Methods and system for performing handover in a wireless communication system

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Methods and system for performing handover in a wireless communication system

IntcrDigital Technology Corporation C. 188885 WO 2007/149509 PCT/US2007/014423

METHODS AND SYSTEM FOR PERFORMINGHANDOVER IN A WIRELESS COMMUNICATION SYSTEM

FIELD OF THE INVENTION

[0003] The present invention is related to wifeless communicationsystems. More particularly, the present invention is related to a method andsystem for performing handover in a long term evolution (LTE) system.

[0004] BACKGROUND

[0006] LTE for the fourth generation (4G) system is now beingconsidered to develop a new radio interface and radio network architecturethat provides a high data rate, low latency, packet optimization, and improvedsystem capacity and coverage. For an LTE system, instead of using codedivision multiple access (CDMA), which is currently being used in a 3Gsystem, orthogonal frequency division multiple access (OFDMA) andfrequency division multiple access (FDMA) are proposed to be used indownlink and uplink transmissions, respectively. By changing in manyaspects in the LTE system, intra-LTE handover procedures and relatedoperations need to be re-considered.

[0006] The user equipment (UE) mobility management in anLTE_ACTIVE mode handles all necessary steps for seamless handover in theLTE system, such as making an intra-LTE handover decision on a sourcenetwork side, (i.e., control and evaluation of UE and evolved Node-B (eNode-B) measurements taking into account UE-specific area restrictions), preparingradio resources on a target network side, commanding the UE to interfacewith new radio resources, releasing radio resources on the source networkside, and the like. The UE mobility management mechanism also handles thetransfer of context data between involved nodes, and the update of noderelations on a control plane (C-plane) and a user plane (U-plane).

[0007] Figure 1 is a signaling diagram of a handover process 100currently proposed for the LTE system. A UE 152 find a source eNode-B 154

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WO 2007/149509 PCT/US2007/014423 perform measurements and exchange measurement reports (step 102). Thesource eNode-B 154 makeR a handover decision based on the measurementreports (step 104). The source eNode-B 154 then sends a handover request toa target eNode-B 156 (step 106). The handover decision and subsequentprocedures before handover completion are performed without involving amobility management entity/user plane entity (MME/UPE) 158, (i.e.,handover preparation messages are directly exchanged between the sourceeNode-B 154 and the target eNode-B 156).

[0008] The target eNode-B 156 performs an admission control for theUE 152 (step 108). If the target eNode-B 156 can accept the UE 152, thetarget eNode-B 156 sends a handover response to the source eNode-B 154(step 110). The source eNode-B 154 sends a handover command to the UE 152(step 112). For seamless handover, a U-plane tunnel is established betweenthe source eNode-B 154 and the target eNode-B 156.

[0009] The UE 152 and the target eNode-B 156 then exchange layer 1and 2 (L1/L2) signaling (step 114). During handover execution, user data mayhe forwarded from the source eNode-B 154 to the target eNode-B 156. Theforwarding may take place in a service dependent and implementation specificway. Forwarding of user data from the source eNode-B 154 to the targeteNode-B 156 should take place as long as packets are received at the sourceeNode-B 154 from the UPE 158.

[0010] After a connection to the target eNode-B 156 is established, theUE 152 sends a handover complete message to the target eNode-B 156 (step116). The target eNode-B 156 Bends a handover complete message to theMME/UPE 158 (step 118). The MME/UPE 158 then sends a handovercomplete acknowledgement (ACK) to the target eNode-B 156 (step 120). Afterthe MME/UPE 158 is informed hy the target eNode-B 156 that the UE 152has gained an access at the target eNode-B 156 by the handover completemessage, the U-plane path is switched by the MME/UPE 158 from the sourceeNode-B 154 to the target eNode-B 156.

[0011] The release of the radio resources at the source eNode-B 154 istriggered by a release resource message sent by the target eNode-B 156 (step -2- WO 2007/149509 PCT/US2007/014423

122). After receiving the release resource message from the target eNode-B 156, the source eNode-B 154 releases the radio resources for the UE 152 (step 124). The UE 152 performs a location update with the MME/UPE 158 (step 126).

[0012] The above intra-LTE handover procedure 100 does not providedetails regarding the handover command, (such as configurations of the UE152 based on the target eNode-B’s requirement), and details regarding UEoperation after the UE receives the handover command, (such as datatransmission between the source eNode-B 154 and the UE 152 and radio linkcontrol (RLC) and hybrid automatic repeat request (HARQ) reset and packetdata convergence protocol (PDCP) sequence number (SN) gap identification bythe UE 152). The above intra-LTE handover procedure 100 also does notprovide details regarding UE timing adjustment for synchronous andasynchronous eNode-Bs and details for efficient target eNode-B scheduling ofresources for UE transmission.

