purpose, the signal received by a terminal is converted by mixer means to the intermediate frequency via a signal generated by the receive oscillator. In this manner an intermediate-frequency signal is obtained having a frequency equal to the difference frequency between that of the received signal and the frequency of the signal generated by the receive oscillator.
In the telecommunication system known from afore-mentioned conference lecture, the send oscillator and the receive oscillator are formed by a laser. The mixer means here consist of an optical directional coupling and a photodiode. The mixer means convert the received light signal having a very high frequency, for example, 10.sup.14 Hz, into an intermediate-frequency signal having a much lower frequency, for example 3 GHz.
A further embodiment of the invention is characterized in that the send oscillator and the receive oscillator comprise a single shared oscillator.
As has already been explained in afore-mentioned conference lecture, the use of only a single oscillator both as the send oscillator and the receive oscillator realizes a considerable saving. In order to make the use of only a single oscillator possible, the frequency of the oscillator of the first terminal must differ from the frequency of the oscillator in the second terminal by a value equal to the intermediate frequency. As a result, both in the receiver of the first terminal and the receiver of the second terminal the desired intermediate-frequency signal is available at the output of the mixer means.
If more than two terminals utilize the same pair of carriers, these two terminals may be divided into two groups; i.e., a group of terminals transmitting a carrier with a first frequency and a group of terminals transmitting a carrier with a second frequency, the difference between the first and second frequencies being equal to the intermediate frequency. In this situation all the terminals from the first group can communicate with all the terminals from the second group.
A further embodiment of the invention is characterized in that the modulation means comprise control means of the send oscillator.
In the prior-art telecommunication system the modulation means comprise an external modulator inserted between the oscillator (laser) and the send means. The use of a single laser as a send oscillator and also as a receive oscillator is further known from the conference lecture "Coherent 565 Mbit/s DPSK Bidirectional Transmission Experiment with Local Transceiver Lasers", Proceedings 15th European Conference on Optical Communication, Vol. 1, Gothenburg, Sep. 10-14, 1989, pp. 417-420, paper ThA21-7.
The external modulators that may be utilized for this purpose are rather expensive, however, and cause considerable attenuation of the signal generated by the oscillator (laser). By modulating the oscillator (laser) directly, the external modulator is no longer necessary.
When a terminal simultaneously transmits and receives, the intermediate-frequency signal coming from the terminal has both the modulation of the signal to be transmitted and the modulation of the received signal. Selection of the modulation method must take into account that the modulation of the transmitted signal should be distinguishable from the modulation of the received signal.
An example of such a modulation method is amplitude modulation with a relatively small depth of modulation. The frequency spectrum of the signal transmitted by a specific terminal then consists of a carrier having two sidebands at a distance from the carrier equal to the frequency of the subcarrier.
If this terminal receives from a further terminal a similar signal having a different carrier frequency and a different subcarrier frequency, the output of the mixer means will be an intermediate-frequency signal essentially consisting of a carrier and two pairs of sidebands. One pair of sidebands comes from the further terminal and the other pair of sidebands comes from the first terminal itself. Although there may also occur mixing products of the sidebands generated by the first and the further terminal, the magnitude of the mixing products may be restricted if the modulation depth is selected to be sufficiently small. The different signals may then be simply separated with the aid of electrical filters.
A further embodiment is characterized in that the modulation means comprise angle modulation means and in that the demodulation means comprise angle demodulation means.
It appears that when angle modulation (for example, phase or frequency modulation) is used no undesired mixing products occur, as a result of which the information transmitted by the two terminals can be separated free of distortion. This will be clarified as follows.
The light carrier signal L.sub.t generated by the laser oscillator in a terminal can be written: EQU L.sub.t =A.multidot.sin(.omega..sub.1 t+.DELTA..psi..multidot.s.sub.1 (t))(1)
In (1) .omega..sub.1 is the angular frequency of the light signal L.sub.t, .DELTA..psi. is the angle modulation constant and s.sub.1 (t) the signal to be modulated on the light signal. The received signal L.sub.r can be written: EQU L.sub.r =B.multidot.sin(.omega..sub.2 t+.DELTA..psi..multidot.s.sub.2 (t))(2)
In (2) .omega..sub.2 is the angular frequency of the signal L.sub.r and s.sub.2 (t) is the baseband signal modulated on the received signal.
In the mixer means an intermediate-frequency signal is determined by summing the two signals L.sub.t and L.sub.r and, subsequently, determining the power of this sum signal. For the sum L of the signals L.sub.t and L.sub.r the following holds: ##EQU1## The mean power .vertline.L.vertline..sup.2 of the signal L is: EQU .vertline.L.vertline..sup.2 =A.sup.2 +B.sup.2 +2 A.multidot.B.multidot.cos{(.omega..sub.0 -.omega..sub.1)t+.DELTA..psi.[s.sub.1 (t)-s.sub.2 (t)]} (4)
Since the mixer means produce an intermediate-frequency signal that is proportional to the variations in the mean power .vertline.L.vertline..sup.2 of the received signal, the following holds for this intermediate-frequency signal S: EQU S=.alpha..multidot.A.multidot.B.multidot.cos{(.omega..sub.0 -.omega..sub.1)t+.DELTA..psi.[s.sub.1 (t)-s.sub.2 (t)]} (5)
Herein .alpha. is a constant.
The intermediate-frequency signal may also be determined in a different manner, for example, by multiplying L.sub.r and L.sub.t, but this will lead to a result similar to the result of formula (5).
If the laser is phase modulated, the signals s.sub.1 and s.sub.2 are the baseband signals (modulated on a subcarrier, if available). At the output of the demodulation means a demodulated signal s.sub.u is then available after phase demodulation, which signal is proportional to the phase of the AC component in the signal .vertline.L.vertline..sup.2. For this demodulated signal it then holds: s.sub.u =C.sub.1 [(s.sub.1 (t)-s.sub.2 (t)], where C.sub.1 is a demodulation constant.
If the laser is frequency modulated, the signals s.sub.1 and s.sub.2 are equal to: ##EQU2## Herein s'.sub.1 (t) and s'.sub.2 (t) are the baseband signals (modulated on a subcarrier, if available). At the output of the demodulation means a demodulated signal s.sub.u is then available after frequency demodulation, which signal is proportional to the amplitude of a signal that is proportional to the first derivative with respect to time of the signal .vertline.L.vertline..sup.2. For this signal it then holds: s.sub.u =C.sub.2 .multidot.(s'.sub.1 (t)-s'.sub.2 (t)), wherein C.sub.2 is a demodulation constant. The relation between the modulated signal and the baseband signal with phase and frequency modulation is described, for example, in the book entitled "Communication Systems" by A. Bruce Carlson, pp. 220-253 (ISBN 0-07-009957-X).
If at least one of the signals s.sub.1 or s.sub.2 (s'.sub.1 or s'.sub.2) comprises a subcarrier-modulated baseband signal, the two baseband signals may simply be separated by means of filters. In addition, from the equations for s.sub.u it follows that only the signals s.sub.1 and s.sub.2 are present in the demodulated signal and that undesired distortion terms are completely absent.
A further embodiment of the invention is characterized in that the further modulation means comprise means for modulating at least two baseband signals on two individual subcarriers, and the further demodulator comprises means for obtaining at least two baseband signals from the demodulated signal.
These measures make it possible to realise a full duplex connection between a first and a second terminal, while facility is provided for a third terminal to send messages to the first or second terminal.