Abstract:
The present invention overcomes the problem of S1 interface failure by redirecting user equipments to another radio basestation, or another RAT. This redirecting may be on a UE-by-UE basis, i.e. when a connection request is received, redirecting that UE, or may involve redirecting all UEs in a cell by broadcasting modified system information. The redirecting may be such that future traffic load, for example as caused by UEs performing tracking area updates, is minimized, by instructing UEs to redirect after a randomized period of time, or by locking the cell after a randomized period of time, for example. In yet further embodiments of the invention, the cell may be locked after a certain number of connection requests have been received. In this way, particularly busy cells can be locked sooner than would otherwise be the case.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a telecommunications system and method, and in particular to a telecommunications system and method for handling connection establishment requests following a network interface failure. 
       BACKGROUND OF THE INVENTION 
       [0002]    As is generally known, telecommunications systems employ a cellular system in which a plurality of radio basestations each maintain one or more “cells” to which user equipments (also known as mobile terminals) are connected. The radio basestations send communications (e.g. control and data signals) to the UEs in the downlink, and receive communications from the UEs in the uplink. The radio basestations further communicate with a core network, which maintains overall control of the telecommunications system. In different telecommunications systems, functionality is split differently between the radio network (i.e. the radio basestations and the mobile terminals, etc) and the core network. 
         [0003]      FIG. 1  shows an exemplary telecommunications system  10 , known as the E-UTRAN (Evolved UMTS terrestrial radio access network) which uses the Long Term Evolution (LTE) standard. The system  10  comprises a plurality of radio basestations (also known as eNode Bs, Node Bs, etc)  12   a ,  12   b ,  12   c , each of which maintains one or more cells (not illustrated). UEs  14   a ,  14   b ,  14   c ,  14   d  within each cell communicate with the corresponding radio basestation  12  of that cell. Also, as is known, cells are grouped together into what are known as “tracking areas”. 
         [0004]    In the E-UTRAN, radio basestations are capable of communicating with one another over interfaces known as X2 interfaces (illustrated as dashed lines in  FIG. 1 ). Each radio basestation  12  further has one or more interfaces with the core network. These are known as S1 interfaces (illustrated as solid lines in  FIG. 1 ). In particular, the radio basestations  12  have one or more S1 interfaces to one or more mobility management entities (MMES)  16   a ,  16   b , as will be described in more detail below. 
         [0005]    The cells of a radio basestation can be connected to different sets of MMEs (i.e. different MME Pools), through use of the tracking area. One MME pool is responsible for a certain tracking area, and a cell is connected to a certain tracking area. In  FIG. 1 , all of the cells of the radio basestations are connected to the same MME Pool (i.e. MMEs  16   a  and  16   b ). However, different cells, of the same radio basestation, may be connected to different tracking areas that are in turn connected to different MME pools. 
         [0006]    A UE generally selects its cell by determining the strongest received signal and, once selected, the UE reads system information through that cell. The system information contains, amongst other things, the tracking area of which the cell is part. 
         [0007]    Once the UE selects a cell, it also performs a tracking area update towards the core network (e.g. the MME) to let the core network know in which tracking area it is located, and to receive a tracking area list of the cells in which, should the UE move to any of them, the UE need not perform a tracking area update. 
         [0008]    Thus, it can be seen that a relatively high amount of traffic may be generated to and from the core network when a UE moves between cells. 
         [0009]    A further problem arises when all of the S1 interfaces of a particular radio basestation have become non-operational. A proposed solution to this problem has been to “lock” the cell, i.e. to effectively turn off the output power of the radio basestation, and thus to stop broadcasting the system information. However, this solution itself causes problems. For example, a user in idle mode that would not be able to re-select to another, functional cell will become aware that the network has developed a fault, in that his or her UE will indicate that there is no network. Furthermore, if an S1 interface becomes non-operational, it will most probably be affecting many radio basestations at the same time. This implies that many UEs will attempt cell-reselection, with some or all of them performing a subsequent tracking area update, at the same time. This could potentially lead to failure in other nodes that become inundated with connection requests, etc, causing the problem to spread to other parts of the network. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of the invention there is provided a radio basestation for serving one or more cells of a telecommunications network, the telecommunications network comprising a core network and a radio access network. The radio basestation comprises one or more interface means for linking the radio basestation with the core network; and means for redirecting user equipments to another radio basestation or another radio access technology (RAT) when each of said one or more interface means is non-operational. 
