Abstract:
A method, apparatus, article of manufacture, and propagated signal, for setting up a call by receiving a value representing a power level for a cell, adjusting the value representing the power level for the cell based on congestion to produce an adjusted value, and setting up the call based on the adjusted value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates the dynamic reduction of telephone call congestion, and more particularly, to the dynamic reduction of telephone call congestion in a mobile telephone system. 
     2. Description of the Related Art 
     In a mobile telephone communication system, such as the global system for mobiles (GSM) telephone communication system, “call setup” occurs over common control channels. As illustrated in the mobile telephone communication system  10  of FIG. 1, a mobile telephone unit  12  is located in one of cell clusters  14 ,  16 , and  18 , each of which includes cells A-G. The mobile telephone unit  12  measures the power level of a number of base stations  20 ,  22 , and  24  in the general vicinity, and reports the power level measurements to a base station controller  26  via the respective base stations  20 ,  22 ,  24 . The base station controller  26  then sets up or assigns a call requested by the mobile terminal unit  12  on the strongest base station  20 ,  22 ,  24 . The base station controller  26  makes the decision for the mobile terminal unit  12  and sends an order to the strongest serving base station  20 ,  22 ,  24  (as reported by the mobile terminal unit  12 ) to set up the desired call. 
     Congestion occurs when too many mobile terminal units are trying to set up calls on a particular cell, (in this case, cell A in cell cluster  16 ) which requires additional channel capacity above its existing configuration. 
     The cluster of cells  16  provides service to the mobile terminal units  12  within their coverage area. In some cases, mobile users may gather with much higher density around one cell in a cluster  16 , causing a much higher blocking probability for that cell, than is present in the neighboring cells. 
     In a particular example, cell A of cell cluster  16  includes an area in which an event, such as a sporting event, is taking place and cell A experiencing congestion levels in excess of a threshold, for example 10% of the calls are not being assigned, whereas the congestion of all other cells B-G in cluster  16  is much less than 10%, for example less than 1%. This congestion prevents some of the mobile telephone units within cell A of cluster  16  from having their calls assigned. As a result, many subscribers are unable to complete their desired calls. 
     SUMMARY OF THE INVENTION 
     The present invention solves this problem by providing a method, apparatus, article of manufacture, and propagated signal which adjusts the value representing the power level for the strongest base station, when the congestion of the strongest server is above a threshold level. Once the level of the strongest server is adjusted, the order of the base stations is then rearranged, and another cell becomes the strongest serving cell at the location where the mobile terminal unit is located, thereby effectively reducing the set up radius of the strongest base station. This reduction impacts the number of subscribers to be served by that cell, and hence, results in a congestion reduction in that cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates several clusters of cells within a network controlled by a single base station controller; 
     FIGS. 2 a  and  2   b  illustrate the base station controller in preferred embodiments of the present invention; 
     FIG. 3 illustrates other elements of the base station controller in another preferred embodiment of the present invention; 
     FIG. 4 illustrates an adjustment to the call setup radius of cell A; 
     FIG. 5 illustrates the percentage reduction in the number of subscribers trying to set up calls as a function of reduction factor; 
     FIG. 6 illustrates an exemplary site configuration for which telephone call congestion is dynamically reduced in one embodiment of the present invention; 
     FIGS. 7-10 illustrate the reduction in blocking for various site configurations; and 
