GB2350020A

GB2350020A - Radio transmission system and method

Info

Publication number: GB2350020A
Application number: GB9911107A
Authority: GB
Inventors: Giles Grange
Original assignee: Domis SA
Current assignee: Domis SA
Priority date: 1997-11-18
Filing date: 1999-05-14
Publication date: 2000-11-15
Anticipated expiration: 2019-05-14
Also published as: DE19921959B4; DE19921959A1; FR2771234A1; GB9911107D0; FR2771234B1; GB2350020B

Abstract

A radio transmission system includes: a transmission module (2) intended to sequentially transmit the same information by modulating at least two carrier frequencies F1 and F2; and a reception module (4) comprising: at least one local oscillator (6) able to oscillate successively at the carder frequencies F1 and F2, and a demodulator intended to restore transmitted information. During transmission, said local oscillator oscillates at F2 when transmission takes place at frequency F1, and oscillates at F1 when transmission takes place at F2 so as to prevent interference between the frequencies. The invention also includes a method of transmitting information by radiofrequency, wherein at least two carrier signals of frequencies F1 and F2 are modulated by the information to be transmitted and then there are transmitted successively the signal of frequency of F1 for a first predetermined period then the signal of frequency F2 for a second predetermined period.

Description

2350020 RADIO TRANSMISSION SYSTEM AND METHOD

The present invention lies in the field of radio transmissions of digital and/or analogue information and more particularly concerns a system and a method for transmitting information having a transmission module intended to sequentially transmit the same information by modulation of at least two carrier frequencies F1 and F2, a reception module of the superheterodyne type comprising at least one local oscillator able to oscillate successively at the carrier frequency F1 and at the carrier frequency F2, and a demodulator intended to restore the transmitted information.

To send information between a transmitter and a receiver, a single carrier signal is traditionally amplitude or frequency modulated by the information to be transmitted. This signal uses a more or less wide channel according to the current regulations and the volume of information to be transmitted.

A major drawback of this solution stems from the fact that the information cannot be transmitted if the channel is busy or if interference arises during the transmission.

Multi-channel transmission systems are known which systematically make a search on an available transmission channel between a plurality of channels before each transmission. These systems require the use of several transmitters and several receivers, each transmitter being adapted to a given transmission channel and being associated with a given receiver. Apart from the fact that these devices are expensive, when interference occurs during the transmission, the information is not transmitted.

European patent application EP 0 370 862 BI describes a system intended to simultaneously transmit information in two logic states through two different transmission 2 channels by modulating two distinct carrier frequencies. In this system, the information to be transmitted must be previously transformed in the transmitter in order to generate two intermediate signals each modulating one of the said carrier frequencies. This system requires the use of at least two transmitters for simultaneously transmitting the two intermediate signals and at least two receivers for receiving the transmitted signals. Because the said intermediate signals are received simultaneously, the transmission channels can be mutually interfered with during transmission because of interference between the carrier signals. In addition, the prior transformation of the information to be transmitted requires the use of an additional stage at the transmitter in order to generate the said intermediate signals. This increases the complexity and cost of this type of transmission device.

A basic object of the invention is to mitigate the drawbacks of the prior art by means of a transmission system and method which are reliable and inexpensive, providing great immunity of the transmission channels used against mutual noise liable to be generated by interference between the carrier signals.

According to the invention, during transmission, the local oscillator of the reception module oscillates successively at the frequency F2 when transmission takes place at the frequency F1 and at the frequency F1 when transmission takes place at the frequency F2, so as to avoid interference between the said frequencies F1 and F2.

Thus, in cases where the transmission frequency is equal to the frequency of the local oscillator, the spectrum received is transposed to the frequency 0 Hz, to which the reception module is insensitive.

According to an important characteristic of the invention, the transmission module successively transmits the frequency F1 for a first predetermined period T and then the 3 frequency F2 during a second predetermined period T, the said successive transmission periods T and T' can be equal and are followed by a time delay period whose duration is equal to the sum of their respective periods during which transmission is stopped.

Other characteristics and advantages of the invention will emerge from the description which follows, taken by way of non-limitative example with reference to the accompanying figures, in which:

- Figure 1 depicts a general block diagram of a transmission system according to the invention; - Figure 2 depicts schematically and partially a receiver of the superheterodyne type known in the prior art; - Figure 3 depicts schematically an interference detection unit with which the system according to the invention is equipped; Figure 4 depicts a transmission timing diagram for the transmission system of Figure Figure I depicts a transmission system including a transmission module 2 intended to sequentially send the same information by modulation of two carrier frequencies F1 and F2, a reception module 4 comprising a local oscillator 6 able to oscillate successively at the carrier frequency F1 and at the carrier frequency F2 and a demodulator 8 intended to restore the transmitted information. As can be seen in Figure 1, the transmission module 2 includes a twochannel transmitter 10 connected to a first control unit 12 intended to switch the said twochannel transmitter alternately onto the frequency F1 and then onto the frequency F2 during transmission. Thus the said two-channel transmitter 10 successively transmits the frequency F1 during a first predetermined period T and then the frequency F2 during a second predetermined 4 period T'. The two successive transmission periods T and T' can be equal and are followed by a time delay period whose duration is equal to their sum. During the time delay period, transmission is stopped. The carriers of respective frequencies F1 and F2 transmitted are shown diagrammatically by arrows in dotted lines.

