HK1126074A - Wireless communication method and system for performing handover between two radio access technologies - Google Patents

Description

Wireless communication method and system for performing handover between two radio access technologies

Technical Field

The present invention relates to wireless communication systems. In particular, the present invention relates to a method and system for supporting handover between a second generation (2G)/third generation (3G) Radio Access Network (RAN) and evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (E-UTRAN) based system.

Background

With the widespread introduction of 3G and Long Term Evolution (LTE) technologies, a major concern is the need to continuously provide services using the old 2/2.5G technologies as well as the 3G and LTE technologies in a seamless manner. However, it takes a long time to be able to match the geographical coverage and network capacity of 3G and LTE based networks to the coverage and capacity achieved by the old 2/2.5G networks. Furthermore, features of 3G and LTE systems may control different sites in the same coverage area, e.g., an LTE cell may be smaller than a cell of 3G and 2/2.5G technologies.

In the absence of 3G or LTE coverage, the user needs to use the old 2/2.5G network and a wireless transmit/receive unit (WTRU) operating in that network will need to support multiple Radio Access Technologies (RATs), thus requiring multi-RAT WTRU capabilities. Not only must the multi-RAT WTRU be able to search for other types of RAT networks at power up, but the multi-RAT WTRU must also be able to reselect network types when moving out of the LTE coverage area.

During inter-RAT handover, a cell/session must be handed over from one RAT network to another without a significant reduction in performance that is apparent to users of dual RAT WTRUs. For General Packet Radio Service (GPRS) capable multi-RAT WTRUs, a packet service connection must also be transferred to another network.

An inter-system handover is a process of maintaining a communication connection when moving from one cell of a first RAT network to another cell of a second RAT network. Since LTE networks are deployed in geographical areas covering older 2/2.5G networks, seamless inter-RAT handover becomes very important for providing uninterrupted service and reachability to users.

Disclosure of Invention

The present invention relates to a method and apparatus for performing handover between UTRAN and E-UTRAN in a wireless communication system. The wireless communication system includes a UTRAN, an E-UTRAN, a 2G/3G core network, and an LTE core network, and at least one wireless transmit/receive unit (WTRU) including an LTE element and a 2G/3G element. In accordance with the present invention, the WTRU is configured to handover a call originated on the UTRAN to the E-UTRAN and vice versa.

An E-UTRAN-based system includes an Access Gateway (AGW) located in the LTE core network that initiates a handover procedure for the WTRU to switch from an E-UTRAN mode to the UTRAN mode. The handover procedure may be initiated by responding to a measurement report sent by the WTRU to the AGW. Upon initiating a handover, the AGW exchanges messages with an Access Server Gateway (ASGW) anchor node located in the LTE core network. The ASGW then exchanges messages with a target SGSN located in the target 2G/3G network over a Gn interface. The Gn interface is an existing protocol for IP-based connections between SGSN and SGSN-GGSN. Upon receiving a handover message from the ASGW, the target SGSN notifies a target Radio Network Controller (RNC). The target RNC then informs the target node B. The target RNC then sends a handover command to the WTRU via the SGSN, ASGW, AGW and LTE NB. The handover command includes a target cell ID and a channel. Upon receipt of the handover command by the WTRU, the WTRU switches channels and establishes a wireless connection with the target node-B on a new channel in UTRAN mode. The target node B then informs the RNC that the handover is complete. The RNC forwards the handover complete message to the ASGW through the SGSN, and the ASGW instructs the AGW to release E-UTRAN radio resources previously used by the WTRU.

Drawings

The invention will be understood in more detail from the following description of preferred embodiments, given by way of example and understood in conjunction with the accompanying drawings, in which:

FIG. 1 is an exemplary block diagram of a dual-mode communication system configured in accordance with the present invention;

FIG. 2 illustrates signaling between system components of FIG. 1 performing a handover process from E-UTRAN to UTRAN; and

figure 3 shows signaling between system components of figure 1 performing a handover process from UTRAN to E-UTRAN.

Detailed Description

The term "wireless transmit/receive unit (WTRU)" as referred to hereinafter includes, but is not limited to, a User Equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a mobile telephone, a Personal Digital Assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. The term "base station" as referred to hereinafter includes, but is not limited to, a node B, a site controller, an Access Point (AP), or any other type of interfacing device operating in a wireless environment.

