JP6224452B2 - User terminal, method, and processor - Google Patents

Description

  The present invention relates to a user terminal, a method, and a processor used in a mobile communication system.

  In LTE (Long Term Evolution) whose specifications are established in 3GPP (3rd Generation Partnership Project), which is a standardization project for mobile communication systems, a random access procedure is used as a procedure for a user terminal to establish a connection with a base station. It is prescribed. The random access procedure includes a contention based and a non-contention based.

  The contention-based random access procedure is used when the non-contention based random access procedure cannot be used, and includes the following four steps.

  As a first step, the user terminal selects any preamble sequence from among a preamble sequence group that can be used for contention-based random access trials, and transmits a random access preamble to the base station using the selected preamble sequence. The random access preamble does not include the user terminal identifier (terminal identifier).

  As a second step, the base station that has received the random access preamble transmits a random access response to the user terminal. The random access response includes a preamble identifier indicating a preamble sequence of a random access preamble received by the base station.

  As a third step, the user terminal that has received a random access response including a preamble identifier corresponding to the random access preamble transmits a connection request message to the base station. The connection request message includes a terminal identifier.

  As a fourth step, the base station that has received the connection request message transmits a contention resolution message to the user terminal. The contention resolution message includes a terminal identifier included in the connection request message received by the base station.

3GPP Technical Specification “TS36.300 V11.7.0” September 2013

  By the way, in the contention based random access procedure, a plurality of user terminals can transmit a random access preamble using the same preamble sequence. Such a situation is called preamble contention (or preamble collision).

  In this case, a plurality of user terminals related to preamble contention respond to one random access response transmitted from the base station, and transmit a plurality of connection request messages to the base station. The base station includes, for example, the terminal identifier included in the connection request message received first in the contention resolution message. As a result, the user terminal that first transmitted the connection request message among the plurality of user terminals related to the preamble contention establishes a connection with the base station.

  On the other hand, a user terminal that is not specified by the contention resolution message starts the random access procedure again from the first step, and preamble contention may occur in the second random access procedure.

  Accordingly, the contention-based random access procedure can increase the time required for the user terminal to establish a connection with the base station (connection processing delay). In particular, it is a serious problem that the connection processing delay becomes long when an emergency call is made.

  Therefore, the present invention aims to improve the contention based random access procedure.

  The user terminal according to the first feature transmits a random access preamble for performing a contention based random access attempt to the base station. The user terminal includes a control unit that performs control to transmit a plurality of random access preambles having different preamble sequences to the base station, triggered by a predetermined event that should increase the success rate of the random access trial.

  The method according to the second feature is a method in a user terminal that transmits a random access preamble for performing a contention based random access attempt to a base station. The method includes a step of transmitting a plurality of random access preambles having different preamble sequences to the base station triggered by a predetermined event that should increase a success rate of the random access attempts.

  A processor according to a third feature is provided in a user terminal that transmits a random access preamble for performing a contention based random access attempt to a base station. The processor performs a process of transmitting a plurality of random access preambles having different preamble sequences to the base station, triggered by a predetermined event that should increase the success rate of the random access attempt.

  According to the present invention, the contention based random access procedure can be improved.

It is a block diagram of the LTE system which concerns on embodiment. It is a block diagram of UE which concerns on embodiment. It is a block diagram of eNB which concerns on embodiment. It is a protocol stack figure of the radio | wireless interface in a LTE system. It is a block diagram of the radio | wireless frame used with a LTE system. It is a sequence diagram which shows a contention based random access procedure. It is a figure for demonstrating the operation | movement outline | summary which concerns on embodiment. It is a flowchart which shows the operation | movement flow of UE which concerns on embodiment. It is a sequence diagram which shows the operation | movement sequence which concerns on embodiment.

[Outline of Embodiment]
The user terminal according to the embodiment transmits a random access preamble for performing a contention based random access attempt to the base station. The user terminal includes a control unit that performs control to transmit a plurality of random access preambles having different preamble sequences to the base station, triggered by a predetermined event that should increase the success rate of the random access trial.

  In an embodiment, the predetermined event is an outgoing emergency call.

  In the first modification of the embodiment, the predetermined event is a failure of the previous random access attempt.

