CA2920995A1 - Method for processing radio link failure in multiple base station connectivity based radio communication system, and apparatus for same - Google Patents

Abstract

The present invention relates to a method and apparatus for communication between a terminal and a base station (BS) in a radio communication system. A multiple BS
connectivity communication method by a BS according to one embodiment comprises:
receiving a radio link failure (RLF) anticipation message associated with a first link from a terminal; searching for a target cell for handover of the terminal according to the RLF
anticipation message; when the target cell is present as a result of the searching, selecting the target cell; receiving a RLF message associated with the first link from the terminal through a second link; and transmitting to the terminal a command to handover to the selected target cell. In an embodiment, in the system where a terminal is connected with one or more BSs, it is possible to efficiently process a RLF and mitigate the effect on application level connectivity.

Description

[DESCRIPTION]
[Title of Invention]
METHOD FOR PROCESSING RADIO LINK FAILURE IN MULTIPLE BASE
STATION CONNECTIVITY BASED RADIO COMMUNICATION SYSTEM, AND APPARATUS FOR SAME
[Technical Field]
1011 An embodiment of the present invention relates to a method and apparatus for communication between a base station and terminals in a radio communication system and, in particular, to a Radio Link Failure handling method and apparatus for multiple connectivity of a terminal to several base stations.
[Background Art]

[02] A 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) (release 11 or earlier) system is based on a single connectivity model in which one User Equipment (UE) connects to one evolved Node B (eNB). In such a system, handling Radio Link Failure (RLF) is based on the detection of RLF by the UE
and, in the RLF state with the base station, based on additional operations of the UE
without any notification of the RLF to the eNB.

[03] FIG. 1 is a diagram exemplifying an RLF handling procedure in a mobile communication system, and FIG. 2 is a signal flow diagram illustrating the RLF

failure handling procedure in a mobile communication system.

[04] Referring to FIG. 1, a UE performs a procedure known as Radio Link monitoring (RLM) to detect RLF. The UE may measure BLER of PDCCH during a predetermined time period. If the BLER drops below a predetermined threshold (Qout) during the predetermined time period, an out-of-sync indicator may be generated in a Physical (PHY) layer. If the PHY layer generates successively a predetermined number of out-of-sync indicators N310 to an RRC layer, the RRC
layer starts an RLF timer T310. RLF may be declared when the RLF timer expires.
After the start of the timer T310, the timer T310 may stop when the PHY layer generates successively a predetermined number of in-sync indicators N311.

[05] Referring to FIG. 2, a UE 210 may detect RLF on a first link at step 251.
The UE 210 may declare RLF upon expiry of the timer T310. Declaring the RLF, the UE

210 may stop all uplink transmissions and deactivate all radio bearers to avoid uplink interference occurrence probability. Then the UE 210 may scan for a suitable target cell 240 at step 252 and transmit a connection reestablishment request message to the target cell 240 at step 253. If the target cell 240 has a UE context already at step 254, it sends the UE 210 a connection reestablishment success message to complete the reestablishment at step 255. After the successful reestablishment, the connection between the UE 210 and the target cell 240 resumes.
1061 However, if the target cell 240 does not have the UE context at step 257, it sends the UE 210 a connection reestablishment failure message to declare reestablishment failure at step 258. After the reestablishment failure, the UE

enters an idle mode at step 259 and triggers a new connection to the target cell 240 at steps 260 and 261. This may cause an application level connection release, which is very undesirable. Further, it should be noted that the target cell 240 has the UE
context only when a handover starts before the occurrence of RLF. Thus the reestablishment procedure is likely to succeed only in such a scenario.
1071 If a predetermined maximum number of random accesses have failed, this may be indicative of the existence of system RLF. Also, if a predetermined maximum number of Radio Link Control (RLC) retransmissions have been completed, this may be indicative of the existence of RLF.
[08] A related art discloses a method in which a LIE scans for an appropriate target cell after declaring RLF and sends a selected target cell an RLF indicator indicating occurrence of RLF with the serving eNB (ID of the serving eNB) in the single connectivity system (3GPP LTE Release 11 or earlier). The target cell sends the serving eNB the relevant information. If such information is received, the serving eNB
may improve handover parameters which may cause RLF and other radio link parameters.
[09] Meanwhile, the terms eNB and cell may be used interchangeably throughout the specification.
[Disclosure of Invention]
[Technical Problem]

