MXPA99007044A - Method and device in a mobile telecommunications system - Google Patents

Abstract

The present invention relates to a method and an apparatus in a mobile telecommunications system for the re-establishment of a speech connection, especially one that risks being interrupted prematurely. A parameter (Q) indicating the quality of the traffic channel (TCH) on which the speech connection is implemented, is compared to a predetermined value (Q1). If the parameter (Q) is lower than said value (Q1) the speech connection (340) is parked temporarily on a dedicated control channel (DCCH) of high quality. When a traffic channel (TCH) again becomes idle (360) the speech connection is handed over (370) to this channel, which may either be associated with the base station originally serving the speech connection or another base station in the mobile telecommunications system.

Description

METHOD AND DEVICE IN A MOBILE TELECOMMUNICATION SYSTEM Technical Field The present invention relates to a method in a mobile telecommunication system for restoration of a connection, especially a risky connection that is prematurely interrupted. The invention also relates to a device for carrying out the method.

Description of the Related Art A cellular, mobile telecommunications system generally has to be able to pass a connection between different cells in the system, ie a transfer between cells, Typical situations in which a transfer between cells will have to be carried out are when a mobile subscriber moves from one cell to another, or in the case of an overload in a cell. In some cases, for example, when one or more channels are exposed to disturbances, it may be advantageous to assign a new channel to an existing connection within the same cell, i.e. to perform an intra-cell handover. If an inter or intracell transfer of a connection is needed, but a new appropriate channel to which the connection can pass can not be found, there is a risk that the connection will be interrupted. The most common reason for a connection interruption is that a mobile subscriber moves to an area where the radio coverage of the communication system is too weak or completely missing. Radio transmitters that disturb in or out of the system can also interfere with the radio channel of a connection and cause premature termination of the connection. A mobile subscriber may also have moved to an overloaded cell in which no unoccupied channels are available. If the connection can not be transferred to a channel in an adjacent cell, with which the mobile subscriber has contact, or if the mobile subscriber has no radio contact with other cells in the system, the connection is interrupted when the signal is made too weak. A connection to a mobile subscriber can also be interrupted even when the subscriber has radio contact with a cell in which unoccupied channels are available. This can happen if a system parameter in the communication system is exceeded. The timing advance parameter in a Time Division Multiple Access (TDMA) system is one of said syntax parameters. The parameter of advance of Time-lapse shows the time by which a mobile station must carry out its transmission in order that its information fits in the frame structure of the service base station, when the distance between a mobile station and the The base station with which it is communicating exceeds a certain value, the timing advance parameter is unable to compensate for the propagation delay between the mobile station and the base station, and the connection is interrupted. A threshold value maxTA, established by the operator of the mobile telecommunications system in order to mark a cell edge of a base station is another system parameter that limits the maximum distance from which a mobile station can communicate with a certain station. of base. If the distance between a mobile station and the base station with which it communicates exceeds maxTA, the connection is interrupted. Reconfiguring a cell usually implies that all traffic transmitted through the cell must cease. This is another reason for the interruption of connections. Of course, a base station can also stop serving connections, in whole or in part, on more unpredictable occasions, such as power failure or other serious problems.

