CALL CONTROL METHOD FOR A RADIO SYSTEM

Abstract: The invention relates to a call control method for a radio system, wherein a semi-duplex type mobile station (MS1, MS2) is commanded so as to clear a call on the radio channel of a base station (BS), by transmitting a disconnect signal to the mobile station (MS1, MS2) over said radio channel, the mobile station leaving the radio channel on receiving said disconnect signal. In order to ensure that the disconnect signal is received by the mobile station, the field intensity received by the base station (BS) on said radio channel is measured immediately after the transmission of the disconnect signal, and the radio channel is released for the next call only when the measured field intensity falls below a predetermined threshold value.

Description: Call control method for a radio system Field of the Invention The invention relates to a call control method for a radio system, wherein a semi-duplex type mobile subscriber radio station is commanded so as to clear a call on a radio channel of a base station, by transmitting a disconnect signal to the mobile sub¬ scriber radio station over said radio channel, the mobile subscriber radio station leaving the radio channel on receiving said disconnect signal. Background of the Invention A semi-duplex type transceiver transmits and receives on the same radio channel and is not able to receive while its own transmitter is in operation. This property of the semi-duplex mobile radio may result in a situation in which the transmitter of the mobile radio is in operation when the call terminates during a call disconnect signal transmitted from the mobile exchange, and does not receive the disconnect signal but remains on the traffic channel even though the mobile exchange has cleared the call and released the traffic channel. The same channel can be allocated to a new call immediately after the release, and so the mobile radio remaining on the traffic channel after a preceding call may undesir¬ ably participate in the new call or at least hear the beginning of the call. Previously, attempts have been made to solve this problem by transmitting special selective dis¬ connect messages on the traffic channel in the beginning of the call, which messages command the mobile radios not associated with the current call to leave the channel. If the transmitter of the mobile radio remaining on the channel after the preceding call is still or again in operation even when such disconnect messages are being transmitted,, the mobile radio does not receive the messages and will not leave the channel. Such disconnect messages can be transmitted continuously during the call, in which case they, however, deteriorate the call quality, causing short breaks. The object of the invention is to avoid the above-mentioned problems of the prior art. Summary of the Invention This is achieved by means of a call control method according to the invention, wherein the field intensity received by the base station on said radio channel is measured immediately after the trans¬ mission of the disconnect signal, and the radio channel is released for the next call only when the measured field intensity falls below a predetermined threshold value. After the disconnect signal has been trans¬ mitted by the mobile exchange on the traffic channel, the base station immediately measures the field intensity on the channel. If the measured value is high enough, it is assumed that the transmitter of the mobile radio is in operation, and the channel is not released for the next call. After the carrier signal of the mobile radio has disappeared from the channel, which is detected as a decrease in the measured field intensity below a predetermined threshold value, a new disconnect signal is first transmitted and the channel is released for the next call. By the procedure according to the invention, the mobile radio cannot remain on the channel when the next call begins, and so no additional disconnect messages need to be transmitted during the call. Brief Description of the Drawings In the following the invention will be de¬ scribed in greater detail by means of illustrating embodiments with reference to the attached figure, which illustrates a radio system in which the inven¬ tion is applicable. Detailed Description of the Invention The radio system shown in the figure comprises at least one fixed transceiver equipment BS, which will be referred to as a base station in this con- nection. The base station usually has several radio channels for communication purposes (pairs of trans¬ mitting and receiving frequencies), a separate trans¬ ceiver unit being assigned for each radio channel. At least one radio channel is a so-called control chan- nel for control signalling between the base station BS and mobile radios stations MS. The other radio channels are usually so-called traffic channels through which actual speech or data connections are established, i.e. the calls between a specific mobile radio and another mobile radio of the same system or e.g. a subscriber of a public switched telephone net¬ work. The base station BS is connected to a mobile exchange MX or the like, through which the calls are switched to the base station BS. The system may con- tain several mobile exchanges MX communicating with each other and e.g. with the public switched tele¬ phone network, and several base stations are usually connected to the same mobile exchange. The operation of the entire system is usually controlled in a centralized manner by means of a special system controller (not shown). One or more mobile transceivers, i.e. mobile radios MSI and MS2 roam freely within the area of the mobile radio system. The mobile radios are of the so- called semi-duplex type, i.e. they transmit and re¬ ceive alternately on one and the same traffic channel frequency. The call set-up and the selection of the base station usually take place by signalling on the control channel. This signalling may be e.g. mobile radio signalling complying with the standard MPT 1327 issued by the British Department of Trade and Industry (DTI). The invention relates to a situation in which a call already set up to a semi-duplex type mobile radio is cleared by signalling on the traffic channel assigned to the call, and the traffic channel is re¬ leased for the next call. According to the invention, this takes place as follows: Assume that the mobile radio station MSI, for instance, has set up a speech or data connection, i.e. call, on a traffic channel Chi via the base station BS to the mobile exchange MX. When the call terminates, the mobile exchange MX transmits a special call disconnect signal or message on the traffic channel Chi to the mobile radio station MSI via the base station BS Λ and when mobile radio station MSI receives the signal or message, it terminates the transmission and reception on the traffic channel Chi, thus releasing the channel for the next call. After the transmission of the dis¬ connect signal the base station BS immediately measures the received field intensity on the traffic or radio channel Chi so as to ensure that the mobile radio MSI has received the disconnect signal and left the channel. If the field intensity measured by the BS on the radio channel Chi does not exceed a pre¬ determined threshold value after the transmission of the disconnect signal, the radio channel is released immediately by the mobile exchange MX. If the field intensity measured by the base station BS exceeds the threshold value, it is assumed that the transmitter of the mobile radio station MSI is still in operation on the channel Chi, as a result of which the traffic channel Chi will not yet be released for the next call. Only when the carrier of the mobile radio MSI again disappears from the traffic channel Chi, which is detected as a decrease in the measured field intensity below said threshold value, a new dis- connect signal is first transmitted and the channel is released for the next call. In this way, it is ensured that the mobile radio MS will not remain on the traffic channel when the next call begins. Certain radio systems set an upper limit for the duration of an uninterrupted transmission of the mobile radio station MS. For instance, the un¬ interrupted duration of transmission shall not exceed 60 seconds according to the above-mentioned MTP 1343 specification. In one embodiment of the invention, the traffic channel (and the associated facilities) is assumed to be defective and is removed from opera¬ tion if the field intensity measured on the traffic channel does not fall below the set limit value during a predetermined period of time after the transmission of the disconnect signal. In the system complying with the MPT 1343 specification this period of time has to be at least about 60 seconds. The invention has been described above by way of example by means of a specific radio system, to which, however, the invention is not intended to be restricted. Essential is only that the mobile radio is of the semi-duplex type and the system clears the call by signalling on the traffic channel. The figure and the description related to it are also in other respects only intended to illustrate the present invention. In its details, the method according to the invention may vary within the scope of the attached claims.

