Abstract:
A system and method are disclosed for providing a warning of, and greater immunity from, an impending call drop in a digital wireless telecommunications system. During normal operations, data is transmitted at an initial information frame rate and a corresponding initial physical frame rate. Upon the data transmission producing errors in excess of a threshold, which indicates an impending call drop, the initial information frame rate is reduced while error correction codes are introduced into the data transmission to maintain the initial physical frame rate. The reduction in the information frame rate produces a subtle yet noticeable change in acoustic voice quality, which serves as a warning of the impending call drop. The introduction of error correction codes into the transmission provides greater immunity to the impending call drop by improving data integrity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to digital wireless telecommunications systems. More particularly, the invention relates to a system and method for warning of, and providing greater immunity from, an impending call drop in a digital cellular telephone system, personal communication service (PCS) network, or equivalent enterprise employing variable frame rate data transmission. 
     2. Description of the Prior Art 
     In analog wireless service networks, such as Advanced Mobile Phone Service (AMPS) systems, a mobile subscriber can anticipate that a call may be dropped (abruptly terminated while conversing) if the subscriber hears static superimposed upon speech. This can occur when the mobile unit traverses the boundary of RF coverage. 
     In digital wireless telecommunications systems, such as CDMA and TDMA cellular telephone networks, the subscriber does not experience impending call drop static. Up until the instant the call is dropped, the voice quality is clear, and then the call terminates abruptly. This unexpected dropping is not desirable. 
     Various prior art proposals have been made to generate an audible tone that serves as an impending call drop warning. Rather than using annoying tone signals, however, it would be preferable if a more subtle call drop warning could be provided to the subscriber. It would be further advantageous if this warning could be implemented with a technique for delaying or even avoiding the impending call drop so as to provide increased call drop immunity. 
     BRIEF SUMMARY OF THE INVENTION 
     The foregoing problem is solved by the system and method of the present invention, which provides a warning of, and greater immunity from, an impending call drop in a digital wireless telecommunications system. During normal operations, data is transmitted in the telecommunications system at an initial information frame rate and a corresponding initial physical frame rate. Upon the data transmission producing errors in excess of a threshold, which is indicative of an impending call drop, the initial information frame rate is reduced while error correction codes are introduced into the data transmission to maintain the initial physical frame rate. The reduction in the information frame rate produces a subtle yet noticeable change in acoustic voice quality, which serves as a warning of the impending call drop. The introduction of error correction codes into the transmission provides greater immunity to the impending call drop by improving data integrity. 
     Typically, the wireless telecommunications system includes a Mobile Switching Center (MSC) coupled to at least one radio base station servicing at least one mobile radio unit. The base station and the mobile unit communicate with each other in a digital transmission mode wherein voice data is digitally encoded by voice encoder/decoders (vocoders) located in the MSC (or the base station) and the mobile unit as a sequence of variable rate information frames. The information frames are packaged by physical layers (which may be implemented as part of the vocoders) into a sequence of physical transmission frames that include the information frames with appended and/or prepended control and error checking bits. 
     In preferred embodiments of the invention, the telecommunications system is programmed to monitor normal transmissions between the base station and the mobile unit for frame errors. During such normal transmissions, the vocoders encode voice data at an initial information frame rate and the physical layers receive the initial rate information frames from the voice encoders and construct physical transmission frames at an initial physical frame rate corresponding to the initial information frame rate. 
     When a call drop is impending, the number of frame errors increases. When it reaches a predetermined threshold, the telecommunications system controls the vocoders to reduce the information frame rate from the initial information frame rate to a reduced information frame rate, such that reduced rate information frames are provided to the physical layers. The telecommunications system then controls the physical layers to add error correction bits to the reduced rate information frames to transmit error correction enhanced physical transmission frames at the initial physical frame rate. 
     The error correction enhanced physical transmission frames produce a characteristic change in speech quality due to the reduced information frame rate. This change in speech quality provides a subtle but effective call drop warning. Preferably, the reduced rate information frame carries no more than half the information of the initial information frame. 