The reception module 4 has a second control unit 14 connected to the local oscillator 6 and intended to switch the tatter respectively onto the frequency F2 when the transmitter 10 transmits the frequency F1 and onto the frequency F1 when the transmitter 10 transmits the frequency F2. An indicator 16 of the transmission frequency is provided on the reception unit for displaying the change in transmission frequency. This indicator 16 can also be incorporated in a sensor, not shown, independent of the reception module.

The sequential reception of the modulated carriers prevents interference between the said frequencies F1 and F2 at the reception module 4. This is because, as shown in Figure 3 illustrating a receiver of the superheterodyne type, a mixer 18 effects a transposition of the received frequencies to lower frequencies which can be used by the demodulator 8. The output signal of the said mixer 18 represents the difference F1 - Fo I (or respectively F2 - Fol) between the frequency to be received F1 (or respectively F2) and the frequency Fol of the local oscillator 6. Thus, in cases where the reception frequency and the frequency of the local oscillator 6 are equal, the output of the said mixer 18 delivers a signal of frequency 0 Hz, that is to say a continuous voltage level which cannot be used by the demodulator 8. Consequently, when the reception module 4 receives a frequency F1 (or respectively F2) whilst the local oscillator 6 is controlled so as to oscillate at the frequency F2 (or respectively Fl), the latter is insensitive to the said frequency F2 (or respectively FI). As a result, when one of the frequencies F1 or F2 is received, the other cannot be received, and vice versa. This has the effect of preventing interference between the two carrier frequencies at the reception module 4.

According to a preferred embodiment of the invention, the reception module 4 has an interference detection unit 20, depicted schematically in Figure 3, comprising a stage 22 for sampling the received carriers, connected to a comparator stage 24 intended to compare the amplitude levels of the said received carriers with an auto-adaptive threshold level previously determined according to the mean noise level, the said comparator stage 24 being connected to a decision stage 26 which actuates an alarm signal when interference is detected.

Figure 5 depicts schematically a timing diagram illustrating a method of transmitting information by radiofrequency in which at least two carrier signals of respective frequencies F1 and F2 are modulated by the information to be transmitted, and then there are successively transmitted the signal of frequency F1 for a first predetermined period T and then the signal of frequency F2 for a second predetermined period T.

As can be seen in Figure 5, the transmission is effected during a periodic cycle containing two successive identical periods during which the two carrier signals are transmitted and two identical successive periods during which transmission is interrupted.

On reception, the carrier signals received are sampled and the amplitude of the said sampled signals is compared with an auto-adaptive threshold level previously determined according to the mean level of noise so as to detect any interference. In the event of interference, an alarm signal is emitted.

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Claims

1. A radio transmission system including a transmission module intended to sequentially transmit the same information by modulating at least two carrier frequencies F1 and F2, a reception module comprising at least one local oscillator able to oscillate successively at the carrier frequency F1 and at the carrier frequency F2, and a demodulator intended to restore the transmitted information, wherein during the transmission, the said local oscillator oscillates at the frequency F2 when transmission takes place at the frequency F1 and at the frequency F1 when transmission takes place at the frequency F2 so as to prevent interference between the said frequencies F1 and F2.

2. A radio transmission system according to Claim 1, wherein the said transmission module successively transmits the frequency F1 during a first predetermined period T and then the frequency F2 during a second predetermined period T.

3. A radio transmission system according to Claim 2, wherein the two successive transmission periods T and T are equal and are followed by a time delay period whose duration is equal to the sum of the periods T and T' during which transmission is stopped.

4. A radio transmission system according to Claim 2 wherein the transmission module has a two-channel transmitter connected to a first control unit intended to switch the said two-channel transmitter alternately onto the frequency F1 and then onto the frequency F2 during transmission.

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5. A radio transmission system according to Claim 2, wherein the reception module has a second control unit intended to switch the local oscillator respectively onto the frequency F2 when the transmitter transmits the frequency F1 and onto the frequency Fl when the transmitter transmits the frequency F2.

6. A radio transmission system according to Claim 5, wherein the reception module has an interference detection unit.

7. A radio transmission system according to Claim 6, wherein the interference detection unit has a stage for sampling the received carriers, connected to a comparator stage intended to compare the amplitude levels of the said received carriers with an autoadaptive threshold level previously determined according to the mean noise level, the said comparator stage being connected to a decision stage which controls an alarm signal when interference is detected.

8. A radio transmission system according to Claim 6, wherein the said reception module has an indicator of the received frequency.

9. A radio transmission system substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of transmitting information by radiofrequency, wherein at least two carrier signals of respective frequencies F1 and F2 are modulated by the information to be 8 transmitted and then there are transmitted successively the signal of frequency F1 for a first predetermined period T and then the signal of frequency F2 for a second predetermined period T.

11. A method according to Claim 10, wherein transmission is effected during a periodic cycle containing two successive identical periods during which the two carrier signals are transmitted and two successive identical periods during which transmission is interrupted.

12. A method according to one of Claims 10 or 11, wherein, on reception, the carrier signals received are sampled and the amplitude of the said sampled signals is compared with an auto-adaptive threshold level previously determined according to the mean noise level so as to detect any interference.

13. A method according to Claim 10 or 11, wherein, in the event of interference, an alarm signal is emitted.

14. A method of transmitting information by radiofrequency, substantially as hereinbefore described with reference to the accompanying drawings.