Abstract:
Performance of a wireless telecommunications systems is enhanced by communication of an actual applied forward link gain and, if necessary, an overload indicator from base stations participating in a soft handoff. Subsequently, a power control system within a selection/distribution unit determines a forward link gain acceptable to all base stations involved in a soft handoff call.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is related to the application of Modris  0 . A. Baum, Eshwar Pittampalli, Roy H. Durdik and Edward F. Berliner entitled “Method for Optimizing Frame Error Rate Settings During Soft Handoffs In A Wireless Telecommunications Network”, which application is assigned to the assignee of the present application and is being filed concurrently herewith.  
         TECHNICAL FIELD  
         [0002]    This invention relates to wireless telecommunications networks, and more particularly, to enhancing the compatibility and performance of such networks during soft handoff procedures.  
         BACKGROUND OF THE INVENTION  
         [0003]    The world-wide proliferation of wireless telecommunications technology presents an opportunity for service providers to profit from an ever-growing demand for convenient, reliable wireless service. As these service providers are well aware, controlling expenses while providing such service poses a significant challenge. Wireless service providers meet this challenge by implementing wireless telecommunications networks comprised of mobile switching centers (MSCs) interconnected to base stations. The MSC completes calls between mobile terminals (that is, any mobile station which uses radio transmission) and other parties. These “other parties” may be other mobile terminals or telephone subscribers served by the public-switched telephone network (PSTN). Each base station is associated with a specific geographic region and serves as an interface between mobile terminals within its region and the MSC.  
           [0004]    One consequence of the explosive use of wireless telecommunications and the emergence of numerous wireless service providers in the world market is the implementation of many, diverse wireless networks. The various systems and protocols deployed by these networks often yields equipment incompatibility affecting overall network performance. An especially troubling manifestation of this incompatibility is the inability of existing wireless networks to reconcile incongruent forward link gains (that is, the power supplied by the base station to active mobile terminals) when more than one base station is involved in supporting a call.  
           [0005]    Multiple base stations are involved in a call during “soft handoff” procedures which routinely occur in code division multiple access (CDMA) wireless telecommunications networks. In a soft handoff situation, each base station handles a certain “leg” of the call. Although multiple base stations serve various legs of the call, there is generally one base station (i.e., the dominant base station) which maintains a more powerful forward link with the mobile terminal than any other base station. Since disparity among the legs of a call is undesirable, unequal forward link power levels may lead to an attempt, on the part of the nondominant base stations, to increase forward link gains in order to compensate for insufficient gain from a dominant base station. Even if the nondominant base stations are successful in increasing forward link gains to match that of the dominant base station, the increase may not significantly enhance the transmission quality of the call. Indeed, unilateral increases in forward link gains by nondominant base stations adversely affect other active mobile terminals being served by those base stations. In other words, increasing forward link power levels by base stations which are not providing the strongest signal to an active mobile terminal in order to match forward link power levels of dominant base stations causes detriment to other calls served by those base stations and provides marginal benefits to the call in question.  
           [0006]    Therefore, there is a need in the art for reconciling forward link power levels among base stations in a wireless telecommunications network.  
         SUMMARY OF THE INVENTION  
         [0007]    This need is addressed and a technological advance is achieved by establishing communication between a centralized power control system, a selection/distribution unit (SDU) and participating base stations so that the SDU is notified of the applied forward link power gain setting and overload status. More particularly, when an overload status is attained by a base station, it informs the SDU of the actual value of the power gain level applied and its power overload status. Once the overload status is detected, no further gain increases by the SDU are allowed until the power overload condition is retracted. Gain decreases, however, are allowed. That is, the subsequent gain values can be less than or equal to the value returned with the most recent response indicating overload.  
