Patent Description:
Mobile communication systems were developed to provide mobile users with communication services. With the rapid advance of technologies, the mobile communication systems have evolved to the level capable of providing high speed data communication service beyond the early voice-oriented services.

Recently, standardization for a Long Term Evolution (LTE) system, as one of the next-generation mobile communication systems, is underway in the <NUM>rd Generation Partnership Project (3GPP). LTE is a technology for realizing high-speed packet-based communications with the data rate of up to <NUM> Mbps, which is higher than the currently available data rate, and its standardization is almost complete.

In line with the completion of the LTE standardization, an LTE-Advanced (LTE-A) system is now under discussion, which improves a transfer rate by combining the LTE communication system with several new technologies. One of such technologies is Carrier Aggregation. The Carrier Aggregation is a technology allowing a terminal to use multiple downlink carriers and multiple uplink carriers unlike the conventional technology of using one downlink carrier and one uplink carrier for data communication.

Currently, the LTE-A is featured with the intra-eNB carrier aggregation only. This restricts applicability of the carrier aggregation function so as to a problem of failing aggregation of macro and pico cells in a scenario where a plurality of pico cells and a macro cell operate in an overlapped manner.

3GPP TSG-RAN WG2 Meeting #<NUM> Agenda Item <NUM>. <NUM> (R2-<NUM>) discusses how to handle RACH failure on SCell. It proposes that the UE shall not trigger reestablishment for SCell RACH failure; that the RACH failure handling should be handled by the eNB; that the UE shall not report "random Access problem" to upper layers for SCell RACH failure case; and that the sync status of SCells in sTAG shall not be changed by the UE itself upon SCell RACH failure.

3GPP TSG-RAN WG2 Meeting #<NUM> Agenda Item <NUM>. <NUM> (R2-<NUM>) discusses potential solutions for SCell RA failure and analyses its impact on the sync state of the corresponding sTAG. It proposes that when UE detects SCell RA failure, UE should not indicate Random Access problem to upper layers; that when UE detects SCell RA failure, UE should not report SCell RA failure to eNB; and that when UE detects SCell RA failure, the related sTAG should be in out-of-sync state.

The present invention has been conceived to solve the above problem and aims to provide an inter-eNB carrier aggregation method and apparatus.

The protection conferred is determined from the claims.

The data transmission method and apparatus of the present invention is advantageous in that a terminal is capable of increasing the probability of fast data transmission/reception through carrier aggregation.

The advantages of the present invention are not limited to the aforesaid, and other advantages not described herein be clearly understood by those skilled in the art from the descriptions below.

Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. Prior to the description of the present invention, the LTE system and carrier aggregation are explained briefly.

<FIG> is a diagram illustrating the architecture of an LTE system according to an embodiment of the present invention.

Referring to <FIG>, the radio access network of the mobile communication system includes evolved Node Bs (eNBs) <NUM>, <NUM>, <NUM>, and <NUM>, a Mobility Management Entity (MME) <NUM>, and a Serving-Gateway (S-GW) <NUM>. The User Equipment (hereinafter, referred to as UE) <NUM> connects to an external network via eNBs <NUM>, <NUM>, <NUM>, and <NUM> and the S-GW <NUM>.

In <FIG>, the eNBs <NUM>, <NUM>, <NUM>, and <NUM> correspond to the legacy node Bs of the UMTS system. The eNBs allow the UE <NUM> to establish a radio channel and are responsible for complicated functions as compared to the legacy node B. In the LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel and thus there is a need of a device for scheduling data based on the state information such as buffer states, power headroom states, and channel states of the UEs, and the eNBs <NUM>, <NUM>, <NUM>, and <NUM> are responsible for scheduling. Typically, one eNB controls a plurality of cells. In order to secure the data rate of up to 100Mbps, 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 <NUM> is an entity to provide data bearers so as to establish and release data bearers under the control of the MME <NUM>. The MME <NUM> is responsible for mobility management of UEs and various control functions and may be connected to a plurality of eNBs.

<FIG> is a diagram illustrating a protocol stack of the LTE system according to an embodiment of the present invention.

Referring to <FIG>, the protocol stack of the LTE system includes Packet Data Convergence Protocol (PDCP) <NUM> and <NUM>, Radio Link Control (RLC) <NUM> and <NUM>, Medium Access Control (MAC) <NUM> and <NUM>, and Physical (PHY) <NUM> and <NUM>. The PDCP <NUM> and <NUM> is responsible for IP header compression/decompression, and the RLC <NUM> and <NUM> is responsible for segmenting the PDCP Protocol Data Unit (PDU) into segments in appropriate size for Automatic Repeat Request (ARQ) operation. The MAC <NUM> and <NUM> is responsible for establishing connection to a plurality of RLC entities so as to multiplex the RLC PDUs into MAC PDUs and demultiplex the MAC PDUs into RLC PDUs. The PHY <NUM> and <NUM> 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.

<FIG> is a diagram illustrating the concept of intra-eNB carrier aggregation.

Referring to <FIG>, an eNB transmits and receives signals through multiple carriers across a plurality of frequency bands. For example, the eNB <NUM> can be configured to use the carrier <NUM> with center frequency f1 and the carrier <NUM> with center frequency f3. If carrier aggregation is not supported, the UE <NUM> has to transmit/receive data using one of the carriers <NUM> and <NUM>. However, the UE <NUM> having the carrier aggregation capability can transmit/receive data using both the carriers <NUM> and <NUM>. The eNB can increase the resource amount to be allocated to the UE with the carrier aggregation capability in adaptation to the channel condition of the UE so as to improve the data rate of the UE <NUM>. The technique of aggregating the downlink and uplink carriers respectively for transmission and reception at one eNB is referred to as intra-eNB carrier aggregation. In any case, however, there may be a need of aggregating the downlink/uplink carriers of different eNBs.

<FIG> is a diagram illustrating the inter-eNB carrier aggregation according to an embodiment of the present invention.

In the exemplary case of <FIG>, the first eNB <NUM> uses the carrier <NUM> with center frequency f1 for transmission/reception, and the second eNB <NUM> uses the carrier <NUM> with center frequency f2 for transmission/reception. If the downlink carrier <NUM> with the center frequency f1 and the downlink carrier <NUM> with the center frequency f2 are aggregated, this means that carriers transmitted by more than one eNB are aggregated for one UE. This is referred to as inter-eNB Carrier Aggregation (CA) in the present invention.

The terms used frequently in the present invention are described hereinafter.

Assuming that a cell is configured to operate on one downlink carrier and one uplink carrier in the conventional concept, the carrier aggregation can be understood as if the UE communicates data via multiple cells. The carrier aggregation increases the peak data rate in proportion to the number of aggregated carriers.

In the following description, if a UE receives data through a certain downlink carrier or transmits data through a certain uplink carrier, this means to receive or transmit data through control and data channels provided in cells corresponding to center frequencies and frequency bands characterizing the carriers. In the present Invention, carrier aggregation may be expressed as configuring a plurality of serving cells with the use of terms such as primary cell (PCell), secondary cell (SCell), and activated serving cell. These terms are used as they are in the LTE mobile communication system and specified in TS36. <NUM> and <NPL>).

In the present invention, the serving cells controlled by the same eNB are defined as a set of serving cells. The set may is classified into one of a primary set and a non-primary set. The primary set is the set of serving cells controlled by the eNB controlling the PCell (primary eNB), and the non-primary set is the set of serving cells controlled by the eNB not controlling the PCell (non-primary eNB). The eNB may notifies the UE whether a serving cell belongs to the primary set or non-primary set in the process of configuring the corresponding serving cell. One UE can be configured with one primary set and one or more non-primary set.

In the following description, the terms 'primary set' and `non-primary set' may be substituted by other terms to help understanding. For example, the terms `primary set,' 'secondary set,' 'primary carrier group, , and 'secondary carrier group' may be used. Even in such a case, however, it should be notice that although the terms are different but used in the same meaning.

<FIG> is a signal flow diagram illustrating the operations of the UE and the eNB for configuring a SCell belonging to the primary set according to an embodiment of the present invention.

Referring to <FIG>, in the mobile communication system made up of the UE <NUM>, the first eNB <NUM>, and the second eNB <NUM>; the first, second, and third cells are controlled by the first eNB <NUM>, and the fourth and fifth cells are control by the second eNB <NUM>. Suppose that the PCell of the UE is the first cell and the first eNB <NUM> configures the second cell as an additional SCell to the UE <NUM>. In the following description, the eNB <NUM> controlling the PCell, i.e. the primary set, is referred to as serving eNB. The eNB <NUM> which is not the serving eNB <NUM> and controls the serving cell of the UE is referred to as drift eNB. That is, the eNB <NUM> controlling the serving cells of the primary set is the serving eNB <NUM>, and the eNB <NUM> controlling the serving cells of the non-primary set is the drift eNB <NUM>. The serving eNB <NUM> and the drift eNB <NUM> may be referred to as the primary eNB <NUM> and non-primary eNB <NUM>, respectively.

The serving eNB <NUM> sends the UE a control message called RRC Connection Reconfiguration including the information on the SCell to be added newly to the UE at step <NUM>. The SCells to be added newly are managed by the serving eNB <NUM> directly and informations thereon, as shown in table <NUM>, are included in the control message.

The Timing Advance Group (TAG) is a set of the serving cells sharing the same uplink transmission timing. A TAG is classified into one of Primary TAG (P-TAG) and Secondary TAG (S-TAG). The P-TAG includes the PCell, and S-TAG includes SCells without PCell). If a certain serving cell belongs to a certain TAG, this means that the uplink transmission timing of the serving cell is identical with those of the other serving cells belonging to the TAG and whether the uplink synchronization is acquired is determined by means of the Timing Advance (TA) timer of the TAG. The uplink transmission timing of a certain TAG is set through a random access process in a serving cell belonging to the TAG and maintained with the receipt of TA command. The UE starts or restart the TA timer of the corresponding TAG whenever the TA command for the corresponding TAG is received. If the TA timer expires, the UE determines that the uplink transmission synchronization of the corresponding TAG has broken and thus suspends uplink transmission until the next random access occurs.