[0013] SUMMARY

[0014] The present invention is related to a method and system forperforming handover in an LTE system. A source eNode-B makes a handoverdecision based on measurements, and sends a handover request to a targeteNode-B. The target eNode-B sends a handover response to the source eNode-B indicating that a handover should commence. The source eNode-B thensends a handover nnmmn-nd to a wireless transmit/receive unit (WTRU). Thehandover command includes at least one of reconfiguration information,information regarding timing adjustment, relative timing difference betweenthe source eNode-B and the target eNode-B, information regarding an initialanherirrling process at the target eNode-B, and measurement information forthe target eNode-B. The WTRU then accesses the target eNode-B andexchanges layer 1/2 signaling to perform downlink synchronization, timingadjustment, and uplink and downlink resource assignment based oninformation included in the handover command. -3- WO 2007/149509 PCT/US2007/014423

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawings wherein: [0017] Figure 1 is a signaling diagram of a handover process currently proposed for the LTE system; and [00IS] Figure 2 is a signaling diagram of an intra-LTE handover processin accordance with the present invention.

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS[0020] When referred to hereafter, the terminology '’WTRU" includesbut is not limited to a UE, a mobile station, a fixed or mobile subscriber unit,a pager, a cellular telephone, a personal digital assistant (PDA), acomputer, or any other type of user device capable of operating in a wirelessenvironment. When referred to hereafter, the terminology "eNode-B” includesbut is not limited to a base station, Node-B, a site controller, an access point(AP), or any other type of interfacing device capable of operating in a wirelessenvironment.

[0021] The present invention provides detailed procedures for signaling and operations at a WTRU and source and target eNode-Bs during intra-LTEhandover both for successful handover and handover failure cases. In asuccessful handover case, new information elements (IEs) are added in boththe handover command message and the handover complete message. In ahandover failure case, new signaling messages are exchanged between asource eNode-B and a target eNode-B.

[0022] Figure 2 is a signaling diagram of an intra-LTE handover process200 in accordance with the present invention. A WTRU 252 and a sourceeNode-B 254 each perform at least one measurement, and the WTRU 252sends a measurement report to the source eNode-B 254 (step 202). The sourceeNode-B 254 makes a handover decision based on the measurement reportand the result of its own measurement (step 204). The source eNode-B 254then sends a handover request to a target eNode-B 256 (step 206). The target -4- WO 2007/149509 PCT/US2007/014423 eNode-B 256 performs an admission control for the WTRU 252 (step 208). If the target eNode-B 256 can accept the WTRU 252, the target eNode-B 256 sends a handover response to the source eNode-B 254 indicating that a handover should commence (step 210). The source eNode-B 254 then sends a handover command to the WTRU 252 (step 212).

[0023] The handover command should include at least one ofreconfiguration information for radio resource control (RRC), radio link control(RLC), medium access control (MAC) and physical (PHY) layer, informationregarding timing adjustment when handing over from the source eNode-B 254to the target eNode-B 256, (i.e., whether the WTRU 252 should performtiming adjustment autonomously or using a random access channel (RACH)procedure, if a RACH is to be used, whether random or dedicated accesssignature will be used, or the like), relative timing difference between eNode-Bs (or cells) for autonomous timing adjustment, information regarding initialradio resource scheduling procedure at the target eNode-B 256, measurementinformation for the target eNode-B 256, and the like. The informationregarding the initial scheduling procedure at the target eNode-B 256 indicateswhether a RACH access procedure should be used for a resource assignmentrequest or the target eNode-B 256 may schedule resources for the WTRU 252without receiving ad explicit resource assignment request from the WTRU252. Alternatively, the measurement and other configuration information maybe sent to the WTRU 252 by the target eNode-B 256 after receiving ahandover complete message from the WTRU 252 at step 226.

[0024] For a seamless handover, a U-plane tunnel is establishedbetween the source eNode-B 254 and the target eNode-B 256. After sendingthe handover command, the source eNode-B 254 may forward the user data tothe target eNode-B 256. The forwarding may take place in a servicedependent and implementation specific way.

[0025] After receiving the handover command from the source eNode-B254, the WTRU 252 may continue to transmit and receive data to and fromthe source eNode-B 254. The data transmission process depends on whethersynchronized handover or non-syuchronized handover is used. -5- WO 2007/149509 PCT/US2007/014423 [0026] When a synchronized handover procedure is used, (i.e., the sourceeNode-B 254 and the target eNode-B 256 are synchronized or the relativetiming difference is known to the WTRU 252), the source eNode-B 254 and theWTRU 252 may continue to transmit and receive data after receiving thehandover command until a pnrtnin handover time (tao) which is signaled viathe handover mm-m and. The transmitted data after receiving the handovercommand is preferably limited to incomplete service data units (SDUs), (i.e.,RLC protocol data unit (PDU)), transmitted before the handover commandwas sent. An RLC control message is sent to the WTRU 252 to indicate asequence number (SN) of a successfully received SDU(s) and an SDU gap.The SN may be a PDCP SN, or other types of SN. An SN common to thesuccessfully received SDU(s) and unsuccessfully received SDU(s) may beincluded in the RLC control message.

