CN117063608A - Apparatus and method for processing non-access stratum message using small data transmission procedure - Google Patents

Abstract

A user equipment having a Non-Access-Stratum (NAS) layer and a radio resource control (radio resource control, RRC) layer is provided, wherein the user equipment is configured to send an indication message from the NAS layer to the RRC layer. The indication message indicates that one or more NAS messages are to be delivered to the RRC layer for transmission to a core network node, and also specifies how the one or more NAS messages are to be transmitted. In response to receiving the indication at the RRC layer, the RRC layer adopts a connection state according to the indication.

Description

Apparatus and method for processing non-access stratum message using small data transmission procedure

Technical Field

The present invention relates to Non-Access-Stratum (NAS) signaling with small data transfer (Small Data Transmission, SDT) procedures. In particular, the invention relates to how small data transfers (Small Data Transmission, SDT) are performed for all NAS messages to be transferred in the network.

Background

In a communication device, a program for sending communications runs across multiple layers of a logical stack within the device's memory and operating system. The different layers of the stack are used to perform the different processes needed to perform the communication.

The non-access stratum (NAS) is a functional layer in the radio communication protocol stack between the core network and the user equipment. One of the functions of the NAS layer is to manage communication session establishment and maintenance as user equipment moves in the communication network. The NAS layer is in contrast to an Access Stratum (AS) layer that is responsible for carrying information over the radio part of the communication network. NAS may also be considered as a protocol for messages transparently transferred over a wireless network between a User Equipment (UE) or User Equipment (e.g., mobile device) and a core node (e.g., mobile switching center, serving GPRS support node, or mobility management entity). Examples of NAS messages include attach or tracking area update messages, authentication messages, or service requests. Once a User Equipment (UE) establishes a wireless connection, the UE communicates with a core node using the wireless connection to coordinate the desired services. The difference is that the access stratum is explicitly used for the session between the UE or mobile device and the wireless network, while the NAS is used for the session between the UE or mobile device and the core network node. NAS messages are passed to the lower radio resource control (radio resource control, RRC) layer for transmission from the UE or mobile device to a node of the network.

NAS messages are typically transmitted by a data pipe or data channel according to a signaling radio bearer format (e.g., SRB1 or SRB 2), with the RRC layer in rrc_connected mode. NAS messages are encrypted using NAS security, and the RRC layer simply transmits the NAS message by carrying the NAS message on the RRC message or including the NAS message in a UL information transfer or DL information transfer RRC message. Thus, the content of the NAS message is not visible to the RRC layer. In general, in order to transmit the NAS message, the UE or the mobile device needs to transition from the rrc_idle mode to the rrc_connected mode at the RRC layer. This mode transition of the UE or mobile device requires a certain amount of power consumption by the UE or mobile device.

Some NAS messages are sent for communication processes that require only one NAS message or a few NAS messages (request and response messages). In this case, the power consumption of the UE to transition from the inactive or idle state to the connected state for transmitting one or only a few NAS messages is particularly inefficient.

It is desirable to develop a mechanism for sending NAS messages that saves power when only a small number of NAS messages are sent.

Disclosure of Invention

According to an aspect, a user equipment having a Non-Access-Stratum (NAS) and a radio resource control (radio resource control, RRC) layer is provided. The user device is configured to: sending an indication message from the NAS layer to the RRC layer, wherein the indication message indicates that one or more NAS messages are to be delivered to the RRC layer for transmission to the core network node, the indication further specifying how to transmit the one or more NAS messages; in response to receiving the indication at the RRC layer, a connection state is employed according to the indication.

The user device may also be configured to: if the indication message specifies that one or more NAS messages are to be transmitted to the core network node through the signaling radio bearer SRB2, causing the RRC layer to initiate a transition from the unconnected state to the connected state; alternatively, if the indication message specifies that one or more NAS messages are to be transmitted to the core network node over SRB2bis, the RRC layer is not caused to initiate a transition from the unconnected state to the connected state.

The user device may also be configured to: if the indication message comprises an explicit instruction for keeping the RRC layer in an unconnected state, continuing to be in the unconnected state; alternatively, if the indication includes an explicit instruction for the RRC layer to initiate a transition from the unconnected state to the connected state, a transition from the unconnected state to the connected state is initiated.

The transition from the unconnected state to the connected state includes performing a conventional restoration process.