     The error correction enhanced physical transmission frames also provide increased error correction robustness for greater immunity from the impending call drop. To that end, the difference between the initial information frame rate and the reduced information frame rate is selected to accommodate sufficient error correction bits to achieve a predetermined number of correctable errors per frame. Preferably, these additional error correcting bits constitute a cyclic error control code, and most preferably, a shortened BCH cyclic code. The error control code is selected so as to correct a predetermined number of correctable errors in a frame that is the same size as the initial information frame. 
     By way of example, assume the initial information frame has n information bits and the reduced information frame is selected to have k information bits. The error correction enhanced physical transmission frame will be capable of carrying n−k additional error correction bits representing a shortened cyclic error control code capable of correcting t errors in a frame of size n′≧n and k′≧k, where n′ is the correctable frame size and k′ is the number of correctable frame information bits  7 . In the event fewer than n−k additional error correction bits are required to correct t errors in a frame of size n′≧n and k′≧k, the error correction enhanced physical transmission frame is constructed to include additional place holder bits. 
     The invention may be implemented without returning to the normal transmission mode until a call is terminated. Alternatively, the initial information frame rate could be resumed in the event the number of frame errors falls below the predetermined threshold. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying Drawing, in which: 
     FIG. 1 is a block diagram of a wireless telecommunications system constructed in accordance with the present invention; 
     FIG. 2 is a block diagram showing a first rate set (Rate Set  1 ) of standard physical transmission frames used for transmitting data in the telecommunications system of FIG. 1; 
     FIG. 3 is a block diagram showing a second rate set (Rate Set  2 ) of standard physical transmission frames used for transmitting data in the telecommunications system of FIG. 1; 
     FIG. 4 is a block diagram showing an error correction enhanced physical transmission frame constructed in accordance with the present invention from a full frame rate physical transmission frame of the first rate set shown in FIG. 2; 
     FIG. 5 is a block diagram showing an error correction enhanced physical transmission frame constructed in accordance with the present invention from a full frame rate physical transmission frame of the second rate set shown in FIG. 3; 
     FIG. 6 is a sequence diagram showing a service option negotiation between a mobile unit and a base station (or a Mobile Switching Center) in accordance with the present invention; and 
     FIG. 7 is a flow diagram illustrating method steps performed by the telecommunications system of FIG. 1 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the Drawing, wherein like reference numbers designate like elements in all of the several views, FIG. 1, illustrates a digital wireless telecommunications system  2  constructed in accordance with a preferred embodiment of the invention. The telecommunications system  2  can be any digital cellular telephone system, PCS network, or equivalent, that has the capability of employing variable frame rate data transmission, including systems utilizing the CDMA and TDMA air interface protocols. The following discussion assumes, by way of example only, that the telecommunications system  2  is a CDMA cellular telephone network. 
     The telecommunications system  2  includes at least one radio base station  4  servicing at least one mobile radio unit  6  within a bounded geographic service area that is commonly referred to as a “cell.” Typically, there would be many bases stations  4  servicing multiple mobile units  6 . 
     A Mobile Switching Center  8  (MSC) controls the operations of the system  2 . It includes an Executive Cellular Processing Complex (ECPC)  9  containing plural data processing devices, including an Executive Cellular Processor (ECP)  10 . The MSC  8  also includes at least one Digital Cellular Switch (DCS)  11  that connects to the ECPC through a DCS/ECP interface  12 . The DCS routes call traffic received from the base station  4  via a switching fabric layer  13  to a land-based telecommunications system, such as the Public Switched Telephone Network (PSTN). A physical layer  14  interfaces with the base station  4  via a packet pipe  15  that carries speech data packets in a broadband mode. The physical layer  14  constructs and deconstructs the physical data frames that are carried in the packet pipe  15 . A speech encoder/decoder (vocoder)  16  constructs and deconstructs information frames containing speech data that are utilized by the physical layer  14  to construct the physical transmission frames. It should be understood that the vocoder  16  and the physical layer  14  could be integrated into a single hardware unit that logically performs the vocoder and physical layer functions, or could be configured as separate hardware units. Use of the separate terms “vocoder” and “physical layer” hereinafter is not intended to signify the separate hardware implementation, but will be understood as also including the single hardware unit embodiment that performs both of the vocoder and physical layer logical functions. It should also be understood that the physical layer  14  and the vocoder  16  could be incorporated into the base station  4 , rather than the MSC  8 , if so desired. 
