Abstract:
A method for use in a source long term evolution (LTE) mobility management entity (MME) and an MME are disclosed. The method includes receiving a relocation request from an evolved Node B (eNB), determining a handover target global system for mobile (GSM)/enhanced data rates for GSM evolution (EDGE) radio access network (GERAN) system for a handover of a wireless transmit/receive unit (WTRU) based on the received relocation request, identifying a serving General Packet Radio Service (GPRS) support node (SGSN) that controls a target GERAN cell, and forwarding the relocation request to a target SGSN.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/029,532 filed Feb. 12, 2008, which issues as U.S. Pat. No. 8,072,936 on Dec. 6, 2011, which claims the benefit of U.S. Provisional Application No. 60/889,383, filed Feb. 12, 2007, the contents of which are hereby incorporated by reference herein. 
    
    
     FIELD OF INVENTION 
     This application is related to wireless communications. 
     BACKGROUND 
     There are different types of wireless communication systems. For example, some wireless communication systems include general packet radio service (GPRS), global system for mobile (GSM)/enhanced data rates for GSM evolution (EDGE) radio access network (GERAN), and the newly introduced long term evolution (LTE) evolved universal terrestrial radio access network (EUTRAN). LTE/EUTRAN system has a different physical layer and a different architecture from those systems preceding it, i.e., GPRS, GERAN, or UTRAN. 
     When a Multi-mode mobile unit is traveling across the geographic coverage of these different systems, it may need to be handed off from one network to another. Since not all networks are identical, a method for supporting the handover between systems would be beneficial. 
       FIG. 1  shows an exemplary diagram of a system  100  including an LTE system architecture. The system  100  shows an LTE/EUTRAN  101  and its evolved packet core  105  interworking with an existing GERAN  102 , UTRAN  103 , and their GPRS Core  104 . The LTE/EUTRAN  101  comprises an E-Node B (not shown) that is connected (S1) to an evolved packet core  105  containing a mobility management entity/user plane entity (MME/UPE)  106  and an inter AS anchor Gateway  107 . The Evolved Packet Core  105  connects (S6) to a home subscriber service (HSS)  111 , and connects (S7) to a Policy and Charging Rules (PCRF)  112 . The inter AS Anchor gateway  107  connects (Gi) to Operator IP Servers (such as IMS, PSS)  110 , connects (S2) to a Non-3GPP IP Access network  108 , and connects (S2) to a WLAN 3GPP IP Access network  109 . The GPRS Core  104  comprises a Serving GPRS Support Node (SGSN) (not shown) which is responsible for Mobility Management, Access Procedures, and User Plane Control. The GPRS Core  104  also comprises a Gateway GPRS Support Node (GGSN), where the network is connected to external networks and other operator servers. The Operator IP Servers  110  may include an IP Multimedia Service Subsystem (IMS) where VoIP and other multimedia services are controlled. The Non-3GPP IP access network  108  includes connections to other technologies that are developed in other standard Forums such as 3GPP2 (CDMA2000) and WiMAX (IEEE 802.16 system). The WLAN 3GPP IP access network  109  has WLANs incorporated into 3GPP systems via interworking architecture defined in 3GPP. 
     SUMMARY 
     A method and apparatus for supporting handover from an LTE/EUTRAN cell to a general packet radio service (GPRS)/global system for mobile communications (GSM)/enhanced data rates for GSM evolution (EDGE) radio access network (GERAN) cell. In one embodiment, a GERAN access procedure during the handover includes sending a packet switched (PS) attach signal. In another embodiment, the GERAN access procedure includes RAN mobility information messages being exchanged between the WTRU and a target base station controller (T-BSC). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings wherein: 
         FIG. 1  shows an example of an LTE general network architecture; 
         FIG. 2  shows an initial state for handoff from an LTE system to a GPRS/GERAN system; 
         FIG. 3  shows a second state for handoff from an LTE system to a GPRS/GERAN system; 
         FIG. 4  shows a third state for handoff from an LTE system to a GPRS/GERAN system; 
         FIG. 5  shows a functional block diagram of a wireless transmit/receive unit and a Node B; 
         FIGS. 6A, 6B, 6C  show a signal flow diagram of a handover procedure including a packet switched (PS) handover signal; and 
         FIGS. 7A, 7B, 7C  show a signal flow diagram of a handover procedure including a relocation detect from a source evolved Node-B (S-ENB) to a target base station controller (T-BSC). 
