Patent Description:
To satisfy a wireless data traffic demand which is growing after a 4th generation (<NUM>) communication system is commercialized, efforts are exerted to develop an advanced 5th generation (<NUM>) communication system or a pre-<NUM> communication system. For this reason, the <NUM> communication system or the pre-<NUM> communication system is referred to as a beyond <NUM> network communication system or a post long term evolution (LTE) system.

To achieve a high data rate, the <NUM> communication system considers its realization in an extremely high frequency (mmWave) band (e.g., <NUM> band). To mitigate a path loss of propagation and to extend a propagation distance in the extremely high frequency band, the <NUM> communication system is discussing beamforming, massive multiple input multiple output (MIMO), full dimensional (FD)-MIMO, array antenna, analog beam-forming, and large scale antenna techniques.

Also, for network enhancement of the system, the <NUM> communication system is developing techniques such as evolved small cell, advanced small cell, cloud radio access network (RAN), ultra-dense network, device to device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), and receive interference cancellation.

Besides, the <NUM> system is working on hybrid frequency shift keying and quadrature amplitude modulation (FQAM) and sliding window superposition coding (SWSC) as advanced coding modulation (ACM) schemes, and filter bank multi carrier (FBMC), non orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as advanced access technologies.

In the <NUM> system, various network structures are discussed for efficiency of the system operation. For example, network virtualization, split of a base station into a central unit (CU) and a distributed unit (DU) are discussed. Accordingly, an operating scheme adequate for the new network structure requires further studies.

A prior art of 3GPP TSG-RAN WG3 Meeting #97bis, R3-<NUM> concerns discussions on initial access procedure within high-layer split including a FINAL DL RRC MESSAGE TRANSFER, to convey RRC message. This document is a document under Art. <NUM>(<NUM>) EPC.

Another prior art of <CIT> concerns a distributed unit rejecting a user connection request. This document is prior art according to Article <NUM>(<NUM>) EPC.

Based on the discussions described above, the present disclosure provides an apparatus and a method for efficiently managing connections in a wireless communication system.

Also, the the present disclosure provides an apparatus and a method for managing connections in an environment where a base station is split into a central unit (CU) and a distributed unit (DU) in a wireless communication system.

Also, the the present disclosure provides an apparatus and a method for processing a control message according to a resource status of a distributed unit of a base station in a wireless communication system.

A first aspect of the present invention is a method performed by a distributed unit, DU, of a base station, comprising: receiving, from a user equipment, UE, a radio resource control, RRC, message comprising a resume request wherein the RRC message indicates a transition to an RRC connected state from an RRC inactive state; determining whether the resume request of the UE is acceptable or not according to a status of layers managed by the DU; in case that the resume request of the UE is acceptable in the DU: transmitting, to a central unit, CU, an initial uplink, UL, RRC message transfer message including the RRC container including the RRC message, the initial UL RRC message transfer message further including an information element to indicate that the resume request of the UE is acceptable in the DU, transmitting, to the CU, a UE context setup response message including resource allocation information for a data radio bearer, DRB, in response to receiving, from the CU, a UE context setup request message for the DRB, and transmitting, to the UE, an RRC connection resume message based on a downlink, DL, RRC message transfer message, in response to receiving, from the CU, the DL RRC message transfer message including the RRC connection resume message based on success of a UE context retrieval by the CU; transmitting, to the UE, a RRC connection setup request message based on the DL RRC message transfer message, in response to receiving, from the CU, the DL RRC message transfer message including the RRC connection setup request message based on failure of a UE context retrieval by the CU; and in case that the resume request of the UE is not acceptable in the DU: transmitting, to the CU, an initial UL RRC message transfer message including an RRC container including the RRC message, the initial UL RRC message transfer message not including the information element, and transmitting, to the UE, an RRC connection reject message based on a DL RRC message transfer message, in response to receiving, from the CU, the DL RRC message transfer message including RRC connection reject message for the resume request.

Preferably, whether the resume request of the UE is acceptable or not is determined according to resource allocation of a media access control, MAC, layer and a physical, PHY, layer for a signaling radio bearer, SRB, in the DU.

Preferably, the initial UL RRC message transfer message includes: information on a cell-radio network temporary identifier, C-RNTI, of the terminal, and information on a user equipment, UE, F1 application identity, UE F1AP ID, for the DU.

A second aspect of the present invention is a method performed by a central unit, CU, of a base station, comprising: receiving, from a distributed unit, DU, an initial uplink, UL, radio resource control, RRC, message transfer message including an RRC container including an RRC message comprising a resume request wherein the RRC message indicates a transition to an RRC connected state from an RRC inactive state; in case that an information element to indicate that the resume request of the UE is acceptable in the DU is not included in the initial UL RRC message transfer message: identifying that the resume request of the UE is not acceptable in the DU, and transmitting, to the DU, a downlink, DL, RRC message transfer message including an RRC connection reject message for the resume request; in case that the information element is not included in the initial UL RRC message transfer message: identifying that a UE context retrieval is successful and DRB resumption is needed, based on the initial UL RRC message transfer message, transmitting, to the DU, a UE context setup request message for a data radio bearer, DRB, receiving, from the DU, a UE context setup response message including a configuration of the DRB; and transmitting, to the DU, a DL RRC message transfer message including an RRC connection resume message; and in case that a UE context retrieval fails, and the information element is included in the initial UL RRC message transfer message: transmitting, to the DU, a DL RRC message transfer message including RRC connection setup request message based on the failure of the UE context retrieval.

A third aspect of the present invention is an apparatus of a distributed unit, DU, comprising: at least one transceiver; and at least one processor configured to implement the first aspect of the present invention.

A fourth aspect of the present invention is an apparatus of a central unit, CU, comprising: at least one transceiver; and at least one processor configured to implement the second aspect of the present invention.

An apparatus and method according to various embodiments of the present disclosure may efficiently manage connections, by processing a control message according to a resource status of a distributed unit of a base station.

Effects obtainable from the present disclosure are not limited to the above-mentioned effects, and other effects which are not mentioned may be clearly understood by those skilled in the art of the present disclosure through the following descriptions.

Terms used in the present disclosure are used for describing particular embodiments and are not intended to limit the scope of other embodiments. A singular form may include a plurality of forms unless it is explicitly differently represented. All the terms used herein, including technical and scientific terms, may have the same meanings as terms generally understood by those skilled in the art to which the present disclosure pertains. Among terms used in the present disclosure, the terms defined in a general dictionary may be interpreted to have the same or similar meanings with the context of the relevant art, and, unless explicitly defined in this disclosure, it shall not be interpreted ideally or excessively as formal meanings. In some cases, even terms defined in this disclosure should not be interpreted to exclude the embodiments of the present disclosure.

In various embodiments of the present disclosure to be described below, a hardware approach will be described as an example. However, since the various embodiments of the present disclosure include a technology using both hardware and software, the various embodiments of the present disclosure do not exclude a software-based approach.

Hereafter, the present disclosure relates to an apparatus and a method for managing connections in a wireless communication system. Specifically, the present disclosure explains a technique for managing connections in an environment where a base station is split into a central unit (CU) and a distributed unit (DU) in the wireless communication system.

Terms indicating signals, terms indicating channels, terms indicating control information, terms indicating network entities, terms indicating components of an apparatus, and terms indicating message and information elements in the message, which are used in the following descriptions, are for the sake of explanations. Accordingly, the present disclosure is not limited to the terms to be described, and may use other terms having technically identical meaning.

In addition, the present disclosure describes various embodiments using terms used in some communication standards (e.g., 3rd generation partnership project (3GPP)), which are merely exemplary for explanations. Various embodiments of the present disclosure may be easily modified and applied in other communication systems.

<FIG> illustrates a wireless communication system according to various embodiments of the present disclosure. <FIG> depicts a base station <NUM>, a terminal <NUM>, and a terminal <NUM>, as some of nodes which use a radio channel in the wireless communication system. While <FIG> depicts only one base station, other base station which is identical or similar to the base station <NUM> may be further included.

The base station <NUM> is a network infrastructure for providing radio access to the terminals <NUM> and <NUM>. The base station <NUM> has coverage defined as a specific geographical area based on a signal transmission distance. The base station <NUM> may be referred to as, besides the base station, an access point (AP), an eNodeB (eNB), a next generation nodeB (gNB), a 5th generation node (<NUM> node), a wireless point, a transmission/reception point (TRP), or other terms having technically identical meaning.

