Abstract:
In a cellular communications system ( 10 ) in which a plurality of mobile subscriber units ( 100 ) attempt to access at least one BTS ( 140 ), which is controlled by a controller ( 150 ), a method of controlling system congestion is provided. The subscriber units ( 100 ) record a number of failed access attempt transmissions ( 210 ). The failed attempt data ( 122, 124 ) is transmitted ( 220 ) to the BTS ( 140 ) in subsequent access messages ( 120 ). From the failed attempt data ( 122, 124 ), the controller ( 150 ) generates an array of access restriction values ( 172 ), which are transmitted to the subscriber units ( 100 ) within the Access Parameters Message ( 170 ). The access restriction values ( 172 ) are used by the subscriber units ( 100 ) to perform one or more persistence tests ( 240, 245, 250 ). The access restriction values ( 172 ) control the probability of the persistence tests ( 240, 245, 250 ) passing for any given access slot. The present method provides dynamic control of system load, allowing for a higher percentage of successful call attempts when the system ( 10 ) is experiencing a heavy load.

Description:
FIELD OF THE INVENTION 
     The present invention relates to communications systems and, more particularly, to a method for adaptive control of access to a wireless telecommunication system. 
     BACKGROUND OF THE INVENTION 
     In cellular communication systems, such as a code division multiple access (CDMA) communication system, a subscriber unit or mobile station and a fixed network unit or base station establish a two-way communication link through forward and reverse radio frequency (RF) communication links. The forward communication links originate from the base station and the reverse communication links from the mobile station. The base station normally communicates simultaneously to a number of mobile stations. An example of such a system is described in the Telecommunications Industry Association/Electronic Industry Association Standard 95B (TIA/EIA/IS-95-B). 
     When too many subscribers access a system at the same time, the system enters a degraded state. The degradation affects both the subscriber and the system. System degradation is due to collisions on the control channel. FIG. 1 is a plot of access channel throughput versus offered traffic measured through the number of access attempts per slot time for an exemplary cellular telecommunication system. As shown in FIG. 1, the system throughput steadily increases up to an ideal point at an offered traffic rate of 1.0. Beyond this point, system degradation is evident. 
     In conventional systems, the media access controller or base station controller broadcasts an access restriction value to the subscriber units on a paging channel in an attempt to keep the system operating on the left side of the throughput curve. The access restriction value is used by each subscriber unit to calculate when to initiate another access attempt. Unfortunately, in conventional systems, the media access controller cannot determine on which side of the throughput curve the subscriber unit is functioning. This is partially due to the fact that in conventional cellular telecommunication systems, it is impossible to determine how many attempts a subscriber made before successfully initiating an access. Therefore, the media access controller indiscriminately broadcasts access restriction values based on a static database configuration in order to cut off excess traffic. 
     Accordingly, it has been considered desirable to develop a new and improved method and apparatus for adaptively controlling access to a shared media that meets the above-stated needs and overcomes the foregoing difficulties and others, while providing better and more advantageous results. 
     For instance, one advantage of the present invention is that it provides a more robust system with better throughput. 
     Another advantage of the present invention is the provision of a cellular communications system which accommodates geographic loading differences. 
     Yet another advantage of the present invention is that more call attempts will succeed when the system is experiencing a heavy load. 
     Still another advantage of the present invention is the provision of a dynamic assignment of access restriction values based on actual load instead of a static database configuration. 
     A further advantage of the present invention is that it selectively reduces less important traffic components, better accommodating all classes of subscriber during emergency situations. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a plot of access channel throughput versus offered traffic measured through the number of access attempts per slot time for an exemplary wireless communication system; 
     FIG. 2 is a diagrammatic illustration of a wireless communication system according to a preferred embodiment of the present invention; 
     FIG. 3 is a simplified block diagram of an exemplary wireless device that incorporates the features of the present invention therein; 
     FIG. 4 is a buffer diagram which illustrates the congestion control method of the present invention; and 
