Method and apparatus for canceling a circuit switched fallback

A method and apparatus may be used to cancel a communication session. The apparatus may transmit a first message to initiate a communication session and then transmit a second message that indicates a request to cancel the communication session. If the apparatus determines that there is an ongoing Packet Switched (PS) session, the apparatus may continue the ongoing PS session on a current Radio Access Technology (RAT) without performing an intersystem change. If the apparatus receives a handover (HO) command, the apparatus may transmit a message to reject the HO command, or it may ignore the HO command.

TECHNOLOGY FIELD

This application is related to wireless communications.

BACKGROUND

It has been a goal in wireless communications to devise a mechanism to enable a wireless transmit/receive unit (WTRU) to place or receive a communication during an ongoing session without service interruption. For example, in a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) scenario, a goal has been to devise a mechanism to enable a WTRU, while it is connected to a packet switched (PS) domain of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), to place (i.e., originating) or receive (i.e., terminating) circuit switched (CS) services or vice versa. Examples of CS services may include at least such services as a CS voice call, a short message service (SMS), location services, or supplementary services.

In some wireless communication deployment scenarios, some services may initially be provided by a particular Radio Access Technology (RAT), in which case the WTRU may be required to perform a handover to a target RAT prior to canceling an initiated service. Performing a handover may lead to service interruption at the source RAT. In systems such as Global System for Mobile communication (GSM), Universal Mobile Telecommunications System (UMTS), or Code Division Multiple Access (CDMA), a user may cancel an ongoing mobile originated (MO) session request before the session setup is completed. For example, in a GSM deployment scenario, a connection management (CM) entity may handle session cancellation procedures. In this example, if a user decides to cancel an initiated session before the session setup is completed, the CM entity may send a cancel message. A handover to the target RAT, however, may be performed in order to cancel the initiated session.

In an LTE example, a WTRU may perform a CS fallback (CSFB) procedure to initiate a CS session during an ongoing PS session. Because a CS session request via LTE may force the WTRU to change RATs, canceling a CS session request when the WTRU is camped on an LTE cell may cause numerous problems.

For example, depending on the time at which the user cancels the ongoing CSFB procedure, signaling messages may have been exchanged or may be in the process of being exchanged between network entities in preparation to execute the CSFB. This may lead to problems such as service interruption, time delay, or unnecessary resource drain. For example, the source and target network nodes may exchange signaling messages to prepare resources for the terminal. Not expecting the WTRU to cancel the request, the mobility management entity (MME) may begin performing actions to complete the procedure by forwarding user context information to the target RAT to which the WTRU will handover. If the WTRU has an ongoing PS session in LTE, the MME may transfer the required context to the target RAT so that resources may be reserved to handoff the PS session. It would therefore be desirable to have a method and apparatus to address WTRU behavior when a session request is canceled before session setup is completed.

SUMMARY

A method and apparatus for canceling a Circuit Switched (CS) Fallback (CSFB) procedure are disclosed. The apparatus may transmit a first message to initiate a CSFB procedure and then transmit a second message that indicates a request to cancel the CSFB procedure. If the apparatus determines that there is an ongoing Packet Switched (PS) session, the apparatus may continue the ongoing PS session on a current Radio Access Technology (RAT) without performing an intersystem change. If the apparatus receives a handover (HO) command, the apparatus may transmit a message to reject the HO command, or it may ignore the HO command.

DETAILED DESCRIPTION

When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, an evolved Node-B (eNB), a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment.

The embodiments described herein may be applicable to any access technology that may support CS or PS sessions. Examples include, but are not limited to, Long Term Evolution (LTE), Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications (GSM), General Packet Radio Service (GPRS), Enhanced Data rates for Global Evolution (EDGE), Code Division Multiple Access (CDMA2000), IEEE 802 technologies such as 802.11, 802.16 and Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS), or any future technology. For purposes of explanation, the various embodiments are described in an LTE context, but the various embodiments may be implemented using any technology that may support CS and/or PS sessions.

