Abstract:
A Radio Resource Control (RRC) message transmission method and apparatus for a wireless communication system operating in TTI and acknowledge mode is configured to reduce interruption time by minimizing retransmission times of RRC message. The message transmission method for a wireless communication system includes generating, at a transmitter, a message; transmitting the message two or more times consecutively from the transmitter to a receiver; analyzing, at a receiver, the message transmitted by the transmitter; transmitting an acknowledge message or a negative-acknowledge from the receiver to the transmitter depending on the analysis result; and terminating, at the transmitter, a retransmission process of the message upon receipt of the acknowledgement message.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims priority to an application entitled “RRC MESSAGE TRANSMISSION METHOD IN WIRELESS COMMUNICATION SYSTEM” filed in the Korean Intellectual Property Office on Jan. 22, 2009 and assigned Serial No. 10-2009-0005351, the contents of which are incorporated herein by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to wireless communications and, in particular, to a Radio Resource Control (RRC) message transmission method and apparatus for a wireless communication system operating in TTI and acknowledge mode. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the wireless communication system operating in Transmission Time Interval (TTI) mode, the transmitter transmits a message at TTI, and the receiver performs Cyclic Redundancy Check (CRC) on the message received from the transmitter and reports the result to the transmitter. That is, the receiver in acknowledge mode determines whether the message has been received successfully and transmits an acknowledgement/negative acknowledgement (ACK/NACK) message to the transmitter based on the determination result. 
         [0004]    In Long Term Evolution (LTE) system, Radio Resource Control (RRC) is responsible for controlling layer 3 signaling between User Equipment (UE) and evolved Node B (eNB), connection establishment and release, broadcast of system information, Radio Bearer establishment/reconfiguration and release, RRC connection mobility procedure, and the like. Typically, the RRC messages are exchanged in TTI mode, and the transmitter performs retransmission process depending on the response transmitted by the receiver. 
         [0005]    A description is made of RRC message transmission procedure.  FIG. 1  illustrates a sequence diagram of the operations of a UE and network entities for a handover procedure in an LTE system. 
         [0006]    Referring to  FIG. 1 , a UE is communicating packet data  111  with a source eNB. During the communication, the source eNB sends a Reference Signal Received Power (RSRP) Detection signal  112  to the UE. Upon receipt of the RSRP Detection signal, the UE performs a handover measurement procedure  121  to  129  with the source eNB. During the handover measurement procedure, the UE performs Scheduling Request, Buffer Status Report, and Measurement Report to the source eNB. The source eNB sends an ACK/NACK message to the source eNB in response to the individual messages. 
         [0007]    If a handover decision has been made, the source eNB performs a handover preparation procedure  131  to  133  with a target eNB. In the handover procedure, the source eNB sends a Handover Request message  131  to the target eNB, and the target eNB sends a Handover Request ACK message to the source message. 
         [0008]    Once the handover has been prepared, the source and target eNBs and UE perform a handover interruption procedure  141  to  150 . During the handover interruption procedure, the source eNB sends an RRC connection reconfiguration message to the UE and transfers a PDCP SN status message to the target eNB. The source eNB also forwards the data to the target eNB. After the establishment of a link between the UE and target eNB, the target eNB and the UE perform packet data communication. Thereafter, the target eNB sends a packet switch request message to a Mobility Management Entity/Source Gateway (MME/SGW) and performs then performs packet data communication with the UE. 
         [0009]    In  FIG. 1 , the messages  123 ,  126 ,  141 , and  148  are RRC messages transmitted in TTI mode, and the receiver sends an ACK/NACK message in response to the individual RRC messages. When a NACK message is received, the transmitter retransmits the corresponding message. 
         [0010]      FIG. 2  illustrates a message flow diagram for an exchange of RRC messages and ACK/NACK messages between a UE and an eNB in a conventional wireless communication system operating in TTI mode. In  FIG. 2 , the transmission party receives an ACK/NACK message in 8 TTIs after the transmission of an RRC message. 
