Abstract:
A method is provided of identifying a handover target femtocell base station from among multiple femtocell base stations. The method comprises: (a) measuring the characteristic of the handover target femtocell base station to provide a first measured value of the characteristic; (b) then identifying as handover candidate femtocell base stations a first set of femtocell base stations all having that first characteristic value; (c) then changing the characteristic of selected femtocell base stations in the first set; (d) then measuring the characteristic of the handover target femtocell base station to provide a further measured value of the characteristic; (e) then identifying as a reduced set of handover candidate femtocell base stations each femtocell base station in the first set having the further measured value of the characteristic; (f) then checking whether the number of femtocell base stations in the reduced set of handover candidate femtocell base stations is one, and upon determining that the number of femtocell base stations in the reduced set is greater than one, taking the reduced set as the first set in repeating steps (c), (d) (e) and (f).

Description:
FIELD OF THE INVENTION 
     The present invention relates to telecommunications, in particular to wireless telecommunications. 
     DESCRIPTION OF THE RELATED ART 
     Wireless telecommunications systems are well-known. Many such systems are cellular, in that radio coverage is provided by a bundle of radio coverage areas known as cells. A base station that provides radio coverage is located in each cell. Traditional base stations provide coverage in relatively large geographic areas and the corresponding cells are often referred to as macrocells. 
     It is possible to establish smaller sized cells within a macrocell. Cells that are smaller than macrocells are sometimes referred to as small cells, microcells, picocells, or femtocells, but we use the term femtocells generically for cells that are smaller than macrocells. One way to establish a femtocell is to provide a femtocell base station that operates within a relatively limited range within the coverage area of a macrocell. One example of use of a femtocell base station is to provide wireless communication coverage within a building. 
     The femtocell base station is of a relatively low transmit power and hence each femtocell is of a small coverage area compared to a macrocell. A typical coverage range is tens of metres. 
     Femtocell base stations have auto-configuring properties so as to support plug- and play deployment by users, for example in which femto base stations may integrate themselves into an existing macrocell network so as to connect to the core network of the macrocell network. 
     Femtocell base stations are intended primarily for users belonging to a particular home or office. Femtocell base stations may be closed access or open access. In femtocell base stations that are closed access, access is restricted only to registered users, for example family members or particular groups of employees. In femtocell base stations that are open access, other users may also use the femtocell base station, subject to certain restrictions to protect the Quality of Service received by registered users. 
     One known type of Femtocell base station uses a broadband Internet Protocol connection as “backhaul”, namely for connecting to the core network. One type of broadband Internet Protocol connection is a Digital Subscriber Line (DSL). The DSL connects a DSL transmitter-receiver (“transceiver”) of the femtocell base station to the core network. The DSL allows voice calls and other services provided via the femtocell base station to be supported. The femtocell base station also includes a radio frequency (RF) transceiver connected to an antenna for radio communications. An alternative to such a wired broadband backhaul is to have a wireless backhaul. 
     Femtocell base stations are sometimes referred to as femtos. 
     Handover of a user terminal from connection to one cell to connection to another cell is common in cellular telecommunications systems. Handovers may be from macrocell base station to macrocell base station, from femto to femto, from femto to macrocell base station (“Handout”) and from macrocell base station to femto (“Hand-in”). 
     Handover, in particular, of a user terminal from connection to a macrocell base station to connection with a femto (“Hand-in”) poses challenges in terms of target disambiguation, in other words, uniquely identifying the target femto for handover from multiple candidates. Lack of information of the identity of the best handover candidate is the heart of the target “ambiguity” problem. This problem gets worse as the density of femtos in any given area increases. Specifically, this problem arises as many femtos have to share just a few primary scrambling codes, so there is much scrambling code reuse, meaning that a primary scrambling code does not identify the femtocell. Even when limited to within the coverage area of a single macrocell, there may be many target femto candidates. 
