Abstract:
Disclosed is a method for maintaining uplink timing synchronization by a User Equipment (UE) in a mobile communication system, without uplink transmission of a preamble from a UE or without transmission of uplink timing difference information from an Evolved Node B (E-NB) to solve the inefficient use problem of radio resources, occurring due to periodic transmission of uplink signaling and downlink signaling during maintenance of the uplink timing synchronization, and a UE apparatus for the same.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Jun. 22, 2006 and assigned Serial No. 2006-56417, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for maintaining uplink timing synchronization (sync) in an Orthogonal Frequency Division Multiplexing (OFDM) system and a User Equipment (UE) apparatus for the same. 
     2. Description of the Related Art 
     The mobile communication scheme can be classified into Time Division Multiplexing (TDM), Code Division Multiplexing (CDM) and Orthogonal Frequency Multiplexing (OFM) schemes according to a multiplexing method. The CDM scheme is most popularly used in the current mobile communication system, and can be subdivided into a synchronous and an asynchronous CDM scheme. Since the CDM scheme basically uses codes, it tends to suffer from a lack of resources due to a limit of orthogonal code resources. Accordingly, an OFDM scheme has emerged as an alternative to the CDM scheme. 
     The OFDM scheme is for transmitting data using multiple carriers, and is a type of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel streams, and modulates each of the parallel streams with a plurality of orthogonal sub-carriers, i.e. sub-carrier channels, before transmission. The OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme, but it maintains orthogonality between multiple sub-carriers during transmission and overlaps frequency spectra. Therefore, the OFDM scheme has high frequency efficiency, is robust against frequency selective fading and multi-path fading, and can reduce Inter-Symbol Interference (ISI) with use of a guard interval. In addition, the OFDM scheme enables simple design of a hardware equalizer and is robust against impulse noises, so it can obtain the optimal transmission efficiency during high-speed data transmission. 
     A Long Term Evolution (LTE) system employing the OFDM scheme is now under discussion in 3 rd  Generation Partnership Project (3GPP) as the next generation mobile communication system that will replace Universal Mobile Telecommunication System (UMTS), which is the 3 rd  generation mobile communication standard. 
       FIGS. 1A and 1B  illustrate examples of a wireless mobile communication system to which reference will be made by the present invention, particularly illustrating examples of a 3GPP LTE system. 
     Referring to  FIG. 1A , a UE  11  indicates a terminal for the 3GPP LTE system, and an Evolved Radio Access Network (E-RAN)  14 , a radio base station device directly participating in communication with a terminal in the existing 3GPP system serves as a Node B for managing cells, and also serves as a Radio Network Controller (RNC) that controls a plurality of Node Bs and radio resources. In the E-RAN  14 , an Evolved Node B (E-NB)  12  and an Evolved RNC (E-RNC)  13  can be separately implemented in the physically different nodes, or can be merged in a single node, in the manner of the existing 3GPP system. Although in the following description the E-NB  12  and the E-RNC  13  are physically merged in a single node of the E-RAN  14 , the same can be applied to when the E-RNC  13  is separately implemented in the physically different node. 
     An Evolved Core Network (E-CN)  15  is a node provided by merging functions of a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) in the existing 3GPP system into one function. The E-CN  15 , interposed between a Packet Data Network (PDN)  16  and the E-RAN  14 , serves as a gateway for allocating an Internet Protocol (IP) address to the UE  11  and connecting the UE  11  to the PDN  16 . Definitions and functions of the SGSN and the GGSN follow the 3GPP standard, and a detailed description thereof will be omitted herein. 
     Referring to  FIG. 1B , an Evolved UMTS Radio Access Network (E-RAN)  110  is simplified to a 2-node configuration of Evolved Node Bs (E-NBs)  120 ,  122 ,  124 ,  126  and  128 , and anchor nodes  130  and  132 . A UE  101 , or a terminal, accesses an IP network by the E-RAN  110 . The E-NBs  120  to  128  correspond to the existing Node Bs in the UMTS system, and are connected to the UE  101  over a wireless channel. Unlike the existing Node Bs, the E-NBs  120  to  128  perform more complex functions. In LTE, because all user traffics, including real-time services such as Voice over IP (VoIP), are serviced over a shared channel, there is a need for devices for gathering status information of UEs and performing scheduling depending thereon, and the E-NBs  120  to  128  manage the devices. 