The terminal <NUM> and the terminal <NUM> each are a device used by a user, and communicate with the base station <NUM> over a radio channel. In some cases, at least one of the terminal <NUM> and the terminal <NUM> may operate without user's involvement. That is, at least one of the terminal <NUM> and the terminal <NUM> is a device which performs machine type communication (MTC), and may not be carried by the user. The terminal <NUM> and the terminal <NUM> each may be referred to as, besides the terminal, a user equipment (UE), a mobile station, a subscriber station, a remote terminal, a wireless terminal, a user device, or other term having a technically equivalent meaning.

The base station <NUM>, the terminal <NUM>, and the terminal <NUM> may transmit and receive radio signals in a millimeter wave (mmWave) band (e.g., <NUM>, <NUM>, <NUM>, <NUM>). In so doing, to improve channel gain, the base station <NUM>, the terminal <NUM>, and the terminal <NUM> may conduct beamforming. Herein, the beamforming may include transmit beamforming and receive beamforming. That is, the base station <NUM>, the terminal <NUM>, and the terminal <NUM> may apply directivity to a transmit signal or a received signal. For doing so, the base station <NUM> and the terminals <NUM> and <NUM> may select serving beams <NUM>, <NUM>, <NUM>, and <NUM> through a beam search or beam management procedure. After the serving beams <NUM>, <NUM>, <NUM>, and <NUM> are selected, communications may be performed using resources which are quasi co-located (QCL) with resources which carry the serving beams <NUM>, <NUM>, <NUM>, and <NUM>.

If large-scale properties of a channel which carries a symbol on a first antenna port may be inferred from a channel which carries a symbol on a second antenna port, the first antenna port and the second antenna port may be said to be QCL. For example, the large-scale properties may include at least one of delay spread, Doppler spread, Doppler shift, average gain, average delay, and spatial receiver parameter.

In the example described with reference to <FIG>, the base station <NUM> has been described as a single device. However, the base station <NUM> may be implemented to be divided into two or more devices. For example, a base station may include one central unit and at least one distributed unit. The structure of the base station divided into the central unit and the distributed unit may be referred to as a 'CU-DU split structure'. The central unit and the distributed unit may be distinguished in terms of protocol layers, and its example is shown in <FIG> below.

<FIG> illustrates a protocol stack of a terminal and a base station in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates the protocol structure for control, that is, of a control plane. Referring to <FIG>, the terminal <NUM> processes layers such as radio frequency (RF), physical (PHY), media access control (MAC), radio link control (RLC), packet data convergence protocol (PDCP), and radio resource control (RRC). A distributed unit <NUM>-<NUM> of the base station <NUM> processes layers such as RF, PHY, MAC, and RLC, and a central unit <NUM>-<NUM> of the base station <NUM> processes layers such as PDCH and RRC. An interface between the distributed unit <NUM>-<NUM> and the central unit <NUM>-<NUM> may be referred to as 'F1'.

<FIG> illustrates a configuration of a central unit of a base station in a wireless communication system according to various embodiments of the present disclosure. The configuration in <FIG> may be understood as the configuration of the distrusted unit <NUM>-<NUM> of the base station <NUM>. A term such as 'portion' or '~ er' used hereafter indicates a unit for processing at least one function or operation, and may be implemented using hardware, software, or a combination of hardware and software.

Referring to <FIG>, the distributed unit includes a wireless communication unit <NUM>, a backhaul communication unit <NUM>, a storage unit <NUM>, and a control unit <NUM>.

The wireless communication unit <NUM> may perform functions for transmitting and receiving signals over a radio channel. For example, the wireless communication unit <NUM> performs a conversion function between a baseband signal and a bit string according to a physical layer standard of the system. For example, in data transmission, the wireless communication unit <NUM> generates complex symbols by encoding and modulating a transmit bit string. Also, in data reception, the wireless communication unit <NUM> restores a receive bit string by demodulating and decoding a baseband signal.

Also, the wireless communication unit <NUM> up-converts the baseband signal to an RF band signal, transmits it via an antenna, and down-converts an RF band signal received via an antenna to a baseband signal. For doing so, the wireless communication unit <NUM> may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a digital to analog convertor (DAC), an analog to digital convertor (ADC), and so on. In addition, the wireless communication unit <NUM> may include a plurality of transmit and receive paths. Further, the wireless communication unit <NUM> may include at least one antenna array including a plurality of antenna elements.

In terms of the hardware, the wireless communication unit <NUM> may include a digital unit and an analog unit, and the analog unit may include a plurality of sub-units according to an operating power and an operating frequency. The digital unit may be implemented with at least one processor (e.g., a digital signal processor (DSP)).

The wireless communication unit <NUM> transmits and receives the signals as stated above. Hence, whole or part of the wireless communication unit <NUM> may be referred to as 'a transmitter', 'a receiver', or 'a transceiver'. Also, in the following, the transmission and the reception over the radio channel is used as the meaning which embraces the above-stated processing of the wireless communication unit <NUM>.

The backhaul communication unit <NUM> provides an interface for communicating with other nodes in the network. That is, the backhaul communication unit <NUM> converts a bit sting transmitted from the distributed unit to another node, for example, to the central unit, another access node, another distributed unit, an upper node, or a core network, to a physical signal, and converts a physical signal received from the another node to a bit string.

The storage unit <NUM> stores a basic program for operating the distributed unit, an application program, and data such as setting information. The storage unit <NUM> may include a volatile memory, a non-volatile memory, or a combination of a volatile memory and a non-volatile memory. The storage unit <NUM> provides the stored data at a request of the control unit <NUM>.

The control unit <NUM> controls general operations of the distributed unit. For example, the control unit <NUM> transmits and receives signals through the wireless communication unit <NUM> or the backhaul communication unit <NUM>. Also, the control unit <NUM> records and reads data in and from the storage unit <NUM>. The control unit <NUM> may execute functions of the protocol stack requested by a communication standard. According to another embodiment, the protocol stack may be included in the wireless communication unit <NUM>. For doing so, the control unit <NUM> may include at least one processor.

According to various embodiments, the control unit <NUM> may manage connections of terminals. For example, the control unit <NUM> may determine whether to accept a procedure requested by a control message of an upper layer, and may control an operation for informing a determination result to the central unit. According to an embodiment, the control unit <NUM> may control to receive the control message processed by the central unit from the terminal, receive a first control message processed by the central unit from the terminal, transmit to the central unit the first control message modified to indicate rejection of the procedure requested by the first control message, and receive a second control message indicating the procedure rejection in response to the modified first control message. According to another embodiment, the control unit <NUM> may control to receive a first control message processed by the central unit from the terminal, transmit the first control message to the central unit, receive a second control message indicating to perform the procedure requested by the first control message, drop the second control message in response to determining to reject the procedure requested by the first control message, and transmit a third control message corresponding to the procedure rejection to the central unit. For example, the control unit <NUM> may control the distributed unit to perform operations according to various embodiments described below.

<FIG> illustrates a configuration of a central unit of a base station in a wireless communication system according to various embodiments of the present disclosure. The configuration illustrated in <FIG> may be understood as the configuration of the central unit <NUM>-<NUM> of the base station <NUM>. A term such as 'portion' or '~ er' used hereafter indicates a unit for processing at least one function or operation, and may be implemented using hardware, software, or a combination of hardware and software.

Referring to <FIG>, the central unit includes a backhaul communication unit <NUM>, a storage unit <NUM>, and a control unit <NUM>.

The backhaul communication unit <NUM> provides an interface for communicating with other nodes in the network. That is, the backhaul communication unit <NUM> converts a bit sting transmitted from the central unit to another node, for example, to the distributed unit, another access node, another central unit, an upper node, or a core network, to a physical signal, and converts a physical signal received from the another node to a bit string. The backhaul communication unit <NUM> transmits and receives the signals as stated above. Hence, whole or part of the backhaul communication unit <NUM> may be referred to as 'a transmitter', 'a receiver', or 'a transceiver'.

The storage unit <NUM> stores a basic program for operating the central unit, an application program, and data such as setting information. The storage unit <NUM> may include a volatile memory, a non-volatile memory, or a combination of a volatile memory and a non-volatile memory. The storage unit <NUM> provides the stored data according to a request of the control unit <NUM>.

The control unit <NUM> controls general operations of the central unit. For example, the control unit <NUM> transmits and receives signals through the backhaul communication unit <NUM>. Also, the control unit <NUM> records and reads data in and from the storage unit <NUM>. The control unit <NUM> may execute functions of a protocol stack required by a communication standard, that is, functions of PDCP and RRC layers. For doing so, the control unit <NUM> may include at least one processor.

