# Patent

## Title
High speed multiplexer with parallel architecture

## Abstract

The field of the invention is that of electronic or electro-optical multiplexers for the transmission of digital data at high speed. The device according to the invention makes it possible to multiplex two NRZ-type, binary-coded signals. It mainly comprises: • A generator of four primary signals; • Means of electronically selecting said primary signals controlled by the two initial signals for generating the final multiplexed signal. The selection means are complemented with a logic block for generating four logic signals, all different, dependent on the two initial signals. The main advantage of this device is to allow for the use of so-called ETDM technologies for the transmission of high-speed multiplexed signals inasmuch as the necessary logic circuits operate mainly at a frequency half that of the multiplexed signal. These electronic transmission technologies are less expensive than the so-called OTDM optical technologies.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of electronic multiplexers for the transmission of digital data at high speed.
2. Description of the Prior Art
To minimize the number of digital data transmission cables, there is an interest in multiplexing the data so as to have one and the same cable carry several digital signals. To multiplex digital data, there are various types of techniques available. One of these techniques that is commonly employed is called TDM, standing for Time-Division Multiplexing. The principle of this technique is illustrated in FIG. 1 in the case of the transmission of two signals DI1 and DI2. The two digital signals having the same rate F are time-division multiplexed by a multiplexer M which transmits a single digital signal DM at a rate 2F over a single channel L, this signal DM is then demultiplexed by a demultiplexer DEM to restore the two initial signals DO1 and DO2 to the rate F. Generally, the multiplexed signal DM is obtained by interleaving the initial signals, every other bit of the multiplexed signal corresponding, for example, to the bits of the first signal DI1, the other bits of the multiplexed signal corresponding to those of the second signal DI2.
When the multiplexed signal DM is transmitted by optical pathways, the multiplexing means are either optical means or electronic means. In the first case, the term OTDM, standing for Optical Time-Division Multiplexing, is used. In the second case, the term ETDM, standing for Electrical Time-Division Multiplexing, is used.
The ETDM technologies are less expensive than the OTDM technologies which are normally reserved for very high speed transmissions. The ETDM technologies are currently limited to 80 gigabits per second. They normally use technologies:

    
    
        For the lower rates, based on Silicon-(Bi)-CMOS, standing for Complementary Metal Oxide Semiconductor;
        For the higher rates, based on SiGe, GaAs or InP.
    
    


Currently, there is a growing need for transmissions at very high rates, greater than 80 Gbits/s, a rate that is not easily accessible to ETDM technologies.
SUMMARY OF THE INVENTION
Thus, the device according to the invention makes it possible simply to double the multiplexing capabilities of the current ETDM technologies. There can thus be obtained high rates corresponding to the current telecommunication needs using less expensive technologies. Naturally, the invention can also apply to the OTDM technologies.
More specifically, the subject of the invention is an electronic circuit for time-division multiplexing two initial NRZ-type, binary-coded signals, each bit of the signals having a duration T, the two signals being able to take respective values defining four states denoted “0, 0”, “0, 1”, “1, 0”, “1,1”, said circuit comprising at least:

    
    
        A generator of four primary signals respectively associated with these said four states:
        
            A first continuous primary signal of amplitude corresponding to the “0” binary level;
            A second continuous primary signal of amplitude corresponding to the “1” binary level;
            A third periodic primary signal comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said third signal having a duration T/2, half the duration of the bits of the initial signals;
            A fourth periodic primary signal comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said fourth signal having a duration T/2, half the duration of the bits of the initial signals, said fourth signal being phase-shifted by a duration of one bit relative to the third signal;
            Electronic selection means controlled by the two initial signals for generating a final signal from the four primary signals, said means being arranged so that, for each duration T for which the two signals define a given state, the final signal results from the selection during this duration of the one of the four primary signals that is associated with said given state.
        