     
    
    
     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 user 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, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
       FIGS. 2-4  show examples of three states of traffic paths and tunnels established between network entities during a handover of a WTRU from an LTE network to a GERAN network. In  FIG. 2 , an initial state  200  is shown for a mobile WTRU  201  moving from an LTE network cell designated as local area LA2/Routing Area RA2, to a GERAN system LA1/RA1. The cells belonging to GERAN systems may constitute different Location Area/Routing Area (LA1/RA1) from those belonging to LTE based cells (LA2/RA2). In certain deployments, although GERAN cells may be co-located with GERAN cells, these cells will remain under different LA/RA configuration due to the difference between the two system architectures. Other WTRUs  254 ,  255  are shown camped on the cell LA2/RA2, and WTRUs  251 ,  252  are camped on the cell LA1/RA1. The WTRU is currently connected to an access gateway  211  via a source Evolved Node B (ENB)  222 , where tunnels  215  and  216  are established for the user data plane. Source mobility management entity (MME)  221  controls mobility and handles user control plane traffic on tunnels  217  and  218 . User control plane traffic is connected on tunnel  219  between the source ENB  222  and the WTRU  201 . The target GERAN system comprises a target SGSN  231 , a target base station controller (BSC)  232 , and a target mobile services switching center/visitor location register (MSC/VLR) entity  233 . 
       FIG. 3  shows an optional second state  300  for tunneling of network entities during the handover of the WTRU  201  from the LTE network cell LA2/RA2 to the GERAN cell LA1/RA1. The WTRU  201  is now migrated to the GERAN cell LA1/RA1. 
     An optional tunnel  225  may be created between the target BSC  232  of the GERAN system and the source ENB  222 . The tunnel  225  may be used to temporarily forward the current pending data transfer between GERAN system and the WTRU via E-Node B while a new connection between the Evolved Core Network  105  and the GPRS Core  104  (i.e., while the backbone procedures to switch traffic is completed). This will ensure that no data is lost during transition. A system operator may choose not to implement this step and go to a complete transition case where no connection is established between the GERAN BSC  232  and ENB  222 . In such a case, forwarding of data occurs at higher layers, between the two core networks on S3 and S4 connections. The user data plane and control plane traffic is carried to the WTRU  201  across the tunnels  235  and  236 , respectively. 
       FIG. 4  state  400  for tunneling of network entities during the handover of the WTRU  201  from the LTE network cell LA2/RA2 to the GERAN cell LA1/RA1. As shown in  FIG. 4 , the traffic switching has occurred in the upper layers such that the GERAN system is now is the network source for user traffic, as shown by GGSN  411 . The WTRU  201  is now connected to the GERAN system GGSN  411  via the Target SGSN  231 , and the target BSC  232 , on user data plane and control plane tunnels  415 ,  416  and  417 , respectively. 
       FIG. 5  is a functional block diagram of a WTRU  510  and a Node B  520 . As shown in  FIG. 5 , the WTRU  510  is in communication with the Node B  520  and both are configured to support handover from GPRS/GERAN to LTE/EUTRAN. 
