Method and device for search on free channel

The present invention relates to a free channel search method and device. It applies to the production of automated radiocommunication systems in which the search for a free channel takes place by the transmission of test patterns successively on determined frequencies.

In these systems, a dialogue between transmitting and receiving radio stations enables the receiver of the receiving station to inform the transmitter of the other station of the frequency to be used preferentially and possibly of the spare frequencies. A corresponding type of standardized exchange is for example described in standard MIL STD 188-141 A. When a communication is established, the stations remain either on the single frequency which has been selected if the dialogue takes place in simplex mode or on the pair of selected frequencies if the dialogue takes place in duplex mode. But when one or the other of the stations judges the quality of transmission insufficient, a new search for a free channel using the emergency frequencies takes place while temporarily interrupting the useful communication from a few seconds to a few tens of seconds. The system must then search for other frequencies when the quality of the backup frequencies which have been previously selected has itself deteriorated during the time of the communication. Of course, these operations take time and are detrimental to the proper routing of communications.

The aim of the invention is to overcome the aforementioned drawback.

To this end, the invention relates to a real-time search method and free channel background task allowing voice or data communications at a fixed frequency or frequency evasion between transmitting and receiving electric radio stations, characterized in that it consists in transmitting during communications the test signals of a transmission channel in silent time intervals where there is no transmission of speech or data and in maintaining a table of usable frequencies depending on its transmission quality.

The subject of the invention is also a device for implementing the method.

The main advantage of the invention is that it makes it possible, by using speech or data transmission silences, to search for free frequencies in background tasks, without disturbing the transmission and by automatically switching to better frequencies or by switching to better frequencies. advising operators that better frequencies are available. The device implemented allows dynamic and permanent refreshing of the frequencies used and in particular in frequency evasion, those of the frequency plane.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the appended figures which represent:

FIG. 1 shows the different steps of the method according to the invention in the form of a flowchart.

Figure 2 a formatting of the test sequence used.

FIGS. 3 and 4 an embodiment of a device for implementing the method according to the invention.

The method according to the invention which is described below makes it possible to use speech silences in a speech radio link to carry out propagation tests on replacement frequencies. Obviously the method applies for the transmission of data or signals other than speech. It takes place according to steps 1 to 7 of the flowchart of figure 1. Stage 1 consists in transmitting the speech to the transmitter through a delay device of determined duration T and in continuously measuring in stage 2 the amplitude of the speech signal. If the measured amplitude has not exceeded a determined threshold S during the period T, the voice signal is not transmitted and the corresponding silence is reconstituted in the receiver. Also during the period T a frequency test is carried out in step 3. This test consists in transmitting instead of the voice signal, a flag signal having characteristics known to the receiver. The flag signal is formed for example by a series of symbols of a digital frequency or phase shift modulation of the type known by the abbreviations m-FSK, m-PSK, GMSK. The flag signal can be followed as shown in Figure 2 by the reference of the frequency to be tested and an indicator to inform the receiver of the direction of the test which can be either the direction of speech transmission or the direction reverse. Otherwise, the test frequency is determined by a predetermined sequence and clocked by synchronized clocks in the transmitter and the receiver. At the end of the transmission of the flag, the transmitter and the receiver switch in step 4 to the test frequency and one of the two transmits, depending on the direction of the test, a digital or analog test signal. The receiver of the test signal then performs in step 5 a measurement of the quality of the signal by determining, for example, and in a known manner, the signal / noise ratio, the number of recognized symbols and the number and amplitude of the paths propagation received. The quality of the test frequency is then compared in step 6 to the quality of the frequency in use for speech transmission, and to those of other frequencies which have already been tested during speech silences.

The receiver then decides in step 7 either to replace automatically or by an operator in step 7a as soon as possible the frequency in use for the transmission of speech by the last frequency tested, or to classify at the step 7b, this as an emergency frequency, either to store in step 7c its quality for possible subsequent use, or finally to classify it 7d as unusable (rejection).

The device for implementing the method which is represented in FIG. 3 allows the transmission of a frequency test sequence in a transmitting station. It comprises a delay device 8 of duration T, coupled to the audio frequency input of a radio transceiver 9 via a switch 10. A vocoder 11 having a transmission delay equal to T can optionally be used. substituted for the delay device 8. A device 12 for measuring the level of the speech signal applied to the input of the delay device 8 is coupled to the input of a sequencer 13 via a device for detecting silence of speech 14. The sequencer 13 ensures the progress of steps 1 to 7 of the process. It controls a generator 15 of the flag signal, a generator 16 of the index of the frequency to be tested and a device 17 coupled to a quality table or frequency table 18 to determine the choice of the frequency to be tested and to control the transmitter. station on that frequency. A clock 19 controls the sequencer 13 and the device 17.

The device which is represented in FIG. 4 where the elements homologous to those of FIG. 2 are represented with the same references, makes it possible to carry out a frequency test in a station receiving a test sequence. It comprises a flag signal detector 20 and a frequency detector to be tested 21 both coupled to the receive output of the transceiver 9. A frequency and half-wave choice logic is coupled at the output of the frequency detector 21 to control in frequency and in alternation the transceiver 9 of the receiving station. A vocoder 23 is coupled by its input to the output of the receiver of the transceiver 9 or to the output of a silence generator 24 through a switch 25. A quality test device 26 is coupled between the receiver output of the transceiver 26 and a memory 27 for recording in a table in memory 27 the results of the quality test. These results are supplied to the frequency and half-wave choice logic 22.

The method and the devices described above can operate either in duplex mode or in half-duplex mode. In the case of duplex links, where normally only one correspondent speaks at a given time, the return direction is mainly composed of silences of sufficient duration to allow a search for a free channel. It is always the receiver of the test sequences who can compare the frequencies, choose the best frequencies and possibly decide to change the corresponding frequency. This simply takes place in the silences of speech following the flags, the flags of a link direction being followed when needed by the desired frequency value or reference with a change indicator. In order to minimize the possible disturbances, the actual change can be deferred until the next silence by being triggered by the corresponding flag.

The same method can be implemented in the half-duplex links, certain silences being assigned to the transmission of their preferred frequency by the receiver. However, this transmission can be deferred without great inconvenience until the next reversal of the direction of transmission of the speech and carried out on this occasion.

In the case of frequency evasion operation, the scrambled frequencies of the frequency plane can be replaced by free frequencies.