HANDOVER METHOD AND CELLULAR RADIO NETWORK

Abstract: The invention relates to a handover method and a cellular radio network. In a handover when a mobile station (120) moves from a serving cell (110) to a neighbouring cell (112), a two-way radio connection (130) between a base station (102) of a serving cell (110) and the mobile station (120) is changed into a two-way radio connection (132) between the base station (104) of the neighbouring cell (112) and the mobile station (120). In accordance with the invention, the base station (104) of the neighbouring cell (112) emulates the base station (102) of the serving cell (110) in the handover by sending a handover command to the mobile station as if it was sent by the base station (102) of the serving cell (110). A method is also presented for calculating the correct transmission time.

Description: HANDOVER METHOD AND CELLULAR RADIO NETWORK FIELD OF INVENTION The present invention relates to a method for ensuring a handover of a mobile station in a cellular radio network, which cellular radio network comprises at least one base station system and at least one mobile station, and the base station system comprises at least two base stations and at least one base station controller, and the base station has a coverage area, i.e. a cell, in which handover when the mobile station moves from a serving cell to a neighbouring cell, a two-way radio connection between the base station of the serving cell and the mobile station is changed into a two-way radio connection between the base station of the neighbouring cell and the mobile station. BACKGROUND ART In a handover, the existing radio connection between a base station of the serving cell and a mobile station is changed into a two-way radio connection between the base station of the adjacent cell, i.e. the neighbouring cell and the mobile station on some other channel without the user detecting what is taking place, or so that at least possible disturbances caused by the handover to the radio connection will remain minimal. It is not, however, always possible to carry out a handover without it being noticed because in an operational environment where the mobile stations move with respect to the base station, the radio connection can be lost due to the fact that the propagation of radio waves is prevented because of a tunnel, a bridge or a building, for example. It could, however, be possible to continue the connection after a short while as the mobile station returns to the coverage area of some other base station. It is therefore possible by a handover to lose connections that could in principle be continued after a short break. One important reason for such losses is that a mobile station moves very fast across the boundary between cells, and there is not enough time to carry out a handover properly. An already lost radio connection is attempted to be recovered by prior art functions for re-establishing a connection. In that case the mobile station selects from the channels known to it the channel whose signal is the strongest, and sends a connection re-establishment request to the base station of this cell. It is a serious problem that the connection may already have been broken on the network level, in which case the connection is inevitably lost. This is because when a connection is lost, a timer will be started at a mobile switching centre. If the mobile switching centre does not have time to receive information about the re-establishment request before the timer expires, it will destroy all the information relating to the lost connection. Even if there was time to re-establish the connection before the timer expires, it may happen that the other party of the call switches off the connection as the connection appears to be broken and the break takes too long. The GSM System for Mobile Communications (Michel Mouly and Marie-Bernadette Pautet, 1992, ISBN:2-9507190-0-0-7) discusses on pages 412 to 415 the call re-establishment function and various uncertainties relating to it and at the end it is wished that it would be solved in a better way in future systems. CHARACTERISTICS OF INVENTION The object of the present invention is to eliminate the problems associated with known solutions. This will be attained with the method as described in the preamble which is characterized in that the base station of the neighbouring cell emulates the base station of the serving cell in the handover. The invention also relates to a cellular radio network comprising at least one base station system and at least one mobile station, and the base station system comprises at least two base stations and at least one base station controller, and the base station has a coverage area, i.e. a cell, in which handover when the mobile station moves from a serving cell to a neighbouring cell, a two-way radio connection between the base station of the serving cell and the mobile station is changed into a two-way radio connection between the base station of the neighbouring cell and the mobile station. The system is characterized in that the base station of the neighbouring cell is arranged to emulate the base station of the serving cell in the handover. Several advantages are attained with the method as described in the invention. A very considerable advantage is that the invention ensures a handover, in which case the uncertain connection re-establishment function need not be used. In this way temporarily broken connections can be more definitely saved. A user of the network sees this as a rise in the quality of service as losses of a connection are avoided in such situations where the connection was previously lost. The system of the invention has the same advantages as described above for the method. The preferred embodiments and the other more detailed embodiments emphasize the advantages of the invention. It is evident that the preferred embodiments and the more detailed embodiments can be combined to one another into different combinations to achieve the intended technical effect. BRIEF DESCRIPTION OF FIGURES In the following, the invention will be explained in more detail with reference to the examples of the accompanying drawings, wherein Figure 1 shows a cellular radio network of the invention in parts essential for the invention, and Figure 2 illustrates a calculation of the time shown in the invention at which time a base station of a neighbouring cell is to send a signal to a mobile station to emulate a base station of a serving cell. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is examined first. The method and the system of the invention can be utilized in any cellular radio network to ensure a handover of a mobile station 120. In the following, the invention will be described in connection with the GSM system without restricting thereto. The cellular radio network typically comprises base station systems. The base station system comprises at least two base stations 102, 104 and at least one base station controller 100. The base station 102, 104 comprises a coverage area, i.e. a cell 110, 112. The mobile station 120 moves in the cell 110, 112. The mobile station 120 can also move across a boundary 114 between the cells 110, 112. There is a two-way radio connection 130 between the base station 102 of the serving cell 110 and the mobile station 120. A handover refers to that when the mobile station 120 moves from the serving cell 110 to the neighbouring cell 112, the two-way radio connection 130 between the base station 102 of the serving cell 110 and the mobile station 112 is changed into a two-way radio connection 132 between the base station 104 of the neighbouring cell 112 and the mobile station 120. In accordance with the invention, the base station 104 of the neighbouring cell 112 emulates the base station 102 of the serving cell 110 in the handover. In the emulation the base station 104 of the neighbouring cell 112 sends a handover command to the base station 120 as if it was sent by the base station 102 of the serving cell 110. In a primary embodiment of the invention, the method is implemented in the following way. First, a connection 130 is established in the normal way, that is, the base station controller 100 activates a connection 130 between the base station 102 of the serving cell 110 and the mobile station 120. Then the base station controller 100 activates at least one base station 104 of the neighbouring cell 112 of the serving cell 110 into a stand-by state by providing it with the specification parameters of the used radio connection 130. The mobile station 120 moves towards some neighbouring cell 112 and the base station 104 of the neighbouring cell 112 set to the stand-by state monitors if the mobile station 120 is detected in its cell 112. When the mobile station 120 moves across the boundary 114 between the serving cell 110 and the neighbouring cell 112, the base station 104 of the neighbouring cell 112 informs the base station controller 100 of the event. Depending on the situation, the base station controller 100 is aware that the connection 130 between the mobile station 120 and the base station 102 of the serving cell 110 is broken, or that a normal handover is impossible to be carried out via the base station 102 of the serving cell 110, or that a successful handover will be more probably carried out via the base station 104 of the neighbouring cell 112, in which case the base station controller 100 retrieves all the information relating to the connection 130 from the base station 102 of the serving cell 110 and transmits the information to the base station 104 of the neighbouring cell 112, and terminates then all traffic relating to the connection 130 at the base station 102 of the serving cell 110. After this, the base station 104 of the neighbouring cell 112 calculates a correct timing advance. The base station controller 100 commands the base station 104 of the neighbouring cell 112 to continue the connection 132 as if it was the base station 102 of the original serving cell 110 by using the information obtained from the base station 102 of the original serving cell 110 relating to the connection 130. The base station controller 100 gives a handover command to the base station 104 of the neighbouring cell 112, then the base station 104 of the neighbouring cell 112 carries out a normal handover procedure and its cell becomes a serving cell 112 for the connection 132. At the end, the base station controller 100 deactivates the resources connected to the connection 130 from the base station 102 of the original serving cell 110. The crossing of the boundary 114 between the cells 110, 112 by the mobile station 120 is detected on the basis of the radio field strength measurement of the mobile station 120 by the base station 104 of the neighbouring cell 112. To reduce the load of the base station 104, the function can be optimized so that the base station controller 100 sets only such base stations 104 to the stand-by state at which base stations 104 it is possible for the mobile station 120 to move fast across the boundary 114 between the cells 110, 112. A margin 140 of the handover area, that is, the area in which the handover will be carried out, is smaller than usual due to the efficiency of the present method. Another method for reducing the load of the base station 104 by optimization is that a value is not calculated for the timing advance correcting the propagation delay caused by the distance between the base station 104 and the mobile station 120 in such a cellular radio network where the cell size is small, because in that case the propagation delay is also very small. In case the presented optimization cannot be used, the calculation is carried out in the following way as described in Figure 2. The base station 102 of the serving cell 110 sends a signal to the mobile station 120 at time T. The mobile station 120 receives the signal sent by the base station 102 of the serving cell 110 at time T1. The mobile station 120 sends the signal to the