Claims: Claims: 1. A call control method for a radio system, wherein a semi-duplex type mobile subscriber radio station (MSI, MS2) is commanded so as to clear a call on a radio channel of a base station (BS), by trans¬ mitting a disconnect signal to the mobile subscriber radio station (MSI, MS2) over said radio channel, the mobile subscriber radio station leaving the radio channel on receiving said disconnect signal, c h a r a c t e r i z e d in that the field inten¬ sity received by the base station (BS) on said radio channel is measured immediately after the trans¬ mission of the disconnect signal, and the radio channel is released for the next call only when the measured field intensity falls below a predetermined threshold value. 2. A call control method according to claim 1, c h a r a c t e r i z e d in that if the field intensity measured on the radio channel does not exceed said threshold value after the transmission of the disconnect signal, the radio channel is released immediately. 3. A call control method according to claim 1 or 2, c h a r a c t e r i z e d in that if the field intensity measured on the radio channel exceeds said threshold value immediately after the transmission of the disconnect signal, a new disconnect signal is transmitted immediately when the measured field intensity falls below said threshold value. 4. A call control method according to any of the preceding claims, c h a r a c t e r i z e d in that if the field intensity measured on the radio channel does not fall below said threshold value within a predetermined period of time after the transmission of the disconnect signal, the radio channel is assumed to be defective. 5. A call control method according to claim 4, c h a r a c t e r i z e d in that said predetermined period of time is at least about 60 seconds.

Inventors: LOEPPOENEN JUSSI, LOEPPOENEN, JUSSI
Applicants: NOKIA TELECOMMUNICATIONS OY
Application year: 1992
Main classification: H04Q7
IPC codes: H04W76, H04W84