The UE <NUM> sends a response message in reply to the control message at step <NUM>. The UE <NUM> establishes forward/downlink synchronization with the second cell, i.e. serving cell <NUM>, at step <NUM>. The forward/downlink is of transmitting from the eNB to the UE, and the reverse/downlink is of transmitting from the UE to the eNB. In the present invention, the terms are used interchangeably. If the downlink synchronization is established in a certain cell, this means that the synchronization channel of the cell is acquired so as to check the downlink frame boundary.

The serving eNB <NUM> may send the UE <NUM> a command to activate the SCell <NUM> at a certain time when determined that the UE has completed the configuration of the SCell <NUM> at step <NUM>. The SCell <NUM> activation command may be Activate/Deactivate MAC Control Element (A/D MAC CE) as a MAC layer control command. The control command is structured in the form of a bitmap of which the first bit corresponds to the SCell <NUM>, the second bit to SCell <NUM>, and the nth bit to SCell n. The bitmap may be the size of <NUM> byte. In this case, <NUM> indices, i.e. from <NUM> to <NUM>, are used in such a way of mapping the second Least Significant Bit (LSB) to the SCell <NUM>, the third LSB to SCell <NUM>, and the last LSB or the Most Significant Bit (MSB) to SCell <NUM>, without use of the first LSB.

The UE <NUM> starts monitoring the physical control channel (carrying Physical Downlink Control Channel (PDCCH) and uplink/downlink transmission resource allocation information) of the SCell after the elapse of a predetermined period from the receipt of the SCell <NUM> activation command at step <NUM>. If the SCell has been acquired synchronization and belonged to a TAG already, the downlink/uplink transmission starts since then. That is, if the downlink transmission resource allocation information is received on the PDCCH, the UE receives downlink data but ignores the uplink transmission resource information although it has bene received. If the SCell belongs to a non-synchronized TAG, the UE waits for the receipt of `random access command' on PDCCH in a SCell belonging to the TAG. The random access command is a value of a predetermined field of the uplink transmission resource allocation information to instruct the UE <NUM> to transmit a preamble in a serving cell. The Carrier Indicator Field of the random access command may carry the identifier of the serving cell for preamble transmission. The random access command instructing transmission of random access preamble is received from the serving cell <NUM> at step <NUM>. As shown in <FIG>, the CIF may indicate the serving cell <NUM> for preamble transmission.

The UE <NUM> monitors PDCCH of the PCell to receive Random Access Response (RAR) in reply to the preamble after transmitting the preamble through the SCell <NUM> at step <NUM>. The RAR may include TA command and other control information. If the preamble is transmitted by the serving eNB <NUM>, it is likely to be efficient to send the response in replay to the preamble through the PCell in various aspects. For example, since the RAR is received only through the PCell, it is possible to reduce the PDCCH monitoring load of the UE. Accordingly, the UE <NUM> monitors the PDCCH of the PCell to receiving RAR at step <NUM>. If a valid response message is received in reply to the preamble, the UE <NUM> assumes that it is possible to transmit uplink signal transmission after the elapse of a predetermined period from that time point. For example, if the valid RAR is received at the subframe n, it is determined that the uplink transmission is possible from the subframe (n+m).

<FIG> is a signal flow diagram illustrating the operations of the UE and the eNB for configuring a SCell belonging to a non-primary set according to an embodiment of the present invention.

At step <NUM>, the serving eNB <NUM> determines to add a SCell to the UE <NUM> at a certain time point. Particularly if the UE is located in the area of a cell controlled by the second eNB <NUM>, the serving eNB <NUM> determines to add the cell controlled by the second eNB <NUM> as a SCell and sends the second eNB <NUM> a control message at step <NUM>. The control message may include the information indicating that the second eNB <NUM> is not identical with the first eNB <NUM>. Here, the second eNB <NUM> which is not identical with the serving eNB <NUM> and controls the serving cell of the UE is referred to as drift eNB (DENB) <NUM>. The control message may include the information as shown in table <NUM>.

If a SCell add request control message is received at step <NUM>, the drift eNB <NUM> determines whether to accept the request in consideration of the current load status at step <NUM>. If it is determined to accept the SCell add request, the drift eNB <NUM> sends the serving eNB <NUM> a SCell add accept message at step <NUM>. At this time, the drift eNB <NUM> generates a control message including the information as shown in table <NUM> and transmits the control message to the serving eNB <NUM>.

If the control message is received at step <NUM>, the serving eNB <NUM> generates an RRC control message instructing the UE <NUM> to add a serving cell at step <NUM>. The RRC control message may include the information as shown in table <NUM>.

The RRC control message of step <NUM> may include the configuration information of a plurality of SCells. The serving cells of the primary and non-primary sets may be configured together. For example, if the second to fifth cells are configured to the UE having the first cell as the PCell, the informations thereon may be arranged in the RRC control message in various orders.

<FIG> is a diagram illustrating a structure of the RRC control message including SCell configuration information according to an embodiment of the present invention.

Referring to <FIG>, the first and second cells have the same uplink transmission timing and forms the P-TAG, the third cell forms the S-TAT <NUM>, and the fourth and fifth cells form the S-TAG <NUM>.

The RRC control message may include SCellToAddModList <NUM>. The SCellToAddModList may include SCellToAddMod <NUM> for the second cell, SCellToAddMod <NUM> for the third cell, SCellToAddMod <NUM> for the fourth cell, and SCellToAddMod <NUM> for the fifth cell.

The SCellToAddMod <NUM>, <NUM>, <NUM>, and <NUM> may include specific information or not depending on the characteristic of the corresponding SCell.

If the SCell belongs to the P-TAG, i.e. if it has the same uplink transmission timing as the PCell, the corresponding SCellToAddMod may not include the information on the TAG. For example, the SCellToAddMod <NUM> for the second cell does not include the information on the TAG. The SCellToAddMod <NUM>, <NUM>, and <NUM> for the SCells belonging to the rest non-P-TAGs may include the TAG identifiers and TA timer values of the TAGs to which the corresponding SCells belong.

The information on at least one of the cells belonging to the non-primary set may include the non-primary set information <NUM>, e.g. non-primary set identifier and C-RNTI for use by the UE in the non-primary set. In the example of <FIG>, the SCellToAddMod <NUM> for the fourth cell includes the non-primary set information <NUM>. Accordingly, whether the corresponding cell belongs to the non-primary set can be determined based on the non-primary set information <NUM>. The information on one of the cells belonging to the non-primary set includes PUCCH configuration information <NUM>. In the example of <FIG>, the SCellToAddMod <NUM> for the fourth cell includes the PUCCH configuration information <NUM>.

To the SCell which belongs to the non-primary set but has no non-primary set information, the non-primary set information of the SCell having the same TAG id is applied. In the exemplary case of <FIG>, although the information on the fifth cell includes no non-primary set information, the UE can check that the fifth cell belongs to the non-primary set based on the non-primary set information of the fourth cell having the same TAG id. The UE can use the non-primary set identifier and C-RNTI of the fourth cell for identifying the fifth cell.

<FIG> is a diagram illustrating a structure of the RRC control message including SCell configuration information according to another embodiment of the present invention.

Referring to <FIG>, the TAG information and non-primary set information may be included at a position not in the SCellToAddMod.

The RRC control message may include SCellToAddModList <NUM>. The SCellToAddModList may include SCellToAddMod <NUM> for the second cell, SCellToAddMod for the third cell, SCellToAddMod for the fourth cell, and SCellToAddMod for the fifth cell. <FIG> shows only the SCellToAddMod <NUM> for the second cell for explanation convenience. The SCellToAddMod <NUM> may include the same types of informations. That is, every SCellToAddMod may include the information such as sCellIndex-r10, cellIdentification-r10, and radioResourceConfigCommonSCell-r10.

The TAG information <NUM>, the non-primary set information <NUM>, and the PUCCH configuration information of PUCCH SCell <NUM> may be included separately.

The TAG information <NUM> may include the TAG identifiers, identifiers of the SCells forming the TAG, and TA timer value. As shown in <FIG>, the TAG information <NUM> may include the information <NUM> notifying that the TAG having the TAG identifier <NUM> includes the SCell <NUM> and that the TA timer is set to the value t1. The TAG information <NUM> also may include the information <NUM> notifying that the TAG having the TAG identifier <NUM> includes the SCell <NUM> and SCell <NUM> and that the TA timer is set to the value t2.

The non-primary set information <NUM> may include the per-non-primary set identifiers, identifiers of the serving cells included in the set, and C-RNTI for use in the corresponding set. For example, the information <NUM> indicating that the non-primary set having the set identifier <NUM> includes the SCell <NUM> and SCell <NUM> and uses the C-RNTI x. The primary set information is determined according to the following rule without explicit signaling.

The serving cells belonging to the primary set include the PCell and the SCells not belonging to any non-primary set. The C-RNTI to be use in the primary set may be the C-RNTI in use by the current PCell.

The non-primary set information <NUM> may include the TAG identifier other than the SCell identifier. This is possible under the assumption that the set and TAG are formed such that one TAG is not formed across multiple sets. For example, the non-primary set configuration information <NUM> may include the information indicating the TAG id <NUM> instead of the information indicating the SCell <NUM> and SCell <NUM> in order for the UE to determine that the SCell <NUM> and SCell <NUM> having the TAG id <NUM> belong to the non-primary set.

