Method and apparatus for managing multiple timing advance groups in mobile communication system supporting carrier aggregation

Abstract

A method and apparatus for managing multiple Timing Advance Groups (TAGs) operating with different timings are provided for use in a Long Term Evolution (LTE) system. The method for managing multiple TAGs at a base station of a wireless communication system supporting carrier aggregation according to an exemplary embodiment of the present invention includes categorizing a plurality of carriers into at least one TAG according to a predetermined rule, assigning a TAG index to each TAG, transmitting the TAG index to a terminal, generating a Timing Advance Command (TAC) for synchronization, and transmitting the TAC to the terminal in a random access process. The method and apparatus for managing TAGs according to exemplary embodiments of the present invention is capable of informing of the timing advance group to which each carrier belongs and managing multiple timing advance groups efficiently without signaling overhead.

Claims

1. A method by a terminal, the method comprising: receiving, from a base station, radio resource control (RRC) connection reconfiguration information for configuring a secondary cell (SCell), wherein the RRC connection reconfiguration information includes an identifier of a timing advance group (TAG) to which the SCell belongs if the SCell does not belong to a primary TAG (PTAG) to which a primary cell (PCell) belongs and the RRC connection reconfiguration information does not include an identifier of a TAG for the SCell if the SCell belongs to the PTAG; receiving, from the base station, a medium access control (MAC) control element (CE) for activating the SCell; monitoring a physical downlink control channel (PDCCH) for the SCell based on the received MAC CE; receiving, from the base station, a PDCCH order for initiating a random access procedure on the SCell based on the monitoring; performing the random access procedure on the SCell based on the PDCCH order; and receiving, from the base station, a MAC CE including a timing advance command (TAC) for a TAG including the SCell based on the random access procedure.

2. The method of claim 1, wherein the PDCCH order includes a random access resource and a random access preamble index.

3. The method of claim 1, wherein the random access procedure is a non-contention based random access procedure.

4. The method of claim 1, wherein the MAC CE including the TAC further includes an identifier of a TAG including the SCell, and wherein the identifier is 0 if the TAG is the PTAG and the identifier is not 0 if the TAG is not the PTAG.

5. A terminal comprising: a transceiver configured to transmit and receive a signal; and a controller configured to: control the transceiver to receive, from a base station, radio resource control (RRC) connection reconfiguration information for configuring a secondary cell (SCell), wherein the RRC connection reconfiguration information includes an identifier of a timing advance group (TAG) to which the SCell belongs if the SCell does not belong to a primary TAG (PTAG) to which a primary cell (PCell) belongs and the RRC connection reconfiguration information does not include an identifier of a TAG for the SCell if the SCell belongs to the PTAG, control the transceiver to receive, from the base station, a medium access control (MAC) control element (CE) for activating the SCell, monitor a physical downlink control channel (PDCCH) for the SCell based on the received MAC CE, control the transceiver to receive, from the base station, a physical downlink control channel (PDCCH) order for initiating a random access procedure on the SCell based on the monitoring, perform the random access procedure on the SCell based on the PDCCH order, and receive, from the base station, a MAC CE including a timing advance command (TAC) for a TAG including the SCell based on the random access procedure from the base station.

6. The terminal of claim 5, wherein the PDCCH order includes a random access resource and a random access preamble index.

7. The terminal of claim 5, wherein the random access procedure is a non-contention based random access procedure.

8. The terminal of claim 5, wherein the MAC CE including the TAC further includes an identifier of a TAG including the SCell, and wherein the identifier is 0 if the TAG is the PTAG and the identifier is not 0 if the TAG is not the PTAG.

9. A method by a base station, the method comprising: transmitting, to a terminal, radio resource control (RRC) connection reconfiguration information for configuring a secondary cell (SCell), wherein the RRC connection reconfiguration information includes an identifier of a timing advance group (TAG) to which the SCell belongs if the SCell does not belong to a primary TAG (PTAG) to which a primary cell (PCell) belongs and the RRC connection reconfiguration information does not include an identifier of a TAG for the SCell if the SCell belongs to the PTAG; transmitting, to the terminal, a medium access control (MAC) control element (CE) for activating the SCell; transmitting, to the terminal, a physical downlink control channel (PDCCH) order for initiating a random access procedure on the SCell; initiating the random access procedure on the SCell based on the PDCCH order; and transmitting, to the terminal, a MAC CE including a timing advance command (TAC) for a TAG including the SCell based on the random access procedure, wherein the PDCCH order is received at the terminal by monitoring a PDCCH for the SCell based on the received MAC CE by the terminal.