FIG. 1is a diagram of a protocol stack100for the control plane between a WTRU110and an Mobility Management Entity (MME)120. The Radio Resource Control (RRC) component130is the main controlling function in the access stratum (AS) and may be responsible for establishing the radio bearers and configuring lower layer components using RRC signaling between the eNB140and the WTRU110. The control plane of the AS handles radio-specific functionality and interacts with the non-access stratum (NAS)150. The embodiments described may use RRC signaling, NAS signaling, or a combination of both to cancel a Circuit Switched Fallback (CSFB) procedure.

In a typical LTE example, a WTRU may request a CSFB procedure to initiate a CS session while camped on an LTE cell. In response, the WTRU may receive a mobility message to move to a target RAT in accordance with the CSFB procedure. The WTRU may perform an intersystem change to move to the target RAT, as indicated in the mobility message, to proceed with a CS session. An intersystem change may include a PS handover (HO) if the WTRU and the target RAT support PS HO.

If a PS HO is to be performed, the network nodes in the source and target RATs may have already communicated the event and possibly prepared resources for PS HO. In this scenario, if a PS session is ongoing in LTE, the data packets may not be forwarded to the WTRU to avoid packet loss during the HO. The data packet path may be switched to the target RAT, if PS HO is to be performed, where the WTRU may continue receiving and/or transmitting its data packets. If PS HO is not supported in the target RAT, the WTRU's PS session may be suspended for the duration of the CS session. Regardless of whether the target RAT supports PS HO, after the CS session ends, the WTRU may resume its PS session again in LTE.

In the example above, the WTRU may cancel the requested CSFB procedure at any point in time after the WTRU transmits the CSFB request. However, the cancellation of the CSFB procedure may not occur until after the intersystem change is performed. According to the prior art, once the CSFB is requested, the cascade of events that follow cannot be stopped and may result in excess signaling.

FIG. 2is a flow diagram of an example method200to cancel a CSFB procedure. Referring toFIG. 2, a WTRU may transmit a message to initiate a CS session210. To cancel the initiated CS session, the WTRU may transmit a message indicating a request to cancel the CS session220. By way of example, the request to cancel may be sent to a mobility management entity (MME). If the there is an ongoing PS session230, the WTRU may continue the PS session on the current RAT without performing an intersystem change240. If there is no ongoing PS session230, the WTRU may remain in Idle Mode250.

FIG. 3is a signal flow diagram of an example method300that may use a modified Extended Service Request (ESR) to reduce the amount of signaling that may occur when a WTRU cancels a CSFB procedure. In this method300, the WTRU310may transmit an ESR320to an MME330in an RRC message to trigger a CSFB procedure. A timer (not shown) may be started when the CSFB procedure is triggered. In response to the triggered CSFB procedure, the MME330may transmit a S1-AP message340to an eNB350. The S1-AP message340may contain a CSFB indicator.

As shown inFIG. 3, the WTRU310may transmit a modified ESR message360to the MME330to indicate the cancellation event for an ongoing CSFB procedure. The modified ESR message360may be transmitted as a NAS message and may be transmitted at any time before an intersystem change or HO message is received at the WTRU310. After transmitting the modified ESR message360, the WTRU310may enter a state, such as an EMM-Service-Request-Initiated-Cancel-Pending state365, for example. The EMM-Service-Request-Initiated-Cancel-Pending state may have an associated timer (not shown).

A timer (not shown) may be associated with the transmitted modified ESR message360. The network may transmit an acknowledgement message380to assure the WTRU310that the cancel request was received and will be processed accordingly. As such, the WTRU310may not fallback to a CS RAT. If there is an active PS session, the session may be resumed in LTE385. In this example, the source and target network nodes may perform signaling to cancel the CSFB procedure, for example by releasing any reserved resources in the target RAT/cell.

The WTRU310may, after canceling the CSFB procedure, stop the timer related to the ESR that initiated the CSFB procedure. This timer may be stopped when the WTRU310transmits the modified ESR message360to cancel the CSFB procedure, or when the WTRU310receives the acknowledgement380indicating that the CSFB procedure is canceled. After the CSFB is canceled, the WTRU may enter an EMM-REGISTERED state (not shown).