         [0011]    Thus, the present invention overcomes the problem of S1 interface failure by redirecting user equipments to another radio basestation, or another RAT. This redirecting may be on a UE-by-UE basis, i.e. when a connection request is received, redirecting that UE, or may involve redirecting all UEs in a cell by broadcasting modified system information. 
         [0012]    According to further embodiments of the invention, the redirecting may be such that future traffic load, for example as caused by UEs performing tracking area updates, is minimized. This may be achieved by instructing UEs to redirect after a randomized period of time, or by locking the cell after a randomized period of time, for example. 
         [0013]    In yet further embodiments of the invention, the cell may be locked after a certain number of connection requests have been received. In this way, particularly busy cells can be locked sooner than would otherwise be the case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the following drawings in which: 
           [0015]      FIG. 1  shows an example of a telecommunications network; 
           [0016]      FIG. 2  shows a radio basestation according to the present invention; 
           [0017]      FIG. 3  shows a user equipment according to the present invention; 
           [0018]      FIG. 4  is a flow chart of a first method according to the present invention; 
           [0019]      FIG. 5  is a flow chart of a second method according to the present invention; and 
           [0020]      FIG. 6  is a flow chart of a third method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The present invention is generally applicable to telecommunications networks of the type illustrated in  FIG. 1 . In particular, the invention is applicable to any telecommunications system having radio basestations, wherein those radio basestations each have one or more interfaces to a core network. Accordingly, the invention is relevant for Long Term Evolution (LTE) and the E-UTRAN, but also to other standards and networks, including but not limited to GSM, UMTS, etc. For consistency, the invention will further be described with relevance to LTE and the E-UTRAN; however, as mentioned above, other systems and standards are also considered to fall within the scope of the invention. 
         [0022]      FIG. 2  illustrates a radio basestation  20  according to the present invention. The radio basestation  20  comprises an antenna  22 , which is capable of receiving and transmitting signals. The antenna  22  is coupled to transmitter/receiver circuitry  24  (or, more generally, transceiver circuitry), and this is in turn connected to processing circuitry  26 . 
         [0023]    The radio basestation  20  further comprises interface circuitry  28  for managing the one or more interfaces with the core network (e.g. S1 interfaces) and, if applicable, with other radio basestations (e.g. X2 interfaces). 
         [0024]      FIG. 3  illustrates a user equipment (or mobile terminal, etc)  30  according to the present invention. The UE 30 comprises an antenna  32 , which is capable of receiving and transmitting signals. The antenna  32  is coupled to transmitter/receiver circuitry  34  (or, more generally, transceiver circuitry), and this is in turn connected to processing circuitry  36 . 
         [0025]    It will be apparent to those skilled in the art that, where they are non-essential for describing operations and processes according to the present invention, many features have been omitted from  FIGS. 2 and 3  for clarity purposes. 
         [0026]    According to the present invention, the cell is kept open, at least for a determined amount of time, after all S1 interfaces are non-operational. During the period of time in which the S1 interfaces are down, but the cell itself is operational, the radio basestation redirects UEs to a cell maintained by another radio basestation, or to another radio access technology (RAT) in which the interfaces with the core network are operational. 
         [0027]    According to one embodiment of the present invention, to be described in greater detail below, UEs are redirected towards another basestation or RAT by means of a redirection message that is issued in response to a connection request by that UE. According to another embodiment, to be described in greater detail below, idle-mode UEs are redirected towards another basestation or RAT by means of the system information broadcast from the basestation. 
         [0028]      FIG. 4  is a flow chart illustrating a method in a basestation of a telecommunications network in accordance with an aspect of the present invention. 
         [0029]    The method starts in step  40 , and then, in step  42 , the last S1 interface between the basestation and the core network becomes non-operational. In step  44 , a timer is initiated in the basestation, which determines the time two before the cell becomes locked. In one embodiment, the timer is initiated with a random value, which may be a random value within predefined limits (e.g. a random value between 30 and 120 seconds). In this way, if multiple basestations lose their S1 interfaces at the same time (as is quite likely if a MME become non-operational, for example), the basestations do not all lock at the same time. Thus any traffic associated with tracking area updates, etc, when a cell locks, is spread out over time. 