     Generally, one E-NB controls a plurality of cells. In addition, the E-NB performs Adaptive Modulation &amp; Coding (AMC) that determines a modulation scheme and a channel coding rate according to channel status of a UE. Similar to High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and Enhanced Dedicated CHannel (E-DCH) of UMTS, even in LTE, Hybrid Automatic Repeat reQuest (HARQ) is performed between the E-NB  120  to  128  and the UE  101 . However, because LTE cannot meet various Quality of Service (QoS) requirements only with HARQ, Outer-ARQ in an upper layer can be performed between the UE  101  and the E-NBs  120  to  128 . HARQ, as is well known, refers to a technique for soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing a reception success rate. In high-speed packet communication, such as HSDPA and EDCH, the HARQ technique is used to increase transmission efficiency. It is expected that to realize a data rate of a maximum of 100 Mbps, LTE will use OFDM as a wireless access technology in a 20-MHz bandwidth. 
       FIG. 2  illustrates an uplink timing synchronization procedure in a 3GPP LTE system to which OFDM is applied. 
     Referring to  FIG. 2 , a first UE (UE 1 ) is located near an E-NB, and a second UE (UE 2 ) is located far from the E-NB. T_pro 1  indicates a propagation delay time in wireless transmission up to the UE 1 , and T_pro 2  indicates a propagation delay time in wireless transmission up to the UE 2 . Because the UE 1  is located nearer to the E-NB compared to the UE 2 , it has less propagation delay time. In  FIG. 2 , T_pro 1  is 0.33 us, and T_pro 2  is 3.33 us. 
     In one cell (indicated by a circle in  FIG. 2 ) of the E-NB, when the UE 1  and the UE 2  are powered on or are in an idle mode, uplink timing synchronization of the UE 1 , the UE 2  and of UEs in the cell, detected by the E-NB, are not matched to each other. Reference numeral  201  indicates timing synchronization for uplink transmission of an OFDM symbol of the UE 1 , and reference numeral  202  indicates timing synchronization for uplink transmission of an OFDM symbol of the UE 2 . When propagation delay times of uplink transmission of the UE 1  and the UE 2  are considered, timings at the E-NB receiving the uplink OFDM symbols are shown by reference numerals  211 ,  212  and  213 . That is, the uplink symbol  201  of the UE 1  is received at the E-NB with a propagation delay time in the timing  212 , and the uplink symbol  202  of the UE 2  is received at the E-NB with a propagation delay time in the timing  213 . 
     Since uplink timing synchronizations for the UE 1  and the UE 2  have not been acquired (matched) yet for the timings  212  and  213 , start timing  211  in which the E-NB receives and decodes an uplink OFDM symbol, timing  212  in which the E-NB receives an OFDM symbol from the UE 1 , and timing  213  in which the E-NB receives an OFDM symbol from the UE 2  are different from each other. Therefore, the uplink symbols transmitted from the UE 1  and the UE 2  serve as interference components to each other, as they have no orthogonality, and the E-NB may not successfully decode the uplink symbols  201  and  202  transmitted from the UE 1  and the UE 2 , due to the interference and the discrepancy between the start timing  211  and the reception timings  212  and  213  of uplink symbols. 
     Therefore, the E-NB matches uplink symbol reception timings of the UE 1  and the UE 2  through the uplink timing synchronization procedure. After completion of the uplink timing synchronization procedure, the E-NB can match the start timing  221  in which it receives and decodes uplink OFDM symbols, the timing  222  in which it receives an uplink OFDM symbol from the UE 1 , and the timing  223  in which it receives an uplink OFDM symbol from the UE 2 . After matching the timings, the E-NB can maintain orthogonality between the uplink symbols transmitted from the UE 1  and the UE 2 , and thus can successfully decode the uplink symbols  201  and  202  transmitted from the UE 1  and the UE 2 . 
       FIG. 3  illustrates an example of an uplink timing synchronization procedure. 
     In step  311 , a UE  301  generates a preamble code to be used in the uplink timing synchronization procedure. If the UE  301  is constructed such that multiple preamble codes can be used in the uplink timing synchronization procedure, the UE  301  generates one of the multiple preamble codes. The ‘preamble code’ is a type of code sequence agreed upon between the UE  301  and an E-NB  302 , and the UE  301  transmits the preamble code over the uplink using radio resources allocated by the E-NB  302  in step  321  (UL SYNC REQ). Upon receipt of the preamble code, the E-NB  302  calculates a correlation between the preamble code and candidate preamble codes available for uplink timing synchronization during a sliding window having a certain constant interval, to find the timing and preamble code indicating the highest correlation. In addition, the E-NB  302  calculates a difference between the then-reception timing and the timing in which it should actually have received the preamble code, and provides in step  322  the UE  301  with an IDentifier (ID) of the found preamble code and information on the uplink timing difference using a response message (UL SYNC RES). In step  331 , the UE  301  changes and updates the uplink transmission timing using the information on the uplink timing difference, received through the response message. From this time one, uplink signaling and data transmission is achieved using the changed and updated uplink timing. 
