Abstract:
A cell handover method and cellular radio system includes at least one indoor subscriber terminal ( 100 ) and cells ( 31  to  38 ) with operating area of each being confined to the interior of a building, and at least one outdoor cell ( 11 ) whose operating area extends from the exterior to the interior of the building, and in which cellular radio system the cell serving the subscriber terminal ( 100 ) will be changed, characterized in that the radio system is configured for cell handover in such a way that cells ( 31  to  38 ) are grouped into cell groups with a gateway cell ( 21  to  24 ) provided around each group, and with one gateway cell ( 21 ) provided at the entrance of the building, the gateway cell ( 21  to  24 ) surrounding the cell ( 31  to  38 ) is entered on the neighbor list of each cell ( 31  to  38 ), other gateway cells ( 22  to  24 ) and an outdoor cell ( 11 ) are entered on the neighbor list of the gateway cell ( 21 ) provided at the entrance, and the gateway cells ( 21  to  24 ) are entered on the neighbor list of the outdoor cell ( 11 ).

Description:
This application is the national phase of international application PCT/FI99/00195 filed Mar. 16, 1999 which designated the U.S. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a cell handover method employed in a cellular radio system comprising at least one indoor subscriber terminal and cells with operating area of each being confined to the interior of a building, and at least one outdoor cell whose operating area extends from the exterior to the interior of the building, and in which cellular radio system the cell serving the subscriber terminal will be changed. 
     BACKGROUND OF THE INVENTION 
     In cellular radio systems, the cell size may vary, for instance, depending on the number of subscriber terminals within the cell area. For instance, in an urban environment, where subscriber terminals are typically densely located in a relatively small area, so-called macro cells, micro cells and pico cells are employed which provide the necessary radio coverage over every serviceable area. The radius of the micro cell coverage area may range from 0.5 to 3 km, for instance. The radius of the pico cell coverage area may range from a few metres to a few tens of metres. For instance, from the view-point of network construction, an office environment can well be compared with the urban environment. 
     In cellular radio systems, the network monitors at all times the location of the subscriber terminal in the network. In order that monitoring would be possible, it is naturally necessary that the subscriber terminal is turned on. The network is typically divided into traffic areas, and some network element, for instance a base station controller, keeps a record of the subscriber terminal movements in different traffic areas. When the subscriber terminal is turned on, it registers to a network base station which transmits a signalling message on the basis of the registration to the base station controller. Thus the base station controller can switch an incoming call or message to a specific subscriber terminal irrespective of its location. If the subscriber terminal moves to the borders of the base station service area or outside the service area, the subscriber terminal has to reregister. If the subscriber terminal moves from one base station service area to another, a base station handover is performed. In the above-described situation, a channel and a cell are generally handed over. 
     In a small-cell area, there are areas and spots between cells where reception of the base station is poor. If the subscriber terminal is located in areas of that kind, problems may arise in call setup. Poor reception may cause interruption of an established connection, for instance. Problems caused by poor service areas are corrected by means of so-called umbrella cells. The umbrella cell is generally a high-power cell as compared with the underlying cells within its coverage area. The umbrella cell can be, for instance, a macro cell or a micro cell, whereby it provides a coverage area for a pico cell which serves as a small cell. In general, the calls are directed to the umbrella cell when all channels of the small-cell base station are busy or the subscriber is outside the small-cell coverage area. 
     In typical cellular radio systems used in office environments, pico cells are provided in predetermined locations in an office building. The cells are preferably located to cover as well as possible all spaces and floors of the building. The macro cells and micro cells surrounding the office building may, however, cause interference to data transmission inside the office building, since transmission powers used outside the office building are typically higher than those used inside. In practice this means that the subscriber terminal located inside the office building may perform a handover to a base station outside the building, and consequently the base station load of the cell outside the building may grow excessively. If the transmission power used indoors is increased in order to try to solve the problem, the overall interference in the cellular radio network increases. 
     When the subscriber terminal is in idle mode, it typically switches to the base station from which it receives the strongest signal. This means that the subscriber terminal which is located in a pico cell and which is in idle mode can select a cell outside the building, if the outdoor cell is also dominating inside the building. 
     In the cellular radio networks, specific C 1  parameters are used, which determine the serving cell of the subscriber terminal, when the subscriber terminal is in idle mode. If the subscriber terminal is in dedicated mode, the cell can be forced to provide service to the subscriber terminal by means of the base station parameters. After the C 1  parameter, the cellular radio networks further employ C 2  parameters, by means of which the serving cell is determined to the subscriber terminal. 
