HK1129982B - Handover in a long term evolution (lte) wireless communication system - Google Patents

Description

Handover in a Long Term Evolution (LTE) wireless communication system

Technical Field

The present invention relates to wireless communication systems. In particular, the present invention relates to a method and system for performing handover in a Long Term Evolution (LTE) system.

Background

Currently, LTE with respect to fourth generation (4G) systems is being conceived in order to develop a completely new radio interface and radio network architecture that provides high data rates, low latency, packet optimization, and improved system capacity and coverage. For LTE systems, rather than using Code Division Multiple Access (CDMA), which is currently used in 3G systems, it is proposed to use Orthogonal Frequency Division Multiple Access (OFDMA) and Frequency Division Multiple Access (FDMA) in downlink and uplink transmissions, respectively. As many aspects in LTE systems have changed, it is necessary to reconsider handover procedures and related operations within LTE.

In LTE _ ACTIVE mode, User Equipment (UE) mobility management handles all necessary steps for seamless handover in LTE systems, such as making LTE-internal handover decisions at the source network (i.e. controlling and evaluating UE and evolved node-B (enodeb) measurement processes taking into account UE-related area restrictions), provisioning radio resources at the target network, ordering the UE to interface with new radio resources, releasing radio resources at the source network, etc. Furthermore, the UE mobility management mechanism handles context data transfer between involved nodes and node relation updates on the control plane (C-plane) and user plane (U-plane).

Fig. 1 is a signaling diagram of a handover process 100 currently proposed for LTE systems. The UE152 performs measurements with the source enodeb 154 and exchanges measurement reports (step 102). The source enode-B154 makes a handover decision based on the measurement report (step 104). The source eNode-B154 then sends a handover request to the target eNode-B156 (step 106). The handover decision procedure and subsequent procedures performed before the handover is over are performed without involving the mobility management entity/user plane entity (MME/UPE)158 (i.e., the handover preparation message is exchanged directly between the source enodeb 154 and the target enodeb 156).

The target eNode-B156 performs admission control for the UE152 (step 108). If the target eNode-B156 can accept the UE152, the target eNode-B156 sends a handover response to the source eNode-B154 (step 110). The source enode-B154 sends a handover command to the UE152 (step 112). For seamless handover, a U-plane tunnel is established between the source eNode-B154 and the target eNode-B156.

The UE152 then exchanges layer one and layer two (L1/L2) signaling with the target eNode-B156 (step 114). During handover execution, user data may be forwarded from the source eNode-B154 to the target eNode-B156. The forwarding process may be performed in a service-dependent and implementation-specific manner. Whenever a packet is received at the source eNode-B154 from the UPE 158, user data forwarding from the source eNode-B154 to the target eNode-B156 should be performed.

After establishing a connection with the target eNode-B156, the UE152 sends a handover complete message to the target eNode-B156 (step 116). The target eNode-B156 sends a handover complete message to the MME/UPE 158 (step 118). The MME/UPE 158 may then send a handover complete Acknowledgement (ACK) to the target eNode-B156 (step 120). After the target eNode-B156 informs the MME/UPE 158 that the UE152 has obtained access on the target eNode-B156 via a handover complete message, the MME/UPE 158 may handover the U-plane path from the source eNode-B154 to the target eNode-B156.

At the source eNode-B154, a radio resource release is triggered by a release resource message sent by the target eNode-B156 (step 122). After receiving the release resource message from the target eNode-B156, the source eNode-B154 releases the radio resources for the UE152 (step 124). UE152 performs a location update in conjunction with MME/UPE 158 (step 126).

The LTE intra-LTE handover process 100 described above does not provide details of the handover command (e.g., configuration of the UE152 based on target enodeb requirements) and details of the UE operation after the UE receives the handover command (e.g., data transmission between the source enodeb 154 and the UE152, Radio Link Control (RLC) and hybrid automatic repeat request (HARQ) resets, and Packet Data Convergence Protocol (PDCP) Sequence Number (SN) gap (gap) identification implemented by the UE 152). In addition, the LTE intra-handover procedure 100 described above also does not provide details of UE timing adjustment for synchronous and asynchronous enode-bs and details of efficient target enode-B resource scheduling for UE transmissions.