     In addition to the components that may be found in a typical WTRU, the WTRU  510  includes a processor  515 , a receiver  516 , a transmitter  517 , and an antenna  518 . The processor  515  is configured to support handover from GPRS/GERAN to LTE/EUTRAN. The receiver  516  and the transmitter  517  are in communication with the processor  515 . The antenna  518  is in communication with both the receiver  516  and the transmitter  517  to facilitate the transmission and reception of wireless data. The processor  515 , receiver  516 , transmitter  517 , and antenna  518  may be configured as a GPRS/GERAN radio transceiver, or configured as an LTE/EUTRAN radio transceiver. Also, although only one processor, receiver, transmitter, and antenna is shown, it should be noted that multiple processors, receivers, transmitters, and antennas may be included in the WTRU  510 , whereby different groupings of processors, receivers, transmitters, and antennas operate in different modes, (e.g., GPRS/GERAN transceiver or LTE/EUTRAN transceiver). 
     In addition to the components that may be found in a typical Node B, the Node B  520  includes a processor  525 , a receiver  526 , a transmitter  527 , and an antenna  528 . The processor  525  is configured to support handover from LTE/EUTRAN to GERAN. The receiver  526  and the transmitter  527  are in communication with the processor  525 . The antenna  528  is in communication with both the receiver  526  and the transmitter  527  to facilitate the transmission and reception of wireless data. 
     In a first embodiment, GERAN access procedures include packet switched (PS) attach signals between the LTE transceiver of the WTRU  510 .  FIGS. 6A-6C  show an exemplary signal diagram of a handover procedure  600  for this embodiment. While the following signals are shown in  FIGS. 6A-6C  and described in a particular sequence, the signals may occur in variations to the sequence in accordance with this embodiment. 
     In the signal diagram of  FIGS. 6A-6C , signals are exchanged among: a dual mode LTE/GERAN WTRU having an LTE transceiver and a GERAN transceiver, each transceiver comprising a receiver and a transmitter; a source e-Node B (S-ENB); a target BSC (T-BSC); a source LTE-MME; a target SGSN; and an LTE UPE/Gateway. The dual mode WTRU in this example includes an LTE and GERAN transceiver. 
     As shown in  FIGS. 6A-6C , user downlink (DL) and uplink (UL) traffic  601   a ,  601   b  is exchanged between the WTRU LTE transceiver, the S-ENB and the LTE UPE/Gateway. The WTRU LTE transceiver performs measurements  605  on LTE frequencies and GERAN frequencies, and transmits a GERAN measurement report signal  606  to the S-ENB. The WTRU may receive a list of different radio access technologies, including GERAN, from the S-ENB to identify the types of frequency measurements to undertake. Intersystem Handover  607  is initiated by the S-ENB, which makes the handover decision based on the measurement report  606 , with GERAN being the target. A relocation request signal  608 , containing the source cell ID and the target cell ID, is transmitted from the S-ENB to the source MME. The source MME makes a determination  609  of the target system cell ID and the SGSN ID by mapping the target cell ID (GERAN) to an SGSN IP address. The source MME forwards the relocation request  610  to the target SGSN, including an international mobile subscriber identity (IMSI), source cell ID and target cell ID. 
     The target SGSN performs a determination  611  of the target BSC ID, and requests the user profile and context if it was not included in signaling message  610 . The target SGSN sends a handover request signal  612  to the T-BSC, including the cell ID, SGSN ID, and the international mobile subscriber identity/temporary mobile subscriber identity (IMSI/TMSI). The T-BSC performs a determination  613  of channel availability and initiates radio access bearer (RAB) establishment. The T-BSC transmits a handoff request ACK  614 , including the IMSI/TMSI, to the Target SGSN. A user plane and control plane tunnel  615  is established between the T-BSC and the Target SGSN. The target SGSN creates an MM state and SM state  616  to prepare for activating packet data protocol (PDP) context information. 