     Currently, offices employ more and more internal information networks, so-called intranet networks, which in principle provide services that are similar to those of the internet networks. The intranet networks are implemented, for instance, by means of computers connected to a LAN network. In the future, the intranet networks operate more and more wirelessly. These wireless office systems are also called WIO systems (Wireless Intranet Office). In the new systems, it is possible for a mobile telephone, for instance, to establish a wireless connection to a transceiver located in a computer, wherefrom the connection signal passes through the LAN network to some other mobile telephone, for instance. The computer system comprising a transceiver that is able to establish a connection to a telephone is called an Intranet Mobile Cluster. 
     When the Intranet Mobile Cluster is installed in the office, the above-mentioned problems with radio resource management increase. In prior art systems, if the telephone that established a connection to a WIO system computer performs a cell handover to a cell outside the building, the load of the outdoor cell increases due to cell handovers. Also the telephone in idle mode inside the building loads the outdoor cell, when the telephone registers to a base station providing the cell outside the building. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The object of the invention is to provide a method and equipment implementing the method so as to solve the above problems. This is achieved with a method of the type presented in the preamble characterized in that the radio system is configurated for cell handover in such a way that cells are grouped into cell groups with a gateway cell provided around each group, and with one gateway cell provided at the entrance of the building, the gateway cell surrounding the cell is entered on the neighbor list of each cell, other gateway cells and an outdoor cell are entered on the neighbor list of the gateway cell provided at the entrance, and the gateway cells are entered on the neighbor list of the outdoor cell. 
     The invention further relates to a cellular radio system comprising at least one subscriber terminal inside the building and cells with operating area of each being confined to the interior of the building, and at least one outdoor cell whose operating area extends from the exterior to the interior of the building, and in which cellular radio system the subscriber terminal changes the serving cell if necessary. 
     The cellular radio network is characterized in that the radio system is configurated for cell handover in such a way that the cellular radio system comprises gateway cells, each surrounding a cell group consisting of cells, with one gateway cell provided at the entrance of the building, each cell comprising on the neighbor list the gateway cell surrounding the cell, the gateway cell at the entrance comprising other gateway cells and an outdoor cell on its neighbor list, and the outdoor cell comprises the gateway cells on its neighbor list. 
     The preferred embodiments of the invention are disclosed in the dependent claims. 
     The invention is based on determining the neighbor lists of the cells such that the cell handover from inside the building to the outdoor cell must be carried out through a specific indoor cell, even though the subscriber terminal were in the operating area of the outdoor cell while inside the building. 
     Several advantages are achieved with the method and system of the invention. The method can reduce the load of the outdoor cell, since cell handovers to the outdoor cell decrease. The system of the invention can be readily applied to systems currently in use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described in greater detail in connection with preferred embodiments, with reference to the accompanying drawings, wherein 
     FIG. 1 shows a cellular radio system in accordance with the invention, 
     FIG. 2 shows a cell, 
     FIG. 3 illustrates handover situations in the cellular radio system, 
     FIG. 4 illustrates neighbor cell listings of the cells, 
     FIG. 5 illustrates an implementation of the cellular radio system in practice. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cellular radio system comprising cells of various sizes, the cells being located in an office building, for instance. The cellular radio system comprises cells  31  to  38 , which are so-called Intranet Personal Cluster cells, provided on various floors of the office building. The Intranet Personal Cluster cells can be pico cells, for instance. The cellular radio system further comprises WIO gateway cells  21  to  24  provided on various floors of the office building. The gateway cell  21  is provided at the entrance  50  of the building. Outside the office building the cellular radio system comprises a micro and/or macro cell  11 , whose radius exceeds the indoor cells  31  to  38 . In the cellular radio system shown in the figure, the micro cell may also serve as a so-called umbrella cell. In general, the umbrella cell transmits a signal with high transmission power, whereby the umbrella cell may become dominating among the other cells of the cellular radio system. 
     FIG. 2 illustrates the structure of the cell  31 . Also the other cells  32  to  38  are in principle similar to the cell  31 . The cell  31  comprises a computer system  200  acting as an Intranet Mobile Cluster in the cell area, the computer system being connected by means of a LAN network, for instance, to other cells  32  to  38  operating on the same principle. In its operating area the cell  31  further comprises a subscriber terminal  100 . The computer  200  may comprise a transceiver  201 , by means of which the computer and the subscriber terminal communicate. In practice, the subscriber terminal  100  and the computer  200  establish a connection by means of a transceiver operating within the frequency range of 2.46 GHz, for instance. Moreover, the connection between the subscriber terminal  100  and the computer  200  can be established by means of an infrared link. In the described Intranet Mobile Cluster, a conventional base station is not necessarily employed at all, even though it would be possible. The infrared connection operates within the frequency range of 2.46 GHz. Further, the connection between the subscriber terminal and the computer can be implemented by means of a cable, for instance. For the connection between the subscriber terminal and the computer to function in practice, the subscriber terminal  100  requires suitable SW and HW implementations, which convert the signalling between the subscriber terminal and the computer mutually compatible. 