         [0011]    Referring to  FIG. 2 , the eNB sends an RRC message  221  to the UE. If the RRC message has been received, the UE checks the normality of received RRC message and sends an ACK/NACK message  213  in response to the RRC message depending on the normality check result. The eNB receives the ACK/NACK message in 8 TTIs after the transmission of the RRC. If the received RRC message is abnormal, the UE sends the NACK message to the eNB in response to the RRC message. Upon receipt of the NACK message, the eNB, as the transmitter, retransmits the RRC message  215  and receives the ACK/NACK message  217  in another 8 TTIs after the retransmission of the RRC message. 
         [0012]    Since the eNB and UE communicate with each other through a radio link, the message transmitted by the eNB or UE can be lost due to the radio link problem or bad communication condition. For example, if the RRC message transmitted at step  211  has been erroneous or lost, the eNB must retransmit the RRC message at step  215 , resulting in procedure delay of 8 TTIs. This means that the message loss causes the retransmission procedure delay equal to the time interval for retransmission in the periodical retransmission mode. 
         [0013]    Assuming that the RRC message of  FIG. 2  is the RRC connection reconfiguration message of  FIG. 1  as a handover command and if the RRC connection reconfiguration message transmitted by the source eNB has been lost, the UE sends the NACK message to request the source eNB for the retransmission of the lost RRC message. Once the NACK message has been received, the source eNB retransmits the corresponding RRC message at a predetermined time, i.e., 8 TTIs after the initial transmission of the RRC message, resulting in handover interruption time. 
         [0014]    There is therefore a need for a method to reduce the handover interruption time by minimizing the retransmission times of RRC message. 
       SUMMARY OF THE INVENTION 
       [0015]    To address the above-discussed deficiencies of the prior art, it is a primary object to provide a RRC message transmission method that is capable of reducing the interruption time in a wireless communication system. 
         [0016]    In accordance with an exemplary embodiment of the present invention, a message transmission method for a wireless communication system includes generating, at a transmitter, a message; transmitting the message two or more times consecutively from the transmitter to a receiver; analyzing, at a receiver, the message transmitted by the transmitter; transmitting an acknowledge message or a negative-acknowledge from the receiver to the transmitter depending on the analysis result; and terminating, at the transmitter, a retransmission process of the message upon receipt of the acknowledgement message. 
         [0017]    Preferably, transmitting the message includes generating duplicates of the RRC message on an RRC layer of the transmitter; and transmitting the duplicates of the RRC message consecutively through a physical layer by a Radio Link Control (RLC) layer or a Media Access Control (MAC) layer. 
         [0018]    Preferably, transmitting the message includes generating, at an RRC layer of the transmitter, a consecutive transmission control information; transferring the RRC message with the consecutive transmission control information from the RRC layer to a Radio Link Control (RLC) layer; generating, at the RLC layer, duplicates of the RRC message based on the consecutive transmission control information; and transmitting, at the RLC layer, the duplicates of the RRC message to the receiver consecutively through a Media Access Control (MAC) and a Physical (PHY) layer. 
         [0019]    Preferably, transmitting the message includes generating, at an RRC layer of the transmitter, a consecutive transmission control information; transferring the RRC message with the consecutive transmission control information from the RRC layer to a Radio Link Control (RLC) layer; transferring the RRC message with the consecutive transmission control information from the RLC layer to a Media Access Control (MAC) layer; generating, at the MAC layer, duplicates of the RRC message based on the consecutive transmission control information; and transmitting, at the MAC layer, the duplicates of the RRC message to the receiver consecutively through a Physical (PHY) layer. 
         [0020]    In accordance with another exemplary embodiment of the present invention, a message transmission method of a user equipment (or base station) in a wireless communication system includes transmitting a message two or more times consecutively at a predetermined interval; receiving a reply at the predetermined interval in response to the message; and terminating, if the reply is an acknowledge message, a retransmission process of the message. 
         [0021]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0023]      FIG. 1  illustrates a sequence diagram of the operations of a UE and network entities for a handover procedure in an LTE system; 
           [0024]      FIG. 2  illustrates a message flow diagram for an exchange of RRC messages and ACK/NACK messages between a UE and an eNB in a conventional wireless communication system operating in TTI mode; 
           [0025]      FIGS. 3 and 4  illustrate message flow diagrams for an exchange of RRC messages and ACK/NACK messages between a UE and an eNB in an RRC message transmission method according to an exemplary embodiment of the present invention; 
           [0026]      FIG. 5  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to an exemplary embodiment of the present invention; 
           [0027]      FIG. 6  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to another exemplary embodiment of the present invention; and 
           [0028]      FIG. 7  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIGS. 3 through 7 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communications system. 