     The target SGSN sends a relocation response  617  including an IMSI and SGSN ID, to the Source MME, which sends a relocation command signal  618 , that includes the TMSI and target BSC ID to the S-ENB. A temporary tunnel  619  to the T-BSC is established by the S-ENB to forward user data to the T-BSC. A handover command  620  is transmitted from the source MME to the S-ENB, and is forwarded to the WTRU LTE transceiver, which recognizes the target GERAN technology amongst others that may be supported, and an initiate/synch radio signal  621 , including a target channel ID, is communicated to the WTRU GERAN transceiver. The GERAN transceiver sends an ACK signal  623 . User data  622  is exchanged on the temporary tunnel  619  between the S-ENB and the T-BSC. A handover complete signal  624  is sent from the WTRU LTE transceiver to the S-ENB. The T-BSC sends a relocation detect signal  625  to the target SGSN. 
     The GERAN access procedure includes a PS attach signal  626  transmitted from the WTRU GERAN transceiver to the T-BSC, which forwards the PS attach signal  627  to the target SGSN. A PS attach accepted signal  628  is returned by the target SGSN to the T-BSC. The WTRU GERAN transceiver is configured to receive a PS attach Accept  629  from the T-BSC, and to respond with a PS attach accept ACK  630 . The T-BSC forwards a PS attach accept ACK signal  631  to the target SGSN. 
     The target SGSN performs an update  633  of the PDP context with the new SGSN TEID, and establishes a tunnel  634  with the LTE UPE/gateway for a GPRS tunneling protocol user plane and control plane (GTP-U and GTP-C). At this stage the user plane path is established for all PDP contexts between the WTRU GERAN transceiver, Target BSC, Target SGSN, and serving gateway. The switch of traffic  638  is complete from the source ENB to the target SGSN. Between the WTRU GERAN transceiver and T-BSC, a tunnel  632  is established for exchange of RAN information and RAB establishment, and user DL/UL traffic  635  is exchanged. 
     A handover complete signal  636  is sent from the target SGSN to the source MME, which sends a release signal  639  to the S-ENB and an HO complete ACK  637  to the target SGSN. The S-ENB performs a release  640  of its resources related to the WTRU, and stops forwarding data. A release ACK  641  is transmitted from the S-ENB to the source MME, and user DL/UL GERAN traffic now flows on tunnel  642  between the WTRU GERAN transceiver and the target BSC, on tunnel  643  between the T-BSC and the target SGSN, and on tunnel  644  between the target SGSN and the GGSN gateway. 
       FIGS. 7A-7C  show a signaling diagram according to a second embodiment in which the GERAN access procedure includes a relocation detect signal from the source ENB and the target BSC, and RAN mobility information signals. In this embodiment, the signals are similar to the first embodiment as shown in  FIGS. 6A, 6B and 6C , except for the following signals which are used in lieu of the PS attach signals  626 - 631  excluded in this embodiment. 
     As shown in  FIGS. 7A-7C , a GERAN access procedure begins the handover complete signal  624 . A relocation detect signal  725  is sent by the S-ENB to the T-BSC, and a relocation detect signal  726  is forwarded from the T-BSC to the target SGSN. RAN mobility information  727  is transmitted by the T-BSC to the WTRU GERAN transceiver, which returns a RAN mobility information ACK  728 . The T-BSC sends a Relocation complete signal  729  to the target SGSN. 
     As described in  FIGS. 1-7C  above, radio resources are prepared in the target 3GPP access system before the WTRU  510  is commanded by the source 3GPP access system to change to the target 3GPP access system. A tunnel is established between the two radio access networks (RANs) (basic service set (BSS)/basic service controller (BSC) and E-Node B) in order to forward the data while the core network resources are assigned. 
     A control interface may exist in the core level between the 2 G/3 G SGSN and corresponding MME to exchange the mobility context and the session context of the Mobile. Additionally, the target system may provide directions to the WTRU as to the radio access requirements, such as the radio resource configuration, target cell system information, and the like. 
     There is an intermediate state during handoff where the DL User plane data is sent from source system to the target system before the User plane is switched directly to the target system in order to avoid the loss of user data, (e.g., by forwarding). Bi-casting may also be used until the 3GPP Anchor determines that it can send DL U-plane data directly to the target system. 
     Although features and elements are described above in particular combinations, each feature or element can 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). 
     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. 
     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.