  In the embodiment, when a plurality of random access responses corresponding to the plurality of random access preambles are received from the base station, the control unit sends a plurality of connection request messages corresponding to the plurality of random access responses to the base station. Control to send to.

  In an embodiment, when a plurality of contention resolution messages specifying terminal identifiers of the user terminals are received from the base station after transmitting the plurality of connection request messages, the control unit corresponds to one contention resolution message. While leaving the connection, control is performed to release the connection corresponding to another contention resolution message.

  In the second modification of the embodiment, when any one of the plurality of connection request messages is selected by the base station and a contention resolution message for the selected connection request message is received from the base station, the control is performed. The unit establishes a connection corresponding to the contention resolution message.

  The method according to the embodiment is a method in a user terminal that transmits a random access preamble for performing a contention-based random access attempt to a base station. The method includes a step of transmitting a plurality of random access preambles having different preamble sequences to the base station triggered by a predetermined event that should increase a success rate of the random access attempts.

  A processor according to an embodiment is provided in a user terminal that transmits a random access preamble for performing a contention-based random access attempt to a base station. The processor performs a process of transmitting a plurality of random access preambles having different preamble sequences to the base station, triggered by a predetermined event that should increase the success rate of the random access attempt.

[Embodiment]
In the following, an embodiment when the present invention is applied to an LTE system will be described.

(System configuration)
FIG. 1 is a configuration diagram of an LTE system according to the embodiment. The LTE system according to the embodiment supports packet-switched voice communication (VoLTE: Voice over LTE).

  As shown in FIG. 1, the LTE system according to the embodiment includes a UE (User Equipment) 100, an E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) 10, an EPC (Evolved Packet Core) 20, and a PDN (PackNet). ) 30.

  UE100 is corresponded to a user terminal. The UE 100 is a mobile communication device, and performs wireless communication with a connection destination cell (serving cell). The configuration of the UE 100 will be described later.

  The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10 includes an eNB 200 (evolved Node-B). The eNB 200 corresponds to a base station. The eNB 200 is connected to each other via the X2 interface. The configuration of the eNB 200 will be described later.

  The eNB 200 manages one or a plurality of cells, and performs radio communication with the UE 100 that has established a connection with the own cell. The eNB 200 has a radio resource management (RRM) function, a user data routing function, a measurement control function for mobility control / scheduling, and the like. “Cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE 100.

  The EPC 20 corresponds to a core network. The EPC 20 includes an MME (Mobility Management Entity) / S-GW (Serving-Gateway) 300. The MME performs various mobility controls for the UE 100. The S-GW performs user data transfer control. The MME / S-GW 300 is connected to the eNB 200 via the S1 interface. Moreover, the EPC 20 includes a PCRF (Policy and Charging Rules Function) / P-GW (PDN Gateway) 400. The PCRF performs QoS control and charging control. The P-GW is a connection point with the PDN 30 and controls user data transfer.

  The PDN 30 corresponds to an IMS (IP Multimedia Subsystem) for the IP multimedia service. The PDN 30 provides a voice call service using SIP.

  FIG. 2 is a block diagram of the UE 100. As shown in FIG. 2, the UE 100 includes an antenna 101, a radio transceiver 110, a user interface 120, a GNSS (Global Navigation Satellite System) receiver 130, a battery 140, a memory 150, and a processor 160. The memory 150 and the processor 160 constitute a control unit. The UE 100 may not have the GNSS receiver 130. Further, the memory 150 may be integrated with the processor 160, and this set (that is, a chip set) may be used as the processor 160 '.

  The antenna 101 and the wireless transceiver 110 are used for transmitting and receiving wireless signals. The radio transceiver 110 converts the baseband signal (transmission signal) output from the processor 160 into a radio signal and transmits it from the antenna 101. Further, the radio transceiver 110 converts a radio signal received by the antenna 101 into a baseband signal (received signal) and outputs the baseband signal to the processor 160.

  The user interface 120 is an interface with a user who owns the UE 100, and includes, for example, a display, a microphone, a speaker, and various buttons. The user interface 120 receives an operation from the user and outputs a signal indicating the content of the operation to the processor 160. The GNSS receiver 130 receives a GNSS signal and outputs the received signal to the processor 160 in order to obtain location information indicating the geographical location of the UE 100. The battery 140 stores power to be supplied to each block of the UE 100.

