Abstract:
A method is provided of controlling transmission power of a picocell base station. The method comprises the steps of the picocell base station: transmitting a signal at a first power; receiving a report from a mobile terminal that the signal is received within a predetermined quality range; dependent upon receiving said report instructing the mobile terminal to report as to the number of neighboring macrocells detected by the mobile terminal; and controlling the transmission power of a further signal dependent upon said number.

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 relative to a macrocell. Cells that are smaller than macrocells are sometimes referred to as microcells, picocells, or femtocells, but we use the term picocells generically for cells that are smaller than macrocells. One way to establish a picocell is to provide a picocell base station that operates within a relatively limited range within the coverage area of a macrocell. One example of use of a picocell base station is to provide wireless communication coverage within a building. 
         [0004]    The picocell base station is of a relatively low transmit power and hence each picocell is small compared to a macrocell. 
         [0005]    Picocell base stations can use broadband internet Protocol connections as “backhaul”, in other words they are connected via picocell base station gateways to, for example, a Universal Mobile Telecommunications System (UMTS) core network, specifically each gateway may be connected to a UMTS Mobile Switching Centre (MSC) and hence a Signaling Gateway Support Node (SGNS). 
         [0006]    Picocell base stations are intended primarily for users belonging to a particular home or office. Picocell base stations may be private access or public access. In picocell base stations of private access type, access is restricted only to registered users, for example family members or particular groups of employees. On the other hand, in public access picocell base stations, other users may also use the picocell base station, subject to certain restrictions to protect the Quality of Service received by registered users. 
         [0007]    Picocell base stations may be used to provide radio coverage where there are coverage “holes” that is areas that macrocells do not cover. However if, the picocell size is set to be too large, in other words, power of transmitted signals from the picocell base station are set to be too high, then the picocell will cause unnecessary interference to neighboring macrocells. 
         [0008]    Conversely, if the picocell size is set to be too small, in other words, powers of transmitted signals from the picocell base station are set to be too low, then the picocell will not achieve the aim of filling the coverage hole. 
         [0009]    Handover success rate may be used as an indicator that a coverage hole is present, however it is not definitive, as other factors, such as the level of network loading and signaling delays, may also cause handover failures. 
       SUMMARY OF THE INVENTION 
       [0010]    The reader is referred to the appended independent claims. Some preferred features are laid out in the dependent claims. 
         [0011]    For example, a method is provided of controlling transmission power of a picocell base station. The method comprises the steps of the picocell base station transmitting a signal at a first power and receiving a report from a mobile terminal that the signal is received within a predetermined quality range. Dependent upon receiving said report the picocell base station instructs the mobile terminal to report as to the number of neighboring macrocells detected by the mobile terminal; and then controls the transmission power of a further signal dependent upon said number. 
         [0012]    Preferred embodiments advantageously enable coverage holes between macrocells to be filled by increasing the transmission power of a picocell base station. Preferred embodiments advantageously enable levels of interference experienced by macrocells to be reduced by decreasing the transmission power of the picocell base station. I this way, a steady-state transmission power can be achieved where said number of neighboring macrocells detected is optimum. 
         [0013]    The process is preferably automatic, in particular of an auto-configuring nature. Advantageously, in preferred embodiments, manual tuning of transmission power level is not required. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An embodiment of the present invention will now be described by way of example and with reference to the drawings, in which: 
           [0015]      FIG. 1  is a diagram illustrating a wireless communications network according to a first embodiment of the present invention, 
           [0016]      FIG. 2  is a diagram illustrating the picocell base station of the network shown in  FIG. 1 , 
           [0017]      FIG. 3  is a flow chart illustrating the transmit power adjustment method used by the picocell base station, 
           [0018]      FIGS. 4   a,    4   b,  and  4   c  illustrate schematically possible picocell base station transmit power levels that may occur, where  FIGS. 4   a  and  4   b  illustrate power levels before adjustment, and  FIG. 4   c  illustrates the power level after adjustment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    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 picocell base stations. Two macrocell base stations  22  for wireless telecommunications are shown for simplicity in  FIG. 1 . 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 respective macrocell base station  22  and the surrounding geography. 
         [0020]    A picocell base station unit (PCBSU)  30  provides wireless communications within a picocell  36 . A picocell is a radio coverage area. The radio coverage area of the picocell  30  is much less than that of the macrocell  24 . For example, the picocell  36  corresponds in size to a user&#39;s home. 