     Steps  341 ,  342  and  343  indicate a process of re-performing the uplink timing synchronization procedure in steps  311 ,  321  and  322  to recheck the changed and updated timing, and can be omitted. 
     The uplink timing synchronization procedure shown in  FIG. 3  should be periodically performed because the UE in the mobile communication system continuously moves, and thus the distance difference between the UE and the E-NB may change over time. When the periodic uplink timing synchronization procedure is performed, the UE periodically generates a preamble code used for the uplink timing synchronization procedure and transmits the preamble code to the E-NB over the uplink, and the E-NB should find an uplink timing difference by receiving and decrypting the periodic uplink preamble code, and provide the uplink timing difference information to the UE over the downlink. Therefore, overhead of the uplink signaling/downlink signaling occurs, causing inefficient use of radio resources. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method of maintaining uplink timing synchronization without uplink transmission of a preamble from a UE or without transmission of uplink timing difference information from an E-NB to solve the inefficient use problem of radio resources, occurring due to periodic transmission of uplink signaling and downlink signaling during maintenance of the uplink timing synchronization, and a UE apparatus for the same. 
     According to the present invention, there is provided a method for maintaining uplink timing sync in a mobile communication system. The method includes transmitting to a Node B an uplink sync request message including a preamble code, receiving from the Node B an uplink sync response message in response to the request message, adjusting uplink timing according to uplink timing adjustment information included in the response message, and storing the adjusted uplink timing as reference uplink timing for uplink signaling or data transmission; after storing the reference uplink timing, periodically measuring downlink timing, and calculating a difference between the measured downlink timing and previously stored reference downlink timing, determining the uplink timing using the calculated difference of the downlink timing and the reference uplink timing, receiving a request message for readjustment of the uplink timing from a Node B that has detected discrepancy of the uplink timing, readjusting the uplink timing using uplink timing adjustment information included in the uplink timing readjustment request message, updating the reference uplink timing with the readjusted uplink timing, and storing the updated reference uplink timing, and when updating the reference uplink timing, updating the last measured downlink timing as the reference downlink timing, and storing the reference downlink timing. 
     According to the present invention, there is provided a user equipment apparatus for maintaining uplink timing sync. The user equipment apparatus includes a message transceiver for receiving a message including uplink timing adjustment information from a Node B, and transmitting uplink data and signaling to the Node B according to uplink timing, a message decrypter for decrypting a message received from the message transceiver to acquire the uplink timing adjustment information, a reference timing manager for setting reference uplink timing according to the acquired uplink timing adjustment information, and setting the reference downlink timing according to the downlink timing, an uplink timing adjuster for periodically adjusting the uplink timing using a difference between the downlink timing and the reference downlink timing, and the reference uplink timing, a timer for counting a period for which the uplink timing is adjusted, and a downlink measurer for measuring the downlink timing according to the period, and providing the measured downlink timing to the uplink timing adjuster and the reference timing manager. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, 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: 
         FIGS. 1A and 1B  illustrate conventional configurations of an OFDM mobile communication system to which reference will be made by the present invention; 
         FIG. 2  illustrates an uplink timing synchronization procedure in an OFDM mobile communication system; 
         FIG. 3  illustrates an example of an uplink timing synchronization procedure; 
         FIG. 4  illustrates a method of maintaining UL timing synchronization in an OFDM system according to the present invention; 
         FIG. 5  illustrates an operation of a UE according to the present invention; and 
         FIG. 6  illustrates a UE apparatus according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for the sake of clarity and conciseness. 
     In the present invention, when a UE acquires UpLink (UL) timing synchronization with an E-NB through an initial UL timing synchronization procedure, the UE sets DownLink (DL) timing during corresponding UL transmission before DL transmission including UL timing difference information from the E-NB, or DL timing during DL transmission including the UL timing difference information, as Reference DL timing, and sets and maintains the then-UL timing adjusted using the UL timing difference information as Reference UL timing. The UE periodically acquires DL timing through measurement on a DL channel, finds a DL timing difference by comparing it with the Reference DL timing, and finds the then-UL timing by applying the DL timing difference to the Reference UL timing. The period can be short, and can be configured by the E-NB. 