10. The method of claim 9, wherein the PDCCH order includes a random access resource and a random access preamble index.

11. The method of claim 9, wherein the random access procedure is a non-contention based random access procedure.

12. The method of claim 9, wherein the MAC CE including the TAC further includes an identifier of a TAG including the SCell, and wherein the identifier is 0 if the TAG is the PTAG and the identifier is not 0 if the TAG is not the PTAG.

13. A base station comprising: a transceiver configured to transmit and receive a signal; and a controller configured to: control the transceiver to transmit, to a terminal, radio resource control (RRC) connection reconfiguration information for configuring a secondary cell (SCell), wherein the RRC connection reconfiguration information includes an identifier of a timing advance group (TAG) to which the SCell belongs if the SCell does not belong to a primary TAG (PTAG) to which a primary cell (PCell) belongs and the RRC connection reconfiguration information does not include an identifier of a TAG for the SCell if the SCell belongs to the PTAG, control the transceiver to transmit, to the terminal, a medium access control (MAC) control element (CE) for activating the SCell, control the transceiver to transmit, to the terminal, a physical downlink control channel (PDCCH) order for initiating a random access procedure on the SCell, initiate the random access procedure on the SCell based on the PDCCH order, and control the transceiver to transmit, to the terminal, a MAC CE including a timing advance command (TAC) for a TAG including the SCell based on the random access procedure, wherein the PDCCH order is received at the terminal by monitoring a PDCCH for the SCell based on the received MAC CE by the terminal.

14. The base station of claim 13, wherein the PDCCH order includes a random access resource and a random access preamble index.

15. The base station of claim 13, wherein the random access procedure is a non-contention based random access procedure.

16. The base station of claim 13, wherein the MAC CE including the TAC further includes an identifier of a TAG including the SCell, and wherein the identifier is 0 if the TAG is the PTAG and the identifier is not 0 if the TAG is not the PTAG.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a diagram illustrating an architecture of a Long Term Evolution (LTE) system according to an exemplary embodiment of the present invention;

(3) FIG. 2 is a diagram illustrating a protocol stack of an LTE system according to an exemplary embodiment of the present invention;

(4) FIG. 3 is a diagram illustrating an exemplary situation of carrier aggregation in an LTE system according to an exemplary embodiment of the present invention;

(5) FIG. 4 is a diagram illustrating a principle of uplink timing synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) based 3rd Generation Partnership Project (3GPP) LTE system according to an exemplary embodiment of the present invention;

(6) FIG. 5 is a diagram illustrating an exemplary network environment having network entities operating on primary and secondary carriers at different locations in a system according to an exemplary embodiment of the present invention supporting carrier aggregation;

(7) FIG. 6 is a signaling diagram illustrating a timing synchronization between a User Equipment (UE) and an evolved Node B (eNB) according to an exemplary embodiment of the present invention;

(8) FIG. 7 is a diagram illustrating the formats of an extended Timing Advance Command (TAC) Medium Access Control (MAC) Control Element (CE) for use in a method according to an exemplary embodiment of the present invention;

(9) FIG. 8 is a flowchart illustrating an eNB procedure of a method according to an exemplary embodiment of the present invention

(10) FIG. 9 is a flowchart illustrating a UE procedure of a method according to an embodiment of the present invention;

(11) FIG. 10 is a block diagram illustrating a configuration of a UE according to an exemplary embodiment of the present invention; and

(12) FIG. 11 is a block diagram illustrating a configuration of an eNB according to an exemplary embodiment of the present invention.

(13) Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(14) The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.

(15) The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

(16) It is to be understood that the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a component surface includes reference to one or more of such surfaces.

(17) FIG. 1 is a diagram illustrating an architecture of a Long Term Evolution (LTE) system according to an exemplary embodiment of the present invention.

(18) Referring to FIG. 1, the radio access network of the mobile communication system includes evolved Node Bs (eNBs) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130. The User Equipment (UE) 135 connects to an external network via eNBs 105, 110, 115, and 120 and the S-GW 130.

(19) In FIG. 1, the eNBs 105, 110, 115, and 120 correspond to legacy node Bs of a Universal Mobile Communications System (UMTS). The eNBs 105, 110, 115, and 120 allow the UE to establish a radio link and are responsible for complicated functions as compared to a legacy node B. In an LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel. Thus, there is a need for an eNB which includes a unit for scheduling data based on state information such as UE buffer conditions, power headroom state, and channel state. Typically, one eNB controls a plurality of cells. In order to secure a data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. Also, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE. The S-GW 130 is an entity to provide data bearers so as to establish and release data bearers under the control of the MME 125. MME 125 is responsible for various control functions and is connected to a plurality of eNBs 105, 110, 115, and 120.