  The memory 150 stores a program executed by the processor 160 and information used for processing by the processor 160. The processor 160 includes a baseband processor that modulates / demodulates and encodes / decodes a baseband signal, and a CPU (Central Processing Unit) that executes programs stored in the memory 150 and performs various processes. . The processor 160 may further include a codec that performs encoding / decoding of an audio / video signal. The processor 160 executes various processes and various communication protocols described later.

  FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, the eNB 200 includes an antenna 201, a radio transceiver 210, a network interface 220, a memory 230, and a processor 240.

  The antenna 201 and the wireless transceiver 210 are used for transmitting and receiving wireless signals. The radio transceiver 210 converts the baseband signal (transmission signal) output from the processor 240 into a radio signal and transmits it from the antenna 201. In addition, the radio transceiver 210 converts a radio signal received by the antenna 201 into a baseband signal (received signal) and outputs the baseband signal to the processor 240.

  The network interface 220 is connected to the neighboring eNB 200 via the X2 interface and is connected to the MME / S-GW 300 via the S1 interface. The network interface 220 is used for communication performed on the X2 interface and communication performed on the S1 interface.

  The memory 230 stores a program executed by the processor 240 and information used for processing by the processor 240. The processor 240 includes a baseband processor that performs modulation / demodulation and encoding / decoding of a baseband signal, and a CPU that executes a program stored in the memory 230 and performs various processes. The processor 240 executes various processes and various communication protocols described later.

  FIG. 4 is a protocol stack diagram of a radio interface in the LTE system. As shown in FIG. 4, the radio interface protocol is divided into the first to third layers of the OSI reference model, and the first layer is a physical (PHY) layer. The second layer includes a MAC (Media Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The third layer includes an RRC (Radio Resource Control) layer.

  The physical layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Between the physical layer of UE100 and the physical layer of eNB200, user data and a control signal are transmitted via a physical channel.

  The MAC layer performs data priority control, retransmission processing by hybrid ARQ (HARQ), random access procedure at the time of establishing RRC connection, and the like. Between the MAC layer of the UE 100 and the MAC layer of the eNB 200, user data and control signals are transmitted via a transport channel. The MAC layer of the eNB 200 includes a scheduler that determines an uplink / downlink transport format (transport block size, modulation / coding scheme) and an allocation resource block to the UE 100. Details of the random access procedure will be described later.

  The RLC layer transmits data to the RLC layer on the receiving side using the functions of the MAC layer and the physical layer. Between the RLC layer of the UE 100 and the RLC layer of the eNB 200, user data and control signals are transmitted via a logical channel.

  The PDCP layer performs header compression / decompression and encryption / decryption.

  The RRC layer is defined only in the control plane that handles control signals. Control signals (RRC messages) for various settings are transmitted between the RRC layer of the UE 100 and the RRC layer of the eNB 200. The RRC layer controls the logical channel, the transport channel, and the physical channel according to establishment, re-establishment, and release of the radio bearer. When there is a connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 is in the RRC connected state, and otherwise, the UE 100 is in the RRC idle state.

  A NAS (Non-Access Stratum) layer located above the RRC layer performs session management, mobility management, and the like.

  FIG. 5 is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Division Multiplexing Access) is applied to the downlink and SC-FDMA (Single Carrier Frequency Multiple Access) is applied to the uplink.

  As shown in FIG. 5, the radio frame is composed of 10 subframes arranged in the time direction. Each subframe is composed of two slots arranged in the time direction. The length of each subframe is 1 ms, and the length of each slot is 0.5 ms. Each subframe includes a plurality of resource blocks (RB) in the frequency direction and includes a plurality of symbols in the time direction. Each resource block includes a plurality of subcarriers in the frequency direction. Among radio resources allocated to the UE 100, a frequency resource can be specified by a resource block, and a time resource can be specified by a subframe (or slot).

  In the downlink, the section of the first few symbols of each subframe is an area mainly used as a physical downlink control channel (PDCCH) for transmitting a control signal. The remaining section of each subframe is an area that can be used as a physical downlink shared channel (PDSCH) mainly for transmitting user data.