According to various embodiments, the control unit <NUM> may manage connections of terminals. According to various embodiments, the control unit <NUM> may manage the connection according to a control message received from the distributed unit. According to an embodiment, the control unit <NUM> may control to receive a first control message generated by the terminal from the distributed unit, and transmit a second control message indicating procedure rejection in response to identifying that the first control message is modified to indicate rejection of a procedure requested by the first control message. For example, the control unit <NUM> may control the central unit to carry out operations to be explained according to various embodiments.

<FIG> illustrates a configuration of a terminal in a wireless communication system according to various embodiments of the present disclosure. The configuration illustrated in <FIG> may be understood as the configuration of the terminal <NUM>. A term such as 'portion' or '~ er' used hereafter indicates a unit for processing at least one function or operation, and may be implemented using hardware, software, or a combination of hardware and software.

Referring to <FIG>, the terminal includes a communication unit <NUM>, a storage unit <NUM>, and a control unit <NUM>.

The communication unit <NUM> may perform functions for transmitting and receiving signals over a radio channel. For example, the communication unit <NUM> performs a conversion function between a baseband signal and a bit string according to a physical layer standard of the system. For example, in data transmission, the communication unit <NUM> generates complex symbols by encoding and modulating a transmit bit string. Also, in data reception, the communication unit <NUM> restores a receive bit string by demodulating and decoding a baseband signal. Also, the communication unit <NUM> up-converts the baseband signal to an RF band signal, transmits it via an antenna, and down-converts an RF band signal received via an antenna to a baseband signal. For example, the communication unit <NUM> may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, and so on.

In addition, the communication unit <NUM> may include a plurality of transmit and receive paths. Further, the communication unit <NUM> may include at least one antenna array including a plurality of antenna elements. In terms of the hardware, the communication unit <NUM> may include a digital circuit and an analog circuit (e.g., radio frequency integrated circuit (RFIC)). Herein, the digital circuit and the analog circuit may be implemented as a single package. In addition, the communication unit <NUM> may include a plurality of RF chains. Further, the communication unit <NUM> may perform beamforming.

The communication unit <NUM> transmits and receives the signals as stated above. Hence, whole or part of the communication unit <NUM> may be referred to as 'a transmitter', 'a receiver', or 'a transceiver'. Also, in the following, the transmission and the reception over the radio channel is used as the meaning which embraces the above-stated processing of the communication unit <NUM>.

The storage unit <NUM> stores a basic program for operating the terminal, an application program, and data such as setting information. The storage unit <NUM> may include a volatile memory, a non-volatile memory, or a combination of a volatile memory and a non-volatile memory. The storage unit <NUM> provides the stored data at a request of the control unit <NUM>.

The control unit <NUM> controls general operations of the terminal. For example, the control unit <NUM> transmits and receives signals through the communication unit <NUM>. Also, the control unit <NUM> records and reads data in and from the storage unit <NUM>. The control unit <NUM> may execute functions of a protocol stack requested by a communication standard. For doing so, the control unit <NUM> may include at least one processor or microprocessor, or part of a processor. Also, part of the communication unit <NUM> and the control unit <NUM> may be referred to as a communication processor (CP).

<FIG> illustrates a configuration of a communication unit in a wireless communication system according to various embodiments of the present disclosure. <FIG> depicts an example of a detailed configuration of the wireless communication unit <NUM> of <FIG> or the communication unit <NUM> of <FIG>. More specifically, <FIG> illustrates components for performing beamforming, as part of the wireless communication unit <NUM> of <FIG> or the communication unit <NUM> of <FIG>.

Referring to <FIG>, the wireless communication unit <NUM> or the communication unit <NUM> includes an encoder and modulator <NUM>, a digital beamformer <NUM>, a plurality of transmit paths <NUM>-<NUM> through <NUM>-N, and an analog beamformer <NUM>.

The encoder modulator <NUM> performs channel encoding. For the channel encoding, at least one of low density parity check (LDPC) code, convolution code, and polar code may be used. The encoder and modulator <NUM> generates modulation symbols by performing constellation mapping.

The digital beamformer <NUM> performs the beamforming on a digital signal (e.g., the modulation symbols). For doing so, the digital beamformer <NUM> multiplies the modulation symbols by beamforming weights. Herein, the beamforming weights are used to change an amplitude and a phase of the signal, and may be referred to as a 'precoding matrix' or a 'precoder'. The digital beamformer <NUM> outputs the digital-beamformed modulation symbols to the transmit paths <NUM>-<NUM> through <NUM>-N. In so doing, according to a multiple input multiple output (MIMO) transmission scheme, the modulation symbols may be multiplexed or the same modulation symbols may be provided to the transmit paths <NUM>-<NUM> through <NUM>-N.

The transmit paths <NUM>-<NUM> through <NUM>-N convert the digital-beamformed digital signals to analog signals. For doing so, the transmit paths <NUM>-<NUM> through <NUM>-N each may include an inverse fast Fourier transform (IFFT) operator, a cyclic prefix (CP) adder, a DAC, and an up-converter. The CP adder is used for an orthogonal frequency division multiplexing (OFDM) scheme, and may be excluded if other physical layer scheme (e.g., filter bank multi-carrier (FBMC)) is applied. That is, the transmit paths <NUM>-<NUM> through <NUM>-N provide an independent signal process for a plurality of streams generated through the digital beamforming. Yet, depending on the implementation, some of the components of the transmit paths <NUM>-<NUM> through <NUM>-N may be used in common.

The analog beamformer <NUM> beamforms the analog signals. For doing so, the digital beamformer <NUM> multiplies the analog signals by the beamforming weights. Herein, the beamforming weights are used to change the amplitude and the phase of the signal. More specifically, the analog beamformer <NUM> may be configured variously, according to a connection structure between the transmit paths <NUM>-<NUM> through <NUM>-N and the antennas. For example, the plurality of the transmit paths <NUM>-<NUM> through <NUM>-N each may be connected with one antenna array. As another example, the plurality of the transmit paths <NUM>-<NUM> through <NUM>-N may be connected with one antenna array. As yet another example, the plurality of the transmit paths <NUM>-<NUM> through <NUM>-N may be adaptively connected with one antenna array, or connected with two or more antenna arrays.

In a system according to various embodiments, the terminal may operate in various states. For example, the terminal may operate in one of an RRC connected state, an RRC idle state, or an RRC inactive state. The RRC connected state may be referred to as an RRC 'active state' or an 'RRC normal state'. In the RRC connected state, all the protocol layers of the terminal are activated, and the terminal may transmit/receive data. In the RRC idle state, the protocol layers of the terminal are deactivated and resources on a core network are released. However, in the RRC idle state, the terminal may perform operations such as broadcast information reception, tracking area (TA) update, paging reception, and so on.

In the RRC inactive state, the protocol layers of the terminal are deactivated, but the connection between the terminal and the core network is maintained. That is, in the RRC inactive state, the connection with the core network is similar to the RRC connected state. The RRC inactive state separately manages the state of the terminal in a radio access network (RAN), separately from managing the idle state in the core network, and thus enables the terminal to maintain access stratum (AS) context and to transit to the connected state faster. With the introduction of the RRC inactive state, various applications such as connectionless data transmission for small data within the inactive state as well as terminal power consumption reduction may be implemented.

The terminal may transit between the RRC connected state, the RRC idle state, and the RRC idle state according to a given condition. For example, if transmission data occurs at the terminal which is operating in the RRC inactive state or the RRC idle state, the terminal may request state transition, by transmitting a message for transiting to the RRC connected state. Examples of the procedure for the state transition are described below with reference to <FIG> and <FIG>.

<FIG> illustrates a signal exchange diagram if context retrieval is successful in RRC state transition in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

Referring to <FIG>, in step <NUM>, the terminal <NUM> operates in the RRC inactive state or the RRC idle state. Next, in step <NUM>, the terminal <NUM> transmits an RRC connection resume request message. That is, according to event occurrence such as generating data to transmit, the terminal <NUM> transmits a message requesting transition to the RRC connected state.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message to the central unit <NUM>-<NUM>. The initial uplink RRC message includes the RRC connection resume request message transmitted from the terminal <NUM>. The RRC connection resume request message received at the distributed unit <NUM>-<NUM> is data including a MAC header, an RLC header, and so on. Hence, for the received data, the distributed unit <NUM>-<NUM> may perform necessary processing in the MAC layer and the RLC layer, and include an RRC message (e.g., the RRC connection resume request) to transmit to the central unit in the initial uplink RRC message, and then transmit to the central unit <NUM>-<NUM>. In addition, the initial uplink RRC message may further include at least one of identification information (e.g., cell-radio network temporary identity (C-RNTI)) of the terminal <NUM>, and information relating to resource allocation configuration of the distributed unit <NUM>-<NUM>. The initial uplink RRC message may be referred to as an 'F1-application protocol (AP) initial uplink RRC message transfer message'.