Disclosure of Invention

The present invention relates to a method and system for performing handover in an LTE system. The source eNode-B makes a handover decision based on the measurements and sends a handover request to the target eNode-B. The target eNode-B sends a handover response to the source eNode-B to indicate that a handover should be initiated. The source eNode-B then sends a handover command to a wireless transmit/receive unit (WTRU). The handover command includes at least one of the following information: reconfiguration information, information about timing adjustments, relative timing differences between the source and target enode-bs, information about initial scheduling processing on the target enode-B, and measurement information about the target enode-B. The WTRU then accesses the target enodeb exchanging layer 1/2 signaling to perform downlink synchronization, timing adjustment, and uplink and downlink resource allocation based on the information contained in the handover command.

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 a signaling diagram of a handover process currently proposed for an LTE system; and

fig. 2 is a signaling diagram of an LTE internal handover process according to the present invention.

Detailed Description

When referred to hereafter, the terminology "WTRU" includes but 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), a computer, or any other user device capable of operating in a wireless environment. The term "enodeb" as referred to below includes, but is not limited to, a base station, a node-B, a site controller, an Access Point (AP), or any other interfacing device capable of operating in a wireless environment.

For the cases of successful handover and handover failure, the present invention provides detailed procedures for signaling and operation on the WTRU, the source and target enode-bs during intra-LTE handover for these cases. In case of a successful handover, a new Information Element (IE) is added in both the handover command message and the handover complete message. In case of handover failure, a new signaling message will be exchanged between the source enode-B and the target enode-B.

Fig. 2 is a signaling diagram of an LTE internal handover process 200 according to the present invention. The WTRU252 and the source enode-B254 both perform at least one measurement process and the WTRU252 sends a measurement report to the source enode-B254 (step 202). The source eNode-B254 makes a handover decision based on the measurement report and its own measurement results (step 204). The source eNode-B254 then sends a handover request to the target eNode-B256 (step 206). The target enode-B256 performs admission control for the WTRU252 (step 208). If the target eNode-B256 is receptive to the WTRU252, the target eNode-B256 sends a handover response to the source eNode-B254 to indicate that a handover should be initiated (step 210). The source enode-B254 then sends a handover command to the WTRU252 (step 212).

The handover command should include at least one of: reconfiguration information for the Radio Resource Control (RRC), Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers, information on timing adjustments when handing over from the source enodeb 254 to the target enodeb 256 (i.e., whether the WTRU252 should autonomously or perform timing adjustments using a Random Access Channel (RACH), whether a random or dedicated access signature is to be used if a RACH is to be used, etc.), relative timing differences between enode-bs (or cells) used for autonomous timing adjustments, information related to the initial radio resource scheduling procedure on the target enodeb 256, measurement information for the target enodeb 256, etc. The information related to the initial radio resource scheduling procedure on the target enodeb 256 indicates whether the RACH access procedure should be used for the resource allocation request, or the target enodeb 256 may schedule resources for the WTRU252 without receiving an explicit resource allocation request from the WTRU 252. Alternatively, the target enode-B256 may send the measurements and other configuration information to the WTRU252 after receiving a handover complete message from the WTRU252 in step 226.

For seamless handover, a U-plane tunnel is established between the source eNode-B254 and the target eNode-B256. After sending the handover command, the source eNode-B254 may forward the user data to the target eNode-B256. The forwarding may be done in a service dependent and implementation specific manner.

After receiving the handover command from the source enodeb 254, the WTRU252 may continue to transmit and receive data to and from the source enodeb 254. The data transfer process then depends on whether synchronous or asynchronous handover is used.

When a synchronous handover procedure is used (i.e., the source eNode-B254 and the target eNode-B256 are synchronous, or the relative timing difference is known to the WTRU 252), the source eNode-B254 and the WTRU252 may continue to transmit and receive data after receiving a handover command until a certain handover time (t) signaled by the handover command has elapsed_HO) Until now. Preferably, data transmitted after receiving the handover command is limited to incomplete Service Data Units (SDUs) (i.e., RLC Protocol Data Units (PDUs)) transmitted before sending the handover command. An RLC control message will be sent to the WTRU252 to indicate the sequence number of one or more SDUs successfully received and the SDU gap. The SN may be either a PDCP SN or other type of SN. The SN common to the successfully received SDU or SDUs and the unsuccessfully received SDU or SDUs may be included in the RLC control message.