     FIG. 3 illustrates a cellular radio system which comprises one micro cell  11 , four so-called gateway cells  21  to  24  and eight Personal Cluster cells  31  to  38 . By means of the cells  21  to  24 ,  31  to  38  the interior of the building is divided into sectors, which enables prevention of potential interference between the micro cell  11  and the indoor network. The cellular radio system further comprises at least one subscriber terminal  100 , which is located in the coverage area of a Personal Cluster cell, for instance. The cells  31  to  38  are located in the gateway cell operating areas in such a way that the gateway cell  21  comprises the cells  31  to  32  in its area, the gateway cell  22  comprises in its area the cell  33 , the gateway cell  23  comprises the cells  34  to  36  in its area and the gateway cell  24  comprises the cells  37  to  38  in its area. The Personal Cluster cells  31  to  38  in the figure constitute a so-called Intranet Personal Cluster and the WIO gateway cells  21  to  24  constitute a so-called Intranet Mobile Cluster. 
     In the cellular radio system of FIG. 3, it is assumed that the cell  24  is located on the top floor of the building. The cell  23  is located one floor below the cell  24 , and the cell  22  is located one floor below the cell  23 . The cell  21  is located in the entrance  50  area on the ground floor of the office building. The operating areas of the cell  21  and the micro cell  11  may overlap. The borders of the operating areas of the cell  21  and the cell  11  may be located outside the office building in such a way, for instance, that when the operating area of the cell  21  ends the operating area of the micro cell  11  begins. At the entrance, part of the operating area of the cell  21  extends indoors. 
     The cell  21  is called a lobby cell, since the cell typically acts in the lobby area at the main entrance. However, the cell  21  does not necessarily act outside the office building at all. The lobby cell  21  can be made to extend coverage over a lift shaft of the building or an eventual staircase inside the building, whereby the channel handovers are not performed until on upper floors. In the above-described manner, the number of channel handovers can be reduced. 
     Let us assume that the subscriber terminal  100  established a connection by means of the computer  200  in the cell  37  area to a subscriber terminal in another cell area, for instance. However, it is not necessary that the subscriber terminal in the cell  37  area established the connection, but the subscriber terminal can also be assumed to be in idle mode. If the subscriber terminal  100  located in the operating area of the cell  37  moves outside the coverage area of the cell  37 , the gateway cell  23  surrounding the cell  37  starts serving the subscriber terminal. 
     Configuration of the cells  21  to  24  in the cellular radio system can be based on GSM throughout the building, since the WIO system communicates with the computer, which enables continuous service to the subscriber terminals in the building. The Personal Cluster cell and the WIO Gateway cell surrounding it belong to the same area (Intranet Location Area) controlled by a WIO Gatekeeper device (WGK)  500 , yet the area is different from the one that the outdoor micro cell  11  belongs to. In principle, the WGK operates in the same manner as the conventional base station controller. However, the operations of the WGK and the base station controller differ in that the WGK is able to switch calls. 
     The location of the gateway cells is of importance to the operation of the cellular radio system. The described cellular radio system in principle comprises two kinds of gateway cells. The gateway cells may serve as transmission links between the Intranet Personal Cluster and the Intranet Mobile Cluster. Moreover, the gateway cell may serve as a transmission link between the Intranet Mobile Cluster and the outdoor cell  11 . 
     FIG. 3 also shows handover situations arisen between cells. In a handover situation the subscriber terminal changes the cell that serves the subscriber terminal for another. In the cellular radio system of the figure, it is possible for the subscriber terminal in the cell  37  area, for instance, to perform a handover to the cell  24 . Likewise, it is possible for the subscriber terminal in the cell  24  area to perform a handover to the cell  37 . It is also possible to perform the cell handover between the adjacent gateway cells. A handover to the outdoor cell  11  can only be performed from the cell  21  of the indoor gateway cells. However, it is possible to perform a handover from the outdoor cell  11  to all indoor gateway cells  21  to  24 . In the cellular radio system, possible cell handovers are determined by means of neighbouring cell lists. 
     Here below is presented Table 1, which clarifies the contents of the neighbor lists of the cells. Configuration of the cellular radio system, which corresponds to Table 1, is shown in FIG.  4 . The table clearly shows that the neighbor list of the outdoor cell  11  includes all indoor gateway cells  21  to  24 . Of the indoor cells, only the cell  21 , which is preferably located in the lobby of the building, includes the outdoor cell  11  on its neighbor list. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Cell 
                 Cell type 
                 Cells on cell neighbour list 
               