In step <NUM>, the central unit <NUM>-<NUM> retrieves context of the terminal <NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message to the distributed unit <NUM>-<NUM>. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message.

Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including the RRC connection resume message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection resume message. In step <NUM>, the terminal <NUM> transmits an RRC connection resume complete message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an uplink RRC message transfer message including the RRC connection resume complete message. Thus, in step <NUM>, the terminal <NUM> transits to the RRC connected state.

<FIG> illustrates a signal exchange diagram if context retrieval fails in RRC state transition in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

Referring to <FIG>, in step <NUM>, the terminal <NUM> operates in the RRC inactive state or the RRC idle state. Next, in step <NUM>, the terminal <NUM> transmits an RRC connection resume request message. That is, according to event occurrence such as generating data to transmit, the terminal <NUM> transmits a message requesting transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message to the central unit <NUM>-<NUM>. The initial uplink RRC message includes the RRC connection resume request message transmitted from the terminal <NUM>, and additionally, may further include at least one of identification information (e.g., C-RNTI) of the terminal <NUM> and resource allocation configuration information of the distributed unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> fails to retrieve context of the terminal <NUM>. The central unit <NUM>-<NUM> determines that the terminal <NUM> is acceptable. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup message including an RRC connection setup message.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection setup message. In step <NUM>, the terminal <NUM> transmits an RRC connection setup complete message. That is, the terminal <NUM> performs a necessary operation to establish an RRC connection, and then transmits a message indicating that the RRC connection is established. Next, in step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message. For example, the UE context setup response message may include an RRC connection setup complete message. Thus, in step <NUM>, the terminal <NUM> transits to the RRC connected state.

Through the procedure of <FIG> or <FIG>, the terminal may transit from the RRC inactive state to the RRC connected state. However, accepting the request for the state transition of the terminal is not always guaranteed. For example, if hardware resources, logical resources, or physical resources, to be allocated, lack, the request for the state transition may be rejected. The case where the request for the state transition is rejected will be described with reference to <FIG>.

<FIG> illustrates a signal exchange diagram if an RRC state transition attempt is rejected in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

In step <NUM>, the central unit <NUM>-<NUM> determines to reject the request of the terminal <NUM>. The request may be rejected for various causes. For example, the central unit <NUM>-<NUM> may reject the request of the terminal <NUM> according to the lack of resources due to congestion. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message. For example, the downlink RRC message transfer message may include an RRC message indicating the rejection for the RRC connection resume. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an RRC connection reject message.

In the above example, the request of the state transition was rejected by the central unit. However, since the base station has the structure divided into the central unit and the distributed unit, the failure of the state transition may be determined by the distributed unit. In other words, the distributed unit may determine whether to accept the request (e.g., the state transition request) of the terminal. This operation may be referred to as 'connection control', 'resource allocation control', 'resource management', 'resource control', 'admission control', and so on.

Since the central unit is higher than the distributed unit, if the central unit determines the rejection, it may conduct the procedure by notifying the determined rejection case. However, if the distributed unit determines the rejection, it may be impossible to process the state transition failure case merely using an existing message or information element (IE), or additional message exchange may be required though the failure case is processed with the existing message. Thus, avoid such inefficiency, a procedure is required to notify the rejection case from the distributed unit to the central unit.

In other words, unlike a conventional base station which is a single object, in a base station structure including two objects such as the central unit and the distributed unit, not only the central unit but also the distributed unit may determine the connection rejection. Hence, the present disclosure provides a method for, if the distributed unit determines the connection rejection, notifying the connection rejection case of the distributed unit to the central unit which is the object separated from the distributed unit.

<FIG> illustrates a flowchart for rejecting a transition before control message transfer of a distributed unit of a base station in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates an operating method of the distributed unit <NUM>-<NUM>.

Referring to <FIG>, in step <NUM>, the distributed unit receives data including a control message from the terminal. At this time, the distributed unit may receive lower layer data (e.g., MAC packet data unit (PDU)) including a control message of an upper layer not processed by the distributed unit. In other words, the distributed unit may receive the control message of the upper layer processed by the central unit. Herein, the control message may be a message of the RRC layer. Further, the control message may be a message requesting state transition of the terminal. Specifically, the control message may be a message requesting transition from the RRC inactive state to the RRC connected state.

In step <NUM>, the distributed unit determines whether to accept the request of the terminal. In other words, the distributed unit determines whether to accept a procedure (e.g., state transition, connection setup, etc.) requested by the terminal. For example, the distributed unit may determine whether to accept the terminal based on available status of available hardware resources (e.g., a buffer, etc.), logical resources (e.g., the maximum number of users to accept), and physical resources (e.g., time resources, frequency resources, etc.). That is, regardless of the layer (e.g., the RRC layer) where the control message is processed, the distributed unit determines whether to accept the terminal based on the status of the layers managed by the distributed unit.

If determining not to accept the request of the terminal, the distributed unit modifies the control message to indicate rejection for the request of the terminal, in step <NUM>. In other words, the distributed unit modifies the control message to indicate that it rejects the procedure requested by the control message. According to an embodiment, the distributed unit may insert a reject indication into the control message. For example, the distributed unit may insert the reject indication by adding, replacing or removing at least one bit having a particular value at a particular position in the control message. As another example, the distributed unit may represent the reject indication, by removing or replacing a specific information element with another value in the control message. That is, according to various embodiments, inserting the reject indication may be understood to embrace not only adding at least one bit, but also replacing or deleting it.

In step <NUM>, the distributed unit transfers the control message to the central unit. For doing so, the distributed unit may generate a transfer message including the modified control message, and transmit the transfer message via an interface between the distributed unit and the central unit.

In the embodiment described with reference to <FIG>, the distributed unit determines whether to accept the request of the terminal. Various operations may be performed to determine whether to accept the request of the terminal. In other words, specific embodiments for determining whether to accept the request of the terminal may be variously defined. The specific embodiments for determining whether to accept the request of the terminal may be variously defined depending which information is based on or what kind of determination is conducted.

According to an embodiment, to determine whether to accept the request of the terminal, the distributed unit may perform in advance an operation performed after receiving the request for context setup from the central unit, before receiving the request for the context setup. For example, before receiving the request for the context setup, the distributed unit may attempt to reserve/allocate resources of the MAC/PHY layer for a signaling radio bearer (SRB) to transmit a control message. Together with the resource reservation/allocation of the MAC/PHY layer, determining whether to accept the request of the terminal may be performed. Specifically, if the resource allocation for the signaling radio bearer is successful, the distributed unit may determine to accept the request for the terminal, and if the resource allocation for the signaling radio bearer fails, the distributed unit may determine not to accept the request for the terminal. If the resource reservation/allocation operation of the MAC/PHY layer is performed in advance, if the state transition is successful as shown in <FIG> or <FIG>, after the request for the context setup is received (e.g., after step <NUM> or after step <NUM>), the operation for the resource reservation/allocation of the MAC/PHY layer may be omitted.

Herein, the signaling radio bearer considered in the resource reservation/allocation of the MAC/PHY layer may include a signaling radio bearer (e.g., SRB1) for a dedicated control channel (DCCH). In this case, the resource reservation/allocation of the MAC/PHY layer for transmitting the control message may include a MAC/PHY resource allocation operation for SRB <NUM> for transferring parameters (e.g., channel quality information (CQI), sounding reference signal (SRS) reporting, etc.) used by the terminal in transmitting an RRC connection setup complete message through the SRB1 to the central unit.

<FIG> illustrates a flowchart of a central unit of a base station in a wireless communication system according to various embodiments of the present disclosure. <FIG> illustrates an operating method of the central unit <NUM>-<NUM>.

Referring to <FIG>, in step <NUM>, the central unit receives a control message from the distributed unit. In other words, the central unit receives the control message generated by the terminal from the distributed unit. Specifically, the central unit receives a transfer message including the control message, and identifies the control message in the transfer message. Herein, the control message may be a message of the RRC layer. Further, the control message may be a message requesting state transition of the terminal. For example, the control message may be a message requesting transition from the RRC inactive state to the RRC connected state.

