Abstract:
A method for implementing a fallback procedure from a packet switched (PS) radio access technology (RAT) to a circuit switched (CS) RAT is disclosed. A paging message is received at a wireless transmit/receive unit (WTRU) via the PS RAT, the paging message including a WTRU identifier and a domain indicator, wherein the domain indicator indicates whether the paging message is related to a service on the CS RAT. If the WTRU identifier in the paging message matches a stored identifier of the receiving WTRU, then a fallback procedure to the CS RAT is performed. The WTRU responds to the paging message using a CS RAT-based identifier assigned to the WTRU.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/506,493, filed Jul. 21, 2009, which issue as U.S. Pat. No. 8,243,725 on Aug. 14, 2012, which claims the benefit of U.S. Provisional Application No. 61/088,397, filed Aug. 13, 2008, the contents of which are hereby incorporated by reference herein. 
     
    
     FIELD OF INVENTION 
       [0002]    This application is related to wireless communications. 
       BACKGROUND 
       [0003]    A goal of the Third (3 rd ) Generation Partnership Project (3GPP) Long Term Evolution (LTE) program is to develop new technology, new architecture, and new methods in new LTE settings and configurations. These features are being developed to provide improved spectral efficiency, reduced latency, and better utilization of the radio resources. These features are intended to provide faster user experiences, richer applications, and improved services, with less cost. 
         [0004]    LTE is a Packet Switched (PS)-only radio technology. It is desirable to support backwards mobility with legacy Global System for Mobile Communications (GSM). For inter-working with legacy Circuit Switched (CS) networks, such as GSM, it was expected that IP Multimedia Core Network Subsystem (IMS) networks would be deployed. Specifically, Voice Call Continuity (VCC) was expected to be the technique used for handing over voice calls from LTE PS networks, using Voice over Internet Protocol (VoIP) techniques, to legacy CS networks. It would be desirable to de-couple IMS deployments from LTE deployments. In other words, it would be desirable to initially use the currently deployed CS infrastructure for voice calls, while deploying LTE for high-speed PS services only. For this reason, it would be desirable for LTE to allow a multi-mode wireless transmit/receive unit (WTRU), such as with LTE and GSM and/or Wideband Code Division Multiple Access (WCDMA), use the LTE network for high-speed PS data traffic while reverting to legacy CS network for voice traffic, without necessarily using any IMS features such as VCC. 
         [0005]    When a WTRU attaches to the Evolved Packet System (EPS) over the E-UTRAN network, the Non Access Stratum (NAS) layer Attach message may include a CS fallback indicator to indicate to the network the need to attach the WTRU in the CS domain as well. The Mobility Management Entity (MME) may then perform the attachment to the CS domain on behalf of the WTRU, before indicating that the process completed via the Attach Accept message, as shown in  FIG. 1 . The Attach procedure for the CS fallback in EPS may be realized based on a combined General Packet Radio Service (GPRS)/International Mobile Subscriber Identifier (IMSI) Attach procedure as specified in the 3GPP standard TS 23.060. 
         [0006]      FIG. 1  is a flow diagram of a method  100  for performing an Attach procedure. In the method  100 , messages are exchanged between a WTRU  102 , an MME  104 , a mobile switching center/visitor location register (MSC/VLR)  106 , and a home subscriber server (HSS)  108 . The WTRU  102  initiates the Attach procedure by transmitting an Attach Request message, including parameters as specified in 3GPP standard TS 23.401 and a CS fallback indicator, to the MME  104  (step  110 ). The CS fallback indicator indicates that the WTRU  102  is capable of using CS fallback and configured to use CS fallback. 
         [0007]    The EPS Attach procedure is performed as specified in 3GPP standard TS 23.401 (step  112 ). The VLR  106  is updated according to the combined GPRS/IMSI Attach procedure in 3GPP standard TS 23.060 if the Attach Request message includes a Combined Update indicator (step  114 ). The VLR number is derived from the Tracking Area Identity (TAT). The MME  104  starts the location update procedure towards the new MSC/VLR upon receiving the first Insert Subscriber Data message from the HSS  108 . This operation marks the WTRU  102  as EPS-attached in the VLR  106 . 
         [0008]    The MME  104  sends a Location Update Request message, such as a new Location Area Identity (LAI), IMSI, MME address, or Location Update Type, to the VLR  106  (step  116 ). A new LAI is determined in the MME  104  based on a mapping from the Tracking Area (TA). A mapped LAI may be to either a GSM EDGE Radio Access Network (GERAN) or a UMTS Terrestrial Radio Access Network (UTRAN). 