The PUCCH SCell's PUCCH configuration information <NUM> may be made up of non-primary set identifier, PUCCH SCell identifier, and PUCCH configuration information. Each non-primary set has one PUCCH SCell. The CSI information for the serving cells belonging to the non-primary set and HARQ feedback information may be transmitted on the PUCCH configured to the PUCCH SCell.

Depending on the embodiment, the PUCCH SCell can be determined according to a predetermined rule without signaling PUCCH SCell identifier explicitly in the PUCCH SCell's PUCCH configuration information <NUM>. For example, the SCell corresponding to the first SCellToAddMod of the SCellToAddModList <NUM> may be assumed as the PUCCH SCell. In the embodiment of <FIG>, the SCell corresponding to the first SCellToAddMod of the SCellToAddModList <NUM> may be determined as the PUCCH SCell. Also, the SCell having the highest or lowest SCell identifier among the SCells of which information includes the SCellToAddMod information in the corresponding RRC control message may be determined as the PUCCH SCell. Such an implicit determination method can be used under the assumption that only one non-primary set exists.

Returning to <FIG>, the UE <NUM> sends the serving eNB <NUM> a response message at step <NUM> and establishes downlink synchronization with the newly configured SCells at step <NUM>. The UE <NUM> acquires System Frame Number (SFN) of the PUCCH SCell among the newly configured SCells at step <NUM>. The SFN is acquired in the process of receiving the system information, i.e. Master Information Block (MIB). The SFN is an integer incrementing by <NUM> at every <NUM> in the range of <NUM> to <NUM>. The UE <NUM> checks the PUCCH transmission timing of the PUCCH SCell based on the SFN and PUCCH configuration information.

Afterward, the UE waits until the SCells are activated. If downlink data or a predetermined control message instructing to activate SCell is received from the serving eNB <NUM> at step <NUM>, the drift eNB <NUM> starts a procedure of activating the SCells.

The drift eNB <NUM> sends the UE <NUM> the A/D MAC CE instructing to activate the SCell, e.g. SCell <NUM>, at step <NUM> and, if the MAC CE is received at the subframe n, the UE <NUM> activates the SCell at subframe (n+m1). However, since the uplink synchronization of the PUCCH SCell is not acquired yet at the subframe (n+m1), both the downlink and uplink transmission/reception are not possible although the SCell has been activated. That is, the UE <NUM> monitors PDCCH of the SCell but ignores the downlink/uplink resource allocation signal although it is received.

The drift eNB <NUM> sends the UE <NUM> a random access command to establish uplink synchronization with the PUCCH SCell at step <NUM>. The random access command includes Carrier Indicator Field (CIF) carrying the identifier of the serving cell for preamble transmission.

The UE <NUM> initiates random access procedure in the PUCCH SCell using a dedicated preamble indicated in the random access command. That is, the UE <NUM> sends a preamble through the SCell at step <NUM> and monitors PDCCH to receive RAR in response thereto. If the UE transmits the preamble in the primary set, the RAR is transmitted through the PCell. Otherwise if the preamble is transmitted in the non-primary set, the UE monitors PDCCH of the SCell in which the preamble has been transmitted or the PUCCH SCell to receive RAR. This is because there is a need of extra information exchange between the drift eNB <NUM> and the serving eNB <NUM> to process the RAR in the PCell. The RAR may be received with the C-RNTI to be used by the UE <NUM> in the non-primary set. It is more efficient to transmit the response message with the C-RNTI because the UE <NUM> also has been allocated the C-RNTI and there is no probability of malfunctioning caused by collision due to the use of the dedicated preamble (i.e. since the eNB knows the UE to which the RAR has to be transmitted based on the dedicated preamble). If the valid response message is received through the SCell in which the preamble has been transmitted or the PUCCH SCell, the UE <NUM> adjusts the uplink transmission timing of the PUCCH SCell and the TAG to which the PUCCH SCell based on the TA command of the response message and activates uplink at a predetermined time point. If the valid TA command or the valid random access response message is received at the subframe n, the predetermined timing becomes the subframe (n+m2). Here, m2 is a predetermined integer.

<FIG> is a flowchart illustrating a random access procedure for inter-eNB carrier aggregation mode according to an embodiment of the present invention.

Referring to <FIG>, the UE <NUM> performs a random access procedure for various reasons. The UE <NUM> may transmit the preamble in the PCell <NUM> or SCell <NUM> of the primary set or the PUCCH SCell <NUM>. The subsequent process is determined depending on the type of the serving cell in which the preamble has been transmitted. Detailed description thereon is hereinafter.

The UE <NUM> without RRC connection selects the first cell <NUM> among a plurality of available cells according to a predetermined rule at step <NUM>. The system information is provided to a plurality of unspecific UEs by means of the control message, i.e. System Information Block (SIB). The SIB2 may include the following informations necessary for the UE <NUM> to perform random access in the corresponding cell.

The following informations are related to random access.

The UE <NUM> takes an action required for the UE without RRC connection (i.e. UE in idle state) in the first cell <NUM>, e.g. action of monitoring the paging channel and measuring neighbor cells. If RRC connection setup necessity occurs at a certain time point (e.g. if the UE <NUM> receives paging or data or control message to be transmitted by the UE <NUM> occurs) at step <NUM>, the UE <NUM> performs RRC connection setup procedure with the first cell <NUM>. In the RRC connection setup procedure, a Signaling Radio Bearer (SRB) for RRC control message exchange between UE and eNB and Data Radio Bearer (DRB) for user data exchange between UE and eNB are configured, and the UE <NUM> and the eNB uplink and downlink data using the radio bearers.

If the random access is triggered by a certain reason at a certain timing, e.g. if the eNB instructs the UE <NUM> to initiate random access or if the UE <NUM> needs to request the eNB for transmission resource, at step <NUM>; the UE <NUM> determines the time period for transmitting the preamble based on the random access-related information acquired from the SIB <NUM> at step <NUM> and transmits the preamble at step <NUM>. The UE <NUM> transmits the preamble at uplink subframe n and starts the ra-window at downlink subframe n+m. The UE <NUM> monitors to receive the response message during the timer period defined as ra-window. Here, m is a value defined in the standard, e.g. <NUM> or <NUM>. The UE <NUM> monitors to determine whether the identifier mapped to the transmission resource used for transmitting the preamble is allocated and, if the identifier is scheduled, receives the RAR at step <NUM> to check whether the identifier mapped to the transmitted preamble is included in the header of the data at step <NUM>. If the identifier mapped to the preamble transmitted by the UE <NUM> is included in the header of the data, the UE <NUM> memorizes uplink transmission resource allocation information (UL grant), Transmission Power Control (TPC), and UL Timing Advance (TA).

Depending on the embodiment, if no valid RAR message is received during the ra-window, the UE <NUM> retransmits the preamble. The number of preamble retransmission is limited by preambleTransMax and, if the random access is not completed even though the preamble has been transmitted as many as preambleTransMax, determines any problem has occurred in uplink and thus initiates RRC connection reestablishment procedure.

The UE applies the UL grant, TPC, and TA included in the valid RAR message to the uplink transmission in the first serving cell at step <NUM>. The UE adjusts the transmission start time of the uplink subframe n of the first serving cell to precede the start time (start boundary) of the downlink subframe of the first serving cell as much as TA, increases or decreases the uplink transmit power of the first serving cell as much as indicated by the TPC, and selects the transmission resource of the first serving cell as the resource for the uplink transmission. Next, the UE performs uplink transmission in the serving cell <NUM> through which the RAR has been received at step <NUM>. The UE <NUM> selects the serving cell <NUM> through which the RAR has been received as the first serving cell <NUM>.

In the situation where carriers are not aggregated, where only one serving cell having uplink exists although the carriers are aggregated, or where only one cell allowed for random access exists although plural serving cells having uplink, the UE <NUM> selects the cell <NUM> through which the RAR has bene received as the serving cell to apply the information carried in the RAR.

The UE <NUM> performs Physical Uplink Shared Channel (PUSCH) transmission by applying the UL grant of the RAR in the first cell <NUM> at step <NUM>. Typically, synchronous HARQ process is applied to the PUSCH transmission. If an HARQ NACK is received in the synchronous HARQ transmission process, retransmission is performed using the same transmission resource as before. The UE <NUM> may perform HARQ transmission maxHARQ-Tx times and, if HARQ transmission fails even after attempts of maxHARQ-Tx times, stops PUSCH transmission. The maxHARQ-Tx is set to prevent the PUSCH transmission for one MAC PDU from being repeated infinitely and determined depending on how the eNB scheduler has considered the channel condition of the UE <NUM> and delay sensibility of the service configured to the UE <NUM>. Total <NUM> types of maxHARQ-Tx parameters can be provided to the UE <NUM>.

If the preamble transmitted at step <NUM> is the dedicated preamble, the UE <NUM> performs PUSCH transmission by applying the second maxHARQ-Tx at step <NUM>. Otherwise if the preamble transmitted at step <NUM> is the random preamble selected by the UE <NUM>, the UE <NUM> performs PUSCH transmission by applying the first maxHARQ-Tx. If the dedicated preamble has been used, this means that the eNB has known the UE <NUM> at the time when the PUSCH has been transmitted and, otherwise if the random preamble has been used, this means that the eNB has not known the UE <NUM> at the time when the PUSCH has been transmitted. The UE <NUM> applies the third maxHARQ-Tx when transmitting PUSCH based on the UL grant acquired in the random access procedure of the primary set SCell. In the case of transmitting PUSCH based on the UL grant for the non-primary set SCell or the UL grant acquired in the random access procedure of the PUSCH SCell, if the transmitted preamble is the random preamble, the UE <NUM> applies the fourth maxHARQ-Tx and, otherwise if the transmitted preamble is the dedicated preamble, the fifth maxHARQ-Tx. The second maxHARQ-Tx and the third maxHARQ-Tx may have the same value. The maxHARQ-Tx is also applied to the normal PUSCH transmission as well as the PUSCH transmission in the random access procedure. The UE may operate as follows in transmitting the PUSCH through a certain serving cell.