Fig. 1 is an exemplary block diagram of a wireless communication system 100 including LTE and 2G/3G components. The system includes at least one multi-RAT WTRU110, E-UTRAN 112, UTRAN 114, LTE core network 116, and 2G/3G core network 136.

In accordance with the present invention, the WTRU110 is configured to handover between the UTRAN 114 and the E-UTRAN 112 or vice versa. The WTRU110 includes an LTE component 118 and a 2G/3G component 120. The WTRU110 operates in either an LTE mode or a 2G/3G mode.

Typically, when the WTRU 100 is operating in LTE mode, the WTRU 100 exchanges messages with the E-UTRAN 112 via an LTE element 118 and an enhanced node B (E-NB)122, and the E-NB 122 exchanges messages with an Access Gateway (AGW)124 located in the LTE core network 116. The AGW124 communicates with an Access Server Gateway (ASGW) anchor node 126.

When the WTRU110 is operating in 2G/3G mode, the WTRU110 exchanges messages with the UTRAN 114 via 2G/3G elements 120 and a Node B (NB)128, and the NB 128 exchanges messages with a Radio Network Controller (RNC) 130. The UTRAN 114 exchanges messages with the SGSN132 and the 2G/3G core network 136 via the RNC 130. The SGSN132 keeps track of the location of the WTRU110 when the WTRU110 is operating in 2G/3G mode.

The 2G/3G core network 136 also includes a GGSN 134. The GGSN134 is a gateway function in the 2G/3G system 136. It assigns an IP address and connects the user to the server of the desired service. The GGSN134 also controls the quality of service (QoS) of the various data flows and connects the wireless system to an IP Multimedia Subsystem (IMS) system. The 2G/3G core network 136 communicates with the LTE core network 116 through the ASGW anchor node 126 and the SGSN 132. ASGW anchor node 126 and SGSN exchange messages over Gn (S4) communication link 138.

Fig. 2 illustrates signaling between components of the system 100 of fig. 1 in accordance with the present invention. In particular, fig. 2 shows a handover procedure from an LTE communication mode to a 2G/3G communication mode.

In the E-UTRAN to UTRAN handover procedure of FIG. 2, the WTRU110 initially operates in LTE mode and sends a measurement report 205 to the AGW124 through the E-NB 122. At step 210, AGW124 triggers a handover procedure based on the information contained in measurement report 205 and sends a relocation request message 217 containing target information to ASGW126 including the target cell ID and target SGSN.

ASGW126 sends a relocation request message 217 containing information about the target cell ID to target SGSN132 at step 215. The target SGSN132 determines the target RNC130 and then signals the target RNC130 at step 220. In step 225, the target RNC130 determines the target NB 128, and the target RNC130 exchanges initial configuration messages with the target NB 128. After the initial configuration message has been exchanged, the target RNC130 sends a Radio Access Bearer (RAB) setup answer 233 to the target SGSN 132.

In step 235, the target SGSN sends a relocation request to AGW124 through ASGW 126. The relocation request includes the target cell ID. At step 240, the AGW initiates a Context Transfer (CT) by sending a context transfer message 242 to the target SGSN132 using ASGW 126. The target SGSN132 forwards the SRNS context to the target RNC130 at step 245. In step 250, the target RNC130 exchanges RAB establishment messages with the target 2G/3G NB 128. Next, the target RNC130 sends a CT complete message 255 to the AGW124 through the ASGW126, and the target RNC130 also sends a CT answer 253 to the target SGSN 132. At step 260, the AGW124 forwards a handover command to the WTRU110 through the E-NB 122 specifying the cell ID and channel number.

At step 265, WTRU110 switches channels and occupies (camp) the new channel specified in the switch command. In step 268, the WTRU uses 2G/3G element 120 to send an RRC connection setup message to 2G/3G NB 128 on the new channel. In step 270, 2G/3G NB 128 exchanges reconfiguration complete messages with target RNC 130. The target RNC130 sends a handover complete message to the target SGSN132 at step 273. The target SGSN completes the handover by sending a handover complete message 277 to ASGW126 in step 275.