[0027] When a non-synchronized handover procedure is used, (i.e., thesource eNode-B 254 and the target eNode-B 256 are not synchronized or therelative timing difference is not known to the WTRU 252), the source eNode-B254 stops transmission as soon as the source eNode-B 254 sends the handoverpn-mmand to the WTRU 252. The WTRU 252 also stops transmission of thedata packets to the source eNode-B 254 as soon as the WTRU 252 receives thehandover command. Alternatively, the source eNode-B 254 may continuetronamiBgion of data packets until the WTRU 252 switches to the targeteNode-B 254.

[0028] After receiving the handover command, the WTRU 252 accessesthe target eNode-B 256 and exchange layer 1/2 (L1/L2) signaling with thetarget eNode-B 256 to perform downlink synchronization, timing adjustment,(i.e., uplink synchronization), and uplink and downlink resource assignmentbased on information included in the handover command.

[0029J For timing adjustment, (i.e., uplink synchronization), the WTRU252 implements one of two options. Preferably, the network decides whichoption to be used.

[0030] In accordance with a first option, the WTRU 252 autonomouslyperforms the timing adjustment based on relative timing difference between -6-

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WO 2007/149509 PCT/US2007/014423 the source eNode-B 254 (or cell) and the target eNode-B 256 (or cells) (step214a). The relative timing difference information is preferably included in thehandover command.

[0031] In accordance with a second option, a conventional RACH accessprocedure is used for the timing adjustment (step 214b). The WTRU sends aEACH preamble to the target eNode-B and the target eNode-B calculatestiming offset based on the transmitted RACH preamble and sends the timingoffset information to the WTRU for uplink synchronization.

[0032] A plurality of RACH preamble signatures with differentorthogonality and different priority may be used, and among the plurality ofRACH preamble signatures, a RACH preamble signature with higherorthogonality, higher priority and/or higher power may be used for thehandover purpose.

[0033] A particular (dedicated) RACH preamble signature may hereserved for the handover purpose to indicate that the sender is a handoverWTRU, (i.e., a WTRU undergoing a handover process). This dedicated RACHpreamble signature is indicated in the handover command. After receiving thereserved RACH preamble signature, the target eNode-B 256 recognizes thatthe sender is a handover WTRU and may provide a priority to the handoverWTRU. This can avoid the random access process which causee a longinterruption time during handover. Alternatively, a RACH message followingthe RACH preamble may explicitly indicate that the sender is a handoverWTRU. A handover WTRU is preferably given a higher priority to access aneNode-B (cell) than a non-handover WTRU due to state transition. The RACHprocedure using the reserved RACH preamble signature may he used in eithersynchronized or non-synchronized eNode-B (or cell) handover. A physicalradio resource allocation for sending the reserved RACH preamble signatureto the target eNode-B 256 may also be included in the handover command toreduce a delay for the random access.

[0034] The random access procedure may be used for different purposes.The random access procedure may be used to initiate communication betweena WTRU and a network which requires a state transit from an LTE_idle state -7- WO 2007/149509 PCT/US2007/014423 to an LTE_active state. The random access procedure may be used for timingadjustment during handover and then for an access request to the new cell.When the random access procedure is used during handover, the delay causedby the random access procedure should be minimized. Therefore, there shouldbe differences, (e.g., giving a priority to a handover WTRU), between therandom access to the target eNode-Β (cell) during handover and the randomaccess to the source eNode-Β (cell) in a non-handover situation because ofstate transition from an LTE-Idle state to an LTE-Active state in the non-handover case. ΪΟΟ35] «After receiving the RACH preamble signature from the WTRU,the target eNode B estimates the timing adjustment value and sends thisvalue back to the WTRU (step 216).

[0036] After performing timing adjustment, (either autonomously or viaa RACH preamble transmission), the WTRU 202 may send a radio resourceassignment request to the target eNode-B 256 (step 218). The request ispreferably Bent via a RACH message following the RACH preamble. Thetarget eNode-B 256 then schedules downlink and uplink resources for theWTRU 252 (step 220). Alternatively, the target eNode-B 256 may scheduleresources for the WTRU 252 without receiving an explicit request from theWTRU 252. The resource scheduling may take place any time after the targeteNode-B 256 admits the WTRU at step 208. For example, for thesynchronized handover procedure, the target eNode-B 256 may schedule theuplink and downlink resources after some pre-defined time (earlier than theexpected time for eNode-Β switching).