In step <NUM>, the central unit identifies whether the control message indicates rejection for the terminal's request. In other words, the central unit determines whether the control message indicates the rejection of the distributed unit. According to an embodiment, the central unit may determine whether the control message indicates the rejection, by identifying whether a reject indication is inserted in the control message. For example, the central unit may identify whether at least one bit having a particular value at a particular position in the control message is added, replaced, or removed. As another example, the central unit may determine whether a particular information element in the control message is removed or replaced by another value. That is, according to various embodiments, inserting the reject indication may be understood to embrace not only adding at least one bit, but also replacing or deleting it.

If the control message indicates the rejection for the terminal's request, the central unit transmit a control message for the rejection, in step <NUM>. That is, the central unit determines connection setup of the terminal, that is, to reject the state transition, according to the determination of the distributed unit, and generates and transmits a control message for notifying this to the terminal. Specifically, the central unit may generate a control message notifying the rejection, generate a transfer message including the control message, and then transmit the transfer message to the distributed unit.

If the control message does not indicate the rejection for the request of the terminal, the central unit determines whether to accept the request of the terminal, in operation <NUM>. The central unit may determine the acceptability based on the status of the layers managed by the central unit. For example, the central unit may determine the acceptability based on whether context of the terminal is stored, and the resource status of the corresponding layers. If not accepting the request of the terminal, the central unit proceeds to step <NUM>.

By contrast, if accepting the request of the terminal, in step <NUM>, the central unit transmits a control message for performing a procedure according to the request. For example, if the terminal requests the state transition, the central unit may generate a control message indicating the state transition, generate a transfer message including the control message, and then transmit the transfer message to the distributed unit. For example, the message indicating the state transition may include a message instructing to set the context or a message instructing to establish an RRC connection.

According to the embodiments described with reference to <FIG> and <FIG>, the rejection for the request of the terminal determined by the distributed unit may be transferred to the central unit. Thus, meaningless signaling for an unnecessary state transition attempt may reduce.

Also, as described above, the reject indication may be defined in various forms. The reject indication which is the information element for informing the central unit of the rejection case is not limited to its name, and any parameter for informing the central unit of the rejection case is sufficient. In addition, it is possible to notify the rejection case, by intentionally omitting and transmitting the corresponding information element. For example, the transfer message (e.g., an initial uplink RRC transfer message) including the reject indication may include at least one of information elements shown in <Table <NUM>>.

<FIG> and <FIG> illustrate the case where the control message for the state transition is received, but a control message for another purpose may be received. Thus, according to another embodiment, the distributed node may further perform an operation of classifying the control message. That is, if receiving a control message, the distributed node may determine whether it is signaling requiring resource allocation such as state transition, and perform the above-mentioned operations in response to the signaling requiring the resource allocation. For example, the distributed node may classify the control message, by examining a bit pattern of a specific position in the control message. According to yet another embodiment, the distributed node may perform an operation related to the reject indication for all control messages without classifying the message, and the central node may selectively use the reject indication.

The operations related to the reject indication as mentioned above may be applied to various procedures. For example, if an RRC connection resume request message is received and the distributed unit determines to reject the RRC connection resume prior to transmitting an initial uplink RRC message transfer message, the aforementioned reject indication may be used to notify the central unit of the RRC connection resume request rejected by the distributed unit. By defining the reject indication, if the distributed unit which is a lower object determines to reject, additional signaling for an operation to share information to the central unit which a higher object may be prevented and procedure latency may be minimized. An example in which the reject indication is used in the RRC connection resume procedure is described with reference to <FIG>, <FIG>, <FIG>, and <FIG>.

<FIG> illustrates a signal exchange diagram if a state transition attempt using an RRC resume procedure is successful in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

In step <NUM>, the distributed unit <NUM>-<NUM> determines admission of the state transition of the terminal <NUM>. For example, the distributed unit <NUM>-<NUM> tries to resource allocation of the MAC/PHY layer for a signaling radio bearer to transmit a control message, and determines success of the resource allocation. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message including the RRC connection resume request message to the central unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> retrieves context of the terminal <NUM>. <FIG> illustrates a case where the context of the terminal <NUM> is stored in the central unit <NUM>-<NUM>. Accordingly, the central unit <NUM>-<NUM> may obtain the context of the terminal <NUM>. That is, the central unit <NUM>-<NUM> may successfully retrieve the context of the UE <NUM> and determine to resume a data radio bearer (DRB).

Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message to the distributed unit <NUM>-<NUM>. Specifically, as obtaining the context of the terminal <NUM>, the central unit <NUM>-<NUM> may allocate a UE F1-AP IE for the terminal <NUM>, and transmit a message requesting context setup. As the context setup is requested from the central unit <NUM>-<NUM>, the distributed unit <NUM>-<NUM> sets the context of the terminal <NUM>. Next, in step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message.

Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection resume message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection resume message. Next, operations for the terminal <NUM> to transit to the RRC connected state are performed. For example, although not depicted in <FIG>, the terminal <NUM> may transmit an RRC connection resume complete message to the distributed unit <NUM>-<NUM>, and the distributed unit <NUM>-<NUM> may transmit an uplink RRC message transfer message including the RRC connection resume complete message to the central unit <NUM>-<NUM>. Thus, the terminal <NUM> may transit to the RRC connected state.

<FIG> illustrates a signal exchange diagram if the state transition attempt using the RRC resume procedure is rejected by the distributed unit before the control message transfer in the wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

In step <NUM>, the distributed unit <NUM>-<NUM> determines rejection for the state transition of the terminal <NUM>. For example, the distributed unit <NUM>-<NUM> attempts resource allocation of the MAC/PHY layer for a signaling radio bearer to transmit a control message, and determines failure of the resource allocation. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message including a reject indication to the central unit <NUM>-<NUM>. That is, if the distributed unit <NUM>-<NUM> determines the RRC connection resume rejection prior to the transmission of the initial uplink RRC message, the distributed unit <NUM>-<NUM> includes the reject indication in the initial uplink RRC message to notify the rejection case to the central unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> omits the retrieve operation for the context of the terminal <NUM>. That is, upon identifying the reject indication, the central unit <NUM>-<NUM> determines that the terminal <NUM> rejects the state transition request without further determination. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a final downlink RRC message transfer message including an RRC connection reject message. That is, the central unit <NUM>-<NUM> receiving the initial uplink RRC message including the reject indication does not perform a new context allocation operation, and directly transmits the downlink RRC message transfer message including the RRC connection resume reject message or the RRC connection reject message to the distributed unit <NUM>-<NUM>.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reject message to the terminal <NUM>. The distributed unit <NUM>-<NUM> receiving the downlink RRC message transfer message including the RRC connection resume reject message or the RRC connection reject message transfers the RRC message to the terminal <NUM>. Accordingly, the terminal <NUM> is finally notified of the failure of the state transition operation to the RRC connected state.

<FIG> illustrates a signal exchange diagram if a state transition attempt using an RRC connection procedure is successful in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustrations purposes only. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

Referring to <FIG>, in step <NUM>, the terminal <NUM> operates in the RRC inactive state or the RRC idle state. Next, in step <NUM>, the terminal <NUM> transmits an RRC connection request message. That is, according to event occurrence such as generating data to transmit, the terminal <NUM> transmits a message requesting transition to the RRC connected state.

In step <NUM>, the distributed unit <NUM>-<NUM> determines admission for the state transition of the terminal <NUM>. For example, the distributed unit <NUM>-<NUM> tries to resource allocation of the MAC/PHY layer for a signaling radio bearer to transmit a control message, and determines success of the resource allocation. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message including the RRC connection resume request message to the central unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> retrieves context of the terminal <NUM>. <FIG> illustrates a case where the context of the terminal <NUM> is stored in the central unit <NUM>-<NUM>. Accordingly, the central unit <NUM>-<NUM> may obtain the context of the terminal <NUM>. That is, the central unit <NUM>-<NUM> may successfully retrieve the context of the UE <NUM> and determine to connect a DRB.

Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection setup message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection message. Next, operations for the terminal <NUM> to transit to the RRC connected state are performed. For example, although not depicted in <FIG>, the terminal <NUM> may transmit an RRC connection setup complete message to the distributed unit <NUM>-<NUM>, and the distributed unit <NUM>-<NUM> may transmit an uplink RRC message transfer message including the RRC connection complete message to the central unit <NUM>-<NUM>. Thus, the terminal <NUM> may transit to the RRC connected state.