In step 280, the ASGW initiates the release operation by sending a release message 282 to the AGW 124. In step 285, E-UTRAN radio resources are released. In step 290, the handover is completed, wherein the WTRU110 and the SGSN132 exchange Routing Area (RA) update and PDP context modification procedures.

Fig. 3 illustrates signaling between components of the system 100 of fig. 1 in accordance with the present invention. In particular, fig. 3 shows a handover procedure from a 2G/3G communication mode to an LTE communication mode.

During the UTRAN to E-UTRAN handover procedure of FIG. 3, the WTRU110 initially operates in 2G/3G mode. The WTRU110 sends a measurement report 305 to the RNC130 through the NB 122. In step 310, the RNC130 triggers handover based on the information contained in the measurement report and sends a relocation required message 313 containing target information to the SGSN 132. In step 315 the SGSN determines the target ASGW and sends a relocation required message 318 to the target ASGW126, the relocation required message 318 including the target cell ID. In step 320, target ASGW126 determines the target AGW and forwards relocation required message 318 to target AGW 124. At step 325, the target AGW124 determines the target E-NB. At step 328, the target AGW124 and the target E-NB 122 exchange initial configuration messages.

At step 330, the AGW initiates the CT by sending a relocation response message 333 to SGSN132 using target ASGW 126. In step 335, the SGSN132 sends a relocation success message to the RNC 130. In step 340, the RNC initiates a SRNS context transfer by sending a SRNS context message 343 using SGSN132 and target ASGW 126. In step 345, the target AGW and the target E-NB 122 exchange RAB setup messages. In step 347, RAB establishment is completed and target AGW124 sends a context answer and reconfiguration complete message 349 to SGSN132 via target ASGW 126. In step 350, the CT is completed and the SGSN132 sends a context complete message 353 to the RNC 130.

In step 355, the RNC130 instructs the WTRU to switch channels by sending a handover command 357 to the WTRU110 using the 2G/3G NB 128. The handover command message 357 includes at least one target cell ID and a channel. At step 360, the WTRU110 switches channels and occupies the new channel. In step 363, the E-NB 122 sends an initial access message to the target AGW124 using the E-UTRAN resources. At step 365, the reconfiguration is complete and the target AGW124 sends a reconfiguration complete message 368 to the SGSN132 via the ASGW. In step 370, the RNC initiates the release operation by sending a release message 373 to the 2G/3 GNB. At step 375, radio resources are released at 2G/3G NB 128. In step 380, the WTRU110 sends a RA update and Packet Data Protocol (PDP) context modification message to the target AGW 124.

Examples

1. A method for Handover (HO) between an E-UTRAN based system and a UTRAN based system, wherein an interface between the two systems is established between an Access Server Gateway (ASGW) anchor node and a Serving GPRS Support Node (SGSN), the method comprising:

then using the interface between ASGW anchor node and SGSN;

an Access Gateway (AGW) initiates a HO procedure to a UTRAN system for a User Equipment (UE);

the UTRAN system sends a relocation response message to the AGW;

the AGW performs relocation; and

the UE terminates the handover.

2. The method of embodiment 1 further comprising an Access Server Gateway (ASGW) forwarding the relocation request to the target RNC.

3. The method according to any of the above embodiments, further comprising the UTRAN system allocating resources for the UE in the target SGSN.

4. The method according to any of the preceding embodiments, further comprising the AGW forwarding the handover command to the UE.

5. The method according to any of the preceding embodiments, further comprising the target SGSN notifying the AGW via AGSW that the handover is completed.

6. The method according to any of the preceding embodiments, further comprising the AGW sending a release message to release the radio resources.

7. The method as in any one of the above embodiments, further comprising the target SGSN sending an update PDP context to the ASGW.

8. The method of any of the above embodiments, further comprising updating the QoS profile.

9. The method of any preceding embodiment, further comprising updating the HSS record.

10. The method according to any of the preceding embodiments, wherein the AGW initiates the handover process based on a measurement report reported by the UE.

11. The method as in any one of the above embodiments, wherein the relocation request is forwarded by the supported SGSN via the ASGW anchor node.

