Abstract:
A method for arranging CRC (Cyclic Redundancy Code) bits within logical transmission units (LTUs) in a CDMA communication system having a physical channel frame comprised of a plurality of LTUs arranged in line, with a physical layer CRC field and tail bits arranged after the LTUs. Each LTU includes a payload and a LTU CRC field for detecting errors in the LTU payload. In the CRC bit arranging method, the CRC field included in odd-numbered LTUs are arranged after the payload and the CRC field included in even-numbered LTUs are arranged before the payload in such a manner that a LTU CRC field of an even-numbered LTU is adjacent to a LTU CRC field of an odd-numbered LTU connected to said even-numbered LTU.

Description:
PRIORITY 
     This application claims priority to an application entitled “Data Communication Device and Method in CDMA Communication System” filed in the Korean Industrial Property Office on Jan. 7, 1999 and assigned Ser. No. 99-873, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a CDMA communication system, and in particular, to a device and method for assembling and de-assembling or decomposing a logical transmission unit (LTU) used for efficient data transmission in a radio environment. 
     2. Description of the Related Art 
     In general, CDMA-2000 systems have a supplemental channel that operates at a high data rate. Although the following description will use the example of a supplemental channel in a CDMA-2000 system, the present invention can apply to any system having two or more cyclic redundancy code (CRC) fields in a transmission frame, as will become clear in the description below. FIG. 1 shows the protocol layer structure of the supplemental channel in the CDMA-2000 system. Although the CDMA-2000 system has several different types of upper layer entities, FIG. 1 shows an RLP (Radio Link Protocol) layer  111  for the upper layer entity, by way of example. The RLP layer  111  assembles data received from an upper layer into an RLP frame. A multiplex sublayer  112  receives the RLP frame(s) from the RLP layer  111  and assembles RLP frame into MuxPDUs. (Multiplex sublayer Protocol Data Units). A supplemental channel physical layer element  113  receives the MuxPDUs from the multiplex sublayer  112  and assembles the MuxPDUs into a supplemental channel (SCH) frame, which is transmitted over the physical channel. 
     The supplemental channel physical layer element  113  is the physical layer of the supplemental channel and refers to the hardware structure of the supplemental channel. The supplemental channel physical layer element  113  receives data transmitted from the multiplex sublayer  112 , fills the payload of a SCH frame with the received data, generates CRC (Cyclic Redundancy Code) bits, and then attaches the CRC bits and a tail of 8 zero (0) bits to the end of the SCH frame. Once assembled, the SCH frame is encoded by the supplemental channel physical layer element  113 , and then transmitted to the receiving side. 
     In FIG. 1, the multiplex sublayer  112  receives transmission data from an upper layer entity, in this case RLP, and fills MuxPDUs with the received data. The multiplex sublayer  112  writes specific information in the MuxPDU header so that the multiplex sublayer on the receiving side will know to which upper layer entity the payload of the received MuxPDU should be transferred. When it is not possible to fill the SCH frame payload with any more MuxPDUs, a particular type of MuxPDU is used to fill the remaining space in the physical frame. Herein, this particular type of MuxPDU will be referred to as a fill-MuxPDU or padding (bits). 
     The above process of assembling MuxPDUs from RLP frames, and then assembling SCH frames from MuxPDUs is performed during transmission. Below, the process of de-assembling or decomposing SCH frames on the receiving end is described. 
     First, upon receipt of a SCH frame, the supplemental channel element  113  performs decoding on the received SCH frame. After decoding, the supplemental channel element  113  calculates the CRC bits for the received payload, and then compares the calculated CRC bits with the CRC bits received with the SCH frame. As noted above, the received CRC bits were calculated and transmitted by the supplemental channel element on the transmission side. If the CRC bits are identical to each other, the supplemental channel element  113  provides the multiplex sublayer  112  with the payload of the received supplemental channel frame along with information indicating that the payload passed the CRC check. If the CRC bits are different from each other, the supplemental channel element  113  provides the multiplex sublayer  112  with the payload of the received supplemental channel frame along with information indicating that the payload failed the CRC check. Upon receipt of information indicating the payload passed the CRC check, the multiplex sublayer  112  examines the provided payload from the beginning to separate the MuxPDUs. On the other hand, upon receipt of information indicating the payload failed the CRC check, the multiplexer sublayer  112  discards the provided payload and informs each upper layer entity that an error frame has been received. 
     FIG. 2 shows the structure of the SCH frame which is transmitted over the supplemental channel. Referring to FIG. 2, the SCH frame is comprised of a payload, a 16-bit physical layer CRC, and 8 tail bits indicating termination of the SCH frame for encoding. As indicated above when the transmission procedure was described, the payload is filled with several MuxPDUs and the remainder is filled with fill-MuxPDUs (padding bits). 
     In any radio transmission system, burst errors occur in the data stream. In the CDMA-2000 system, where the SCH frame is transmitted using a convolutional encoding method, burst errors occur from place to place inside the payload of the SCH frame, because of the long length of the SCH frame. In this case, because there are many MuxPDUs in the SCH frame, there will be some MuxPDUs that have no errors as well as the one or more MuxPDUs that have errors. It is much more efficient to separate the error-less MuxPDUs instead of discarding them, and transfer all of them to the upper layer as correctly received data. 