     In known systems, messages passed from the macrocell base station towards the core network during preparation for handover do not enable the best target femto to be uniquely identified. The user terminal receives paging signals from various femtos and reports to the macrocell base station the scrambling code of the best quality received signal and the best target femto. However, that code is used by many other femtos also, so does not clearly identify to the macrocell base station which is the best target femto. 
     Some methods of macrocell base station to femto handover (hand-in) are known. In some known approaches, the femto is closed access, meaning that only a few user terminals are permitted to connect to the femto and these user terminals are listed in an access control list according to their unique identifiers, namely their respective International Mobile Subscriber Identity (IMSI). The list of handover candidates is then greatly reduced as the only femtos that may be considered as target candidates are those having that user terminal on their respective access control list as a permitted user. 
     Where the femtos are open access, femtos do not have an access control list. However it is possible to determine handover candidates using characteristics of the handover source macrocell and also characteristics of femtos. Specifically, only femtos are selected that are within the macrocell and have the same scrambling code as that identified by the user terminal and informed to the macrocell base station. For example, in a system where femtos use six scrambling codes distributed in equal proportions among the femtos, then the size of the candidate list is correspondingly reduced six-fold. Handover is then attempted to all of the femtos that remain on the candidate list. Once one of these successfully takes on the connection with the user terminal, that successful femto informs the network, which then instructs all of the other femtos on the list to cease their handover acceptance attempts. As there are usually many femtos on the reduced list of handover candidates, resources are wasted in the multiple unsuccessful handover attempts that are made. 
     SUMMARY 
     The reader is referred to the appended independent claims. Some preferred features are laid out in the dependent claims. 
     An example of the present invention is a method of identifying a handover target femtocell base station from among multiple femtocell base stations, the method comprising: 
     (a) measuring a characteristic of the handover target femtocell base station to provide a first measured value of the characteristic; 
     (b) then identifying as handover candidate femtocell base stations a first set of femtocell base stations all having that first characteristic value; 
     (c) then changing the characteristic of selected femtocell base stations in the first set; 
     (d) then measuring the characteristic of the handover target femtocell base station to provide a further measured value of the characteristic; 
     (e) then identifying, as a reduced set of handover candidate femtocell base stations, each femtocell base station in the first set having the further measured value of the characteristic; 
     (f) then checking whether the number of femtocell base stations in the reduced set of handover candidate femtocell base stations is one, and upon determining that the number of femtocell base stations in the reduced set is greater than one, taking the reduced set as the first set in repeating steps (c), (d) (e) and (f). 
     The characteristic is preferably primary scrambling code. 
     In preferred embodiments the time varying values of the characteristic are sent by a handover source base station controller to a femto-gateway in order for the femto-gateway to identify the target femto. 
     Some preferred embodiments provide a solution to macro to femto handover (“hand-in”). 
     In preferred embodiments, the unique handover target femto can be identified among a large number of femtos residing in a macrocell but sharing few primary scrambling codes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example and with reference to the drawings, in which: 
         FIG. 1  is a diagram illustrating a wireless communications network according to a first embodiment of the present invention, 
         FIG. 2  is a diagram illustrating an example femtocell base station deployment within one macrocell shown in  FIG. 1 , 
         FIG. 3  is a diagram illustrating a serving RNC and a femto gateway of the network shown in  FIGS. 1 and 2 , 
         FIG. 4  is a diagrammatic illustration of how the scrambling codes of handover target candidate femtos are changed over time in order to uniquely identify the best target, and 
         FIG. 5  is a message sequence diagram illustrating operation of the serving RNC and the femto gateway and communications with the user terminal and femto handover target candidates in the example shown in  FIG. 4 , and 
         FIG. 6  is an illustration of a femtocell base station having two radiating cells, one for handover target identification and the other for voice/data services. 
     