<FIG> illustrates a signal exchange diagram if the state transition attempt using the RRC connection procedure is rejected by the distributed unit before the control message transfer in the wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

In step <NUM>, the distributed unit <NUM>-<NUM> determines rejection for the state transition of the terminal <NUM>. For example, the distributed unit <NUM>-<NUM> attempts resource allocation of the MAC/PHY layer for a signaling radio bearer to transmit a control message, and determines failure of the resource allocation. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message including a reject indication to the central unit <NUM>-<NUM>. That is, if the distributed unit <NUM>-<NUM> determines the RRC connection reject prior to the transmission of the initial uplink RRC message, the distributed unit <NUM>-<NUM> includes the reject indication in the initial uplink RRC message to notify the rejection case to the central unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> omits the retrieve operation for the context of the terminal <NUM>. That is, upon identifying the reject indication, the central unit <NUM>-<NUM> determines that the terminal <NUM> rejects the state transition request without further determination. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a final downlink RRC message transfer message including an RRC connection reject message. That is, the central unit <NUM>-<NUM> receiving the initial uplink RRC message including the reject indication does not perform a new context allocation operation, and directly transmits the downlink RRC message transfer message including the RRC connection reject message to the distributed unit <NUM>-<NUM>.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reject message to the terminal <NUM>. The distributed unit <NUM>-<NUM> receiving the downlink RRC message transfer message including the RRC connection reject message transfers the RRC message to the terminal <NUM>. Hence, the terminal <NUM> is finally notified of the failure of the state transition operation to the RRC connected state.

As stated above, the rejection state determined by the distributed unit may be transferred to the central unit using the reject indication. However, before the initial uplink RRC message transfer message is transmitted, whether to accept the terminal may not be determined. That is, after the initial uplink RRC message transfer message is transmitted, the rejection case may be determined by the distributed unit. In this case, since the reject indication is not included, if the terminal is not rejected by the central unit, the state transition procedure may proceed. In addition, if the admission rejection of the terminal is determined before the initial uplink RRC message transfer message is transmitted but the reject indication is omitted, the same case may occur. If the state transition procedure is proceeded by the determination latency of the rejection case or the omission of the reject indication, this means occurrence of inefficiency due to unnecessary signaling. Thus, the present disclosure provides a method for a case where the reject indication is not transmitted.

<FIG> illustrates a flowchart for rejecting state transition after control message transfer of a distributed unit of a base station in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only. <FIG> illustrates an operating method of the distributed unit <NUM>-<NUM>.

Referring to <FIG>, in step <NUM>, the distributed unit receives data including a control message from the terminal. Herein, the control message may be a message of the RRC layer. Further, the control message may be a message requesting state transition of the terminal. For example, the control message may be a message requesting the transition from the RRC inactive state to the RRC connected state.

In step <NUM>, the distributed unit transfers the control message to the central unit. For doing so, the distributed unit may generate a transfer message including the control message, and transmit the transfer message via an interface between the distributed unit and the central unit.

In step <NUM>, the distributed unit receives a control message for the procedure from the central unit. If the procedure requested by the terminal is the state transition, the request for the state transition of the terminal is accepted by the central unit, and thus the distributed unit may receive a downlink RRC message transfer message including the control message for the state transition. For example, the control message may include a context setup request message or an RRC connection setup message.

In step <NUM>, the distributed unit determines whether to accept the request of the terminal. For example, the distributed unit may determine whether to accept the terminal based on available statuses of available hardware resources (e.g., a buffer, etc.), logical resources (e.g., the maximum number of users to accept), and physical resources (e.g., time resources, frequency resources, etc.). That is, regardless of the layer (e.g., the RRC layer) where the control message is processed, the distributed unit determines whether to accept the request based on the status of the layers managed by the distributed unit. The operation of step <NUM> may be understood as an operation for identifying the determination before step <NUM> or a new determining operation.

If determining not to accept the request of the terminal, in step <NUM>, the distributed unit transmits a control message for aborting the procedure to the central unit. That is, the distributed unit does not transfer the control message to the terminal, but generates and transmits a control message corresponding to the rejection. Thus, the procedure (e.g., the state transition) requested by the terminal may be aborted.

If determining to accept the request of the terminal, the distributed unit transfers a control message for the procedure to the terminal, in step <NUM>. That is, the distributed unit may transmit the control message to the terminal, so that the procedure for the state transition is conducted.

As in the embodiment described with reference to <FIG>, unnecessary procedure proceeding may be prevented though the reject indication is not used. In other words, even if the reject indication is omitted as shown in <FIG>, the quick procedure termination may be achieved according to the determination of the DU. Herein, the omission of the reject indication may occur for various causes, for example, if the admission rejection is determined but an error occurs in the message modification, or if it takes a long time (e.g., a time limit for transmitting the control message) to determine whether to accept, or if a case for determining whether to accept is changed. An example of applying the procedure of <FIG> to the RRC connection resume procedure is described with reference to <FIG>.

<FIG> illustrates a signal exchange diagram if an RRC state transition attempt is rejected by a distributed unit after control message transfer in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only. <FIG> illustrates signal exchanges between the terminal <NUM>, the distributed unit <NUM>-<NUM> of the base station <NUM>, and the central unit <NUM>-<NUM> of the base station <NUM>.

In step <NUM>, the terminal <NUM> transmits an RRC connection resume request message. That is, according to event occurrence such as generating data to transmit, the terminal <NUM> transmits a message requesting transition to the RRC connected state.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message transfer message to the central unit <NUM>-<NUM>. The initial uplink RRC message transfer message includes the RRC connection resume request message transmitted from the terminal <NUM>. In addition, the initial uplink RRC message may further include at least one of identification information (e.g., C-RNTI) of the terminal <NUM> and information relating to resource allocation configuration of the distributed unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> retrieves context of the terminal <NUM>. <FIG> illustrates a case where the context of the terminal <NUM> is stored in the central unit <NUM>-<NUM>. Accordingly, the central unit <NUM>-<NUM> may obtain the context of the terminal <NUM>. That is, the central unit <NUM>-<NUM> may successfully retrieve the context of the UE <NUM>, and determine to resume a data radio bearer.

In step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including the RRC connection resume to the distributed unit <NUM>-<NUM>. Specifically, upon obtaining the context of the terminal <NUM>, the central unit <NUM>-<NUM> may allocate a UE F1-AP IE for the terminal <NUM>, and transmit a message requesting the context setup.

In step <NUM>, the distributed unit <NUM>-<NUM> determines rejection for the terminal <NUM>, and drops the RRC connection resume message. For example, this step may be a case in which the distributed unit <NUM>-<NUM> determines the rejection before step <NUM> but the insertion of the reject indication is omitted, or a case in which the rejection for the terminal <NUM> is determined after step <NUM>. In other words, this step may be a case in which the distributed unit <NUM>-<NUM> determines to reject the RRC connection resume after transmitting the initial uplink RRC message transfer message or a case in which the distributed unit <NUM>-<NUM> determines the rejection before transmitting the initial uplink RRC message transfer message but the operation of adding the reject indication to the initial uplink RRC message transfer message is omitted. Hence, the distributed unit <NUM>-<NUM> does not transfer the RRC connection resume message to the terminal <NUM>, but drops it.

In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup reject message. For example, the distributed unit <NUM>-<NUM> may generate the UE context setup reject message and transmit an uplink RRC message transfer message including the UE context setup reject message. If the distributed unit which is the lower object determines the rejection after transmitting the initial uplink RRC message, or determines before transmitting the initial uplink RRC message but the reject indication is omitted, the distributed unit <NUM>-<NUM>, which receives the downlink RRC transfer message including the RRC connection resume message, may perform the rejection operation by the distributed unit <NUM>-<NUM> for the RRC connection resume request, by transmitting the UE context setup reject message to the central unit <NUM>-<NUM>.

In step <NUM>, the central unit <NUM>-<NUM> removes the context of the terminal <NUM>. That is, the central unit <NUM>-<NUM> terminates the state transition procedure of the terminal <NUM>. Next, in step <NUM>, the state transition procedure of the terminal <NUM> may be performed again.