     In order to exploit this efficiency, the MuxPDUs are grouped into larger units, called logical transmission units (LTUs). The LTU has a specific size and has a CRC field for indicating whether or not the LTU has an error. Therefore, when the physical layer CRC check on the SCH frame shows an error, the multiplex sublayer  112  won&#39;t discard the entire payload. Instead, the multiplex sublayer  112  separates out the individual LTUs in the SCH frame payload, and performs an LTU CRC check on each individual LTU. When there is an error, the multiplex sublayer  112  discards the LTU. Otherwise, when there is no error, the multiplex sublayer  112  separates out the MuxPDUs included in the LTU and provides the data to the upper layers. 
     Using this LTU function, each LTU has a CRC field, which is additionally filled into the SCH frame payload. This means that the room in the payload for MuxPDUs is reduced by the size of the CRC field. Therefore, there is a need for a method of maximizing the number of MuxPDUs for transmission while maintaining the existing MuxPDU size. In addition, it is proposed that a 16-bit, or 2-byte, CRC field should be used for byte alignment in arranging the LTU in the payload of the physical channel frame. The preferred embodiment of the present invention proposes a 12-bit LTU CRC field so that more MuxPDUs can be placed in the payload of the SCH frame. In addition, the bytes in the MuxPDUs retain byte alignment, even though the CRC field is 1 and a ½ bytes long. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a device and method for effectively arranging a CRC field in a CDMA communication system. 
     It is another object of the present invention to provide an LTU assembling device and method which retains byte alignment without reducing the size of a MuxPDU, thereby increasing efficiency, in a communication system. 
     It is another object of the present invention is to provide a byte alignment method using at least two logical CRC fields in one physical frame. 
     To achieve the above and other objects, there is provided a method for arranging CRC fields within logical transmission units, (LTUs) in a CDMA (Code Division Multiple Access) communication system having a physical channel frame comprised of a plurality of LTUs arranged in line, followed by the physical layer CRC field and tail bits. Each LTU has a payload containing data and a LTU CRC field for error correction of the data. In the LTU CRC arranging method, the LTU CRC field included in odd-numbered LTUs is placed in the back of the LTU and the CRC field included in even-numbered LTUs is placed in the front of the LTU. Because each odd-numbered LTU is followed by an even-numbered LTU, the two CRC fields form a three byte boundary between adjacent LTUs, resulting in the retention of byte alignment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagram illustrating the protocol stack for a supplemental channel in a CDMA communication system; 
     FIG. 2 is a diagram illustrating the structure of a physical channel frame in a CDMA communication system; 
     FIG. 3 is a diagram illustrating a multiplex sublayer which assembles and de-assembles logical transmission units (LTUs) in a CDMA communication system according to an embodiment of the present invention; 
     FIG. 4 is a flow chart illustrating a procedure for assembling a supplemental channel frame payload in a CDMA communication system according to an embodiment of the present invention; 
     FIG. 5 is a flow chart illustrating a procedure for assembling an LTU in a CDMA communication system according to an embodiment of the present invention; 
     FIG. 6 is a diagram illustrating the structure of a odd-numbered LTU generated in a CDMA communication system according to an embodiment of the present invention; 
     FIG. 7 is a diagram illustrating the structure of an even-numbered LTU generated in a CDMA communication system according to an embodiment of the present invention; 
     FIG. 8 is a flow chart illustrating a procedure for de-assembling a supplemental channel frame payload in a CDMA communication system according to an embodiment of the present invention; and 
     FIG. 9 is a flow chart illustrating a procedure for de-assembling an LTU in a CDMA communication system according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     FIG. 3 shows a multiplex sublayer which assembles and de-assembles LTUs in a CDMA communication system according to an embodiment of the present invention. The structure of FIG. 3 applies to both mobile stations and base stations. 
     Referring to FIG. 3, the supplemental channel element  313  and the upper layer entity, in this case, RLP  311 , have the same operation as those of FIG.  1 . The remaining 6 elements comprise the multiplex sublayer according to an embodiment of the present invention. All the functions of this embodiment of the present invention are performed by a multiplex sublayer controller  312 . FIG. 3 shows 5 newly added devices. An LTU table  314  stores the number of LTUs which can fill the payload of a SCH frame according to different data rates of the supplemental channel. An example of what a LTU table would store is given below in Table 1. If the LTU function is not to be used, the number of LTUs is set to ‘0’. An LTU counter  315  counts the current number of LTUs in the LTU assembling and de-assembling process. A first register (REGISTER N)  316  stores the number of LTUs which can fill the payload of a SCH frame according to the current data rate of the transmitted or received supplemental channel. A CRC generator  317  generates a 12-bit CRC field by receiving the bits in the payload of the LTU. Here, the multiplex sublayer controller  312  directs the CRC generator  317  to generate the CRC field for the payload of the LTU, in the LTU assembling and de-assembling process. A second register (REGISTER PASS)  318  stores the CRC check results of the physical layer, provided from the supplemental channel element  313 . For example, when the SCH frame has passed the CRC check, the second register  318  stores ‘1’, and, if the SCH frame does not pass the CRC check, the second register  318  stores ‘0’ and provides the CRC check results to the multiplex sublayer controller  312 . 
     Table 1 below shows an example of the LTU table  314 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Data Rate of SCH (bps) 
                 LTU Number 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 9600 
                 0 
               