When an unsynchronized handover procedure is used (i.e., the source eNode-B254 and the target eNode-B256 are not synchronized or the relative timing difference is not known to the WTRU 252), the source eNode-B254 will immediately stop transmitting once the source eNode-B254 sends a handover command to the WTRU 252. In addition, the WTRU252 may stop transmitting data packets to the source enode-B254 immediately upon receipt of the handover command by the WTRU 252. Alternatively, the source eNode-B254 may continue to transmit data packets until the WTRU252 switches to the target eNode-B256.

Upon receiving the handover command, the WTRU252 accesses the target eNode-B256 and exchanges layer 1/2 (L1/L2) signaling with the target eNode-B256 to perform downlink synchronization, timing adjustment (i.e., uplink synchronization), and uplink and downlink resource allocation based on information contained in the handover command.

For timing adjustment (i.e., uplink synchronization), the WTRU252 implements one of two options. Preferably, the network will determine which option to use.

According to a first option, the WTRU252 autonomously performs timing adjustments based on the relative timing difference between the source enode-B254 (or cell) and the target enode-B256 (or cell) (step 214 a). Preferably, the relative timing difference information is contained in the handover command.

According to a second option, a conventional RACH access procedure will be used for timing adjustment (step 214 b). The WTRU sends a RACH preamble to the target enodeb, and the target enodeb calculates a timing offset based on the calculated RACH preamble and sends the timing offset information to the WTRU for uplink synchronization.

A plurality of RACH preamble signatures with different orthogonality and different priority may be used and among them, a RACH preamble signature with higher orthogonality, higher priority and/or higher power may be used for handover.

For handover purposes, a specific (dedicated) RACH preamble signature may be reserved for it to indicate that the sender is a WTRU performing handover (i.e., a WTRU undergoing a handover process). This dedicated RACH preamble signature is indicated in the handover command. After receiving the reserved RACH preamble signature, the target enode-B256 recognizes the sender as a WTRU performing a handover and may provide priority to the WTRU performing the handover. This avoids random access processes that generate long interruption times during handover. Alternatively, the RACH message following the RACH preamble may explicitly indicate that the sender is a WTRU performing handover. Preferably, the WTRU performing the handover will be provided a higher priority for accessing an enode-B (cell) due to the state transition than a non-handover WTRU. The RACH procedure using the reserved RACH preamble signature can be used in both synchronized and unsynchronized enode-B (or cell) handovers. The physical radio resource allocation used to send the reserved RACH preamble signature to the target enodeb 256 may also be included in the handover command in order to reduce the delay of the random access.

The random access procedure may be used for different purposes. The random access procedure may be used to initiate communication between the WTRU and the network that requires a state transition from an LTE _ idle (idle) state to an LTE _ active state. The random access procedure may also be used for timing adjustment during handover and then for access requests involving new cells. When the random access procedure is used in the handover procedure, a delay caused by the random access procedure should be minimized. Thus, due to the state transition from LTE _ idle state to LTE _ active state in non-handover case, there should be a difference between random access for the target enodeb (cell) during handover and random access for the source enodeb (cell) in non-handover case (e.g., providing priority to the WTRU performing the handover).

After receiving the RACH preamble signature from the WTRU, the target enodeb estimates the timing adjustment value and sends this value back to the WTRU (step 216).

After performing timing adjustments (either autonomously or via RACH preamble transmission), the WTRU202 may send a radio resource allocation request to the target enodeb 256 (step 218). Preferably, the request is sent via a RACH message following the RACH preamble. The target enode-B256 then schedules downlink and uplink resources for the WTRU252 (step 220). Alternatively, the target enode-B256 may schedule resources for the WTRU252 without receiving an explicit request from the WTRU 252. The resource scheduling may be performed at any time thereafter after the target enode-B256 grants the WTRU in step 208. For example, for a synchronous handover procedure, the target eNode-B256 may schedule uplink and downlink resources after a predetermined time (earlier than expected for eNode-B handover) has elapsed.

The target enode-B256 sends the uplink resource allocation to the WTRU252 (step 222). This uplink resource will be used for sending a handover complete message in step 226 instead of for data transmission. The WTRU252 preferably resets the RLC and HARQ parameters after receiving an uplink resource allocation from the target enodeb 256 (step 224). Alternatively, the WTRU252 may reset the RLC and HARQ parameters after receiving and processing the handover command in step 212. These parameters associated with the transmission to the target eNode-B256 (or cell) are included in the handover command.