1101 The communication methods of a UE and an eNB according to an embodiment of the present invention aim to provide a mechanism for the efficient handling of RLF
that will reduce the impact of RLF on the application level connection in a system in which one UE connects to one or more eNBs.
1111 The objects of the present invention are not limited to the aforesaid, and other objects not described herein with be clearly understood by those skilled in the art from the descriptions below.
[Solution to Problem]
1121 In order to achieve the above objects, a multi-base station connectivity communication method of a base station according to an embodiment of the present invention includes receiving an RLF expected message related to a first link from a terminal; searching for a target cell for handover of the terminal based on the RLF
expected message; selecting, when a target cell exists as a search result, the target cell;
receiving an RLF message related to the first link from the terminal through a second link; and transmitting a handover command to the terminal for handover to the selected target cell.
1131 Preferably, the method further includes selecting, when no target sell exists as a search result, a base station for single connectivity to the second link and transmitting a single connectivity switching message to the terminal for single connectivity with the selected base station.
[14] Also, in order to achieve the above objects, a multi-base station connectivity communication method of a terminal according to an embodiment of the present invention includes transmitting an(RLF expected message related to a first link to a base station, transmitting an RLF message related to the first link to the base station through a second link, and receiving a handover command for handover to the selected target cell based on the RLF expected message.
1151 Preferably, the method further includes receiving a signal connectivity switching message from the base station through the second link, and switching to a single connectivity mode a single base station based on the received signal connectivity switching message.

[16] Also, in order to achieve the above objects, a multi-base station connectivity communication method of a base station according to an embodiment of the present invention includes receiving an RLF message related to a first link from a terminal through a second link; searching for a target cell for handover of the terminal based on the RLF message; selecting, when a target cell exists as a search result, the target cell;
and transmitting a handover command to the terminal for handover to the selected target cell.
[17] Preferably, the method further includes selecting, when no target sell exists as a search result, the base station for single connectivity through the second link and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
[18] Also, in order to achieve the above objects, a multi-base station connectivity communication method of a terminal according to an embodiment of the present invention includes transmitting an RLF message related to a first link to a base station through a second link and receiving a handover command for handover to a target cell selected by the base station based on the RLF message.
[19] Preferably, the method further includes receiving a single connectivity switching message from the base station through the second link and switching to a single connectivity mode with a single base station based on the received single connectivity switching message.
[20] Also, in order to achieve the above objects, a multi-base station connectivity communication method of a base station according to an embodiment of the present invention includes receiving an RLF message related to a first link from a terminal through a second link, selecting a base station for single connectivity through the second link based on the RLF message, and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
[21] Also, in order to achieve the above objects, a multi-base station connectivity communication method of a terminal according to an embodiment of the present invention includes transmitting an RLF message related to a first link to a base station through a second link, receiving a single connectivity switching message from the base station through the second link, and switching to a single connectivity mode with a single base station based on the received signal connectivity switching message.
[22] Also, in order to achieve the above objects, a base station of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with other base stations and terminals and a control unit which controls receiving an RLF expected message related to a first link from a terminal; searching for a target cell for handover of the terminal based on the RLF expected message; selecting, when a target cell exists as a search result, the target cell; receiving an RLF message related to the first link from the terminal through a second link; and transmitting a handover command to the terminal for handover to the selected target cell.
[23] Also, in order to achieve the above objects, a terminal of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with base stations and a control unit which controls transmitting an RLF expected message related to a first link to a base station, transmitting an RLF message related to the first link to the base station through a second link, and receiving a handover command for handover to the selected target cell based on the RLF expected message.
[24] Also, in order to achieve the above objects, a base station of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with other base stations and terminals and a control unit which controls receiving an RLF message related to a first link from a terminal through a second link; searching for a target cell for handover of the terminal based on the RLF message; selecting, when a target cell exists as a search result, the target cell; and transmitting a handover command to the terminal for handover to the selected target cell.
[25] Also, in order to achieve the above objects, a terminal of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with base stations and a control unit which controls transmitting an RLF message related to a first link to a base station through a second link and receiving a handover command for handover to a target cell selected by the base station based on the RLF message.
[26] Also, in order to achieve the above objects, a base station of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with other base stations and terminals and a control unit which controls receiving an RLF message related to a first link from a terminal through a second link, selecting a base station for single connectivity through the second link based on the RLF message, and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
[27] Also, in order to achieve the above objects, a terminal of performing a multi-base station communication according to an embodiment of the present invention includes a communication unit which communicates with base stations and a control unit which controls transmitting an RLF message related to a first link to a base station through a second link, receiving a single connectivity switching message from the base station through the second link, and switching to a single connectivity mode with a single base station based on the received signal connectivity switching message.
[Advantageous Effects of Invention]
[28] The communication methods of a UE and an eNB according to an embodiment of the present invention are capable of providing a mechanism for the efficienthandling of RLF that will reduce the impact of RLF on the application level connection in a system in which one UE connects to one or more eNBs.
[29] The advantages of the present invention are not limited to the aforesaid, and other advantages not described herein will be clearly understood by those skilled in the art from the descriptions below.

