Abstract:
A method is provided of identifying which one of a plurality of femtocell base stations neighbouring a first femtocell base station is to be the target for handover of a call connection with a user terminal from the first base station. The neighbour femtocell base stations use the same primary scrambling code as each other. The femtocell base stations use code-division multiple access, CDMA. Each of the neighbour femtocell base stations transmits a respective broadcast signal that include timing information such that each neighbour femtocell base station transmits a broadcast signal having different timing information to that of the other neighbour femtocell base stations. The user terminal receives broadcast signals, determines the timing information of at least one of the received broadcast signals and reports the timing information to the first femtocell base station. The first femtocell base station identifies the target femtocell base station for handover from the timing information.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to telecommunications, in particular to wireless telecommunications. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    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. 
         [0003]    It is possible to establish smaller sized cells within a macrocell. Cells that are smaller than macrocells are sometimes referred to as 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. 
         [0004]    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. 
         [0005]    Femtocell base stations are intended primarily for users belonging to a particular home or office. Femtocell base stations may be private access or public access. In femtocell base stations that are private access, access is restricted only to registered users, for example family members or particular groups of employees. In femtocell base stations that are public access, other users may also use the femtocell base station, subject to certain restrictions to protect the Quality of Service received by registered users. 
         [0006]    One known type of Femtocell base station uses a broadband Internet Protocol connection “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. 
         [0007]    Femtocell base stations are sometimes referred to as femtos. 
         [0008]    One problem of femtocell base stations, particular those involving code division multiple access, such as those in accordance with current Third Generation Partnership Project (3GPP) Universal Mobile Telecommunications System (UMTS) standards, is that there are few primary scrambling codes (PSCs) available. Primary scrambling codes are used to distinguish between neighbouring base stations. Typically, there are between six and sixteen PSCs that are available to the femtocells. The PSC to be used by each femtocell base station is selected by an automatic configuration process based on measurements made by that femtocell base station. For example, a femto detects from received signals which PSCs are in use around the femto, and selects a PSC that the femto has not detected as being in use, else selects the PSC of the weakest signal received. In consequence, there is typically use of each of the available primary scrambling codes by multiple femtos, in other words much primary scrambling code “re-use”. 
         [0009]    As is known, user terminals, currently in a cell, report measurements, to the base station of that cell, that the user terminals have made as to signals received from other nearby base stations. This is so as to determine whether it is appropriate to handover the call connection with the user terminal from that cell to another cell from which a stronger signal is received. 
         [0010]    With this level of PSC reuse, there is a strong chance that user terminals served by a femto will detect signals from multiple different cells having the same primary scrambling code. There is no unique cell identifier, Cell ID, reported in the user terminal&#39;s measurement report according to current 3GPP standards. Accordingly, there is ambiguity or uncertainty as to which of the neighbouring femtos using that PSC provides the best signal to the user terminal and so should b handover target. 
         [0011]    A known approach is to seek to handover to all of the femtocells that are identified as having the primary scrambling code of the best received signal. This is inefficient because only one of the handover attempts is permitted to succeed. Network resources that are used in the handover attempts to the other femtocells than the one that is successful, are wasted. 
       SUMMARY OF THE INVENTION 
       [0012]    The reader is referred to the appended independent claims. Some preferred features are laid out in the dependent claims. 
         [0013]    A method is provided of identifying which one of a plurality of femtocell base stations neighbouring a first base station is to be the target for handover of a call connection with a user terminal from the first base station, the femtocell base stations using code-division multiple access, CDMA, wherein the neighbour femtocell base stations use the same primary scrambling code as each other; 
         [0014]    each of the neighbour femtocell base stations transmitting a respective broadcast signal that include timing information such that each neighbour femtocell base station transmits a respective broadcast signal having different timing information to that of the other neighbour femtocell base stations; 
         [0015]    the user terminal receiving at least one of the broadcast signals, 
         [0016]    wherein 
         [0017]    the first base station being a first femtocell base station; 
         [0018]    the user terminal determining the timing information of at least one of the received broadcast signals and reporting the timing information to the first femtocell base station; and 
         [0019]    the first femtocell base station identifying the target femtocell base station for handover from the timing information. 
         [0020]    The inventors found a way to differentiate neighbour femtocell base stations that use the same Primary scrambling code as each other by considering differences in timing applied to signals on their broadcast channels. This enables a best neighbour handover candidate to be uniquely identified. 
         [0021]    In preferred embodiments, a measurement report from a user terminal provides the identifying timing information. 
         [0022]    Preferred embodiments enable a neighbour femtocell base station to be uniquely identified for handover despite the Primary scrambling code “reuse” that occurs in femtocell base station deployments. This reduces the resources used in signalling as part of handover attempts to multiple neighbours, only one of which will be successful. Accordingly, in preferred embodiments, delays and blocked or dropped calls are reduced. 