         [0009]    The VLR  106  creates an association with the MME  104  by storing the MME address (step  118 ). The VLR  106  performs a location update procedure in the CS domain (step  120 ). The VLR  106  responds to the MME  104  with a Location Update Accept message, such as a VLR Temporary Mobile Subscriber Identity (TMSI) (step  122 ). The MME  104  sends an Attach Accept message, including parameters as specified in 3GPP standard TS 23.401, a Location Area Identity (LAI), and a VLR TMSI (if allocated), to the WTRU (step  124 ). The existence of a LAI (and a VLR TMSI, if allocated) indicates a successful attachment to the CS domain. 
         [0010]      FIG. 2  is a flow diagram of a method  200  for a WTRU terminating a call while in Idle mode. In the method  200 , messages are exchanged between a WTRU  202 , an eNB  204 , a MME  206 , a radio network controller (RNC) or base station controller (BSC)  208 , a MSC/VLR  210 , a HSS  212 , and a gateway mobile switching center (GMSC)  214 . 
         [0011]    The method  200  begins with the GMSC  214  receives an initial address message (IAM; step  220 ). The GMSC  214  retrieves the routing information of the terminating WTRU by using the Send Routing Info (SRI) procedures (step  222 ). The GMSC  214  sends the IAM to the MSC  210  on the terminating side (step  224 ). 
         [0012]    The MME  206  receives a paging (IMSI, VLR TMSI, or Location Information) message from the MSC  210  over a SGs interface (step  226 ). The TMSI (or IMSI) received from the MSC  210  is used by the MME  206  to find the S-TMSI, which is used as the paging address on the radio interface. If the location information is reliably known by the MME  206  (i.e., the MME stores the list of TAs), the MME  206  pages the WTRU  202  in all the TAs. If the MME  206  does not have a stored TA list for the WTRU  202 , the MME  206  should use the location information received from the MSC to page the WTRU. If pre-paging is deployed, this procedure takes place before step  224 , immediately after the MSC  210  receives the MAP_PRN message from the HSS  212 . 
         [0013]    The MME  206  sends a paging message to each eNB  204  (step  228 ). The paging message includes a suitable WTRU identity (i.e., S-TMSI or IMSI) and a core network (CN) domain indicator that indicates which domain (CS or PS) initiated the paging message. In this case, the domain indicator is set to “CS” by the MME  206 . The eNB  204  pages the WTRU  202  (step  230 ), and the paging message contains a suitable WTRU identity (i.e., S-TMSI or IMSI) and a CN domain indicator. 
         [0014]    The WTRU  202  establishes an RRC connection and sends an Extended Service Request (CS fallback indicator) to the MME  206  (step  232 ). The WTRU  202  indicates its S-TMSI in the RRC signaling. The Extended Service Request message is encapsulated in RRC and S1 AP messages. The CS fallback indicator indicates to the MME  206  that CS fallback for the WTRU  206  is required. The MME  206  sends S1 AP: Initial WTRU Context Setup (including WTRU capabilities, CS fallback indicator, and other parameters) to indicate the eNB  204  to move the WTRU  202  to UTRAN/GERAN (step  234 ). 
         [0015]    For the next action, there are two options, the choice depending on whether the target RAT has PS handover (HO) capability or not. If the target RAT has PS HO capability, then upon receipt of the Initial WTRU Context Setup message with a CS fallback indicator, the eNB  204  may optionally solicit measurement reports from the WTRU  202  to determine the target cell to which PS handover will be performed (step  236 ). A PS handover is then performed and as part of the PS handover, the WTRU  202  receives a HO from E-UTRAN Command that may contain a CS fallback indicator, which indicates to the WTRU  202  that the handover was triggered due to a CS fallback request. If the HO from E-UTRAN Command contains a CS fallback indicator and the WTRU  202  fails to establish a connection to the target RAT, then the WTRU  202  considers that the CS fallback has failed. 
         [0016]    If the target RAT has no PS HO capability, then upon receipt of the Initial WTRU Context Setup message with a CS fallback indicator, the eNB  204  may optionally solicit measurement reports from the WTRU  202  to determine the target cell to redirect the WTRU  202  to (step  236 ). After that, the eNB  204  releases the RRC Connection with redirection information to change to a CS-capable RAT (including RAT, frequency, and cell information). As an option, the inter-RAT system information might be provided by the eNB  204  using the Network Assisted Cell Change (NACC) procedure for GERAN. In this case, the WTRU  202  receives in inter-RAT cell change order that may contain a CS fallback indicator, which indicates to the WTRU  202  that the cell change order was triggered due to a CS fallback request. If the inter-RAT cell change order contains a CS fallback indicator and the WTRU  202  fails to establish a connection to the target RAT, then the WTRU  202  considers that the CS fallback has failed. 