- 6 -[Brief Description of Drawings]
[30] FIG. 1 is a diagram exemplifying an RLF handling procedure in a mobile communication system.
[31] FIG. 2 is a signal flow diagram illustrating the RLF failure handling procedure in a mobile communication system.
[32] FIG. 3 is a diagram illustrating exemplary dual connectivity according to an embodiment of the present invention.
[33] FIG. 4 is a signal flow diagram illustrating a method for handling RLF in the dual connectivity according to an embodiment of the present invention.
[34] FIG. 5 is a signal flow diagram illustrating a method for handling RLF in the dual connectivity according to another embodiment of the present invention.
[35] FIG. 6 is a signal flow diagram illustrating a method for handling RLF in the dual connectivity according to another embodiment of the present invention.
[36] FIG. 7 is a flowchart illustrating an RLF handling method when the link of the MeNB is in the RLF state in the procedure of FIG. 6.
[37] FIG. 8 is a flowchart illustrating an RLF handling method when the link of the SeNB is in the RLF state in the procedure of FIG. 6.
[38] FIG. 9 is a signal flow diagram illustrating an early RLF indication procedure in a single connectivity system.
[39] FIG. 10 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to an embodiment of the present invention.
[40] FIG. 11 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to another embodiment of the present invention.
[41] FIG. 12 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to another embodiment of the present invention.
[42] FIG. 13 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to another embodiment of the present invention.

- 7 -[43] FIG. 14 is a signal flow diagram illustrating a method for switching from the single connectivity to the dual connectivity according to an embodiment of the present invention.
[44] FIG. 15 is a signal flow diagram illustrating a method for switching from the single connectivity to the dual connectivity according to another embodiment of the present invention.
[45] FIG. 16 is a block diagram illustrating the UE according to an embodiment of the present invention.
[46] FIG. 17 is a block diagram illustrating the eNB according to an embodiment of the present invention.
[47] FIG. 18 is a signal flow diagram illustrating an RLF indicator transmission procedure between a UE and an MeNB according to an embodiment of the present invention.
[48] FIG. 19 is a signal flow diagram illustrating an RLF indicator transmission procedure between a UE and an MeNB according to another embodiment of the present invention.
[49] FIG. 20 is a signal flow diagram illustrating a method for transmitting an RLF
indicator including an RLF cause value according to an embodiment of the present invention.
[50] FIG. 21 is a signal flow diagram illustrating a method for transmitting an RLF
indicator included in the measurement report configuration according to an embodiment of the present invention.
[51] FIG. 22 is a signal flow diagram illustrating a method for transmitting an RLF
indicator included in a measurement report configuration according to another embodiment of the present invention.
[52] FIGs. 23 to 25 are diagrams illustrating exemplary MAC CEs according to an embodiment of the present invention.
[Mode for the Invention]
[53] Exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings.