If a PUSCH transmission time arrives in a certain serving cell, the UE operates to determine the maxHARQ-Tx to be applied.

The UE <NUM> performs downlink data reception and uplink data transmission with the first cell <NUM>.

If the data amount of the UE <NUM> increases, the eNB controlling the first cell <NUM> may determines to add a SCell to the UE <NUM> so as to increase the data rate. The eNB determines to add a primary set SCell <NUM> to the UE at step <NUM>.

If the location of the transmission/reception device of the SCell <NUM> to be added newly differs from the location of the transmission/reception device of the PCell <NUM>, e.g. if the PCell <NUM> is a macro cell and if the SCell <NUM> is an cell formed with an RRH, the eNB provides the UE <NUM> with the SCell configuration information along with the information necessary for performing random access in the SCell <NUM> at step <NUM>. At this time, the random access-related information for use in the SCell <NUM> is provided partially other than wholly so as to be used along with the information used in the PCell <NUM>. For example, the preambleTransMax and random access transmission resource information for use in the SCell <NUM> are provided to be used along with the ra-ResponseWindowSize used in the PCell <NUM>. The preambleTransMax and random access transmission resource information are the parameters for use in controlling preamble transmission and set preferably to the values dedicated to the serving cell in which the preamble is to be transmitted. Whereas, the ra-ResponseWindowSize is associated with the serving cell through which the RAR is to be received other than the serving cell through which the preamble has bene transmitted and thus set preferably to the value defined in the PCell <NUM>. For explanation convenience, the preambleTransMax of the primary set SCell <NUM> is referred to as preambleTransMax2.

The UE <NUM> receives a PDCCH order instructing to initiate random access from the SCell <NUM> as the second cell at step <NUM>. The PDCCH order is a command instructing the UE <NUM> to initiate random access procedure in a predetermined serving cell and specified in TS36. <NUM> in detail. The PDCCH order is transmitted in Downlink Control Information (DCI) format 1A, and CRC is scrambled with the C-RNTI of the corresponding UE. Each filed is coded as shown in table <NUM>.

Upon receipt of the PDCCH order at subframe [n], the UE <NUM> applies the preamble indicated by means of the preamble index to transmit the preamble in the second cell <NUM> at subframe [n+x1]. The subframe number x1 is an integer greater than a predetermined x and is a value corresponding to the first valid PRACH occasion since [n+x]. x denotes the time required for the UE to take an action for transmitting the preamble and is set to a relatively large value in consideration of a low-end UE having low processing capability. This parameter is set to <NUM> in the current standard. The valid PRACH occasion denotes the PRACH occasion allowed for the UE to transmit preamble among the PRACH occasions defined based on the PRACH configuration information and is determined based on the PRACH mask index. The PRACH mask index has been specified in TS36.

After transmitting the random access preamble in the SCell, the UE <NUM> monitors the PDCCH of the PCell <NUM> to determine whether RAR is received during the ra-window. Here, the window size may be set to the ra-ResponseWindowSize of the PCell <NUM> which has been acquired at step <NUM>.

Typically, if the preamble has been transmitted through a SCell, it is intuitive to receive the RAR through the SCell. The RAR is addressed to a specific identifier called RA-RNTI unlike the user data. Accordingly, in order to receive the RAR in the SCell, the UE has to monitor to receive the RA-RNTI as well as C-RNTI in the SCell, and this causes a problem of increasing complexity of the UE <NUM>. However, if the RAR reception is restricted to the PCell, it is necessary just to monitor to receive the C-RNTI in the SCell, resulting in avoidance of increase of the complexity of the UE. Accordingly, although the UE <NUM> has transmitted the preamble in the SCell <NUM> at step <NUM>, the UE <NUM> monitors the PDCCH to receive the RAR transmitted with the RA-RNTI of the PCell <NUM> during the ra-window defined as ra-ResponseWindowSize for the PCell <NUM>. If no valid RAR is received, the UE <NUM> retransmits the preamble in the SCell <NUM>. If it fails completing the random access even after a predetermined number of retransmission attempts, the UE <NUM> stop transmitting preamble so as to prevent uplink transmission from being performed.

If a valid RAR is received through the PCell <NUM> at step <NUM>, the UE <NUM> applies the UL grant, TPC, and TA included in the valid random access response message to the uplink transmission in the second serving cell <NUM> at step <NUM>. The second serving cell <NUM> is the serving cell <NUM> through the UE has transmitted the preamble other than the serving cell <NUM> through which the RAR has been received. The UE <NUM> adjusts the transmission start time of uplink subframe n to precede the start time (start boundary) of the downlink subframe n as much as TA. The UE also increases or decreases the uplink transmit power in the second cell <NUM> as much as indicated by the TPC. The RAR may include a <NUM>-bit TPC as shown in table <NUM>.

Typically, the TPC relates to the PUSCH transmit power control of the serving cell <NUM> through which the RAR has been received. However, the UE may perform random access in several cells and, if the UE has transmitted the preamble through the SCell <NUM>, the TPC is of PUSCH transmit power control of the serving cell <NUM> through which the preamble has been transmitted other than the serving cell <NUM> through which the RAR has been received.

The UE <NUM> selects the transmission resource of the second serving cell <NUM> as the resource for uplink transmission. Next, the UE perform uplink transmission at step <NUM>.

As described above, if carriers are not aggregated, if only one serving cell having uplink exists although the carriers are aggregated, or if only one cell allowed for random access exists although plural serving cells having uplink, the serving cell to which the information included in the RAR is applied is the cell through which the RAR is received, i.e. the PCell <NUM>. Otherwise if the preamble is transmitted through the SCell <NUM> or if the random access is performed in the SCell <NUM> as well as the PCell <NUM>, and if information included in the RAR is applied to the cell <NUM> through which the RAR has been received, the goal of the random access procedure may not be achieved. The UE <NUM> which has one serving cell allowed for random access maintains its operation because the convention operation of applying the information included in the RAR to the serving cell through which the RAR has been received, but the UE <NUM> having multiple serving cell allowed for random access applies the information included in the RAR to the serving cell <NUM> through which the preamble has been transmitted instead of applying the information included in the RAR to the serving cell <NUM> through which the RAR has been received. That is, if the RAR is received in response to the preamble, the UE <NUM> having a plurality cell allowed for performing random access applies the TA, TPC, and UL grant included in the RAR to the serving cell <NUM> through which the preamble has been transmitted other than the serving cell <NUM> through which the RAR has been received.

The UE <NUM> performs Physical Uplink Shared Channel (PUSCH) transmission in the second cell <NUM> by applying the UL grant of the RAR at step <NUM>. At this time, the UE <NUM> controls the PUSCH transmission by applying the third maxHARQ-Tx or the second maxHARQ-Tx acquired at step <NUM>.

The eNB determines to configure the cell <NUM> of the non-primary set to the UE <NUM> at step <NUM>. The serving eNB performs a predetermined procedure with the drift eNB and transmits the control information for configuring the SCell <NUM> of the primary set to the UE <NUM> at step <NUM>. The non-primary set SCell <NUM> may be a PUCCH SCell. The control information may be transmitted to the UE through the RRC Connection Reconfiguration message. The control message may be configured as shown in table <NUM> or <FIG> or <FIG> and may include the information such as preambleTansMax3, ResponseWindowSize2, non-primary set information, the fourth maxHARQ-Tx, and the fifth maxHARQ-Tx. Here, the non-primary set information may include an indicator indicating that the eNB to which the SCell <NUM> belongs differs from the eNB to which the PCell <NUM> belongs.

As aforementioned, the random access information of the SCell is defined differently for the primary set and the non-primary set. The random access-related information is summarized in table <NUM>.

The UE <NUM> transmits the preamble using predetermined frequency/time resource of the PUCCH SCell <NUM> at a predetermined time point at step <NUM>.

After transmitting the random access preamble in the PUSCH SCell <NUM>, the UE <NUM> monitors PDCCH of the PUCCH SCell <NUM> to determine whether an RAR is received during the ra-window having a size determined based on the ResponseWindowSize2 at step <NUM>. Since the PCell <NUM> and PUCCH SCell <NUM> are controlled by different eNBs, it is inefficient to receive the response message through the PCell <NUM> in response to the preamble transmitted through the PUCCH SCell <NUM>. Accordingly, when the preamble has been transmitted through a SCell, if the SCell is a primary set SCell <NUM>, the UE <NUM> attempts receipt of the random access response message through the PCell <NUM> and, if the SCell is the PUCCH SCell <NUM>, attempts receipt of the random access message through the corresponding cell <NUM>. If no valid RAR is received through the PUCCH SCell <NUM>, the UE retransmits the preamble through the PUCCH SCell <NUM>. If it fails to complete the random access procedure even after a predetermined number of retransmission attempts, the UE <NUM> stops transmitting the preamble and takes a predetermined action. The predetermined number of retransmissions is equal to the preambleTransMax3.

If the valid RAR is received through the PUCCH SCell <NUM> at step <NUM>, the UE <NUM> applies the UL grant, TPC, and TA included in the valid RAR to the uplink transmission through the serving cell <NUM> at step <NUM>. The serving cell <NUM> is the serving cell through which the RAR has been received, i.e. the PUCCH SCell <NUM>. The UE <NUM> adjusts the transmission start time of uplink subframe n to precede the start time (start boundary) of the downlink subframe n as much as TA. The UE also increases or decreases the uplink transmit power in the PUCCH SCell <NUM> as much as indicated by the TPC.

The UE <NUM> selects the transmission resource of a predetermined serving cell, i.e. PUCCH SCell <NUM>, for uplink transmission. The UE performs uplink transmission at step <NUM>.