(20) FIG. 2 is a diagram illustrating a protocol stack of an LTE system according to an exemplary embodiment of the present invention.

(21) Referring to FIG. 2, the protocol stack of the LTE system includes Packet Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control (RLC) 210 and 235, Medium Access Control (MAC) 215 and 230, and Physical (PHY) 220 and 225. The PDCP 205 and 240 is responsible for IP header compression/decompression, and the RLC 210 and 235 is responsible for segmenting the PDCP Protocol Data Unit (PDU) into segments of appropriate size for Automatic Repeat Request (ARQ) operation. ARQ is a technique for determining whether the packet transmitted by the transmitter is received by the receiver successfully and for retransmitting the packets which were received erroneously. The MAC 215 and 230 is responsible for establishing connection to a plurality of RLC entities so as to multiplex RLC PDUs into MAC PDUs and demultiplex the MAC PDUs into RLC PDUs. The PHY 220 and 225 performs channel coding on the MAC PDU and modulates the MAC PDU into OFDM symbols to transmit over radio channel or performs demodulating and channel-decoding on the received OFDM symbols and delivers the decoded data to the higher layer. Also, the PHY layer uses Hybrid ARQ (HARQ) for additional error correction by transmitting 1 bit information indicating for positive or negative acknowledgement from the receiver to the transmitter. The 1 bit corresponding to a positive acknowledgement a negative acknowledgement indicator is referred to as HARQ ACK/NACK information. The downlink HARQ ACK/NACK corresponding to uplink transmission is transmitted through a Physical Hybrid-ARQ Indicator Channel (PHICH), and the uplink HARQ ACK/NACK is transmitted through a Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).

(22) FIG. 3 is a diagram illustrating an exemplary situation of carrier aggregation in an LTE system according to an exemplary embodiment of the present invention.

(23) Referring to FIG. 3, typically an eNB can use multiple carriers transmitted and receive in different frequency bands. For example, the eNB 305 can be configured to use the carrier 315 with center frequency f1 and the carrier 310 with center frequency f3. If carrier aggregation is not supported, the UE 330 has to transmit/receive data using one of the carriers 310 and 315. However, if carrier aggregation is supported, then the UE 330 having the carrier aggregation capability can transmit/receive data using both the carriers 310 and 315. The eNB 305 can increase the amount of the resource to be allocated to the UE having the carrier aggregation capability in adaptation to the channel condition of the UE so as to improve the data rate of the UE.

(24) By taking notice that a cell is configured with one downlink carrier and one uplink carrier in a conventional communication system, the carrier aggregation can be understood as if the UE communicates data via multiple cells. With the use of carrier aggregation, the maximum data rate increases in proportion to the number of aggregated carriers.

(25) In the following description, the phrase the UE receives data through a certain downlink carrier or transmits data through a certain uplink carrier corresponds to transmission or reception data through control and data channels provided in a cell corresponding to center frequencies and frequency bands of the downlink and uplink carriers. Although the description is directed to an LTE mobile communication system for explanation convenience, exemplary embodiments of the present invention can be applied to other types of wireless communication systems supporting carrier aggregation.

(26) FIG. 4 is a diagram illustrating a principle of uplink timing synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) based 3rd Generation Partnership Project (3GPP) LTE system according to an exemplary embodiment of the present invention.

(27) Referring to FIG. 4, the UE#1 is located in close proximity to the eNB while the UE#2 is located at a relatively further proximity from the eNB. T_pro1 indicates the first propagation delay time to the UE#1, and T_pro2 indicates the second propagation delay to the UE#2. As shown in FIG. 4, the UE#1 is located near the eNB as compared to the UE#2 and thus the UE#1 has a relatively short propagation delay (e.g., T_pro1 is 0.333 us, T_pro2 is 3.33 us).

(28) When the UE#1 and UE#2 power on or operate in idle mode within a cell of the eNB, the uplink timing of the UE#1, the uplink timing of the UE#2, and the uplink timings of other UEs detected by the eNB in the cell may fail in synchronization (e.g., the uplink timings of the various UEs in the cell may fail to be synchronized with each other).