    
    
     DETAILED DESCRIPTION 
     We now describe a network including femtocell base stations then look in greater detail at macrocell base station to femto handover, particularly how to uniquely identify the target femto where primary scrambling codes are shared by multiple femtos. 
     Network 
     As shown in  FIGS. 1 and 2 , a network  10  for wireless communications, through which a user terminal  34  may roam, includes two types of base station, namely macrocell base stations and femtocell base stations (the latter being sometimes called “femtos”). One macrocell base station  22  is shown in  FIGS. 1 and 2  for simplicity. Each macrocell base station has a radio coverage area  24  that is often referred to as a macrocell. The geographic extent of the macrocell  24  depends on the capabilities of the macrocell base station  22  and the surrounding geography. 
     Within the macrocell  24 , each femtocell base station  30  provides wireless communications within a corresponding femtocell  32 . A femtocell is a radio coverage area. The radio coverage area of the femtocell  32  is much less than that of the macrocell  24 . For example, the femtocell  32  corresponds in size to a user&#39;s office or home. 
     As shown in  FIG. 1 , the network  10  is managed by a radio network controller, RNC,  170 . The radio network controller, RNC,  170  controls the operation, for example by communicating with macrocell base stations  22  via a backhaul communications link  160 . The radio network controller  170  maintains a neighbour list which includes information about the geographical relationship between cells supported by base stations. In addition, the radio network controller  170  maintains location information which provides information on the location of the user equipment within the wireless communications system  10 . The radio network controller  170  is operable to route traffic via circuit-switched and packet-switched networks. For circuit-switched traffic, a mobile switching centre  250  is provided with which the radio network controller  170  may communicate. The mobile switching centre  250  communicates with a circuit-switched network such as a public switched telephone network (PSTN)  210 . For packet-switched traffic, the network controller  170  communicates with serving general packet radio service support nodes (SGSNs)  220  and a gateway general packet radio support node (GGSN)  180 . The GGSN then communicates with a packet-switch core  190  such as, for example, the Internet. 
     The MSC  250 , SGSN  220 , GGSN  180  and operator IP network constitute a so-called core network  253 . The SGSN  220  and GGSN  180  are connected by an operator IP network  215  to a femtocell controller/gateway  230 . 
     The femtocell controller/gateway  230  is connected via the Internet  190  to the femtocell base stations  30 . These connections to the femtocell controller/gateway  230  are broadband Internet Protocol connections (“backhaul”) connections. 
     In  FIG. 2 , three femtocell base stations  30  and corresponding femtocells  32  are shown for simplicity. 
     It is possible for a mobile terminal  34  within the macrocell  24  to communicate with the macrocell base station  22  in known manner. When the mobile terminal  34  enters into a femtocell  32  for which the mobile terminal is registered for communications within the femtocell base station  30 , it is desirable to handover the connection with the mobile terminal from the macrocell to the femtocell. In the example shown in  FIG. 2 , the user of mobile terminal  34  is a preferred user of the nearest  32 ′ of the femtocells  32 . 