In this method, specific considerations and exceptions may be made since a timer may have started when the initial ESR triggered the CSFB procedure. Moreover, after transmitting the ESR320, the WTRU310may enter an EMM-Service-Request-Initiated state. One exception may be that the WTRU310remains in this state, but restarts its timer and awaits a new acknowledgement message. Another option may be to enter a new substate within the current state and initiate an associated timer.

The contents of the modified ESR message described inFIG. 3are shown inFIG. 4. Referring toFIG. 4, the modified ESR message400may contain a Protocol Discriminator field410, a Security Header Type field420, an Extended Service Request Message Identity field430, a Service Type field440, a NAS Key Set Identifier field450, an MME Temporary Mobile Subscriber Identity (M-TMSI) field460, and a CSFB Response field470. The Service Type field440may indicate the type of service that the WTRU is requesting and may be modified to include a request to cancel an ongoing CSFB procedure. The details of the possible values of this field are shown below in Table 1.

As an alternative to transmitting a modified ESR message360, the WTRU may transmit a NAS message500with the contents as shown inFIG. 5. As shown inFIG. 5, the NAS message500may contain a Protocol Discriminator Information Element (IE)510, a Security Header Type IE520, a Service Withdraw Identity IE530, a Withdraw Type IE540, a NAS Key Set Identifier IE550, and/or other optional IEs560. The Withdraw Type IE may be used to indicate a service that the WTRU is requesting to cancel. The Withdraw Type IE may be four bits and contain the information as shown in Table 2 below.

TABLE 2Service Withdraw Type IEService Withdraw Type Value Bits4321Description0000Mobile originating CS fallback0001Mobile terminating CS fallback0010Allocate dedicated Evolved PacketSystem (EPS) bearer requestxxxxOthersAll other values may be reserved

FIG. 6is a flow diagram of an alternate method to cancel a CSFB procedure. Referring toFIG. 6, the WTRU610may transmit a message to the MME615to initiate a CS session. Upon receiving the message to initiate a CS session620, the MME615may notify a target RAT to begin reservation of PS and/or CS resources (not shown). In the meantime, the MME615may receive a message that indicates a request to cancel the CSFB procedure625. If the MME615receives this message after notifying the target RAT to setup a CS session, the MME615may notify the target RAT of the request to cancel the CSFB procedure630. If the MME615receives this message before notifying the target RAT to setup the CS session, the MME may not notify the target RAT of the request to cancel the CSFB procedure. The target RAT may transmit an intersystem change or HO message (not shown) if the notification of the request to cancel the CSFB procedure is not received in time.

Referring again toFIG. 6, the WTRU610may determine whether an intersystem change or HO message is received635. If an intersystem change or HO message is not received, the WTRU610may continue a PS session on the current RAT640without performing an intersystem change or HO. If there is no ongoing PS session645, the WTRU610may remain in Idle Mode650.

If the WTRU610receives an intersystem change or HO message, the WTRU610may transmit a message to reject the intersystem change or HO655. The message to reject the intersystem change or HO655may be transmitted as a NAS message to the MME615and/or to the eNB (not shown) in the target RAT as an RRC message. Thus, both the RRC and NAS components in the network may receive a request to cancel the CSFB procedure. For both of these alternatives, the WTRU610may transmit the messages at any time, for example, before or after receiving a message to perform an intersystem change or HO to a target RAT. The RRC and NAS components in the network may acknowledge the receipt of their respective messages by transmitting another message to the WTRU. The NAS and the RRC components may also take specific actions to cancel the CSFB procedure. For example, the MME may contact other network nodes, such as a Serving General Packet Radio Service (GPRS) Support Node (SGSN), to cancel any preparations for an intersystem change or HO. Similar actions may be triggered by the RRC in the eNB.

FIG. 7is a flow diagram of an example method700to cancel a CSFB procedure by performing an additional intersystem change or HO. The WTRU may perform an intersystem change or HO to the target RAT and directly return to LTE by performing another intersystem change or HO. If the CS session is not canceled in LTE, it may be canceled in the target RAT.