[0037] The target eNode-B 256 sends an uplink resource assignment tothe WTRU 252 (step 222). This uplink resource is used for sending ahandover complete message at step 226, not for data transmission. TheWTRU 252 preferably resets RLC and HARQ parameters after receiving theuplink resource assignment from the target eNode-B 256 (step 224).Alternatively, the WTRU 252 may reset the RLC and HARQ parameters afterreceiving and processing the handover command at step 212. These -8-

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WO 2007/149509 PCT/US2007/014423 parameters related to transmission to the target eNode-B 256 (or cell) areincluded in the handover command.

[0038] The WTRU 252 sends a handover complete message to the targeteNode-B 256 (step 226). The WTRU 252 preferably includes a starting uplinkPDCP SN to be transmitted in the handover complete message. Optionally,the WTRU 252 may send an RLC control message to the target eNode-B 256after the handover complete message to indicate the successfully transmittedSDUs and an SDU gap.

[0039] The target eNode-B 256 sends uplink and downlink resourcescheduling information for data transmission and an RRC message to theWTRU (step 228). The RRC message includes at least one of radio accessbearer (RAB) reconfiguration information, a starting PDCP SN in thedownlink, an RLC control message, and measurement related information.Some or all of the above information may optionally be sent as part of thehandover command or the first packet from the target eNode-B 256.

[0040] The target eNode-B 256 sends a handover complete message tothe MME/UPE 258 to inform that the WTRU 252 has gained an access at thetarget eNode-B 256 (step 230). The MME/UPE 258 then sends a handovercomplete acknowledgement (ACK) to the target eNode-B 256 and switches theU-plane data path from the source eNode-B 254 to the target eNode-B 256(step 232). A release of the radio resources at the source eNode-B 254 istriggered by a release resource message sent by the target eNode-B 256 (step234). After receiving the message from the target eNode-B 256, the sourceeNode-B 254 releases the radio resources for the WTRU 252 (step 236).

[0041] A handover failure case is explained hereinafter by referring toFigure 2. When the WTRU 252 is not able to handover successfully, theWTRU 252 may resort to a radio link (RL) failure or a cell reselectionprocedure. If the handover command fails at step 212, the source eNode-B 254informs the target eNode-B 256 of such a failure. The target eNode-B 256schedules any uplink and downlink resources to the WTRU 252 after step 208.When performing cell reselection in a handover failure case, the WTRU 252may first try to access the originally connected cell within the source eNode-B -9-

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WO 2007/149509 PCT/US2007/014423 254. If this fails, the WTRU 252 may try to access other cells within thesource eNode-B. If this also fails, then the WTRU 252 may try to access toother cells not included in the source eNode-B based on the measurementresult.

[0042] The source eNode-B 254 maintains a timer to time out if thehandover complete message is not received alter a predetermined time afterthe handover command failure. The source eNode-B 254 may reset RRCcontext, PDCP context, RLC and HARQ parameters related to the WTRU 252if the handover failure timer expires. The source eNode-B then releases theradio resources for the WTRU 252.

[0043] When cell reselection is performed by the WTRU 252, the sourcecell or eNode-B identity (ID) is sent by the WTRU 252 to any eNode-B as partof the LTE-radio network temporary identity (RNTI) information for thedetection if the WTRU 252 accesses the original cell or any other cells. At thesource eNode-B, the source eNode-B’s MAC layer informs its RRC layer of thehandover failure if the MAC layer detects failed transmission of handovercommand.

[0044] Embodiments.

[0045] 1. A method for performing handover in a wireless ηητπτηηπΊnation system.

[0046] 2. The method of embodiment 1 comprising a WTRU and a source eNode-B performing measurements.

[0047] 3. The method of embodiment 2 comprising the source eNode- B τη «king a handover decision based on the measurements.

[0048] 4. The method of embodiment 3 comprising the source eNode- B sending a handover request to a target eNode-B.

[0049] 5. The method of embodiment 4 comprising the target eNode- B Bending a handover response to the source eNode-B indicating that ahandover should commence.

[0050] 6. The method of embodiment 5 comprising the source eNode- B Bending a handover command to the WTRU, the handover commandincluding at least one of reconfiguration information, information regarding -10-

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WO 2007/149509 PCT/US2007/014423 timing adjustment, relative timing difference between the source eNode-B andthe target eNode-B; information regarding an initial radio resource schedulingprocedure at the target eNode-B, and measurement information for the targeteNode-B.