The connection of a certain mobile station, which has a low priority, may have to be interrupted because a new connection having a higher priority is established in the cell in which the connection is established. If all connections in functions in the cell have a higher priority than the first connection, when the new connection is established, the first connection will have to be assigned to a traffic channel in another cell. If there is no unoccupied capacity in the other cells with which the mobile station has contact, the connection is interrupted. Usually, connections to transmit data are assigned a lower priority than speech connections in which speech is transmitted in real time. There are two principally different ways to reduce the fraction of interrupted connections in a mobile telecommunications system. It is possible to try to prevent interrupted connections, and to the extent possible, try to re-establish the connection that is at risk of being interrupted soon, or that has recently been interrupted. Preventive actions can focus on increasing radio coverage, improving quality and / or improving capacity. The radius coverage can be increased by introducing more base stations. improved antennas and / or increased output power of the base stations. the quality can be improved by refining the signal coding or increasing the channel spacing in the system. The connection capacity of the system can be increased by increasing the frequency scale of the system or by reducing the channel spacing, which of course in turn can result in reduced quality. All the preventive actions cited involve high costs for the telecommunications operator, while none of the actions will guarantee that no connection will ever be prematurely interrupted. Solutions that seek to reestablish interrupted connections, or connections that are likely to be interrupted soon, offer a relatively inexpensive complement to the aforementioned preventive actions. From the U.S. Patent Specification, A. 4 811 380, a method is known to prevent the loss of a connection in the handover. If a connection between a first base station and a mobile station is transferred to a second base station in the mobile station does not receive a handover confirmation from the second base station, the mobile station will use a signaling channel from the established channel of the second base station, to send a request for the restoration of the connection to the second base station. The second base station responds by transmitting a handover confirmation to the mobile station and the connection continues, served by the second base station. European Patent Application EP, A 690 650 describes a method in a cellular communication system in which A transfer request from a mobile station is sorted into a row until a channel is left unoccupied in the cell to which the connection must be passed. In accordance with this method, the channels are reserved in the target cell (ie, the cell to which the connection is to be transferred) for the mobile stations in the row, so that, for example, the establishment of new connections in the target cell is prevented if all unoccupied channels in this cell have already been reserved for mobile stations in a row in the surrounding cells. In this way, the risk of a connection being interrupted in connection with a handover is reduced. International Patent Application WO, A, 96/07287 describes a method in which the handover of a low quality connection in a first cell , in the case of an emergency, a second cell is forced that has a higher signal strength. When a normal handover occurs, the signal strength in the target cell must exceed the signal strength in the original cell (ie, the cell from which the handover was initiated) with a certain hysteresis value. In an emergency situation, this hysteresis value can be reduced so that the handover will occur at an earlier stage, reducing the risk of the connection being interrupted. None of the solutions mentioned above indicates the possibility of temporarily stopping a connection on a dedicated signaling channel while waiting for a traffic channel to be idle. The method of compliance with US, A, 4 81i 380 employs a control channel, but through this channel only one request is transmitted for restoration of an interrupted call. The method described is based on the mobile station indicating on the control channel that its speech connection has been interrupted. The mobile station then makes contact with the stationary system for a restoration of the connection. This restoration of an interrupted speech connection takes quite a long time, therefore, there is a risk that at least one of the connected subscribers (Subscriber A or B) will interrupt the call before the system has succeeded Reset the connection.

SUMMARY OF THE INVENTION The present invention presents a solution to the problem of interrupted speech connections in a mobile telecommunications system. In particular, the invention is intended to reduce the risk of a total interruption of a call in connection with the handover, but also seeks to reduce the fraction of speech connections that are interrupted due to other reasons. These other reasons may be, for example, a system parameter that is exceeded, low signal quality caused by disturbances and blocking due to temporary overload. An object of the present invention in this way is to minimize the fraction of speech connections completely lost in a telecommunication system. Another object of the invention is to shorten the time during which the paid load transmission between a mobile station and the stationary system is prevented in connection with the temporary interruption of a speech connection. Still another object of the invention is to inform the subscribers (subscribers A and B) connected through a speech connection that has been temporarily interrupted, that the connection will be restored in a short time to reduce the risk that any of the subscribers leave the connection, It is also an object of the invention to reduce the fraction of speech connections that have to be disconnected due to the establishment of speech connections with a higher priority. According to one embodiment of the method of the invention, a parameter indicates that a certain speech connection is very likely to be disconnected. If the parameter has a value too low this indicates that the speech connection as soon as possible can be passed on to a channel that has a higher quality than the current channel. While a better traffic channel is expected to be available for the speech connection, it is temporarily transferred (parks) to a dedicated unoccupied control channel. Unfortunately, the dedicated control channel does not offer any possibility of communicating payload information. between the connected subscribers, but at least the connection is not disconnected. As soon as a traffic channel of sufficiently high quality remains unoccupied, the connection on this channel is restored.