               
                   
                 19200 
                 0 
               
               
                   
                 38400 
                 2 
               
               
                   
                 76800 
                 4 
               
               
                   
                 153600 
                 8 
               
               
                   
                 14400 
                 0 
               
               
                   
                 28800 
                 0 
               
               
                   
                 57600 
                 2 
               
               
                   
                 115200 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     In Table 1, for a data rate of 9600 bps, the LTU number is 0. This means that the payload of the SCH frame is filled in the normal fashion, without using the LTU function, i.e., using only MuxPDUs without attaching the LTU CRC. 
     Further down in Table 1, for a data rate of 38400 bps, the LTU number is 2. This means that two LTUs, each one comprised of MuxPDUs and an LTU CRC, are used to fill the payload of the SCH frame. 
     FIG. 4 shows a procedure for assembling the payload of a SCH frame in the multiplex sublayer according to an embodiment of the present invention. Referring to FIG. 4, the multiplex sublayer controller  312  reads the number of LTUs according to the data rate of the supplemental channel (SCH) from the LTU table  314 , and then stores the read LTU number in the first register  316 , in step  411 . At step  413 , it is determined if the value in the first register  316  is ‘0’. If the value is ‘0’, it is not necessary to assemble LTUs. Thus, in step  415 , the multiplex sublayer controller  312  creates the payload in the same manner as in the existing SCH frame payload creating method. Thereafter, in step  423 , the multiplex sublayer controller  312  transfers the created SCH frame payload to the supplemental channel element  313 . 
     Otherwise, if the value stored in the first register  316  is not ‘0’ at step  413 , the multiplex sublayer controller  312  proceeds to step  417  to perform a SCH frame payload creating process according to the present invention. After the payload of the SCH frame is filled with LTUs in step  417 , the multiplex sublayer controller  312 , in step  419 , determines whether the payload of the SCH frame is completely filled. If the payload is not completely filled, the multiplex sublayer controller  312  fills the remaining part with ‘0’ bits for padding in step  421 , and then proceeds to step  423 . Otherwise, if the payload is completely filled, the multiplex sublayer controller  312  transfers the created SCH frame payload to the supplemental channel element  313 , in step  423 . 
     FIG. 5 shows the LTU assembling process of step  417  in FIG. 4 in greater detail. Referring to FIG. 5, the multiplex sublayer controller  312  sets the LTU counter  315  to ‘1’ in step  511 . In step  513 , the multiplex sublayer controller  312  determines whether the LTU counter  315  has an odd value. If the LTU counter  315  has an odd value, the multiplex sublayer controller  312  assembles an odd-numbered LTU, starting at step  517 ; but if the LTU counter  315  has an even value, the multiplex sublayer controller  312  assembles an even-numbered LTU, starting at step  523 . 
     With regard to the odd-numbered LTU assembling process, the multiplex sublayer controller  312  starts filling the LTU with MuxPDUs at the beginning of the LTU in step  517 . There is at least one MuxPDU per LTU, and the number of MuxPDUs is determined according to the LTU size. When the payload of the LTU is filled with the MuxPDUs, the multiplex sublayer  312  directs the payload of the LTU to the CRC generator  317  in step  519 , thus enabling the CRC generator  317  to calculate the 12 CRC bits for the LTU payload. Once the CRC generator  317  has generated the 12 CRC bits for the LTU payload, the multiplex sublayer controller  312  attaches the 12-bit CRC field in back of the LTU payload in step  521 , and proceeds to step  529 . Thus, the CRC field of every odd-numbered LTU is at the very end of the LTU. 
     With regard to the even-numbered LTU composing process, the multiplex sublayer controller  312  starts filling the LTU with MuxPDUs beginning at the 13 th  bit, in step  523 . Thus, the first 12 bits of the LTU are reserved for the CRC field. After filling the LTU payload with MuxPDUs, the multiplex sublayer controller  312  directs the LTU payload to the CRC generator  317  in step  525 , thus enabling the CRC generator  317  to calculate the 12 CRC bits for the LTU payload. Once the CRC generator  317  generates the 12 CRC bits for the LTU payload, the multiplex sublayer controller  312  places the 12-bit CRC field in the reserved 12-bit blank in the front of the LTU in step  527 , and proceeds to step  529 . Thus, the CRC field of every even-numbered LTU is at the very start of the LTU. 