     The base station includes a Receive/Transmit (RX/TX) unit  17  that receives reverse radio channel signals  18  from the mobile unit  6  and transmits forward radio channel signals  19  to the mobile unit  6 . The RX/TX unit  17  is also a baseband/narrowband converter. It transmits and receives narrow band signals to and from the mobile unit  6 . These narrow band signals are converted to and from baseband signals that the RX/TX unit  17  exchanges with a Channel Element (CE) unit  20 , which performs baseband signal processing. A Channel Cluster Controller (CCC)  21  provides an interface between the CE unit  20  (and other CE units) and the packet pipe  15 . A controller  22  controls the operations of the RX/TX unit  17 , the CE unit(s)  20 , and the CCC  21 . It maintains a data link  23  to the ECPC  9 , which the ECPC  9  uses to control the base station  4  to set up and maintain a call. 
     The mobile unit  6  includes an RX/TX unit  24  that acts as a counterpart to the base station RX/TX unit  17 . A physical layer  25  constructs and deconstructs physical data frames that are utilized by the RX/TX unit  24 . A speech encoder/decoder (vocoder)  26  constructs and deconstructs information frames containing speech data that are utilized by the physical layer  25  to construct the physical transmission frames. Again, the physical layer  25  and the vocoder  26  could be implemented as logical functions performed by a single hardware unit or as separate hardware units, and the use of the separate terms “vocoder” and “physical layer” is not intended to signify the separate hardware implementation to the exclusion of the integrated implementation. A programmed controller  27  manages the operations of the mobile unit  6 , including the RX/TX unit  24 , the physical layer  25  and the vocoder  26 . 
     The base station and the mobile unit communicate with each other in a digital transmission mode wherein voice data is digitally encoded by the vocoders  16  and  26  located in the MSC  8  (or the base station  4 ) and the mobile unit  6 , respectively. The voice data is encoded as a sequence of variable rate information frames. These information frames are then packaged by the physical layers  14  and  25  located in the MSC  8  (or the base station  4 ) and the mobile unit  6 , respectively, into physical transmission frames that include the information frames with appended and/or prepended control and error checking bits. 
     FIG. 2 illustrates a first rate set (Rate Set  1 ) of physical transmission frames and FIG. 3 illustrates a second rate set (Rate Set  2 ) of physical transmission frames constructed by the physical layers  14  and  25 . By way of example, the frames are each 20 milliseconds long but vary in the amount of information and control bits they carry. FIG. 2 shows that Rate Set  1  includes a fall rate frame  40  carrying 9600 bits per second (bps), a half rate frame  42  carrying 4800 bps, a quarter rate frame  44  carrying 2400 bps, and an eighth rate frame  46  carrying 1200 bps. FIG. 3 shows that Rate Set  2  includes a full rate frame  50  carrying 14400 bps, a half rate frame  52  carrying 7200 bps, a quarter rate frame  54  carrying 3600 bps, and an eighth rate frame  56  carrying 1800 bps. 
     By varying the frame rate, the telecommunications system  2  is able to adjust channel capacity to support changes in the number of mobile units  6  handled by the base station  4 . The vocoders  16  and  26  automatically adjust the frame rate based upon input voice characteristic information, as is known in the art. As is also known, the procedure for adjusting the frame rate can be modified by negotiating a “service option” between the base station  4  and the mobile unit  6  in a procedure known as “service option negotiation.” The control computer  20  in the base station  4  includes a state machine that manages service option negotiations on behalf of the base station  4 . The controller  46  negotiates on behalf of the mobile unit  6 . Logic within the physical layers  14  and  25 , and within the vocoders  16  and  26 , implements the agreed-upon service option at the base station  4  and mobile unit  6 , respectively. 
     As seen in FIGS. 2 and 3, each physical transmission frame includes a number of information bits representing an information frame constructed by the vocoders  16  and  26 . In the full rate and half rate frames of Rate Set  1 , and in all of the frames of Rate Set  2 , the information frame is appended with a Frame Quality Indicator area “F” containing error checking bits, such as a cyclic redundancy check (CRC) frame check sequence (FCS). Each frame in Rate Set  1  and Rate Set  2  is also appended with encoder tail bits “T.” Each frame in Rate Set  2  is prepended with an erasure indicator bit “E.” 
     FIGS. 2 and 3 illustrate that the number of information bits transmitted in a physical transmission frame decreases with the frame rate, as does the transmitted energy per frame. Tables 1 and 2 summarize these relationships: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 TRANSMITTED ENERGY PER FRAME 
               
