GB1579645A

GB1579645A - Telecontrol message transmission system

Info

Publication number: GB1579645A
Application number: GB2041977A
Authority: GB
Original assignee: Societe Anonyme de Telecommunications SAT
Current assignee: Societe Anonyme de Telecommunications SAT
Priority date: 1976-05-21
Filing date: 1977-05-16
Publication date: 1980-11-19
Also published as: DE2722420B2; FR2352356B1; DE2722420C3; DE2722420A1; FR2352356A1

Description

(54) TELECONTROL MESSAGE TRANSMISSION SYSTEM (71) We, SOCIETE ANONYME DE TELECOMMUNICATIONS, a French Company of 40 Avenue de New-York, Paris, 16 eme - France do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a telecontrol message transmission system utilizing the correlation detection method.

The correlation detection transmission method is well known and comprises transmitting a signal obtained by the addition of a random signal x(t) with same delayed by a time lapse Ti, namely x(t - Ti), and, at the receiving end, filtering the autocorrelation signal obtained by multiplication of the received signal with the latter delayed by the time lapse Tj.

Such a method, however, fails to free from sine signals originating from jammers. Such signals generate at the output of the autocorrelating circuit a continuous term of substantial power from which the term corresponding to the message cannot be discriminated.

The object of the invention is to provide a correlation detection transmission system free from the recited defect.

There is provided according to the invention a telecontrol order transmission system comprising, at the transmitting end, means for generating random signals x(t), circuit means for alternately delivering the sum and the difference of signals x(t) and x(t - Ti), said circuit means being switched at the rate of a frequency fc, and, at the receiving end, circuit means for autocorrelating the received signal with the same delayed by time lapse Ti, and a band-pass filter with a bandwidth centered about frequency fc and narrower than that of signal x(t) connected to the output of said autocorrelating means.

The output of the autocorrelating circuit is therefore an alternating signal + RX(O), with R,(O) being the correlation function of signal x(t), such a signal is very easy to discriminate from the continuous signal originating from a jammer by means of a filter of frequency fc and of narrow bandwidth.

The invention will be better understood from the following description, taken in conjunction with the annexed drawings.

In the drawings: Figure 1 shows in diagrammatic form a transmission system in accordance with an -embodiment of the invention, Figure 2 shows in detail the switching means provided at the transmitting end, and Figure. 3 shows diagrammatically a system for transmitting m telecontrol orders.

Referring to Figure 1, a so called white noise generator 1 delivers a signal x(t) with a bandwidth B centered at a frequency fO. The spectral density Sx(f) and the autocorrelation function RX(t) of s(t) are expressed by the known relations - So if S0 - (f) zf, B B = 2 if f0 2 (f) 0 + 2 Rx(t) = SoB sin x BT cos 2 #: fo# :t ' COS 2 BT Signal x(t) from generator 1 is passed through a delay line 2 to be delayed by a time Tj.

Time Tj will be chosen far greater than 1/B to facilitate the correlation detection.

The delayed signal x(t - Ti) is alternately added to, and subtracted from signal x(t) delivered by generator 1 in an adder-subtractor 3 which is switched at the rate of a signal N of frequency fc, fc being very small as compared with 1/to. The resulting signal s(t) is transmitted and then passed at the receiving end through a band-pass filter 4 with a bandwidth B centered at frequency fO.

Signal y(t) from filter 4 and same delayed by time Ti in a delay line 5 are multiplied in a mixer 6 which effects the autocorrelation operation y(t).y(t - Ti) and delivers signal z(t).

While device 3 adds x(t) and x(t - Tj) at the transmitting end, the average value of signal x(t) is E (z(t)) = E (y(t). y(t - Tis)) = Rx(O) While device 3 subtracts x(t - Ti) from x(t), the average value of signal z(t) is E (z(t)) = - Rx(O) The output of mixer 6 is therefore a squarewave signal with an amplitude of + RX(O).

The signal z(t) is passed through a band -pass filter 7 of frequency fc having a bandwidth b very narrow as compared with B. Filter 7 yields at its output a sine signal whose power in known to be 8 2 a2 RX(O) It will be understood that a jamming signal in sine wave form would not interfere with the detection of the telecontrol message. Such a jamming signal would furnish at the output of mixer 6 a continuous signal which is not capable of interfering with alternating signal z(t) and which is filtered off by filter 7.

The following values are indicated to illustrate the invention fo = 10 MH,, B = 500 kHz fc = 90 Hz, b = 12,5 Hz Conventional amplitude detector 8 and threshold circuit 9 are provided at the output of filter 7 to prevent detection of false orders.

Figure 2 shows in more detailed manner an embodiment of the delay line 2 and adder-subtracter 3. Signals x(t) and x(t - Ti) are added or subtracted at a point G of a resistor arrangement and the resulting signal s(t) is transmitted after frequency transposition is achieved. A switch I actuated by an order signal N at frequency foe passes alternately x(t - Ti) and - x(t - Ti). Switch I connects point G alternately to the two terminals of the secondary winding of a transformer having its center earthed.