         [0017]    If the WTRU  202  obtains the LA/RA information of the new UTRAN/GERAN cell (e.g., based on the system information or redirection information) and the LA/RA of the new cell is different from the one stored in the WTRU  202 , it performs a LA Update or a Combined RA/LA update procedure if the target system operates in Network Mode of Operation (NMO) I (step  238 ). The WTRU  202  responds with a page response message to the MSC  210  as follows. 
         [0018]    If the target RAT is UTRAN or GERAN Iu mode, the WTRU  202  establishes an RRC connection and responds to the paging message in an RRC Initial Direct Transfer message. The CN domain indicator is set to “CS” in the Initial Direct Transfer message. When received at the RNC  208 , the Paging Response message is sent in an RANAP Initial WTRU message to the MSC  210  (step  242 ). 
         [0019]    If the target RAT is GERAN A/Gb mode, the WTRU  202  establishes an RR connection by using the procedures specified in 3GPP TS 44.018 (i.e., the WTRU  202  requests and is assigned a dedicated channel, where it sends a Set Asynchronous Balanced Mode (SABM) containing a layer 3 Service Request message=PAGING RESPONSE to the BSS and the BSS responds by sending a UA). After establishing the main signaling link as described in 3GPP TS 44.018, the WTRU  202  enters either Dual Transfer Mode or Dedicated Mode and the CS call establishment procedure completes. When received at the BSC  208 , the Paging Response message is sent in a BSSAP COMPLETE LAYER 3 INFORMATION message to the MSC. The BSS should be prepared to receive a PAGING RESPONSE even when a corresponding PAGING REQUEST has not been sent by this BSS. Also, the MSC  210  should be prepared to receive a paging response after a relatively long time from when the CS paging message was sent (step  226 ). 
         [0020]    In case the MSC serving the 2G/3G cell is the same as the MSC that served the WTRU  202  while camped on LTE, it stops the paging response timer and establishes the CS connection (step  244 ). 
         [0021]    If the MSC that receives the paging response is different from the MSC that sent the paging request and if the LA Update or Combined RA/LA Update was not performed, the MSC rejects the page response by releasing the A/Iu-cs connection (step  246 ). The RNC/BSC  208  in turn releases the RRC/RR connection (step  248 ). The RRC/RR release triggers the WTRU  202  to perform a LA Update (step  250 ) as follows. If the target system operates in Network Mode of Operation (NMO) I, then the WTRU  202  performs a combined RA/LA Update. When the target system operates in NMO I, if the WTRU  202  is still in UTRAN/GERAN after the CS voice call is terminated and if a combined RA/LA Update has not already been performed, the WTRU  202  performs a combined RA/LA Update procedure. This procedure is used to create a Gs association between the MSC/VLR  210  and the SGSN and to release the SGs association. 
         [0022]    If the target system operates in NMO II or III, then the WTRU  202  performs a LA Update towards the MSC  210 . The LA Update triggers the Roaming Retry for CS fallback procedure. When the target system operates in NMO II or III, if the WTRU  202  is still in UTRAN/GERAN after the CS voice call is terminated and if a LA Update has not already been performed, the WTRU  202  performs a LA Update procedure. This procedure is used to release the SGs association between the MSC/VLR  210  and the MME  206 . 
         [0023]    It is noted that if the WTRU is in Idle mode, then paging may be initiated, because the network is not aware of the location and attachment status of the WTRU. If the WTRU is in Connected mode, meaning that the WTRU is attached to the network, paging the WTRU is not needed and the network may easily reach the WTRU via a dedicated message. One example of a dedicated message is a “CS service notification” message. 
         [0024]      FIG. 3  is a flow diagram of a method  300  for the preparation phase of a mobile originated (MO) call in active mode. In the method  300 , messages are exchanged between a WTRU  302 , an eNB  304 , a BSS  306 , an MME  308 , an MSC  310 , a SGSN  312 , and a serving GW  314 . The WTRU  302  sends a CS Call Request message to the eNB  304  (step  320 ). The eNB  304  may optionally request a measurement report from the WTRU  302  to determine the target GERAN/UTRAN cell to which PS handover will be performed (step  322 ). 
         [0025]    When the WTRU  302  is moving, it may be necessary to handover the WTRU to a different eNB and/or BSS to maintain the connection between the WTRU and the network (i.e., perform a relocation process). The eNB  304  sends a Relocation Required message to the MME  308  (step  324 ). The MME  308  forwards a Relocation Request message to the SGSN  312  (step  326 ). The SGSN  312  sends a PS Handover Request message to the BSS  306  (step  328 ). The SGSN  312  reserves radio resources in a target BSS (step  330 ) and the target BSS creates a Target BSS to Source BSS Transparent Container (step  332 ). The BSS  306  sends a PS Handover Request Acknowledge message to the SGSN  312  (step  334 ). The SGSN  312  forwards a Relocation Response message to the MME  308  (step  336 ). After this step is completed, the WTRU  302  has been handed over to the target BSS. 