12. The method according to any of the preceding embodiments, wherein the UTRAN system sends a relocation response message to the AGW via the ASGW.

13. The method as in any one of the above embodiments, wherein the AGW specifies a RAN technology, a channel number, a RA, and a LA for the UE.

14. The method as in any one of the above embodiments, wherein the AGW sends the SRNS context to the target SGSN via the ASGW.

15. The method as in any one of the above embodiments, wherein the UE sends a reconfiguration complete message to the target SGSN.

16. The method according to any of the preceding embodiments, wherein a handover is performed from an E-UTRAN-based system to a UTRAN-based system.

17. The method according to any of the preceding embodiments, wherein the measurement report is sent from the UE to the AGW via the E-node B.

18. The method according to any of the preceding embodiments, wherein the AGW initiates the handover trigger.

19. The method as in any one of the above embodiments, wherein the ASGW determines a target SGSN.

20. The method as in any one of the above embodiments wherein the target SGSN determines the target RNC.

21. The method as in any one of the above embodiments, wherein the target RNC determines the target node B.

22. A method as in any preceding embodiment wherein the initial configuration is determined between a UTRAN node B and a target RNC.

23. The method as in any one of the above embodiments, wherein the AGW initiates the context transfer.

24. The method as in any one of the above embodiments, wherein the AGW transmits the context to the ASGW, and the ASGW transmits the context to the target SGSN.

25. The method as in any one of the above embodiments, wherein the target RNC sends a RAB establishment response to the target SGSN.

26. The method as in any one of the above embodiments, wherein the target SGSN sends a target cell ID relocation response to the ASGW.

27. The method as in any one of the above embodiments wherein the target SGSN forwards the SRNS context to the target RNC.

28. The method as in any one of the above embodiments, wherein RAB establishment occurs between the node B and the target RNC.

29. The method as in any one of the above embodiments, wherein the target RNC sends a Context Transfer (CT) acknowledgement to the target SGSN.

30. The method as in any one of the above embodiments, wherein the target SGSN sends a CT complete signal to the ASGW.

31. The method as in any one of the above embodiments, wherein the ASW sends the HO command to the E-node B with the target cell ID and channel.

32. The method as in any one of the above embodiments, wherein the E-node B sends the HO information to the UE along with the cell ID and channel.

33. The method as in any one of the above embodiments, wherein the UE switches channels and occupies a new channel.

34. The method as in any one of the above embodiments, wherein the UE sends the RRC connection setup to the node B.

35. The method as in any one of the above embodiments, wherein the reconfiguration complete signal is sent between the node B and the target RNC.

36. The method as in any one of the above embodiments, wherein the handover complete signal is sent from the target RNC to the target SGSN.

37. The method of embodiment 36 wherein the HO complete signal is sent from the target SGSN to the ASGW.

38. The method as in any one of the above embodiments, wherein the ASGW initiates the release.

39. The method according to any of the preceding embodiments, wherein the ASGW initiates the release of the AGW.

40. The method as in any one of the above embodiments, wherein the AGW releases E-UTRAN radio resources.

41. The method as in any one of the above embodiments, wherein the routing area update and PDP context modification procedures occur between the UE and the target SGSN.

42. A method as in any of the embodiments 1-15 wherein a handover is made from a UTRAN-based system to an E-UTRAN-based system.

43. The method as in any one of embodiments 42 wherein the measurement report is sent from the UE to the target RNC via the node B.

44. A method as in any of embodiments 42-43 wherein a target RNC sends a HO trigger to a target SGSN.

45. A method as in any of embodiments 42-44 wherein a target SGSN determines a target ASGW.

46. A method as in any of embodiments 42-45 wherein a target SGSN sends a relocation request to an ASGW.

47. The method as in any one of embodiments 42-46 wherein the ASGW determines a target AGW.

48. A method as in any of embodiments 42-47 wherein the ASGW forwards a relocation request to the AGW.

49. The method as in any one of embodiments 42-48 wherein the AGW determines the target E-node B.

50. The method as in any one of embodiments 42-49 wherein initial configuration occurs between an E-node B and an AGW.

51. The method as in any one of embodiments 42-50 wherein the AGW initiates the context transfer.

52. A method as in any of embodiments 42-51 wherein the ASGW sends a relocation response to the target SGSN.

53. The method as in any one of embodiments 42-52 wherein the target SGSN forwards a relocation response to the target RNC.