The unconnected state may be an inactive state or an idle state.

The user equipment may be configured to send the indication message as part of an uplink RRC early data request message on the CCCH during a mobile-initiated early data transfer (Mobile Originated Early Data Transmission, MO-EDT) or similar procedure defined for a 3G, 4G or 5G mobile communication system.

The user device may be configured to send the indication message as part of a control plane cellular internet of things (Cellular Internet of Things, CIoT) procedure in a new air interface (NR).

The indication message may be a separate message or may be a field appended to one or more NAS messages.

The user device may be configured to send a further indication message from the RRC layer to the NAS layer, the further indication message indicating whether a current cell node to which the user device is connected supports a small data transfer (small data transmission, SDT) function.

The user device may be configured to include the indication message as part of an uplink RRC message sent from the user device to the network.

The user device may be configured to receive the indication as part of a downlink RRC message transmitted from the network to the user device.

According to another aspect, there is provided a method for performing radio resource control in a user equipment having a NAS layer and an RRC layer, the method comprising:

sending an indication message from the NAS layer to the RRC layer indicating that one or more NAS messages are to be delivered to the RRC layer for transmission to the core network node, the indication further specifying how the one or more NAS messages are to be transmitted; and responding to the receiving of the indication message at the RRC layer, and adopting the connection state of the RRC layer according to the indication.

According to another aspect, there is provided a network apparatus for operating as a cell node in a network comprising one or more user devices and one or more further cell nodes, wherein the network apparatus is configured to operate as a relay node in response to receiving an indication of a partial context from another cell node, the indication of the partial context being configured to perform NAS messaging between the user device and a core network node connected to the network apparatus.

The network device may be configured to: receiving an uplink RRC message from the user equipment, the uplink RRC message including an indication message indicating that one or more NAS messages are to be transmitted to a core network node while an RRC layer of the user equipment remains in an unconnected state; and transmitting a NAS signaling transfer instruction to another cell node of the network in response to receiving the uplink RRC message, wherein the NAS signaling transfer instruction comprises an instruction message for indicating that an RRC layer of the user equipment maintains an unconnected state.

The network device is operable to operate as a relay node by applying RLC configuration and exchanging uplink and downlink NAS messages with another cell node of the network for transmission to the core network in response to receiving an indication of the partial context. For part of the UE context, only RLC contexts for signaling radio bearer SRB2 and/or signaling radio bearer SRB2bis are sent instead of for SDT DRB, as part of the UE context is not used in the current procedure.

The NAS signaling indication may include an indication message indicating the RRC layer of the user equipment as XnAP: a portion of the retrieve UE context request message remains unconnected and the procedure is only used to transmit NAS messages.

In response to receiving XnAP: retrieving a UE context request message, the network device may not relocate the UE's context and may send part of the UE context only for RLC contexts for SRB2 and/or SRB2bis, but not for SDT DRB, wherein the XnAP: retrieving the UE context request message includes an indication message indicating that the RRC layer of the user equipment remains unconnected and that the procedure is only used to transmit NAS messages.

The RRC layer of the user equipment may be used to trigger the use of a shortened timer to complete NAS message transmission.

According to another aspect, a network is provided, the network comprising one or more user devices according to any one of claims 1 to 11 and one or more network apparatuses according to any one of claims 12 to 16.

Drawings

The invention will now be described by way of example with reference to the accompanying drawings. In the drawings:

fig. 1 shows a schematic diagram of a proposed indication message sent between a NAS layer and an RRC layer of a user equipment.

Fig. 2 illustrates an example messaging diagram for delivering NAS messages from a user device to a cell node of a network using an SDT procedure.

Fig. 3 illustrates an example messaging diagram for transmitting NAS messages from a user device to a cell node of a network.

Fig. 4 illustrates an example messaging diagram for a user device transmitting between cell nodes of a network.

Detailed Description

A procedure is presented herein that saves UE power when only a small number of NAS messages are processed or sent. In particular, the proposed procedure enables transmission of NAS messages without requiring the UE to transition the RRC layer, for example, from an inactive state to a connected state. In the rrc_inactive state, sending NAS messages for certain procedures may save power for the device.

When the RRC layer is in rrc_inactive, a small amount of user plane data may be transmitted. However, this mechanism has not been used to support the transmission of signaling radio bearer SRB messages with small data transfer (small data transmission, SDT). In particular, SDT has not been used to transport NAS messages.