- 8 -[54] Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention.
This aims to omit unnecessary description so as to make clear the subject matter of the present invention.
[55] Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention.
Exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. Furthermore, terms used herein are defined by taking functions of the present invention into account and can be changed according to the practice or intention of users or operators. Therefore, definition of the terms should be made according to overall disclosures set forth herein.
[56] FIG. 3 is a diagram illustrating exemplary dual connectivity according to an embodiment of the present invention.
1571 Referring to FIG. 3, discussions are underway for dual connectivity as one of the key technologies for improving user efficiency and achieving high mobility robustness in the 3GPP LTE Release 12. In such a dual connectivity scenario, the UE
may connect to a macro cell and a pico cell simultaneously. However, whether or not simultaneous communication with the two cells is possible may depend on the capability of the UE. For the macro and pico cells to be helpful to the UE, they should be in harmony. In such a system, the macro cell acts as a mobility anchor and hides pico cell change from a core network in a way that causes a significant reduction in signaling load from the core network. The pico cell may handle a large amount of traffic, while the macro cell may handle more important traffic such as VoIP
traffic.
One cell (macro cell) may serve as a Master eNB (MeNB) and the other (pico cell) may serve as a Secondary eNB (SeNB).
1581 At this time, it is preferable for the UE in an RRC_CONNECTED state to stay in as many connections as possible.
[59] RLF on one link may cause loss of the dual connectivity mode and thus encumber the ongoing connection. Accordingly, it is important for the RLF
handling technology in the dual connectivity system to make it possible for the UE to stay in the dual connectivity mode even when one of the radio links is in the RLF
state.

- 9 -[60] A detailed description is made of the RLF handling procedure in the dual connectivity system hereinafter.
[61] In the dual connectivity, one or both of an MeNB link and an SeNB link may be in the RLF state. As described below, when one link is in the RLF state, this may be referred to as "single link RLF" or "partial RLF". When both the links are in the RLF state, this may be referred to as "dual link RLF" or "complete RLF". As described above, the RLF link scenarios may be categorized as follows.
[62] 1. Single link RLF
[63] a. MeNB is in RLF state [64] b. SeNB is in RLF state [65] 2. Dual link RLF
[66] a. Both of MeNB and SeNB links are in RLF state [67] Since two links (one for MeNB and the other for SeNB) exist in the dual connectivity, even though one link is in the RLF state, a UE that has one good link may handle efficiently RLF.. That is, the good link can be used to handle the RLF that occurs on the other link.
[68] In Carrier Aggregation (CA) of 3GPP LTE Rel. 10/11, SCell link quality may be determined based on the Channel Quality Indicator (CQI) report. Since it is assumed that the SCell may be deactivated depending thereon, Radio Link Monitoring (RLM) to the SCell is not performed. It is configured that CQIs of all cells are reported to the PCell through only a Physical Uplink Control Channel (PUCCH) carrying CQI. However, whether the MeNB can have a CQI report of the SeNB in the dual connectivity has not been agreed yet (scenario lb). Also, if a connection between the MeNB and the UE does not have a good radio quality, it is unclear how the MeNB
has its CQI (scenario la).
[69] In the CA of Rel. 10/11, the PUCCH is transmitted only in the Pcell;
however, in the case that the same access cannot be made in the dual connectivity, non-ideal backhaul may influence the operation.
[70] In the dual connectivity, the CA RLF handling of Rel 10/11 is applied only to a case where the MeNB (PCell) link is in the RLF state and may mean perform reestablishment of all connections including the connections of the SeNB
(SCell). If