  In the uplink, both ends in the frequency direction in each subframe are regions used mainly as physical uplink control channels (PUCCH) for transmitting control signals. Also, the 6 resource blocks in the center in the frequency direction in each subframe are areas that can be used as physical random access channels (PRACH) for transmitting random access preambles. The other part in each subframe is an area that can be used mainly as a physical uplink shared channel (PUSCH) for transmitting user data.

(Contention based random access procedure)
Prior to establishing an RRC connection with the eNB 200, the UE 100 performs random access to the eNB 200 in the MAC layer.

  FIG. 6 is a sequence diagram showing a contention based random access procedure. The contention based random access procedure is used when the non-contention based random access procedure cannot be used, and includes the following four steps (steps S1 to S4).

  As shown in FIG. 6, in step S1, the UE 100 randomly selects one of the preamble sequences from a group of preamble sequences that can be used for contention-based random access attempts. Information on preamble sequence groups that can be used for contention-based random access attempts is included in system information broadcast by the eNB 200. UE 100 transmits a random access preamble (Random Access Preamble) to eNB 200 using the selected preamble sequence by RACH (Random Access Channel). The random access preamble does not include the identifier (terminal identifier) of the UE 100.

  In step S <b> 2, the eNB 200 that has received the random access preamble transmits a random access response (Random Access Response) to the UE 100. Here, eNB200 estimates the uplink delay between UE100 based on the random access preamble received from UE100. Moreover, eNB200 determines the radio | wireless resource allocated to UE100. The random access response includes a timing correction value based on a delay estimation result, information on the determined assigned radio resource, a preamble sequence identifier (preamble identifier) received from the UE 100, and the like.

  In step S3, the UE 100 that has received a random access response including a preamble identifier corresponding to the random access preamble transmits a connection request message (RRC Connection Request) to the eNB 200. The connection request message is a message transmitted / received in the RRC layer, and may be referred to as a message 3 (Msg3) or a scheduled transmission. The connection request message includes a terminal identifier. The terminal identifier is a TMSI (Temporary Mobile Subscriber Identity) unique to each UE.

  In step S4, the eNB 200 that has received the connection request message transmits a contention resolution message (Contention Resolution) to the UE 100. The contention resolution message is a message transmitted and received at the RRC layer, and may be referred to as message 4 (Msg4). The contention resolution message includes a terminal identifier included in the connection request message received by the eNB 200. Specifically, the conflict resolution message includes the connection request message itself as a conflict resolution ID. The UE 100 determines that the random access procedure is completed by receiving a contention resolution message including the connection request message (contention resolution ID) transmitted by itself.

  In the contention-based random access procedure described above, a plurality of UEs 100 can transmit a random access preamble using the same preamble sequence. Such a situation is called preamble contention (or preamble collision).

  In this case, the plurality of UEs 100 related to the preamble contention respond to one random access response transmitted from the eNB 200 and transmit a plurality of connection request messages to the eNB 200. The eNB 200 includes, for example, the terminal identifier included in the connection request message received first in the conflict resolution message. As a result, among the plurality of UEs 100 related to the preamble contention, the UE 100 that first transmitted the connection request message establishes a connection with the eNB 200.

  On the other hand, the UE 100 that is not specified by the contention resolution message, that is, the UE 100 that cannot normally receive the contention resolution message, restarts the random access procedure from step S1 after the elapse of a predetermined time (referred to as “backoff time”). become. Preamble contention may also occur in the second random access procedure. Therefore, in the contention based random access procedure, the time required for the UE 100 to establish a connection with the eNB 200 (that is, a connection processing delay) can be long. In particular, when an emergency call by VoLTE (hereinafter referred to as “VoLTE emergency call”) is originated, a long connection processing delay is a serious problem.

(Operation according to the embodiment)
Next, operations according to the embodiment will be described.

(1) Operation Overview FIG. 7 is a diagram for explaining an operation overview according to the embodiment.

  As shown in FIG. 7, a plurality of UEs (UEs 100-1 to 100-3) in an RRC idle state are located within the coverage area of the eNB 200, and the plurality of UEs 100 perform contention-based random access procedures to the eNB 200. Assume that the situation is performed simultaneously. The UE 100-1 is a UE that attempts to make a VoLTE emergency call to a called terminal provided in an organization (emergency call receiving organization) such as the police, fire department, or emergency. The UEs 100-2 and 100-3 are UEs that intend to perform communication other than the VoLTE emergency call.