To enable the transport of NAS messages, SDT needs to be supported for existing signaling radio bearer SRB1 and signaling radio bearer SRB2 carrier types that are typically used for NAS messages. Such development means that NAS messages can be transmitted using SDT functionality without switching to rrc_connected state. However, in some cases where a large number of NAS messages are to be sent, such as during a continuous communication session, this may be undesirable and thus may cause problems for certain types of NAS procedures.

An apparatus and method are presented herein for indicating whether NAS messages to be transmitted from an RRC layer can be transmitted while the RRC layer remains in an unconnected state, an idle or inactive state. Depending on whether the NAS message belongs to a NAS procedure that can securely utilize SDT functionality. Thus, such NAS messages may be sent without the UE having to transition the RRC layer from idle or inactive mode to connected mode. Such NAS messages may be identified and indicated to the RRC layer, providing an efficient control mechanism and reducing power consumption of the UE.

For example, NAS procedures can be divided into two types. For the class 1 NAS procedure, the UE transitions from rrc_idle to rrc_connected mode, performs the NAS procedure, and then returns to rrc_idle. For the class 2 NAS procedure, the UE transitions from rrc_idle to rrc_connected mode, performing the NAS procedure. The NAS procedure in turn may trigger other NAS procedures such as authentication followed by packet data unit or PDU session establishment and exchange of user plane data. For such class 2 procedures, the UE remains in rrc_connected mode until the session or call is released. It should be noted that the terms "class 1" and "class 2" for NAS procedure classification are only used to explain the concept that only certain NAS procedures can be performed in rrc_inactive mode. In other words, the use of class 1 and class 2 NAS procedures and like terms are purely for illustration purposes and do not limit the invention to these terms.

The proposed method or the present invention is to provide a mechanism for efficiently delivering class 1 NAS messages for certain procedures while maintaining the unconnected state rrc_inactive using SDT framework. While for other class 2 NAS messages, the Non-Access Stratum (NAS) may provide an indication to transition to the rrc_connected state. Without this proposed mechanism to indicate to the RRC layer when to switch state, the RRC layer in the UE does not know when NAS messages can be transmitted while maintaining the rrc_inactive state, but always switches to the rrc_connected state when NAS messages are transmitted.

Since only some NAS messages may or need to be transmitted in rrc_inactive state, the NAS layer may provide an implicit or explicit indication to the AS or RRC layer indicating whether the UE should remain in an unconnected state or transition to a connected state.

AS previously described, the AS/RRC layer cannot determine when to perform a state transition because the NAS message is encrypted by the NAS security mechanism and thus cannot be decoded by the AS/RRC layer.

The proposed approach is to provide a mechanism for efficiently delivering specific types of NAS messages, referred to herein as class 1 or first type NAS messages, using the SDT framework while maintaining the rrc_inactive state.

That is, the RRC layer of the user equipment or UE may be configured to assume a connected state in response to an indication of how to transmit one or more NAS messages. Thus, the RRC layer may be configured to maintain the unconnected state in response to at least one form of indication. For example, the indication may be in the form of an indication that SRB2 is to be used for transmitting NAS messages, which means or specifies that the NAS message to be transmitted is of class 2. Furthermore, in another example, the indication that SRB2 is not used to transmit NAS messages means that class 1 NAS messages are to be transmitted. If it can be assumed that class 1 NAS messages are to be transmitted according to this indicated form of transmission type, it can also be assumed that the RRC layer can remain unconnected for transmission, e.g. by SDT mechanism. In contrast, the use of SRB2 typically triggers the RRC layer to transition to the connected state as part of the existing process of transmitting NAS messages.

Examples of class 1 NAS procedures are UE-initiated periodic registration updates (due to having a predefined period of inactivity), UE-initiated de-registration procedures, and UL/DL NAS messages for positioning (including reporting and assistance data).

Examples of class 2 NAS procedures are UE-triggered service request, UE-initiated PDU session establishment, UE-initiated PDU session modification, and UE-initiated PDU session release.

While the UE remains in rrc_inactive state, a class 1 NAS procedure may be initiated and completed. However, for class 2 NAS procedures, the UE should immediately transition to rrc_connected state, AS these procedures typically involve extensive AS/RRC and NAS signaling and setup, modification or release procedures of the user plane. For network initiated class 2 NAS procedures, the network may also perform paging and put the UE into a connected state.

