Abstract:
Signals of different cells in a cellular wireless communications system are observed during associated intervals to predict a signal behavior in future. A target cell is selected by observing over time signal characteristics of potential target cells. Thereafter a target cell is selected by using the observed characteristics to predict which potential target cell will in future satisfy certain criteria.

Description:
BACKGROUND 
     1. Field 
     The present invention relates generally to methods, devices and systems for reselecting and then handing over a mobile communications device from a first cell to a second cell in a cellular wireless communications system. More particularly, although not exclusively, aspects and embodiments of the invention relate to criteria for selecting a second cell while a mobile station is ‘camped’ on, or otherwise interacting with and/or controlled by, a first cell. Particular aspects and embodiments of the present invention are well suited for use in a cellular wireless communications system which supports packet switched communications, for example according to the General Packet Radio Service (GPRS) standard, but are not limited to such an application. 
     2. Background 
     It is well known that cellular wireless communication systems generally comprise a number (often large) of radio transceivers, or base stations, that define service areas or cells. The schematic diagram in  FIG. 1  of the accompanying drawings, illustrates a system  100  comprising four base stations  120  defining respective cells  110 . The cells typically overlap in order to ensure continuous coverage of service in the service areas. This is desirable for many reasons, not least because cellular systems are designed specifically to accommodate users as they move around within the system. In principle, mobile communications devices  130  interact with various base stations as the devices move through the respective cells  110  of the system  100 . 
     One of the goals of a cellular wireless communication system is to enable a mobile communications device, which will be referred to herein for convenience as a “mobile station”, to remain connected to the system even when the user is moving through the system from one cell to another. Traditionally, the mobile station has been a so-called “mobile phone” or “cellular phone,” although, with advances in technology, a mobile station may be any one or more of a wide range of devices from solely voice devices to solely data devices. A mobile station may be anything from a traditional radio pager or mobile phone, though faxes, personal data assistants (PDAs), and music players, to computers, or any combination of these. This list is, of course, far from exhaustive. Indeed, although the term “mobile station” is used herein, the term is also intended to encompass devices that may not be user-operated or even user-operable, for example the device could be a wireless ‘data card’ or the like, which is within another kind of apparatus. 
     Early cellular systems were circuit switched systems. That it to say, for each call the system created a circuit that reserves a channel for the user for the duration of the call. This is an inefficient use of resources, especially for bursty data. As technology has advanced, newer cellular systems have moved away from circuit switching to packet switching in which bursts of data are sent only when needed. Consequently, cellular systems have become more suitable for the transmission of data, which tends to be transmitted in bursts rather than a continuous stream. 
     As already mentioned each cell in a cellular system is defined and served by a base station. As a mobile station is moved from the service area defined by one cell into that defined by another, the system and the mobile station must break the connection with one base station and establish a connection with another base station whilst minimizing the connection loss between the mobile station and the system. This operation is sometimes known as a cell reselection, a handoff or a handover. For simplicity of description only herein, the term “reselection” will be used as a generic term to describe the operations involved with a mobile station or equivalent moving from operating with one base station to operating with another base station; and the reader should import an alternative term, such as “handover”, “handoff” or the like, if the context so dictates. The term “camped on” is commonly used, and will be used hereinafter, to describe the base station with which, and respective cell in which, a mobile station is operating. That is, a cell reselection involves a mobile station moving from being camped on one cell to being camped on another cell. 
     Typically, a cell reselection can be initiated either by the mobile station or by the cellular system. How reselection is initiated can depend on factors such as the kind of cellular system, its mode of operation and on the capabilities of a mobile station. In any event, reselection is typically initiated either as a result of a service degradation, which tends to lead to increased power consumption requirements, or there being an opportunity to improve the service, which would lead to reduced power consumption requirements. Especially since many mobile stations operate from battery power, an opportunity to reduce power consumption, thereby improving power efficiency, is usually advantageous. Service degradation can result from factors such as increasing distance between a mobile station and a base station or natural or man-made obstructions such as hills or buildings respectively. 