  Each of the UEs 100-1 to 100-3 transmits a random access preamble for performing a contention based random access attempt to the eNB 200. The UE 100-1 transmits, to the eNB 200, a plurality of random access preambles having different preamble sequences, triggered by a predetermined event that should increase the success rate of the random access attempt. In an embodiment, the predetermined event that should increase the success rate of random access attempts is the origination of a VoLTE emergency call.

  UE100-1 which is going to transmit a VoLTE emergency call transmits several random access preambles from which a preamble series differs to eNB200. On the other hand, each of the UEs 100-2 and 100-3 trying to perform communication other than the VoLTE emergency call transmits one random access preamble to the eNB 200.

  In this way, the UE 100-1 transmits a plurality of random access preambles to the eNB 200, so that even if some of the plurality of random access preambles compete with the UEs 100-2 and 100-3, The access preamble does not compete with UEs 100-2 and 100-3. Therefore, UE100-1 can establish RRC connection with eNB200 by the said other random access preamble. Therefore, it is possible to prevent the connection processing delay of the UE 100-1 attempting to make a VoLTE emergency call from becoming long.

  In the embodiment, when receiving a plurality of random access responses corresponding to a plurality of random access preambles from the eNB 200, the UE 100-1 transmits a plurality of connection request messages corresponding to the plurality of random access responses to the eNB 200. Since the UE 100-1 cannot determine the success or failure of the random access procedure until the contention resolution message is normally received, it is necessary to transmit a plurality of connection request messages when receiving a plurality of random access responses from the eNB 200.

  In the embodiment, when the UE 100-1 receives a plurality of contention resolution messages specifying the terminal identifier of the UE 100-1 from the eNB 200 after transmitting the plurality of connection request messages, the UE 100-1 creates a connection corresponding to one contention resolution message. Release connections corresponding to other conflict resolution messages while leaving. In the current specification, it is not assumed that one UE 100 establishes a plurality of RRC connections at the same time. By leaving only the connection corresponding to one contention resolution message, an unexpected error can be prevented. it can.

(2) Operation Flow FIG. 8 is a flowchart showing an operation flow of the UE 100 according to the embodiment. In the initial state of this flow, it is assumed that the UE 100 is in the RRC idle state.

  As illustrated in FIG. 8, in step S <b> 101, the processor 160 of the UE 100 determines whether or not an emergency call transmission operation has been performed on the user interface 120. When there is no emergency call transmission operation (step S101; NO), in step S102, the processor 160 performs a normal operation.

  If there is an emergency call transmission operation (step S101; YES), in step S103, the processor 160 performs a contention based random access procedure using a plurality of random access preambles. The sequence of the contention based random access procedure will be described later.

  In step S104, the processor 160 determines whether or not a plurality of RRC connections have been established by a contention based random access procedure using a plurality of random access preambles. When a plurality of RRC connections are established (step S104; YES), in step S105, the processor 160 releases another RRC connection while leaving one RRC connection.

  At step S106, the processor 160 originates a VoLTE emergency call using the established RRC connection. In step S107, when a call connection (session) is established by calling a VoLTE emergency call, the processor 160 transmits and receives voice data using the established call connection.

(2) Operation Flow FIG. 9 is a sequence diagram showing an operation sequence according to the embodiment. Here, differences from the above-described operation sequence will be mainly described.

  As shown in FIG. 9, in step S11, the UE 100-1 accepts an emergency call transmission operation.

  In step S12, the UE 100-1 transmits a plurality of random access preambles having different preamble sequences to the eNB 200 using an emergency call transmission operation as a trigger. For example, the UE 100-1 randomly selects a plurality of preamble sequences from a group of preamble sequences that can be used for contention-based random access trials, and transmits the plurality of random access preambles to the eNB 200 using the selected plurality of preamble sequences. .

  In FIG. 9, the UE 100-1 uses the first random access preamble (step S12-1) using the preamble sequence A, the second random access preamble (step S12-2) using the preamble sequence B, and the preamble sequence. The case where the 3rd random access preamble (step S12-3) using C is continuously transmitted to eNB200 is illustrated. The transmission intervals of these multiple random access preambles may be defined in advance, or may be selected randomly by the UE 100-1.