         [0023]    In some embodiments, where a femtocell base station can only handle a few calls at a time, the reduced chance of calls being blocked is particularly significant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    An embodiment of the present invention will now be described by way of example and with reference to the drawings, in which: 
           [0025]      FIG. 1  is a diagram illustrating a wireless communications network according to an embodiment of the present invention, 
           [0026]      FIG. 2  is a diagram illustrating allocation of primary scrambling codes to femtocells in the network shown in  FIG. 1 , 
           [0027]      FIG. 3  is a diagram illustrating one of the femtocell base stations shown in  FIGS. 1 and 2 , and 
           [0028]      FIG. 4  is a diagram illustrating an example of a handover candidate femtocell being identified amongst the femtocells shown in  FIG. 2  that use the same primary scrambling code. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    We now describe a network including femtocell base stations then look in greater detail at a femtocell base station and the relevant operation of the femtocell base station. 
       Network 
       [0030]    As shown in  FIG. 1 , in one communication network  20 , there are two types of base stations (often denoted BS or BSs): macrocell base stations and femtocell base stations. One macrocell base station  22  for wireless telecommunications is shown for simplicity in  FIG. 1 . The 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. 
         [0031]    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 home. In  FIG. 1 , three femtocell base stations  30  and corresponding femtocells  32  are shown for simplicity. 
         [0032]    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. 1 , the user of mobile terminal  34  is a preferred user of the nearest  32 ′ of the femtocells  32 . 
         [0033]    As shown in  FIG. 1 , the femtocell base stations  30  are connected via broadband Internet Protocol connections (“backhaul”)  36  to a core network (not shown in  FIG. 1 ) and hence the rest of the telecommunications “world” (not shown in  FIG. 1 ). 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). 
         [0034]    As shown in  FIG. 2 , the femtocells  32  provide contiguous coverage, for example within an office building, and have six available primary scrambling codes. As shown in  FIG. 2 , in this example, these six codes, denoted PSC 1 ,PSC 2 , . . . , PSC 6  are distributed so as to avoid neighbouring cells having the same primary scrambling a first femtocell  32   a  uses code PSC 2 , and has six neighbouring femtos. Of these, two femtocells  32   b ,  32   c  that neighbour the first femtocell  32   a  use the same code PSC 1  as each other. 
         [0035]    As regards the other femtocells  32   d , 32   e , 32   f , 32   g  that neighbour the first femtocell  32   a , these use the other five primary scrambling codes. Specifically, femtocell  32   d  uses PSC 3 , femtocell  32   e  uses PSC 4 , femtocell  32   f  uses PSC 6 , and femtocell  32   g  uses PSC 5 . 
       Femtocell Base Stations 
       [0036]    As shown in  FIG. 3 , the femtocell base station  30  consists of a radio transmitter-receiver, sometimes referred to as a radio transceiver  40 . The radio transceiver  40  is connected to a broadcast channel (BCH) signal generator  42 . The broadcast signal generator  42  includes a Cell System Frame Number (SFN) generator  44 , and Primary scrambling code (PSC) information  46 . The SFN generator is connected to a clock-setting stage  48  that is connected to a backhaul transceiver  50 . The backhaul transceiver is connected to a backhaul line  56 , for example one using Internet Protocol and connected to the Internet (not shown). 
         [0037]    The radio transceiver  40  is also connected to a processor  52  of measurement reports received from a user terminal  34 . The processor  52  is also connected to an identifier  54  of a handover target femtocell base station. 
       Broadcast Channel  
       [0038]    In use, the BCH signal generator  42  sends broadcast signals that include both Primary scrambling code and current System Frame Number (SFN). SFN is a clock signal that increments from 0 to 255 every 10 mS, then repeats. The SFN has a timing offset which is set by the clock-setting stage  48  that receives a clock adjustment control signal via the backhaul line  56  and backhaul transceiver  50  from a central control node (not shown) which is part of the core network (not shown). 
         [0039]    During the configuration of each femtocell base station, each femtocell base station is assigned a timing offset by the central control node, and also receives, from the central node, information of timing offsets of neighbouring femtocell base stations, in addition to their corresponding scrambling codes and cell identities (Cell IDs). Each femtocell base station stores this information that it receives. 
         [0040]    The central control node gives different timing offsets to femtocell base stations. In particular, femtocell base stations that have the same Primary scrambling code and are within a group of neighbouring, handover candidate, femtocells are assigned different timing offsets. 