     One known kind of reselection operation requires a mobile station to monitor the signaling level and suitability of cells that neighbor the cell on which the mobile station is camped, which will be referred to hereinafter as the “serving cell”, and compare the monitored service levels with the signaling level and suitability of the serving cell. Then, if the signaling level and suitability of a neighboring cell is deemed by the mobile station to be better than that of the serving cell, for at least a predefined period of time (say, five seconds), the mobile station initiates a reselection to the respective neighboring cell, which becomes the new serving cell. Such an operation is described in an ETSI Technical Specification document 145 008 v4.16.0, Digital Cellular telecommunications system (phase 2+); Radio subsystem link control (3GPP TS 45.008, version 4.16.0, release 4, section 6.6.2). 
     SUMMARY 
     The present inventors have appreciated that, according to the prior art, unnecessary cell reselection operations can occur. Since cell reselection operations can consume a significant amount of power and/or result is a significant break in communications during an established voice call or other connection, the present inventors have appreciated that it would be advantageous to attempt to avoid unnecessary cell reselection operations. Aspects and embodiments of the invention are, therefore, aimed at avoiding unnecessary cell reselection operations. 
     According to a first aspect of the present invention, there is provided a cell reselection method for selecting a target cell in a cellular wireless communications system, the method including monitoring over time signal characteristics of cells, including at least potential target cells, and selecting a target cell by using the monitored characteristics to predict which potential target cell will in future satisfy certain criteria. 
     According to a second aspect of the present invention, there is provided a cellular wireless communications system comprising plural cells, including a serving cell and plural potential target cells, and a mobile station operable according to the cellular wireless communications system, the system comprising: a first process to monitor over time signal characteristics of cells, including at least potential target cells; and a second process to select a target cell by using the monitored characteristics to predict which potential target cell will in future satisfy certain criteria. 
     According to a third aspect of the present invention, there is provided a cellular mobile communications apparatus adapted for operation in a cellular wireless communications system, the apparatus comprising: a receiver arranged to receive signals of cells, the signals having certain characteristics; and a processor arranged to operate processes for selecting a target cell, the processes comprising: a first process to monitor over time the signal characteristics of cells, including at least potential target cells; and a second process to select a target cell by using the monitored characteristics to predict which potential target cell will in future satisfy certain criteria. 
     According to a fifth aspect of the present invention, there is provided a communication device in which characteristics of signals of different sources are monitored over respective time periods and a source is selected for communication when the monitored characteristic satisfies certain criteria at a time after its respective time period. 
     According to a sixth aspect of the present invention, there is provided a transceiver in which signals of different communications nodes in a communications network are observed during associated intervals to predict a signal behavior in future, which is used to identify at least one node suitable for subsequent communication with the transceiver. 
     The above and further features, aspects and embodiments of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of embodiments of the invention given by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic diagram showing a cellular wireless communications system; 
         FIG. 2  is a high-level block diagram showing the main components in a cellular wireless communications system; 
         FIG. 3  is a graph showing a comparison of signaling parameter C 2  levels for a serving cell and neighbor cells; 
         FIG. 4  is shows the same data as in  FIG. 3  and includes, in addition, straight best-fit lines; 
         FIG. 5  is a flow chart illustrating a cell reselection operation according to one exemplary embodiment of the present invention; 
         FIG. 6  is a flow chart illustrating a cell reselection operation according to a second exemplary embodiment of the present invention; and 
         FIG. 7  is a block diagram showing the main functional components of a typical mobile station that may be configured to operate in accord with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the schematic diagram in  FIG. 2  of the accompanying drawings, there is shown a high level block diagram of a typical wireless cellular communications system, for example as shown in  FIG. 1 . For the purposes of illustration, the system includes only four base stations  120 , which provide access to the system for mobile stations  130 , for example mobile telephone handsets. Each base station  120  is controlled by a controller  210  and each controller  210  is connected to a core network  220  of the system, via an appropriate communications infrastructure. Each controller  210  can control one base station  120  but typically a controller controls more than one base station. The core network  220  in general contains the infrastructure, components and functionality for controlling the controllers  210 ; routing calls and connections of all kinds from and to mobile stations  130 ; routing calls and connections from mobile stations  130  to other systems and terminating equipment; and receiving calls and connections, from other systems and terminating equipment, which are intended for mobile stations. Examples of other terminating equipment are traditional telephone equipment  230 , which are connected to via a PSTN  234  and PSTN gateway  238  of the core network  220 , Internet servers  240 , which are connected to via an Internet gateway  244  and the Internet  248 , and other telecommunications systems or services (not shown), such as voicemail or corporate networks respectively. 