  Here, the description will be made assuming that the second random access preamble using the preamble sequence B competes with a random access preamble transmitted by another UE (UE 100-2 or UE 100-3).

  In step S13, the eNB 200 that has received the plurality of random access preambles from the UE 100-1 transmits a plurality of random access responses to the plurality of random access preambles to the UE 100-1. In FIG. 9, the eNB 200 performs the first random access response (step S13-1) for the first random access preamble, the second random access response (step S13-2) for the second random access preamble, and the third Exemplifies a case of transmitting a third random access response (step S13-3) to the random access preamble to the UE 100-1.

  In step S14, the UE 100-1 that has received a plurality of random access responses from the eNB 200 transmits a plurality of connection request messages (Msg3) corresponding to the plurality of random access responses to the eNB 200. In FIG. 9, the UE 100-1 receives a first connection request message (step S14-1) corresponding to the first random access response, and a second connection request message (step S14-) corresponding to the second random access response. 2) and a case where the third connection request message (step S14-3) corresponding to the third random access response is transmitted to the eNB 200.

  Here, the second random access preamble using the preamble sequence B is competing with the random access preamble transmitted by the other UE, and the eNB 200 first receives the connection request message from the other UE. The description will be made assuming that the second connection request message is not selected by the eNB 200. Here, the contention of the second random access preamble and the random access preamble transmitted by another UE is that the preamble sequence B of the second random access preamble and the preamble sequence of the random access preamble transmitted by the other UE are different. Occurs when they are the same series.

  In step S15, the eNB 200 that has received a plurality of connection request messages from the UE 100-1 has a contention resolution message (Msg4) for a connection request message corresponding to a random access preamble in which no contention has occurred among the plurality of connection request messages. Is transmitted to UE100-1. In FIG. 9, the eNB 200 transmits the first contention resolution message (step S15-1) and the third connection for the first connection request message without transmitting the second contention resolution message for the second connection request message. The case where the 3rd contention resolution message (step S15-3) with respect to a request message is transmitted to UE100-1 is illustrated.

  In step S <b> 16, the UE 100-1 receives the contention resolution message including the connection request message transmitted by itself, and establishes an RRC connection corresponding to the contention resolution message with the eNB 200. In FIG. 9, the UE 100-1 establishes with the eNB 200 a first RRC connection corresponding to the first contention resolution message and a third RRC connection corresponding to the third contention resolution message.

  In step S <b> 17, the UE 100-1 releases the other RRC connection while leaving one RRC connection among the plurality of established RRC connections. As a rule for selecting an RRC connection to be released, for example, a rule for releasing an RRC connection established later or a rule for releasing an RRC connection having a lower quality (reception field strength, etc.) of a received contention resolution message is used. Can do. FIG. 9 illustrates a case where the UE 100-1 releases the third RRC connection while leaving the first RRC connection.

  In step S18, the UE 100-1 transmits a VoLTE emergency call using the established RRC connection (first RRC connection).

(Summary of embodiment)
As described above, when the UE 100-1 attempting to make a VoLTE emergency call transmits a plurality of random access preambles to the eNB 200, a part of the plurality of random access preambles competes with another UE. However, other random access preambles do not compete with other UEs. Therefore, UE100-1 can establish RRC connection with eNB200 by the said other random access preamble. Therefore, it is possible to prevent the connection processing delay of the UE 100-1 from becoming long.

[Modification 1]
In the above-described embodiment, the predetermined event serving as a trigger for transmitting a plurality of random access preambles is a VoLTE emergency call. However, the trigger for transmitting a plurality of random access preambles is not limited to the transmission of a VoLTE emergency call, but may be other events. The number of random access preambles to be transmitted may be changed according to the importance of transmission. For example, when the destination is an emergency organization (such as the police), the number of random access preambles to be transmitted is 3, and then when the destination is a destination registered in the memory 150 as an important destination, transmission is performed. The number of random access preambles is 2, and when the destination is not an emergency organization (such as the police) or a destination registered as an important destination, the number of random access preambles to be transmitted may be 1.

