Abstract:
The current invention relates to the registration of mobile stations in a mobile communications network. It has particular relevance to handovers and roaming between base stations of an unlicensed radio access network ( 110 ) and those of a conventional cellular network ( 120 ). As the cells in an unlicensed radio access network have a rather small size, it means that handover potentially can occur far more frequently than for a conventional cellular network. This can result in an undesired ‘ping-pong’ effect, i.e. that the mobile station is switching back and forth between two radio cells too frequently. The current invention reduces this effect by preventing the mobile station to register to the unlicensed radio access network during a modifiable period of time ( 306 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
   The current invention relates to the registration of mobile stations in a mobile communications network. It has particular relevance to handovers and roaming between base stations of an unlicensed radio access network and those of a conventional cellular network. 
   DESCRIPTION OF RELATED ART 
   Conventional (licensed) cellular networks may be extended by including access networks that utilize a low power unlicensed-radio interface to communicate with mobile stations. These access networks are designed to be used together with the core elements of a standard public mobile network. The access network is constructed so that the core elements, such as mobile switching centers MSC of the public mobile network views the unlicensed-radio access network as a conventional base station subsystem, BSS. A mobile access network of this kind and the mobile station for use with this access network is described in the European patent specification EP 1207708. The mobile access network consists of an access controller that connects to the core network and one or several low power access points. The access points are connected to the access controller via a broadband packet-switched network. The low power and resultant low range of the unlicensed-radio interface means that several such access networks may be provided in relatively close proximity. The use of an already existing broadband network to connect the access points and the access controller greatly facilitates the installation of the access network, permitting a subscriber to install the access network in his own home himself, for example. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless LAN and Bluetooth. An adapted mobile station capable of operating over both the standard air interface (e.g. the Um interface) and the unlicensed-radio interface means that the subscriber requires only one phone for all environments. 
   A number of companies within the mobile telecom industry have jointly developed a set of open specifications for the unlicensed radio access network. Specifications of particular relevance are UMA Architecture (Stage 2) R1.0.0 and UMA Protocols (Stage 3) R1.0.0. (UMA=Unlicensed Mobile Access Network). 
   In unlicensed radio access networks it is the mobile station that initiates handover rather than the base station subsystem, BSS. This precludes the need to configure information on neighboring cells for each access point in the unlicensed radio access network. 
   A consequence of allowing the mobile station alone to control the handovers is that the network itself has little control and consequently is more vulnerable to frequent handovers. 
   In order to limit the signaling and processor load, conventional handover algorithms generally include some form of hysteresis, which prevents handover occurring too frequently. However, this is not entirely effective when unlicensed radio access networks are included in the mobile network, as the small size of the unlicensed radio frequency cells means that handover can potentially occur far more frequently. This can result in a ‘ping-pong’ effect, i.e. that the mobile station is switching back and forth between two radio cells too frequently. In the UMA specifications this problem has been addressed by introducing a time-supervision (an access network reselection timer TU3910) in the mobile station. When the mobile station leaves and de-registers from its associated unlicensed radio access, a timer is started in the mobile station. As long as the timer is running, the mobile station is expected not to return and register to the unlicensed radio access again, unless the mobile station has detected loss of coverage of the conventional cellular network. The value of the timer (which typically is fixed and predefined) is received by the mobile station in a register accept message from the access network controller (in the UMA specification also referred to as an UNC, UMA Network Controller). When the timer expires, the mobile station can register again if necessary. 
   SUMMARY OF THE INVENTION 
   The solution known from prior art is however far from sufficient. One problem is that handovers between two mobile communications access networks, of which one is an unlicensed radio access network, both connected to a conventional cellular network, is a rather new scenario and the experience of this is limited. As the unlicensed radio access cells are relatively small, it is expected that the probability of a ‘ping-pong’ effect between two different cells is significantly higher than in conventional cellular networks. 
   To choose a proper value of the access network reselection timer is difficult. A too small value does not significantly reduce the ‘ping-pong’ effect and a too large value limits the use of the unlicensed radio access network. 
   Another problem is that a mobile context stored in the access network controller is erased each time the mobile station de-registers. The context of a mobile station is the information required to identify and locate the mobile station. Having this limitation, the unlicensed radio access network cannot keep any history about the mobile station&#39;s behavior. Without this history, it is not possible for the access network controller to detect and act upon an undesired or an unexpected high frequency of handovers and location updates. 