         [0026]      FIG. 4  is a flow diagram of a method  400  for the execution phase of a MO call in active mode. In the method  400 , messages are exchanged between a WTRU  402 , an eNB  404 , a BSS  406 , an MME  408 , an MSC  410 , a SGSN  412 , and a serving GW  414 . Uplink and downlink payload PDUs are exchanged between the WTRU  402  and the eNB  404 , utilizing the serving GW  414  as needed (step  420 ). The MME  408  sends a Relocation Command to the eNB  404  (step  422 ), which triggers the eNB  404  to send a Handover from E-UTRAN Command to the WTRU  402  (step  424 ). 
         [0027]    The WTRU  402  and the eNB  404  perform a GERAN A/Gb access procedure (step  426 ) and the WTRU  402  sends an XID Response message to the BSS  406  (step  428 ). The BSS  406  sends a PS Handover Complete message to the SGSN  412  (step  430 ) and forwards the XID Response message to the SGSN  412  (step  432 ). Also at this time, it is possible for the WTRU  402  to send uplink packet data to the eNB  404  (step  434 ). 
         [0028]    If the target RAT is GERAN, the WTRU  402  sends a SABM message with a Connection Management (CM) Service Request to the BSS  406  (step  436 ). The BSS  406  forwards complete Layer 3 information along with the CM Service Request to the MSC  410 , which indicates that CS resources have been allocated in the GERAN cell (step  438 ). The BSS  406  responds to the WTRU  402  by sending a UA with the CM Service Request, which positively acknowledges the SABM message (step  440 ). The WTRU  402  then initiates a CS call establishment procedure (step  442 ). 
         [0029]    Part of the processing of the Handover from E-UTRAN Command (step  424 ) includes the SGSN  412  receiving a Handover Complete message. Upon receiving the Handover Complete message, the SGSN  412  sends a Forward Relocation Complete message to the MME  408  to indicate completion of the PS handover procedure (step  444 ). The MME  408  responds to the SGSN  412  by sending a Forward Relocation Complete Acknowledge message to the SGSN  412  (step  446 ). 
         [0030]    The SGSN  412  sends an Update PDP Context Request message to the serving GW, including a new SGSN address, a tunnel endpoint identifier (TEID), and a negotiated quality of service (QoS) (step  448 ) and optionally sends IP packets to the serving GW  414  (step  450 ). The serving GW  414  updates the PDP context fields and sends an Update PDP Context Response message (including the TEID) to the SGSN  412  (step  452 ). At this point, the serving GW  414  sends new incoming downlink IP packets to the SGSN  412  instead of the source eNB (step  454 ). The downlink IP packets are then forwarded to the BSS  406  (step  456 ) and ultimately to the WTRU  402  (step  458 ). 
         [0031]    The WTRU  402  and the SGSN  412  perform an XID negotiation procedure for LLC ADM (step  460 ). The WTRU  402  and the SGSN  412  may also perform an XID negotiation procedure for LLC ABM (step  462 ). It is noted that one or both of the XID negotiation procedures (for ADM and ABM) may be performed, depending on the LLC layer parameters to be negotiated. The WTRU  402  triggers a routing area update procedure when it is possible to send uplink data packets (step  464 ). 
         [0032]    Even with the foregoing procedures, there are two problems that need to be addressed: a combined location area update/tracking area update and the identity of the WTRU that is used for paging. 
         [0033]    1. Combined Location Area Update/Tracking Area Update 
         [0034]    One problem relates to the combined Location Area Update (LAU)/Tracking Area Update (TAU) procedure necessary to remain attached to the CS domain while in the E-UTRAN. In legacy GPRS, a routing area (RA) was a subset of a location area (LA). The system information would broadcast both the location area identity (LAI) and the routing area identity (RAI). A change in LA implied a change in RA, but not vice versa. In LTE System Information, there is no provision to broadcast the LA code of the surrounding legacy GSM/UMTS CS network. In addition, there is not supposed to be a pre-defined relationship between TAs and LAs. Therefore, this problem may be stated: how does the WTRU determine that a combined TAU/LAU procedure needs to be triggered? 
         [0035]    2. WTRU Identity for Paging 
         [0036]    A second problem relates to the WTRU identity for paging. In LTE, the WTRU identity for paging is the S Temporary Mobile Subscriber Identity (S-TMSI). But the WTRU is not supposed to respond to paging messages via the IMSI in LTE. In legacy GSM, the WTRU may be paged using either the TMSI or the IMSI. 