Fig. 1 shows a schematic diagram of the proposed method 100. NAS layer 102 is shown above RRC layer 104, wherein sending indication 106 according to the arrow direction from NAS layer 102 to RRC layer 104 is depicted. The NAS layer 102 and the RRC layer 104 form part of a user equipment or user equipment, which is part of a network comprising one or more user equipment and one or more network devices (e.g. cell nodes or core network nodes). The user device is configured to send an indication message 106 from the NAS layer 102 to the RRC layer 104, wherein the indication message 106 indicates that one or more NAS messages are to be delivered to the RRC layer 104 for transmission to the core network node. The indication also specifies how to transmit one or more NAS messages. In response to receiving the indication message 106 at the RRC layer 104, the RRC layer 104 adopts the connected state according to the indication.

In an embodiment, the indication 106 may be an indication provided to the RRC layer 104 that no NAS message is transmitted using SRB 2. The indication may take the form of a data pipe or data channel specifically indicating that the NAS message is to be transmitted over "non-SRB 2" and may be referenced by a dedicated tag (e.g., SRB2 bis). The tag used to indicate that the NAS message is to be sent over a data pipe or data channel other than SRB2 may be selected as any suitable tag for this purpose. Thus, when the UE maintains the rrc_inactive state, a separate SRB may be used to transmit the NAS message. SRB2-bis may be established for use with SRB2 and may be used to carry NAS class 1 procedure messages while the UE remains in rrc_inactive state.

Thus, SRB2 can only be used to transport NAS messages for class 2 procedures, which in turn triggers a state transition to rrc_connected. This is because, as described above, for these procedures, multiple NAS messages are expected to be exchanged with the network and may also involve the establishment, modification or release of the user plane.

Thus, a mechanism is provided. By this mechanism, the NAS layer can indicate on which SRB or which SRB format/type the NAS message should be transmitted or which SRB format/type to utilize, which in turn indicates to the RRC layer whether the NAS message to be transmitted can or can be transmitted in rrc_inactive mode, simply by indicating the carrier type of the NAS message. This is thus an implicit indication to the RRC layer, indicating in what state the RRC layer may operate to transmit NAS messages. The implicit indication to trigger the transition of state to RCC Connected mode is achieved by the instruction to transmit NAS messages on SRB 2. The implicit indication to keep the RRC layer in rrc_inactive mode is achieved by the instructions for transmitting NAS messages over SRB2 bis.

The user device may be configured to: if the indication specifies that one or more NAS messages are to be transmitted to the core network node via SRB2, causing the RRC layer to initiate a transition from the unconnected state to the connected state; alternatively, if the indication specifies that one or more NAS messages are to be transmitted to the core network node over SRB2bis, the RRC layer is not caused to initiate a transition from the unconnected state to the connected state.

In an embodiment, the indication may comprise an explicit instruction message 106 sent from the NAS layer 102 to the RRC layer 104, the explicit instruction message 106 instructing the RRC layer to assume the connected state directly according to the content of the indication. That is, the indication may indicate that the RRC layer remains in an unconnected state or that the RRC layer initiates a transition from the unconnected state to the connected state. Thus, the user device is to: if the indication message includes an explicit instruction to keep the RRC layer in the unconnected state, it continues to be in the unconnected state. Thus, the user device is also used to: if the indication includes explicit instructions to cause the RRC layer to initiate a transition from the unconnected state to the connected state, a transition from the unconnected state to the connected state is initiated. The transition from the unconnected state to the connected state may include performing a conventional restoration process.

When the user equipment or device is in rrc_inactive mode (also referred to as an unconnected state), the NAS layer may thus cause the UE to perform transmission of a class 1 NAS procedure message while maintaining the rrc_inactive state by explicitly instructing the RRC layer to maintain the unconnected state. For UE-initiated class 2 procedures, the NAS layer may thus provide an explicit indication to the AS/RRC layer to transition to rrc_connected.

In this case, the indication is a message comprising said instructions. The indication is appended in one or more NAS messages to be transmitted to the core network node. The indication message may be a separate message appended to one or more NAS messages. The explicit indication message may be attached by being embedded in the RRC restoration request message, or an UL information transfer message of the RRC uplink message may be concatenated to the RRC restoration request message. That is, among other possible methods for carrying NAS messages, either embedded in the RRC resume request or appended in the uplink with the RRC resume message as UL information transfer data.