[0051] 7. The method of embodiment 6 wherein the reconfiguration information is for at least one of an RRC layer, an RLC layer, a MAC layerand a physical layer.

[0052] 8. The method as in any one of embodiments 6-7, wherein the handover command indicates that the target eNode-B schedules resource forthe WTRU based on a BACH access procedure.

[0053] 9. The method as in any one of embodiments 6-3, wherein the handover command indicates that the target eNode-B schedules resource forthe WTRU without receiving an explicit resource assignment request from theWTRU.

[0054] 10. The method as in any one of embodiments 6-9, further comprising the source eNode-B forwarding user data to the target eNode-B.[0055] 11. The method of embodiment 10 wherein the forwarding of the user data is performed in a service dependent and implementation specificway.

[0056] 12. The method as in any one of embodiments 6-9, wherein the WTRU and the source eNode-B continue to transmit and receive data after theWTRU receives the handover command.

[0057] 13. The method of embodiment 12 wherein the WTRU and the source eNode-B continue to transmit and receive the data until a handovertime that is signaled via the handover command.

[0058] 14. The method as in any one of embodiments 12-13, wherein the data transmitted is an incomplete SDU.

[0059] 15. The method of embodiment 14 wherein the source eNode-B sends an RLC message to the WTRU including an SN to indicate asuccessfully received SDU and an unsuccessfully received SDU.

[0060] 16. The method of embodiment 15 wherein the SN is a PDCP SN or a common SN. -11-

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WO 2007/149509 PCT/US2007/014423 [0061] 17. The method as in any one of embodiments 6-9, wherein the source eNode-B stops transmission of data to the WTRU as soon as the sourceeNode-B sends the handover command to the WTRU, and the WTRU stopstransmission of data to the source eNode-B as soon as the WTRU receives thehandover command.

[0062] 18. The method as in any one of embodiments 6-9, wherein the source eNode-B continues transmission of data until the WTRU switches tothe target eNode-B.

[0063] 19. The method as in any one of embodiments 6-18, further comprising the WTRU performing timing adjustment with the target eNode-B.

[0064] 20. The method of embodiment 19 wherein the WTRU autonomously performs the timing adjustment based on relative timingdifference between the source eNode-B and the target eNode-B.

[0065] 21. The method as in any one of embodiments 19-20, wherein the relative timing diffarenna information is included in the handovercommand.

[0066] 22. The method as in any one of embodiments 19-21, wherein the WTRU uses a RACH access procedure for the timing adjustment.

[0067] 23. The method of embodiment 22 wherein a plurality of RACH preamble signatures with different orthogonality and different priorityare used, and among the plurality of RACH preamble signatures, a RACHpreamble signature with higher orthogonality, higher priority and higherpower is used for handover purpose.

[0068] 24. The method of embodiment 23 wherein a particular RACH preamble signature is reserved for the handover purpose.

[0069] 25. The method of embodiment 24 wherein the reserved RACH preamble signature is indicated in the handover command.

[0070] 26. The method as in any one of embodiments 6-25, further comprising the target eNode-B assigning an uplink resource for transmissionof a handover complete message for the WTRU. -12-

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WO 2007/149509 PCT/US2007/014423

[0071] 27. The method of embodiment 26 wherein the target eNode-B schedules the uplink resource based on a resource assignment request fromthe WTRU.

[0072] 28. The method of embodiment 27 wherein the resource assignment request is sent via a RACH.

[0073] 29. The method of embodiment 26 wherein the target eNode-B schedules the uplink resource without receiving a request from the WTRU.[0074] 30. The method of as in any one of embodiments 6-29, further comprising the WTRU resetting RLC and HARQ after receiving the uplinkresource from the target eNode-B.

[0075] 31. The method as in any one of embodiments 6-29, further comprising the WTRU resetting RLC and HARQ after receiving the handovercommand.

[0076] 32. The method as in any one of embodiments 6-31, further comprising the WTRU sending a handover complete message to the targeteNode-B, the handover complete message including an uplink PDCP SN to hetransmitted.

[0077] 33. The method of embodiment 32 further comprising the WTRU sending an RLC control message to the target eNode-B after thehandover complete message to indicate a successfully transmitted SDU andan SDU gap.

[0078] 34. The method as in any one of embodiments 6-33, further comprising the target eNode-B sending uplink and downlink schedulinginformation for data transmission and RRC message to the WTRU, the RRCmessage including at leaet one of RAB reconfiguration information, a startingPDCP SN start in a downlink, an RLC control message, and measurementrelated information.

[0079] 35. The method as in any one of embodiments 6-34, further comprising the WTRU performing an RL failure procedure when the handovercommand is not successfully delivered.