base station 102 of the serving cell 110 at time T2 in which timing advance TA has been taken into account. The base station 104 of the neighbouring cell 112 is aware of distance D1 from the base station 104 of the neighbouring cell 112 to the base station 102 of the serving cell 110, indicated as the signal propagation delay. The base station 104 of the neighbouring cell 112 receives the signal sent at time T by the base station 102 of the serving cell 110 to the mobile station 120 at time T. The base station 104 of the neighbouring cell 112 calculates when the base station 102 of the serving cell 110 sent the signal, i.e. time T so that D1 is reduced from T. The base station 104 of the neighbouring cell 112 calculates time T2 when the mobile station 120 sends a signal to the base station 102 of the serving cell 110 so that half of TA, i.e. TA/2 is reduced from time T. The base station 104 of the neighbouring cell 112 receives the signal sent by the mobile station 120 to the base station 102 of the serving cell 110 at time T2'. The base station 104 of the neighbouring cell 112 calculates distance D2 to the mobile station 120, indicated as the signal propagation delay so that T2 is reduced from time T2'. At the end the base station 104 of the neighbouring cell 112 calculates time T3 when the base station 104 of the neighbouring cell 112 sends a signal to the mobile station 120 as if it was the base station 102 of the serving cell 110 by using parameters T1 and D2 so that D2 is reduced from T1. The method is used in a cellular radio network where the change required by the method for prior art solutions is that the base station 104 of the neighbouring cell 112 is used to emulate the base station 102 of the serving cell 110 in the handover. In that case new functions in the system are: the base station 104 of the neighbouring cell 112 is arranged to monitor at least one mobile station 120 in the serving cell 110. Furthermore, the base station 104 of the neighbouring cell 112 and/or base station controller 100 is arranged to receive information sent by the base station 102 of the serving cell 110 relating to the radio connection 130, and to calculate the time when the base station 104 of the neighbouring cell is to send a signal to the mobile station 120. Monitoring is carried out by an existing measuring receiver of the base station 104, by a normal receiver or by a receiver of a tester, for example. The reception of information relating to the radio connection 130 is carried out in the base station 104 of the neighbouring cell 112 and/or base station controller 100 as in the prior art parameter processing. Information relating to the radio connection 130 comprises e.g. a description of the radio channel, a training sequence, a timing advance and an encryption key. The calculation of the transmission time is carried out in the base station 104 of the neighbouring cell 112 and/or base station controller 100 in the manner described above. At its simplest, the invention is implemented as a software. The software can be stored then into a memory included in the base station 104 and/or base station controller 100 and it is processed in a processor included in the equipment. The application can also be implemented by general or signal processors or a separate logic. Although the invention has been explained above with reference to the example of the appended drawings, it is evident that the invention is not restricted thereto, but it can be modified in various ways within the scope of the inventive idea disclosed in the accompanying claims.

Claims: CLAIMS 1. A method for ensuring a handover of a mobile station in a cellular radio network, which cellular radio network comprises at least one base station system and at least one mobile station (120), and the base station system comprises at least two base stations (102, 104) and at least one base station controller (100), and the base station (102, 104) has a coverage area, i.e. a cell (110, 112), in which handover when the mobile station (120) moves from a serving cell (110) to a neighbouring cell (112), a two-way radio connection (130) between the base station (102) of the serving cell (110) and the mobile station (120) is changed into a two-way radio connection (132) between the base station (104) of the neighbouring cell (112) and the mobile station (120), c h a r a c t e r i z e d in that the base station (104) of the neighbouring cell (112) emulates the base station (102) of the serving cell (110) in the handover. 2. A method according to claim 1, c h a ra c t e r i z e d in that in the emulation the base station (104) of the neighbouring cell (112) sends a handover command to the mobile station (120) as if it was sent by the base station (102) of the serving cell (110). 3. A method according to claim ^ c h a ra c t e r i z e d in that the base station controller (100) activates a connection between the base station (102) of the serving cell (110) and the mobile station (120), the base station controller (100) activates at least one base station (104) of the neighbouring cell (112) of the serving cell (110) into a stand-by state by providing it with the specification parameters of the used radio connection (130), the mobile station (120) moves towards some neighbouring cell (112) and the base station (104) of the neighbouring cell (112) set to the stand-by state monitors if the mobile station (120) is detected in its own cell (112), when the mobile station (120) moves across a boundary (114) between the serving cell (110) and the neighbouring cell (112), the base station (104) of the neighbouring cell (112) informs the base station controller (100) of the event, the base station controller (100) is aware that the connection (130) between the mobile station (120) and the base station (102) of the serving cell (110) is broken, or that a normal handover is impossible to be carried out via the base station (102) of the serving cell (110), or that a successful handover will be more probably