To provide such an explicit indication message 106, the NAS layer 102 needs to first determine which NAS procedure is being initiated. This may be done when the UE is in rrc_inactive state. The NAS layer 102 can then notify the RRC layer 104 of an instruction (e.g., "continue in rrc_inactive" or "initiate transition to rrc_connected state") via the explicit indication message 106 accordingly, as follows.

In addition to the indication from the NAS layer 102 to the RRC layer 104, it may be desirable to provide further indications from the RRC layer 104 to the NAS layer 102 in the opposite direction. The RRC layer may in turn provide an indication message to the NAS layer 102 as to whether SDT functionality is supported within the cell node to which the UE is currently connected. This may be helpful to the NAS layer because the SDT function is a mechanism for the RRC layer to transmit NAS messages while maintaining the unconnected state. Such NAS message transmission with the RRC layer in an unconnected state may fail if the cell network node does not support the SDT function.

The explicit indication message of the necessary RRC layer state may be extended to MO-EDT procedure for control plane cellular internet of things (Control Plane Cellular Internet of Things, CIoT). For example, when the upper layer has requested that the rrc_connection mode of the mobile originated data be established or restored, a mobile originated early data transfer (Mobile Originated Early Data Transmission, MO-EDT) is triggered. In this example, the uplink user data may be transmitted in NAS messages. The uplink user data and NAS message may be concatenated in an uplink RRC message (UL RRC early data request message) on the CCCH. Similarly, downlink user data may optionally be transmitted and concatenated with NAS messages in a downlink RCC message (DL RRC early data complete message) on the CCCH. Such a mechanism may provide enhancements to existing MO-EDT procedures, where similar mechanisms may be used to carry NAS messages, and the NAS layer may indicate to the RRC layer whether the UE needs to enter the rrc_connected state or whether rrc_idle may be maintained while NAS messaging is performed. This may be achieved using an explicit indication message as described above. Thus, the user device may be configured to send an indication message during the MO-EDT procedure as part of an uplink RRC early data request message on the CCCH.

The explicit indication message 106 of the necessary RRC layer 104 state may be extended to a Control Plane (CP) cellular internet of things (cellular internet of things, CIoT) for use in a new air interface (NR), which is an alternative name for the 5G radio interface. Thus, the user device may be configured to send an indication message during the MO-EDT procedure as part of an uplink RRC early data request message on the CCCH. In the case of introducing the CP CIoT scheme described above (similar to the scheme used in LTE) for NR, the mechanism presented herein may be used to carry NAS messages, and when NAS messaging is performed using explicit indication as described above, NAS layer 102 may indicate to RRC layer 104 in indication message 106 whether the UE needs to enter rrc_connected state or whether rrc_idle state may be maintained.

Thus, depending on the implementation scenario, the unconnected state may be an inactive state or an idle state.

The proposed method can be described as a method of performing radio resource control in a user equipment having a NAS layer and an RRC layer. Such a method may include sending an indication message from the NAS layer to the RRC layer indicating that one or more NAS messages are to be delivered to the RRC layer for transmission to the core network node. The indication may also specify how to transmit one or more NAS messages. The method may continue by employing a connection state of the RRC layer according to the indication in response to receiving the indication message at the RRC layer. How the one or more NAS messages are transmitted may include indicating the carrier that will be used to transmit the NAS message, e.g., via SRB2 or SRB2bis, as described above. Alternatively, how the NAS message is transmitted may include explicit instructions to the RRC layer regarding the connection state that the RRC layer should use when transmitting the NAS message, as described herein.

Fig. 2 shows a messaging diagram for a class 1 NAS procedure using an SDT procedure to transfer NAS messages from a UE 202 to a cell node 204 of the network. Such transmission utilizes the proposed indication message 106 in the connection setup message transmitted between the UE 202 and the cell node 204. For example, initially the UE 202 is in an unconnected state in rrc_inactive 206. The SDT procedure starts, but in this case the SDT procedure can support the sending of NAS messages. A series of messages may then be exchanged between UE 202 and cell node 204. For example, these messages may include MSG 1 preamble-SDT indication 210, MSG RAR (grant of msg3+ data) 212, MSG 3RRC resume request+srb2 message (UL information transfer) 214, and MSG 4RRC release+srb2 message (DL information transfer) 216 message. To transmit NAS messages for class 1 procedures in rrc_inactive mode, an uplink NAS message may be included in UL information transmission data and transmitted together with an uplink RRC message (e.g., RRC resume message 214). The downlink NAS message may be included in the DL information transfer data and transmitted together with a downlink RRC message (e.g., RRC release message 216).