The WTRU252 sends a handover complete message to the target enode-B256 (step 226). Preferably, the WTRU252 includes the initial uplink PDCP SN to be transmitted in the handover complete message. Alternatively, the WTRU252 may send an RLC control message to the target enode-B256 after the handover complete message to indicate successfully transmitted SDUs and SDU gaps.

The target enode-B256 sends uplink and downlink resource scheduling information for data transmission and an RRC message to the WTRU (step 228). The RRC message includes at least one of: radio Access Bearer (RAB) reconfiguration information, starting PDCP SN in downlink, RLC control messages, and measurement related information. Alternatively, some or all of the above information may be sent from the target eNode-B256 as part of the handover command or as a first packet.

The target enodeb 256 sends a handover complete message to the MME/UPE258 to signal that the WTRU252 has gained access on the target enodeb 256 (step 230). The MME/UPE258 may then send a handover complete Acknowledgement (ACK) to the target enodeb 256 and may handover the U-plane data path from the source enodeb 254 to the target enodeb 256 (step 232). At the source eNode-B254, a radio resource release is triggered by a release resource message sent by the target eNode-B256 (step 234). After receiving the message from the target eNode-B256, the source eNode-B254 releases the radio resources for the WTRU252 (step 236).

The case of handover failure will be explained below with reference to fig. 2. When the WTRU252 fails to successfully handover, the WTRU252 may resort to a Radio Link (RL) failure or a cell reselection procedure. If the handover command in step 212 fails, the source eNode-B254 signals the failure to the target eNode-B256. The target enode-B256 schedules uplink and downlink resources to the WTRU252 after step 208. When performing cell reselection processing in case of handover failure, the WTRU252 may first attempt to access the initially connected cell inside the source enodeb 254. If the process fails, the WTRU252 may attempt to access other cells within the source eNode-B. If the process also fails, the WTRU252 may attempt to access other cells not included in the source eNode-B based on the measurements.

The source enode-B254 may maintain a timer to generate an interrupt (time out) when a handover complete message is not received after a handover command fails and a predetermined time elapses. The source enode-B254 may reset the RRC context, PDCP context, RLC and HARQ parameters associated with the WTRU252 if the handover failure timer expires (expire). The source enode-B then releases the radio resources for the WTRU 252.

When the WTRU252 performs cell reselection processing, a source cell or enodeb Identification (ID) is sent by the WTRU252 to any enodeb as part of LTE Radio Network Temporary Identity (RNTI) information to detect whether the WTRU252 accesses an initial cell or other cell. At the source eNode-B, if the MAC layer detects a failed handover command transmission, the MAC layer of the source eNode-B signals the failure of the handover to its RRC layer.

Examples

1. A method for performing handover in a wireless communication system.

2. The method of embodiment 1, comprising: the WTRU and the source enode-B perform measurements.

3. The method of embodiment 2, comprising: the source enode-B makes a handover decision based on the measurements.

4. The method of embodiment 3, comprising: the source eNode-B sends a handover request to the target eNode-B.

5. The method of embodiment 4, comprising: the target eNode-B sends a handover response to the source eNode-B to indicate that a handover should be initiated.

6. The method of embodiment 5, comprising: the source eNode-B sends a handover command to the WTRU, the handover command including at least one of: reconfiguration information, information on timing adjustment, relative timing difference between source and target enode-bs, information related to an initial radio resource scheduling procedure on the target enode-B, and measurement information on the target enode-B.

7. The method of embodiment 6, wherein reconfiguration information is for at least one of: RRC layer, RLC layer, MAC layer, and physical layer.

8. The method as in any one of embodiments 6-7 wherein the handover command instructs the target eNode-B to schedule resources for the WTRU in accordance with a RACH access procedure.

9. The method as in any one of embodiments 6-8 wherein the handover command instructs the target eNode-B to schedule resources for the WTRU without receiving an explicit resource allocation request from the WTRU.

10. The method as in any one of embodiments 6-9, further comprising: the source enode-B forwards the user data to the target enode-B.

11. The method according to embodiment 10, wherein the forwarding of user data is performed in a service-dependent and implementation-specific manner.

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 the WTRU receives the handover command.

13. The method of embodiment 12 wherein the WTRU and the source enodeb continue to transmit and receive data until a handover time signaled via a handover command has elapsed.

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

15. The method of embodiment 14 wherein the source enode-B sends an RLC message to the WTRU including SNs to indicate successfully received SDUs and unsuccessfully received SDUs.