[0080] 36. The method as in any one of embodiments 6-35, wherein the source eNode-B maintains a timer to time out if a handover complete -13- WO 2007/149509 PCT/US2007/014423 message is not received until a predetermined time after the handover command is not successfully delivered.

[0081] 37. The method of embodiment 36 wherein the source eNode-B resets RRC context» PDCP context, RLC and HARQ parameters related to the WTRU if the timer expires.

[0082] 38. The method as in any one of embodiments 6-37, further comprising the WTRU performing a cell reselection procedure when thehandover command is not successfully delivered.

[0083] 39. The method of embodiment 38 wherein the WTRU first tries to access an originally connected cell in the source eNode-B.

[00841 40. The method of embodiment 39 wherein the WTRU tries to

access another cell in the source eNode-B if the WTRU fails to access theoriginally connected celL

[0085] 41. The method of embodiment 40 wherein the WTRU tries to access another cell not included in the source eNode-B if the WTRU fails toaccess said another cell in the source eNode-B.

[0086] 42. The method as in any one of embodiments 38-41» wherein the WTRU sends a source eNode-B ID to the target eNode-B during cellreselection.

[0087] 43. A wireless communication system for performing handover,

[0088] 44. The system of embodiment 43 comprising a WTRU mnfigured to perform measurement and send a measurement repot.

[0089] 45. The system of embodiment 44 comprising a target eNode- B.

[0090] 46. The system of embodiment 45 comprising a source eNode- B configured make a handover decision based on the measurement report,send a handover request to the target eNode-B, and send a handovercommand to the WTRU after receiving a handover response from the targeteNode-B indicating that a handover should commence, wherein the handovercommand includes at least one of reconfiguration information, infoxmationregarding riming adjustment, relative riming difference between the source -14-

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WO 2007/149509 PCT/US2007/014423 eNode-B and the target eNode-B; information regarding an initial radioresource scheduling procedure at the target eNode-B, and measurementinformation for the target eNode-B.

[0091] 47. The system of embodiment 46 wherein the reconfiguration information is for at least one of an RRC layer, an RLC layer, a MAC layerand a physical layer.

[0092] 48. The system as in any one of embodiments 46-47, wherein the handover command indicates that the target eNode-B schedules resourcefor the WTRU using a RACH access procedure.

[0093] 49. The system as in any one of embodiments 46-48, wherein the handover command indicates that the target eNode-B schedules resourcefor the WTRU without receiving an explicit resource assignment request fromthe WTRU.

[0094] 50. The system as in any one of embodiments 46-49, wherein the source eNode-B is configured to forward user data to the target eNode-Bafter sending the handover command to the WTRU.

[0095] 51. The system of embodiment 50 wherein the forwarding of the user data is performed in a service dependent and implementation specificway.

[0096] 52. The system as in any one of embodiments 46-51, wherein the WTRU and the source eNode-B continue to transmit and receive data afterthe WTRU receives the handover command.

[0097] 53. The system, as in any one of embodiments 46-51, wherein the WTRU and the source eNode-B continue to transmit and receive the datauntil a handover time that is signaled via the handover command.

[0098] 54. The system as in any one of embodiments 52-53, wherein the data transmitted is an incomplete SDU.

[0099] 55. The system of embodiment 54 wherein the source eNode-B sends an RLC message to the WTRU including an SN to indicate asuccessfully received SDU and an unsuccessfully received SDU.

[00100] 56. The system of embodiment 55 wherein the SN is a PDCP SN or a common SN. -15-

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WO 2007/149509 PCT/US2007/014423 [00101] 57. The system as in any one of embodiments 46-51, wherein the source eNode-Β stops transmission of data to the WTRU as soon as thesource eNode-Β sends the handover command to the WTRU, and the WTRUstops transmission of data to the source eNode-Β as soon as the WTRUreceives the handover command.

[00102] 58. The system as in any one of embodiments 46-51, wherein the source eNode-Β continues transmission of data until the WTRU switchesto the target eNode-B.

[00103] 59. The system as in any one of embodiments 46-58, wherein the WTRU is configured to perform timing adjustment with the target eNode-B.

[00104] 60. The system of embodiment 59 wherein the WTRU is configured to autonomously perform the timing adjustment based on relativetiming difference between the source eNode-Β and the target eNode-B.

[00105] 61. The system of embodiment 60 wherein the relative timing difference information is included in the handover command.

[00106] 62. The system as in any one of embodiments 59-61, wherein the WTRU is configured to use a EACH access procedure for the timing adjustment.

[00107] 63. The system of embodiment 62 wherein plurality of RACH preamble signatures with different orthogonality and different priority areused, and among the plurality of RACH preamble signatures, a RACHpreamble signature with higher orthogonality, higher priority and higherpower is used for handover purpose.