- 10 -the reestablishment is not successful, it may be necessary to terminate and reestablish all connections. Such an approach cannot still justify that the SeNB has a good link but is in the RLF state. (*l_ ) [71] A reestablishment procedure may be successful only when a cell in which the reestablishment is performed has already been prepared to accommodate the UE.
This may not be the case where the connection in progress is not influenced in a non-handover situation.
[72] In contrast, there is no legacy procedure for handling RLF of an SeNB
(SCell).
[73] In an RLF handling method, it may be possible to handle the RLF
independently on the link in the RLF state. For example, the UE finds a suitable target cell and attempts reestablishment after declaring RLF to the SeNB in the scenario lb.
If the target cell already has the UE context, the reestablishment may be successful.
Otherwise, the ongoing connections on the link of the SeNB (eNB with the link on which RLF has occurred) should be removed to perform initial connection establishments (as initial dual connectivity mode establishment). Such a method is suitable for scenario lb and may also be applied to scenario la.
1741 Alternatively, if the reestablishment fails, it may be possible to switch to a single connectivity with an eNB having a good link.
[75] The above described RLF handling methods may have a negative influence on application level connections and thus may increase the loss of the dual connectivity mode and the switching between the dual connectivity and the single connectivity.
[76] The present invention proposes a method of using a good link (link which is not in the RLF state) for notifying the MeNB of the RLF failure state of the other link.
The MeNB may prepare an alternative cell instead of the cell of the link in the RLF
state. Thereafter, the MeNB may perform triggering such that the UE is handed over to the prepared cell instead of performing re-establishment. This may prevent the loss of the dual connectivity mode and maintain continuous connections in a smooth manner. Alternatively, in the case where any suitable alternative cell is not found, the MeNB may prepare a cell having a good link to provide the single connectivity mode to the UE (from cells in connection with the provision of the dual connectivity to the

- 11 -UE). In order to detect the RLF from the link, an RLM process may be performed on the link.
[77] The above-described method may be applied in both the RLF scenario la and scenario lb. At this time, the UE context is required to be maintained until a predetermined time after the RLF timer T310 expires. Further, in order to quickly prepare the alternative cell, an RLF indicator may be transmitted to the MeNB
through another good link before the RLF is declared on one link.
[78] FIG. 18 is a signal flow diagram illustrating an RLF indicator transmission procedure between a UE and an MeNB according to an embodiment of the present invention.
[79] Referring to FIG. 18, a UE 1810 is dually connected to an MeNB 1820 and an SeNB 1830 according to an embodiment of the present invention at step 1851.
The UE 1810 performs an RLM procedure to at least one SeNB 1830 (as well as to the cell of the MeNB 1820) at step 1852. Afterward, if the UE 1810 detects RLF in the cell of the SeNB 1830, it sends the PCell of the MeNB 1820 an RLF indicator including RLF
cause information at step 1854. After transmitting the RLF indicator, the UE

stops all uplink transmissions including the configured PUCCH at step 1855 and releases the data radio bearers existing in the cell in which the RLF is detected at step 1856.
[80] FIG. 19 is a signal flow diagram illustrating an RLF indicator transmission procedure between a UE and an MeNB according to another embodiment of the present invention.
[81] Referring to FIG. 19, a UE 1910 is dually connected to an MeNB 1920 and an SeNB 1930 at step 1951. The UE 1910 performs RLM to one cell of the SeNB 1930 configured as a pathloss reference (as well as to the PCell of the MeNB 1920) at step 1952. The cell configured as the pathloss reference may be referred to as a pSCell or special cell of the SeNB 1930. If RLF is detected in the pSCell at step 1953, the UE
1910 sends the PCell of the MeNB 1920 an RLF indicator including RLF cause information. After transmitting the RLF indicator, the UE 1910 stops all uplink transmissions including the configured PUCCH at step 1955 and releases the data radio bearers existing in the cell in which the RLF is detected at step 1956.
According

- 12 -to an embodiment of the present invention, the PUCCH may be configured in the cell which has been configured as the pathloss reference.
[82] According to another embodiment of the present invention, the RLF
indicator for one link may be transmitted to the MeNB through the other link good in quality at predetermined timings as follows.
[83] 1. Before RLF is declared (before T310 expires) [84] a. RLF expected indication [85] 2. When RLF is declared (when T310 expires) [86] a. RLF declaration indication [87] If the RLF indicator is received, the MeNB may prepare an alternative cell for serving the UE on behalf of the cell in which RLF is expected (or has occurred). This can avoid connection reestablishment.
[88] FIG. 4 is a signal flow diagram illustrating a method for handling RLF in the dual connectivity according to an embodiment of the present invention.
[89] Referring to FIG. 4, the RLF handling method according to an embodiment of the present invention is characterized in that if the RLF is declared in the dual connectivity the dual connectivity is replaced with the single connectivity to an eNB
having a good link.
[90] When RLF is declared for one of the two radio links (e.g. when the T310 timer expires), the result may be notified to the MeNB through the second link (i.e.
link having a good connection state). Then the MeNB may prepare an eNB having a good link to serve the UE in the single connectivity mode and control the UE to switch to the single connectivity mode.
[91] In detail, RLF may occur on the first link at step 451. For example, RLF
may be declared when the timer T310 expires.
[92] Afterward, the UE 410 sends the MeNB 420 a RLF message at step 452. At this time, the RLF message may be transmitted to the MeNB 420 through the second link having a good connection state.
[93] According to an embodiment, the UE 410 may send the MeNB 420 a message including an indicator indicating occurrence of RLF. According to an embodiment, the RLF indicator may be defined as a new indicator.