(29) Reference number 401 denotes uplink OFDM symbol transmission timing of the UE#1, and reference number 403 denotes uplink OFDM symbol transmission timing of the UE#2. By taking notice of the uplink transmission propagation delays of the UE#1 and UE#2, the eNB may receive the uplink OFDM symbols at the timings as denoted by reference numbers 405, 407, and 409. The UE#1's uplink symbol transmitted at the timing 401 is received by the eNB at the timing 407 with propagation delay while the UE#2's uplink symbol transmitted at the timing 403 is received by the eNB at the timing 409 with propagation delay.

(30) In FIG. 4, because the timings 407 and 409 correspond to timings before synchronization between the uplink transmission timings of the UE#1 and UE#2, the uplink OFDM symbol reception and decoding start timing 405 of the eNB, the UE#1's uplink OFDM symbol reception timing 407, and the UE#2's uplink OFDM symbol reception timing 409 are different among each other. In other words, the timings 405, 407, and 409 appear offset because they timings are not synchronized with each other.

(31) In this case, the uplink symbols transmitted by the UE#1 and UE#2 have no orthogonality so as to interfere with each other and, as a consequence, the eNB is likely to fail decoding the uplink symbols transmitted, at the timing 401 and 403, by the UE#1 and UE#2 due to the interference and the mismatch between the uplink symbol reception timings 407 and 409.

(32) Uplink timing synchronization is a procedure for acquiring the eNB's uplink symbol reception timings with the UE#1 and UE#2 and, if the uplink timing synchronization procedure is completed, it is possible to acquire the synchronization among the eNB's uplink OFDM symbol reception and decoding start timing, UE#1's uplink OFDM symbol reception timing, and UE#2's uplink OFDM symbol reception timing as denoted by reference numbers 411, 413, and 415. In the uplink timing synchronization procedure, the eNB transmits Timing Advance (TA) information to the UEs to notify the UEs of the timing adjustment amount. For example, the eNB may transmit the TA information in the Timing Advance Command MAC Control Element (TAC MAC CE) or in the Random Access Response (RAR) message in response to the random access preamble transmitted by the UE for initial access.

(33) If the TA information is received, the UE starts a time alignment timer (timeAlignmentTimer or TAT). The TAT is the timer for verifying the validity of the TA. That is, the TA is valid before the expiry of the TAT, however, the validity of the TA is not guaranteed after the expiry of the TAT. If additional TA information is received afterward, the TAT restarts and, if the TAT expires, TA information received from the eNB after the expiry of the TAT is regarded as invalid so as to set the uplink communication with the eNB. By acquiring synchronization among the transmission timings as described above, it is possible to maintain the orthogonality between the uplink symbols of the UE#1 and UE#2 such that the eNB can decode the uplink symbols from the UE#1 and UE#2 at the timings 401 and 403 successfully.

(34) FIG. 5 is a diagram illustrating an exemplary network environment having network entities operating on primary and secondary carriers at different locations in a system according to an exemplary embodiment of the present invention supporting carrier aggregation.

(35) Referring to FIG. 5, the Remote Radio Heads (RRHs) 503 operating on frequency band F2 507 are deployed around the macro eNB 501 using frequency band F1 505. If the UE is connected to both the macro eNB and the RRH and is located near the RRH and if the UE transmits signal via the RRH, the signal can reach the RRH at an appropriate timing even when there is a little delay due to the short distance. However, the signal transmitted to the macro eNB fails to reach the macro eNB at an appropriate timing due to the long distance. In order to overcome this problem, the UE may operate with aggregated carriers to synchronize multiple uplink transmission timings.

(36) For this purpose, exemplary embodiments of the present invention propose a method in which the eNB categorizes the carriers having similar uplink timings into a group. This group is referred to as Timing Advance Group (TAG). Further, there is a need for a method for managing the TAG groups efficiently.

(37) Exemplary embodiments the present invention provide a method for informing the UE of a TAG to which each UE belongs and for controlling transmission of each TAG's TAC MAC CE to the UE efficiently for timing synchronization.