     FIG. 5 is a flow chart which illustrates a method of accessing a base transceiver station in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 2, a wireless communication system  10 , such as a direct sequence code division multiple access (DS-CDMA) digital radiotelephone system is provided. Base transceiver stations (BTSs)  22 ,  32 ,  42  may communicate with mobile device  24 , operating within coverage area  20 , served by BTS  22 . Similarly, BTSs  22 ,  32 ,  42  may communicate with a mobile device  34  operating within coverage area  30 , served by BTS  32 . BTSs have fixed locations, such locations chosen to provide overlapping coverage areas. BTSs  22 ,  32 ,  42  are coupled to a media access controller, such as a base station controller (BSC)  50 , which includes, among other things, a processor  52  and a memory  54 . The BSC  50  is, in turn, coupled to a mobile switching center (MSC)  60 , also including, among other things, a processor  62  and a memory  64 . The BSC and MSC operate according to well-known methods and are commercially available from Motorola, Inc. While the present invention is being described with respect to a wireless communication system, it is to be appreciated that it is applicable to any system in which a plurality of subscriber units compete for access to a shared media, such as an internet-based and/or ethernet-based system. 
     Multiple access wireless communication between BTS&#39;s  22 ,  32 ,  42  and mobile devices  24 ,  34 ,  44  occurs via radio frequency (RF) channels over which digital communication signals such as voice, data, and video are transmitted. Base-to-mobile device communications are said to occur on a forward-link channel  70 , while mobile-to-base communications are referred to as being on a reverse-link channel  80 . A communication system using CDMA channelization is described in detail in EIA/TIA Standard IS95-B. 
     As shown in FIG. 2, a communication signal  70  is transmitted on an IS-95B forward-link channel, such as a paging channel or traffic channel from a BTS, such as BTS  32  to mobile device  34 . Alternately, a communication signal  80  may be transmitted via an IS-95B reverse-link channel, such as an access channel or a traffic channel, by mobile device  34  to its source BTS  32 . Similarly, other mobile devices may receive and/or transmit communication signals with other base transceiver stations. 
     With reference now to FIG. 3, there is shown a simplified block diagram of an exemplary mobile subscriber unit or wireless device  24 ,  34 ,  44 . The wireless device  24 ,  34 ,  44  can be a telephone, a cable telephony interface device, a cellular or PCS radiotelephone, a cordless radiotelephone, a radio, a personal digital assistant (PDA), a pager, a palm-top computer, a personal computer, etc. Accordingly, as used herein, wireless device refers to each of these devices and their equivalents. 
     The device  24 ,  34 ,  44  includes a transceiver  70 , transceiver antenna  72 , microprocessor-based controller  74 , memory  76 , keypad  78 , transducers  80  (e.g. microphone, speaker), an analog-to-digital converter  82 , and a processor  84 . The wireless device  24 ,  34 ,  44  is adapted to communicate (i.e. transmit and receive communication signals such as data and voice) over a public switched telephone network (PSTN) via a cellular radiotelephone system, such as a CDMA cellular radiotelephone system, global system for mobile communication (GSM) cellular radiotelephone system, etc. 
     With reference now to FIG. 4, there is shown a second simplified block diagram of an exemplary subscriber unit or wireless device  100 . The subscriber unit  100  can be a telephone, a cable telephony interface device, a cellular or PCS radiotelephone, a cordless radiotelephone, a radio, a personal digital assistant (PDA), a pager, a palm-top computer, etc. Accordingly, as used herein, subscriber unit or wireless device refers to each of these devices and their equivalents. 
     In one embodiment of the present invention, the subscriber unit  100 , which includes a processor  110  and a controller  112 , periodically transmits an access message  120  on an Access Channel  130  to a base transceiver station (BTS)  140 , which is controlled by a controller  150 . Access messages may include unsolicited registrations, data burst messages, mobile originations, and the like. Because of the sporadic nature of cellular traffic, access attempts often fail because there are a greater number of access attempts than available time slots (FIG.  1 ). In the case of a failed access attempt, the subscriber unit  100  keeps a count of failed access attempts over a given period of time. This data is stored in a subscriber unit memory  160 , which consists of a failed attempts memory  162  and a time period memory  164 . It is to be appreciated that access attempts consist of probe sequences executed by the subscriber unit  100  before successfully accessing the CDMA cellular system. One skilled in the art will appreciate that CDMA systems allow a mobile subscriber unit up to sixteen probe sequences before aborting its attempt to access. 
     In one preferred embodiment, the access message  120  from the subscriber unit  100  contains two new data fields, namely, a number of failed attempts data field  122  and a time period data field  124 . In another embodiment, the subscriber unit  100  calculates a sliding average of failed access attempts per unit time and transmits this field to the BTS  140 . It is to be appreciated that the failed attempts data field  122  and the time period data field  124  may be optional within the Access Message, allowing pre-existing subscriber units to co-exist with subscriber units employing the present invention. In other words, the functionality of the present invention is not lost in an environment in which legacy subscriber equipment co-exists with a statistically sufficient population of subscriber equipment employing the present invention. 