16. The method as in embodiment 15 wherein the SN is a PDCP SN or a common SN.

17. The method as in any one of embodiments 6-9 wherein the source eNode-B stops transmitting data to the WTRU upon the source eNode-B sending a handover command to the WTRU, and the WTRU stops transmitting data to the source eNode-B upon the WTRU receiving the handover command.

18. The method as in any one of embodiments 6-9 wherein the source eNode-B continues to transmit data until the WTRU switches to the target eNode-B.

19. The method as in any one of embodiments 6-18, further comprising: the WTRU performs timing adjustment in conjunction with the target enodeb.

20. The method of embodiment 19 wherein the WTRU autonomously performs timing adjustments based on a relative timing difference between the source enodeb and the target enodeb.

21. The method as in any one of embodiments 19-20 wherein the relative timing difference information is included in the handover command.

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

23. The method according to embodiment 22, wherein a plurality of RACH preamble signatures with different orthogonality and different priority are used, and among the plurality of RACH preamble signatures, a RACH preamble signature with higher orthogonality, higher priority and higher power is used for handover.

24. The method as in embodiment 23 wherein a specific RACH preamble signature is reserved for handover.

25. The method as in embodiment 24 wherein the reserved RACH preamble signature is indicated in a handover command.

26. The method as in any one of embodiments 6-25, further comprising: the target eNode-B allocates uplink resources for the WTRU for transmitting the handover complete message.

27. The method as in embodiment 26 wherein the target enode-B schedules uplink resources based on a resource allocation request from the WTRU.

28. The method as in embodiment 27 wherein the resource allocation request is sent via a RACH.

29. The method of embodiment 26 wherein the target enode-B schedules uplink resources without receiving a request from the WTRU.

30. The method as in any one of embodiments 6-29, further comprising: the WTRU resets RLC and HARQ after receiving uplink resources from the target enodeb.

31. The method as in any one of embodiments 6-29, further comprising: the WTRU resets RLC and HARQ after receiving the handover command.

32. The method as in any one of embodiments 6-31, further comprising: the WTRU sends a handover complete message to the target eNode-B, wherein the handover complete message includes an uplink PDCP SN to be transmitted.

33. The method of embodiment 32, further comprising: the WTRU sends an RLC control message to the target enode-B after the handover complete message to indicate the successfully transmitted SDUs and SDU gaps.

34. The method as in any one of embodiments 6-33, further comprising: the target eNode-B sends uplink and downlink scheduling information for data transmission to the WTRU along with an RRC message, wherein the RRC message includes at least one of: RAB reconfiguration information, starting PDCP SN starting in downlink, RLC control messages and measurement related information.

35. The method as in any one of embodiments 6-34, further comprising: the WTRU performs an RL failure procedure when the handover command is not successfully delivered.

36. The method as in any one of embodiments 6-35 wherein the source eNode-B includes a timer for interrupting when a handover complete message has not been received for a predetermined time after a handover command has not been successfully delivered.

37. The method as in embodiment 36 wherein the source enode-B resets the RRC context, PDCP context, and WTRU-related RLC and HARQ parameters if the timer expires.

38. The method as in any one of embodiments 6-37, further comprising: when the handover command is not successfully delivered, the WTRU performs a cell reselection procedure.

39. The method of embodiment 38 wherein the WTRU first attempts to access an initially connected cell in a source enode-B.

40. The method of embodiment 39 wherein the WTRU attempts to access other cells in the source eNode-B if the WTRU fails to access the initially connected cell.

41. The method of embodiment 40 wherein if the WTRU fails to access the other cell in the source eNode-B, the WTRU attempts to access the other cell not included in the source eNode-B.

42. The method as in any one of embodiments 38-41 wherein the WTRU sends a source eNode-B ID to a target eNode-B during cell reselection.

43. A wireless communication system for performing a handover.

44. The system of embodiment 43, comprising: a WTRU configured to perform measurements and send measurement reports.

45. The system of embodiment 43, comprising: target enode-B.

46. The system of embodiment 45, comprising: a source eNode-B configured to make a handover decision based on the measurement report, send a handover request to the target eNode-B, send a handover command to the WTRU after receiving a handover response from the target eNode-B to indicate that a handover should be initiated, wherein the handover command includes at least one of: reconfiguration information, information on timing adjustment, relative timing difference between source and target enode-bs, information related to an initial radio resource scheduling procedure on the target enode-B, and measurement information on the target enode-B.