(38) In an exemplary embodiment of the present invention in which one PCell (or first cell) and three SCells 1, 2 and 3 (or second cells) exist, if the PCell and the SCell 3 have different uplink timings and if the SCell 1 and SCell 2 have the uplink timings similar to each other but different than those of the PCell and SCell 3, then the PCell, the SCell 1 and SCell 2, and the SCell 3 can be categorized into TAG1, TAG2, and TAG3 respectively. In other words, the PCell may be categorized into TAG1, the SCell 1 and SCell 2 may be categorized into TAG2, and SCell 3 may be categorized into SCell 3. The PCell is the cell operating on the primary frequency through which the initial connection establishment and connection re-establishment procedures and handover procedure can be performed. The TAG including the PCell is referred to as PTAG and the TAG including only SCells is referred to as SCell. That is, the eNB transmits the TA information in TAC MAC CE or RAR through TAG 2 for adjusting uplink timing, and the UE adjusts uplink timings for both the SCell 1 and SCell 2 according to the TAC MAC CE. Also, the uplink timings for the TAG1 and TAG3 are adjusted according to the information included in the TAC MAC CE for the corresponding TAG.

(39) In order to adopt the concept of TAG, the eNB must inform the UE as to which carriers belong to the same TAG. The simplest approach for informing the UE of such information is to transmit the index of the TAG, to which the secondary carrier belong, in the SCell configuration provided to the UE. In case of the PCell, the highest TAG index value is allocated without explicit indication. That is, if the TAG index starts from 0, the TAG index of the PTAG including the primary carrier is 0. According to exemplary embodiments of the present invention, this method must add the TAG index to the configuration information of each secondary carrier, thereby resulting in increase of signaling overhead. According to other exemplary embodiments of the present invention, another approach is to provide the TAG index in the configuration information of only one secondary subcarrier among the subcarriers belonging to the same TAG. Although the actual TAG indices of other secondary carriers, the TAG to which the secondary cell belongs is indicated implicitly. The primary carrier (PCell) is allocated the highest TAG index without extra indication. That is, if the TAG index starts from 0, the TAG index of the PTAG to which the primary carrier belongs is 0. Although actual TAG indices are provided for the secondary carriers, it is possible to indicate the TAG to which each secondary carrier belongs implicitly. For instance, in the previous example, the SCell 1 and SCell 2 belong to the same TAG. However, the eNB provides the TAG index only in the configuration information of the SCell 1. If the configuration of the SCell 2 received from the eNB does not include a TAG index, then the TAG of the SCell 2 follows the TAG of the SCell 1 configured before the SCell. With this approach, it is possible to provide the information on the TAG to which each secondary carrier belongs without including the TAG index in the configurations of each respective secondary carrier.

(40) The timing synchronization is performed using the TAC MAC CE on a per TAG basis. If the TAC MAC CE is transmitted per TAG, the number of TAC MAC CEs increases in proportional to the number of the TAG so as to be transmitted to the UE as distributed across MAC PDUs. The timing synchronization performed using the TAC MAC CE on a per TAG basis leaves room for improvement in terms of signaling optimization. If it is possible to transmit the TAC MAC CEs of multiple TAGs in fewer extended TAC MAC CEs, this can improve the resource efficiency. For example, the resource efficiency may be improved if the TAC MAC CEs of multiple TAGs are transmitted in one extended TAC MAC CE. Such an extended TAC MAC CE can be configured in various formats, and exemplary embodiments of the present invention propose several efficient formats of the extended TAC MAC CE.

First Exemplary Embodiment

(41) FIG. 6 is a signaling diagram illustrating a timing synchronization between an UE and an eNB according to an exemplary embodiment of the present invention.

(42) Referring to FIG. 6, the eNB 605 decides to add three secondary cells (i.e. SCell 1, SCell 2, and SCell 3) to transmit data to the UE 600 at step 610. The SCell indices are set to 1, 2, and 3. The uplink timings of the SCell 1 and SCell 2 are identical with each other but differ from the uplink timing of the SCell 3. Also, the uplink timing of the PCell differs from all of the SCells. The eNB sets the TAG index of the PCell to 0. The SCell 1 and SCell 2 are configured with the TAG index 1, and the SCell 3 is configured with tag index 2. The eNB generates per-SCell configuration information. At this time, only the SCell 1 and SCell 3 are configured with respective TAG indices while the SCell 2 is not configured with any TAG index. The UE regards the TAG of the SCell 2 as being identical to the SCell 1. In order to follow this rule, it is necessary to assign the contiguous indices to the SCells having the same uplink timing. Among the contiguous indices, the TAG index is included with the configuration information on the SCell having the highest index among contiguous indices of the SCells.

(43) The eNB 605 transmits to the UE 600 a RRC Connection Reconfiguration message including the configuration information of the respective SCells and the TAG index at step 615. Table 1 provides an example of the SCell configuration information included in the RRC Connection Reconfiguration message. For example, the eNB 605 provides the UE 600 with a SCell list to be added or released in the RRC Connection Reconfiguration message. The RRC Connection Reconfiguration message also includes the SCell configuration information per SCell to be added. As another example, the configuration information may also include the TAG index as described above.