   The present invention improves the known solution by keeping the mobile context alive during a certain time (a context keep-alive time) after the mobile station de-registers from the unlicensed access network. A further improvement is achieved by allowing the access network controller to modify the value of the access network reselection timer when the mobile station registers again. 
   Somewhat more in detail, when a mobile station registers to the unlicensed radio access network for the first time, the context is created and an initial value of the access network reselection timer is set in the access network controller. This value is sent to the mobile station in a register accept message. When the mobile station de-registers, the context is kept and a time supervision is started in the access network controller, a context keep-alive timer. The value of the context keep-alive timer is always larger than the value of the access network reselection timer. If the mobile station has not registered again when the context keep-alive timer expires, the context is erased in the access network controller. If the mobile station registers again when the context still exists, the access network controller will stop the context keep-alive timer. According to the further improvement the access network controller will set a new value (e.g. a larger value) of the access network reselection timer. This new timer value is sent to the mobile station in the register accept message in the same way as for the initial value. 
   The inventive concept works both for registrations in active mode (i.e. for handover) and in idle mode (i.e. in roaming and location update situations). 
   The objective of the current invention is therefore to reduce the frequency of handovers and location updates between different networks and thereby reducing signaling and processing load in the involved network elements. 
   The main advantage of the invention is that the access network controller can keep a history and act upon an undesirable high frequency of registrations and de-registrations. By modifying the time period when the mobile station is not allowed to register, the ‘ping-pong’ effect between cells can be further reduced. 
   Another advantage is that the invention is compatible with existing unlicensed radio interfaces as the mobile station stores and acts upon the received value of the access network reselection timer received from the access network controller as before. 
   The invention will now be described in more detail and with preferred embodiments and referring to accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram that describes a mobile communications network involving network elements from an unlicensed radio access network and a conventional cellular network including a conventional radio access network. 
       FIG. 2  is a block diagram that describes a mobile communications network involving network elements from two unlicensed radio access networks. 
       FIG. 3  is a flow chart that describes the registration procedure for a mobile station in an unlicensed access network according to the inventive concept. 
       FIG. 4  is a block diagram that describes the involved network elements and the relevant information flow between these network elements in one embodiment of the invention. 
       FIGS. 5 and 6  are tables that describe some examples of different schemes of access network reselection timers that can be used in the registration procedure. 
       FIG. 7  is a block diagram that describes an embodiment of how the different schemes of timers are implemented in the access controller. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   The block diagram in  FIG. 1  schematically depicts the network elements of two mobile communication networks of which one is a conventional mobile telecommunication network  100 ,  120  and the other is an unlicensed radio access network  110 . 
   The conventional (licensed) mobile telecommunications network (such as a GSM network) is divided into a core network portion  100  and a conventional access network portion  120 . The access network portion  120  is connected to an MSC (Mobile Switching Center)  101  and a SGSN (Serving GPRS Support Node)  102  in the core network portion  100 . The access network portion  120  is also called BSS, Base Station System and is divided into two entities, a BSC, Base Station Controller  122  and a BTS, Base Transceiver System  121 . 
   The unlicensed radio access network  110  is connected to a MSC  104  and a SGSN  103  in the core network portion  100  in the same manner as for the BSS  120 . The MSC  101  and the MSC  104  could either be different MSC&#39;s or be one and the same as well as SGSN  102  and SGSN  103  could be either different SGSN&#39;s or one and the same. 
   The unlicensed radio access network  110  is further divided into an Access Network Controller (or using the UMA standard term UNC, UMA Network Controller)  113 , a broadband network  112  and one or several AP, Access Points  111 ,  114 . 
   The Access Network Controller  113  appears to the core network portion  100  as if it is a BSS in the same way as BSS  120 . 
   The Access Point  111  provides the radio link to a Mobile Station  130  using an unlicensed radio spectrum. The radio link could use for example technologies such as DECT, Bluetooth or Wireless LAN&#39;s (IEEE 802.11 etc.). 
   The broadband network  112  provides connectivity between the Access Network Controller  113  and the Access Point  111  and can be based on for example IP, ATM or other broadband technologies. 
   The Mobile Station  130  includes a multi-mode radio supporting for example both a conventional cellular radio spectrum and an unlicensed radio spectrum. The Mobile Station  130  has the capability to switch between the BTS  121  and the Access Point  111 . 
