Abstract:
In the AM RLC reset procedure of a wireless communication system, a delayed Reset ACK PDU will cause un-synchronization between the Sender and the Receiver. This invention of method and system checks for arriving of the first and the second Reset PDUs and the first and the second Reset ACK PDUs first, then selectively updates the status-related variables and HFN values to synchronize the Sender and the Receiver.

Description:
CROSS REFERENCE APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/338,148, filed on Nov. 13, 2001. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to a wireless communication system and methods. More particularly, the invention relates to a robust RLC reset method and system in a wireless communication environment.  
           [0003]    In a wireless communication system, all communication contents will be packaged in Protocol Data Unit (PDU) format. Refers to FIG. 1, a typical PDU consists a number of bytes (octets), where various bit-size fields are defined. For example, shown in FIG. 1, the one-bit D/C field  12  indicates whether the type of an AM PDU is a data or a control PDU. The 3-bits PDU TYPE field  14  indicates what kind of control type the PDU is. The 1-bit Reset Sequence Number (RSN)  16  is used to indicate the sequence of the transmitted Reset PDU. If this Reset PDU is a retransmission of an original Reset PDU, the RSN value is same as the original Reset PDU. Otherwise, the RSN value is toggled to the next RSN value. Its initial value is 0. The value will be reinitialized every time the RLC is re-established. But it will not be reinitialized when the RLC is reset. The 3-bits Reserved  1  (R 1 ) field  18  is reserved for future functions. The 20-bits Hyper Frame Number Indictor (HFNI) field  20  is used to indicate the Hyper Frame Number (HFN), which helps to track the synchronization between a Sender and a Receiver. A Sender can be a User Equipment (UE) or an UTRAN (Universal Terrestrial Radio Access Network) and so is a Receiver. And the last field—the PAD field  22  is used to make sure the minimum length of the PDU. In general, a transmission from the UE to the UTRAN is called an Uplink transmission (UL) while the transmission from the UTRAN to the UE is called a Downlink transmission (DL).  
           [0004]    Under certain conditions in an Acknowledge Mode (AM), either a Sender or a Receiver will initiate a reset procedure if one sends too many retries—the number of retries has exceeded the maximum number of retransmission, or one receives a PDU with erroneous sequence number. As shown in FIG. 2, in a normal AM RLC (Radio Link Control) reset procedure, a Sender  30  initiates a reset procedure during transmission. The Sender  30  sends a Reset PDU (stage  34 ) to the Receiver  32 , then the Receiver  32  returns a corresponding RESET ACK PDU (stage  36 ) to the Sender  30 . Using the reset procedure, the HFN numbers and status-related STATE variables between the Sender  30  and the Receiver  32  will be re-synchronized, so will be the communication between them.  
           [0005]    [0005]FIG. 3 illustrates the RLC reset procedure in more detail, using an UE as a Sender  40  and an UTRAN as a Receiver  42 . When the reset condition occurs with this configuration, the Sender  40  will initiate a reset procedure. Assume at stage  44 , the Sender  40  has its UL Hyper Frame Number (UL HFN)=x and its DL Hyper Frame Number (DL HFN)=y 1  (stage  44 ). Meantime the Receiver  42  has its UL HFN=x 1  and its DL HFN=y (stage  46 ). The Sender  40  prepares a Reset PDU with its HFNI=x and RSN=0. The Sender in stage  48  passes the Reset PDU (RSN=0, and HFNI=x) down to the lower communication layers e.g., MAC or Physical Layer, where this Reset PDU (RSN=0, and HFNI=x) will be sent through a designated connecting channel to the Receiver  42 . Afterward, the Sender  40  in stage  50  stops sending or receiving data through its regular communication channel. Once the Receiver  42  receives the particular Reset PDU (RSN=0, and HFNI=x), it will return a Reset ACK PDU (RSN=0, and HFNI=y) through the designated connecting channel to the Sender  42  (stage  52 ). Afterward, the Receiver  42  in stage  54  also resets its STATE variables. Then the Receiver starts sending DL AM PDUs with DL HFN=y+1 and receiving UL AM PDU with UL HFN=x+1, where y is the value of the HFNI field of the Reset ACK PDU. Upon receiving the Reset ACK PDU (RSN=0, and HFNI=y) from the Receiver  42 , the Sender  40  will reset its STATE variables and start to send and receive data with its UL HFN=x+1 and DL HFN=y+1 (stage  56 ). Therefore, the Hyper Frame Numbers (HFNs) of the Sender  40  and the Receiver  42  are synchronized with UL HFN=x+1 and DL HFN=y+1.  