           [0008]    In the preferred embodiment, a MSC includes a SDU equipped with a central power control system. Multiple base stations participating in a soft handoff provide the SDU with actual forward link power gain settings and, if applicable, overload status. This information is transmitted over an SDU/base station communication protocol described in the patent application of Michael F. Dolan, Thomas L. McRoberts, Eshwar Pittampalli and Thomas T. Towle entitled “Wireless Telecommunications System For Improving Performance And Compatibility”, Ser. No. 08/881,192, and incorporated by reference herein. Applied forward link gain and overload status information is received and stored in the SDU power control system which monitors and determines all forward link power levels associated with other base stations participating in active calls. When an overload status alert message is received, the SDU is prohibited from increasing forward link gains until the power overload condition is retracted. Simultaneously, the SDU requests all soft handoff participating legs to adjust their respective forward link gain settings to match that of the overloaded leg.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1A is an illustrative embodiment of a wireless telecommunications system in which the present invention may be practiced;  
         [0010]    [0010]FIG. 1B is a simplified block diagram depicting the mobile switching center and two base stations shown in FIG. 1A;  
         [0011]    [0011]FIGS. 2A and 2B are message flow diagrams illustrating the exchange of messages in the wireless telecommunications system of FIGS. 1A &amp; 1B in accordance with the preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1A shows wireless telecommunications system  100  including mobile switching center (MSC)  102  and a plurality of base stations interconnected to the mobile switching center. More particularly, base stations  104 ,  106 ,  108 ,  110 ,  112 ,  114  and  116  are interconnected to MSC  102  via interconnection trunks  105 ,  107 ,  109 ,  111 ,  113 ,  115  and  117 , respectively. Each interconnection trunk  105 ,  107  . . .  117  includes three separate communication links between each base station and MSC  102 . These links are described in detail in FIG. 1B. Wireless telecommunications system  100  is a CDMA system in which multiple base stations may serve various legs of a call. In this regard, MSC  102  allocates and monitors base station resources to properly administer information exchange during a mobile call.  
         [0013]    In wireless telecommunications system  100 , each base station is associated with a particular geographic region in which it is assumed that it will serve as the dominant base station for mobile calls in that region. More particularly, base station  104  is the dominant base station associated with geographic region  120 , base station  106  is the dominant base station for geographic region  122 , base station  108  dominates geographic region  124 , base station  110  is the dominant base station for geographic region  126 , base station  112  is the dominant base station associated with geographic region  128 , base station  114  is associated with geographic region  130 , and base station  116  is the dominant base station associated with geographic region  132 . Also shown in FIG. 1A are locations where geographic regions of base stations overlap. One overlap location is identified as intersecting region  142 . In intersecting region  142 , mobile terminal  150  may be served by multiple base stations (most likely base station  106  and base station  108 ). Although multiple base stations may serve a mobile terminal, there is normally one base station which maintains a stronger forward link with the mobile terminal during a call.  
         [0014]    MSC  102  determines the allocation of base station resources for a particular mobile terminal. Accordingly, when an active mobile terminal is in an overlap geographic region, MSC  102  administers a soft handoff among base stations. Soft handoff occurs when the power level of the signal between the mobile terminal and another base station is increased above a predetermined threshold. Soft handoff resource allocation procedures must account for the different operating parameters of each base station involved. One such operating parameter is the power gain applied to the forward link by the base stations.  
         [0015]    [0015]FIG. 1B is a simplified block diagram of MSC  102  and base stations  106  and  108 . The system architecture shown in FIG. 1B is described in more detail in the patent application of Michael F. Dolan et al. entitled “Wireless Telecommunications System for Improving Performance and Reliability”, Ser. No. 08/881,192, which is incorporated by reference herein.  
         [0016]    In this embodiment, MSC  102  comprises control processor  204 , switch fabric  209  and SDU  208  including power control system  206 . Switch fabric  209  is interconnected to SDU  208  via link  235 .  
         [0017]    Base station  106  includes call controller  212 , interconnection processor  214  and channel element  216 . Channel element  216  is interconnected to the call controller and interconnection processor via links  237 . Also shown is base station  108  including interconnection processor  224  and call controller  222  which are interconnected to channel element  226  via links  229 . In this embodiment, mobile station  260  is being simultaneously served by base station  106  and  108 .  