     When the UL timing has discrepancies due to abrupt unstableness of the DL channel, the discrepancy is detected by the E-NB and UL timing difference information can be signaled to the UE over the DL. Upon receipt of the UL timing difference information transmitted over the DL, the UE changes/updates Reference UL timing to the UL timing adjusted using the UL timing difference information, and changes/updates Reference DL timing to DL timing during corresponding UL transmission before DL transmission including the UL timing difference information, or to DL timing during DL transmission including the UL timing difference information. 
     To prevent the discrepancy of UL timing due to abrupt unstableness of the DL channel, the UE finds a difference between an average value of DL timing, calculated through measurement for the period, and the Reference DL timing, or finds a difference between the DL timing and the Reference DL timing through periodic measurement on the DL channel. When the difference between the average value of DL timing, calculated through measurement on the DL channel for the period, and the Reference DL timing has an abnormally large value, it can be disregarded. 
     The term ‘UL timing’ used throughout the specification, which is the timing in which the UE transmits signaling/data over the UL, can be indicated as a timing offset (in units of actual time such as μs, or in such units as slots, symbols, subframes or frames) with respect to a DL frame to which the UE has matched a DL sync channel, and the ‘DL timing’ indicates DL frame timing acquired through a DL sync channel. 
       FIG. 4  illustrates generally a method of finding UL timing according to the present invention, and more particularly a method of finding UL timing by applying a difference between DL timing and Reference DL timing to Reference UL timing. 
     Referring to  FIG. 4 , steps  411 ,  421  and  422  indicate execution of an initial UL timing synchronization procedure. That is, in step  411 , a UE  401  generates a preamble code to be used for a UL timing synchronization procedure. If the UE  401  is constructed such that multiple preamble codes can be used in the UL timing synchronization procedure, the UE  401  generates one of the multiple preamble codes. The ‘preamble code’ is a type of code sequence agreed upon between the UE  401  and an E-NB  402 , and the UE  401  transmits the preamble code over the uplink using radio resources allocated by the E-NB  402  in step  421  (UL SYNC REQ). Upon receipt of the preamble code, the E-NB  402  calculates a correlation between the received preamble code and candidate preamble codes available for UL timing synchronization during a sliding window having a constant interval, to find the timing and preamble code indicating the highest correlation. In addition, the E-NB  402  calculates a difference between the then-reception timing and the timing in which it should actually have received the preamble code, and provides in step  422  the UE  401  with an ID of the preamble code and information on the UL timing difference using a response message (UL SYNC RES). 
     In step  431 , the UE  401  adjusts UL timing using the UL timing difference information received through the response message, and sets it as Reference UL timing. In addition, the UE  401  sets DL timing in step  412  as Reference DL timing. 
     Here, an operation order of step  431  is subject to change. That is, if steps  411 ,  421  and  422  are re-performed in the manner of steps  341 ,  342  and  343  of  FIG. 3  to recheck UL timing adjusted according to the UL timing difference information received in step  422 , Reference DL timing is reset to the DL timing during the re-performed UL preamble code transmission, and Reference UL timing is reset to the UL timing adjusted according to the newly received UL timing difference information. 
     After completion of the UL timing adjustment in step  431 , the UE  401  performs in step  432  UL signaling/data transmission with the adjusted UL timing. 
     In  FIG. 4 , for the DL timing measurement of step  412  for Reference DL timing setting, although when the UL timing difference information was received over the DL in step  422 , DL timing for the DL at the time (step  421 ) that the UL transmission has occurred before the DL reception is measured herein by way of example, the present invention does not exclude any other possible time for which the DL timing for setting Reference DL timing is measured. For example, DL timing for the DL at the time (step  422 ) that UL timing difference information was received over the DL, can be set as Reference DL timing. 
     In steps  441  and  451 , the UE  401  measures DL timing at each time through measurement for a period T, calculates a difference between the measured DL timing and the Reference DL timing, and calculates/adjusts the UL timing by applying the DL timing difference to the Reference UL timing. That is, without signaling exchange with the E-NB  402 , the UE  401  finds DL timing through periodic measurement, finds a timing difference between the DL timing and Reference DL timing by comparing the DL timing with Reference DL timing, and then finds UL timing by applying the timing difference to the Reference UL timing. 