As in the various embodiments described above, in the state transition procedure, the procedure may be terminated quickly by the rejection determination of the distributed unit. The above-described embodiments illustrate the RRC connection resume procedure, but the above-described embodiments may be also applied to an initial access procedure and an RRC reestablishment procedure. Embodiments relating to the RRC reestablishment procedure will be described with reference to <FIG> and <FIG>, embodiments relating to the RRC resume procedure will be described with reference to <FIG> and <FIG>, and embodiments relating to the initial access procedure will be described with reference to <FIG> and <FIG>.

<FIG> illustrates a signal exchange diagram for an RRC reestablishment procedure in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> allocates resources. Herein, the resources may include a signaling radio bearer. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers a corresponding RRC message (e.g., the RRC connection reestablishment request message) in a non-UE related F1-AP initial uplink RRC message transfer message. In addition, the initial uplink RRC message transfer message may include C-RNTI of the UE <NUM>, and may further include at least one of information relating to resource allocation configuration (e.g. radio Resource Config Dedicated) and a reject indication. Next, the procedure divides into a first successful case <NUM>, a second successful case <NUM>, a first failure case <NUM>, and a second failure state <NUM>.

According to the first successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves UE context stored in the distributed unit <NUM>-<NUM> and the central unit <NUM>-<NUM>. Hence, the central unit <NUM>-<NUM> may assign a CU UE F1AP identifier (ID) for the terminal <NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reestablishment message. That is, the RRC connection reestablishment message is encapsulated in the downlink RRC message transfer message. Additionally, the downlink RRC message transfer message may further include information (e.g., previous CU UE F1AP ID, previous DU UE F1AP ID) for retrieving the UE context. In step <NUM>, the distributed unit <NUM>-<NUM> retrieves the UE context according to the information received in step <NUM>, and transmits the RRC connection reestablishment message. In step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment complete message to the distributed unit <NUM>-<NUM>. In step <NUM>, the distributed unit <NUM>-<NUM> encapsulates the RRC message, that is, the RRC connection reestablishment complete message, in the F1-AP uplink RRC message transfer message, and transmits the uplink RRC message transfer message to the central unit <NUM>-<NUM>.

According to the second successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves the UE context stored in the central unit <NUM>-<NUM>. That is, the second successful case <NUM> is an example corresponding to the case where the UE context is not stored in the distributed unit <NUM>-<NUM>. In step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including data radio bearer related information. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reestablishment message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment message. In step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment complete message. In step <NUM>, the distributed unit <NUM>-<NUM> encapsulates the RRC message, that is, the RRC connection reestablishment complete message, in the F1-AP uplink RRC message transfer message, and transmits the uplink RRC message transfer message to the central unit <NUM>-<NUM>.

According to the first failure case <NUM>, in step <NUM>, since the UE context is not stored in the central unit <NUM>-<NUM>, the central unit <NUM>-<NUM> fails to retrieve the UE context. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reestablishment reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment reject message to the terminal <NUM>.

According to the second failure case <NUM>, in step <NUM>, since the distributed unit <NUM>-<NUM> has determined not to accept one non-guaranteed bit rate (GBR) data radio bearer, the central unit <NUM>-<NUM> determines to reject the the state transition. That is, the second failure case <NUM> an example corresponding to a case where the reject indication is included in the initial uplink RRC message transfer message transmitted in step <NUM>. That is, the second failure case <NUM> is the case where the distributed unit <NUM>-<NUM> determines the RRC connection reject before transmitting the initial uplink RRC message transfer message, and the distributed unit <NUM>-<NUM> includes the reject indication in the initial uplink RRC message transfer message to notify the rejection case. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including the RRC connection reestablishment reject message. In other words, the central unit <NUM>-<NUM> receiving the initial uplink RRC message transfer message including the reject indication does not perform a new UE context assignment operation and directly transmits a downlink RRC message transfer message including the RRC connection reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment reject message to the terminal <NUM>. That is, the distributed unit <NUM>-<NUM> receiving the downlink RRC message transfer message including the RRC connection reject message transfers the corresponding RRC message to the terminal <NUM>, so that the terminal <NUM> is finally notified of the transition operation failure to the RRC connected state.

<FIG> illustrates another signal exchange diagram for an RRC reestablishment procedure in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers a corresponding RRC message (e.g., the RRC connection reestablishment request message) in a non-UE related F1-AP initial uplink RRC message transfer message. Next, the procedure divides into a first successful case <NUM>, a second successful case <NUM>, a first failure case <NUM>, and a second failure state <NUM>.

According to the first successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves UE context stored in the distributed unit <NUM>-<NUM> and the central unit <NUM>-<NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including information for retrieving the UE context. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reestablishment message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment message. In step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment complete message. In step <NUM>, the distributed unit <NUM>-<NUM> encapsulates the RRC message, that is, the RRC connection reestablishment complete message, in an F1-AP uplink RRC message transfer message, and transmits the uplink RRC message transfer message to the central unit <NUM>-<NUM>.

According to the second successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves the UE context stored in the central unit <NUM>-<NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including data radio bearer related information. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including the RRC connection reestablishment message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment message. In step <NUM>, the terminal <NUM> transmits an RRC connection reestablishment complete message. In step <NUM>, the distributed unit <NUM>-<NUM> encapsulates the RRC message, that is, the RRC connection reestablishment complete message, in the F1-AP uplink RRC message transfer message, and transmits the uplink RRC message transfer message to the central unit <NUM>-<NUM>.

According to the second failure case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves the UE context stored in the central unit <NUM>-<NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including data radio bearer related information. At this time, the distributed unit <NUM>-<NUM> determines that the terminal <NUM> is unacceptable, and transmits a UE context setup failure message, in step <NUM>. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including the RRC connection reestablishment reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment reject message to the terminal <NUM>.

The procedures of <FIG> and <FIG> are all related to the RRC connection reestablishment. The procedure of <FIG> has a technical advantage in that it may reduce latency of the connection reestablishment in some cases, and the procedure of <FIG> has a technical advantage in that there is no operation of adding the resource allocation configuration information (e.g., radio Resource Config Dedicated) and the reject indication.

<FIG> illustrates a signal exchange diagram for an RRC resume procedure in a wireless communication system according to claimed embodiments of the present disclosure.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection resume request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> allocates resources. Herein, the resources may include a signaling radio bearer (SRB). In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers the corresponding RRC message (e.g., RRC connection resume request message) in a non-UE related F1-AP initial uplink RRC message transfer message. In addition, the initial uplink RRC message transfer message may include C-RNTI of the terminal <NUM>, and may further include at least one of resource allocation configuration information (e.g. radio Resource Config Dedicated) and a reject indication. Next, the procedure divides into a successful case <NUM>, a successful fallback case <NUM>, and a failure case <NUM>.

According to the first successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves UE context stored in the distributed unit <NUM>-<NUM> and the central unit <NUM>-<NUM>, and determines to resume at least one data radio bearer. Accordingly, the central unit <NUM>-<NUM> may allocate a CU UE F1AP ID for the terminal <NUM>. In step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message including resource allocation information (e.g., radio Resource Config Dedicated) for at least one data radio bearer. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including the RRC connection resume message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection resume message. In step <NUM>, the terminal <NUM> transmits an RRC connection resume complete message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an uplink RRC message transfer message including the RRC message, that is, the RRC connection reestablishment complete message, to the central unit <NUM>-<NUM>.

According to the successful fallback case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> fails to retrieve the UE context and determines to accept an access request. Accordingly, the central unit <NUM>-<NUM> may assign a CU UE F1AP ID for the terminal <NUM>. Next, in step <NUM>, the central unit <NUM>-<NUM> generates an RRC connection setup message, and encapsulates and transmits the RRC connection setup message in a downlink RRC message transfer message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection setup message. In step <NUM>, the terminal <NUM> transmits an RRC connection setup complete message. In step <NUM>, the distributed unit <NUM>-<NUM> encapsulates the corresponding RRC message in an F1-AP uplink RRC message transfer message, and transmits the F1-AP uplink RRC message transfer message.

According to the failure case <NUM>, in step <NUM>, since the distributed unit <NUM>-<NUM> has included the reject indication, the central unit <NUM>-<NUM> omits the context retrieval. The second failure case <NUM> is an example corresponding to a case where the reject indication is included in the initial uplink RRC message transfer message transmitted in step <NUM>. That is, the second failure case <NUM> is the case where the distributed unit <NUM>-<NUM> determines the RRC connection reject before transmitting the initial uplink RRC message transfer message, and the distributed unit <NUM>-<NUM> includes the initial uplink RRC message transfer message to notify the rejection case. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reestablishment reject message. In other words, the central unit <NUM>-<NUM> receiving the initial uplink RRC message transfer message including the reject indication does not perform a new UE context assignment operation, and directly transmits a downlink RRC message transfer message including the RRC connection reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reestablishment reject message to the terminal <NUM>. That is, the distributed unit <NUM>-<NUM> receiving the downlink RRC message transfer message including the RRC connection reject message transfers the corresponding RRC message to the terminal <NUM>, so that the terminal <NUM> is finally notified of the transition operation failure to the RRC connected state.