Referring toFIG. 7, the WTRU may initiate a CS session710as described in the examples above. The WTRU may then perform an intersystem change or HO and camp on the CS RAT720. The WTRU may then cancel the CS session730while on the CS RAT. The WTRU may cancel the CS session prior to performing an intersystem change or HO to the CS RAT. Based on the knowledge of the cancellation, the WTRU may abort the establishment of the CS session735by not transmitting a CM Service Request message. If there is an ongoing PS session740, the WTRU may perform an intersystem change or HO to the PS RAT750. If there is no ongoing PS session, the WTRU may return to Idle Mode760. In this example, the WTRU may return to LTE by sending a fake measurement to indicate that the CS RAT's signal strength is weak compared to that of LTE. Alternatively, the WTRU may perform an Idle Mode cell reselection to LTE.

FIG. 8is a flow diagram of an example method to cancel a CSFB procedure by ignoring a message to perform an intersystem change or HO. Referring toFIG. 8, the WTRU may transmit a message to initiate a CS session810before transmitting a request to cancel the CSFB820. In response to transmitting the request to cancel the CSFB820, the WTRU may receive an intersystem change or HO message830that includes an indicator. The indicator may specify whether the intersystem change or HO is due to the CSFB. The WTRU may ignore the intersystem change or HO message840based on the knowledge the WTRU transmitted a request to cancel the CSFB. If there is an ongoing PS session850, the WTRU may continue the PS session860on the current RAT without performing an intersystem change or HO. If there is no ongoing PS session850, the WTRU may remain in Idle Mode870.

There may be an associated timer, such as a T304, with the intersystem change or HO message. If the timer expires prior to completing the intersystem change or HO, the WTRU may initiate a connection re-establishment procedure. The WTRU may not perform the intersystem change or HO until the timer expires. Alternatively, the WTRU may set the value of the timer T304 to zero after receiving the intersystem change or HO message to directly initiate a connection re-establishment procedure. The WTRU may then resume its services over LTE.

FIG. 9shows a Long Term Evolution (LTE) wireless communication system/access network900that includes an Evolved-Universal Terrestrial Radio Access Network (E-UTRAN)905. The E-UTRAN905includes a WTRU910and several evolved Node-Bs, (eNBs)920. The WTRU910is in communication with an eNB920. The eNBs920interface with each other using an X2 interface. Each of the eNBs920interface with a Mobility Management Entity (MME)/Serving GateWay (S-GW)930through an S1 interface. Although a single WTRU910and three eNBs920are shown inFIG. 9, it should be apparent that any combination of wireless and wired devices may be included in the wireless communication system access network200.

FIG. 10is a block diagram of an example LTE wireless communication system1000including the WTRU910, the eNB920, and the MME/S-GW930. As shown inFIG. 10, the WTRU910, the eNB920and the MME/S-GW930are configured to perform a method for canceling a CSFB.

In addition to the components that may be found in a typical WTRU, the WTRU910includes a processor1016with an optional linked memory1022, at least one transceiver1014, an optional battery1020, and an antenna1018. The processor1016is configured to perform a method for canceling a CSFB. The transceiver1014is in communication with the processor1016and the antenna1018to facilitate the transmission and reception of wireless communications. In case a battery1020is used in the WTRU910, it powers the transceiver1014and the processor1016.

In addition to the components that may be found in a typical eNB, the eNB920includes a processor1017with an optional linked memory1015, transceivers1019, and antennas1021. The processor1017is configured to perform a method for canceling a CSFB. The transceivers1019are in communication with the processor1017and antennas1021to facilitate the transmission and reception of wireless communications. The eNB920is connected to the Mobility Management Entity/Serving GateWay (MME/S-GW)930which includes a processor1033with an optional linked memory1034.

A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, Mobility Management Entity (MME) or Evolved Packet Core (EPC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software including a Software Defined Radio (SDR), and other components such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a Near Field Communication (NFC) Module, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any Wireless Local Area Network (WLAN) or Ultra Wide Band (UWB) module.