54. The method as in any one of embodiments 42-53 wherein the target RNC initiates a Serving Radio Network Subsystem (SRNS) Context Transfer (CT).

55. The method as in any one of embodiments 42-54 wherein the target RNC sends a SRNS CT signal to the target SGSN.

56. A method as in any of embodiments 42-55 wherein a target SGSN forwards a SRNS CT signal to an ASGW.

57. The method as in any one of embodiments 42-56 wherein the AGW and the E-node B complete a Radio Access Bearer (RAB) establishment.

58. A method as in any of embodiments 42-57 wherein the ASGW sends a CT acknowledgement/reconfiguration complete message to the target SGSN.

59. The method as in any one of embodiments 42-58 wherein the target SGSN completes the CT.

60. A method as in any of the embodiments 42-59 wherein the target SGSN sends a CT complete message to the target RNC.

61. The method as in any one of embodiments 42-60 wherein the target RNC directs the UE to switch channels.

62. A method as in any of embodiments 42-61 wherein a target RNC sends a HO command and channel with a cell ID to a node B.

63. The method as in any one of embodiments 62 wherein a node B forwards a HO command to a UE.

64. A method as in any of embodiments 42-63 wherein a UE switches channels and occupies a new channel.

65. The method as in any one of embodiments 42-64 wherein the UE performs initial access to the AGW through the E-node B.

66. The method as in any one of embodiments 42-65 wherein the AGW completes the reconfiguration.

67. A method as in any of embodiments 42-66 wherein the ASGW sends a reconfiguration complete message to the target SGSN.

68. The method as in any one of embodiments 42-67 wherein the target RNC initiates the release.

69. The method as in any one of embodiments 42-68 wherein the target RNC sends a release to the node B.

70. A method as in any of embodiments 42-69 wherein a node B releases radio resources.

71. A method as in any of embodiments 42-70 wherein TA update and PDP context are modified between the UE and the AGW.

72. A WTRU configured in accordance with the UE in any of the above embodiments.

Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The methods or flow charts provided in the present invention may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of the computer-readable storage medium include read-only memory (ROM), random-access memory (RAM), registers, buffer memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM discs and Digital Versatile Discs (DVDs).

For example, suitable processors include: 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, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) circuit, any 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, terminal, base station, radio network controller, 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, and BluetoothA module, a Frequency Modulation (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) module.

Claims (27)

1. A method for performing handover of a wireless transmit/receive unit (WTRU) from an evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (E-UTRAN) to a universal mobile telecommunications system terrestrial radio access network (UTRAN) in a wireless communication system including a dual Radio Access Technology (RAT) WTRU, a UTRAN, an E-UTRAN, a second generation/third generation (2G/3G) core network, and a Long Term Evolution (LTE) network, the method comprising:

initiating a handover process at an Access Gateway (AGW) based on a measurement report sent by the WTRU;

sending a relocation request from the AGW to a target radio network controller located in the target UTRAN;

allocating resources for the WTRU in the target UTRAN;

sending a relocation response from the UTRAN to the AGW;

sending a handover command from the AGW to the WTRU;

performing a Serving Radio Network Subsystem (SRNS) relocation by sending a SRNS context to the target RNC;

sending a reconfiguration complete message from the WTRU to a target serving General Packet Radio Service (GPRS) support node (SGSN);

releasing radio resources on the E-UTRAN by sending a release message to the E-UTRAN; and

sending an updated Packet Data Protocol (PDP) context from the target SGSN to an Access Server Gateway (ASGW) anchor node.

2. The method of claim 1 wherein the relocation request is sent from the AGW to the target RNC via the ASGW anchor node and the SGSN.

3. The method of claim 2 wherein the ASGW acts as a source SGSN.

4. The method of claim 1 wherein allocating resources for the WTRU in the target UTRAN comprises allocating resources in the target SGSN.

5. The method of claim 1 wherein the relocation response message is sent from the UTRAN to the AGW via the ASGW anchor node.

6. The method of claim 1, wherein the handover command includes information about the radio access technology, channel coding, routing area, and positioning area for a new connection.