           [0006]    In the case that the expected Reset ACK PDU is lost during transmission, as shown in FIG. 4, the sender  60  has UL HFN=x and DL HFN=y 1  while the receiver has UL HFN=x 1 , and DL HFN=y as shown in stages  64  and  66 . In stage  68 , a reset condition triggered, the Sender  60  sends the 1 st  Reset PDU with RSN=0 and HFNI=x to the Receiver  62  through a designated connecting channel. Then the Sender  60  will stop sending and receiving data from the regular channels (stage  70 ). The Receiver  62  receives the 1 st  Reset PDU and responds with the 1 st  Reset ACK PDU with RSN=0 and HFNI=y in stage  72 . Once the Receiver  60  sends out the corresponding Reset ACK PDU (RSN=0 and HFNI=y), it will reset its STATE variables and update its HFNs with UL HFN=x+1 and DL HFN=y+1 in stage  78 . Nevertheless, the return Reset ACK PDU is lost (stage  74 ), after a predetermined time out period expired (Reset time-out), the Sender  60  will send the another (2 nd ) Reset PDU (RSN=0, and HFNI=x) as shown in stage  80 . Upon receiving the 2nd Reset PDU (RSN=0 and HFNI=x), the Receiver shall respond by returning a corresponding Reset ACK PDU (RSN=0, and HFNI=y+1 (the current highest HFN) stage  82 ). Next at the stage  84 , the Receiver  62  updates its UL HFN=x+1 and DL HFN=y+2. When the Sender  60  receives the Reset ACK PDU (RSN=0 and HFNI=y+1) before the second Reset Time-out, the Sender  60  will reset its STATE variables and starts to send and receive data with UL HFN=x+1 and DL HFN=y+2 (stage  86 ). The communication resumes a normal operation and the HFNs of the Sender  60  and the Receiver  62  are synchronized.  
           [0007]    Nevertheless, in some cases that the responded Reset ACK PDU is not lost but delayed during the radio transmission. Such delay could happen during the lower layer transmitting scheduling. When the logical channel of this responded Reset ACK PDU has lower transmitting priority than other logical channels that have data to be transmitted. Therefore, as shown in FIG. 5, the Sender  90  does not receive the expected Reset ACK PDU, which still is in the return pipeline, before the time-out expired (stages  98 ,  102  and  104 ). The Sender  90  sends another Reset PDU out again (stage  106 ). Nevertheless, the Sender  90  eventually receives the delayed Reset ACK PDU and another Reset ACK PDU (stages  106 ,  108  and  114 ), which responded to the resend Reset PDU and is considered as “out-of-date”. The prior art suggests that the Sender will discard the “out-of-date” Reset ACK PDU (stage  112 ). At the stage  116 , the Receiver  92  starts to send and receive data with UL HFN=x+1 and DL HFN=y+2. While the Sender  90  is ready to send and receive data with its UL HFN=x+1 and DL HFN=y+1 (stage  112 ). It is clear that the DL HFNs between the Sender  90  and the Receiver  92  are out of synchronization.  
         SUMMARY  
         [0008]    Accordingly, in order to obviate the limitations and drawbacks encountered in the prior art, the present invention resolves the problems caused by the delayed Reset PDU in the AM RLC reset procedure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Following drawings with reference numbers and exemplary embodiments are referenced for explanation purpose.  
         [0010]    [0010]FIG. 1 illustrates the data structure of a regular AM Reset PDU and a regular Reset ACK PDU;  
         [0011]    [0011]FIG. 2 illustrates a simple normal AM RLC Reset procedure;  
         [0012]    [0012]FIG. 3 illustrates a normal AM RLC Reset procedure in more detail;  
         [0013]    [0013]FIG. 4 illustrates an AM RLC Reset procedure with a lost 1 st  Reset ACK PDU;  
         [0014]    [0014]FIG. 5 illustrates an AM RLC Reset procedure with a delayed Reset ACK PDU;  
         [0015]    [0015]FIG. 6 a  illustrates a modified AM RLC Reset procedure implemented at the Receiver Side.  
         [0016]    [0016]FIG. 6 b  illustrates a modified AM RLC Reset procedure implemented at the Sender Side.  
         [0017]    [0017]FIG. 6 c  illustrates another modified AM RLC Reset procedure implemented at the Receiver Side.  