         [0037]    In case the WTRU performed a combined EPS/IMSI Attach or a Combined TAU/LAU procedure, the MSC/VLR would either assign a TMSI to the WTRU or use the WTRU&#39;s IMSI for paging. A TMSI is unique only within the LA where it was assigned. Because there is no relationship between LAs and TAs, a WTRU may, in Idle Mode, cross LAs where the previously assigned TMSI may be invalid.  FIG. 5  shows a conceivable scenario where the WTRU, while being in the same TA (TA 1 ), crosses between two different LAs, such as LA 1  and LA 2 , without performing a TA update procedure toward the MME. The problem in this situation is how to properly page the WTRU. 
         [0038]    During the Attach procedure for connecting to LTE services (as shown in  FIG. 1 ), the network must allocate an ID number to the WTRU (a Global Unique Temporary Identifier (GUTI) which includes the S-TMSI). All paging messages to the WTRU use the S-TMSI as the identifier for the WTRU. Under existing procedures, if the network pages the WTRU using the IMSI, the WTRU is supposed to detach from the network and then reattach to the network, to create the appropriate parameters in the network&#39;s database so that the WTRU may be paged using the IMSI. As one example, in GSM, the WTRU always has an IMSI and may also have a TMSI, if assigned by the network. But, the VLR does not have to allocate a TMSI to the WTRU. In LTE, the MME must allocate a GUTI, which includes the S-TMSI. If the VLR does not allocate a TMSI to the WTRU, and sends back the WTRU&#39;s IMSI in the Attach Accept message, then the WTRU must delete any TMSI that it had stored. As a result, the WTRU may only be paged by the IMSI. But if the WTRU may only be paged by the IMSI in the CS domain, there is a problem, because in LTE, the WTRU cannot be paged by the IMSI, and has to be paged by the S-TMSI. 
       SUMMARY 
       [0039]    A WTRU may be paged by using the assigned IMSI. A bit may be added to existing messaging to indicate that the CN domain is CS, so that the WTRU knows it is receiving a CS page using the IMSI. When the WTRU falls back (i.e., leaves E-UTRAN) to GERAN or UTRAN, it must use the IMSI in the paging response message to the MSC/VLR. In addition, the WTRU indicates a combined LAU/TAU when it is registered in both the PS and the CS domains and the mobility management procedure for a tracking area update has been triggered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings, wherein: 
           [0041]      FIG. 1  is a signal diagram of an attach procedure for CS fallback; 
           [0042]      FIG. 2  is a signal diagram of a method for a WTRU terminating a call while in Idle mode; 
           [0043]      FIG. 3  is a signal diagram of the preparation phase of a MO call in active mode; 
           [0044]      FIG. 4  is a signal diagram of the execution phase of a MO call in active mode; 
           [0045]      FIG. 5  is a diagram of an example relationship between location areas and tracking areas; 
           [0046]      FIG. 6  shows an example wireless communication system including a plurality of wireless transmit/receive units (WTRUs) and an evolved Node B (eNB); 
           [0047]      FIG. 7  is an example functional block diagram of a WTRU and the eNB of  FIG. 6 ; 
           [0048]      FIG. 8  is a flowchart of a method for triggering a combined TAU/LAU procedure; 
           [0049]      FIG. 9  is a flowchart of another method for triggering a combined TAU/LAU procedure; 
           [0050]      FIG. 10  is a flowchart of a method for triggering a LAU procedure; 
           [0051]      FIG. 11  is a flowchart of a method for determining a paging identity for a WTRU; 
           [0052]      FIG. 12  is a flowchart of a second method for determining a paging identity for a WTRU; 
           [0053]      FIG. 13  is a flowchart of a third method for determining a paging identity for a WTRU; and 
           [0054]      FIG. 14  is a flowchart of a fourth method for determining a paging identity for a WTRU. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    When referred to hereafter, the term “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 user device capable of operating in a wireless environment. When referred to hereafter, the term “base station” includes, but is not limited to, a Node B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
         [0056]    A person having ordinary skill in the art should understand that the method and apparatus provided herein are applicable to any access technology which uses circuit switched or packet switched data, such as 3GPP LTE, WCDMA, GSM, GPRS, Enhanced Data for GSM Evolution (EDGE), Code Division Multiple Access (CDMA) 2000 (and related technologies), and the IEEE 802 family of technologies, for example 802.11, 802.16, and WiMAX. 