             
          
           
               
                   
                 FRAME RATE 
                 TRANSMITTED ENERGY 
               
               
                   
                   
               
               
                   
                 FULL 
                 E 
               
               
                   
                 HALF 
                 1/2*E 
               
               
                   
                 QUARTER 
                 1/4*E 
               
               
                   
                 EIGHTH 
                 1/8*E 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 INFORMATION BITS PER FRAME (TABLE 2) 
               
             
          
           
               
                 FRAME RATE 
                 RATE SET 1 
                 RATE SET 2 
               
               
                   
               
               
                 FULL 
                 172  
                 267 
               
               
                 HALF 
                 80 
                 125 
               
               
                 QUARTER 
                 40 
                  55 
               
               
                 EIGHTH 
                 16 
                  21 
               
               
                   
               
             
          
         
       
     
     In accordance with the present invention, a new service option is provided that produces a subtle change in voice quality which alerts the mobile subscriber that a call may be dropped because the mobile unit  6  is in a fringe area such as a cell boundary or RF coverage hole. At the same time, the new service option implements increased robustness in the base station-mobile unit communications channel to provide enhanced call drop immunity. 
     The controller  22  and the CE unit  20  at the base station  4  monitor normal variable frame rate transmissions (i.e., before the new service option is invoked) between the base station  4  and the mobile unit  6  for frame errors. During such normal transmissions, the vocoders  16  and  26  encode voice data at an initial information frame rate and the physical layers  14  and  25  receive the initial rate information frames from the voice encoders and construct physical transmission frames at an initial physical frame rate corresponding to the initial information frame rate. Assume for example, that the physical layers  14  and  25  are initially transmitting physical transmission frames at a full frame rate incorporating  172  information bits under Rate Set  1  or 267 information bits under Rate Set  2 . 
     When a call drop is impending, the number of frame errors increases as the signal quality decreases. When the number of frame errors reaches a predetermined threshold (provided by the ECPC  9 ), the new service option is negotiated and the vocoders  16  and  26  are locked at a reduced information frame rate relative to the initial information frame rate. In the preferred embodiment of the invention, the vocoders  16  and  26  are locked in the half-rate mode. Significantly, when the reduced rate information frames are provided to the physical layers  14  and  25 , they do not reduce the physical frame rate in accordance with the reduced information frame rate. Rather, the physical layers  14  and  25  maintain the original physical frame rate. To pad the missing bits of the physical transmission frames, the physical layers  14  and  25  append error correction bits to the reduced rate information frames and construct error correction enhanced physical transmission frames utilizing the initial physical frame rate. 