     There are various kinds of wireless cellular communications systems, which operate according to various different standards. Such systems and standards include, but are not limited to, GSM, GPRS and third generation standards such as UMTS and WCDMA. The diagram in  FIG. 2  is intended to be generic, and apply, at least functionally, to all such standards and systems. 
     Particular embodiments of the present invention relate to cell reselection in a GPRS system. According to the GPRS terminology, a base station  120  is commonly referred to as a base transceiver station (BTS) and the controller  210  is commonly referred to as a base station controller (BSC). The combination of BTS and BSC is commonly referred to as the base station subsystem (BSS). Hereafter, while GPRS components will be referred in order to describe particular embodiments of the present invention, it will be appreciated that the principles taught apply equally to other kinds of wireless cellular communications systems, such as GSM and 3G. 
     Turning now to  FIG. 3 , the graph therein shows three curves, which illustrate exemplary signaling relationships between a mobile station, its serving cell (scell), on which the mobile station is camped, and two neighboring cells (ncell 1  and −ncell 2 ). The signaling relationships are characterized by a signaling parameter, C 2 , the value of which provides an indication of the strength of signals received by the mobile station from the respective cells. In general, a higher value of C 2  indicates a stronger signal between the mobile station and the cell. 
     It will be evident that alternative embodiments of the present invention may rely on deriving and/or monitoring different parameters and characteristics of systems and mobile stations, insofar as the parameters and characteristics relate in some way to the likely signaling performance or capability between the mobile station and a base station or similar. 
     In known GPRS systems, a mobile station monitors the C 2  values of all cells that are classed as neighbors of the serving cell. Each cell identifies which other cells are classed neighbors and a list of neighbors is communicated to a mobile station, by a new serving cell, during or soon after a cell reselection operation. The mobile station attempts to monitor the C 2  values for the serving cell and all neighbor cells during the time the device is camped on the serving cell. A mobile station generates C 2  values in a pre-defined way, for example as described in section 6.4 of the aforementioned ETSI document, by evaluating various characteristics of signals received from the neighbor cells; although the most important characteristic is typically signal power. Typically, a mobile station will scan for neighbor cell signals periodically, for example every second, or as otherwise defined by a control program of the mobile station, in order to monitor the C 2  levels. 
     Referring to the curves in the graph in  FIG. 3 , it is shown that the C 2  value of scell fluctuates between about 30 and 33 for around six seconds and then decreases over the remaining four seconds to about 27.5. This may be as a result of the mobile station moving away from the scell BTS and towards the ncell 1  BTS. At around four seconds, the C 2  value for ncell 1  becomes higher than the C 2  value of scell. The C 2  value of ncell 1  increases steadily up until about nine seconds and then begins to drop. By seven seconds, the C 2  value for ncell 2  also becomes higher than the C 2  value of scell and remains higher for the remaining period shown. However, the C 2  value for ncell 2  does not exceed the C 2  value of ncell 1 . In principle, it would appear to make sense for the mobile station to select ncell 1  as a new serving cell in order to improve power-efficiency. In practice, this is exactly what happens according to prior art reselection operations. Specifically, according to the prior art, as soon as the mobile station detects that the C 2  value of ncell 1  is higher than the C 2  value of scell (which, according to the graph, is when four seconds have lapsed), the mobile station starts a timer running. Then, if, after the timer expires (for example after another five seconds), the situation remains the same, at around nine seconds, a reselection to ncell 1  is initiated by the mobile station. 