     FIGS. 11-12 illustrate the congestion reduction factor as a function of a number of initiated call attempts per busy hour for two different sites. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates several clusters of cells within a network controlled by base station controller  26 . Each cell  12 A-G,  14 A-G,  16 A-G provides service to mobile terminals within their coverage area. In some cases, mobile users may gather with a much higher density around one of the cells in a cluster, causing a much higher blocking probability for that cell than its neighbors. 
     For example, assume that cell A is experiencing congestion levels in excess of a threshold value (such as 10%), whereas the congestion level in all other cells in that cluster is much less than 10% (such as &lt;1%). 
     As set forth above, congestion occurs when too many mobile terminals are trying to set up calls on a particular cell (in this case, cell A of cluster  16 ), which requires additional channel capacity above its existing configuration. 
     Call setup in GSM occurs over common control channels, where the mobile terminal unit  12  measures the power level of a number of bases stations  20 ,  22 ,  24  in the general vicinity, and reports these measurements to the base station controller (BSC)  26 . The call is then set up on the strongest server. The BSC  26  makes the decision for the mobile terminal unit  12  and sends an order to the strongest serving base station (as reported by the mobile terminal unit  12 ) to set up a call. 
     As illustrated in FIG. 2 a,  the BSC  26  includes at least a processor  30  and an article of manufacture  32 . The article of manufacture  32  further includes a storage medium  34  and an executable computer program  36 . The executable computer program  36  includes the instructions to dynamically reduce the call congestion. In another embodiment, as illustrated in FIG. 2 b,  the executable computer program  36  is provided as part of an externally supplied propagated signal  38 . 
     As illustrated in FIG. 3, the base station  26  also includes a radio transceiver  40 . The radio transceiver  40  is an electronic unit inside the base station  26  that contains a transmitter and receiver. In one embodiment, the radio transceiver  40  includes a transmitter  42 , a transmit RF power amplifier  44 , a receiver  46 , a diversity receiver  48 , and control circuitry  50 . The output of the transmit RF power amplifier  44  is supplied to a transmit antenna (not shown) and the outputs of the receiver  46  and diversity receiver  48  are provided to receive antennas (also not shown). 
     The present invention includes a modification to the executable computer program  36  in the BSC  26  so that, upon congestion of a cell A above some threshold level, an adjustment is applied to the level of the strongest server. Once the level of the strongest server is reduced, the order of the servers is then re-arranged, and another cell becomes the strongest serving cell at the location where the mobile is located. Thus, effectively, the setup radius of the original strongest server has been reduced. This reduction will impact the number of subscribers to be served by that cell, and hence, result in a reduction in congestion. 
     In a preferred embodiment, the adjustment ΔR is the result of a constant decrease of “−zdB” in received signal levels, as measured by the mobile terminal unit  12 . This produces a reduction in the setup radius of the original strongest server, as illustrated in FIG.  4 . However, the adjustment “−zdB” need not be a fixed dB reduction. In fact, the adjustment could be a different type of fixed adjustment or an adjustment which varies in some manner, such as, over time. 
     In order to prevent the system from oscillating, i.e. activation and deactivation as the congestion level goes up and down, two thresholds are defined, a lower value and an upper value. The reduction is activated in the BSC  26  when congestion of one of it&#39;s cells is above some level, i.e., 20%, and not deactivated until congestion is reduced below another level, i.e., 5%. This prevents any oscillatory behavior. 
     The effect of reduction of the call setup radius can be seen from the reduction in the path loss. The GSM standard recommends the use of Hata Model for propagation calculations. The model can be simply stated as one which predicts the path loss L in dB as 
     