   The radio coverage of a transceiver in a mobile network is also known as a cell. A cell  125  illustrates the radio coverage of the transceiver in BTS  121  and a cell  115  illustrates the radio coverage of the transceiver in the Access Point  111 . 
   The current invention relates to the situation when the Mobile Station  130  is roaming inside or in close proximity to an area  140 , which is covered by both cells  115  and  125 . 
   The invention is also applicable to a situation involving a plurality of different unlicensed radio access networks. In  FIG. 2  the Mobile Station  130  is roaming around in or close to an area  240 , which is covered by the cell  115  and a cell  225 . The latter belonging to an unlicensed radio access network  220 . The two unlicensed radio access networks are here connected to the same MSC  103  and SGSN  104  but this is not a necessary requirement for the implementation of the invention. 
   Common for both situations is that when the Mobile Station  130  registers to the unlicensed radio access network  110 , it sends a Register Request message RR to the Access Network Controller  113 . If not already created, the Access Network Controller  113  creates a mobile context CT and sends a Register Accept message RA to the Mobile Station  130 . The Register Accept message RA includes among others a value of an Access Network Reselection Timer T 1  that will be stored in the Mobile Station  130 . When the Mobile Station  130  de-registers from the unlicensed radio access network  110 , it sends a Deregister message DR to the Access Network Controller  113 . In the latter is stored a value of a Context Keep-alive Timer T 2 , the function of which will be explained below. 
   A method for registering the mobile station  130  according to an embodiment of the current invention is described in connection with  FIGS. 3   a,b,c . In  FIG. 3   a , the Access Network Controller  113  receives the Register Request message RR from the Mobile Station  130  in a step  301 . In a step  302  a check is done if the context CT is already created within the Access Network Controller  113 . If not, in an alternative N, the context CT is created in a step  303 . The Access Network Controller  113  does also set an initial value of the Access Network Reselection Timer T 1 , step  304 , and sends in a step  307  the value to the Mobile Station  130  in the Register Accept message RA. When applying the invention in relation to the UMA specifications, timer T 1  can correspond to timer TU3910. 
   When, in  FIG. 3   b , the Mobile Station  130  deregisters (message DR) in a step  308  from the Access Network Controller  113 , the context CT is kept in a step  309  and the Context Keep-alive timer T 2  is started in a step  310  in the Access Network Controller  113 . 
   If, in  FIG. 3   a , the context CT already was created when the Mobile Station  130  registers in the step  301 , the Access Network Controller  113  stops the Context Keep-alive Timer T 2  in a step  305  and sets in a step  306  a new value of the Access Network Reselection Timer T 1 . This new value is sent to the Mobile Station  130  in the Register Accept message RA in the step  307 . 
   If, in  FIG. 3   c , the Mobile Station  130  has deregistered from the Access Network Controller  113  and does not register again before the Context Keep-alive Timer T 2  expires, step  311 , the context CT in the Access Network Controller  113  is erased in a step  312 . At this stage the procedure described above will start all over again when the Mobile Station  130  registers in the step  301 . 
   The values of the Access Network Reselection Timer T 1  and the Context Keep-alive Timer T 2  can be used according to a specific scheme. In a simplified embodiment the value of the Access Network Reselection Timer T 1  can for example be incremented each time the Mobile Station  130  registers to the unlicensed radio access networks. This can be implemented in the Access Network Controller  113  by using a counter variable. This counter variable is incremented with a fixed value for each registration occasion and the value of the Access Network Reselection Timer T 1  is set to this incremented value. 
   However in a more flexible embodiment the timer values are set according to a scheme that consist of one or several tables with timer values stored in the Access Network Controller  113 . These tables are further detailed and explained in  FIGS. 5 and 6 . Common to both embodiments is that the value of the Access Network Reselection Timer T 1  always is smaller than the value of the Keep-alive Timer T 2 . 
   The registration procedure described above does allow for a situation where one and the same mobile station has a plurality of simultaneous contexts in different network access controllers. Referring to  FIG. 2 , the same Mobile Station  130  could for example have simultaneous contexts in both the Access Controller  113  and in an Access Controller  223 . 