         [0018]    Base stations  106  and  108  maintain established interconnection links to MSC  102 . More particularly, SDU  208  (including power control system  206 ) is interconnected to call controller  212  and interconnection processor  214  of base station  106  via interconnection links  243  and  239 , respectively. MSC control processor  204  is interconnected to call controller  212  of base station  106  via interconnection link  231 . Similarly, SDU  208  is interconnected to call controller  222  and interconnection processor  224  of base station  108  via interconnection links  245  and  241 , respectively. Control processor  204  is interconnected to call controller  222  of base station  108  via interconnection link  233 . In other words, each base station maintains three separate interconnection links to MSC  102 . Together, the three links between each base station and MSC  102  form interconnection trunks  105 ,  107  . . .  117  described in FIG. 1A. Further, call controller  212  of base station  106  and call controller  222  of base station  108  are interconnected via link  205 .  
         [0019]    Power control system  206  is responsible for monitoring forward link gains of all base stations in wireless telecommunications system  100 . More particularly, power control system  206  receives forward link gain data from base station interconnection processors  214  and  224 . During a soft handoff procedure, a base station sends a handoff required request to MSC  102  so that the MSC may allocate resources to another base station for a call. If a forward link gain overload message is received in power control system  206  from a base station during soft handoff, the power control system determines a forward link gain acceptable to all base stations involved in the call. Further, power control system  206  accommodates a base station&#39;s inability to increase forward link gains until the overload situation has been retracted by the base station.  
         [0020]    [0020]FIGS. 2A and 2B are message flow diagrams illustrating the exchange of messages in wireless telecommunications system  100  during a soft handoff procedure in accordance with the preferred embodiment of the present invention.  
         [0021]    For purposes of example, assume that mobile terminal  260  is located in intersecting region  142  where it is being simultaneously served by base stations  106  and  108  during a soft handoff. In alternative embodiments, more than two base stations may serve a mobile terminal in an intersecting region and soft handoffs may occur among base stations served by different MSCs. The soft handoff procedure begins when base station  106  extends a handoff request message to MSC  102 . MSC  102  receives the handoff request message and determines which base station is most capable of handling the call. In this example, MSC  102  determines that base station  108  is the base station to be added for a soft handoff with base station  106  to support the active call.  
         [0022]    [0022]FIG. 2A is a message flow diagram in a soft handoff state depicting the messaging required to inform the power control system of the actual forward link power gain applied at base stations having radio facilities used in a call. For purposes of example, assume that the messages described below are exchanged within wireless telecommunications system  100  as shown in FIG. 1. In this example, power control system 106  receives a message from each base station involved in the soft handoff. These messages contain information about the actual forward link gain applied to the call from base stations  106  and  108 . In this example, neither base station is in overload condition.  
         [0023]    [0023]FIG. 2B is a soft handoff message flow diagram depicting the messaging required to inform the power control system of the actual forward link power gain applied at base stations having radio facilities used in a call in which at least one of those base stations is experiencing an overload or an equipment limitation condition. For the purposes of example, assume that the messages described below are exchanged within wireless telecommunications system  100  as shown in FIG. 1.  
         [0024]    In the preferred embodiment, power control system  106  extends a message to each base station requesting the forward link power gain setting to be established at a level “X1”. Subsequently, power control system  106  receives a message including information about the actual forward link gain applied to the call from base station  106 . Base station  106  indicates the applied forward link gain is “X1” and no overload situation exists. Base station  108 , however, suggests applying a forward link gain at level “X2” which is less than “X1”. Also base station  108  indicates it is in an overload condition. Power control system  206  receives the forward link data and determines an appropriate forward link gain. In this example, power control system  206  can change power gain settings in all legs but cannot exceed “X2” on any leg. Accordingly, power control system  206  establishes the forward link gain setting at level “X2” for base stations  106  and  108 . This power level is not increased until base station  108  comes out of its “overload” condition by extending a “no overload” message to the SDU. In alternative embodiments, power control system  206  may extend a default forward link gain to all participating base stations whenever an overload condition exists.  
         [0025]    In this manner, communication of power level limits of a base station to a power control system is established. As a result, base stations are circumvented from increasing power to the forward link and thereby unnecessarily causing system performance degradation. More particularly, the SDU in an MSC coordinating a soft handoff procedure determines a forward link gain which is acceptable to all base stations involved in the process. Advantageously, unnecessary increases in powered levels to the forward link are avoided so that base stations in a wireless telecommunications network may efficiently use resources.  
         [0026]    Although the present invention has been described with respect to an illustrative embodiment, those skilled in the art will recognize that numerous other arrangements may be devised without departing from the scope of the invention.