(44) TABLE-US-00001 TABLE 1 RRCConnectionReconfiguration-v1020-IEs :: = SEQUENCE { sCellToReleaseList-r10ScellToReleaseList-r10OPTIONAL,-- Need ON sCellToAddModList-r10ScellToAddModList-r10OPTIONAL,-- Need On nonCriticalExtensionSEQUENCE { }OPTIONAL,-- Need OP

(45) The UE 600 transmits an RRC Connection Reconfiguration Complete message to the eNB 605 to notify the eNB 605 of the successful receipt of the RRC Connection Reconfiguration message at step 620.

(46) The UE 600 applies to each SCell the corresponding SCell configuration information received from the eNB 605 at step 625. The configuration may include the SCell configuration information with TAG index or not. Because the TAG index is included in the configuration informations of the PCell and SCell 3 but not the SCell 2, the UE determines that the PCell and the SCell 3 belong to different TAGs and determines that the SCell 2 belongs to the same TAG as the SCell 1. Consequently, the SCell 1 and SCell 2 belong to the TAG which is different from the TAG to which the SCell 3 belongs. However, the UE regards the SCell 1 and SCell 2 as belonging to the same TAG.

(47) The eNB 605 transmits SCell activation MAC CE to the UE 600 to activate the configured SCells at step 630. In an LTE radio communication system adopting the carrier aggregation technique, the data transmission does not occur with the configuration information simultaneously. For example, in order to start actual data transmission, it is necessary to first activate the configured SCells. The SCell activation is triggered by the MAC CE and, once the SCells are activated, the UE 600 starts operations necessary for PDCCH monitoring in the SCells, SRS transmission, and CQI/RMI/RI report for data transmission.

(48) Upon receipt of the Activation MAC CE, the UE 600 activates the corresponding SCell at step 635. Upon the activation of the SCell, a sCellDeactivationTimer starts.

(49) The eNB 605 performs timing synchronization for TAG 1 to transmit data. For this purpose, the UE transmits a PDCCH order to the UE 600 to trigger random access procedure at step 640. The PDCCH order includes the preamble to be used by the UE 600 and the random access resource information in order for the UE 600 to perform random access procedure without contention.

(50) Upon receipt of the PDCCH order from the eNB 605, the UE 600 starts random access procedure at step 645. In the random access procedure, the eNB 605 provides the UE 600 with the Timing Advance Command (TAC) such that the UE 600 synchronizes the uplink timing based on the corresponding information.

(51) The UE 600 acquires timing synchronization of the SCell 1 and SCell 2 belonging to TAG 1 through the random access procedure at step 650.

(52) Next, the data transfer takes place via the SCell 1 and SCell 2 at step 655 with the uplink timing acquired at steps 645 and 650.

(53) Afterward, the eNB 605 performs timing synchronization for TAG 2 to transmit data at step 660.

(54) Upon receipt of the PDCCH order for TAG 2 at step 660, the UE 600 performs random access procedure to acquire time synchronization at step 665.

(55) The UE 600 acquires time synchronization for the SCell 3 of the TAG 2 at step 670.

(56) Once the time synchronization is acquired, the uplink data transfer may take place through the SCell 3 as well as the SCell 1 and the SCell 2 at step 675. At this time, the uplink data transfer follows the uplink timing acquired at steps 660 and 665.

(57) Afterward, the eNB 680 detects the necessity of time synchronization again at step 680. As time progresses since the initial synchronization procedure, the time synchronization becomes unreliable. As an example, the necessity of time synchronization may be determined based on a specific timer. This timer is referred to as Time Alignment Timer (TAT). The TAT starts or restarts with the acquisition of the time synchronization. If the timer expires, the system determines that the time synchronization has been lost. Accordingly, the eNB 604 then transmits the TAC MAC CE to the UE 600 before the expiry of the timer so as to acquire time synchronization. The TAC MAC CE includes Timing Advance Command (TAC), and the UE 600 shifts the timing as much as the time duration indicated by the TAC.

(58) Finally the eNB 605 transmits an extended TAC MAC CE to the UE 600 at step 685. As described above, because it is more efficient to transmit the TAC MAC CEs for multiple TAGs in one extended TAC MAC CE, exemplary embodiments of the present invention propose transmission of an extended TAC MAC CE. The extended TAC MAC CE can be implemented in various formats, and exemplary embodiments of the present invention propose several efficient extended TAC MAC CE formats.