         [0057]      FIG. 6  shows an example wireless communication system  600  including a plurality of WTRUs  610  and an eNB  620 . As shown in  FIG. 6 , the WTRUs  610  are in communication with the eNB  620 . It should be noted that, although an example configuration of WTRUs  610  and an eNB  620  is depicted in  FIG. 6 , any combination of wireless and wired devices may be included in the wireless communication system  600 . 
         [0058]      FIG. 7  is an example functional block diagram  700  of a WTRU  610  and the eNB  620  of the wireless communication system  600  of  FIG. 6 . As shown in  FIG. 7 , the WTRU  610  is in communication with the eNB  620 . 
         [0059]    In addition to the components that may be found in a typical WTRU, the WTRU  610  includes a processor  612 , a receiver  614 , a transmitter  616 , and an antenna  618 . The receiver  614  and the transmitter  616  are in communication with the processor  612 . The antenna  618  is in communication with both the receiver  614  and the transmitter  616  to facilitate the transmission and reception of wireless data. The processor  612  is configured to perform a method of maintaining circuit switched continuity in an E-UTRAN. 
         [0060]    In addition to the components that may be found in a typical Node B, the Node B  620  includes a processor  622 , a receiver  624 , a transmitter  626 , and an antenna  628 . The receiver  624  and the transmitter  626  are in communication with the processor  622 . The antenna  628  is in communication with both the receiver  624  and the transmitter  626  to facilitate the transmission and reception of wireless data. The processor  622  is configured to perform a method of maintaining circuit switched continuity in an E-UTRAN. 
         [0061]    Triggers for Performing a Combined TAU/LAU 
         [0062]    The following triggers for performing a combined tracking area update (TAU)/location area update (LAU) may be used individually or in any combination. 
         [0063]    System Information Indication 
         [0064]    The E-UTRAN system information may carry an indication of the LA of the surrounding CS network, such as GSM or UMTS. This information may, for example, be carried on E-UTRAN System Information Blocks, such as SIB 1 . The information may indicate whether the LA is valid for a GERAN network or a UTRAN network. This information may indicate to the WTRU whether a combined TAU/LAU procedure is required in this area. 
         [0065]    A WTRU that is currently in the E-UTRAN and in IDLE mode may initiate a combined TAU/LAU procedure when it detects a change in the LA broadcast by the System Information Block (SIB) in the E-UTRAN.  FIG. 8  is a flowchart of a method  800  for triggering a combined TAU/LAU procedure. The method  800  begins with the WTRU receiving the LA of the surrounding CS network (step  802 ). A determination is made whether the WTRU is currently in the E-UTRAN (step  804 ). If the WTRU is not currently in the E-UTRAN, then the method terminates (step  806 ). If the WTRU is currently in the E-UTRAN, a determination is made whether the WTRU is in Idle mode (step  808 ). If the WTRU is not in Idle mode, then the method terminates (step  806 ). If the WTRU is in Idle mode, then a determination is made whether the WTRU has detected a change in the LA (step  810 ). If the WTRU has not detected a change in the LA, then the method terminates (step  806 ). If the WTRU has detected a change in the LA, then the WTRU initiates a combined TAU/LAU procedure (step  812 ) and the method terminates (step  806 ). 
         [0066]    Timer 
         [0067]    The MSC/VLR may assign a timer to the WTRU when it performs an IMSI Attach procedure or a LAU Request procedure over the E-UTRAN. The timer may continue running in the NAS, even if the Evolved Packet System Mobility Management (EMM) sub-layer is in EMM-Connected mode or in EMM-Idle mode, as long as the WTRU is registered in the EPS domain, the CS domain, or both. When the timer expires, the WTRU may initiate a combined TAU/LAU procedure. 
         [0068]      FIG. 9  is a flowchart of method  900  for triggering a combined TAU/LAU procedure. The method  900  begins with the WTRU performing an IMSI Attach procedure or a LAU Request procedure over the E-UTRAN (step  902 ). After one of these procedures is performed, a timer is set by the MSC/VLR (step  904 ). Once the timer expires (step  906 ), the WTRU initiates a combined TAU/LAU procedure (step  908 ) and the method terminates (step  910 ). 
         [0069]    Subscriber Identity Module (SIM) Card 
         [0070]    The MSC/VLR may inform the WTRU about a new LAI to be stored on the WTRU&#39;s SIM card via the MME without the WTRU initiating a LAU request. As an example, in connected mode, if the MME determines that the WTRU is in an area belonging to a different LAI than the last registered LA, it may initiate the LAU on behalf of the WTRU. When doing so, the MME sends the new LAI and IMSI to the WTRU. The WTRU identity may be the TMSI, in case the MME creates and holds an association between the WTRU IMSI and the TMSI. The network may then configure the WTRU with a new LAI (and optionally a new TMSI) in any NAS message. 