47. The system of embodiment 46 wherein the reconfiguration information is for at least one of: RRC layer, RLC layer, MAC layer, and physical layer.

48. The system as in any one of embodiments 46-47 wherein the handover command instructs the target eNode-B to schedule resources for the WTRU in accordance with a RACH access procedure.

49. The system as in any one of embodiments 46-48 wherein the handover command instructs the target eNode-B to schedule resources for the WTRU without receiving an explicit resource allocation request from the WTRU.

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-B after sending a handover command to the WTRU.

51. The system of embodiment 50 wherein the forwarding of user data is performed in a service dependent and implementation specific manner.

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 after the WTRU receives the handover command.

53. The system as in any one of embodiments 46-51 wherein the WTRU and the source eNode-B continue to transmit and receive data until a handover time signaled via a handover command has elapsed.

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

55. The system of embodiment 54 wherein the source enode-B sends an RLC message to the WTRU including SNs to indicate successfully received SDUs and unsuccessfully received SDUs.

56. The system as in embodiment 55 wherein the SN is a PDCP SN or a common SN.

57. The system as in any one of embodiments 46-51 wherein the source eNode-B stops transmitting data to the WTRU upon the source eNode-B sending a handover command to the WTRU, and wherein the WTRU stops transmitting data to the source eNode-B upon the WTRU receiving the handover command.

58. The system as in any one of embodiments 46-51 wherein the source eNode-B continues to transmit data until the WTRU switches to the target eNode-B.

59. The system as in any one of embodiments 46-58 wherein the WTRU is configured to perform timing adjustment in conjunction with a target enodeb.

60. The system as in embodiment 59 wherein the WTRU is configured to autonomously perform timing adjustments based on a relative timing difference between the source eNode-B and the target eNode-B.

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

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

63. The system of embodiment 62 wherein multiple RACH preamble signatures with different orthogonality and different priority are used and of the multiple RACH preamble signatures, the RACH preamble signature with higher orthogonality, higher priority and higher power is used for handover.

64. The system as in embodiment 63 wherein a specific RACH preamble signature is reserved for handover.

65. The system as in embodiment 64 wherein the reserved RACH preamble signature is indicated in a handover command.

66. The system as in any one of embodiments 46-65 wherein the target eNode-B is configured to allocate uplink resources for the WTRU for transmitting the handover complete message.

67. The system as in embodiment 66 wherein the target enode-B is configured to schedule uplink resources based on a resource allocation request from the WTRU.

68. The system as in embodiment 67 wherein the resource allocation request is sent via a RACH.

69. The system as in embodiment 66 wherein the target enode-B is configured to schedule uplink resources without receiving a request from the WTRU.

70. The system as in any of embodiments 66-69 wherein the WTRU is configured to reset RLC and HARQ after receiving uplink resources from the target eNode-B.

71. The system as in any one of embodiments 46-70 wherein the WTRU is configured to reset RLC and HARQ after receiving a handover command.

72. The system as in any embodiments 46-71 wherein the WTRU is configured to send a handover complete message to the target enodeb, wherein the handover complete message includes an uplink PDCP SN to be transmitted.

73. The system as in embodiment 72 wherein the WTRU is configured to send an RLC control message to the target enode-B after the handover complete message to indicate successfully received SDUs and SDU gaps.

74. The system as in any of embodiments 46-73 wherein the target eNode-B is configured to send uplink and downlink scheduling information for data transmission to the WTRU along with an RRC message comprising at least one of: RAB reconfiguration information, starting PDCP SN starting in downlink, RLC control messages and measurement related information.

75. The system as in any one of embodiments 46-74 wherein the WTRU is configured to perform an RL failure procedure when the handover command is not successfully delivered.

76. The system as in any one of embodiments 46-75 wherein the source eNode-B includes a timer for interrupting when a handover complete message has not been received a predetermined time after a handover command has not been successfully delivered.

77. The system as in embodiment 76 wherein the source enode-B resets the RRC context, PDCP context, and WTRU-related RLC and HARQ parameters if the timer expires.

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

79. The system of embodiment 78 wherein the WTRU first attempts to access an initially connected cell in the source enode-B.

80. The system of embodiment 79 wherein if the WTRU fails to access the initially connected cell, the WTRU attempts to access other cells in the source enode-B.

81. The system of embodiment 80 wherein the WTRU attempts to access other cells not included in the source enode-B if the WTRU fails to access the other cells in the source enode-B.