         [0018]    [0018]FIG. 6 d  illustrates another modified AM RLC Reset procedure implemented at the Sender Side. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    This invention develops a method and system to resolve this potential problem caused by receiving redundant Reset ACK PDUs. Assume that the sender initiates a reset procedure in the AM RLC mode when a reset condition is triggered, whether there is too many retries or received a PDU with erroneous Sequence Number. The Sender sends a first Reset PDU. Upon a time-out period expired, without receiving the responded Reset ACK PDU, The Sender sends a second Reset PDU, which has the same RSN value and HFNI values as the first Reset PDU has. When the Receiver receives either Reset PDU, as described in the prior art, the Receiver will automatically send back one Reset ACK package with updated HFN values, reset itself and update its STATE variables and its HFN values each time. Now the HFN of the transmitting side of the Receiver (DL HFN if UTRAN is the Receiver, UL HFN if UE is the Receiver) has been updated twice, i.e., incremented twice. Meantime, by discarding the out-of-date 2 nd  Reset ACK PDU the Sender will reset only once of its HFN of its receiving side (DL HFN if UE is the Sender, UL HFN if UTRAN is the Sender) based on the value of the first ACK PDU, which is different from the Receiver&#39;s current transmitting HFN value as shown in FIG. 5. This is why the Sender and the Receiver encounter the un-synchronization problem when the Sender receives two Reset ACK PDUs (one delayed) in the current AM RLC reset procedure prior art.  
         [0020]    Thus, this invention modifies the way the second Reset ACK PDU is handled either in the Sender or in the Receiver. Implemented at the Receiver side only as shown in FIG. 6 a , when the Receiver  132  receives the first Reset PDU, it works exactly as the prior art does. In addition, the Receiver  132  will check in-coming Reset PDUs to test whether it is the 1 st  or the 2 nd  Reset PDU (stage  144 ). The Receiver  132  will react to the 1 st  Reset ACK PDU just likes the prior art does, resets STATE variables, starts sending and receiving data, updates its HFNs values (stage  152 ). When the second Reset PDU is received, the Receiver  132  does not reset its STATE variables nor update its HFN values. The Receiver  132  will send out a second Reset ACK PDU, which has the same HFN value and RSN as the first Reset ACK PDU has (stage  146 ). Once the Sender received the second Reset ACK PDU, the Sender will simply discard the second Reset ACK PDU as the prior art does (stage  154 ). At this point, Both HFN values and STATE variables are synchronized between the Sender and the Receiver.  
         [0021]    The problem can also be solved from the Sender  160  as shown in FIG. 6 b . Instead of discarding an out-of-date 2 nd  Reset ACK PDU, in this method the Sender will accept the 2 nd  Reset ACK PDU and reset the STATE variables (stage  182 ,  184  and  186 ). So at the end of the reset procedure we will have UL HFN=x+1 and DL HFN=y+2 in the Sender, which are the same as what in the Receiver  162 .  
         [0022]    In addition to the above-mentioned method, assume that the Receiver has responded to the first received Reset PDU and sent out a corresponding Reset ACK PDU. As shown in FIG. 6 c , up to this stage both the Sender  200  and the Receiver  202  work the same as the prior art does. Except at the Receiver  202  side, the Receiver  202  will check if it received the 2 nd  Reset PDU within a predetermined time period after it sent out the 1 st  Reset ACK PDU (stage  222 ). The predetermined time period can be i.e., a time period for the two-way radio transmission delay between the Sender and the Receiver. If the Receiver receives the second Reset PDU within that predefined period, the Receiver should discard the second Reset PDU. Because based on timing, the Sender did not wait long enough time for the arriving of the first responded Reset ACK PDU before the Sender sends the second Reset PDU. Therefore, the Receiver  202  should ignore and discard the second Reset PDU and process other Reset PDUs as usual (stage  222 ,  224 ). Of course, the same principle can be implemented at the Sender side. The modification is shown in FIG. 6 d . After the Sender  240  has sent the second Reset PDU (stage  258 ), in stage  260 , the Sender  240  will check if it received the Reset ACK PDU within a predetermined time period after it sent out the 2 nd  Reset PDU. As before the predetermined time period can be a time period for the two-way radio transmission delay between the Sender and the Receiver. If it is within that period, the Sender should discard this newly arrived Reset ACK PDU (stage  264 ). Because based on the timing, this Reset ACK PDU apparently is not the responded ACK PDU for the second Reset PDU. It must be the delayed response for the first Reset PDU from the Receiver. The Sender  240 , in stage  262  upon receiving the second Reset ACK PDU, will reset its STATE variables and update its HFNs values based on the 2 nd  Reset ACK PDU. Afterward, the Sender and the Receiver are synchronized.