A method in accordance with this invention in this manner is characterized by that described in claim 1. In accordance with a preferred embodiment of the method, an indication is transmitted to the connected subscribers that the speech connection is temporarily transferred to a channel. dedicated control. This indication is intended to prevent subscribers from interrupting the connection and is only generated in the case when the information communicated through the connection is human speech in real time. One embodiment of the proposed apparatus includes a control unit that receives a set of parameters for a certain connection from the base station controller serving the connection. From the set of parameters, the control unit generates a combined parameter that describes the quality of the speech connection. This parameter is compared with a certain threshold value in the control unit, and if it is lower than the threshold value, the control unit generates a first signal, which is returned to the controller of the base station. When the first signal is received, the base station control preferably allocates a new traffic channel to the connection. If that channel is not available, the connection is transferred to a dedicated control channel in another cell, as a temporary reposition for the traffic channel in which the connection was previously established, but in which now, due to one or the other root, can no longer be maintained. The control unit detects whether an unoccupied traffic channel is found with the base station serving the connection, i.e., (1) the base station through which the speech connection was originally established if the handover does not occur. is An option and if the time has not yet come to make a handover, or (2) if the handover has been completed at the base station to which the connection has been crossed. As soon as the service base station has an unoccupied traffic channel, the control unit assigns this channel to the connection and the payment payload communication in the connection can then resume, The apparatus according to the invention has the particularities features disclosed in claim 17. The proposed invention minimizes the fraction of connections that are lost in the telecommunications system where it is used by passing connections that are at risk of being disconnected to high quality control channels before they are interrupted. the connections, Compared with the solutions of the prior art, the solution according to the invention also reduces the time during which the transmission of the payload between a mobile station and the stationary system is prevented due to the low channel quality . This is because the connection is never completely interrupted, but only stationed in a temporary channel, whose quality is greater than that of the original channel. Restoring an interrupted connection generally takes more time than transferring a connection from one channel to another. The invention also offers the possibility of informing connected subscribers through a connection that is temporarily stationed -on a dedicated control channel, that the connection will be restored in a short time. This reduces the risk of any of the subscribers actively ending the connection. The invention also results in a reduced fraction of forced connections to disconnect due to the establishment of competitive connections with a higher priority. Low priority connections can be parked on dedicated control channels while waiting for unoccupied channel capacity - and thus do not have to be disconnected. The invention also reduces the fraction of blocked connections caused by temporary overload in a mobile telecommunications system. Since some connections during the handover process are parked on dedicated unoccupied control channels, various channels are released and can be used for connection establishment.

Brief Description of the Drawings Figure 1 shows a mobile telecommunications system, known per se, in which the invention is applied; Figure 2 illustrates the use of the method of the invention in logical channels in a TDMA system; Figure 3 illustrates, in a flow graph, the method according to the invention. Figure 4 shows an embodiment of the apparatus according to the invention. The invention will now be described in greater detail by means of preferred embodiments and with reference to the accompanying drawings. The solution refers to a Global System for Mobile Communication (GSM), but is generally applicable in any mobile telecommunications system in which traffic channels and dedicated control channels are used.