82. The system as in any embodiments 78-81 wherein a WTRU sends a source eNode-B ID to a target eNode-B during cell reselection.

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

84. The enodeb of embodiment 83 comprising: a transceiver for transmitting and receiving data to and from the WTRU.

85. The enodeb of embodiment 84 comprising: a measurement unit to perform channel measurements for the WTRU.

86. An eNode-B as in any one of embodiments 84-85, the eNode-B comprising: a handover controller configured to make a handover decision based on the measurement report and send a handover command to the WTRU, send a handover request to the target eNode-B, and send a handover command to the WTRU after receiving a handover response from the target eNode-B to indicate that a handover should be initiated, wherein the handover command includes at least one of: reconfiguration information, information on timing adjustment, relative timing difference between source and target enode-bs, information related to an initial radio resource scheduling procedure on the target enode-B, and measurement information on the target enode-B.

87. The enodeb of embodiment 86 wherein the reconfiguration information is for at least one of: RRC layer, RLC layer, MAC layer, and physical layer.

88. An eNode-B as in any of embodiments 86-87 wherein the handover controller controls the transceiver to transmit and receive data to and from the WTRU after the WTRU receives the handover command.

89. An eNode-B as in any of embodiments 86-87 wherein the handover controller controls the transceiver to transmit and receive data to and from the WTRU prior to the handover time signaled via the handover command.

90. The enodeb of embodiment 89 wherein the transmitted data is an incomplete SDU.

91. An eNode-B as in any of embodiments 86-87 wherein the handover controller controls the transceiver to stop data transmission when a handover command is sent to the WTRU.

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

93. The WTRU of embodiment 92 comprising: a transceiver for transmitting and receiving data to and from the eNode-B.

94. The WTRU of embodiment 93, comprising: a measurement unit for performing the measurement.

95. The WTRU as in any one of embodiments 93-94 comprising: a controller for performing a handover from a source eNode-B to a target eNode-B in accordance with a handover command received from the source eNode-B, wherein the handover command includes at least one of: reconfiguration information, information on timing adjustment, relative timing difference between source and target enode-bs, information related to an initial radio resource scheduling procedure on the target enode-B, and measurement information on the target enode-B.

96. The WTRU of embodiment 95 wherein the reconfiguration information is for at least one of: RRC layer, RLC layer, MAC layer, and physical layer.

97. A WTRU as in any one of embodiments 95-96 wherein a controller controls a transceiver to transmit and receive data to and from a source enode-B after the WTRU receives a handover command.

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

99. A WTRU as in any one of embodiments 95-96 wherein a controller controls a transceiver to stop data transmission to a source enode-B upon receipt of a handover command by the WTRU.

100. A WTRU as in any one of embodiments 95-99 wherein the controller is configured to perform timing adjustment in conjunction with a target enodeb.

101. The WTRU of embodiment 100 wherein the controller autonomously performs timing adjustments based on a relative timing difference between the source enode-B and the target enode-B.

102. The WTRU as in embodiment 100 wherein the controller uses a RACH access procedure for timing adjustment.

103. The WTRU of embodiment 102 wherein a plurality of RACH preamble signatures with different orthogonality and different priority are used and among the plurality of RACH preamble signatures, a RACH preamble signature with higher orthogonality, higher priority and higher power is used for handover.

104. The WTRU of embodiment 103 wherein a specific RACH preamble signature is reserved for handover.

105. The WTRU of embodiment 104 wherein the reserved RACH preamble signature is indicated in a handover command.

106. A WTRU as in any of embodiments 95-105 wherein a controller sends a resource allocation request to a target eNode-B for scheduling of uplink resources.

107. The WTRU of embodiment 106 wherein the resource allocation request is sent via a RACH.

108. A WTRU as in any one of embodiments 95-107 wherein a controller performs a cell reselection procedure when a handover command is not successfully received.

109. The WTRU of embodiment 108 wherein the controller first attempts to access an initially connected cell in the source enode-B.

110. The WTRU of embodiment 109 wherein if the WTRU fails to access the initially connected cell, the WTRU attempts to access other cells in the source enode-B.

111. The WTRU of embodiment 110 wherein if the WTRU fails to access the other cell in the source enode-B, the WTRU attempts to access the other cell not included in the source enode-B.

112. The WTRU as in any one of embodiments 108-111 wherein the controller sends the source enode-B ID to the target enode-B during cell reselection.