     As shown in  FIG. 2 , the femtocell base stations  30  are connected via the broadband Internet Protocol connections (“backhaul”)  36  to the core network (not shown in  FIG. 2 ) and hence the rest of the telecommunications “world” (not shown in  FIG. 2 ). The “backhaul” connections  36  allow communications between the femtocell base stations  30  through the core network (not shown). The macrocell base station is also connected to the core network (not shown in  FIG. 2 ). 
     Identifying the Femtocell Base Station that is to be the Handover Target 
     As shown in  FIG. 3 , the serving RNC  170 ′ of the macrocell base station  22  to which the voice or data call is connected includes a femto handover candidate list  300  and a Radio Access Network Application Part (RANAP) processor  302 . The femto gateway  230 , which appears as an RNC interface to the core network, is connected to multiple femtos  30  and also includes a RANAP processor  304 . Operation is described below. 
     Process 
     The process of reducing the number of femtos in the candidate list is repeated (iterated) until the best handover target is uniquely identified. The process involves varying Primary Scrambling Codes over time to provide information to uniquely identify the best handover target femto. 
     Informed by measurement reports provided by a user terminal as to signal strength of neighbouring cells, the source RNC  170 ′ may decide to seek to handover the connection with the user terminal from the macrocell base station to a femto. Upon making that decision, the source RNC sends a handover request, specifically a RANAP relocation request, to the target femto gateway, which acts as a target RNC. 
     If the femto candidate list has greater than one candidate, then the target femto gateway  23  reacts by instructing a subset of the femto candidates to change primary scrambling code in a predetermined way, and a relocation failure message is sent from the target femto gateway to the source RNC. A further measurement report from the user terminal is made causing the candidate list to be reduced. Steps of user terminal measurement and primary code variation are repeated until the candidate list is of just one candidate, at which time the target femto is uniquely identified and so the handover proceeds. 
     An example is provided below. 
     Four Candidate Example 
     As shown in  FIG. 4 , consider as an example a macrocell coverage area within which twelve femtos are situated, those femtos sharing three Primary Scrambling Codes (PSCs). As shown in  FIG. 4 , at a first time instance t o , four femtos A, B, C, D use the first PSC 1 , another four femtos use the second PSC 2 , and another four femtos use the third PSC 3 . 
     It has been identified, by radio measurements taken by the user terminal  34  that is in call connection with the macrocell base station  22 , that the PSC of the base station which is the best handover target is PSC 1 . However the issue is then to identify which of the four femtos A, B, C, D in the macrocell that use PSC 1  is the best handover target. 
     The PSCs of selected handover candidates are changed over time, in such a way as to enable the best handover candidate to be uniquely identified. In this example, as shown in  FIG. 4  at a time t 1 , femto C and femto D are both changed to PSC 2 . As shown in  FIG. 4 , then at a subsequent time t 2 , femto B is changed to PSC 2  and femto C is changed back to PSC 1 . Thus at times t 0 , t 1 , t 2 , the sequences of scrambling codes for the four candidate femtos areas are as shown in Table 1. This enables the femto handover target to be uniquely identified. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Femto 
                 PSC at time t 0   
                 PSC at time t 1   
                 PSC at time t 2   
               