Description of the Preferred Modes Figure 1 shows a mobile telecommunications system 100, known per se, for example a GSM system, which includes mobile stations MS1-MS3, base stations BS1-BS5, base station BSC controllers, MSC centers of switching mobile services and a MSC gateway or interchange network GMSC- through which the mobile telecommunications system 100 is connected to an external communication network 10 such as the Public Services Telephone Network (PSTN). A mobile MSI station communicates payload comprising human speech or data through a traffic channel to a base station BS1. An A subscriber that calls thus achieves a speech connection with a called B subscriber. A first calling subscriber Ai can communicate by means of the mobile MSI station and a first called subscriber Bi can be a subscriber in the external communication network 10, A second subscriber A2 calling in his place can be connected to the communication network 10 external and a second subscriber B2 called may be located in the mobile telecommunications system 100. Alternatively, of course, both, the calling subscriber and the called subscriber B2 can communicate through the mobile stations MSI and MS3 respectively in the mobile telecommunications system 100. In the latter case at least two traffic channels are needed - since each mobile station MSI, MS3 requires at least one separate traffic channel, when a mobile MSI station, communicating in a first traffic channel with a first station BS1 of base, moves away from this base station and closer to a second base station BS2, the mobile station MSI probably reaches a point where the signal strength of the second base station BS2 exceeds the signal strength of the base station BS2. the first BS1 base station. When this difference in signal strength exceeds a certain value of the hysteresis, the speech connection is normally transferred to the second base station BS2, that is, the mobile MSI station is assigned to a second traffic channel with the second station BS2. base-and the speech connection continues in this second traffic channel. If - when the handover starts, no unoccupied traffic channel is found with the second base station BS2, there is a risk that the speech connection to the first mobile MSI station is interrupted, since the signal quality of the the first base station will probably deteriorate when the first mobile station moves away from this base station. In accordance with the proposed method, the - le ¬ Speech connection in said situation is transferred to a dedicated unoccupied control channel. Primarily, a dedicated control channel from the established channel of the second base station BS2 is selected, since the signal strength of a channel in this established channel is probably higher than with the first base station BS1. The handover procedure proceeds while the speech connection is stationed in the control channel. As soon as a traffic channel is left unoccupied with the second base station BS2, the speech connection is assigned to this channel. In exceptional cases the speech connection can be assigned to a traffic channel with the first base station BS1 if there is u <a channel with sufficiently high quality. If, on the other hand, it is considered that a speech connection has the risk of being interrupted in a short time, when the handover is not an option, preferably a dedicated control channel is selected in the original cell for temporary parking of the connection. speaks. Figure 2 illustrated as logical channels (LCH) in a TDMA system - such as the GSM system, can be divided into dedicated channels (DCH), intended to be assigned to a single user, and common channels (CCH) that can be used simultaneously by different users. The common CCH channels in turn are divided into non-selective channels, or Broadcast BCH Channels- and Common Control CCCH Channels, the broadcast channels comprising a Frequency Correction FCCH Channel, a SCH Synchronization Channel, and a general channel for non-selective transmission, the BCCH Broadcast Correction Channel, and the common control CCCH channels comprising a Call PCH Channel, a Random Access RACH Channel, and an Access Grant Channel (AGCH). The dedicated DCH channels are traffic TCH channels and dedicated DCCH control channels. The traffic TCH channels can be divided into Half Regime Channels (HTCH) and Full Regime FRCH Channels. The dedicated DCCH control channels are in turn divided into a Fast Associated Control Channel (FACCH) for the immediate control of a traffic channel, a Slow Associated Control Channel (SACCH) for less urgent transmission of information regarding a channel of traffic, (such as measurement data) and a Dedicated Control Channel (SDCCH) Only used to signal in connection with the establishment of a connection on a traffic TCH channel. In a GSM system, the method according to the invention is preferably implemented by temporarily transferring 210 a speech connection on an original traffic TCH channel - which is at risk of being interrupted, to the Dedicated Control Solo Channel (SDCCH) corresponding to the TCH Channel of original Traffic. When a new Traffic TCH Channel is left unoccupied, the speech connection is transferred to this new TCH channel, which may belong to the same base station as the original Traffic TCH Channel (and may even be identical to the traffic TCH channel). original) or another base station. A channel with full data rate (FRCH) after parking a pass-through normally to a full data rate FRCH channel and a half-rate channel HRCH channel is normally transferred to a half-rate HRCH channel, but the inter-channel pass-through Of different kinds, of course, it is also noticeable. Ert an alternative analogue or digital mobile telecommunications system, of course it is possible, instead of parking the speech connection on a DGCH channel of arbitrary dedicated signaling, the function of which corresponds to the signaling channel SDCCH dedicated only in GSM. A flow chart of the method according to the invention is shown in Figure 3. In a first step 300, it is determined whether a speech connection has a set interruption risk or not, by comparing a quality Q Q parameter for the traffic TCH channel. of the speech connection with a predetermined Qi value. If the