This application claims benefit of the foreign application priority of UNITED KINGDOM application No. 0301546.8 filed on Jan. 22, 2003. 
     FIELD OF THE INVENTION 
     The present invention relates to cellular communication systems. The present invention relates in particular, but not exclusively, to data collection for frequency planning for cellular communication systems based on measurement reports. Cellular communications systems to which the present invention may be applied include, for example, Global System for Mobile Telecommunication (GSM), Universal Mobile Telecommunications System (UMTS), and General Packet Radio Service (GPRS) systems. 
     BACKGROUND OF THE INVENTION 
     In a cellular communications system, the area over which service is provided is divided into a number of smaller areas called cells. Typically each cell is served from a base transceiver station (BTS) which has an antenna or antennas for transmission to and reception from a large number of user stations, normally mobile stations, e.g. mobile telephones. Examples of established harmonised cellular radio communications systems are Global System for Mobile Telecommunication (GSM) and General Packet Radio Service (GPRS) systems. A further harmonised standard being introduced is the Universal Mobile Telecommunications System (UMTS). 
     In cellular communication systems, multiple frequency channels are used in each cell, as well as used in different cells. A technique known as frequency hopping is used within cells to provide diversity. The choice of which frequencies are to be used in which cells plays an important role in the efficiency and quality of service in a system. For example, adjoining cells are prone to interfere with each other, so appropriate selection of frequencies may reduce such interference. 
     However, in order to achieve efficient usage of resources and provide service to large numbers of users, practically it is necessary to employ frequency plans that may involve potential interference. Various methods are known for testing systems to acquire data to enable frequency plans to be improved. One example is drive testing. Another example is to collect data from measurement reports that are anyway produced in the operation of the cellular communication system. Such measurement reports are produced by mobile stations and transmitted to control units of the system infrastructure. 
     The measurement reports comprise details of e.g. signal strength measured by the mobile station on the frequency or frequencies it is communicating with in its own cell. In addition, the measurement reports comprise similar details of frequencies that correspond to certain neighbouring cells. The mobile station is informed of which frequencies (and in effect which neighbouring cells) to measure and report by virtue of these being on a so-called neighbour list, determined by a control unit of the infrastructure of the system and communicated to the user station. The neighbour list is primarily provided to enable preparation for handover; however of interest here is the fact that measurements of the neighbouring cells and frequencies can be used in frequency planning. 
     In GSM systems, the neighbour list is transmitted to the mobile stations on a control channel called the Slow Associated Control Channel (SACCH), and hence the neighbour list may conveniently be called a SACCH neighbour list. Neighbours included for the primary purpose of potential handover planning are known as configured neighbours. For the purpose of providing measurement reports for collecting data for frequency planning, extra measurement frequencies are added to the SACCH neighbour list. The extra frequencies are known as test frequencies or test neighbours. 
     Conventionally, the same SACCH neighbour list is transmitted to all user stations in a given cell. Also, conventionally, the test frequencies employed in a given cell are changed once a day, so that over the course of a number of days all the frequencies it is desired to test may be included. 
     In each system there is a predetermined limit as to the total number of neighbour cells/frequencies that may be set. In GSM this limit is 32. However, there may be a large number of configured neighbours in a given cell, leaving only a small number of test frequencies that may be employed each day. Thus, the conventional approach is often slow and inefficient. 
     Moreover, by requiring different test frequencies to be tested on different days, the resulting data may not be properly representative of the real life situation, as statistical affects due to different communications traffic on different days may distort the results. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a method of determining frequency planning measurement data in a cellular communications system, as claimed in claim  1 . 
     In a further aspect, the present invention provides apparatus for determining frequency planning measurement data in a cellular communications system, as claimed in claim  3 . 
     The present invention tends to improve the quality of the data collected, by recording data for more frequencies over a shorter time period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a small part of a GSM cellular communication system; 
         FIG. 2  shows a table of frequency channels allocated against neighbour positions over the course of three calls; and 
         FIG. 3  is a table showing which of the frequency channels shown in  FIG. 2  will have been measured during the calls shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  is schematic illustration of a small part of a GSM cellular communication system  1  having a coverage area formed by a number of cells  12 - 22 . A conventional schematic representation showing a hexagonal cell pattern has been used to depict the cell areas, but in practice the shape and size of each cell will be different. 
     In each cell, there is a base station system (BSS)  24 - 34 . In this embodiment each BSS  24 - 34  is an integrated base station controller (BSC) and base transceiver station (BTS). In practice large numbers of mobile stations (MSs) will receive service from the infrastructure shown In  FIG. 1 . However, for clarity, only a few such MSs are shown, namely MSs  42 ,  44 ,  46  and  48 . 
     The BSSs  24 - 34  are each coupled to a Mobile Services Switching Centre (MSC)  50 . The MSC  50  is coupled to an Operations and Management Centre (OMC)  49  and to a Public Switched Telephone Network (PSTN)  51 . 