     An advantage of creating odd- and even-numbered LTUs in this fashion will be described in reference to FIGS. 6 &amp; 7, before returning to step  529  on FIG.  5 . FIGS. 6 and 7 show the odd-numbered LTU and the even-numbered LTU generated in the above procedure. The odd-numbered LTU has the 12-bit LTU CRC field following the LTU payload as shown in FIG. 6, and the even-numbered LTU has the 12-bit LTU CRC field preceding the LTU payload as shown in FIG.  7 . By always having the number of LTUs in a SCH frame equal a multiple of 2 (as shown in Table 1), the CRC field of the odd-numbered LTU is always paired with the CRC field of an even-numbered LTU. In this arrangement, since each CRC field is comprised of 12 bits, the two CRCs will equal 3 bytes. Because the MuxPDUs are all aligned based on the length of a byte, it is usually not possible to use a CRC field that has a fraction of byte. However, in the preferred embodiment of the present invention, it is possible to byte-align the starting point of the MuxPDUs by consecutively connecting the two 12-bit, or 1 and a ½ byte, CRCs. Thus, it is possible to byte-align all the MuxPDUs, even though the individual CRC fields are not byte unit based. 
     Returning to FIG. 5, after the LTU assembling process, the multiplex sublayer controller  312  determines in step  529  whether the required number of LTUs have been assembled. That is, it is determined whether the value of the LTU counter  315  is identical to the value stored in the first register  316 . If it is, the multiplex sublayer controller  312  has completed the LTU assembling process, and thus returns to step  419  of FIG.  4 . If the LTU counter is not equal to the first register  316  at step  529 , the multiplex sublayer controller  312  increases the LTU counter  315  by one at step  515 , and returns to step  513  to repeat the above LTU assembling process. 
     FIG. 8 shows a procedure for receiving the payload of a SCH frame in the multiplex sublayer according to an embodiment of the present invention. Referring to FIG. 8, the multiplex sublayer controller  312  stores, in step  811 , information about the physical layer CRC check results in the second register  318 . The supplemental channel element  313  provides the information about the physical layer CRC check results. Also in step  811 , the multiplex sublayer controller  312  reads the LTU number suitable for the current data rate of the supplemental channel from the LTU table  314  and stores it in the first register  316 . Thereafter, the multiplex sublayer controller  312  determines in step  813  whether the value stored in the first register  316  is ‘0’. 
     If the value stored in the first register  316  is ‘0’, the multiplex sublayer controller  312  proceeds to step  815  since it is not necessary to use the LTU function. In step  815 , if the second register  318  indicates that the SCH frame payload passed the physical layer CRC check, the multiplex sublayer controller  312  separates the MuxPDUs out from the received SCH frame payload, and then separates data blocks out from the respective MuxPDUs, as is done in the existing SCH frame payload de-assembling method. Thereafter, the multiplexer sublayer controller  312  transfers the separated data blocks to the appropriate upper layer entity (e.g., the RLP layer) destination. If the second register  318  indicates that the SCH frame payload did not pass the physical layer CRC check, the multiplex sublayer controller  312  informs the upper layer entity that a corrupted frame has been received, as in the existing SCH frame payload de-assembling method. 
     If the value stored in the first register  316  is not ‘0’ at step  813 , the multiplex sublayer controller  312  proceeds to step  817  to perform the LTU de-assembling process according to an embodiment of the present invention. The LTU de-assembling process according to an embodiment of the present invention is shown in FIG.  9 . 
     Referring to FIG. 9, the multiplex sublayer controller  312  sets the LTU counter  315  to ‘1’ in step  911 . Then the multiplex sublayer controller  312  determines in step  913  whether the LTU counter  315  has an odd value. If the LTU counter  315  has an odd value, the multiplex sublayer controller  312  proceeds to step  917  to perform the odd-numbered LTU de-assembling process; and, if the LTU counter  315  has an even-numbered value, the multiplex sublayer controller  312  proceeds to step  929  to perform the even-numbered LTU de-assembling process. 