     The system congestion data  122 ,  124 , i.e. the failed attempts per unit time, is passed to the access controller  150  for processing. From this data, the controller  150  generates an array of access restriction values. In one embodiment, the controller computes a sliding average of failed access attempts per unit time and uses the sliding average as an index into a lookup table of access restriction values. In another embodiment, the controller calculates an array of access restriction values from the transmitted system congestion data  122 ,  124 . In a preferred CDMA system, the access restriction values consist of an array of PSIST or persistence values. As is discussed more fully below, PSIST values are broadcast to and used by the subscriber units within a given coverage area in the calculation of persistence delay values. Those skilled in the art will appreciate that different PSIST values may be transmitted for different mobile overload classes. In other words, different PSIST values are broadcast to be used for different traffic components based on the observed number of access probe sequences before the access channel request is satisfied. 
     Once the controller  150  generates the access restriction values, an Access Parameters Message  170  is broadcast on a Paging Channel  180  to all subscriber units within a given coverage area. Artisans will appreciate that the Access Parameters Message  170  defines the parameters used by the subscriber units transmitting to the BTS  140  along the Access Channel  180 . Within the Access Parameters Message  170  is an access restriction values field  172 , which contains the array of access restriction values calculated by the controller  150 . 
     With reference now to FIG.  5  and continued reference to FIG. 4, a method for accessing a BTS is provided. In one embodiment, a subscriber unit  100  transmits an access message  120  to the BTS  140  (step  200 ). During the access process, i.e. the executing of one or more probe sequences, the subscriber unit  100  records the number of failed access attempts (step  210 ) over a given period of time before successful access to the system. At step  220 , the subscriber unit transmits the failed attempt data to the BTS. In one embodiment, the failed attempt data is transmitted within the access message. In an alternate embodiment, the failed attempt data is transmitted to the BTS in a separate message. 
     As is discussed above, the controller  150  receives the failed access attempt data and calculates an array of access restriction values that are broadcast continuously to all subscriber units within the given coverage area. These access restriction values are received (step  230 ) by the subscriber unit. The access restriction values are used by the subscriber unit to perform one or more persistence tests (step  250 ). More particularly, the subscriber unit generates a persistence delay (PD) value (step  245 ) using the access restriction values. 
     In a preferred embodiment, the subscriber unit receives an array of PSIST values, such as those provided in Table 1, which are used to compute the PD value in a conventional manner. In addition, the subscriber unit generates a pseudorandom backoff value (step  240 ) in a conventional manner. The PD value (step  245 ) and backoff value (step  240 ) are used to perform the persistence test at step  250 . More particularly, the calculated persistence delay value is compared to the backoff value. If the PD value is less that the backoff value, the persistence test (step  250 ) fails. If the persistence test fails, the subscriber unit waits until the next access slot and performs another persistence test. Persistence tests are performed until the test is passed. If the PD value is less that the backoff value, the persistence test passes and the subscriber unit transmits an access message (step  260 ) to the BTS at relatively low power. As illustrated in Table 1, it is to be appreciated that the access restriction values that the subscriber unit receives from the controller controls the probability of the persistence test passing for any given access slot. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary PSIST Acess Restrictin Values in a CDMA System 
               
             
          
           
               
                   
                 Probe Sequence # 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 PSIST Value 
                 0 
                 4 
                 8 
                 12 
                 16 
                 20 
                 24 
                 28 
                 32 
                 36 
                 40 
                 44 
                 48 
                 52 
                 63 
               
               
                 Reduction in Traffic 
                 1 
                 2 
                 4 
                 8 
                 16 
                 32 
                 64 
                 128 
                 256 
                 512 
                 1k 
                 2k 
                 4k 
                 8k 
                 4 
               
               
                   
               
             
          
         
       
     
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.