         [0030]    In the following description, the present invention provides a method for reducing transmission delay caused by RRC message retransmission. In an exemplary embodiment of the present invention, the same RRC message is retransmitted repeatedly to increase the reception probability at the receiver. This means to reduce the number of retransmissions of an RRC message in TTI mode. 
         [0031]    In a wireless communication system according to the present invention, the RRC message transmission method can be implemented in three different ways. In one embodiment, the RRC entity of a transmitter transmits multiple duplicates of an RRC message and checks the ACK/NACK message transmitted by the receiver. In another embodiment, the RRC entity of the transmitter generates an RRC message having information on the consecutive transmission of the RRC message and a corresponding Radio Link Control (RLC) entity transmits multiple duplicates of the RRC message based on the consecutive transmission information and checks the ACK/NACK message transmitted by the receiver. In yet another embodiment, the RRC entity of the transmitter generates an RRC message with the information on the consecutive transmission and a corresponding Media Access Control (MAC) entity transmits multiple duplicates of the RRC message based on the consecutive transmission information and checks the ACK/NACK message transmitted by the receiver. 
         [0032]    The RRC message transmission method according to an exemplary embodiment of the present invention processes the duplicates of an RRC message as multiple consecutive RRC messages virtually and transmits the consecutive RRC messages within a period of one or more TTIs. The consecutive RRC message retransmission according to an exemplary embodiment of the present invention does not need to modify the other layers and procedures specified in the standard. The RRC (or RLC) entity of the transmitter checks the response message transmitted by the receiver and, if the RRC message has been received successfully at the receiver (i.e., the ACK message has been received), skips retransmission of the RRC message and drops the response message received after this. That is, the RRC (or RLC or MAC) entity stops retransmission of the RRC message upon receipt of the first ACK message transmitted in response to the RRC message. 
         [0033]      FIGS. 3 and 4  illustrates message flow diagrams for an exchange of RRC messages and ACK/NACK messages between a UE and an eNB in an RRC message transmission method according to an exemplary embodiment of the present invention. In the exemplary embodiment of  FIGS. 3 and 4 , the eNB is the transmitter and the UE is the receiver. However, the RRC message transmission method of the present invention can be applied when the UE is a transmitter and the eNB is a receiver. Although the RRC message is transmitted at an interval of TTI in  FIGS. 3 and 4 , the transmission interval can be configured to more than one TTI. Although it is depicted that the RRC message is transmitted 4 times in  FIGS. 3 and 4 , the number of the transmissions of the same RRC message can be configured to 2 to 8 times since the RRC message and the ACK/NACK message are exchanged at an interval of the 8 TTIs. 
         [0034]    Referring to  FIG. 3 , the eNB as the transmitter transmits the RRC message 4 times at TTI. That is, the eNB transmits the RRC message  311  and repeats the transmission of the RRC at TTI  313  to  317 . Accordingly, the UE as the receiver can receive the same RRC messages transmitted at steps  313  to  317 . 
         [0035]    If the RRC message has been received, the UE transmits an ACK/NACK message depending on whether the RRC message is erroneous. That is, the UE analyzes the RRC message transmitted at step  311  and transmits, if the RRC message is received successfully, an ACK message and otherwise, a NACK message at step  321 . In this manner, the UE checks the RRC messages transmitted at steps  313  to  317  and transmits the ACK/NACK message in response to the corresponding RRC messages. 
         [0036]    If an ACK message has been received, this means that the RRC message is successfully and thus the eNB ignores the ACK/NACK messages following the ACK message. That is, if it has been detected that the UE received the RRC message successfully by means of the ACK message, the eNB terminates the retransmission process upon receipt of the ACK message. 