     A cell reselection operation, by its nature, can cause a significant disruption to communications. This is at least in part because, according to the GPRS standard, as soon as a mobile station reselects to a new cell, the mobile station can spend as long as eight seconds reading broadcast information before camping onto the new cell. 
     The present inventors have appreciated that it can be inefficient to initiate cell reselection to the first neighbor cell that appears to have an improved C 2  value. The graph in  FIG. 4  includes the same C 2  curves as in  FIG. 3  and, in addition, a trend line is shown for each curve. The trend lines have been calculated using the C 2  data and have been projected into the future, up until 15 seconds. The trend line for scell clearly shows a steady decline in C 2  power. In addition, the trend lines for ncell 1  and ncell 2  show a steady increase in respective C 2  powers. However, significantly, the trend lines provide a clear indication that the C 2  power of ncell 2  is likely to increase above that of ncell 1  soon after nine seconds. If this turns out to be what happens in practice then a cell reselection at nine seconds, which would occur according to the prior art, from scell to ncell 1 , would be followed soon after (probably about 5-7 seconds later) by another cell reselection operation, from ncell 1  to ncell 2 . In other words, the trend lines indicate that a cell reselection operation at nine seconds is unnecessary. 
     Preferred embodiments of the present invention use historic C 2  data or the like, as shown in  FIG. 4 , to predict which neighbor cell is likely to be the best one to move to, which may not be the one that would otherwise be selected using known selection criteria. 
     An embodiment of the present invention will now be described in detail, wherein, in a GPRS system, a cell reselection operation uses historic C 2  values to predict and then select the most appropriate neighbor cell to move to. It will, however, be appreciated that the principles are equally applicable in GPRS systems and in other kinds of wireless cellular communications systems, wherein the reselection may be initiated by the BSS (or equivalent), the core network or by a mobile station. 
     A cell reselection operation according to an exemplary embodiment of the present invention will now be described in more detail with reference to the flow diagram in  FIG. 5 . In a first step  500 , a mobile station becomes camped on a new serving cell (scell) and acquires its necessary upstream signaling information and neighbor cell information from the respective BTS, in a known way. In a next step  510 , the mobile station scans for signals from the serving cell and the respective neighboring cells and derives the respective C 2  values. In a next step  520 , the mobile station stores C 2  values associated with successfully scanned signals. In this example, at least the ten most recent values of C 2  are stored for each neighbor cell. Then, in step  530 , the mobile station uses the stored C 2  values to generate a trend of the C 2  values, as will be considered in more detail below. In a next step  540 , the mobile station determines whether a C 2  value of any neighboring cell is better than the C 2  value of the serving cell. If the result of the determination is positive for any neighboring cell (that is, the neighboring cell has a higher C 2  value than the serving cell), then, in a next step  550 , the mobile station starts a timer, which the mobile station associates with the respective neighbor cell. If a respective timer is already running then no additional action occurs and the timer is left running. If, on the other hand, the result of the determination is negative for any particular neighboring cell (that is, the neighboring cell has a lower C 2  value than the serving cell), then, in a step  555 , any respective running timer is stopped and reset. If no timer is running, then no action occurs. 
     In other words, according to the present exemplary embodiment, for each scan operation, a timer is started (or permitted to continue) for any neighboring cell which has a better C 2  value than the source cell. A timer runs until a predetermined expiry time, unless, before or on expiry, the C 2  value drops below the C 2  value of the source cell, in which case the timer is stopped and reset (or otherwise cancelled). Obviously, in the example provided, a timer is not started for a neighboring cell unless its C 2  becomes better than the C 2  value of the source cell. 
     Next, in a step  560 , the mobile station determines whether any timer has expired. In this example, the timers are set to expire after five seconds. In other examples, a different expiry time may be selected. In the step  560 , if no timer has expired, then the process returns to the step  510 , whereat the mobile station again scans for signals from the source cell and neighbor cells, and the process repeats. 
