Patent Description:
The Third Generation Partnership Project (3GPP) defines a long-term evolution (LTE) architecture, which provides high data rate, low latency, packet optimization and improved system capacity and coverage. In an LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred to as user equipments (UEs).

LTE technology supports packet based services only. However fallback is specified for circuit switched (CS) services as well. CS Fallback (CSFB) defines a mechanism for using a CS legacy network (e.g. the Global System for Mobile Communications (GSM)/Wideband Code Division Multiple Access (WCDMA)) to provide voice and traditional CS-domain services (e.g. voice call, a CS short message service (SMS), Unstructured Supplementary Service Data (USSD), among others). To provide these CS services, a UE may reuse the CS infrastructure when the UE is served by E-UTRAN.

When a CSFB capable UE selects an E-UTRAN, there is a combined Tracking Area Update/Location Area Update (TAU/LAU) with an international mobile subscriber identity (IMSI) attach procedure initiated. Further, when a CSFB capable UE camping on an E-UTRAN selects a tracking area that is different from one stored in it, there is a combined TA/LA update. Further, when a timer for periodic TA/LA expires, there is a combined TA/LA updating. Such procedures may result in mobile originated (MO) call failure. <CIT> relates to a user terminal having requested a circuit switched fallback service procedure, that detects that a network controlled radio access technology change related to the circuit switched fallback service procedure is not performed.

Accordingly, there is provided a method, a user equipment, and a computer readable medium according to the claims.

The present systems and methods will be better understood with reference to the drawings, in which:.

The present disclosure provides a method at a user equipment comprising: detecting that a circuit switched (CS) call has been requested at the user equipment; if a most recent registration attempt or registration update attempt with a network of a first radio access technology (RAT) resulted in an abnormal state, registering with a network of a second RAT; and initiating the CS call with the network of the second RAT.

The present disclosure further provides a user equipment comprising a processor configured to: detect that a circuit switched (CS) call has been requested at the user equipment; if a most recent registration attempt or registration update attempt with a network of a first radio access technology (RAT) resulted in an abnormal state, register with a network of a second RAT; and initiate the CS call with the network of the second RAT.

The long term evolution architecture is primarily packet based and this may present a challenge for circuit-switched services such as voice or short message service (SMS). One option to deal with such limitation in LTE is referred to as circuit switched fallback (CSFB), where a user equipment will fallback to a network using a second generation (<NUM>) or a third generation (<NUM>) radio access technology (RAT) for circuit switched calls while camping on the LTE network for packet switched services.

The UE will fallback to the <NUM> or <NUM> RAT on either a mobile originated (MO) or a mobile terminated (MT) call.

In order to accommodate the circuit switched fallback, the mobile switching center (MSC) server for the <NUM> or <NUM> RAT communicates with the mobility management entity (MME) for the LTE network.

Circuit switched fallback is provided for in the <NPL>, the contents of which are incorporated herein by reference. In particular the system architecture is provided in Figure <NUM>-<NUM> of the 3GPP TS <NUM> specification and is reproduced herein with regard to <FIG>.

Reference is now made to <FIG>, which illustrates one embodiment of a mobile communication system that includes a <NUM>/<NUM> RAT, as well as an LTE RAT.

In particular, a UE <NUM> is a multi-mode UE capable of communicating with both a network of an LTE RAT as well as a network of a <NUM>/<NUM> RAT. As seen in <FIG>, UE <NUM> communicates with either, or both of a universal mobile telecommunications system (UMTS) terrestrial radio access network (UTRAN) <NUM> or a GSM Edge Radio Access Network (GERAN) <NUM> for circuit switched services. UE <NUM> further communicates with an evolved-UTRAN (E-UTRAN) <NUM> for the LTE services.

Both UTRAN <NUM> and GERAN <NUM> communicate with a serving general packet radio service (GPRS) support node (SGSN) <NUM> for packet switched services and with MSC server <NUM> for circuit switched services.

The E-UTRAN communicates with MME <NUM>.

In the CSFB system, MSC server <NUM> further communicates with MME <NUM>.

During normal operation, UE <NUM> will camp on the LTE system. If a mobile terminated call is received then MSC server <NUM> will notify MME <NUM>, which will then send a page to UE <NUM> through E-UTRAN <NUM> to transition to UTRAN <NUM> or GERAN <NUM>. The UE <NUM> will then transition for the duration of the circuit switched call. At the end of the circuit switch call the UE then transitions back to the LTE system.

Packet switched services may either be suspended for the duration of the circuit switched call or may be transferred to SGSN <NUM>, where the UE <NUM> will continue to receive packet data, although at a lower speed than from the LTE system.