The NAS procedure message may be carried on SRB2 for a NAS message for a class 2 procedure as described above, or on SRB2bis for a NAS message for a class 1 procedure as described above. The uplink NAS messaging using the SDT mechanism may transmit an implicit indication message 106 to the network 104 to keep the UE 102 rrc_inactive during the downlink NAS messaging.

That is, the use of SRB2bis may also instruct the network to keep the UE rrc_inactive during DL NAS signaling. When the network transmits NAS messages to the UE as part of NAS signaling, SRB2bis may be used to indicate to the network that the UE should not be expected or requested to transition to a connected state to receive NAS messages as part of the downlink RCC message.

Whenever the UE selects a cell node with small data transfer (small data transmission, SDT) functionality enabled, the RRC layer will inform the NAS layer of this. Furthermore, if the mobile user device moves out of coverage of that cell node and selects other cell nodes that do not enable or support SDT functionality, the RRC layer may again indicate this to the NAS layer.

Fig. 3 shows a messaging diagram for transferring NAS messages from a UE 202 to a cell node 204 of a network for a class 2 NAS procedure. This type of transmission may also utilize the indication message mechanism set forth above. To transmit NAS messages for class 2 procedures, the UE 202 may initiate a legacy RRC recovery procedure based on either the explicit indication message 106 sent from the NAS layer 102 or the implicit indication message 106 sent from the NAS layer 102.

Fig. 3 first shows that the current state 206 of the UE 202 is rrc_inactive. At 302, the proposed indication message 106 is sent from the NAS layer 102 and instructs the RRC layer 104 to transition to the RRC connected state and initiate a conventional recovery procedure. Messages may then be exchanged between the UE and the cell node 204. The exchange may include messages such as MSG 1 preamble 304, MSG 2RAR (grant of MSG 3) 306, MSG 3RRD resume request 308, and MSG 4RRC resume 310 message. In a next stage 312, the ue 202 has transitioned to rrc_connected mode. UE 202 may then exchange further messages with cell node 204, e.g., by sending MSG 5RRC resume complete 314 and UL information transfer 316 messages, to complete the transfer. Then, AS and NAS signaling in RRC Connected state 318 is established between UE 202 and cell node 204.

Thus, the user device may be configured to include the indication message as part of an uplink RRC message sent from the user device to the network. Similarly, the user device may be configured to receive the indication as part of a downlink RRC message transmitted from the network to the user device.

NAS procedure processing with SDT in the network may be enhanced when involving transfer of a UE from one cell node to other cell nodes of the network. The class 1 NAS procedure applicable to SDT can be further optimized at the network side as follows. For a class 1 NAS procedure, the UE, in case of a cell node different from its last serving cell node, resuming the data transfer session, the indication of NAS signaling transfer may be included at XnAP: the UE context request message is retrieved. Based on the indication, the last serving cell node may decide not to relocate the context of the session. This may be because, for example, the current procedure is only used for NAS signaling. In this case, the last serving cell node may provide a partial UE context, where only RLC context is included as an indication of SRB2 or SRB2 bis. This may not be useful for SDT DRBs because no part of the UE context is used in such a procedure.

Thus, NAS procedure processing with SDT in the UE may also be enhanced. The class 1 NAS procedure applicable to SDT can be further optimized on the UE side. Since class 1 NAS procedures have well-defined traffic patterns, which typically include one request and one response message, such procedures can use a single SDT procedure. Thus, NAS signaling may be exclusively performed using a different timer than T319 or T319', which is a shortened version of these timers. That is, a timer is started when an RRC resume request message is transmitted in the UE and stopped when an RRC release message is received. Thus, the UE may start timers of different lengths depending on whether it performs a single NAS signaling transmission or a single or multiple small data transmissions.

Fig. 4 shows an example of a user device or user equipment 202 transmitting between cell nodes 204 and 402 of a network. An indication of the state that the RRC layer should be in to perform ongoing NAS messaging may be included in the NAS signaling message to facilitate such delivery while enabling the RRC layer to assume the appropriate connection state.