  In the first modification of the embodiment, the predetermined event serving as a trigger for transmitting a plurality of random access preambles is a failure of the previous random access attempt. That is, the UE 100 transmits a plurality of random access preambles to the eNB 200 in the same manner as in the above-described embodiment, triggered by the previous random access procedure being unsuccessful and performing the random access procedure again. As a result, the success rate of the random access trial to be performed anew can be increased, so that it is possible to prevent the connection processing delay from becoming long.

  In the first modification of the embodiment, the number of random access preambles to be transmitted may be increased according to an increase in the number of times the random access procedure has failed continuously. For example, the first random access procedure fails and two random access preambles are transmitted when the second random access procedure is performed. Then, the second random access procedure also fails, and three random access preambles are transmitted when the third random access procedure is performed. Thereby, the success rate of a random access trial can be raised in steps. Here, instead of increasing the random access preamble one by one, the random access preamble may be increased in accordance with a magnification (for example, two times) according to the destination.

[Modification 2]
In the embodiment described above, when receiving a plurality of connection request messages including the same terminal identifier, the eNB 200 transmits a plurality of contention resolution messages for the plurality of connection request messages.

  However, when the eNB 200 receives a plurality of connection request messages including the same terminal identifier, the eNB 200 selects one of the plurality of connection request messages and transmits a contention resolution message for the selected connection request message. Also good.

  In the second modification of the embodiment, when any one of the plurality of connection request messages is selected by the eNB 200, and the UE 100 receives the contention resolution message for the selected connection request message from the eNB 200, the UE 100 Establish only the connection corresponding to the resolution message. Thereby, it can avoid beforehand that UE100 establishes a some RRC connection.

[Other Embodiments]
In the above-described embodiment, a case where a plurality of RRC connections are not established / maintained between the UE 100-1 and the eNB 200 is illustrated, but the specification is such that a plurality of RRC connections can be established / maintained between the UE 100-1 and the eNB 200. If the change is made, a plurality of RRC connections may be established and maintained between the UE 100-1 and the eNB 200.

  In the above-described embodiment, the case where the present invention is applied to the LTE system is mainly described. However, the present invention is not limited to the LTE system, and the present invention may be applied to a system other than the LTE system.

  DESCRIPTION OF SYMBOLS 10 ... E-UTRAN, 20 ... EPC, 30 ... PDN, 100 ... UE, 101 ... Antenna, 110 ... Radio transceiver, 120 ... User interface, 130 ... GNSS receiver, 140 ... Battery, 150 ... Memory, 160 ... Processor 200 ... eNB 201 ... antenna 210 ... radio transceiver 220 ... network interface 230 ... memory 240 ... processor 300 ... MME / S-GW 400 ... PCRF / P-GW

Claims (6)

A user terminal that transmits a random access preamble for performing contention based random access attempts to a base station,
A control unit that performs control to transmit a plurality of random access preambles having different preamble sequences to the base station using a predetermined event that should increase the success rate of the random access trial as a trigger ,
When a plurality of random access responses corresponding to the plurality of random access preambles are received from the base station, the control unit transmits a plurality of connection request messages corresponding to the plurality of random access responses to the base station The user terminal characterized by performing .   The user terminal according to claim 1, wherein the predetermined event is an emergency call.   The user terminal according to claim 1, wherein the predetermined event is a failure of a previous random access attempt. When any one of the plurality of connection request messages is selected by the base station and a contention resolution message for the selected connection request message is received from the base station, the control unit adds the contention resolution message to the contention resolution message. 4. The user terminal according to claim 1, wherein a corresponding connection is established. A method in a user terminal for transmitting a random access preamble to a base station for performing contention based random access attempts, comprising:
Sending a plurality of random access preambles having different preamble sequences to the base station triggered by a predetermined event that should increase the success rate of the random access attempts ;
Transmitting a plurality of connection request messages corresponding to the plurality of random access responses to the base station when receiving a plurality of random access responses corresponding to the plurality of random access preambles from the base station. A method characterized by. A processor provided in a user terminal that transmits a random access preamble for performing a contention based random access attempt to a base station,
A process of transmitting a plurality of random access preambles having different preamble sequences to the base station, triggered by a predetermined event that should increase the success rate of the random access trial ,
When a plurality of random access responses corresponding to the plurality of random access preambles are received from the base station, a process of transmitting a plurality of connection request messages corresponding to the plurality of random access responses to the base station is performed. Processor.

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