For a mobile originated call, the UE <NUM> may transition to UTRAN <NUM> or GERAN <NUM> for the duration of the call. Again, packet switched services may either be suspended or transitioned to SGSN <NUM>.

The typical process for a mobile originated call (without PS handover for simplicity) is provided in <FIG> follows Figure <NUM>-<NUM> of the 3GPP TS <NUM> Specification.

In particular, a UE <NUM> communicates with a LTE eNB <NUM>, which in turn communicates with MME <NUM>.

The fallback for UE <NUM> has a base station subsystem/radio network subsystem (BSS/RNS) <NUM> which is controlled by MSC <NUM>. Data on the fallback RAT is controlled through SGSN <NUM> and bearers are controlled using serving gateway/packet data network gateway (SGW/PGW) <NUM>.

For a fallback based on a mobile originated call, UE <NUM> first provides a non-access spectrum (NAS) extended service request <NUM> to eNB <NUM>, which forwards the message to MME <NUM>.

In response, the MME provides a UE context modification request with a CS fallback indicator, as shown by message <NUM>. The eNB <NUM> sends a response message <NUM> back to MME <NUM>.

Depending on the configuration, the MME <NUM> may also send a message <NUM> to MSC <NUM> indicating that a CM service request message is due.

As shown by block <NUM>, an optional measurement report may be solicited by MME <NUM> to determine a target GERAN/UTRAN cell for redirection.

In the embodiment of <FIG>, it is assumed that a network supports inter-RAT cell change and thus eNB <NUM> may send message <NUM> to provide the inter-RAT cell change to a neighbor GERAN cell, for example. The message may optionally contain a network assisted cell change (NACC).

eNB <NUM> may then send a UE context release request to MME <NUM>, as shown by message <NUM>.

The UE's context is then released, as shown by block <NUM>. The UE then changes RATs and may perform one of, or both of, a location area update (LAU) and a routing area update (RAU), as shown by block <NUM>.

The UE then suspends packet services by sending a suspend message <NUM> to BSS/RNS <NUM> which forwards the message to SGSN <NUM>. The suspension is then coordinated between MME <NUM> and SGSN <NUM>, as shown by arrow <NUM> and the bearers are then updated, as shown by arrow <NUM>. As indicated above, the example of <FIG> is for no PS handover.

At this point, UE <NUM> sends a CM service request to BSS/RNS <NUM>, as shown by arrow <NUM> and BSS/RNS <NUM> sends the circuit switched message with the CM service request to MSC <NUM>, as shown by arrow <NUM>.

If the service request is rejected with reject cause IMSI unknown in visitor location registry (VLR), the process proceeds into block <NUM>. For example, this may occur if the MSC has changed and the UE did no update its registration. In other cases, after the rejection with the cause IMSI unknown in VLR, any mobile terminated call subsequent to this may also fail due to the unknown registration state of the device.

If the process proceeds into block <NUM> then a service reject message <NUM> is forwarded from MSC <NUM>, through BSS/RNS <NUM> to UE <NUM> and a location area update, as shown by block <NUM> is then performed.

After the location area update, or if no service rejection is received, the mobile originated call may occur as shown by arrow <NUM>.

After the call, a routing area update may occur, as shown by block <NUM>.

As will be appreciated, the rejection of the service at block <NUM> will cause the MO call to fail, and may cause poor user experience. One cause of a rejection may be the UE and network elements being in different states, as provided below.

Reference is now made to <FIG>, where the process begins at block <NUM>. In block <NUM> the UE starts with a successful LAU procedure in a <NUM>/<NUM> RAT such as a UTRAN/GERAN establishing a public land mobile network/location area code (PLMN/LAC) or the UE being successfully camped in E-UTRAN.

The process next proceeds to block <NUM> in which a check is made to determine one or more of three factors: <NUM>) whether the UE is configured to transition from a state of having a RRC connection with the UTRAN/GERAN system to a state of having RRC connection with the LTE system. In other words, the UE has an Inter-RAT re-selection to E-UTRAN; <NUM>) whether the UE selects a non-stored TA; and <NUM>) whether the timer for periodic TA/LA updating has expired. If no, the process loops at block <NUM>, and otherwise to process proceeds to block <NUM>.

As a result of Inter-RAT re-selection at block <NUM>, a combined TAU/LAU with IMSI attach procedure may be initiated to register the UE. In some cases, the TAU/LAU may fail due to radio link failure RLF, release of RRC (radio resource control protocol) connection from the RAT, or any other reason. The result may be abnormal termination of the registration. In other cases, combined tracking area (TA)/location area (LA) updating may result in the abnormal termination.