carried out via the base station (104) of the neighbouring cell (112), the base station controller (100) retrieves all the information relating to the connection (130) from the base station (102) of the serving cell (110) and transmits the information to the base station (104) of the neighbouring cell (112) and then terminates all traffic relating to the connection (130) at the base station (102) of the serving cell (110), the base station (104) of the neighbouring cell (112) calculates a correct timing advance, the base station controller (100) commands the base station (104) of the neighbouring cell (112) to continue the connection (132) as if it was the base station (102) of the original serving cell (110) by using the information obtained from the base station (102) of the original serving cell (110) relating to the connection (130), the base station controller (100) gives a handover command to the base station (104) of the neighbouring cell (112), then the base station (104) of the neighbouring cell (112) carries out a normal handover procedure and its cell (112) becomes a serving cell (112) for the connection (132), at the end, the base station controller (100) deactivates the resources connected to the connection (130) from the base station (102) of the original serving cell (110). 4. A method according to claim 3, characterized in that the crossing of the boundary (114) between the serving cell (110) and the neighbouring cell (112) by the mobile station (120) is deduced on the basis of the radio field strength measurement of the mobile station (120) by the base station (104) of the neighbouring cell (112). 5. A method according to claim 3, characterized in that the base station controller (100) sets only such base stations (104) to the stand-by state at which base stations (104) it is possible for the mobile station (120) to move fast across the boundary (114) between the cells (110, 112). 6. A method according to claim 3, characterized in that a margin (140) of the handover area is smaller than usual. 7. A method according to claim 3, characterized in that a value is not calculated for the timing advance correcting the propagation delay caused by the distance between the base station (102, 104) and the mobile station (120) in such a cellular radio network where the cell size is small, because in that case the propagation delay is also very small. 8. A method according to claim 3, c h a r a c t e r i z e d in that the base station (102) of the serving cell (110) sends a signal to the mobile station (120) at time T, the mobile station (120) receives the signal sent by the base station (102) of the serving cell (110) at time T1 , the mobile station (120) sends the signal to the base station (102) of the serving cell (110) at time T2 in which timing advance TA has been taken into account, the base station (104) of the neighbouring cell (112) is aware of distance D1 from the base station (104) of the neighbouring cell (112) to the base station (102) of the serving cell (110), indicated as the signal propagation delay, the base station (104) of the neighbouring cell (112) receives the signal sent at time T by the base station (102) of the serving cell (110) to the mobile station (120) at time T', the base station (104) of the neighbouring cell (112) calculates when the base station (102) of the serving cell (110) sent the signal, that is, time T by using parameters T' and D1 so that D1 is reduced from T, the base station (104) of the neighbouring cell (112) calculates time T2 when the mobile station (120) sends a signal to the base station (102) of the serving cell (110) by using parameters T and TA so that half of TA, i.e. TA/2 is reduced from time T, the base station (104) of the neighbouring cell (112) receives the signal sent by the mobile station (120) to the base station (102) of the serving cell (110) at time T2', the base station (104) of the neighbouring cell (112) calculates distance D2 to the mobile station (120), indicated as the signal propagation delay, by using parameters T2 and T2' so that T2 is reduced from time T2', the base station (104) of the neighbouring cell (112) calculates time T3 when the base station (104) of the neighbouring cell (112) sends a signal to the mobile station (120) as if it was the base station (102) of the serving cell (110) by using parameters T1 and D2 so that D2 is reduced from T1. 9. A cellular radio network comprising at least one base station system and at least one mobile station (120), and the base station system comprises at least two base stations (102, 104) and at least one base station controller (100), and the base station (102, 104) has a coverage area, i.e. a cell (110, 112), in which handover when the mobile station moves from a serving cell (110) to a neighbouring cell (112), a two-way radio connection between the base station (102) of the serving cell (110) and the mobile station (120) is changed into a two-way radio connection (132) between the base station (104) of the neighbouring cell (112) and the mobile station (120), characterized in that the base station (104) of the neighbouring cell (112) is arranged to emulate the base station (102) of the serving cell (110) in the handover. 10. A system according to claim 9, characterized in that the base station (104) of the neighbouring cell (112) is arranged to monitor at least one mobile station (120) in the serving cell (110). 11. A system according to claim 9, characterized in that the base station (104) of the neighbouring cell (112) and/or base station controller (100) is arranged to receive the information sent by the base station (102) of the serving cell (110) relating to the radio connection (130), and to calculate the time when the base station (104) of the neighbouring cell (112) is to send a signal to the mobile station (120).

Inventors: HOKKANEN PETRI, HOKKANEN, PETRI
Applicants: HOKKANEN PETRI, NOKIA TELECOMMUNICATIONS OY, HOKKANEN, PETRI
Application year: 1997
Main classification: H04Q7
IPC codes: H04W36, H04W88