         [0021]    It is possible for a mobile terminal  44  within the macrocell  24  to communicate with the macrocell base station  22  in known manner. When the mobile terminal  44  enters into a picocell  36  for which the mobile terminal is registered for communications within the picocell base station  30 , it is desirable to handover the connection with the mobile terminal from the macrocell to the picocell. In the example shown in  FIG. 1 , the user of mobile terminal  44  is a registered user of the picocell base station  36 . 
       Picocell Base Station 
       [0022]    As shown in  FIG. 2 , the picocell base station  30  includes a transmitter  46  and a receiver  48 , a pilot signal generator  50  and a processor  52 . The pilot signal generator  50  is connected to the transmitter  46 . The processor  52  includes a picocell edge detector  54 , a neighbor cell measurement controller  56 , and a picocell pilot power controller  58 . The picocell edge detector is connected to the receiver  48  and neighbor cell measurement controller  56 . The picocell pilot power controller  58  is connected to the pilot signal generator  50 . 
       Operation of the Picocell Base Station 
       [0023]    As shown in  FIG. 3 , the picocell base station  30  follows a process as follows. 
         [0024]    The process starts in that the mobile terminal, in dedicated state (i.e. call-connected) sends periodic measurement reports of signal to interference ratio (Ec/Io) of received pilot signals from the picocell base station. The pilot signal to interference ratio is a measure of the quality of the currently used channel. 
         [0025]    The reports are received by the receiver  48  and processed by the picocell edge detector  54 . 
         [0026]    At some point, the mobile terminal  44  sends a so-called “Event 2d” report (step a) indicating a pilot Ec/Io in a range −19 dB&lt;pilot Ec/Io&lt;−15 dB. 
         [0027]    Upon the picocell edge detector  54  determining from the report that the signal to interference ratio (Ec/Io) of received pilot signals from the picocell base station is in the range −19 dB&lt;pilot Ec/Io&lt;−15 dB, the mobile terminal is considered to be at the picocell  32  edge, so the picocell edge detector  54  sends a control signal to the neighbor cell measurement controller  56 . The controller  56  then sends via the transmitter  46  a control message asking (step b) the mobile terminal to report as to the number of neighboring macrocells the mobile terminal detects. 
         [0028]    This report from the mobile terminal as to number is received by the receiver  48  and passed to the picocell pilot power controller  58 . The picocell pilot power controller  58  determines (step c) whether or not the reported number of macrocells is greater than three. 
         [0029]    If yes (step d) , the picocell pilot power controller  58  controls the pilot signal generator  50  to reduce (step e) the pilot power by a predetermined amount and a return is then made (step f) to the start. 
         [0030]    If no (step g), a determination is made (step h) in the picocell pilot power controller  58  as to the whether or not the reported number of macrocells is three. If no (step i) then the picocell pilot power controller  58  controls the pilot signal generator  50  to increase (step j) the pilot power by a predetermined amount and a return is then made (step k) to the start. On the other hand, if yes (step l)i.e. the number of macrocells detected is three, then the pilot signal power is not adjusted and process ends (step m). 
         [0031]    Of course the process progresses as each mobile terminal continues to send periodic measurement reports of signal to interference ratio (Ec/Io) of received pilot signals from the picocell base station and at some further points in time, a mobile terminal  44  sends a report (step a) indicating a pilot Ec/Io in a range −19 dB&lt;pilot Ec/Io&lt;−15 dB. 
         [0032]    As illustrated in  FIG. 4   a,  in the scenario that the mobile terminal  44  at the picocell  36  edge detects less than three macrocells  24 , it is considered that the level of picocell base station  30  pilot power is too low such that there is a coverage hole to be “filled”, so an increase in pilot power is appropriate, so as to expand the picocell. 
         [0033]    As illustrated in  FIG. 4   b,  in the scenario that the number of macrocells  24  detected by the mobile terminal  44  at the picocell  36  edge is more than three, then it is considered that the level of picocell base station  30  pilot power is too high such that unacceptable interference to the macrocells  24  will occur, so a reduction in pilot power is appropriate, so as to shrink the picocell. 
         [0034]    The process typically converges after a few “iterations”. Each iteration requires a mobile terminal at the then current picocell edge to trigger the process by a measurement report indicating received pilot power within the predetermined range. Accordingly, different iterations may involve different mobile terminals. 
         [0035]    As illustrated in  FIG. 4   c,  the converged scenario is where three macrocells  24  are detected by the mobile terminal  44  at the edge of the picocell  30 . 
       General 
       [0036]    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.