               
                   
               
             
             
               
                 11 
                 outdoor cell 
                 21, 22, 23, 24, 31-38 
               
               
                 21 
                 lobby gateway 
                 11 ,22, 23, 24, 31, 32 
               
               
                 22 
                 WIO gateway 
                 21, 23, 33 
               
               
                 23 
                 WIO gateway 
                 21, 22, 24, 34, 35, 36 
               
               
                 24 
                 WIO gateway 
                 21, 22, 23, 37, 38 
               
               
                 31 
                 Personal Cluster 
                 21 
               
               
                 32 
                 Personal Cluster 
                 21 
               
               
                 33 
                 Personal Cluster 
                 22 
               
               
                 34 
                 Personal Cluster 
                 23 
               
               
                 35 
                 Personal Cluster 
                 23 
               
               
                 36 
                 Personal Cluster 
                 23 
               
               
                 37 
                 Personal Cluster 
                 24 
               
               
                 38 
                 Personal Cluster 
                 24 
               
               
                   
               
             
          
         
       
     
     Since all indoor gateway cells  21  to  24  appear on the neighbor list of the outdoor cell  11 , it is possible to return the subscriber terminal to the Personal Cluster cell level even in a case when the subscriber terminal is turned on at such a point in the building where the outdoor cell is dominating. If the cellular radio system employs SoLSA functionality (SOLSA=Support of Localized Service Area), it is possible to simplify the neighbor list of the outdoor cell  11  in such a way that the Personal Cluster cells  31  to  38  are deleted from the neighbor list. Deletion of the Personal Cluster cells from the neighbor list is possible, since the subscriber terminal has predetermined priorities to use SoLSA cells when the SoLSA functionality is applied. Thus the subscriber terminal can be forced to establish a connection to the transceiver  201 , even though its signal would not be dominating. 
     FIG. 5 illustrates one implementation alternative of the cellular radio system in accordance with the invention. The cellular radio system comprises a base station  250 , a first base station controller  300 , a mobile exchange  400  and a so-called WIO Gatekeeper device  500 . The base station comprises TRX units, of which some constitute the outdoor cell  11 . In this case, providing a gateway cell  22  is based on WIO function only. When the subscriber terminal  100  establishes a connection to a computer in the Personal Cluster cell  22 , the connection signal is transmitted to the WIO Gatekeeper device  500  via a LAN network  20 . Prior to transmitting the signal to the device  500 , the signal is given an IP address, on the basis of which the signal is routed to the desired destination. In the ILR register, the subscriber terminal has the IP address of the cell to which the signal was transmitted. 
     The radio system of FIG. 5 also comprises an ILR register  510  (ILR=Intranet Location Register) and an HLR register  520  (HLR=Home Location Register). The HLR register comprises information on which location area the subscriber terminal is updated to. When the mobile telephone is brought in a building where the WIO system in accordance with the invention is employed, the mobile telephone connects to a specific Intranet Mobile Cluster. In the above situation, the IP address data of the mobile telephone SIM card are updated in the ILR register. 
     Even though the invention is described in the above with reference to the example in the accompanying drawings, it is obvious that the invention is not restricted thereto, but it can be modified in a variety of ways within the scope of the inventive idea disclosed in the attached claims.