<FIG> is a flowchart illustrating the UE operation for random access according to an embodiment of the present invention.

Referring to <FIG>, the UE acquires the random access-related information at step <NUM>. There is the random access-related information as follows.

That is, the UE may acquire the random access-related information such as PCell random access transmission resource information (PCell prach-ConfigIndex, etc.), primary set SCell random access transmission resource information (SCell prach-ConfigIndex, etc.), non-primary set SCell (e.g. PUCCH SCell) random access transmission resource information (PUCCH SCell prach-ConfigIndex, etc.), ra-ResponseWindowSize, ra-ResponseWindowSize2, preambleTransMax1, preambleTransMax2, preambleTransMax3, maxHARQ-Tx <NUM>, maxHARQ-Tx <NUM>, maxHARQ-Tx <NUM>, maxHARQ-Tx <NUM>, and maxHARQ-Tx <NUM>.

The aforementioned informations are acquired through various control messages such as system information block and dedicated control messages at various time points. For example, the PCell random access transmission resource information, ra-ResponseWindowSize, and preambleTransMax1 can be acquired through the systeminformationblock2 of the PCell, and the maHARQ-Tx <NUM> through the RRC Connection Setup message. Also, the primary set SCell random access transmission resource information, PUCCH SCell random access transmission resource information, ra-ResponseWindowSize2, preambleTransMax2, preambleTransMax3, maxHARQ-Tx <NUM>, maxHARQ-Tx <NUM>, and maxHARQ-Tx <NUM> may be acquired through the RRC connection reconfiguration message.

The random access procedure is triggered at step <NUM>. For example, if a high priority data occurs at the UE, if the eNB commands to perform random access, or if the UE needs to reestablish the RRC connection, the random access procedure may be triggered.

The UE identifies the valid PRACH occasion arriving soonest using the random access transmission resource information and transmits the preamble at the PRACH occasion at step <NUM>. The preamble transmit power is set in consideration of the downlink pathloss of the serving cell through which the preamble is transmitted.

The UE monitors the PDCCH of the PCell or the SCell through which the preamble has been transmitted to receive the response message during a predetermined period in replay to the preamble at step <NUM>. The predetermined period is the ra-window. For example, if the preamble has been transmitted through the PCell, the UE determines the maximum size of the ra-window by applying the ra-ResponseWindowSize acquired from the system information of the corresponding serving cell and monitors the PDCCH of the PCell during the ra-window. If the preamble has been transmitted through the primary set SCell, the UE determines maximum size of the ra-window by applying the ra-ResponseWindowSize acquired from the system information of a predetermined serving cell not identical with the corresponding serving cell, e.g. PCell, and monitors the PDCCH of the PCell during the ra-window. If the preamble has been transmitted through the non-primary set SCell or the PUCCH SCell, the UE determines the maximum size of the ra-window by applying ra-ResponseWindowSize2 and monitors the PDCCH of the PUCCH SCell during the ra-window.

If a valid RAR is received during the ra-window, the procedure goes to step <NUM>. If a valid RAR is received, this means that the UE has decoded the scheduling information with the RA-RNTI mapped to the transmission resource used by the UE for transmitting the preamble through the PDCCH of the PCell or the PUCCH SCell during the period specified by the ra-window and the Random Access Preamble ID (RAPID of the received RAR matches that of the preamble transmitted by the UE.

The UE checks whether it is possible to retransmit the preamble and, if possible, retransmits the preamble at step <NUM> and returns the procedure to step <NUM>. Depending on the embodiment, in the PCell, if the number of preamble transmission times is not greater than preableTransMax2, it is possible to retransmit the preamble. In the PUCCH SCell, if the number of preamble transmission times is not greater than preambleTransMax3, it is possible to retransmit the preamble. The operation in the case where the number of preamble retransmission times is greater than a predetermined maximum allowed times is described in detail with reference to <FIG> later.

The UE determines whether the cell through which the preamble has been transmitted is the PCell or a SCell at step <NUM>. If the preamble has been transmitted through the PCell, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If the cell through which the preamble has been transmitted is the PCell, the UE applies TA to the serving cell through which the RAR has been received. For example, the UE moves up the uplink subframe boundary of the serving cell through which the RAR has been received as much as TA as compared to the downlink subframe boundary of the corresponding serving cell.

The UE configures the uplink transmit power of the serving cell through which the RAR has been received using the TPC included in the RAR at step <NUM>. In more detail, the UE calculates the PUCCH transmit power of the serving cell by accumulating the transmit power control values indicated by the TPC.

The UE transmits the PUSCH in uplink of the serving cell through which the RAR has been received by applying the UL grant included in the RAR at step <NUM>. If the preamble transmitted at step <NUM> is the dedicated preamble, the maxHARQ-Tx <NUM> is applied to the PUSCH transmission and, otherwise if the preamble transmitted at step <NUM> is the random preamble, the maxHARQ-Tx <NUM> is applied to the PUSCH transmission.

If it is determined that the cell through which the UE has transmitted the preamble is a SCell at step <NUM>, the UE determines whether the SCell through which the preamble has been transmitted is a primary set SCell at step <NUM>. If the SCell is a primary set SCell, the procedure goes to step <NUM> and, otherwise if the cell is non-primary set cell, e.g. PUCCH SCell, the procedure goes to step <NUM>.

If the SCell through which the preamble has been transmitted is a primary set SCell, the UE applies, at step <NUM>, the TA to the serving cell through which the preamble has been transmitted at step <NUM> other than the serving cell through which the RAR has been received. That is, the UE moves up the uplink subframe boundary of the serving cell through which the preamble has been transmitted as much as TA as compared to the downlink subframe boundary of the serving cell through which the preamble has been transmitted. The uplink transmission timing is applied to the uplink of all the serving cells belonging to the same TAG as the serving cell through which the preamble has been transmitted. That is, the TA is applied to all of the serving cells belonging to the same TAG as the serving cell through which the preamble has been transmitted. TAG denotes a set of one or more serving cells sharing the same uplink transmission timing. If the serving cells belonging to one TAG share the same uplink transmission timing, this means that the uplink transmission timings of the serving cells are identical with each other and these serving cells establish or loose the uplink synchronization simultaneously. Also, the uplink transmission timings thereof are adjusted simultaneously. A TAG is classified into one of primary TAG and secondary TAG. The primary TAG is the TAG including the primary cell, and the secondary TAG is the TAG composed of only the secondary cells. The primary TAG allows random access only in the primary cell while the secondary TAG allows random access only in a predetermined secondary cell. When adding a SCell, the eNB notifies the UE whether the SCell belongs to the primary TAG or the secondary TAG and, if it belongs to the secondary TAG, which secondary TAG.

The UE sets the uplink transmit power of the serving cell through which the preamble has been transmitted at step <NUM> using the TPC included in the RAR at step <NUM>. In more detail, the UE calculates the PUSCH transmit power of the serving cell by accumulating the transmit power adjustment values indicated by the TPC.

The UE transmits PUCCH in uplink of the serving cell through which the preamble has been transmitted by applying the UL grant included in the RAR at step <NUM>. At this time, the maxHARQ-Tx <NUM> is applied to PUSCH transmission.

If it is determined that the SCell through which the preamble has been transmitted is not a primary set SCell at step <NUM>, the UE applies the TA to the serving cell through which the RAR has been received or the serving through which the preamble has been transmitted at step <NUM>.

The UE sets the uplink transmit power of the serving cell through which the RAR has been received using the TPC included in the RAR at step <NUM>. In more detail, the UE calculates the PUSCH transmit power of the serving cell by accumulating the transmit power adjustment value indicated by the TPC.

The UE transmits the PUSCH in the uplink of the serving cell through which the RAR has been received or the serving cell through which the preamble has been transmitted by applying the UL grant included in the RAR at step <NUM>. At this time, if the preamble transmitted at step <NUM> is the dedicated preamble, the maxHARQ-Tx <NUM> is applied to the PUSCH transmission and, otherwise if the preamble is a random preamble, the maxHARQ-Tx <NUM> is applied to the PUSCH transmission.

<FIG> is a flowchart illustrating the UE operation associated with the random access failure according to an embodiment of the present invention.

In order to prevent the UE from retransmitting the preamble infinitely, a parameter called preambleTransMax can be used. As described above, a plurality of preambleTransMax can be configured to one UE, and the UE operates differently depending on the type of the cell through which the preamble is transmitted.

Referring to <FIG>, random access failure may occurs even after the maximum allowed number of preamble transmission attempts. If the random access procedure is initiated, the UE initializes the PREAMBLE_TRANSMISSION_COUNTER to <NUM>. If it is necessary to retransmit the preamble, e.g. if no valid RAR is received during the ra-window or if the contention resolution fails, the UE increments the PREAMBLE_TRANSMISSION_COUNTER by <NUM>. If the PREAMBLE_TRANSMISSION_COUNTER becomes equal to a value acquired by adding <NUM> to the preambleTransMax, the UE determines that a problem has occurred in the random access procedure and thus takes a necessary action.

As described above, different preambleTransMax can be applied to PCell, SCell, primary set, and non-primary set. The preambleTransMax applied to the PCell (hereinafter, referred to as preambleTransMax1) is acquired from the system information of the PCell, and the PreambleTransMax applied to the primary set SCell (hereinafter, referred to PreambleTransMax2) is acquired from the SCell add control message, e.g. RRC connection reconfiguration message. The non-primary set SCell, e.g. PreambleTransMax applied to the PUCCH SCell (hereinafter, referred to as PreambleTransMax3) is classified into two types. The first PreambleTransMax3 is acquired from the non-primary set SCell or PUCCH SCell add control message, e.g. RRC connection reconfiguration message. Whereas the second PreambleTransMax3 is acquired form the system information of the PUCCH SCell. In association with the random access of the UE through the PUCCH SCell, the first PreambleTransMax3 is applied in the initial random access procedure, and the second PreambleTransMax3 is applied after the system information is acquired from the PUCCH SCell. The UE may apply the first PreambleTransMax3 for the random access in the non-primary set which is not the PUCCH SCell. The first PreambleTransMax3 of the PUCCH SCell may differ from the first PreambleTransMax3 of the non-primary set SCell.