[00108] 64. The system of embodiment 63 wherein a particular RACH preamble signature is reserved for the handover purpose.

[00109] 65. The system of embodiment 64 wherein the reserved RACH preamble signature is indicated in the handover command.

[00110] 66. The system as in any one of embodiments 46-65, wherein the target eNode-Β is configured to assign an uplink resource for transmissionof a handover complete message for the WTRU. -16- WO 2007/149509 PCT/US2007/014423

[00111] 67. The system of embodiment 66 wherein the target eNode-B is configured to schedule the uplink resource based on a resource assignment request from the WTRU.

[00112] 68. The system of embodiment 67 wherein the resource assignment request is sent via a RACH.

[00113] 69. The system of embodiment 66 wherein the target eNode-B is configured to schedule the uplink resource without receiving a request fromthe WTRU.

[00114] 70. The system as in any one of embodiments 66-69, wherein the WTRU is configured to reset RLC and HARQ after receiving the uplinkresource from the target eNode-B.

[00115] 71. The system as in any one of embodiments 46-70, wherein the WTRU is configured to reset RLC and HARQ after receiving the handovercommand.

[00116] 72. The system as in any one of embodiments 46-71, wherein the WTRU is configured to send a handover complete message to the targeteNode-B, the handover complete message including an uplink PDCP SN to betransmitted.

[00117] 73. The system of embodiment 72 wherein the WTRU is configured to send an RLC control message to the target eNode-B after thehandover complete message to indicate a successfully transmitted SDU andan SDU gap.

[00118] 74. The system as in any one of embodiments 46-73, wherein the target eNode-B is configured to send uplink and downlink schedulinginformation for data transmission and RRC message to the WTRU, the RRCmessage including at least one of RAB reconfiguration information, a startingPDCP SN start in a downlink, an RLC control message, and measurementrelated information.

[00119] 75. The system as in any one of embodiments 46-74, wherein the WTRU is configured to perform an RL failure procedure when thehandover command is not successfully delivered. -17- WO 2007/149509 PCT/US2007/014423 [00120] 76, The system as in any one of embodiments 46-75, wherein the source eNode-B includes a timer to time out if a handover complete message is not received until a predetermined time after the handover command is not successfully delivered.

[00121] 77. The system of embodiment 76 wherein the source eNode-B resets RRC context, PDCP context, RLC and HARQ parameters related to theWTRU if the timer expires.

[00122] 78. The system as in any one of embodiments 46-77, wherein the WTRU is configured to perform a cell reselection procedure when thehandover command is not successfully delivered.

[00123] 79. The system of embodiment 78 wherein the WTRU first tries to access an originally connected cell in the source eNode-B.

[00124] 80. The system of embodiment 79 wherein the WTRU tries to access another cell in the source eNode-B if the WTRU fails to access theoriginally connected cell.

[00126] 81. The system of embodiment 80 wherein the WTRU tries to access another cell not included in the source eNode-B if the WTRU fails toaccess said another cell in the source eNode-B.

[00126] 82. The system as in any one of embodiments 78-81, wherein the WTRU is configured to send a source eNode-B ID to the target eNode-Bduring cell reselection.

[00127] 83. An eNode-B for performing handover in a wireless communication system.

[00128] 84. The eNode-B of embodiment 83 comprising a transceiver for transmitting and receiving data to and from a WTRU.

[00129] 85. The eNode-B of embodiment 84 comprising a measurement unit for performing measurements on a channel for the WTRU.

[00130] 86. The eNode-B as in any one of embodiments 84-85, comprising a handover controller configured to make a handover decisionbased on the measurements and send a handover command to the WTRU, thehandover command including at least one of reconfiguration information,information regarding timing adjustment, relative timing difference between -18-

<img img-format="tif" img-content="drawing" file="IL196054AD000210.tif" id="idf0010" />

WO 2007/149509 PCT/US2007/014423 a source eNode-B and a target eNode-B; information regarding an initial radioresource scheduling procedure at the target eNode-B, and measurementinformation for the target eNode-B.

[00131] 87. The eNode-B of embodiment 86 wherein the reconfiguration information is for at least one of an RRC layer, an RLC layer,a MAC layer and a physical layer.

[00132] 88. The eNode-B as in any one of embodiments 86-87, wherein the handover controller controls the transceiver such that data is transmittedand received to and from the WTRU after the WTRU receives the handovercommand.

[00133] 89. The eNode-B as in any one of embodiments 86-87, wherein the handover controller controls the transceiver such that data is transmittedand received to and from the WTRU until a handover time that is signaled viathe handover command.