     With regard to the odd-numbered LTU de-assembling process, the multiplex sublayer controller  312  defines the portion in the beginning of the LTU as the LTU payload, and separates out the LTU CRC field at the end, in step  917 . Then the multiplex sublayer controller  312  determines in step  919  whether physical layer CRC check was passed by analyzing the output of the second register  318 . If the physical layer CRC check is passed, the multiplex sublayer controller  312  proceeds to step  927  to directly de-assemble the LTU into MuxPDUs without performing a LTU CRC check. The MuxPDU de-assembling process is as follows. Because the MuxPDU has a specific size, the multiplex sublayer controller  312  separates out the MuxPDUs from the received payload, then separates out the data blocks from the MuxPDUs, and lastly transfers the separated data blocks to the appropriate upper layer entity. In this example, the upper layer entity is RLP. 
     If the physical layer CRC check has failed, the multiplex sublayer controller  312  proceeds to step  921  to perform a LTU CRC check on the LTU. That is, the multiplex sublayer controller  312  directs the LTU payload to the CRC generator  317 , thus enabling the CRC generator  317  to generate a 12-bit CRC. Thereafter, in step  923 , the multiplex sublayer controller  312  determines whether the just-generated 12-bit CRC is identical to the 12-bit LTU CRC field which was received with the present LTU payload. If the CRC bits are identical to each other, the multiplex sublayer controller  312  proceeds to step  927  to directly de-assemble the LTU payload into MuxPDUs. If the CRC bits do not match in step  923 , the multiplex sublayer controller  312  proceeds to discard the corrupted LTU in step  925 , and then proceeds to step  939 . 
     With regard to the even-numbered LTU de-assembling process, the multiplex sublayer controller  312  separates out the first 12 bits of the LTU as the LTU CRC field and defines the following portion as the LTU payload, in step  929 . Thereafter, in step  931 , the multiplex sublayer controller  312  determines whether the physical layer CRC check was passed by analyzing the output of the second register  318 . If the physical layer CRC check was passed, the multiplex sublayer controller  312  proceeds to step  927  to directly de-assemble the LTU into MuxPDUs without performing a LTU CRC check. 
     If it is determined the physical layer CRC check has failed in step  431 , the multiplex sublayer controller  312  proceeds to step  933  to perform a LTU CRC check on the LTU. That is, the multiplex sublayer controller  312  directs the LTU payload to the CRC generator  317 , thus enabling the CRC generator  317  to generate a 12-bit CRC. Thereafter, in step  935 , the multiplex sublayer controller  312  determines whether the just-generated 12-bit CRC is identical to the 12-bit LTU CRC field which was received with the current LTU payload. If the CRC bits are identical to each other, the multiplex sublayer controller  312  proceeds to step  927  to directly de-assemble the LTU payload into MuxPDUs. If the CRC bits are not identical, the multiplex sublayer controller  312  proceeds to discard the current LTU in step  437 , and then proceeds to step  939 . 
     After either odd-numbered or even-numbered LTU de-assembly or discarding, the process comes to step  439 . At step  439 , the multiplex sublayer controller  312  determines whether the required number of LTUs have been de-assembled. If the value of the LTU counter  315  is identical to the value stored in the first register  316 , the multiplex sublayer controller  312  ends the LTU de-assembly process. If the LTU counter  315  is not identical to the first register  316 , the multiplex sublayer controller  312  increases the LTU counter  315  by one in step  915 , and returns to step  913  to repeat the LTU de-assembling process for the next LTU. 
     As described above, the novel LTU assembly and de-assembly method, which uses a 12-bit CRC field according to an embodiment of the present invention, transmits an increased amount of data by reducing the portion occupied by the LTU CRC field and facilitates efficient data transmission and reception through byte alignment. 
     Although the preferred embodiment used the example of a supplemental channel in a CDMA-2000 system, the present invention can apply to any system having two or more CRC fields in a transmission frame. 
     While the invention has been shown and described with reference to a certain preferred embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.