If a situation where the PREAMBLE_TRANSMISSION_COUNTER becomes equal to the value acquired by adding <NUM> to the preambleTransMax in the PCell, the PREAMBLE_TRANSMISSION_COUNTER becomes equal to the value acquired by adding <NUM> to the preambleTransMax2 in the primary set SCell, or the PREAMBLE_TRANSMISSION_COUNTER becomes equal to the value acquired by adding <NUM> to the preambleTransMax3 in the non-primary set SCell or PUCCH SCell occurs in performing random access, the procedure goes to step <NUM>.

The UE determines whether the serving cell through which the preamble has been transmitted belongs to the primary set or the non-primary set at step <NUM>. If the cell is belongs to the primary set, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If the serving cell through which the preamble has been transmitted belongs to the primary set, the UE determines whether the serving cell is the PCell or SCell at step <NUM>. If the serving cell is the SCell, the procedure goes to step <NUM> and, otherwise the serving cell is the PCell, step <NUM>.

If it is determined that the serving cell through which the preamble has been transmitted is the SCell at step <NUM>, the UE performs the first procedure at step <NUM>.

Otherwise if it is determined that the serving cell through which the preamble has been transmitted is the PCell at step <NUM>, the UE performs the second procedure at step <NUM>.

If it is determined that the serving cell through which the preamble has been transmitted belongs to the non-primary set at step <NUM>, the UE determines whether the serving cell is a PUCCH SCell at step <NUM>. If the serving cell is the PUCCH SCell, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If it is determine that that serving cell through which the preamble has been transmitted is not a PUCCH SCell at step <NUM>, the UE performs the first procedure at step <NUM>.

Otherwise if the serving cell through which the preamble has been transmitted is a PUCCH SCell at step <NUM>, the UE performs the third procedure at step <NUM>.

If the first procedure is performed, this means that the serving cell through which the preamble has been transmitted, i.e. the serving cell having a problem in random access, is a SCell belonging to the primary set or non-primary set SCell but not PUCCH SCell. The random access problem is the problem of the corresponding SCell and affects no influence to the PCell, PUCCH SCell, or other sets. Accordingly, the UE stops transmitting preamble and, if SRS is configured to the corresponding SCell, the SRS too in the first procedure (at this time the SRS transmission resource is released) without further actions.

If the second procedure is performed, this means that the serving cell through which the preamble has been transmitted, i.e. the serving cell having a problem in random access, is the PCell. If a random access problem has occurred, this means that there is significant information in downlink and uplink of the PCell and thus the UE reestablishes the current RRC connection. This means that the UE stops downlink/uplink operations in the PCell and starts the RRC connection reconfiguration procedure. The UE takes the PUCCH SCell first in consideration as the cell to perform the RRC connection reconfiguration procedure. If the RRC connection reconfiguration is performed, this means to select a cell having the downlink channel quality higher than a predetermined threshold to transmit the RRC connection reconfiguration request message. If the eNB controlling the cell has the information on the UE, the eNB sends the UE the RRC connection reconfiguration message through the cell, and the UE continues communication maintaining the current configuration in the cell. If it has no UE information, the eNB sends the UE the RRC connection reconfiguration reject message, and the UE transitions to the idle state and initiates the RRC connection setup procedure.

If the third procedure is performed, this means that the serving cell through which the preamble has been transmitted, i.e. the serving cell having a problem in random access, belongs to a non-primary set and the PUCCH SCell. If the serving cell through which the preamble has been transmitted, i.e. the serving cell having a problem in random access, belongs to the non-primary set and is the PUCCH SCell, the random access error may cause significant problem in downlink or uplink of the PUCCH SCell and thus it is impossible to perform data transmission any longer in the corresponding non-primary set as well as the PUCCH SCell. This is because the PUCCH transmission is impossible in the corresponding non-primary set. In this case, the UE may perform the third procedure. The third procedure may be composed of the following steps.

<FIG> is a flowchart illustrating the UE operation associated with expiry of TA timer according to an embodiment of the present invention.

The time (timeAlignmentTimer, hereinafter referred to TAT) is set and runs per TAG. When the TA timer of a certain TAG expires, the UE may operate differently depending on whether the TAG is of primary set TAT or non-primary set TAG.

The TAT (timeAlignmentTimer) of a certain TAG expires at a certain time point at step <NUM>. The TAT is set per TAG. The TAG of a TAG starts first in the initial random access procedure of the TAG and restarts whenever a TA command for the TAG is received. While the TAT does not run, the uplink signal transmission is prohibited with the exception of the preamble transmission in the corresponding TAG. If the TAG expires, this means that no TA command has been received for the TAG during the period specific by the TAT.

The UE flushes the HARQ buffers of the serving cells belonging to the corresponding TAG at step <NUM>. This is done to prevent non-adaptive HARQ retransmission from being performed in the corresponding cell.

The UE determines whether the serving cells belonging to the TAG are primary set serving cells or non-primary set serving cells at step <NUM>. If the serving cells are the primary set serving cells, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If the serving cells belonging to the TAG are the non-primary set serving cells, the UE determines whether PUCCH SCell exists among the serving cells belonging to the TAG at step <NUM>.

If the PUCCH SCell exists among the serving cells belonging to the TAG at step <NUM>, the procedure goes to step <NUM> to perform the first procedure.

If the PUCCH SCell does not exist among the serving cell belonging to the TAG at step <NUM>, the procedure goes to step <NUM> to perform the second procedure.

If the serving cells belonging to the TAG are the primary set serving cells at step <NUM>, the UE determines whether the corresponding TAG is P-TAG or S-TAG at step <NUM>.

If the corresponding TAG is the P-TAG at step <NUM>, the procedure goes to step <NUM> to perform the third procedure.

If the corresponding TAG is the S-TAG at step <NUM>, the procedure goes to step <NUM> to perform the second procedure.

The first procedure is taken when the serving cells of the TAG of which TAT has expired are the serving cells of the non-primary set (when TAG is the TAG of non-primary set, or the TAG is not the TAG of the primary set) and the PUCCH SCell exists among the cells belonging to the TAG. In this case, the UE stops transmitting PUCCH and SRS in the PUCCH SCell and releases the PUCCH and SRS transmission resources. The UE also stops transmitting SRS in the rest SCells belonging to the TAG and releases the SRS transmission resource. For reference, if a plurality TAGs are configured in the non-primary set, the PUCCH SCell exists in one of the TAGs.

The second procedure is taken when the serving cells of the TAG of which TAT has expired are the serving cells of the non-primary set and no PUCCH SCell exists among them or when the serving cells of the TAG of which TAT has expired are the serving cells of the primary set and no PCell exists among them. In this case, the UE stops transmitting SRS in the SCells belonging to the TAG and releases the SRS transmission resource.

The third procedure is taken when the serving cells of the TAG of which TAG has expired are the serving cells of the primary set and the PCell exists among them. In this case, the UE stops transmitting PUCCH and SRS in the PCell and releases the PUCCH and SRS transmission resources. The UE also releases the SRS resources of the SCells belonging to the P-TAG. If Semi-Persistent Scheduling (SPS) is in use, the UE releases the SPS resource. That is, the UE releases the configured uplink grant and the configured downlink assignment. The SPS is a technique of allocating transmission resource semi-persistently to minimize the use of the transmission resource allocation signal for the serving generating small packets periodically. At this time, the transmission resource allocated once can be used until a control signal is received or a predetermined condition is fulfilled for release of the resource.

<FIG> is a flowchart illustrating the UE operation of transmitting Scheduling Request (SR) in the primary set and non-primary set according to an embodiment of the present invention.

A UE has to request an eNB for transmission resource in order to transmit data in uplink. The UE may request for the transmission resource using the SR transmission resource allocated to itself or the random access procedure. The transmission resource request using the SR transmission resource is referred to as Dedicated-Scheduling Request (D-SR) , and the transmission resource request using the random access procedure is referred to as Random Access-Scheduling Request (RA-SR) procedure. The SR transmission resource is allocated as a part of the PUCCH transmission resource. The PUCCH transmission resource can be allocated to the UE in the PCell or the PUCCH SCell, and the UE may be allocated one or more SR transmission resources at a certain time point.

The UE has to define the scheme of selecting the SR transmission resource to be used. In the case that the non-primary set is configured to the UE, it is preferred to transmit data through the non-primary set if possible. The serving cells of the non-primary cells are likely to be the pico cells other than macro cells and, since the pico cell operate at low uplink transmit power as compared to the macro cells, it is preferred to transmit data through the non-primary set in view of the power consumption. <FIG> is directed to the method of selection SCell prior to PCell for transmitting SR, if the SR transmission resource has been allocated for use in the SCell, to minimize the power consumption of the UE.

At step <NUM>, Buffer Status Report is triggered at the UE. The BSR is the control information which the UE reports its buffer state to the eNB using one of the short BSR and long BSR formats. The BSR may carry the Buffer Status (BS) of at one and up to <NUM> Logical Channel Group (LCG). The short BSR is used when there is one LCG having the data to be transmitted and is composed of the LCG identifier and BS. The long BSR is used to report the buffer status of four LCGs and contains the BSs of the LCGs in an order of the LCG identifiers. The LCG is a set of the logical channel grouped under the control of the eNB, and the logical channels have similar logical channel priorities. The buffer status of the LCG is the sum of the buffer status related to the logical channels included in the LCG and shows the data amount that can be transmitted among the data of RLC transmission buffer, retransmission buffer, PDCP transmission buffer of the logical channels. The BSR may be triggered periodically or when a predetermined condition is fulfilled, e.g. when the data having a priority higher than that of the currently stored data occurs. The former is referred to as periodic BSR, and the latter is referred to as regular BSR.