     FIGS. 4 and 5 illustrate error correction enhanced physical transmission frames  60  and  70  corresponding to Rate Set  1  and Rate Set  2 , respectively. Frame  60  has  80  information or “speech coder bits” corresponding to the half rate frame  42  shown in Rate Set  1  of FIG.  2 . However, Frame  60  is 192 bits long in accordance with the full rate frame  40  shown in Rate Set  1  of FIG.  2 . Frame  70  has 125 information or “speech coder bits” corresponding to the half rate frame  52  shown in Rate Set  2  of FIG.  3 . However, Frame  70  is 288 bits long in accordance with the full rate frame  50  shown in Rate Set  2  of FIG.  3 . 
     The error correction enhanced physical transmission frames  60  and  70  produce a characteristic change in speech quality due to the reduced information frame rate. This change in speech quality serves as a subtle but effective call drop warning. For best results, the reduced rate information frame does not carry more than one-half the information of the initial information frame. For example, if the information frame is initially a full rate frame, it is reduced to a half rate frame, and so on. 
     The frames  60  and  70  also provide increased error correction robustness for greater call drop immunity. To that end, the difference between the initial information frame rate and the reduced information frame rate is selected to accommodate sufficient error correction bits to achieve a predetermined number of correctable errors per frame. Preferably, the additional error correcting bits constitute a cyclic error control code, and most preferably, a shortened BCH cyclic code, with the cyclic encoding and decoding being performed by the physical layers  14  and  25  using the usual circuitry associated with such computations. 
     The error control code is selected so as to correct a predetermined number of correctable errors in a frame that is the same size as the initial information frame. By way of example, assume the initial information frame (e.g., the one generated without vocoder locking) has n information bits and the reduced information frame is selected to have k information bits. The error correction enhanced physical transmission frame (e.g.,  60  or  70 ) will be capable of carrying n−k error correction bits representing a shortened cyclic error control code capable of correcting t errors in a frame of size n′≧n and k′≧k, where n′ is the correctable frame size and k′ is the number of correctable frame information bits. In the event that fewer than n−k error correction bits are required to correct t errors in a frame of size n′≧n and k′≧k, the error correction enhanced physical transmission frame (e.g.  60  or  70 ) is constructed to include additional place holder pad bits. As shown in FIG. 5, there are  7  such pad bits in the error correction enhanced physical transmission frame  70 , which was constructed by adding a shortened BCH control code of 135 bits. Table 3 below provides an example of how to determine the required number of error correction bits using, by way of example, full rate physical transmission frames enhanced with a shortened BCH control code: 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 METHOD FOR DETERMINING NUMBER OF 
               
               
                 ERROR CONTROL BITS 
               
             
          
           
               
                   
                   
                   
                   
                 Shortened 
                 Shortened 
                   
               
               
                 Rate Set 
                 n 
                 k 
                 n − k 
                 n 
                 k 
                 t 
               
               
                   
               
             
          
           
               
                 1 
                 172 
                 80 
                 92 
                 255 
                 163 
                 12 
               
               
                 2 
                 267 
                 125 
                 142 
                 311 
                 376 
                 15 
               
               
                   
               
             
          
         
       
     