- 13 -[94] According to an embodiment, the UE 410 may send the MeNB 420 a legacy Radio Resource Control (RRC) Connection Reestablishment message through the second link. The RRC Connection Reestablishment message may be transmitted when the T310 expires as in the legacy 3GPP LTE. For example, a Random Access Channel (RACH) code may be allocated to the UE 410 for such a purpose. The RRC
Connection Reestablishment message may be handled like the RLF indicator.
[95] As denoted by reference number 450, the L2/L3 context for the first link may be maintained during a predetermined period. The L2/L3 context is required to be maintained during a predetermined period to prepare the single connectivity replacement. Then the Li context for the first link may be released.
[96] Afterward, the MeNB 420 communicates with the SeNB 430 to have the UE
410 prepared for the single connectivity with an eNB having a link good in quality at step 453.
[97] For example, the MeNB 420 may have a good radio link while the SeNB 430 may be in the RLF state. At this time, the MeNB 420 may allow the UE 410 to switch from the dual connectivity with both the MeNB 420 and SeNB 430 to the single connectivity with the MeNB 420. At this time, it may be necessary to transfer the context of the flow serviced by the SeNB 430 to the MeNB 420. A detailed description thereof is made later.
[98] The MeNB operates to switch to the single connectivity at step 454 and sends the UE 410 a single connectivity switching message at step 455. At this time, the flow information changed at step 453 may also be transferred.
[99] Afterward, the UE 410 performs switching to the single connectivity with the eNB having a good link at step 456.
[100] The method according to the embodiment of FIG. 4 may delay handling the RLF on the link of the SeNB 430 and thus be suitable for the scenario lb in which the SeNB 430 has the RLF on the link thereof while the MeNB 420 has a good link, but it is not limited to this case.
[101] FIG. 5 is a signal flow diagram illustrating a method for handling RLF
in the dual connectivity according to another embodiment of the present invention.

- 14 -[102] Referring to FIG. 5, the RLF handling method according to an embodiment of the present invention is characterized in that the UE 510 sends the MeNB 520 an RLF
message after expiry of the RLF timer T310 through a good link in order for the MeNB 520 to prepare an alternative cell for serving the UE 510 in the dual connectivity.
[103] Upon receipt of the RLF message, the MeNB 520 determines whether there exists a target cell 540 suitable for handover. If there is no suitable target cell 540, the MeNB 520 may perform appropriate measurement to select and prepare a target cell 540 suitable for the handover. Then the MeNB 520 may command the UE 510 to perform the handover to the prepared suitable target cell 540.
[104] Otherwise if there is no target cell suitable for the handover, the MeNB

may prepare an eNB corresponding to the good link through which a service is provided in the single connectivity mode. Then the MeNB 520 may command the UE

510 to operate in the single connectivity mode with the selected eNB.
[105] In detail, RLF occurs on the first link at step 551. For example, RLF
may be declared when the timer T310 expires.
[106] Afterward, the UE 510 sends the MeNB 520 an RLF message at step 552. At this time, the RLF message may be transmitted to the MeNB 520 through the second link having a good connection state. At this time, it is necessary to retain the L2/L3 context for the first link during a predetermined time period in order to perform handover or to prepare switching to the single connectivity; thus, there is a need of a new timer such as a context retention timer. Meanwhile, the Li context for the first link is released.
[107] According to an embodiment, the UE 510 may send the MeNB a message including an indicator indicating the occurrence of RLF. According to an embodiment, the RLF indicator may be defined as a new indicator.
[108] According to an embodiment, the UE 510 may send the MeNB 520 a legacy RRC Connection Reestablishment message through the second link. The RRC
Connection Reestablishment message may be transmitted when the T310 expires as in the legacy 3GPP LTE. For example, a RACH code may be allocated to the UE 410 for such a purpose. The RRC Connection Reestablishment message may be handled like