The UE determines whether the triggered BSR is the periodic BSR or the regular BSR at step <NUM>. If the regular BSR is triggered, the procedure goes to step <NUM> and, otherwise if the periodic BSR is triggered, step <NUM>.

If the triggered BSR is the periodic BSR, the UE waits for the transmission resource allocation for use in transmitting the BSR at step <NUM>.

Otherwise if the triggered BSR is the regular BSR, the UE starts the procedure of requesting for the transmission resource allocation for BSR transmission at step <NUM>. This is done because that regular BSR is required to be transmitted to the eNB immediately unlike the periodic BSR.

The UE determines whether the SR transmission resource has been allocated for the PCell and SCell (e.g. PUSCH SCell) at step <NUM>. If so, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If the SR transmission resource has been allocated for the PCell and SCell at step <NUM>, the UE transmits the SR using the SR transmission resource of the serving cell nearest to the current location of the UE among the set (or serving cell) to which the SR transmission resource has been allocated. The UE may determine the serving cell having the least pathloss as the nearest serving cell among the serving cells to which the SR transmission resource has been allocated.

If the SR transmission resource has not been allocated to the PCell and SCell at step <NUM>, the UE determines whether there is a serving cell to which the SR transmission resource has been allocated at step <NUM>.

If there is any serving cell to which the SR transmission resource has been allocated at step <NUM>, the UE transmits the SR using the SR transmission resource of the serving cell to which the SR transmission resource has been allocated at step <NUM>.

Otherwise if there is no serving cell to which the SR transmission resource has been allocated at step <NUM>, the UE initiates random access in the serving cell nearest to the current location of the UE (i.e. the serving cell having the least pathloss) among the serving cell available for random access (e.g. PCell and PUCCH SCell) at step <NUM>.

<FIG> is a flowchart illustrating the UE operation of transmitting Scheduling Request (SR) through the primary set and non-primary set according to another embodiment of the present invention.

When the inter-eNB carrier aggregation is configured, the logical channels can be processed per set. For example, the logical channel of the serving generating small data and sensitive to transmission delay and jitter such as VoIP may be processed through the serving cell of the primary set, and the logical channel of the serving generating large data such as FTP may be processed through the serving cell of the non-primary set. As described above, the eNB may instruct the UE to process a part of the DRB in the serving cell of the non-primary set. The logical channel processed in the serving cell of the primary set is referred to as primary set logical channel, and the logical channel processed in the serving cell of the non-primary set is referred to as non-primary set logical channel. The eNB may notify the UE of the primary set logical channel and non-primary set logical channel using the control message such as RRC connection reconfiguration message. At this time, it is possible to notify of the non-primary set logical channel explicitly while the rest logical channel are configured as the primary set logical channel.

The regular BSR is triggered when high priority data occurs. At this time, the UE selects the serving cell to transmit SR depending on whether the BSR is triggered by the data on the logical channel of the primary set or the data on the logical channel of the non-primary set.

Referring to <FIG>, if the regular BSR is triggered at step <NUM>, the UE determines whether the regular BSR is triggered for the reason of the data of the primary set logical channel (or primary set LCG) or the data of the non-primary set logical channel (or non-primary set LCG) at step <NUM>. If the regular BSR is triggered for the reason of the primary set logical channel data, the procedure goes to step <NUM> and, otherwise if the regular BSR is triggered for the reason of the non-primary set logical channel data, step <NUM>.

If the regular BSR is triggered for the reason of the primary set logical channel data, the UE determines whether the TAT of the P-TAG is running at step <NUM>.

If the TAT of the P-TAG is running, the uplink signal transmission is prohibited with the exception of preamble and thus the UE starts the random access procedure in the PCell at step <NUM>. If a valid RAR message is received, the UE adjusts the uplink transmission timing by applying the TA indicated by the RAR message and transits the regular BSR through the PCESS using the uplink transmission resource.

If the TAT of the P-TAG is running at step <NUM>, the UE determines whether SR transmission resource is allocated on PUCCH of the PCell at step <NUM>. If no SR transmission resource is not allocated on PUCCH of the PCell, the procedure goes to step <NUM> and, otherwise, step <NUM>.

If it is determined that the SR transmission resource is allocated on the PUCCH of the PCell at step <NUM>, the UE starts SR transmission procedure in the PCell at step <NUM>.

If it is determined that no SR transmission resource is allocated on the PUCCH of the PCell at step <NUM>, the UE starts random access procedure in the PCell at step <NUM>.

If it is determined that the regular BSR is triggered for the reason of the data of the non-primary set logical channel at step <NUM>, the UE determines whether the TAT of the TAG to which the PUCCH SCell belongs is running at step <NUM>. If the TAT of the TAG to which the PUCCH SCell belongs is running, the procedure goes to step <NUM> and otherwise, step <NUM>.

If it is determined that the TAT of the TAG to which the PUCCH SCell belongs is running at step <NUM>, the UE determines whether any SR transmission resource is allocated on the PUCCH of the PUCCH SCell at step <NUM>. If no SR transmission resource is allocated on the PUCCH of the PUCCH SCell, the UE starts the SR transmission procedure at step <NUM>. The SR transmission procedure is described in detail with reference to <FIG>.

If it is determined that the TAT of the TAG to which the PUCCH SCell belongs is not running at step <NUM> or if it is determine that no SR transmission resource is allocated on the PUCCH of the PUCCH SCell at step <NUM>, the UE starts the random access procedure in the SCell at step <NUM>. In more detail, the UE starts the random access procedure in the SCell allowed for random access among the SCells of the non-primary set. If a valid RAR message is received through the SCell, the UE adjusts the uplink transmission timing and transmits the regular BSR through the serving cell of the non-primary set using the uplink transmission resource. The SCell allowed for random access in the non-primary set may be the PUCCH SCell.

<FIG> is a flowchart illustrating the UE operation of transmitting Scheduling Request (SR) in the primary set and non-primary set according to another embodiment of the present invention.

Referring to <FIG>, the UE starts the SR transmission procedure at step <NUM>. The UE determines whether any incomplete SR at step <NUM>. If there is any incomplete SR, the procedure goes to step <NUM> and, otherwise, step <NUM> to end the SR transmission procedure.

The SR is triggered along with the regular BSR and regarded as not completed before being canceled. The SR is classified into one of primary SR and non-primary SR. If the BSR triggered along with the SR is the BSR of the primary set (i.e. if the BSR includes the primary set logical channel buffer status and triggered by the data of the primary set logical channel), the SR is the primary SR. If the BSR triggered along with the SR is the BSR of the non-primary set (i.e. if the BSR includes the non-primary set logical channel buffer status and triggered by the data of the non-primary set logical channel), the SR is the non-primary SR. the primary SR cancellation condition and the non-primary SR cancellation condition are as follows.

A MAC PDU to be transmitted through the primary set, the PDU includes a BSR, and the BSR reflects the buffer status up to the time point when the last event that triggered the primary set BSR (MAC PDU for the primary set is assembled and this PDU includes a BSR which includes buffer status up to (and including) the last event that triggered a primary set BSR).

A MAC PDU to be transmitted through the non-primary set is generated, the PDU includes a BSR, and the BSR reflects the buffer status up to the time point when the last event that triggered the non-primary set BSR (MAC PDU for the non-primary set is assembled and this PDU includes a BSR which includes buffer status up to (and including) the last event that triggered a non-primary set BSR).

If there is any incomplete SR at step <NUM>, the UE determines whether the SR is triggered along with the BSR of the primary set at step <NUM>.

Or the UE determines whether the SR is triggered for PUCCH of the PCell. If the SR is triggered for the primary set or the PCell, the procedure goes to step <NUM>. Otherwise if the SR is triggered along with the BSR of the non-primary set or for the PUCCH of the PUCCH SCell, non-primary set, or PUSCH SCell, the procedure goes to step <NUM>.

If it is determined that the SR is triggered along with the BSR of the primary set or for the PCell at step <NUM>, the UE determines whether there is any serving cell allocated available uplink transmission resource among the serving cells of the primary set within the current TTI at step <NUM> (check if UL-SCH is available for a transmission on the primary set serving cell). If there is available transmission resource, the procedure returns to step <NUM> and, otherwise, step <NUM>.

At step <NUM>, the UE checks the three conditions as follows to determine whether the SR transmission is possible through the PUCCH of the PCell within the current TTI.

The measurement gap is the time duration which the eNB configures to the UE to perform measurement on other frequencies and in which the UE does not transmit /receive signal.

The sr-ProhibitTimer aims to prevent the UE from transmitting the SR so frequently and starts when the SR is transmitted. The sr-ProhibitTimer of the primary set (or PCell) and the sr-ProhibitTimer of the non-primary set (or PUCCH SCell) operate separately and may be set to different values. Both the sr-ProhibitTimer's are determined by the eNB and notified to the UE through a control message such as the RRC connection reconfiguration method.

If all of the three conditions are fulfilled, the procedure goes to step <NUM> and, otherwise at least one of the three conditions is not fulfilled, returns to step <NUM>.

The UE compares the SR_COUNTER and the dsr-TransMax at step <NUM>. If the SR COUNTER is less than the dsr-TransMax, the procedure goes to step <NUM> and, otherwise, step <NUM>.

The SR COUNTER denotes the number of SR transmission times of the UE, and the UE increments this variable by <NUM> whenever the SR is transmitted and initializes the variable when the SR is cancelled. The dsr-TransMax is a variable for preventing the SR from being repeated infinitely and is informed to the UE through a control message such as RRC connection reconfiguration message.