     If, however, in the step  560 , the mobile station establishes that a timer has expired, in a step  570 , the mobile station predicts future C 2  values for neighbor cells, in this case five seconds in the future, and determines whether the neighbor cell having the expired timer is predicted to have the best C 2  value, and hence the best signaling capability, five seconds into the future. If the result of the test is positive, then the mobile station selects that neighboring cell as a target cell and, in a step  590 , initiates a reselection operation to that cell. Of course, the future time could be chosen to be more or less than five seconds. 
     If, on the other hand, the mobile station determines that the neighbor cell having the expired timer does not have the best predicted C 2  value five seconds into the future, then, in a next step  580 , the mobile station compares the predicted C 2  values of all valid neighbor cells and selects, as the target cell, the neighbor cell that does have the best predicted C 2  value at that future point in time. In this example, a neighbor cell is ‘valid’ for selection if, at step  580 , it has a current C 2  value higher than the C 2  value of the serving cell. In this example, a neighbor cell that is valid for selection will also by definition have a timer running or also just expired. In other words, according to this criterion, a reselection operation can occur only to select a neighbor cell having a running or just expired timer. 
     In other examples, a criterion might be that a neighbor cell is valid for selection even if, at step  580 , it does not have a C 2  value higher than the serving cell C 2  value. For instance, according to such examples, a neighbor cell might be selected as the target cell due to its having the fastest rising C 2  value and, hence, the highest predicted C 2  value at a selected future time, even if, at step  580 , it does not have a current C 2  value higher than the C 2  value of the serving cell, and no respective timer running. Deciding which criterion to apply to which classes of neighbor cell are valid for selection is one of many system design considerations available for consideration. 
     In a next step  590 , the mobile station initiates a reselection operation, to reselect to and camp on the selected target cell. The process then repeats from step  500 . 
     The cell reselection operation will now be tested against the C 2  level information shown in the graph in  FIG. 4 . In this example, both neighbor cells have a C 2  value higher than the C 2  value of the serving cell, so would both be valid for selection under either preceding exemplary criterion. 
     As indicated in the graph in  FIG. 5 , the process has two main loops, wherein operation in a particular loop is dictated by the test in step  560 . For example, until such time as a timer expires, the process operates in a loop A and, when a timer expires, operation moves to a loop B (for at least one iteration). With reference to the graph in  FIG. 4 , a first timer, which is associated with ncell 1 , is started, due to the test in step  540 , at around four seconds, when the C 2  value of ncell 1  becomes higher than the C 2  level of the serving cell, scell. Then, at seven seconds, the C 2  level of ncell 2  becomes higher than the C 2  level of the serving cell, and a second timer is started. Operation of the process remains in loop A until about nine seconds, when the five-second timer associated with ncell 1  expires. At nine seconds, the test in step  560  becomes positive and loop B comes into operation. In step  570 , the process predicts future C 2  values and finds that the C 2  value for ncell 1  is not predicted to be the best C 2  value at time T 1 , at around 14 seconds on the graph, which is an additional five seconds after the timer has expired. It is apparent from the graph that, at 14 seconds, the C 2  value of ncell 2  is just higher than the C 2  value of ncell 1 . This can be confirmed by substituting 14 seconds (as the y value), into the line equations shown on the graph, which gives a predicted C 2  value of ncell 1  as 40.2 and a predicted C 2  value of ncell 2  as 40.4. Additionally, it is clear from the graph that, after 14 seconds, the C 2  value of ncell 2  is predicted to increase at a steeper slope than the C 2  value of ncell 1 . Thus, in step  580 , the process determines that ncell 2  is predicted to have the best C 2  value at 14 seconds and selects ncell 2  as the target cell. As a result, in step  590 , the mobile station initiates reselection to ncell 2 , at nine seconds, while the second timer is still running. 
     Thus, according to the exemplary process of  FIG. 5  and the data in the graph in  FIG. 4 , cell reselection occurs at around nine seconds, to reselect from the current serving cell to the neighboring cell ncell 2 . This avoids an undesirable cell reselection operation, to neighboring cell ncell 1 , which would otherwise occur. 