- 15 -the RLF indicator. Accordingly, the RRC Connection Reestablishment message may be used, instead of the RLF indicator, to identify a link on which the RLF has occurred.
[109] Afterward, if the MeNB 520 does not have a valid measurement report received recently from the UE 510, it sends the UE 510 a measurement command at step 553. As denoted by reference number 554-1, if the MeNB 520 has a valid measurement report received from the UE 510, it is not necessary to transmit the measurement command. The UE 510 performs measurement and sends the MeNB 520 a measurement report according to the measurement command at step 554.
[1101 According to an embodiment, the MeNB 520 may configure a one shot measurement when the RLF indicator is received. According to an embodiment, when the RLF indicator is received, the MeNB 520 may configure the one shot measurement if the first link is a macro link or may configure the normal measurement if otherwise.
[111] If a measurement report is received from the UE 510 through steps 553 and 554 or a valid measurement report already exists, the MeNB 520 prepares a cell for handover at step 555. At this time, the MeNB 520 may prepare the target cell suitable for the handover of the UE 510 based on the measurement report. The target eNB 540 may be a pico eNB, but it is not limited thereto and may also be a macro eNB.
[112] If no target cell suitable for the handover is found as denoted by reference number 556-1, the MeNB 520 communicates with the SeNB 530 to prepare the UE
510 for switching to the single connectivity with an eNB having a good link at step 556.
[113] According to an embodiment, the MeNB 520 may select one target cell 540 suitable for the handover of the UE 510 among a plurality of cells listed in the measurement report. If no suitable cell is found, the MeNB 520 may prepare the second eNB for the single connectivity. For example, the MeNB 520 may have a good radio link while the SeNB 530 may be in the RLF state. At this time, the MeNB

may allow the UE 510 to switch from the dual connectivity with the MeNB 520 and SeNB 530 to the single connectivity with the MeNB 520. At this time, it may be

- 16 -necessary to transfer the context of the flow serviced by the SeNB 530 to the MeNB
520. A detailed description thereof is made later.
[114] According to an embodiment, the MeNB 520 may select one target cell 540 suitable for the handover of the UE 510 among a plurality of cells listed in the measurement report. If no suitable cell is found, the MeNB 520 may prepare an alternative cell for the handover. The alternative cell may be determined semi-statically by the MeNB 520 and/or SeNB 530. According to an embodiment, the alternative cell may be dynamically designated by a first eNB as the MeNB 520.
If no cell is configured as the alternative cell, the MeNB 520 may prepare a second eNB for the single connectivity mode. As described above, it is necessary to transfer the context of the flow serviced by the first eNB to the second eNB. A detailed description is made later.
[115] If the target cell 540 suitable for the handover is prepared of step 555 at step 557, the MeNB 520 controls to perform handover to the prepared target cell 540 at step 558. For example, the MeNB 520 may send the UE 510 a handover command. At this time, the handover command may be transmitted through the second link having a good connection state. According to an embodiment, the prepared target cell 540 may be at least one of a cell indicated in the measurement report transmitted by the UE
510, an alternative cell, and second eNBs having good links.
[116] Otherwise, if no target cell 540 suitable for handover is selected, the MeNB
520 sends the UE 510 a single connectivity switching message to operate in the single connectivity with the second eNB at step 560. At this time, the flow information changed at step 556 may also be transferred together [117] The method according to the embodiment of FIG. 5 may delay handling the RLF on the link of the SeNB 530 and thus be suitable for the scenario lb in which the SeNB 530 has RLF on the link thereof while the MeNB 520 has a good link but limited to this case.
[118] FIG. 6 is a signal flow diagram illustrating a method for handling RLF
in the dual connectivity according to another embodiment of the present invention.
[119] Referring to FIG. 6, the RLF handling method according to an embodiment of the present invention is characterized in that a UE 610 sends an MeNB an RLF