In the example of <FIG>, it is assumed that abnormal termination of the registration occurred, as shown by block <NUM>. The UE will detect the abnormal termination of the registration and may, according to one embodiment of the present disclosure, consider the UE to be in an abnormal state with regards to registration. As used herein, the "abnormal state" refers to any state in which the UE and the network are not properly registered with each other, and may, for example, be due to RLF, release of RRC connection, or other causes during the registration process.

As a result of the abnormal termination of registration at block <NUM>, the Evolved Packet System (ESP) mobility management (EMM) state is set to REGISTERED and the EMM sub state is EMM_REGISTERED_ATTEMPTING_ TO_UPDATE.

The CSFB call bit may be set to TRUE because the active RAT is E-UTRAN. Therefore, when a CS-domain service is requested the UE will undergo CSFB to establish CS-domain service.

A check at block <NUM> is then made for a MO CS-call (or other CS-domain service) request. If no MO CS-call is made, the process proceeds back to block <NUM>.

If a MO CS call request is made, since the registration was abnormally terminated, a PLMN selection may be initiated in the gateway due to the MO call request. However, in this case the PLMN/LAC found may be the same as the UE had at the last successful registration at block <NUM>. As a result no location update is initiated.

Based on the above, the UE and network registration may be unsynchronized as a result of abnormal termination of the registration at block <NUM>. The network may have deregistered the UE IMSI due to the previous TAU attempt that did not complete. The UE, on the other hand, assumes it has successfully registered in the same PLMN/LAC as in block <NUM>.

The TAU procedure was unsuccessful during the Inter-RAT reselection to E-UTRAN, therefore, the EMM was not fully registered to replace the registration information from the UTRAN/GERAN. The EMM may be in ATTEMPT TO UPDATE state and cannot handle the MO call request at block <NUM>.

As a result of the MO call request the UE may initiate the call establishment procedure (CM SERVICE REQUEST <NUM> from <FIG>) at block <NUM>. The network may reject the CS call attempt (CM SERVICE REJECT, cause <NUM>, IMSI unknown in VLR), as received by the UE at block <NUM>.

As a result of the call request at block <NUM> and call failure at block <NUM> the UE may initiate a LAU request and re-register successfully, as shown by block <NUM>. However, in this case the call attempt has already been aborted or has failed, resulting in a poor user experience.

Based on the above, although the radio resource release during the combined TAU/LAU IMSI attach (or combined TA/LA updating) is triggered by the network conditions, according to one embodiment of the present disclosure the UE may handle the conditions in order to establish the voice call successfully. In particular, in accordance with the present disclosure, the non-access stratum state machine at the UE may be modified to ensure that if a registration attempt is abnormally ended while a voice call attempt is detected, the UE may initiate a further registration attempt, even if the tracking area code (TAC)/location area code (LAC) provided in a broadcast channel message is the same as the last successfully registered TAC/LAC.

Reference is now made to <FIG>. In particular, the process of <FIG> starts at block <NUM> with a precondition that the UE starts with a successful LAU procedure in a <NUM>/<NUM> RAT such as a UTRAN/GERAN establishing a public land mobile network/location area code (PLMN/LAC) or the UE is successfully camped in E-UTRAN. The process then proceeds to block <NUM> in which a check is made to determine one or more of three factors: <NUM>) whether the UE is configured to transition from a state of having a RRC connection with the UTRAN/GERAN system to a state of having RRC connection with the LTE system. In other words, the UE has an Inter-RAT re-selection to E-UTRAN; <NUM>) whether the UE selects a non-stored TA; and <NUM>) whether the timer for periodic TA/LA updating has expired. If no, the process loops at block <NUM>, and otherwise to process proceeds to block <NUM>.

As a result of Inter-RAT re-selection at block <NUM>, a combined TAU/LAU with IMSI attach procedure may be initiated to register the UE. In some cases, the TAU/LAU may fail due to radio link failure RLF, release of RRC (radio resource control protocol) connection from the network, or any other reason. The result may be abnormal termination of the registration. In other cases, combined tracking area (TA)/location area (LA) updating may result in the abnormal termination.

Again, in the example of <FIG>, it is assumed that abnormal termination of the registration occurred, as shown by block <NUM>. The UE will detect the abnormal termination of the registration and may, according to one embodiment of the present disclosure, consider the UE to be in an abnormal state with regards to registration.