     A second, alternative exemplary embodiment of the present invention will now be described with reference to the flow diagram in  FIG. 6 . As shown, steps  500  to  560  of the graph are essentially the same as the same-numbered steps of the graph in  FIG. 5 , and will not be described again. However, after step  560 , the process is different. In particular, when the test in step  560  becomes positive, as a result of a timer expiring, a next step  670  predicts C 2  values, at a specified future time, for each neighbor cell that is associated with a running timer and for each neighbor cell that is associated with an expired timer (but not for a neighbor cell associated with a stopped or reset timer). Then, step  670  determines if any neighbor cell, for which a timer is still running, has the best predicted C 2  value at the specified future time. If the result is positive, the process returns to step  510 . In other words, the process holds-off from reselecting to a new serving cell while any neighbor cell, having a running timer, is predicted to have the best C 2  value at the specified future time. 
     If, in the alternative case, step  670  determines that a neighbor cell associated with an expired timer has the best predicted C 2  value at the selected future time, then, in step  680 , reselection is initiated to that neighbor cell. In this example, in essence, reselection is held-off if a neighbor cell, for which a timer is still running after another timer has expired, is projected to have the best C 2  value at the future time. 
     As with the first exemplary embodiment, neighbor cells may be valid for selection according to different criteria. For example, a neighbor cell may be valid for selection only if its respective C 2  value, at step  670 , is higher than the C 2  value of the serving cell. Alternatively, a neighbor cell may be deemed valid for selection irrespective of the respective C 2  level at step  670 . In the latter case, all neighbor cells would be potential candidates for reselection on the basis of projected future C 2  level. 
     A cell reselection operation according to the second exemplary embodiment will now be tested against the C 2  level information shown in the graph in  FIG. 4 . As before, both neighbor cells have a C 2  value higher than the C 2  value of the serving cell, so would both be valid for selection under either preceding exemplary criterion. 
     As indicated in the graph in  FIG. 6 , the process has three main loops, wherein operation in a particular loop is dictated by the test in step  560  and the test in step  670 . For example, until such time as a timer expires, the process operates in a loop A and, when a timer expires, operation moves to a loop B or a loop C. With reference to the graph in  FIG. 4 , a first timer, which is associated with ncell 1 , is started, due to the test in step  540 , at around four seconds, when the C 2  value of ncell 1  becomes higher than the C 2  level of the serving cell, scell. Then, at seven seconds, the C 2  level of ncell 2  becomes higher than the C 2  level of the serving cell, and a second timer is started. Operation of the process remains in loop A until about nine seconds, when the five-second timer associated with ncell 1  expires. At nine seconds, the test in step  560  becomes positive. Then, in step  670 , the process determines that the best projected future C 2  value, at a time T 1 , belongs to neighbor cell ncell 2 . As neighbor cell ncell 2  still has an associated timer running, the process enters loop C, and iterates therein until 12 seconds have elapsed, at which point the second timer expires. At this point, step  670  determines that no other timers are running and that ncell 2  has the best predicted future C 2  value at a time T 2 , which is five seconds later, at 17 seconds. Accordingly, loop B is selected and step  680  initiates a reselection operation to ncell 2  at around 12 seconds. In other words, the reselection operation is held-off for a further three seconds after a corresponding reselection operation of the first exemplary embodiment would have occurred. 
     Thus, according to the exemplary process of  FIG. 6  and the data in the graph in  FIG. 4 , cell reselection occurs at around 12 seconds, to reselect from the current serving cell to the neighboring cell ncell 2 . This, again, avoids an undesirable cell reselection operation, to neighboring cell ncell 1 , which would otherwise occur. 
     There are a number of parameters in the foregoing process that can be varied to meet different requirements. For example, the number of historic C 2  values that is used to plot trends can be increased or reduced, depending upon how sensitive the prediction needs to be to more recent C 2  values. In addition, timers may be set to expire before or after five seconds, where a longer time could increase the accuracy of the prediction but risk delaying reselections for too long. Furthermore, the predicted C 2  values might be predicted for before or after an additional five seconds, where a longer time would tend to be less accurate but a shorter time might lead to an increased number of unnecessary or undesirable reselection operations. All such parameters can be varied individually or collectively according to experimentation or system modeling. 