It might be contemplated to use time Tj as the parameter indicative of the telecontrol message. Times Tj, however, have to be of great value, which involves technological difficulties in respect of delay lines.

It is therefore preferred to employ a switching frequency as such a parameter. This is shown in Figure 3.

The switching frequency fc is selected among m different frequencies by selecting one signal of frequency fci among m signals of respective frequencies foci, fc2, fc3 .. fc fcm fed to switch device 3, and the frequency fci is recovered at the receiving end by providing m band-pass filters 71, 72, 73 . having narrow bandwidths centered about the various frequencies foci, fc2, fc3 ...., respectively, and arranged in parallel at the output of mixer 6.

The particular filter at the output of which a signal occurs corresponds to the selected frequency which defines the telecontrol order.

WHAT WE CLAIM IS: 1. A telecontrol order transmission system comprising, at the transmitting end, means for generating random signals x(t), circuit means for alternately delivering the sum and the difference of signals x(t) and x(t - Tj), said circuit means being switched at the rate of a frequency fc, and, at the receiving end, circuit means for autocorrelating the received signal with the same delayed by time lapse Tj, and a band-pass filter with a bandwidth centered about frequency fc and narrower than that of signal x(t), connected to the output of said autocorrelating means.

2. A system according to claim 1, wherein the switching frequency fc is selected among m freqencies foci,"' fci"' fcm and, at the receiving end, m band-pass filters transmitting respective frequencies fcj are arranged in parallel at the output of the autocorrelating circuit means, the selection of a switching frequency defining the telecontrol order.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of frequency fc, fc being very small as compared with 1/to. The resulting signal s(t) is transmitted and then passed at the receiving end through a band-pass filter 4 with a bandwidth B centered at frequency fO.

Signal y(t) from filter 4 and same delayed by time Ti in a delay line 5 are multiplied in a mixer 6 which effects the autocorrelation operation y(t).y(t - Ti) and delivers signal z(t).

While device 3 adds x(t) and x(t - Tj) at the transmitting end, the average value of signal x(t) is E (z(t)) = E (y(t). y(t - Tis)) = Rx(O) While device 3 subtracts x(t - Ti) from x(t), the average value of signal z(t) is E (z(t)) = - Rx(O) The output of mixer 6 is therefore a squarewave signal with an amplitude of + RX(O).

The signal z(t) is passed through a band -pass filter 7 of frequency fc having a bandwidth b very narrow as compared with B. Filter 7 yields at its output a sine signal whose power in known to be

8 2 a2 RX(O) It will be understood that a jamming signal in sine wave form would not interfere with the detection of the telecontrol message. Such a jamming signal would furnish at the output of mixer 6 a continuous signal which is not capable of interfering with alternating signal z(t) and which is filtered off by filter 7.

The following values are indicated to illustrate the invention fo = 10 MH,, B = 500 kHz fc = 90 Hz, b = 12,5 Hz Conventional amplitude detector 8 and threshold circuit 9 are provided at the output of filter 7 to prevent detection of false orders.

Figure 2 shows in more detailed manner an embodiment of the delay line 2 and adder-subtracter 3. Signals x(t) and x(t - Ti) are added or subtracted at a point G of a resistor arrangement and the resulting signal s(t) is transmitted after frequency transposition is achieved. A switch I actuated by an order signal N at frequency foe passes alternately x(t - Ti) and - x(t - Ti). Switch I connects point G alternately to the two terminals of the secondary winding of a transformer having its center earthed.

It might be contemplated to use time Tj as the parameter indicative of the telecontrol message. Times Tj, however, have to be of great value, which involves technological difficulties in respect of delay lines.

It is therefore preferred to employ a switching frequency as such a parameter. This is shown in Figure 3.

The switching frequency fc is selected among m different frequencies by selecting one signal of frequency fci among m signals of respective frequencies foci, fc2, fc3 .. fc fcm fed to switch device 3, and the frequency fci is recovered at the receiving end by providing m band-pass filters 71, 72, 73 . having narrow bandwidths centered about the various frequencies foci, fc2, fc3 ...., respectively, and arranged in parallel at the output of mixer 6.

The particular filter at the output of which a signal occurs corresponds to the selected frequency which defines the telecontrol order.

WHAT WE CLAIM IS: 1. A telecontrol order transmission system comprising, at the transmitting end, means for generating random signals x(t), circuit means for alternately delivering the sum and the difference of signals x(t) and x(t - Tj), said circuit means being switched at the rate of a frequency fc, and, at the receiving end, circuit means for autocorrelating the received signal with the same delayed by time lapse Tj, and a band-pass filter with a bandwidth centered about frequency fc and narrower than that of signal x(t), connected to the output of said autocorrelating means.
2. A system according to claim 1, wherein the switching frequency fc is selected among m freqencies foci,"' fci"' fcm and, at the receiving end, m band-pass filters transmitting respective frequencies fcj are arranged in parallel at the output of the autocorrelating circuit means, the selection of a switching frequency defining the telecontrol order.
3. A telecontrol order transmission system substantially as hereinbefore described with

reference to and as illustrated in the accompanying drawings.