7. The method of claim 1, wherein the SRNS context message is sent to the target RNC via the ASGW.

8. The method of claim 1, wherein sending the update PDP context message comprises updating a QoS profile.

9. The method of claim 1 wherein sending the update PDP context message comprises updating a Home Subscriber Service (HSS).

10. The method of claim 1 wherein the ASGW and the SGSN communicate over a Gn interface.

11. A method for performing a handover of a wireless transmit/receive unit (WTRU) from an evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (E-UTRAN) to a universal wireless communication system terrestrial radio access network (UTRAN) in a wireless communication system including a dual Radio Access Technology (RAT) WTRU, a UTRAN, an E-UTRAN, a second generation/third generation (2G/3G) core network, and a Long Term Evolution (LTE) network, the method comprising:

triggering a handover on a radio network controller based on measurements received from the WTRU;

determining a target evolved node B (E-NB);

initiating a context transfer at a target Access Gateway (AGW);

allocating required radio resources for the WTRU in the E-UTRAN;

sending a handover command to the WTRU;

switching channels in the WTRU in response to the handover command;

sending an initial access message from the WTRU to the target AGW;

initiating a release operation in a serving General Packet Radio Service (GPRS) support node (SGSN) to release radio resources used by the WTRU; and

updating Packet Data Protocol (PDP) context information with the target AGW.

12. The method of claim 11, wherein the determining a target E-NB comprises determining a target access server gateway node, ASGW, anchor node.

13. The method of claim 11, wherein the determining a target E-NB comprises determining a target ASG.

14. The method of claim 11, wherein the handover command includes information about a radio access technology, a channel number, a routing area, and a positioning area.

15. The method of claim 11 wherein the ASGW and SGSN communicate over a Gn interface.

16. A wireless communication system for performing handover of a radio transmit/receive unit between a Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN) and an evolved UTRAN (E-UTRAN), the system comprising:

a dual Radio Access Technology (RAT) WTRU;

a second generation/third generation (2G/3G) core network including a Serving Gateway Support Node (SGSN);

a UTRAN connected to the 2G/3G core network via a SGSN;

a Long Term Evolution (LTE) network including an Access Server Gateway (ASGW) anchor node;

an E-UTRAN connected to the LTE core network via an ASGW anchor node; wherein the SGSN and the ASGW communicate via an existing Gn interface.

17. The system of claim 16 wherein the UTRAN includes at least one radio network controller for communicating with node bs and the SGSN and for triggering a handover from UTRAN to E-UTRAN based on measurements received from the WTRU.

18. The system of claim 17 wherein the WTRU exchanges messages with the node B when the WTRU is operating in 2G/3G mode.

19. The system of claim 16 wherein the LTE core network includes an Access Gateway (AGW) for communicating between at least one evolved node B (E-NB) and an ASGW anchor node and for triggering a handover from the E-UTRAN to the UTRAN based on measurements received from the WTRU.

20. The system of claim 19 wherein the WTRU exchanges messages with the E-NB when the WTRU is operating in LTE mode.

21. A dual mode wireless transmit/receive unit (WTRU) configured to perform handover between a Long Term Evolution (LTE) network and a second generation/third generation (2G/3G) network, the WTRU comprising:

an LTE element to communicate with an LTE core network; and

a 2G/3G element to communicate with a 2G/3G core network, wherein the LTE core network and the 2G/3G network communicate over a Gn interface.

22. The WTRU of claim 21 wherein the LTE element communicates with the LTE core network through an evolved node B (E-NB) located in an evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (E-UTRAN).

23. The WTRU of claim 22 wherein the E-NB communicates with the LTE core network through an Access Gateway (AGW).

24. The WTRU of claim 23 wherein the AGW communication is connected to the Gn interface through the Access Server Gateway (ASGW) anchor node.

25. The WTRU of claim 21 wherein the 2G/3G element communicates with LTE having a node b (nb) located in the Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN).

26. The WTRU of claim 21 wherein the NB communicates with the 2G/3G core network through a Serving Gateway Support Node (SGSN) located in the 2G/3G network.

27. The WTRU of claim 21, wherein the SGSN is connected to the Gn interface.