     Considering, for example, step  530  in more detail, one way of generating a trend is to take the stored, historic C 2  values and construct a straight line of best fit for each neighbor cell, as illustrated in the graph in  FIG. 4 . Then, for each best fit line, the line is projected to a particular point in time in the future and used to predict a C 2  value at the particular time. One way of predicting future C 2  values will now be described using known linear regression techniques, thereby avoiding having to construct a graph as such. 
     If a set of data is expected to have a linear correlation, which is an assumption applied to the C 2  data in the present exemplary embodiment over the period of time selected, then it is not necessary to plot actual data points on a graph in order to determine the constants m (slope) and b (y-intercept) of the straight line equation y=mx+b. Instead, a statistical treatment known as linear regression can be applied to a series of data points to determine these constants. 
     In particular, given a set of data (x i , y i ) with n data points, the slope m, y-intercept b and a correlation coefficient, r, can be determined using the following equations: 
     
       
         
           
             
               
                 
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     It should be noted that the limits of the summation, which are i to n, and the summation indices on x and y have been omitted, for reasons of clarity only. 
     In the present example, the ten stored C 2  values for each neighboring cell are treated as y values and the respective scan times are treated as x values, which are used in the foregoing equations to derive values of m and b for each neighboring cell. Then, the predicted C 2  value, y T , for each neighboring cell at a particular future time T 1 , is generated by substituting T 1  as the value of x into the equation y=mx+b—where m and b are now known—and calculating the value of y. Thus, the predicted future value of C 2  at time T 1  for each neighboring cell can be generated, in order to facilitate a cell reselection to a neighbor cell that has the best predicted C 2  value at a pre-determined future point in time. 
     It is anticipated that other techniques could be used to predict future C 2  values, for example employing a weighted straight line fitting, where later C 2  values are weighted to be more influential in the trend than earlier values, or even a curve fitting (such as logarithmic, exponential, power or polynomial) algorithm. Clearly such techniques would be more processor intensive on a mobile terminal, and it is expected that simple straight-line fitting would be appropriate for most situations. 
     The functional components of an exemplary mobile station  700  are illustrated in the block diagram in  FIG. 7 . The device in this example might be a mobile telephone handset. Embodiments of the present invention can be enacted by such a device. The device generally comprises an embedded processor  705 , for controlling the overall operation of the device  700 . The processor  705  has associated memory, including ROM  720 , RAM  715  and non-volatile memory  720 , for example for storing a control program of the device, application programs and/or an address book. Some or all of the memory might be separate from the processor. The device includes an antenna  725 , which is connected to transmit/receive circuit  730 , which communicates signals to and from the processor  705  via a modem  735 . The device is arranged to interact with a base station according to several protocols, for example GSM, GPRS and/or 3G, which are supported by respective application programs, which are typically stored in a protocol module area  740  of non-volatile memory of the device. An interface module  740  facilitates communications with a keypad  745 , a subscriber identity module (SIM)  750  and a display screen  755  of the device. An audio module  760  supports a speaker  765  and a microphone  770 . A graphics processor  775  is included for processing graphics, for example for display on the display screen  755  and, in this example, the device includes a camera module  790 . 
     A device of the kind shown in  FIG. 7  is generally known in the prior art and it is typically an application program that needs to be arranged to control the device to operate in accord with embodiments of the present invention. For example, in embodiments of the present invention that operate in a GPRS cellular wireless communications system, a GPRS application program is arranged to operate generally in accord with one of the flow diagrams of  FIG. 4  or  FIG. 5 , at least insofar as a cell reselection operation is concerned. In any event, at least a subset of the main components of the device in  FIG. 7 , as shown within the dotted line  785 , may be provided as a single chip device, or as plural chips or components, which can be installed in a mobile station to operate according to embodiments of the present invention. 
     Having thus described the invention by reference to the embodiments shown in the drawings it is to be well understood that the embodiments in question are by way of example only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the appended claims and equivalents thereof.