         [0041]    The user terminal  34  is connected to the femtocell base station  30 . The user terminal  34  acts to monitor signals having other Primary scrambling codes than the Primary scrambling code of the femtocell base station to which it is connected. The user terminal  34  is instructed to send a measurement report when a signal having another Primary scrambling code is received with a sufficiently high quality as to be suitable for handover. 
         [0042]    There may be several femtocell base stations using the same Primary scrambling code that are potential “targets” for handover. Accordingly, the measurement report includes information on the timing difference between the “source” femtocell base station and the particular potential “target” femtocell base station. This timing difference is directly related to the difference between the offset applied to the Sequence Frame Number (SFN) in the source femtocell base station and the offset applied to the SFN in the potential target femtocell base station. For example, the timing difference is CFN source -SFN target  where CFN source =SFN source +a known constant. 
         [0043]    As shown in  FIG. 4 , three of the femtocells  32   a , 32   b ,  32   c  are such that the first femtocell base station  30   a  is the controlling (“source”) femtocell base station for user terminal  34  and uses Primary scrambling code PSC 2 . Two femtocell base stations  30   b ,  30   c  are neighbours to the first femtocell base station  30   a  and use the same Primary scrambling code, namely PSC 1 . The femtocell base stations have different timing offsets set by the central control node against a known time point. In this example, second femtocell  30   b  has an offset of 128 frames, which is 1280 mS, because 1 frame lasts 10 milliseconds, in similar fashion, first femtocell base station  30   a  has an offset of 5, which corresponds to 50 mS. Third femtocell base station  30   c    
         [0044]    Upon receiving a signal of sufficient quality that includes Primary scrambling code PSC 1 , the user terminal  34  sends a measurement report to the first femtocell base station  30   a  that indicates that a received signal with PSC 1  had become the strongest of the received signals from the monitored set of neighbouring femtocells base stations, so handover is appropriate. 
         [0045]    The first femtocell base station  30   a  then takes the timing difference recorded in the measurement report and compares that against the information that it has received from the central node and stored as to the timing offsets assigned to its neighbouring femtocell base stations. For example, the measurement report includes a timing difference of say 1233 mS. Comparing this measured timing difference of 1233 mS to the expected timing differences (ignoring the propagation delays, that are small) of 1280-50=1230 mS for the second femtocell base station  30   b  and 300-50=250 mS for the third femtocell base station  30   c , the second femtocell base station  30   b  is identified as the best handover candidate. 
       Random Setting of Timing Offsets 
       [0046]    As an alternative to the setting of timing offsets by a central control node, in some otherwise similar embodiments, each femtocell, upon booting-up, sets the timing offset applied to SFN by selecting the offset randomly from between  0  and  255  seconds. Due to this randomness, there is a likelihood that two neighbouring femtocell base stations that use the same Primary scrambling code have sufficiently different timings to allow a confident identification of which is the correct best handover candidate. One option in this scenario is that the first femtocell base station will query the neighbour as to its timing offset using so-called “layer  3 ” signalling of known type. Specifically, the first femtocell base station knows its own timing offset and queries the neighbouring femtocell base stations as to their timing offsets. 
         [0047]    Another option is for the first femtocell base station to determine timing offsets by receiving broadcast signals from neighbours. Specifically, the first femtocell base station measure the timing offset of a neighbour femtocell base station directly, for example by switching its receiver to monitor the broadcast signal of neighbour, and comparing the measured timing with its own timing so as to calculate the timing offset of the neighbour. In this situation, the first femtocell base station may also decode a Cell ID in a received broadcast signal. The Cell ID uniquely identifies the neighbour that sent the signal. Accordingly, the first femtocell base station associates the measured timing offset of the neighbour with the neighbour&#39;s Cell ID. In accordance with current UMTS standards, user terminals do not report Cell ID, but a femtocell base station may. 
         [0048]    Another option, where femtocell base stations randomly select their own timing offsets from within the range  0  to  2 . 55  seconds, is learning the timing offsets of neighbour femtocell base stations from measurement reports provided by user terminals and history as to handover success. Specifically, the first (“source”) femtocell base station, having a set of neighbouring base stations using the same Primary scrambling code as each other, attempts to handover a call connection to a user terminal to each of the neighbouring base stations. Upon one of the handover attempts being successful, the “source” femtocell base station records the measurement report from the user terminal that the “source” femtocell base station received just prior to the handover. Subsequently the “source” femtocell base station considers the value of timing offset calculated from that measurement report as the timing offset of the neighbouring base station to which the handover was successful. Importantly, upon a further measurement report from a user terminal providing a timing offset that is close to this value, the source femtocell base station then makes, as a first choice, a handover attempt to this neighbour femtocell base station; in other words, this neighbour femtocell base station is considered the handover “target”. 
         [0049]    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,