         [0030]    Therefore, in another embodiment, the timer may be initiated according to one value within a predetermined pattern. That is, different basestations may have different preset timer values that ensure that, if a plurality of basestations all lose their 51 interfaces at the same time, cells are locked at different times thereafter. 
         [0031]    Thus, in step  46 , while the S1 interfaces are all non-operational, the basestation receives a connection request from a UE. In one embodiment, the connection request is an RRC Connection Request message from the UE to establish an RRC (Radio Resource Control) connection with the radio access network (i.e. the radio basestation). The UE can use the RRC connection to signal to the core network. The core network signalling may be just that—signalling (e.g. Tracking Area Update)—or it may be a request to setup one or more bearers to be used for userplane transfer (e.g. Internet browsing or Voice over IP service. In that case, the core network will request the setup of the bearers in the radio basestation. In step  48 , it is determined whether the timer has reached t lock  and, if not, the UE making the connection request is redirected to another radio basestation or to another radio access technology in step  50 . In one embodiment, the UE is redirected with a standardized message. In order to achieve compatibility with legacy standards, a known standardized message may be adapted to include a redirection instruction, for example, the RRC ESTABLISHMENT REQUEST REJECT message. 
         [0032]    The redirection message may include information to help the UE redirect to a specific RAT or basestation, such as a list of RATs or a list of cells to which the UE can redirect. 
         [0033]    If it is determined in step  48  that the timer has reached t lock , the cell is locked in step  52 , i.e. the radio basestation is powered down and no new connection requests are processed. 
         [0034]    Thus, according to this method, a cell stays open for a certain period of time after its last S1 interface becomes non-operational. During this time, any connection requests that are received are redirected to another radio basestation, or to another RAT. 
         [0035]      FIG. 5  is a flow chart illustrating another method in a radio basestation according to the present invention. 
         [0036]    The method starts in step  60 , and then, in step  62 , the last S1 interface between the basestation and the core network becomes non-operational. In step  64 , a counter “i” is set to zero, and in step  66  a connection request is received from a UE. In response to receiving a connection request, in step  68  the counter i is stepped by 1, and in step  70  it is determined whether the value in the counter i has reached a predetermined threshold i max . If the threshold has not been reached, the UE is redirected substantially as described above with reference to step  50  of the preceding method, and the process moves back to step  66 . If i max  has been reached, the cell is locked in step  74 . 
         [0037]    Thus, in this embodiment, the number of connection requests received whilst the S1 interfaces are non-operational, but before the cell has locked, is counted. Once the number of connection requests reaches a predetermined threshold, the cell is locked. In this way, particularly busy cells can be locked in a shorter period of time, thus increasing user satisfaction of the telecommunication network. 
         [0038]    It will be apparent that the methods described with respect to  FIGS. 4 and 5  are not mutually exclusive, and indeed may beneficially operate at the same time. Thus, in one embodiment, when the last S1 interface for a cell becomes non-operational, both a timer and a counter are initiated. Thus, the timer ensures that the cell will be become locked after a certain amount of time; however, particularly busy cells may reach their count threshold i max  before the timer has reached t lock , and therefore the cell may advantageously be locked more quickly in that case. 
         [0039]      FIG. 6  is a flow chart showing a method in a basestation according to a yet further aspect of the present invention. 
         [0040]    The method starts in step  80 , and then, in step  82 , the last S1 interface between the basestation and the core network becomes non-operational. In step  84 , a modified system information is broadcast throughout the cell, and in step  86 , the method ends. 
         [0041]    Conventional system information includes information on frequency bands, scheduling information, and other important information that will be known to those skilled in the art. According to an embodiment of the present invention, the system information is modified when the last S1 interface becomes non-operational to include redirection information that instructs idle-mode UEs to redirect to another radio basestation, or to another radio access technology (RAT). 
         [0042]    In one embodiment, UEs may be instructed to redirect in a random time within a specified range. For example, the specified range may be hardcoded within a standard, or within the UE itself, or the range may be communicated within the system information. In this way the UEs do not all redirect at the same time, and the load on the system is spread out over a period of time. 
         [0043]    When redirecting to another RAT, it may be advantageous to redirect UEs to a specific RAT with which there are no problems identified. For example, an operator employing LTE may use another 2G or 3G operator as overlay, and these may be more robust in the event of an interface failure. 
         [0044]    Again, the method described with respect to  FIG. 6  may be employed in any combination with those described with respect to  FIGS. 4 and 5 . 
         [0045]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.