Dynamic reduction of telephone call congestion

[0002] 1. Field of the Invention

[0003] 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.

[0004] 2. Description of the Related Art

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] FIG. 1 illustrates several clusters of cells within a network controlled by a single base station controller;

[0011] FIGS. 2aand 2billustrate the base station controller in preferred embodiments of the present invention;

[0012] FIG. 3 illustrates other elements of the base station controller in another preferred embodiment of the present invention;

[0013] FIG. 4 illustrates an adjustment to the call setup radius of cell A;

[0014] FIG. 5 illustrates the percentage reduction in the number of subscribers trying to set up calls as a function of reduction factor;

[0015] FIG. 6 illustrates an exemplary site configuration for which telephone call congestion is dynamically reduced in one embodiment of the present invention;

[0016] FIGS. 7-10 illustrate the reduction in blocking for various site configurations; and

[0017] 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.

[0018] FIG. 1 illustrates several clusters of cells within a network controlled by base station controller 26. Each cell 12A-G, 14A-G, 16A-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.

[0019] 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 <1%).

[0020] 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.

[0021] 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.

[0022] As illustrated in FIG. 2a,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. 2b,the executable computer program 36 is provided as part of an externally supplied propagated signal 38.

[0023] 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).

[0024] 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.

[0025] 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.

[0026] 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'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.

[0027] 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

[0028] L=La+Lb*Log(R)  (1)

[0029] 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

[0030] Lb=44.9−6.55*Log(hB)  (2)

[0031] where hB is the GSM antenna height above ground in meters. A differential form of the loss path yields

[0032] ΔR/R=ΔL/Lb  (3)

[0033] Using terms z=ΔL, and 44.9−6.55*Log10(hB) for Lb: $\begin{array}{cc}\Delta R/R=\frac{z\left(\mathrm{dB}\right)}{44.9-6.55*\mathrm{Log}\left(\mathrm{hB}\right)}& \left(4\right)\end{array}$

[0034] 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 $\begin{array}{cc}\mathrm{\Delta \lambda }/\lambda =2*\Delta R/R=\frac{2*z}{44.9-6.55*\mathrm{Log}\left(\mathrm{hB}\right)}& \left(5\right)\end{array}$

[0035] A plot of equation (5) is shown in FIG. 5.

[0036] 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 $\begin{array}{cc}P\left(\lambda \right)=\frac{\left[{\left(\lambda T/60\right)}^N\right]N!}{\Sigma \left[\lambda T/{60}^X\right]/x!x=0,N}& \left(6\right)\end{array}$

[0037] 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.

[0038] 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.

[0039] 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:

[0040] E=T/60.  (7)

[0041] 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.

[0042] 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.

[0043] 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.

[0044] As described above, the present invention reduces congestion on a congested cell within a cluster of un-congested cells, or much less congested cells.

[0045] 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.

[0046] 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.

[0047] 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.