If a MO CS call request is made the process proceeds from block <NUM> to block <NUM> in which a registration attempt with a network of the <NUM>/<NUM> RAT is initiated. The registration attempt is in the form of an LAU. Further, a follow_on_request flag is used to instruct a network not to release the radio resource when the LAU proceed is complete. In fact in the LAU accepted message, the network will set a corresponding flag (follow_on_proceed) to <NUM> to let the UE know that it can now send the CM_SERVICE_REQUEST. If the follow_on_request flag is not set to <NUM>, the network will release the radio resource connection after the LAU accepted. Thus the follow_on_request flag is used to optimize the registration process when a CS call is pending.

In one embodiment a check may be made after the registration at block <NUM> to ensure success and if the registration is not successful the process could be ended or block <NUM> could be repeated.

From block <NUM> the process proceeds to block <NUM> in which the pending call is initiated. In this case, since registration occurs, the CS call establishment succeeds in the embodiment of <FIG> , avoiding the poor user experience described above with regards to <FIG>.

The above embodiments may be implemented by any UE. One exemplary device is described below with regard to <FIG>.

UE <NUM> is typically a two-way wireless communication device having voice and/or data communication capabilities. UE <NUM> generally has the capability to communicate with other computer systems on the Internet. Depending on the exact functionality provided, the UE may be referred to as a data messaging device, a two-way pager, a wireless e-mail device, a cellular telephone with data messaging capabilities, a wireless Internet appliance, a wireless device, a mobile device, or a data communication device, as examples.

Where UE <NUM> is enabled for two-way communication, it may incorporate a communication subsystem <NUM>, including both a receiver <NUM> and a transmitter <NUM>, as well as associated components such as one or more antenna elements <NUM> and <NUM>, local oscillators (LOs) <NUM>, and a processing module such as a digital signal processor (DSP) <NUM>. As will be apparent to those skilled in the field of communications, the particular design of the communication subsystem <NUM> will be dependent upon the communication network in which the device is intended to operate. The radio frequency front end of communication subsystem <NUM> can be any of the embodiments described above.

Network access requirements will also vary depending upon the type of network <NUM>. In some networks network access is associated with a subscriber or user of UE <NUM>. A UE may require a removable user identity module (RUIM) or a subscriber identity module (SIM) card in order to operate on a network. The SIM/RUIM interface <NUM> is normally similar to a card-slot into which a SIM/RUIM card can be inserted and ejected. The SIM/RUIM card can have memory and hold many key configurations <NUM>, and other information <NUM> such as identification, and subscriber related information.

When required network registration or activation procedures have been completed, UE <NUM> may send and receive communication signals over the network <NUM>. As illustrated in <FIG>, network <NUM> can consist of multiple base stations communicating with the UE. Further, as described above, a multi-mode UE may communicate with base stations of different networks <NUM>, for example a WCDMA and an LTE network.

Signals received by antenna <NUM> through communication network <NUM> are input to receiver <NUM>, which may perform such common receiver functions as signal amplification, frequency down conversion, filtering, channel selection and the like. A/D conversion of a received signal allows more complex communication functions such as demodulation and decoding to be performed in the DSP <NUM>. In a similar manner, signals to be transmitted are processed, including modulation and encoding for example, by DSP <NUM> and input to transmitter <NUM> for digital to analog conversion, frequency up conversion, filtering, amplification and transmission over the communication network <NUM> via antenna <NUM>. DSP <NUM> not only processes communication signals, but also provides for receiver and transmitter control. For example, the gains applied to communication signals in receiver <NUM> and transmitter <NUM> may be adaptively controlled through automatic gain control algorithms implemented in DSP <NUM>.

UE <NUM> generally includes a processor <NUM> which controls the overall operation of the device. Communication functions, including data and voice communications, are performed through communication subsystem <NUM>. Processor <NUM> also interacts with further device subsystems such as the display <NUM>, flash memory <NUM>, random access memory (RAM) <NUM>, auxiliary input/output (I/O) subsystems <NUM>, serial port <NUM>, one or more keyboards or keypads <NUM>, speaker <NUM>, microphone <NUM>, other communication subsystem <NUM> such as a short-range communications subsystem and any other device subsystems generally designated as <NUM>. Serial port <NUM> could include a USB port or other port known to those in the art.

Some of the subsystems shown in <FIG> perform communication-related functions, whereas other subsystems may provide "resident" or on-device functions. Notably, some subsystems, such as keyboard <NUM> and display <NUM>, for example, may be used for both communication-related functions, such as entering a text message for transmission over a communication network, and device-resident functions such as a calculator or task list.