        
    
    


Typically, the output signal is equal to:

    
    
        The first primary signal if the state of the initial signals is “0, 0”;
        The second primary signal if the state of the initial signals is “1, 1”;
        The third primary signal if the state of the initial signals is “1, 0”;
    
    


The fourth primary signal if the state of the initial signals is “0, 1”.
The other “primary signal-state” associations do, however, remain possible. It is then appropriate in each case for the chosen association to be taken into account in the demultiplexing operations performed on reception.
Advantageously, the electronic circuit comprises a logic block for generating four logic signals, respectively associated with the four states of the initial signals, a logic state of a logic signal being 1 when the initial signals define a state associated with said logic signal and 0 in the other cases; the selection means can comprise means for calculating the sum of the four products of the logic signals by the primary signals.
Advantageously, the signals are optical signals and the selection means are electronically-controlled optical gates.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood and other advantages will become apparent from reading the description that follows, given by way of non-limiting example, and using the appended figures in which:
FIG. 1 represents the general principle of a two-channel multiplexing/demultiplexing assembly;
FIG. 2 represents the general principle of the multiplexing circuit according to the invention;
FIG. 3 represents the temporal variations of the different signals implemented by the multiplexing circuit according to the invention;
FIG. 4 represents a variant of the multiplexing device according to the invention.



MORE DETAILED DESCRIPTION
FIG. 2 represents the general principle of the multiplexing circuit according to the invention, for multiplexing two initial NRZ (Non-Return to Zero) type, binary-coded signals DI1 and DI2. The bits that make up these signals have a temporal duration T corresponding to a rate F equal to 1/T. The multiplexing circuit mainly comprises:

    
    
        A generator G of four primary signals;
        Electronically-activated means SEL of selecting said primary signals controlled by the two initial signals DI1 and DI2 for generating the final multiplexed signal DM.
    
    


As an example, the control of the selection means is obtained via a logic block BL for generating four logic signals, all different, S0, S1, S2 and S3, dependent on the two initial signals DI1 and DI2.
FIG. 3 represents the temporal variations of the different signals necessary to the invention in the case of two signals DI1 and DI2. In this figure, the time t is on the X axis and the intensity of the various signals on the Y axis. The fine vertical lines are separated by a duration equal to a half-period T/2. The thick parts of the signals D and C represent the useful parts used to “construct” the multiplexed signal.
As illustrated in FIG. 3, the four primary signals obtained from the generator G are as follows:

    
    
        A first continuous primary signal C0, of amplitude corresponding to the “0” binary level;
        A second continuous primary signal C1, of amplitude corresponding to the “1” binary level;
        A third periodic primary signal CK comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said third signal having a duration T/2, half the duration of the bits of the initial signals;
        A fourth periodic primary signal CAK comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said fourth signal having a duration T/2, half the duration of the bits of the initial signals, said fourth signal being phase-shifted by a duration of one bit relative to the third signal.
    
    


Of course, the generation of the first two signals C0 and C1 poses no technical problem. Although the signals CK and CAK are at a frequency twice that of the initial signals, their generation also poses no technical problems inasmuch as they correspond to simple periodic signals that can be generated at high frequency.
During a period T, the pair of signals DI1 and DI2 can have only one of the following four possible states: “0, 0”, “0, 1”, “1, 0”, “1, 1”. Consequently, the multiplexed signal corresponding to these two signals necessarily corresponds to one of the four primary signals.
The object of the selection means is to select the correct primary signal corresponding to the different states of the initial signals. To this end, it is possible to use a logic block which, based on the initial signals, generates four logic signals S0, S1, S2 and S3, respectively associated with the four states of the initial signals, the logic state of a logic signal being 1 when the initial signals define a state associated with said logic signal and 0 in the other cases. These signals are represented in FIG. 3. For example, S0 is “1” when the two signals DI1 and DI2 are “0, 0” and S0 is “0” when the two signals DI1 and DI2 are “0, 1”, “1, 0” and “1, 1”. The generation of these logic signals, which is done at the frequency F, poses no technical implementation problems.
Thus, the four logic signals are representative of the state of the signals DI1 and DI2. Each of the four logic signals controls the selection of one of the four primary signals. The selected primary signal corresponds to the state of the signals DI1, and DI2. As an example, the logic operation performed is as follows:

DM=C0.S0+CK.S1+CAK.S2+C1.S3

The synchronization of the primary signals CK and CAK relative to the selection signals is essential to the correct operation of the circuit. So-called D flip-flop (D.FF) devices can be used to resolve this problem. It is, however, possible that the transitions between the succession of the primary signals forming the multiplexed signal are not perfect. In this case, they can be filtered at the output of the selection means.
It is also possible to use another arrangement. In practice, the multiplexed signal is made up of the succession of the bits BD1 of the first signal D1 followed by the bits BD2 of the second signal D2 to form the temporal succession BD1−BD2−BD1+T−BD2+T−BD1+2T . . .
The multiplexed signal can be obtained in a different way. Each bit BD2 of the second signal D2 is delayed by a temporal duration equal to the duration of one bit. The result is a delayed signal D2−T. A multiplexed signal is then produced by the succession of the bits BD2−T of the delayed signal followed by the bits BD1 of the signal D1 to form the temporal succession BD2−T−BD1−BD2−BD1+T−BD2+T . . . This second multiplexed signal is, as can be seen, identical to the first, apart from a temporal delay equal to a half-period. However, it has been generated from different primary signals which add a noise that is necessary different from that of the initial multiplexed signal. By summing these two multiplexed signals, the noise is thus reduced significantly.
As an example, the device represented in FIG. 4 makes it possible to carry out this function. It comprises:

    
    
        A generator G of the four primary signals C0, C1, CK and CAK;
        Two logic blocks BL1 and BL2;
        Two selection means SEL1 and SEL2 linked to the generator and to the logic blocks;
        Two delay circuits, the first LRT generating a first delay equal to a period T and the second LRT/2 generating a second delay equal to a half-period T/2;
        An analogue summer S.A.
    
    


Operation is as follows:
Each logic block has two inputs denoted E1 and E2. The assembly is such that, in the time-division multiplex obtained from each selection means SEL1 or SEL2, the bits corresponding to the signal received by the first input E1 of the associated logic block BL1 or BL2 precede those corresponding to the signal received by the second input E2. The signals D1 and D2 are respectively connected to the inputs E1 and E2 of the logic block BL1 and are respectively connected to the inputs E2 and E1 of the logic block BL2, the signal D2 being delayed in this case by a period T by the first delay circuit LRT.
The logic blocks BL1 and BL2 each generate four logic signals S0, S1, S2 and S3 which control the primary signals of the selectors SEL1 and SEL2.
At the output of the selectors, two multiplexed signals are obtained, DM1 and DM2, identical but phase-shifted by a half-period. The second delay circuit is used to put them in phase and the analogue summer to add them together. A multiplexed signal DM is then obtained, with reduced noise.
The circuits according to the invention can be implemented by conventional electronic means on initial electronic signals.
It is also possible to perform a multiplexing producing an optical multiplex from optical primary signals obtained by means, for example, of electro-optical modulators. In this latter case, the selection means are electronically-controlled optical gates.

## Claims

1. An electronic multiplexing circuit for time-division multiplexing two initial NRZ-type, binary-coded signals, each bit of the signals having a duration T, the two signals being able to take respective values defining four states denoted “0, 0”, “0, 1”, “1, 0”, “1, 1”, said circuit comprising:
a generator of four primary signals respectively associated with said four states:
a first continuous primary signal of amplitude corresponding to the “0” binary level;
a second continuous primary signal of amplitude corresponding to the “1” binary level;
a third periodic primary signal comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said third signal having a duration T/2, half the duration of the bits of the initial signals;
A fourth periodic primary signal comprising a succession of bits alternating between the “0” and “1” binary levels, each bit of said fourth signal having a duration T/2, half the duration of the bits of the initial signals, said fourth signal being phase-shifted by a duration of one bit relative to the third signal; and
electronic selection means for selecting said primary signals controlled by the two initial NRZ signals for generating a final multiplexed signal from the four primary signals, said means being arranged so that, for each duration T for which the two initial NRZ signals define a given state, the final multiplexed signal results from the selection during the duration T of the one of the four primary signals that is associated with said given state.

2. The electronic multiplexing circuit according to claim 1, wherein the association between the primary signals and the states of the initial signals is as follows:
first primary signal associated with the state “0, 0”;
second primary signal associated with the state “1, 1”;
third primary signal associated with the state “1, 0”;
fourth primary signal associated with the state “0, 1”.

3. The electronic multiplexing circuit according to claim 1, wherein the electronic circuit further comprises a logic block for generating four logic signals, respectively associated with the four states of the initial signals, the logic state of a logic signal being 1 when said initial signals define a state associated with said logic signal and 0 in other cases.

4. The electronic multiplexing circuit according to claim 3, wherein the selection means comprise means for calculating the sum of the four products of the logic signals by the primary signals.

5. The electronic multiplexing circuit according to claim 1, wherein the signals are optical signals and the selection means are essentially electronically-controlled optical gates.