(59) FIG. 7 is a diagram illustrating the formats of an extended TAC MAC CE for use in a method according to an exemplary embodiment of the present invention.

(60) Referring to FIG. 7, Part (a) of FIG. 7 shows a TAC MAC CE having a fixed size. The size of the TAC MAC CE is determined to accommodate the TACs of all TAGs. In order to indicate the TAG for which the TAC is transmitted, a TAG index 700 is inserted before the TAC 705. If the number of allowed TAGs is 4, two bits are required for the TAG index field to discriminate among all of the TAGs. In order to carry the total 4 TACs, the extended TAC MAC CE has a length of 4 bytes.

(61) Part (b) of FIG. 7 shows a TAC MAC CE having variable size. If the TAC MAC CE has a variable size, the UE must be informed that the MAC CE has a variable length. The number of TAGs and information on the corresponding TAGs to be carried in the TAC MAC CE can be configured through a Radio Resource Control (RRC) message. For example, the TAC MAC CE can be included in the RRC Connection Reconfiguration message for adding/modifying/releasing SCells. The number of TAGs can be indicated in a MAC sub-header corresponding to the TAC MAC CE. Because the time synchronization operation is not always performed for all of the TAGs, it is possible to transmit to the UE only the TACs for the TAGs requiring time synchronization.

(62) Part (c) of FIG. 7 shows a TAC MAC CE with information on the number of TAGs. This TAC MAC CE formation includes a size information field/number of included TAGs and reserved bits 710 for match by unit of byte according to the sizes of the TAG index fields influenced by the maximum allowed number of TAGs.

(63) Part (4) of FIG. 7 shows a TAC MAC CE with extension bits for indicating the size of TAC MAC CEs. If a TAC for another TAG follows another TAC, this is indicated by an extension (E) bit 715. For example, if the E bit is set to 0, then such a setting may indicate that no more TAC information follows; and otherwise, if the E bit is set to 1, then such a setting may indicate that the TAC for another TAG follows.

(64) FIG. 8 is a flowchart illustrating an eNB procedure of a method according to an exemplary embodiment of the present invention.

(65) The eNB decides to add SCells for a UE at step 800. Next, the eNB determines whether the SCell 1 belongs to the same TAG as the PCell at step 805. In other words, at step 805, the eNB determines whether the SCell 1 has the same uplink timing as the PCell. If it is determined that the SCell having the first SCell index belongs to the same TAG as the PCell at step 805, then the eNB adds no TAG index to the configuration information on the SCell at step 810. If it is determined that the SCell having the first SCell index does not belong to the same tag as the PCell at step 805, the eNB adds the next TAG index to the configuration information on the SCell 1.

(66) Next, the eNB performs next process from the SCell having the second SCell index at step 820. The eNB determines whether the i.sup.th SCell belongs to the same TAG as the (i?1).sup.th SCell at step 825. If it is determined that the i.sup.th SCell belongs to the same TAG as (i?1).sup.th SCell at step 825, then the eNB skips adding a TAG index to the configuration information on the i.sup.th SCell at step 830. Otherwise, if it is determined that the i.sup.th SCell does not belong to the same TAG as (i+1).sup.th SCell at step 825, then the eNB adds a new TAG index to the configuration information on the i.sup.th SCell at step 835.

(67) The eNB determines whether the index assignment has been done for all of the SCells available for the UE at step 840 and, if so, the procedure goes to the next step. Otherwise, if there is any SCell remaining for which index assignment must be completed, the procedure returns step 820. Afterward, the eNB includes the TAG index information into each SCell configuration information at step 845. Next, the eNB includes the SCell configuration information into the RRC Connection Reconfiguration message at step 850 and transmits the RRC Connection Reconfiguration message to the UE at step 855.

(68) FIG. 9 is a flowchart illustrating a UE procedure of a method according to an exemplary embodiment of the present invention.

(69) The UE receives the RRC Connection Reconfiguration message transmitted by an eNB at step 900.

(70) Next, the UE determines whether the RRC Connection Reconfiguration message includes SCell configuration information at step 905. If no SCell configuration information is included, then the UE terminates the procedure. If the SCell configuration is included, then the UE sets the TAG index of the PCell to the highest value at step 910. That is, if the TAG index value starts from 0, the TAG index of the PCell is set to 0.