       
           L=La+Lb* Log( R )  (1) 
       
     
     The term La is a function of the urbanization level, base station antenna height, mobile terminal unit antenna height, and the RF frequency. Hence, the term La varies for each of the urban classes; urban, suburban, quasi-rural, and rural. The term Lb is given by 
     
       
           Lb= 44.9−6.55*Log( hB )  (2) 
       
     
     where hB is the GSM antenna height above ground in meters. A differential form of the loss path yields 
     
       
           ΔR/R=ΔL/Lb   (3) 
       
     
     Using terms z=ΔL, and 44.9−6.55*Log10( hB ) for  Lb :                Δ                   R   /   R       =       z        (   dB   )         44.9   -     6.55   *     Log        (   hB   )                     (   4   )                                
     Assuming the traffic is evenly distributed within the congested cell, the percentage reduction in the number of subscribers trying to set up calls on cell A becomes                Δλ   /   λ     =       2   *   Δ                   R   /   R       =       2   *   z       44.9   -     6.55   *     Log        (   hB   )                       (   5   )                                
     A plot of equation (5) is shown in FIG.  5 . 
     In order to estimate the reduction in blocking probability due to call setup radius reduction, assume that call setup traffic is modeled by the Erlang B model, where the probability of blocking is                P        (   λ   )       =         [       (     λ                   T   /   60       )     N     ]          N   !             Σ        [     λ                   T   /     60   X         ]       /     x   !                         x   =   0     ,   N                 (   6   )                                
     where λ is the rate of calls in the Busy Hour, T is the average call duration in minutes, and N is the total number of available channels. 
     FIG. 6 illustrates an exemplary site configuration for which telephone call congestion is dynamically reduced in one embodiment of the present invention. The site configuration illustrated in FIG. 6 includes three sectors, a, b, and c which point in directions of 30°, 150°, and 270°, respectively, relative to geographic north. As illustrated in FIG. 6, each of sectors a, b, and c includes four radio transceivers  40 . As a result, the site configuration illustrated in FIG. 6 is denoted as a (4, 4, 4) configuration. Similarly, a (2, 2, 2) site configuration would have only two radio transceivers  40  in each of sectors a-c. 
     FIGS. 7 and 8 illustrate the reduction in blocking for various site configurations as a function of the total number of initiated calls in the busy hour, with 44 Milli-Erlangs/subscriber, and FIGS. 9 and 10 illustrate the same curves with 22 Milli-Erlangs/subscriber. Milli-Erlangs/subscriber is a unit of traffic usage. In particular, one Erlang is one call that lasts sixty minutes, two calls that last for thirty minutes each, or three calls that last for twenty minutes, etc. If the duration of a call is T minutes, then the Erlangs, E, is determined by: 
     
       
         E=T/60.  (7) 
       
     
     For example, 44 Milli-Erlangs/subscriber is equal to 0.044 Erlangs per subscriber, and therefore, E equals 0.044. Using equation (7), one can solve for the duration of a call T, where T equals E×60 or (0.044)×60 =2.64 minutes. 
     The plots indicate that congestion reduction at cell A (which is the congested site) is greater for higher configurations ((4, 4, 4) and higher) sites than for lower configuration sites ((2, 2, 2) and less), and is more effective when the Milli-Erlangs/subscriber is lower. 
     The congestion reduction factor is plotted in FIGS. 11 and 12. These figures show that the present invention is more effective in high configuration sites, and in areas where the subscriber behavior exhibits small channel holding times. As an example, consider a cell configured as a (5,5,5) site, with a traffic load from subscribers within its service area such that the total number of call attempts in the busy hour is 2600 calls. Using FIG. 11, the congestion at that cell is reduced by 80%. Further, the reduction factor is 26% for 44 the Milli-Erlang/subscriber case, as shown in FIG.  12 . 
     As described above, the present invention reduces congestion on a congested cell within a cluster of un-congested cells, or much less congested cells. 
     This technique can be activated only when congestion levels in a cell reaches an upper threshold level, and will not be deactivated unless the congestion has been reduced to a lower threshold. Having two thresholds prevents a state of oscillation of the feature, i.e. activation then deactivation. 
     Although the above-identified preferred embodiment has been described in conjunction with a GSM system, the present invention is also applicable to other communication systems, such as North American TDMA systems or CDMA systems. Similarly, the preferred embodiment of the present invention has been described as taking place in a base station controller of the GSM system. However, the power level adjusting technique described above could also be performed in other elements of the GSM system, such as the base station. Similarly, in other architectures which include other elements, the power level adjusting technique of the present invention could also be performed in a mobile switching system (MSC), selection and distribution unit (SDU), or other applicable architectural element. Also although the preferred embodiment as described above, utilizes the congestion value as the parameter for determining call setup, other parameters may also be utilized, such as assignment rates, setup rates, and blocking rates. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.