         [0037]    Referring to  FIG. 4 , the eNB as the transmitter transmits the same RRC message 4 times at TTI  351 ,  353 ,  355 , and  357 ). In the example of  FIG. 4 , the RRC message transmitted at steps  351  and  353  are erroneous and the RRC message transmitted at step  355  is delivered to the UE successfully. Accordingly, the UE transmits the NACK messages to the eNB in response to the individual RRC messages  361  and  363 . The eNB receives all of the 4 ACK/NACK messages transmitted by the UE in response to the RRC messages and then determines whether to retransmit the RRC message depending on analysis result about the 4 ACK/NACK messages. Since the ACK message has been received in response to the RRC message transmitted at step  355 , the eNB recognizes that the UE received the RRC message successfully  365 . 
         [0038]    Accordingly, the eNB stops retransmission process upon receipt of the ACK message at step  365 . In this manner, the transmitter transmits the same RRC message multiple times and, if at least one of the multiple transmissions of the same RRC message is responded by means of the ACK message, the transmitter stops retransmission process, thereby reducing the RRC message retransmission delay and improving the service quality. 
         [0039]      FIG. 5  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to an exemplary embodiment of the present invention. In the exemplary embodiment of  FIG. 5 , an eNB transmits an RRC message, and a UE receives the RRC message transmitted by the eNB and transmits an ACK/NACK message in response to the RRC message. 
         [0040]    The radio protocol stack of the LTE system includes a Radio Resource Control (RRC) layer, a Radio Link Control (RLC) layer, a Media Access Control (MAC) layer, and a Physical (PHY) layer. As shown in  FIG. 5 , the eNB includes an RRC entity  402 , an RLC entity  404 , a MAC entity  406 , and a PHY entity  408 ; and the UE includes an RRC entity  452 , an RLC entity  454 , a MAC entity  456 , and a PHY entity  458 . 
         [0041]    The RRC entities  405  and  452  control layer 3 are responsible for signaling between the eNB and the UE, link connection and release, broadcast of system information, establishment/modification/release of radio bearers, and RRC connection mobility procedures. The RLC entities  404  and  454  are responsible for segmentation of the RRC packets transferred by the corresponding RRC entities  402  and  452  into Protocol Data Units (PDUs) in size appropriate for Automatic Repeat Request (ARQ) operation. The MAC entities  406  and  456  are connected to multiple RLC entities to multiplex the RLC PDUs to be transmitted into MAC PDUs and demultiplex the received MAC PDUs into RLC PDUs. The PHY entities  408  and  458  are responsible for performing channel coding and modulation on the MAC PDUs to be transmitted and transmit the OFDM symbols over the radio channel and performing channel demodulation and decoding on the received OFDM symbols and deliver the decoded MAC PDUs to the MAC entities  406  and  456 . 
         [0042]    In  FIG. 5 , the RRC entity  402  generates multiple duplicates of an RRC message and transfers the duplicates of the RRC message to the RLC entity. The solid line arrows denote the paths of the RRC message transmitted by the transmitter (i.e., the eNB in  FIG. 5 ), and the dotted line arrows denote the paths of the ACK/NACK message transmitted by the receiver (i.e., the UE in  FIG. 5 ). 
         [0043]    The RRC entity  402  of the eNB transmits the RRC message via the RLC entity  404 , the MAC entity  406 , and the PHY entity  408 ; and the RRC entity  452  of the UE receives the RRC message via the PHY entity  458 , the MAC entity  456 , and the RRC entity  454 . Here, the RRC message is transmitted repeatedly at TTI (or at an interval of two or more TTIs) as shown in  FIG. 3 . That is, the RRC entity  402  transfers the duplicates of the RRC message to the lower layer (i.e., RLC entity). 
         [0044]    The RRC entity  452  of the UE checks the duplicates of the RRC message received in series and determines whether the RLC message has been received normally. If the RRC message has been received normally, the RRC entity  452  transfers an ACK message to the lower layers. Otherwise, if the RRC message has been received abnormally, the RRC entity  452  transfers a NACK message to the lower layers. The ACK/NACK message is transmitted in the reverse order of the RRC message as denoted by the dotted line arrows so as to be received by the RRC entity  402  of the eNB. The RRC entity  402  of the eNB may receive multiple ACK/NACK messages in response to the duplicates of the RRC message. If an ACK message has been received, this means that the UE has received the RRC message and thus the RRC entity  402  of the eNB stops the retransmission process. Only if all the ACK/NACK messages received in response to the duplicates of the RRC message have been NACK messages, the RRC entity  402  of the eNB performs retransmission. 