         [0071]      FIG. 10  is a flowchart of a method  1000  for triggering a LAU procedure. The method  1000  begins with the WTRU moving into an area with a new LAI (step  1002 ). The MME initiates an LAU procedure on behalf of the WTRU (step  1004 ). The MME sends the new LAI and new IMSI to the WTRU (step  1006 ) and the method terminates (step  1008 ). 
         [0072]    EPS/IMSI Attached 
         [0073]    If the WTRU is EPS/IMSI attached and detects a condition for triggering a TAU request, the WTRU combines the TAU request with a LAU request. One condition for triggering the TAU request is when the WTRU enters an area where the TAI is not in the list of TAs previously registered by the WTRU, meaning that the WTRU is entering a new area. When this occurs, the WTRU needs to perform a registration update, which is a combined TAU/LAU procedure. 
         [0074]    Paging Identity for the WTRU 
         [0075]    The following techniques for determining a paging identity for the WTRU may be used individually or in any combination. 
         [0076]    IMSI/TMSI Mapping 
         [0077]    The MME may keep a mapping between the IMSI, the TMSI (if allocated), and the corresponding S-TMSI. If the MSC/VLR initiates paging in E-UTRAN for MT services using the IMSI, the MME may map the IMSI to the corresponding S-TMSI. The MME may then page the WTRU in E-UTRAN using the S-TMSI and an indication on the page message of the cause value, such as CS service, CS fallback, MT CS call, MT Short Message Service (SMS), etc. If the WTRU detects that the page message is directed to an identity assigned by upper layers for the WTRU (a TMSI or an IMSI), it initiates an Extended Service Request message on the E-UTRAN network, and sets the cause value for the service request to page response. If the WTRU is re-directed (falls back) to GSM/UMTS CS, the WTRU sends a page response using the IMSI. 
         [0078]      FIG. 11  is a flowchart of a method  1100  for determining a paging identity for a WTRU. The MME maintains a mapping between the IMSI, the TMSI, and the S-TMSI (if assigned; step  1102 ). The MSC/VLR initiates paging of the WTRU in the E-UTRAN using the IMSI (step  1104 ). The MME maps the IMSI to the corresponding S-TMSI (step  1106 ). The MME pages the WTRU in the E-UTRAN using the S-TMSI and a cause value (step  1108 ). A determination is made at the WTRU whether the S-TMSI in the page message is the receiving WTRU&#39;s S-TMSI (step  1110 ). In one embodiment, the WTRU compares the S-TMSI in the page message with the S-TMSI stored at the WTRU, for example, on the Universal Subscriber Identity Module (USIM) card. If the S-TMSI in the page message is not the receiving WTRU&#39;s S-TMSI, meaning that the page message is not intended for the receiving WTRU, then the page message is discarded (step  1112 ) and the method terminates (step  1114 ). 
         [0079]    If the S-TMSI in the page message is the receiving WTRU&#39;s S-TMSI (step  1110 ), then the WTRU initiates an Extended Service Request message on the E-UTRAN (step  1116 ). The WTRU then falls back to GSM or UMTS for the CS service (step  1118 ). After completing the fall back to CS, the WTRU sends a page response message using the IMSI (step  1120 ) and the method terminates (step  1114 ). 
         [0080]    IMSI Paging 
         [0081]    If the MSC/VLR initiates paging in the E-UTRAN for MT services using the IMSI, the MME may page the WTRU in the E-UTRAN using the IMSI. If the WTRU detects that the cause for the page message is related to a CS service, for example, CS fallback, a MT CS call, MT SMS, a CS domain indicator, etc., it may not ignore the page request nor detach from the E-UTRAN, but instead initiates an Extended Service Request message on the E-UTRAN network, and sets the cause value for the service request to page response. If the WTRU is re-directed (falls back) to GSM/UMTS CS, the WTRU sends a page response using the IMSI. 
         [0082]      FIG. 12  is a flowchart of a method  1200  for determining a paging identity for a WTRU. The method  1200  begins with the MSC/VLR initiating paging of the WTRU in the E-UTRAN using the IMSI (step  1202 ). The MME pages the WTRU in the E-UTRAN using the IMSI (step  1204 ). The WTRU detects a cause value associated with the page message (step  1206 ) and determines whether the page message is related to a CS service (step  1208 ). The paging message includes a WTRU identifier (such as the IMSI) and a domain indicator (set to either CS or PS). If the page message is not related to a CS service, meaning that the domain indicator is set to PS, then the WTRU reattaches to the E-UTRAN (step  1210 ) and the method terminates (step  1212 ). 