     When UL timing is discrepant due to abrupt unstableness of the DL channel, the E-NB  402  detects the discrepancy in step  471 , and transmits UL timing difference information to the UE  401  using a message in step  481 , thereby sending a request for readjustment of the UL timing to the UE  401 . Upon receipt of a UL timing readjustment instruction, the UE  401  resets the DL timing for the corresponding UL transmission time (step  461  in  FIG. 4 ) as Reference DL timing, and resets the UL timing adjusted using the UL timing difference information received through a message in step  481 , as Reference UL timing. 
     Although not shown in  FIG. 4 , a difference between an average value of DL timing measured for the period T and the Reference DL timing can be applied to the Reference UL timing in steps  441  and  451 . In addition, when the average value of DL timing measured for the period T is used for finding a difference from the Reference DL timing as described above, the abnormally great difference can be disregarded without application. Even when the DL timing measured in steps  411  and  451  of  FIG. 4 , other than the average value of DL timing measured for the period T, is used for finding the difference with the Reference DL timing, the abnormally great difference can be disregarded without application. In this case, the previously acquired UL timing can be used as it is. 
       FIG. 5  illustrates an operation of a UE to which an embodiment of the present invention is applied. 
     Referring to  FIG. 5 , in step  501 , the UE receives UL timing difference information (UL timing adjustment info or UL timing advance info) from an E-NB. In step  511 , the UE sets or updates DL timing during UL transmission as Reference DL timing before reception of the UL timing difference information of step  501 , and stores the DL timing. In step  512 , the UE adjusts the UL timing using the UL timing difference information received in step  501 . In step  513 , the UE sets or updates the UL timing adjusted in step  512  as Reference UL timing, and stores the Reference UL timing. In step  521 , the UE determines whether a period of adjusting UL timing has arrived. 
     If the UL timing adjustment period has arrived, the UE measures DL timing through measurement in step  531 , calculates a difference between the measured DL timing and the Reference DL timing in step  532 , and then adjusts UL timing by applying the calculated difference to the Reference UL timing in step  533 . However, if the adjustment period of the UL timing has not arrived, the UE maintains the last adjusted UL timing in step  541 . 
     Although not shown in  FIG. 5 , as described above, the UE does not exclude any other possible time for which it sets Reference DL timing in step  511 . For example, the UE can set, as Reference DL timing, the DL timing for which it has received UL timing difference information over the DL. 
       FIG. 6  illustrates a UE apparatus to which an embodiment of the present invention is applied. 
     Referring to  FIG. 6 , the UE includes a message transceiver  601 , a message decrypter  611 , a Reference UL/DL timing manager  621 , a DL measurer  631 , a timer  641  and a UL timing adjuster  651 . 
     The message transceiver  601  receives signaling including UL timing difference information from an E-NB. The message decrypter  611  decrypts a message received from the message transceiver  601  to detect UL timing difference information, and the UL timing difference information is delivered to the UL timing adjuster  651  via the Reference UL/DL timing manager  621 . The UL timing adjuster  651  adjusts UL timing using the UL timing difference information received via the Reference UL/DL timing manager  621 , and updates the adjusted UL timing as Reference UL timing. The updated Reference UL timing is used for later setting the timing for UL message transmission via the message transceiver  601 . 
     Upon receipt of the UL timing difference information via the message transceiver  601  and the message decrypter  611 , the Reference UL/DL timing manager  621  updates the DL timing measured by the DL measurer  631  as Reference DL timing. The DL timing measured by the DL measurer  631  is DL timing when the UE transmitted the corresponding UL message before it receives a message including UL timing difference information. The timer  641  manages a UL timing update period for matching UL timing synchronization, at which period the timer  641  reports arrival of the period to the DL measurer  631  and the UL timing adjuster  651 . The DL measurer  631  acquires the DL timing, and the UL timing adjuster  651  calculates a timing difference by comparing the acquired DL timing with the Reference DL timing stored in the Reference UL/DL timing manager  621 , and adjusts UL timing by applying the timing difference to the Reference UL timing stored in the Reference UL/DL timing manager  621 . 
     As is apparent from the foregoing description, according to the present invention, the UE stores UL timing and DL timing for the time at which it has acquired initial UL timing synchronization, as Reference UL timing and Reference DL timing, respectively, periodically finding DL timing, calculating a difference by comparing the DL timing with the Reference DL timing, and maintaining UL timing by applying the difference to the Reference UL timing. By doing so, the number of signaling transmissions between the UE and the E-NB is reduced, thereby reducing overhead of UL/DL signaling and enabling efficient use of radio resources. 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, 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.