               
                   
                   
               
             
             
               
                   
                 A 
                 PSC1 
                 PSC1 
                 PSC1 
               
               
                   
                 B 
                 PSC1 
                 PSC1 
                 PSC2 
               
               
                   
                 C 
                 PSC1 
                 PSC2 
                 PSC1 
               
               
                   
                 D 
                 PSC1 
                 PSC2 
                 PSC2 
               
               
                   
                   
               
             
          
         
       
     
     As shown in  FIG. 5 , the user terminal  34  in the coverage area of the macrocell base station (not shown in  FIG. 5 ) sends (step a) a measurement report via the macrocell base station to its serving RNC  170 ′ which is the source RNC as regards handover. The measurement report includes an identifier of the PSC of the best handover candidate as being PSC 1 . 
     A handover request including an identifier of PSC 1  is sent (step b) to the target femto gateway  230 . The femto gateway recognises PSC 1  as one of the few primary scrambling codes reserved for femtos. In consequence, the femto gateway determines (step c) that of the twelve candidate femtos in the macrocell coverage area, only the four A, B, C, D then using PSC 1  are still candidates. 
     The gateway  230  then selects which femtos are to change primary scrambling code so as to aid handover target identification. In this example, femto C and D are selected (step d) and instructions are sent (step e, f) to each of them to change to PSC 2 . Femto C changes (step h) to PSC 2 . Femto D changes (step g) to PSC 2 . As no unique handover target femto is identified the gateway then sends (step i) a relocation failure message to the serving RNC  170 ′ 
     A further measurement report is then received (step j) by the RNC  170 ′ from the user terminal  34 . This further measurement report includes an identifier of the PSC of the best handover candidate as being PSC 2 . A handover request including an identifier of PSC 2  is sent (step k) to the target femto gateway  230 . In consequence, the femto gateway determines (step l) that of the four previously-identified candidate femtos A, B, C, D, only two, namely C and D, are then using PSC 2  so are still candidates. 
     The gateway  230  then selects which of the femtos are to change primary scrambling code. In this example, femto B and C are selected (step m) and instructions are sent (step n, o) to each of them to change to PSC 2 . Femto B changes (step p) to PSC 2 . Femto C changes (step q) to PSC 1 . As no unique handover target femto was identified in that cycle, the gateway sends (step r) a relocation failure message to the serving RNC  170 ′. 
     The next measurement report from the user terminal identifies (step s) the PSC of the best handover candidate femto as PSC 2 . A handover request including an identifier of PSC 2  is sent (step t) to the target femto gateway  230   
     The gateway  230  identifies (step u) from the reduced candidate list of femto C and Femto D, that only femto D then uses PSC 2 , so femto D is the uniquely identified candidate for handover. In other words, consistent with the measurement reports at those three times t 0 , t 1 , t 2 , only femto D had the PSC 1  at t o , PSC 2  at t 1  and PSC 1  at t 3 . The gateway  230  then sends (step v) a handover request (namely a RANAP relocation request) to femto D. Femto D replies (step w) with a handover request accept message. Handover (step x) is then undertaken. 
     Minimising Impact on Served User Terminals 
     Whilst varying the PSC of a femto in seeking to identify the best handover target for a macro-connected user terminal, it is desirable for detrimental effects to be minimised on other user terminals that are either camped in idle mode on the femto or in active connection with the femto. 
     In the example described above, this is done by each femto generating two different, but overlapping, coverage areas. As shown in  FIG. 6 , there is a service coverage area  61  that provides normal cellular service and a handover assisting coverage area  63  which assists by cycling through PSCs under the control of the femto gateway. In use, once the target femto has been uniquely identified, a user terminal is handed over from the macrocell base station to the handover assistance cell  63 . After a short time, the user terminal is then handed over to the regular service cell  61 . Moving between the two cells is controlled by setting cell reselections and handover parameters appropriately, and in some examples, also setting the relative transmit powers of the two coverage areas  61  and  63 . This means that hand-in from the macro to the femto service cell  61  is a two-stage process: first handover to the handover assistance cell  63  then handover from there to the service cell  61 . 
     An alternative is that before its Primary Scrambling Code (PSC) is changed, the neighbour list of a femto is updated to include the newly assigned PSC. The femto then sends a command over its broadcast channel for connected user terminals to reread the PSC so as to be up to date. Then when the PSC of the femto actually changes, user terminals in idle mode switch to camping on that femto using the new PSC and all user terminals in active mode reconfigure their radio resources, by way of a physical channel reconfiguration, to use the new PSC in their connections with that femto. Essentially from the perspective of a user terminal, these processes appear like a handover or relocation. 
     Some Other Examples 
     By way of a further example, if six primary scrambling codes are reserved for femtos in a network, and there are (6 to the power of 5=) 776 femtos, by appropriate changing of primary scrambling codes over time such that each femto has a unique sequence of primary scrambling codes, unique identification of the correct target femto is possible with just five handover request rejections by the target femto gateway acting as an RNC. 
     In an example which consists of a macrocell having a radius of 1 kilometre and one femto per 500 square metres, there are 6280 femtos inside the macrocell. As this is less than 7776, with only five handover request rejections the target femto is uniquely identified. 
     In an example network, there are 512 different primary scrambling codes available but typically few are allocated to femtos. These few are a common set of PSCs reserved for femtos in any macrocell. Alternatively the set of primary scrambling codes allocated to femtos can be specific to each macrocell. 
     The particular examples described above involve altering, at times, primary scrambling codes. Another configuration parameter, or other configuration parameters may be used in addition, or instead, in order to uniquely identify the handover target. For example, carrier frequency may be used. 
     General 
     The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     A person skilled in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Some embodiments relate to program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. Some embodiments involve computers programmed to perform said steps of the above-described methods.