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, examples of which 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 disks and Digital Versatile Disks (DVDs), for execution by a general purpose computer or a processor.

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 video circuit, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, 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 (20)

1. A method for performing a handover, the method comprising:

a wireless transmit/receive unit (WTRU) performs measurements;

the WTRU sending a measurement report to a source eNode-B, wherein the source eNode-B makes a handover decision based on the measurement report, wherein the source eNode-B sends a handover request to a target eNode-B, and wherein the source eNode-B receives a handover response from the target eNode-B, the handover response including an indication that a handover should begin;

the WTRU receiving a handover command from the source eNode-B, the handover command including at least one of: reconfiguration information, information on timing adjustment, a relative timing difference between the source and target eNode-Bs, information on an initial radio resource scheduling procedure at the target eNode-B, and measurement information for the target eNode-B; and

the WTRU performs a handover to the target eNode-B using information included in the handover command.

2. The method of claim 1 wherein the handover command instructs the target eNode-B to schedule resources for the WTRU in accordance with a Random Access Channel (RACH) access procedure.

3. The method of claim 1 wherein the handover command indicates that the target eNode-B schedules resources for the WTRU without receiving an explicit resource allocation request from the WTRU.

4. The method of claim 1, further comprising:

the WTRU performs timing adjustment in conjunction with the target eNode-B.

5. The method of claim 4 wherein the WTRU uses a Random Access Channel (RACH) access procedure for the timing adjustment.

6. The method according to claim 5, wherein a plurality of RACH preamble signatures with different orthogonality and different priority are used, and among the plurality of RACH preamble signatures, a RACH preamble signature with higher orthogonality, higher priority and higher power is used for handover.

7. The method of claim 6, wherein a specific RACH preamble signature is reserved for the handover.

8. The method of claim 7, wherein the reserved RACH preamble signature is indicated in the handover command.

9. The method of claim 1, further comprising:

the WTRU resets Radio Link Control (RLC) and hybrid automatic repeat request (HARQ) after receiving the handover command.

10. The method of claim 9, further comprising:

the WTRU sends a handover complete message to the target eNode-B, the handover complete message including a uplink Packet Data Convergence Protocol (PDCP) Sequence Number (SN) to be transmitted.

11. The method of claim 10, further comprising:

the WTRU sends an RLC control message to the target eNode-B after sending the handover complete message to indicate a successfully transmitted Service Data Unit (SDU) and an SDU gap.

12. An eNode-B for performing a handover, the eNode-B comprising:

a transceiver for transmitting and receiving data to and from a wireless transmit/receive unit, WTRU;

a measurement unit to perform channel measurements for the WTRU;

a handover controller configured to make a handover decision based on the measurements and send a handover command to the WTRU, the handover command comprising at least one of: reconfiguration information, information on timing adjustment, relative timing difference between source and target eNode-Bs, information on initial radio resource scheduling procedure at the target eNode-B, and measurement information for the target eNode-B.

13. A wireless transmit/receive unit, WTRU, for performing a handover, the WTRU comprising:

a transceiver for transmitting and receiving data to and from an evolved node-B (eNode-B);

a measurement unit for performing measurement; and

a controller for performing a handover from a source eNode-B to a target eNode-B using information included in a handover command received from the source eNode-B, the handover command including at least one of: reconfiguration information, information on timing adjustment, a relative timing difference between the source eNode-B and the target eNode-B, information on an initial radio resource scheduling procedure at the target eNode-B, and measurement information for the target eNode-B.

14. The WTRU of claim 13 wherein the controller controls the transceiver to cease data transmission to the source enode-B upon receipt of the handover command by the WTRU.

15. The WTRU of claim 13 wherein the controller is configured to perform timing adjustment in conjunction with the target enodeb.

16. The WTRU of claim 15 wherein a plurality of random access channel, RACH, preamble signatures with different orthogonality and different priority are used, and among the plurality of RACH preamble signatures, a RACH preamble signature with higher orthogonality, higher priority and higher power is used for handover.

17. The WTRU of claim 16 wherein a specific RACH preamble signature is reserved for the handover.

18. The WTRU of claim 17 wherein the reserved RACH preamble signature is indicated in the handover command.

19. The WTRU of claim 13 wherein the controller sends a resource allocation request to the target enodeb to schedule uplink resources.

20. The WTRU of claim 19 wherein the resource allocation request is sent via a RACH.