[00134] 90. The eNode-B of embodiment 89 wherein the data transmitted is an incomplete SDU.

[00135] 91. The eNode-B as in any one of embodiments 86-87, wherein the handover controller controls the transceiver such that data transmission isstopped ae soon as the handover command is sent to the WTRU.

[00136] 92. A WTRU for performing handover in a wireless communication system.

[00137] 93. The WTRU of embodiment 92 comprising a transceiver for transmitting and receiving data to and from an eNode-B.

[00138] 94. The WTRU of embodiment 93 comprising a measurement unit for performing measurements.

[00139] 95. The WTRU as in any one of embodiments 93-94, comprising a controller for performing handover from a source eNode-B to atarget eNode-B in accordance with a handover command received from thesource eNode-B, the handover command including at least one ofreconfiguration information, information regarding timing adjustment,relative timing difference between the source eNode-B and the target eNode- -19-

<img img-format="tif" img-content="drawing" file="IL196054AD000211.tif" id="idf0011" />

WO 2007/149509 PCT/US2007/014423 B; information regarding an initial radio resource scheduling procedure at thetarget eNode-B, and measurement information for the target eNode-B.

[00140] 96. The WTRU of embodiment 95 wherein the reconfiguration information is for at least one of an RRC layer, an RLC layer, a MAC layerand a physical layer.

[00141] 97. The WTRU as in any one of embodiments 95-96, wherein the controller controls the transceiver such that data is transmitted andreceived to and from the source eNode-B after the WTRU receives thehandover command.

[00142] 98. The WTRU of embodiment 97 wherein the data transmitted is an incomplete SDU.

[00143] 99. The WTRU as in any one of embodiments 95-96, wherein the controller controls the transceiver such that data transmission to thesource eNode-B stops as soon as the WTRU receives the handover command.[00144] 100. The WTRU as in any one of embodiments 95-99, wherein the controller is configured to perform timing adjustment with the targeteNode-B.

[00145] 101. The WTRU of embodiment 100, wherein the controller autonomously performs the timing adjustment based on relative timingdifference between the source eNode-B and the target eNode-B.

[00146] 102. The WTRU of embodiment 100, wherein the controller uses a RACH access procedure for the timing adjustment.

[00147] 103. The WTRU of embodiment 102, wherein a plurality of RACH preamble signatures with different orthogonality and different priorityare used, and among the plurality of RACH preamble signatures, a RACHpreamble signature with higher orthogonality» higher priority and higherpower is used for handover purpose.

[00148] 104. The WTRU of embodiment 103, wherein a particular RACH preamble signature is reserved for the handover purpose.

[00149] 105. The WTRU of embodiment 104, wherein the reserved RACH preamble signature is indicated in the handover command. -20-

<img img-format="tif" img-content="drawing" file="IL196054AD000212.tif" id="idf0012" />

WO 2007/149509 PCT/US2007/014423 [00150] 106. Tha WTRU as in any one of embodiments 95-105, wherein the controller sends a resource assignment request to the target eNode-B forscheduling an uplink resource.

[00151] 107. The WTRU of embodiment 106, wherein the resource assignment request is sent via a RACH.

[00152] 108. The WTRU as in any one of embodiments 95-107-, wherein the controller performs a cell reeelection procedure when the handovercommand is not successfully received.

[00153] 109. The WTRU of embodiment 108, wherein the controller first tries to access an originally connected cell in the source eNode-B.

[00154] 110. The WTRU of embodiment 109, wherein the controller tries to access another cell in the source eNode-B if the WTRU fails to accessthe originally connected cell.

[00155] 111. The WTRU of embodiment 110, wherein the WTRU tries to access another cell not included in the source eNode-B if the WTRU fails toaccess said another cell in the source eNode-B.

[00156] 112. The WTRU as in any one of embodiments 108-111, wherein the controller sends a source eNode-B ID to the target eNode-Bduring cell reselection.

[00157] Although the features and elements of the present invention aredescribed in' the preferred embodiments in particular combinations, eachfeature or element can he used alone without the other features and elementsof the preferred embodiments or in various combinations with or withoutother features and elements of the present invention. The methods or flowcharts provided in the present invention may be implemented in a computerprogram, software, or firmware tangibly embodied in a computer-readablestorage 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 disksand removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). -21-

<img img-format="tif" img-content="drawing" file="IL196054AD000213.tif" id="idf0013" />

WO 2007/149509 PCT/US2007/014423 [00158] Suitable processors include, by way of example, a generalpurpose processor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one or moremicroprocessors in association with a DSP core, a controller, a microcontroller,Application Specific Integrated Circuits (ASICs), Field Programmable GateArrays (FPGAs) circuits, any other type of integrated circuit GC), and/or astate machine.

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