   In  FIG. 1  and  FIG. 2 , it is also possible that several Access Points  111 ,  114  are connected to the same Access Network Controller  113 . In this case each Access Point  111 ,  114  is covering its own unique cell. The handover between these cells is done on a lower layer of the radio interface and does not interfere with the registration procedure described in the current invention. The Access Network Controller  113  is however informed by the Mobile Station  130  when it changes Access Point  111 ,  114 . 
     FIG. 4  describes a sample of the information flow between the Mobile Station  130  and the Access Network Controller (UNC)  113 . When the Mobile Station  130  determines that it should register to the unlicensed mobile access network, it sends a Register Request message RR 1  to the Access Network Controller  113 . Assuming that no context CT is created, the Access Network Controller  113  creates such a context CT and sends an initial value (30 seconds) of the Access Network Reselection Timer T 1  in a Register Accept message RA 1  to the Mobile Station  130 . When the Mobile Station  130  determines in a step  401  that it should leave the unlicensed mobile access network, it sends a De-register message DR to the Access Network Controller  113 . In step  402  the Access Network Controller  113  keeps the context CT and starts a Context Keep-alive Timer T 2 . The Mobile Station  130  will not be allowed to register again until, in a step  403 , the Access Network Reselection Timer T 1  expires. When the Mobile Station  130  again determines that it should register to the unlicensed mobile access network it sends a new Register Request message RR 2  to the Access Network Controller  113 . As the context CT still exists, the Access Network Controller  113  will send a new value (60 seconds) of the Access Network Reselection Timer T 1  in a Register Accept message RA 2  to the Mobile Station  130 . 
   An arbitrary number of different schemes for setting the Access Network Reselection Timer T 1  and the Keep-alive Timer T 2  are possible and two examples are found in the tables in  FIGS. 5 and 6  respectively. For each registration occasion  501 , there is a corresponding timer value  502 , and for each registration occasion  601  there is a corresponding timer value  602 . In order to point out the right timer value for each registration occasion, a corresponding counter is implemented in the Access Network Controller  113 . The counter is incremented at each new registration occasion  501 ,  601 . 
   In  FIG. 5 , the Keep-alive Timer T 2  is set to 500 seconds and the initial value of the Access Network Reselection Timer T 1  is set to 30 seconds. For each new registration occasion while the context exists, each subsequent timer value is increased proportionally by multiplying the previous value by two until it reaches the value 480 seconds. 
   In  FIG. 6  it is assumed that the Keep-alive Timer T 2  is set to 250 seconds. The initial timer value of the Access Network Reselection Timer T 1  is set to 30 seconds. For the two subsequent registration occasions the timer values are still set to 30 seconds but for each following registration occasion, the timer value is incremented by 30 seconds until it reaches the value 240 seconds. 
   By using these flexible schemes, the ‘ping-pong’ effect between different cells can be further reduced as the schemes can be modified in order to ‘tune in’ the timer values to fit different installations. Each Access Network Controller  113  could have individual schemes for that particular Access Network Controller if necessary. It also possible that the Access Network Controller  113  stores several schemes, each coupled to the cell the mobile station last visited or coupled to the Access Point that is currently used. The UMA specification includes that the identity of these cells and Access Points are communicated from the Mobile Station  130  to the Access Network Controller  113  in the registration procedure. 
   When setting the timer values in the schemes it is again important that the value of the Access Network Reselection Timer T 1  always is set a value smaller than the value of the Keep-alive Timer T 2 . It may also be necessary to take into account other timers used in the unlicensed radio access network ( 110 ) not mentioned here. 
   In one embodiment of the invention the schemes can be set and modified from an operation and maintenance center OMC.  FIG. 7  is a block diagram describing the Access Network Controller  113 , a network management interface  730  and an Operation and Maintenance Center  720 . The Access Network Controller  113  is connected to the Operation and Maintenance Center  720  through the network management interface  730 . In the Access Network Controller  113  a scheme  710  of values of the Access Network Reselection Timer T 1  and the Keep-alive Timer T 2  are stored in a memory area  711 . For each scheme  710  there is also a corresponding counter variable  709  stored in the memory area  711 . The counter variable  709  is used to point out the position of the current value of Access Reselection Timer T 1  and is incremented at each new registration occasion. The memory area  711  is further accessible by a processor  712 . From the Operation and Maintenance Center  720  it is possible access the processor  712  and memory area  711  and thereby it is possible to set and modify the content of the memory area  711  including the scheme  710 .