To perform the above-described transfer of UEs between cell nodes, the cell node or network device 204 may be configured to process the above-described indication message relating to RRC layer connection state when incorporated in a NAS signaling transfer message.

Thus, the network device may be used to operate as a cell node in a network comprising one or more user equipment and one or more further cell nodes 402 for processing the indication messages as described above. The network device 204 is operable to operate as a relay node in response to receiving a partial context indication from another cell node 402 for performing NAS messaging between the user equipment 202 connected to the network device 204 and the core network node. The partial context includes only RLC contexts for SRB2 and/or SRB2bis, not for SDT DRBs, because the partial context is not used in the current procedure.

Thus, the network apparatus 204 is operable to receive an uplink RRC message from the user device 202, the uplink RRC message including an indication message indicating that one or more NAS messages are to be transmitted to the core network node while the RRC layer of the user device remains in an unconnected state. That is, the uplink RRC message may include information of the above-described indication message regarding the RRC layer connection state of the UE. The network device may be further configured to transmit, in response to receiving the uplink RRC message, a NAS signaling transfer indication to another cell node 402 of the network, the NAS signaling transfer indication further comprising an indication message indicating that the RRC layer of the user equipment remains in an unconnected state, and the procedure is only used for transfer of NAS signaling.

As a result of indicating to the other cell node 402 that the RRC layer of the UE will remain in an unconnected state, the other cell node may decide not to relocate the UE's context based on the NAS signaling indication to the other cell node of the network, but rather provide part of the context of SRB2 and/or SRB2bis to the current cell node 204. Thus, in this case, the current cell node or network device 204 may operate as a relay node. In response to receiving an indication of the partial context, the RLC configuration may be initiated by applying the configuration and exchanging uplink and downlink NAS messages with another cell node 402 of the network for transmission to the core network.

For the above relay node configuration to be applied to the network device, the NAS signaling indication may include an indication message indicating the RRC layer of the user equipment as XnAP: a portion of the retrieve UE context request message remains unconnected and this procedure is being used to transmit NAS messages. In response to receiving XnAP: retrieving the UE context request message, the network device is further operable to trigger completion of NAS message transmission using a shortened timer, wherein the XnAP: the retrieve UE context request message includes an indication message indicating that the RRC layer of the user equipment remains in an unconnected state.

Currently, SDT cannot support all types of NAS procedures, because for some NAS procedures, the UE needs to transition to rrc_connected state as soon as possible. Thus, the proposed mechanism enables the NAS procedure to be divided into two classes, e.g. class 1 and class 2, by NAS layer, where class 1 is suitable for transmissions using SDT, whereas class 2 requires the RRC layer of the UE to transition to RRC Connected state. The UE NAS layer can determine which NAS procedure is being initiated when the UE is in rrc_inactive state and inform the RRC layer, accordingly, by an indication message, whether to keep rrc_inactive mode or initiate a transition to rrc_connected mode. The above mechanisms provide a number of implementation examples for transmitting NAS messages of either process type over the air interface with explicit or implicit indication messages sent from the NAS layer. Possible enhancements to the SDT-compliant NAS procedure handling between the network cell node and the UE are also described above.

The proposed method provides a mechanism that can efficiently transfer NAS messages for class 1 NAS procedures while enabling the UE RRC layer to maintain rrc_inactive unconnected state using SDT framework. Without the proposed mechanism, the RRC layer in the UE does not keep the rrc_inactive unconnected state when sending any NAS message, but instead transitions to the rrc_connected state every time. This continuous transition of the RRC layer into and out of the connected state is a situation where the power usage of the used device is inefficient. Furthermore, the possible enhancements of the SDT procedure for NAS signaling within the network and UE set forth above make NAS signaling efficient by using the SDT framework, thus providing network benefits in terms of reduced signaling and thus further improvements to the UE in terms of power savings.

Applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features. Such features or combinations can be implemented as a whole based on the present description, without regard to whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims, as is common to those of ordinary skill in the art. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims (18)

1. A user equipment (202) having a Non-Access-Stratum (NAS) (102) and a radio resource control (radio resource control, RRC) layer (104), the user equipment being configured to:

sending an indication (106) from the NAS layer to the RRC layer, wherein the indication (106) indicates that one or more NAS messages are to be passed to the RRC layer for transmission to a core network node, the indication further specifying how to transmit the one or more NAS messages;

and in response to receiving the indication at the RRC layer, adopting a connection state according to the indication.