         [0083]    If the page message is related to a CS service, meaning that the domain indicator is set to CS (step  1208 ), then the WTRU initiates an Extended Service Request message on the E-UTRAN (step  1214 ). The WTRU then falls back to GSM or UMTS for the CS service (step  1216 ). After completing the fall back to CS, the WTRU sends a page response message using the IMSI (step  1218 ) and the method terminates (step  1212 ). 
         [0084]    S-TMSI Paging 
         [0085]    The MME may keep a mapping between the IMSI, TMSI (if allocated), and the corresponding S-TMSI. If the MSC/VLR initiates paging in E-UTRAN for MT services using the TMSI, the MME map the TMSI to the corresponding S-TMSI. The MME then pages the WTRU in the E-UTRAN using the S-TMSI and includes an indication on the page message of the cause value, such as, CS service, CS fallback, a MT CS call, MT SMS, etc. If the WTRU detects that the page message is directed to an identity assigned by upper layers for the WTRU, it initiates an Extended Service Request message on the E-UTRAN network, and sets the cause value for the service request to page response. If the WTRU is re-directed (falls back) to GSM/UMTS CS, the WTRU sends a page response using the IMSI. 
         [0086]      FIG. 13  is a flowchart of a method  1300  for determining a paging identity for a WTRU. The MME maintains a mapping between the IMSI, the TMSI, and the S-TMSI (if assigned; step  1302 ). The MSC/VLR initiates paging of the WTRU in the E-UTRAN using the TMSI (step  1304 ). The MME maps the TMSI to the corresponding S-TMSI (step  1306 ). The MME pages the WTRU in the E-UTRAN using the S-TMSI and includes a cause value in the page message (step  1308 ). A determination is made at the WTRU whether the S-TMSI in the page message is the receiving WTRU&#39;s S-TMSI (step  1310 ). If the S-TMSI in the page message is not the receiving WTRU&#39;s S-TMSI, meaning that the page message is not intended for the receiving WTRU, then the page message is discarded (step  1312 ) and the method terminates (step  1314 ). 
         [0087]    If the S-TMSI in the page message is the receiving WTRU&#39;s S-TMSI (step  1310 ), then the WTRU initiates a Extended Service Request message on the E-UTRAN (step  1316 ). The WTRU then falls back to GSM or UMTS for the CS service (step  1318 ). After completing the fall back to CS, the WTRU sends a page response message using the IMSI (step  1320 ) and the method terminates (step  1314 ). 
         [0088]    IMSI Paging 
         [0089]    The MME may keep a mapping between the IMSI, TMSI (if allocated), and the corresponding S-TMSI. If the MSC/VLR initiates paging in E-UTRAN for MT services using the TMSI, the MME maps the TMSI to the corresponding S-TMSI. The MME then pages the WTRU in the E-UTRAN using the IMSI and includes an indication on the page message of the cause value, for example, CS service, CS fallback, a MT CS call, MT SMS, etc. If the WTRU detects that the page message is directed to an identity assigned by upper layers for the WTRU, it may not ignore the page request nor detach from E-UTRAN, but instead it initiates an Extended Service Request message on the E-UTRAN network, and sets the cause value for the service request to page response. When the WTRU is re-directed (falls back) to GSM/UMTS CS the WTRU sends a page response using the IMSI. 
         [0090]      FIG. 14  is a flowchart of a method  1400  for determining a paging identity for a WTRU. The MME maintains a mapping between the IMSI, the TMSI, and the S-TMSI (if assigned; step  1402 ). The MSC/VLR initiates paging of the WTRU in the E-UTRAN using the TMSI (step  1404 ). The MME maps the TMSI to the corresponding IMSI (step  1406 ). The MME pages the WTRU in the E-UTRAN using the IMSI and includes a cause value in the page message (step  1408 ). A determination is made at the WTRU whether the IMSI in the page message is the receiving WTRU&#39;s IMSI (step  1410 ). If the IMSI in the page message is not the receiving WTRU&#39;s IMSI, meaning that the page message is not intended for the receiving WTRU, then the page message is discarded (step  1412 ) and the method terminates (step  1414 ). 
         [0091]    If the IMSI in the page message is the receiving WTRU&#39;s IMSI (step  1410 ), then the WTRU initiates a Extended Service Request message on the E-UTRAN (step  1416 ). The WTRU then falls back to GSM or UMTS for the CS service (step  1418 ). After completing the fall back to CS, the WTRU sends a page response message using the IMSI (step  1420 ) and the method terminates (step  1414 ). 
         [0092]    Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided herein may be implemented in a computer program, software, or firmware incorporated in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
         [0093]    Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
         [0094]    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, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, 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 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.