     In operation, the BSSs  24 - 34  receives radio signals from the MSs and transmits radio signals to the MSs. The MSC  50  provides interconnection and routing, both within the cellular communication system  10  (in co-operation with other MSCs, not shown, covering other areas) and to external elements via the PSTN  51 . The OMC  49  is used by the operator of the cellular communication system  1  to configure and maintain the system  1 . 
     Each BSS  24 - 34  allocates and transmits neighbour lists to MSs receiving service in the corresponding cell. This is implemented by databases and software at the respective BSSs, under instruction from the OMC  49 . The neighbour lists are transmitted from the BSSs to the MSs using the Slow Associated Control Channel (SACCH). 
     In this embodiment, the BSSs  24 - 34  have been adapted to offer, and provide for, a different way of forming the SACCH neighbour lists, as will be described in more detail below. 
     However, this adaptation may be implemented in any suitable manner. A new module may be added to a conventional BSS. The module may consist of a single discrete entity added to a conventional BSS, or may alternatively be formed by adapting existing parts of a conventional BSS, for example by reprogramming of a one or more processors therein. As such the required adaptation may be implemented in the form of processor-implementable instructions stored on a storage medium, such as a floppy disk, hard disk, PROM, RAM or any combination of these or other storage media. Furthermore, whether a separate entity or an adaptation of existing parts or a combination of these, the module may be implemented in the form of hardware, firmware, software, or any combination of these. 
     It is also within the contemplation of the invention that such adaptation of the means for forming SACCH neighbour lists may alternatively be controlled, implemented in full, or implemented in part, by a module added to or formed by adaptation of any other suitable part of the cellular communication system  1  or other cellular communication systems. For example, this may be implemented instead at separate BTSs or BSCs when the BSSs are of a form comprising separate BTSs and BSCs. As another example, this may be implemented at OMCs, in situations where OMCs of systems can directly implement neighbour lists. Further, in the case of other system infrastructures or layouts, implementation may be at any appropriate system node such as any other appropriate type of base station, base station controller etc. Alternatively various parts of the process and means for forming the adapted neighbour lists can be carried out by various components distributed at different locations or entities within the above described cellular communication system  1  or any other suitable cellular communication network or system. 
     The way in which, in this embodiment, the BSSs  24 - 34  form the SACCH neighbour lists will now be described, with reference to  FIGS. 2 and 3 . 
     In this embodiment, the BSSs  24 - 34  each form and allocate SACCH neighbour lists on a per call basis. Thus, in terms of an overview, the SACCH neighbour list is formed and allocated separately each time a call is conducted to/from each MS. In this embodiment, a different SACCH neighbour list is formed for each call in each cell. 
     This process will now be described by use of a simplified example.  FIG. 2  shows a table of frequency channels  206  allocated against neighbour positions  202  over the course of three calls  204  (call numbers  1 ,  2  and  3  in the table) serviced by BSS  28  with respect to MSs in its cell, i.e. a first call conducted by/for MS  44 , a second call conducted by/for MS  46 , and a third call conducted by/for MS  48 . In this example these calls are taking place at the same time (i.e. overlap in time, although not necessarily starting or ending at the same time), although the process described below may also be applied to different calls at different times. 
     In this simplified example, for the sake of clarity, only 12 neighbour positions  202 , numbered  1 - 12  are included. However, as mentioned earlier, under GSM a maximum of 32 are usually employed. Thus, in this example, in any neighbour list, only twelve frequencies (i.e. frequency channels) may be included. Also, for the sake of clarity, the number of available frequencies (frequency channels) is limited in this simplified example to sixteen, made up of the eight frequencies called  1 - 8  in  FIG. 2  and the eight frequencies called  12 - 19  in  FIG. 2 . In this example, the frequencies  1 - 8  and  12 - 19  are used to indicate how there are usually gaps in the frequency channels employed, e.g. here frequencies  9 - 11  are unavailable. 
     In this example, for the first call, the BSS  28  forms, allocates and transmits (to the MS  44  conducting the first call) a first neighbour list  208 . The first neighbour list  208  comprises configured frequencies  1 ,  3 ,  4 ,  7 ,  12  and  17  in neighbour positions  1  to  6  respectively. (Strictly speaking, these are the Broadcast Control Channel (BCCH) parts of the frequency entries in the neighbour list, which in full also include Base Transceiver Station Identity Code (BSIC) parts. However, such details are well known to the skilled person in the art of cellular communications systems, in particular GSM, and need not be described further here for the purpose of explaining the present invention.) The first neighbour list  208  further comprises test frequencies  2 ,  5 ,  6 ,  8 ,  13  and  14  in neighbour positions  7  to  12  respectively. 
     For the second call, the BSS  28  forms, allocates and transmits (to the MS  46  conducting the second call) a second neighbour list  210 . The second neighbour list  210  comprises configured frequencies  1 - 4 ,  3 - 5 ,  4 - 7 ,  74 ,  124  and  17 - 3  in neighbour positions  1  to  6  respectively. These are the same configured frequencies/neighbour positions as in the first neighbour list  208 . However, the second neighbour list  210  further comprises test frequencies  15 ,  16 ,  18 ,  19 ,  2  and  5  in neighbour positions  7  to  12  respectively i.e. overall the second neighbour list  210  is different to the first neighbour list  208 . 