2. The user device of claim 1, wherein the user device is further configured to:

if the indication specifies that the one or more NAS messages are to be transmitted to the core network node over a signaling radio bearer, SRB2, causing the RRC layer to initiate a transition from an unconnected state to a connected state;

or alternatively

If the indication specifies that the one or more NAS messages are to be transmitted to the core network node over SRB2bis, the RRC layer is not caused to initiate a transition from the unconnected state to the connected state.

3. The user device of claim 1, wherein the user device is further configured to: if the indication includes an explicit instruction to cause the RRC layer to remain in an unconnected state, continuing to be in the unconnected state; or alternatively

If the indication includes an explicit instruction to cause the RRC layer to initiate a transition from an unconnected state to a connected state, then initiating a transition from the unconnected state to the connected state.

4. A user device according to claim 3, wherein said transition from said unconnected state to said connected state comprises performing a conventional recovery procedure.

5. A user device according to any of the preceding claims, characterized in that the unconnected state is an inactive state or an idle state.

6. The user device according to any of claims 3 to 5, characterized in that the user device is configured to send the indication during a mobile-initiated early data transfer (Mobile Originated Early Data Transmission, MO-EDT) procedure as part of an uplink RRC early data request message on the CCCH.

7. The user device of claim 6, wherein the user device is configured to send the indication as part of a control plane cellular internet of things (Cellular Internet of Things, CIoT) procedure in a New Radio (NR).

8. The user device according to any of claims 3 to 7, characterized in that the indication is a separate message appended to the one or more NAS messages.

9. The user equipment according to any of the preceding claims, characterized in that the user equipment is configured to send a further indication from the RRC layer to the NAS layer indicating whether the current cell node to which the user equipment is connected supports a small data transfer (small data transmission, SDT) function.

10. A user device according to any of the preceding claims, characterized in that the user device is adapted to include the indication as part of an uplink RRC message sent from the user device to the network.

11. A user device according to any of the preceding claims, characterized in that the user device is arranged to receive the indication as part of a downlink RRC message transmitted from the network to the user device.

12. A network device (204), characterized in that the network device (204) is arranged to operate as a cell node in a network comprising one or more user equipment (202) and one or more further cell nodes (402), wherein the network device is arranged to operate as a relay node in response to receiving an indication of a partial context from another cell node, the indication of the partial context being arranged to perform NAS messaging between the user equipment and a core network node connected to the network device.

13. The network device of claim 12, wherein the network device is to:

receiving an uplink RRC message from the user equipment, the uplink RRC message including an indication message indicating that one or more NAS messages are to be transmitted to a core network node while the RRC layer of the user equipment remains in an unconnected state;

in response to receiving the uplink RRC message, transmitting a NAS signaling transfer indication to the other cell node of the network, the NAS signaling transfer indication comprising an indication message indicating that the RRC layer of the user equipment remains unconnected and receiving the NAS message from the user equipment.

14. The network device according to claim 12 or 13, characterized in that the network device is adapted to operate as a relay node by applying RLC configuration and exchanging uplink and downlink NAS messages with said other cell node of the network for transmission to the core network in response to receiving said indication of partial context.

15. The network apparatus according to claim 13 or 14, wherein the NAS signaling indication comprises an indication message indicating the RRC layer of the user equipment as XnAP: a portion of the retrieve UE context request message remains in the unconnected state and the procedure is being used for NAS messaging.

16. The network device of claim 15, wherein in response to receiving XnAP: retrieving a UE context request message, the network device to trigger completion of NAS message transmission using a shortened timer, wherein the XnAP: the retrieve UE context request message includes an indication message indicating that the RRC layer of the user equipment maintains the unconnected state.

17. A network comprising one or more user devices (202) according to any of claims 1 to 11 and one or more network equipment (204) according to any of claims 12 to 16.

18. A method for performing radio resource control in a user equipment having a NAS layer (102) and an RRC layer (106), the method comprising:

sending an indication (106) from the NAS layer to the RRC layer, wherein the indication (106) indicates that one or more NAS messages are to be passed to the RRC layer for transmission to a core network node, the indication further specifying how to transmit the one or more NAS messages;

and responding to the receiving of the indication at the RRC layer, and adopting the connection state of the RRC layer according to the indication.