quality parameter Q is lower than the value i, the risk of the connection being interrupted within a short time is considered to be high, and in this case, therefore, in step 310 it is checked whether there is a unoccupied traffic channel that can be used. If an idle and appropriate traffic TCH channel exists, the speech connection is assigned to this TCH channel in step 320, before the procedure returns to the first step 300. If, on the other hand, the traffic TCH channel available for the connection speech can not be found, at step 330 it is checked if at least one unoccupied dedicated control channel is found that can be used. As soon as a channel is left unoccupied, in step 340, the speech connection is assigned to this DCCH channel. Normally, there is an appropriate DCCH dedicated control channel that can be used immediately. In step 350, subscribers A and B that are connected through the speech connection (i.e. subscriber A and B, respectively) that the connection is being parked (and thus temporarily interrupted) but that the connection it will be restored in a short time. This can be indicated by transmitting (1) a text message - SO - electronic device that in the GSM system can be transmitted using the Short Message Service (SMS), or (2) a tone message or (3) comfort noise. The electronic text message is only transmitted to subscribers in the mobile telecommunications network, while the other methods of indication are applicable to any kind of subscriber. An alternative to transmitting an electronic text message to the mobile subscriber is to allow the indication to consist of a voice message recorded in the mobile station, which is triggered by a signal from the system. All types of mentioned indications are transmitted on the dedicated control SDCCH channel and are transmitted only if the communicated payload is human speech in real time. Otherwise, the indication may corrupt the reported data. In the next step 360 it is determined whether at least one traffic channel TCH is idle or not to which the speech connection can be passed. The investigation is carried out by monitoring a parameter F. A parameter value F greater than zero means that there is an unoccupied traffic TCH channel that can be used. This traffic TCH channel may either belong to the original cell, that is, it may be a traffic TCH channel in the established channel of the base station that originally served the connection, or it may belong to the target cell, i.e. , can be a traffic TCH channel served by the base station to which the connection has been traded. In step 370 final, the speech connection is assigned to a traffic channel TCH as soon as, in step 360- it has been found that said channel is available. The method then returns to the first step 300. One embodiment of the apparatus according to the invention is illustrated in Figure 4. A control unit CU comprises a processing unit PU and a comparison unit C. The processing unit PU receives a set of parameters Pc, TA and BER from a base BSC controller, the parameters being indicative of different quality parameters for a traffic channel. A first parameter Pc in the game provides a value for the measured signal resistance, which has been measured either in the uplink traffic channel by a base station or in its downlink by a mobile station, and reported by the controller BSC base station through a base station. A second parameter TA in the game denotes a current timing advance value for the mobile station using the traffic channel and a third Bit Error Rate parameter, BER, indicates the bit error rate of the traffic channel. The PU processing unit calculates, from one or more of the Pc parameters, TA-BER in the game a combined Q parameter. The combined 0 parameter is derived in principle with the inverse of a Boolean OR combination of one or more of the parameter values Pc, TA and BER (Q = Pc + TA + l / BER). Each parameter Pc, TA and BER, therefore, must fill a respective minimum condition in order to obtain an acceptable value of the combined Q parameter. Q is proportional to the signal resistance Pc and inversely proportional to the bit error rate BER. The parameter value TA must be less than or equal to a maximum value. A high value of the combined Q parameter means that the channel quality is good. Comparison unit C compares the parameter value Q with a predetermined Oi value. If the parameter Q is equal to or greater than Qi a first signal 0 is fed to the PU processing unit. If not, the comparison unit C feeds a second signal 1 to the processing unit PU which then transmits a signal D to the base station controller BSC. The PU processing unit in the control unit CU also receives a fourth parameter p, indicating the priority of the speech connection established in the traffic channel. The PU processing unit compares the number of established speech connections that have a higher priority than the p-priority of the current speech connectionEoU , to the number of traffic channels associated with the service base station. If the number of the established speech connection that has a higher priority than the current speech connection is greater than or equal to the number of traffic channels associated with the service base station, this corresponds to the combined Q parameter that is less than the default Qi value. The signal D constitutes an order to transfer the speech connection to a dedicated unoccupied control channel as soon as one is idle (which is usually immediately). Simultanly, an indication signal i is output from the control unit CU, the signal being transmitted to the subscribers connected through a dedicated control channel and informing them that the conversion is temporarily stationed, with the proviso that the connection is a speech connection. The PU processing unit simultanly starts to register a variable F. If F is greater than or equal to zero, this means that an appropriate traffic channel is available. When F has a positive value, the control unit CU instructs the base station controller BSC to pass the speech connection to a traffic channel. It is feasible to integrate the control unit CU with a base station controller BSC, but it can also be placed separately or in connection with another unit in the mobile telecommunications system.