         [0045]      FIG. 6  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to another exemplary embodiment of the present invention. In the exemplary embodiment of  FIG. 6 , an eNB transmits an RRC message, and a UE receives the RRC message transmitted by the eNB and transmits an ACK/NACK message in response to the RRC message. 
         [0046]    In  FIG. 6 , the RRC entity  402  of the eNB generates and transfers an RRC message to the RLC entity  404 , and the RLC entity  404  generates multiple duplicates of the RRC message to be transmitted to the UE. The RRC entity  402  of the eNB transfers the RRC message with consecutive transmission control information to the lower layer, i.e., the RLC entity  404 . If the RRC message with the consecutive transmission control information has been received, the RLC entity  404  reserves the TTIs virtually as if multiple RRC messages are received from the RRC entity  402  in series based on the consecutive transmission control information. Thereafter, the RLC entity  404  transmits the RRC message preset number of times (e.g., 4 times as shown in  FIG. 3 ) at TTI and thus the RRC message is delivered to the RRC entity  452  of the UE through the path as denoted by the solid line arrows repeatedly. 
         [0047]    The RLC entity  454  of the UE analyzes the RRC messages received at TTI to evaluate the normality of the RRC message. If the RRC message has been received normally in a TTI, the RLC entity  454  transfers an ACK message to the lower layers. Otherwise, if the RRC message has been received abnormally, the RLC entity  454  transfers a NACK message to the lower layers. The ACK/NACK message is delivered to the RLC entity  404  of the eNB through the path as denoted by the dotted line arrows. The RLC entity of the eNB may receive the NACK messages up to the number of the RRC message transmissions. If an ACK message has been received in a TTI, this means that the UE has received the RRC message normally and thus the RLC entity  404  of the eNB stops the retransmission process. That is, the RLC entity  404  of the transmitter performs retransmission of the corresponding RRC message only when the NACK messages as many as the number of consecutive transmissions of the same RRC message. 
         [0048]      FIG. 7  illustrates operations of a receiver and a transmitter for supporting an RRC message transmission method according to another exemplary embodiment of the present invention. In the exemplary embodiment of  FIG. 7 , an eNB transmits an RRC message, and a UE receives the RRC message transmitted by the eNB and transmits an ACK/NACK message in response to the RRC message. 
         [0049]    In  FIG. 7 , the RRC entity  402  of the eNB generates and transfers an RRC message with consecutive transmission control information to the RLC entity  404 , and the RLC entity  404  transfers the RRC message with the consecutive transmission control information to the MAC entity  406 . If the RRC message with the consecutive transmission control information has been received, the MAC entity  406  reserves the TTIs virtually as if multiple RRC messages are received from the RRC entity  402  in series base on the consecutive transmission control information. Thereafter, the MAC entity  406  transmits the RRC message preset number of times at TTI and thus the RRC message is delivered to the RRC entity  452  through the path as denoted by the solid line arrows repeatedly. 
         [0050]    The MAC entity  456  of the UE analyzes the RRC messages received at TTI to evaluate the normality of the RRC message. If the RRC message has been received normally in a TTI, the MAC entity  456  transfers an ACK message to the lower layer. Otherwise, if the RRC message has been received abnormally in a TTI, the MAC entity  456  transfers a NACK message to the lower layer. The ACK/NACK message is delivered to the MAC entity  406  of the eNB through the path as denoted by the dotted line arrows. The MAC entity of the neb may receive the NACK messages up to the number of the RRC message transmissions. If an ACK message has been received in a TTI, this means that the UE has received the RRC message normally and thus the MAC entity  406  of the eNB stops the retransmission process. That is, the MAC entity  406  of the transmitter performs retransmission of the corresponding RRC message only when the NACK messages as many as the number of consecutive transmissions of the same RRC message. 
         [0051]    As described above, the RRC message transmission method of the present invention is advantageous to reduce the interruption time by minimizing the retransmission times of RRC message. 
         [0052]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.