Method and device for search on free channel

p0001Method and device research on free channel.

p0002The present invention relates to a method and a search feature on free channel. It applies to the realization of automated radio communication systems in which the search for a free channel occurs through the emission of test patterns successively on specific frequencies.

p0003In these systems a dialogue between sending and receiving radio stations allows the receiver of the receiving station to inform the issuer of the other station's frequency to be used preferentially and possibly emergency frequencies. A corresponding type of standardized exchange is for example described in MIL STD 88-141 1 A. When a connection is established, the stations are either on the single frequency that was selected when the dialogue takes place in either simplex the couple selected frequencies if the dialogue is in duplex mode. But when either of the stations deems insufficient transmission quality, a new free search channel using emergency frequencies held by momentarily interrupting the useful communication from seconds to tens of seconds. The system must then search for other frequencies when the quality of emergency frequencies that were previously selected has it even worse during the time of the communication. Naturally these operations take time and are detrimental to the proper routing of communications.

p0004The object of the invention to overcome the aforementioned drawback. To this end, the invention relates to a real time search method and clear channel background allowing voice communication or data fixed frequency or frequency hopping between stations transmitting and receiving radio power, characterized in it consists in transmitting over the communications test signals to a transmission channel in silence time intervals where there is no transmission of speech or data and maintain a table of usable frequencies based on its transmission quality. The invention also relates to a device for carrying out the method.

p0005The invention mainly advantage that it allows using transmission silences speech or data to search for free frequencies in background tasks without disrupting the transmission and switching automatically to better frequencies or preventing operators that better frequencies are available. The device used allows a dynamic and permanent refresh frequencies including frequency hopping, those of the frequency plan.

p0006Other features and advantages of the invention will become apparent with the following description with reference to the accompanying figures:

p0007FIG 1 the different steps of the method according to the invention in the form of a flowchart.

p0008Figure 2 a format test sequence used.

p0009Figures 3 and 4 an embodiment of a device for carrying out the method according to the invention.

p0010The method of the invention described below allows the use in a radio link for voice, speech pauses to perform propagation tests on alternate frequencies.

p0011Of course the process is applied for transmission of data or other than speech signals. It takes place according to steps 1 to 7 of the flow chart of Figure 1. Step 1 is to transmit speech to the transmitter through a fixed-term delay device T and measure continuously in step 2 the amplitude of the voice signal. If the measured amplitude has not exceeded a predetermined threshold S during the period T, the voice signal is not transmitted and the corresponding silence is restored in the receiver. Also during the duration T of a frequency test is performed in step 3. This test consists in transmitting in place of the speech signal, a "flag" signal having known characteristics of the receiver. The "flag" is constituted for example by a series of symbols of a digital frequency modulation or phase shift type known as m-FSK signal Abbreviations, m-PSK, GMSK. The "flag" signal can be tracked in the manner shown in Figure 2 the reference frequency to be tested and an indicator to inform the sense of the test receiver which may be either the direction of speech transmission is reverse. Failing the test frequency is determined by a predetermined sequence and timed by clocks synchronized in the transmitter and receiver. After the transmission of the flag the transmitter and the receiver are switched at step 4 of the test frequency and one of the two outputs, depending on the sense of the test, a digital or analog test signal. The receiver of the test signal then performed in step 5 a measure of the signal quality by determining, for example, and in 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 being used for the transmission of speech, and other frequencies that have already been tested during speech silences.

p0012The receiver then decides at step 7 is to replace or automatically by an operator at step 7a as soon as possible the frequency in use for the transmission of speech by the last frequency tested, either to close to the step 7b it as a backup frequency, or storing in step 7c quality for future use, or finally to classify ia 7d unusable (rejection).

p0013The device for implementing the method which is represented in Figure 3 enables the emission frequency of a test sequence in a transmitting station. It comprises a delay means 8 of the duration T, coupled to the audio input of a radio transceiver 9 via a switch 1 0. A vocoder 1 1 having a transmission delay equal to T can optionally be substituted delay device 8. a measuring device 1 2 of the level of the speech signal applied to the input of the delay means 8 is coupled to the input of a sequencer 1 3 via a speech silence detection device 14. the sequencer 1 3 ensures the sequence of steps 1-7 of the process. It controls a generator 1 5 of the "flag" signal, a generator 1 6 of the index of the frequency to be tested and a device 1 7 coupled to a quality table or table 18 frequencies to determine the choice of frequency to test and control the transmitter station on that frequency. A clock 19 provides control of the sequencer 13 and the device 17.

p0014The device shown in Figure 4 in which elements corresponding to those of Figure 2 are shown with the same references, allows a test frequency in a receiving station of a test sequence. It comprises a detector of the "flag" signal 20 and a frequency detector 21 coupled to test both at the reception output of the transceiver 9. A logical choice of frequency and PTT is coupled at the output of detector frequency 21 to control frequency and duplex transceiver 9 of the receiving station. A vocoder 23 is coupled at its input the output of the transceiver receiver 9 or the output of a silence generator 24 through a switch 25. A quality testing device 26 is coupled between the output receiver of the transceiver 26 and a memory 27 for recording in a memory table 27 of the quality of test results. These results are provided to the logic frequency selection and PTT 22.

p0015The method and devices described above can be used either for duplex or half-duplex. 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 of free channel. It is always the receiver test sequences which can compare the frequencies to choose the best frequencies and possibly decide to change frequency corresponding sound. This takes place merely in the speech pauses following flags, the flags of a connecting direction being followed if necessary of the value or the desired reference frequency with a change indicator. To make minimum possible disruption, the actual change can be deferred until the next silence being triggered by the "flag" correspondent.

p0016The same process can be used in the connections of the PTT, some rests being allocated to the transmission of their preferred frequency by the receiver. However, this transmission can be postponed without great drawback to the next reversal of the direction of transmission of speech and performed on this occasion.

p0017In the case of a frequency hopping operation, the frequency of the scrambled channel plan can be replaced by free frequencies.