Claims (5)

1. - A method in a mobile telecommunications system in which the payload information is communicated in traffic channels, the control information is communicated in dedicated control channels and a speech connection between a first subscriber and a second subscriber is establishes at least one traffic channel, to re-establish the speech connection if it is at risk of being interrupted, characterized by the following steps: a parameter indicates that the speech connection has the risk of being interrupted, the speech connection is transferred from from at least one first traffic channel to a dedicated control channel if the parameter is less than a predetermined value - and the speech connection is again transferred from the dedicated control channel to at least one second traffic channel as soon as it becomes available one of said channels.

2. A method according to claim 1, characterized in that the payment load information is human speech in real time, the first and second subscribers obtain an indication that said speech connection is temporarily transferred to a control channel dedicated and that the indication is provided in a dedicated control channel.

3. A method according to claim 2, characterized in that the indication is a tone message.

4. A method according to claim 2, characterized in that the indication is comfort noise.

5. A method according to claim 2, characterized in that the subscriber or subscribers of the first and second subscribers being subscribers in the mobile telecommunications system, receive an indication in the form of an electronic text message. 6 - A method according to claim 2, characterized in that the subscriber or subscribers of the first and second subscribers being subscribers in the mobile telecommunications system, receive the indication by a special signal from the mobile telecommunications system that triggers a voice message in the mobile station. 7. A method according to claim 1, characterized in that the payload information is data, that is, other information than human speech in real time. 8. A method according to any of claims 1-7, characterized in that the parameter is based on a signal resistance of the at least one first traffic channel in which the speech connection is originally maintained. 9. A method according to claim 8, wherein the at least first traffic channel is established in the telecommunications system between a certain mobile station and at least one base station, characterized in that the signal strength is measured by the mobile station and reported to the base station. 10 - A method according to claim 8, wherein the at least one first traffic channel is established in the telecommunications system between a certain mobile station and at least one base station. 11. A method according to any of claims 1-8, wherein the at least one first traffic channel is established in the telecommunications system between a certain mobile station and at least one base station, characterized in that the parameter is based on a system parameter, which is related to the at least one first traffic channel in which the speech connection is maintained. 12. A method according to claim 11, characterized in that the system parameter is constituted by a timing advance value for the mobile station. 13. A method according to any of claims 1-8, 11 or 12, characterized in that the parameter is based on a help bit error rate at least a first traffic channel in which the connection is maintained of speech. 14. A method according to any of the claims 1-8 μ 11-13, characterized in that the parameter is based on a priority of the connection relationship and speaks to the priority of other speech connections that are served by a particular base station. 15. A method according to claim 14, characterized in that the relationship is constituted by the established speech connection number that has a higher priority than the priority of the speech connection in relation to the number of associated traffic channels. with the base station, and that the parameter is less than a predetermined value if the number of speech connections established that have a higher priority than the priority of the speech connection is greater than or equal to the number of associated traffic channels. the base station, 16.- A method according to any of the preceding claims, characterized in that said last first traffic channel is separated from at least one second traffic channel. 17, - An apparatus in a mobile telecommunications system comprising at least one base station controller, two base stations and a number of mobile stations, in which the payload information is communicated through Traffic Channels , over which control information is communicated through dedicated control channels and in which a speech connection through a first subscriber and a second subscriber is established on at least one traffic channel between a certain mobile station and a station base in order to restore the speech connection if it is at risk of being interrupted, which includes a control unit, characterized in that a base station controller serving the speech connection provides the control unit with a set of parameters which indicates characteristics of the speech connection. that the control unit generates a parameter based on the set of parameters, the parameter indicating the quality of the speech connection, that the control unit compares the value of the parameter with a predetermined threshold value - whereby the control unit generates a first signal to the base station controller if the value of the parameter is lower than the threshold value, that the base station controller when receiving the first signal passes the speech connection of the traffic channel to a dedicated control channel if there is no alternative idle traffic channel, for which the value of the parameter exceeds that of the traffic channel, that the control unit detects if there is an idle traffic channel with the base station serving the speech connection and that the control unit when the unoccupied channel is found- passes the speech connection to this channel. 18. An apparatus according to claim 17, characterized in that the control unit d comprises a processing unit that generates the parameter and a comparison unit that compares the value of the parameter with the threshold value output to a result of the comparison to the processing unit. 19. An apparatus according to claim 17 or 18, characterized in that the control unit in connection with the generation of the first signal also generates a second signal that is transmitted to the base station and comprises a message indicating to the first subscriber and second subscriber that the speech connection is temporarily transferred to a dedicated control channel.