If the SR COUNTER is not less than the dsr-TransMax, the UE determines that the SR transmission has failed and takes a necessary action at step <NUM>. The action is described in detail with reference to <FIG>.

If the SR COUNTER is less than the dsr-TransMax, the UE transmits the SR through the PUCCH of the PCell, increments the SR COUNTER by <NUM>, and starts sr_Prohibit_Timer at step <NUM>, and then returns the procedure to step <NUM>.

If it is determined the SR is not triggered along with the BSR of the primary set or not for the PCell at step <NUM>, the UE determines whether there is any serving cell having uplink transmission resource available among the serving cells of the non-primary set within the current TTI (check if UL-SCH is available for a transmission on the corresponding non primary set serving cell). If there is any cell having transmission resource available, the procedure returns to step <NUM> and, otherwise, step <NUM>.

At step <NUM>, the UE checks the three conditions as follows to determine whether the SR transmission is possible through the PUCCH of the PUCCH SCell within the current TTI.

At step <NUM>, the UE compares the SR_COUNTER_NP and the dsr-TransMax NP. If the SR COUNTER NP is less than the dsr-TransMax_NP, the procedure goes to step <NUM> and, otherwise, step <NUM>.

The SR COUNTER NP denotes the number of SR transmission times of the UE, and the UE increments this variable by <NUM> whenever the SR is transmitted and initializes the variable when the SR is cancelled. The dsr-TransMax_NP is a variable for preventing the SR from being repeated infinitely and is informed to the UE through a control message such as RRC connection reconfiguration message.

If the SR COUNTER NP is not less than the dsr-TransMax_NP, the UE determines that the SR transmission has failed and takes a necessary action at step <NUM>. The action is described in detail with reference to <FIG>.

If the SR COUNTER NP is less than the dsr-TransMax_NP, the UE transmits the SR through the PUCCH of the PCell, increments the SR COUNTER NP by <NUM>, and starts sr_Prohibit_Timer_NP at step <NUM>, and then returns the procedure to step <NUM>.

<FIG> is a flowchart illustrating the UE operation when the Scheduling Request (SR) transmission has failed according to an embodiment of the present invention.

Referring to <FIG>, the SR transmission failure occurs at step <NUM>. The UE determines whether the SR transmission failure has occurred in the PCell or the PUCCH SCell at step <NUM>. That is, the UE checks whether the SR transmission failure is determined based on the result of comparison between SR COUNTER and dsr-TransMax or the result of comparison between SR COUNTER NP and dsr-TransMax_NP.

If it is determined that the SR transmission failure has occurred in the PCell (or the primary set), the procedure goes to step <NUM>. Otherwise if the SR transmission failure has occurred in the PUCCH SCell (or non-primary set), the procedure goes to step <NUM>.

If the SR transmission failure occurs in the PCell, this means that there is any problem in uplink transmission of the PCell. If the SR transmission failure occurs in the PUCCH SCell, this means that there is any problem in the uplink transmission of the PUCCH SCell. For example, the problem may be uplink transmission power configuration error. In this case, there is a need of taking an action for the corresponding set as well as the corresponding cell.

If the SR transmission failure has occurred in the PCell (or primary set), the UE releases the PUCCH transmission resource of the PCell at step <NUM> and releases the SRS transmission resource of all the serving cells of the primary set (or SRS transmission resource of the P-TAG serving cells) at step <NUM>. The UE releases the configured transmission resource, i.e. configure uplink grant and configured downlink assignment, at step <NUM>. Next, the UE starts random access in the PCell at step <NUM>. As described above, the SR transmission failure may be caused by the uplink transmission power configuration error, and the uplink transmission power may be reconfigured through power ramping in the random access procedure.

If the SR transmission failure has occurred in the PUCCH SCell (non-primary set), the UE releases the PUCCH transmission resource of the PUCCH cell at step <NUM> and releases the SRS transmission resource of all the serving cells of the corresponding non-primary set (or the SRS resource of the serving cells belonging to the same TAG as the PUCCH SCell) at step <NUM>. The UE generates an RRC control message for reporting the SR failure in the PUCCH SCell at step <NUM>. The control message may include the identifier of the PUCCH SCell in which the SR transmission failure has occurred and SR transmission power information (e.g. average or maximum value of the transmit power applied to the SR transmission or the information indicating whether the SR transmission power is greater than the maximum transmit power). The UE starts the SR transmission procedure of the PCell to transmit the RRC control message promptly and transmits the RRC control message to the primary set serving cell.

<FIG> is a block diagram illustrating a configuration of the UE according to an embodiment of the present invention.

Referring to <FIG>, the UE according to an embodiment of the present invention includes a transceiver <NUM>, a controller <NUM>, a multiplexer/demultiplexer <NUM>, a control message processor <NUM>, and various higher layer processors <NUM> and <NUM>.

The transceiver <NUM> receives data and predetermined control signals on the downlink channel of the serving cell and transmits data and predetermined control signals on the uplink channel. In the case that a plurality of serving cells is configured, the transceiver <NUM> transmits/receives data and control signals through the plural serving cells.

The multiplexer/demultiplexer <NUM> multiplexes the data generated by the higher layer processors <NUM> and <NUM> and the control message processor <NUM> and demultiplexes the data received by the transceiver <NUM>, the demultiplexed data being delivered to the higher layer processors <NUM> and <NUM> or the control message processor <NUM>.

The control message processor <NUM> is an RRC layer entity which takes an action necessary for processing the control message received from the eNB. For example, the control message processor <NUM> processes the received random access-related information and delivers the processing result to the controller.

The higher layer processors <NUM> and <NUM> are established per service. The higher layer processor processes the data generated by the user service such as File Transfer Protocol (FTP) and Voice over Internet Protocol (VoIP), the processing result being delivered to the multiplexer/demultiplexer <NUM>, and processes the data from the multiplexer/demultiplexer <NUM>, the processing result being delivered to the higher layer service application.

The controller <NUM> controls the transceiver <NUM> and the multiplexer/demultiplexer <NUM> to perform uplink transmission using appropriate resource at an appropriate timing based on the scheduling command, e.g. uplink grants, received by the transceiver <NUM>.

The controller controls overall operations associated with random access and SR transmission. In more detail, the controller performs control operations of the UE as described with reference to <FIG>. For example, the controller may control receiving an SCell add command including configuration information on the secondary serving cell (SCell) to be added from the primary serving cell (PCell), transmitting a preamble to the SCell to be added, receiving a random access response (RAR) message from the SCell to be added, and applying, when the SCell add command includes an indicator indicating that the first eNB to which the SCell to be added belongs differs from the eNB to which the PCell belongs, a parameter included in the RAR for the SCell in which the RAR has been transmitted.

<FIG> is a block diagram illustrating an eNB according to an embodiment of the present invention.

The eNB includes a transceiver <NUM>, a controller <NUM>, a multiplexer/demultiplexer <NUM>, a control message processor <NUM>, various higher layer processors <NUM> and <NUM>, and a scheduler <NUM>.

The transceiver transmits data and predetermined control signals on the downlink channel of the serving cell and receives data and predetermined control signals on the uplink channel. In the case that a plurality of carriers is configured, the transceiver <NUM> transmits/receives data and control signals through the plural carriers.

The multiplexer/demultiplexer <NUM> is responsible for multiplexing data generated by the higher layer processors <NUM> and <NUM> and the control message processor <NUM> or demultiplexing the data received by the transceiver <NUM>, the demultiplexed data being delivered to the control message processor <NUM> or the controller <NUM>. The control message processor <NUM> processes the control message transmitted by the UE and takes a necessary action or generates a control message to be transmitted to the UE, the generated control message being delivered to the lower layer.

The higher layer processors <NUM> and <NUM> are established per service and processes the data from the S-GW or other eNB into RLC PDU, the RLC PDU being delivered to the multiplexer/demultiplexer <NUM>, and processes the RLC PDU from the multiplexer/demultiplexer <NUM> into PDCP SDU, the PDCP SDU being transmitted to the S-GW or other eNB.

The scheduler allocates transmission resource to the UE at an appropriate timing in consideration of the UE buffer status and channel status and controls the transceiver to process the signal to be transmitted to the UE and transmit the signal.

The controller controls overall operations associated with the random access and SR transmission. In more detail, the controller performs control operations of the eNB as described with reference to <FIG>. For example, the controller may control transmitting a SCell add command including configuration information on the secondary serving cell (SCell) to be added to the UE. The controller also may control receiving the preamble transmitted by the UE and transmitting, when the first eNB to which the SCell to be added belongs differs from the second eNB to which the PCell belongs, a random access response (RAR) message to the UE.

Although preferred embodiments of the invention have been described using specific terms, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense in order to help understand the present invention. It is obvious to those skilled in the art that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention.

Claim 1:
A method performed by a terminal in a wireless communication system, the method comprising:
identifying (<NUM>) that a random access problem has occurred, in case that a number of transmissions of a random access preamble by the terminal is greater than a maximum transmission number;
performing (<NUM>) a first procedure in case that the random access preamble is transmitted on a primary cell of a first cell group and the random access problem has occurred, the first cell group being a set of serving cells controlled by a first base station that controls the primary cell; and
performing (<NUM>) a second procedure in case that the random access preamble is transmitted on a cell configured with a physical uplink control channel, PUCCH, of a second cell group and the random access problem has occurred, the second cell group being a set of serving cells controlled by a second base station that does not control the primary cell,
wherein performing (<NUM>) the first procedure further comprises initiating a radio resource control, RRC, connection re-establishment procedure, and
wherein performing (<NUM>) the second procedure comprises: transmitting, on the first cell group, information to report a problem associated with a connection of the second cell group, the information including a type of the problem.