Operating system software used by the processor <NUM> may be stored in a persistent store such as flash memory <NUM>, which may instead be a read-only memory (ROM) or similar storage element (not shown). Those skilled in the art will appreciate that the operating system, specific device applications, or parts thereof, may be temporarily loaded into a volatile memory such as RAM <NUM>. Received communication signals may also be stored in RAM <NUM>.

As shown, flash memory <NUM> can be segregated into different areas for both computer programs <NUM> and program data storage <NUM>, <NUM>, <NUM> and <NUM>. These different storage types indicate that each program can allocate a portion of flash memory <NUM> for their own data storage requirements. Processor <NUM>, in addition to its operating system functions, may enable execution of software applications on the UE. A predetermined set of applications that control basic operations, including at least data and voice communication applications for example, will normally be installed on UE <NUM> during manufacturing. Other applications could be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storage medium. The computer readable storage medium may be a tangible or in transitory/non-transitory medium such as optical (e.g., CD, DVD, etc.), magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM) application having the ability to organize and manage data items relating to the user of the UE such as, but not limited to, e-mail, calendar events, voice mails, appointments, and task items. Naturally, one or more memory stores would be available on the UE to facilitate storage of PIM data items. Such PIM application may have the ability to send and receive data items, via the wireless network <NUM>. Further applications may also be loaded onto the UE <NUM> through the network <NUM>, an auxiliary I/O subsystem <NUM>, serial port <NUM>, short-range communications subsystem <NUM> or any other suitable subsystem <NUM>, and installed by a user in the RAM <NUM> or a non-volatile store (not shown) for execution by the processor <NUM>. Such flexibility in application installation increases the functionality of the device and may provide enhanced on-device functions, communication-related functions, or both. For example, secure communication applications may enable electronic commerce functions and other such financial transactions to be performed using the UE <NUM>.

In a data communication mode, a received signal such as a text message or web page download will be processed by the communication subsystem <NUM> and input to the processor <NUM>, which may further process the received signal for output to the display <NUM>, or alternatively to an auxiliary I/O device <NUM>.

A user of UE <NUM> may also compose data items such as email messages for example, using the keyboard <NUM>, which may be a complete alphanumeric keyboard or telephone-type keypad, or a virtual keyboard, among others, in conjunction with the display <NUM> and possibly an auxiliary I/O device <NUM>. Such composed items may then be transmitted over a communication network through the communication subsystem <NUM>.

For voice communications, overall operation of UE <NUM> is similar, except that received signals would typically be output to a speaker <NUM> and signals for transmission would be generated by a microphone <NUM>. Alternative voice or audio I/O subsystems, such as a voice message recording subsystem, may also be implemented on UE <NUM>. Although voice or audio signal output is generally accomplished primarily through the speaker <NUM>, display <NUM> may also be used to provide an indication of the identity of a calling party, the duration of a voice call, or other voice call related information for example.

Serial port <NUM> in <FIG> may be implemented in a personal digital assistant (PDA)-type UE for which synchronization with a user's desktop computer (not shown) may be desirable, but is an optional device component. Such a port <NUM> would enable a user to set preferences through an external device or software application and would extend the capabilities of UE <NUM> by providing for information or software downloads to UE <NUM> other than through a wireless communication network. The alternate download path may for example be used to load an encryption key onto the device through a direct and thus reliable and trusted connection to thereby enable secure device communication. As will be appreciated by those skilled in the art, serial port <NUM> can further be used to connect the UE to a computer to act as a modem or for charging purposes.

Other communications subsystems <NUM>, such as a short-range communications subsystem, is a further optional component which may provide for communication between UE <NUM> and different systems or devices, which need not necessarily be similar devices. For example, the subsystem <NUM> may include an infrared device and associated circuits and components or a Bluetooth™ communication module to provide for communication with similarly enabled systems and devices. Subsystem <NUM> may further include non-cellular communications such as WiFi or WiMAX, or near field communications (NFC).

Claim 1:
A method performed by a user equipment (<NUM>) comprising:
detecting (<NUM>) that a circuit switched, CS, service has been requested at the user equipment;
determining if a most recent registration attempt or registration update attempt with a network of a first radio access technology, RAT, resulted in an abnormal state (<NUM>);
if the abnormal state is detected, reselecting to a network of a second RAT and registering (<NUM>) with the network of the second RAT even if a tracking area code or a location area code of a last successful network registration matches the tracking area code or location area code in a broadcast message from the network of the second RAT, wherein the registering includes a location area update, and wherein the registering includes transmission of a follow-on-request flag to instruct the network of the second RAT not to release radio resources following completion of the location area update; and
initiating (<NUM>) the CS service with the network of the second RAT.