     The “Shortened n” and “Shortened k” table entries are the actual frame size and number of information bits, respectively, that can be corrected (up to a maximum of “t” errors) if a shortened cyclic code of not more than “n−k” bits is employed in accordance with shortened BCH control code theory. A convenient look up table of “shortened n,” “shortened k,” and “t” values, as well as the polynomials that can be used to generate the control code values inserted in the error correction enhanced physical transmission frames of the present invention, is set forth in “Error Control Coding Fundamentals and Applications, by Shu Lin and Daniel J. Costello, Jr., Prentice-Hall, Inc. (1983) at pages 585-586. 
     The Lin-Costello look-up table shows that the generator polynomial “750 041 551 007 560 255 157 472 451 460 1” (where each digit represents 3 bits using octal representation) can be used to generate a 92 bit shortened BCH code that corrects t=12 erroneous bits in a frame of size shortened n=255, having shortened k=163 information bits. By coincidence, this 92 bit shortened BCH code perfectly matches the n−k=92 bit positions available in the error correction enhanced physical transmission frame  60 . The shortened n=255 and the shortened k=163 values are more than sufficient to handle the original information frame size of n=172 and the reduced information frame size of k=80. 
     The Lin-Costello look-up table shows that the generator polynomial “111 674 470 652 172 553 222 716 260 714 621 621 010 673 320 3” can be used to generate a 135 bit shortened BCH code that corrects t=15 erroneous bits in a frame of size shortened n=511, having shortened k=376 information bits. This 135 bit shortened BCH code, however, is 7 bits less than n−k=142 bit positions available in the error correction enhanced physical transmission frame  70 . Thus, frame  70  includes the 7 pad bits shown in FIG.  5 . The shortened n=511 and the shortened k=376 values are more than sufficient to handle the original information frame size of n=267 and the reduced information frame size of k=125. 
     FIG. 6 illustrates the process by which the new service option of the present invention is negotiated. Initially, the base station  4 , having determined via its controller  22  that the Frame Error Rate (FER) exceeds some threshold (provided by the ECPC  9 ), causes a service request to be sent to the mobile unit  6  at 80. The mobile unit  6  responds at 82 by either accepting or denying the service request. Assuming the mobile unit  6  accepts the service request, the base station  4  causes a service connection sequence to be invoked at 84 and commences an action timer. After the action timer expires at 86, the mobile unit  6  signals at 88 that the service connection is complete. 
     The invention may be implemented such that the telecommunications system  2  does not return to the normal transmission mode once the new service option has been implemented. Alternatively, the initial variable frame rate service option could be resumed in the event the number of frame errors falls below the predetermined threshold. This alternative is illustrated in FIG. 7, which shows the method steps performed by the controller  22  in the base station  4 , in conjunction with the MSC  8 , in accordance with the invention. 
     Beginning at  90 , the controller  22  determines in step  92  whether the service provider that operates the telecommunications system  2  has subscribed to the new service option of the present invention. If the provider has not, then the base station  4  sends control signals to MSC  8  and the mobile unit  6  in step  94  to operate the vocoders  16  and  26  according to a standard variable frame rate. If step  92  produces a true result, indicating the new service option has been enabled, the base station  4  begins checking the frame errors against a threshold in step  96 . If the errors do not exceed the threshold, the process proceeds to step  94  for normal variable rate frame transmission. If the error threshold is found to be exceeded in step  96 , negotiation for the new service option is invoked in step  98  in accordance with FIG.  6 . Following a successful service option negotiation in step  98 , the base station  4  sends control signals to the MSC  8  and the mobile unit  6  to lock the vocoders  16  and  26  at a reduced information frame rate in step  100 . In step  102 , a test is made to determine whether the new service option is to remain in effect for the duration of the call. If it is, the process loops through step  102  until the call is completed. If the new service option is not configured to remain in effect throughout a call in step  102 , the process begins testing the frame errors in step  104  to determine if they fall below the threshold. The process continues testing in step  104  until the frame errors fall below the threshold, at which point service option negotiation is invoked in step  106  to return to the original variable frame rate mode. Once negotiation is complete, the process returns to step  94  and operates under the standard variable frame rate service option. 
     Accordingly, a system and method for providing warning of, and greater immunity from, an impending call drop in a variable frame rate digital wireless system has been described. While various embodiments have been disclosed, it should be apparent that many variations and alternative embodiments would be apparent to those skilled in the art in view of the teachings herein. For example, if available processing power is limited, the number of error correction code bits could be reduced (by utilizing a smaller error correction code), in which case pad bits would be inserted to fill out the error correction enhanced physical transmission frames. It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.