<FIG> illustrates another signal exchange diagram for an RRC resume procedure in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection resume request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers the corresponding RRC message (e.g., the RRC connection resume request message) in a non-UE related F1-AP initial uplink RRC message transfer message. Next, the procedure divides into a successful case <NUM> and a failure case <NUM>.

According to the successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> successfully retrieves UE context stored in the distributed unit <NUM>-<NUM> and the central unit <NUM>-<NUM>, and determines to resume at least one data radio bearer. Alternatively, the central unit <NUM>-<NUM> fails to retrieve the UE context, and determines to accept an access request. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including a cause value. In step <NUM>, the distributed unit <NUM>-<NUM> allocates resources. Herein, the resources may include a signaling radio bearer (SRB). In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message including resource allocation information (e.g., radio Resource Config Dedicated) of at least one data radio bearer. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection resume message or an RRC connection setup message. For example, the RRC connection resume message may be included if the context retrieve is successful in step <NUM>, and the RRC connection setup message may be included if the context retrieve fails. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection resume message or the RRC connection setup message. In step <NUM>, the terminal <NUM> transmits an RRC connection resume complete message or an RRC connection setup complete message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an uplink RRC message transfer message including the RRC message, that is, the RRC connection reestablishment complete message or the RRC connection setup complete message to the central unit <NUM>-<NUM>.

According to the failure case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> succeeds in retrieving the UE context, or determines to accept the access. Hence, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including a cause value. In step <NUM>, the distributed unit <NUM>-<NUM> fails to allocate resources. That is, the distributed unit <NUM>-<NUM> determines that the terminal <NUM> is not acceptable. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup failure message. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reject message to the terminal <NUM>.

The procedures of <FIG> and <FIG> are all related to the RRC connection resume. The procedure of <FIG> has technical advantages of allowing flexible implementation and reducing latency for the resume. The procedure of <FIG> has a technical advantage of providing an integrated procedure for the successful case.

<FIG> illustrates a signal exchange diagram for an initial access procedure in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> allocates resources. Herein, the resources may include a signaling radio bearer (SRB). In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers the corresponding RRC message (e.g., the RRC connection request message) in a non-UE related F1-AP initial uplink RRC message transfer message. In addition, the initial uplink RRC message transfer message may include C-RNTI of the terminal <NUM>, and may further include at least one of resource allocation configuration information (e.g. radio Resource Config Dedicated) and a reject indication. Next, the procedure divides into a successful case <NUM> and a failure case <NUM>.

According to the successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection setup message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an RRC connection setup message. In step <NUM>, the terminal <NUM> transmits an RRC connection setup complete message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an uplink RRC message transfer message including the RRC connection setup complete message.

According to the failure case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reject message to the terminal <NUM>. That is, the terminal <NUM> is finally notified of the transition operation failure to the RRC connected state.

<FIG> illustrates another signal exchange diagram for an initial access procedure in a wireless communication system according to various embodiments of the present disclosure. These embodiments are described for illustration purposes only.

Referring to <FIG>, in step <NUM>, the terminal <NUM> transmits an RRC connection request message for transition to the RRC connected state. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an initial uplink RRC message. That is, the distributed unit <NUM>-<NUM> includes and transfers the corresponding RRC message (e.g., the RRC connection request message) in a non-UE related F1-AP initial uplink RRC message transfer message. Next, the procedure divides into a successful case <NUM> and a failure case <NUM>.

According to the successful case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including a cause value. In step <NUM>, the distributed unit <NUM>-<NUM> allocates resources. Herein, the resources may include a signaling radio bearer (SRB). In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup response message including resource allocation information (e.g., radio Resource Config Dedicated) of at least one data radio bearer. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection setup message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection setup message. In step <NUM>, the terminal <NUM> transmits an RRC connection setup complete message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits an uplink RRC message transfer message including the RRC message, that is, the RRC connection setup complete message to the central unit <NUM>-<NUM>.

According to the failure case <NUM>, in step <NUM>, the central unit <NUM>-<NUM> transmits a UE context setup request message including a cause value. In step <NUM>, the distributed unit <NUM>-<NUM> fails in the resource allocation. That is, the distributed unit <NUM>-<NUM> determines that the terminal <NUM> is not acceptable. In step <NUM>, the distributed unit <NUM>-<NUM> transmits a UE context setup failure message. In step <NUM>, the central unit <NUM>-<NUM> transmits a downlink RRC message transfer message including an RRC connection reject message. In step <NUM>, the distributed unit <NUM>-<NUM> transmits the RRC connection reject message to the terminal <NUM>.

The methods according to the embodiments described in the claims or the specification of the disclosure may be implemented in software, hardware, or a combination of hardware and software.

As for the software, a computer-readable storage medium storing one or more programs (software modules) may be provided. One or more programs stored in the computer-readable storage medium may be configured for execution by one or more processors of an electronic device. One or more programs may include instructions for controlling the electronic device to execute the methods according to the embodiments described in the claims or the specification of the disclosure.

Such a program (software module, software) may be stored to a random access memory, a non-volatile memory including a flash memory, a read only memory (ROM), an electrically erasable programmable ROM (EEPROM), a magnetic disc storage device, a compact disc (CD)-ROM, digital versatile discs (DVDs) or other optical storage devices, and a magnetic cassette. Alternatively, it may be stored to a memory combining part or all of those recording media. A plurality of memories may be included.

Also, the program may be stored in an attachable storage device accessible via a communication network such as Internet, Intranet, local area network (LAN), wide LAN (WLAN), or storage area network (SAN), or a communication network by combining these networks. Such a storage device may access a device which executes an embodiment of the present disclosure through an external port. In addition, a separate storage device on the communication network may access the device which executes an embodiment of the present disclosure.

In the specific embodiments of the disclosure, the elements included in the disclosure are expressed in a singular or plural form. However, the singular or plural expression is appropriately selected according to a proposed situation for the convenience of explanation, the disclosure is not limited to a single element or a plurality of elements, the elements expressed in the plural form may be configured as a single element, and the elements expressed in the singular form may be configured as a plurality of elements.

Claim 1:
A method performed by a distributed unit, DU, of a base station, comprising:
receiving (<NUM>), from a user equipment, UE, a radio resource control, RRC, message comprising a resume request wherein the RRC message indicates a transition to an RRC connected state from an RRC inactive state;
determining (<NUM>, <NUM>, <NUM>) whether the resume request of the UE is acceptable or not according to a status of layers managed by the DU;
in case that the resume request of the UE is acceptable (<NUM>, <NUM>) in the DU:
transmitting (<NUM>), to a central unit, CU, an initial uplink, UL, RRC message transfer message including an RRC container including the RRC message, the initial UL RRC message transfer message further including an information element to indicate that the resume request of the UE is acceptable in the DU,
transmitting (<NUM>), to the CU, a UE context setup response message including resource allocation information for a data radio bearer, DRB, in response to receiving (<NUM>), from the CU, a UE context setup request message for the DRB, and transmitting (<NUM>), to the UE, an RRC connection resume message based on a downlink, DL, RRC message transfer message, in response to receiving (<NUM>), from the CU, the DL RRC message transfer message including the RRC connection resume message based on success (<NUM>) of a UE context retrieval by the CU;
transmitting (<NUM>), to the UE, an RRC connection setup request message based on a DL RRC message transfer message, in response to receiving (<NUM>), from the CU, the DL RRC message transfer message including the RRC connection setup request message based on failure (<NUM>) of a UE context retrieval by the CU; and
in case that the resume request of the UE is not acceptable (<NUM>) in the DU:
transmitting (<NUM>), to the CU, an initial UL RRC message transfer message including an RRC container including the RRC message, the initial UL RRC message transfer message not including the information element, and
transmitting (<NUM>), to the UE, an RRC connection reject message based on a DL RRC message transfer message, in response to receiving (<NUM>), from the CU, the DL RRC message transfer message including an RRC connection reject message for the resume request.