(71) Afterward, the UE determines whether the configuration information on the SCell having the first SCell index includes a TAG index at step 915. If the TAG index is included at step 915, then the UE regards that the SCell as belonging to a new TAG different from the TAG to which the PCell belongs at step 920. Otherwise, if the TAG index is included at step 915, then the UE regards that the SCell as belonging to the same TAG as the PCell at step 925.

(72) Next, the UE performs the next step operation for the SCell having the second SCell index at step 930 and then determines whether the configuration information of the i.sup.th SCell includes a TAG index at step 935. If the TAG index is included, then the UE regards that the i.sup.th SCell as belonging to a new TAG different from that of the (i?1).sup.th SCell at step 940 such that the corresponding TAG is indicated by the included TAG index. Otherwise, if no TAG index is included, then the UE regards that the i.sup.th SCell as belonging to the same TAG as the (i?1).sup.th SCell at step 945.

(73) Finally, the UE determines whether this operation has been performed for all of the SCells allowed to the UE at step 950 and, if so, the UE terminates the procedure. Otherwise, the procedure goes to step 930.

Second Exemplary Embodiment

(74) As described above, when transmitting to the UE the SCell configuration of each secondary carrier, the TAG indices of the secondary carriers are included in the corresponding configurations of all the secondary carriers. In contrast to other exemplary embodiments of the present invention, some exemplary embodiments may be such that there is no SCell configuration having no TAG index. According to such exemplary embodiments of the present invention, however, the TAG index of the PCell is not informed explicitly but rather set to the highest value. That is, if the TAG index starts from 0, then the TAG index of the PTAG to which the PCell (i.e. the primary carrier) belongs is set to 0. FIG. 6 illustrates examples of the contrasting exemplary embodiments of the present invention. For example, at steps 615 and 625 of FIG. 6, the TAG index denoted with 2.sup.nd embodiment corresponds to an exemplary embodiment of the present invention in which the TAG index of the PCell is not informed explicitly.

(75) FIG. 10 is a block diagram illustrating a configuration of a UE according to an exemplary embodiment of the present invention.

(76) Referring to FIG. 10, the UE transmits/receives data generated by a higher layer device 1005 and control messages generated by a control message processor 1007. When transmitting control signal and/or data to the eNB, the UE multiplexes the control signal and/or data by means of the multiplexer/demultiplexer 1003 under the control of the controller 1009. When receiving control signal and/or data from the eNB, the UE receives the physical signal by means of the transceiver 1001, demultiplexes the received signal by means of the multiplexer/demultiplexer 1003, and delivers the demultiplexed signal to the corresponding higher layer device 1005 or control message processor 1007.

(77) According to exemplary embodiments of the present invention, if a TAC MAC CE is received, the control message processor delivers the TAC MAC CE to the carrier aggregation processor 1011. The carrier aggregation processor 1011 commands the controller 1009 to apply the TAC for the corresponding TAG.

(78) FIG. 11 is a block diagram illustrating a configuration of an eNB according to an exemplary embodiment of the present invention.

(79) Referring to FIG. 11, the eNB transmits/receives data generated by a higher layer device 1105 and control messages generated by a control message generator 1107. In transmission mode, the data is multiplexed by the multiplexer/demultiplexer 1103 and then transmitted through the transceiver 1101 under the control of the controller 1109. In reception mode, the physical signal is received by the transceiver 1101, demultiplexed by the multiplexer/demultiplexer 1103, and then delivered to the higher layer device 1105 or the control message processor 1107 according to the message information under the control of the controller 1109.

(80) According to exemplary embodiments of the present invention, the higher layer device 1105 determines whether to allow the UE to use the SCells. If the UE is allowed to use the SCells, the higher layer device 1105 analyzes the TAG to which each SCell belongs and sends the analysis result to the control message processor 1107. The control message processor 1107 includes the TAG index in SCell configuration information for a corresponding SCell, encapsulates all configuration information in the RRC Connection Reconfiguration message and sends the RRC Connection Reconfiguration message to the multiplexer/demultiplexer 1103. If it is necessary to perform the time synchronization for multiple TAGs, the carrier aggregation processor 1111 sends the TAC values of the respective TAGs to the control message processor 1107. The control message processor 1107 includes the TAC values of all the TAGs in the extended TAC MAC CE and sends the extended TAC MAC CE to the multiplexer/demultiplexer 1103.

(81) As described above, the timing advance group management method and apparatus according to exemplary embodiments of the present invention is capable of informing of the timing advance group to which each carrier belongs and managing multiple timing advance groups efficiently without signaling overhead.

(82) While the invention has been shown and described with reference to certain exemplary embodiments 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 and their equivalents.