     For the third call, the BSS  28  forms, allocates and transmits (to the MS  48  conducting the third call) a third neighbour list  212 . The third neighbour list  212  comprises configured frequencies  1 - 4 ,  3 - 5 ,  4 - 7 ,  7 - 4 ,  12 - 4  and  17 - 3  in neighbour positions  1  to  6  respectively. These are the same configured frequencies/neighbour positions as in the first neighbour list  208  and the second neighbour list  210 . However, the third neighbour list  212  further comprises test frequencies  6 ,  8 ,  13 ,  14 ,  15  and  16  in neighbour positions  7  to  12  respectively i.e. overall the third neighbour list  212  is different to both the first neighbour list  208  and the second neighbour list  210 . 
     Thus, for each of the three calls, a different neighbour list is employed. 
     A benefit of the frequency allocation described above with reference to  FIG. 2  may be seen with reference to  FIG. 3 .  FIG. 3  is a table showing which of the above described frequencies  206  will have been measured during the above described calls  204  (this is indicated in  FIG. 3  by an “X” in each case). As can be seen, all 16 frequencies (i.e.  1 - 8  and  12 - 19 ) will have been measured. This is in contrast to the fact that if the same neighbour list has been used for all three calls (as per the prior art), only some of the frequencies would have been measured. (Of course, in some applications, not all frequencies will be measured, however there will still tend to be an advantageous increase in the number of frequencies measured compared to the number measured with a common unchanging neighbour list.) 
     In the above described embodiment, a different SACCH neighbour list is formed for each call in each cell. However, there are a number of ways in which a less extreme variation is employed, whilst still making effective use of the concept of forming and allocating the SACCH neighbour list on a per call basis. 
     For example, it may be that all possible combinations are tested in a given time before it is wished to stop testing. In this case some or all of the transmitted combinations may be used again. 
     As another example, a subset of combinations of neighbour lists may be provided which are believed to cover all variables required to be assessed, and these may be repeated on a cyclical basis. 
     As another example, the process may be applied to substantially all calls, but not absolutely all calls. Calls may be omitted from variation of neighbour list for any one or any combination of reasons, for example certain calls may be excluded due to processing limitations, random errors, requirement to provide a constant neighbour list for some other reason, and so on. Another possibility is calls to certain MSs may be excluded from variation. 
     As another example, the process may be applied to some but not all the cells of the cellular communications system. 
     As another example, the process may be applied for some period or periods during the day, but not all day. 
     As another example, the neighbour list may be varied for plural numbers of calls rather than every call. For example, a given neighbour list may be used for two calls, then changed to a new list used for a further two calls, and so on. Any appropriate number of calls for such a regime (i.e. as opposed to every one or every two calls) may be used according to the requirements of the particular system or situation under consideration. 
     Also, any two or more of the above examples may occur in combination. 
     Thus, the terminology “on a per call basis” as used herein is to be understood as encompassing different ways of varying the neighbour lists according to different calls being serviced, rather than being independent of calls, and rather than being uniformly implemented irrespective of which calls are being processed. 
     Although the above embodiment has been described in terms of a GSM system, it will be appreciated that the present invention may be embodied in any suitable cellular communications system in which neighbour positions, or equivalent characteristics, may be used to provide measurement data for frequency planning. 
     The present invention may in appropriate circumstances provide, inter alia, one or more of the following advantages: 
     the data collection time required to provide a given amount of useful input for frequency planning is reduced; 
     the total amount of data required to be collected to provide a given amount of useful input for frequency planning is reduced; 
     reduced data analysis time; 
     reduced hardware and software requirements to process data due to less data; 
     reduction in cycle time required to provide a given amount of useful input for frequency planning, thereby providing more representative frequency data corresponding more closely to actual cell traffic.
A method, and apparatus for, determining frequency planning measurement data in a cellular communications system ( 1), comprising: allocating neighbor lists ( 208, 210, 212) to provide measurement data for frequency planning; wherein the neighbor lists ( 208, 210, 212) are allocated on a per call basis. Thus different neighbor lists ( 208, 210, 212) may be allocated by the same base station ( 28) to different mobile stations ( 44, 46, 48) for respective calls ( 204) that overlap in time. A different neighbor list ( 208, 210, 212) may be allocated for substantially each call ( 204) conducted by a base station ( 28) of the cellular communication system ( 1), compared to respective directly preceding calls or may be allocated for substantially each of a predetermined number of calls conducted by a base station ( 28) of the cellular communications system ( 1), compared to the predetermined number of directly preceding calls. At least substantially all test frequencies ( 206) for the cell ( 16) served by the base station ( 28) may thereby be covered over a plurality of calls ( 204).