- 17 -expected message through a link with a good connection state before expiry of an RLF
timer T310 in order for the MeNB 620 to prepare an alternative cell for serving the UE 610 in the dual connectivity.
[120] The RLF expected message may be transmitted upon start of the RLF timer T310 or after a predetermined time period from the start of the RLF timer 310 to secure the time for early RLF recovery.
[121] Upon receipt of the RLF expected message, the MeNB 620 may determine whether any target cell 640 suitable for handover exists. If a suitable target cell 640 does not exist, it may perform measurement appropriately to determine the target cell 640 suitable for the handover. Afterward, if RLF occurs (e.g. if the timer expires), the UE 610 sends the master eNB 620 a message, e.g. RLF message, through a link with a good connection state to indicate explicitly the occurrence of the RLF.
Then the MeNB 620 may command the UE 610 to perform handover to the prepared target cell 640 based on the detection of the RLF.
[122] Otherwise if no target cell 640 suitable for handover is found, the MeNB

may prepare an eNB corresponding to the good link serving the UE 610 in the single connectivity mode. Then the MeNB 620 may command the UE 610 to switch to the single connectivity mode with the selected eNB based on the detection of the RLF.
[123] In detail, RLF is expected on a first link at step 651. For example, RLF
may be expected when an RLF timer T310 starts.
[124] Next, the UE 610 sends the MeNB 620 an RLF expected message at step 652.

At this time, the RLF expected message may be transmitted to the MeNB 620 through the second link having a good connection state.
[125] According to an embodiment, the UE 610 may send the MeNB 620 a message including an RLF-expected indicator indicating expectation of RLF. According to an embodiment, the RLF-expected indicator may be defined as a new indicator.
[126] According to an embodiment, if the timer T310 starts, the UE 610 may send the MeNB 620 the RLF expected message for the first link.
[1271 According to an embodiment, if the timer T310 starts, an RLF preparation timer as a new timer may start. If the RLF preparation timer expires, the LIE
610 may send the MeNB 620 the RLF expected message for the first link. The time of the RLF

- 18 -preparation timer may be shorter than that of the RLF timer T310. If out-of-sync indicators are reported successively during a predetermined period after the start of the RLF timer, the UE may send the MeNB 620 the RLF expected message.
11281 According to an embodiment, a new Reference Signal Received Power (RSRP) threshold may be determined as a RLF report threshold value. At this time, if the RLF
report threshold value is fulfilled after the start of the timer T310, the UE
610 may send the MeNB 620 the RLF expected message for the first link.
11291 According to an embodiment, a hysteresis may be defined based on the RLF

preparation timer and RLF report threshold. At this time, if the average of the RSRP
for the RLF preparation timer stays below the RLF report threshold, the UE 610 may send the MeNB 620 the RLF expected message for the first link.
[130] According to an embodiment, the CQI reports for the first link may be transmitted on the second link and they act as RLF situation indicators. The MeNB
620 and SeNB 630 may detect the RLF situation based on such reports.
[131] If the MeNB 620 has no valid measurement report received recently from the UE 610 after the receipt of the RLF expected message for a related layer, it sends the UE 610 a measurement command at step 653. However, if the MeNB 620 has the measurement report received from the UE 610 which is still valid as denoted by reference number 654-1, such a measurement command may not be required. The UE

610 performs measurement and sends the MeNB 620 a measurement report according to the measurement command from the MeNB 620.
[132] According to an embodiment, if the RLF-expected indicator is received, the MeNB 620 may configure one short measurement. According to an embodiment, when the RLF-expected indicator is received, the MeNB 520 may configure the one shot measurement if the first link is a macro link or may configure the normal measurement if otherwise.
[133] If a measurement report is received from the UE 610 through steps 653 and 654 or a valid measurement report already exists, the MeNB 620 prepares a cell for handover at step 555. At this time, the MeNB 520 may prepare the target cell suitable for the handover of the UE 510 based on the measurement report. The target

- 19 -eNB 640 may be a pico eNB, but it is not limited thereto and may also be a macro eNB.
[134] Meanwhile, if no target cell suitable for the handover is found as denoted by reference number 656-1, the MeNB 620 communicates with the SeNB 630 to prepare the UE 510 for switching to the single connectivity with an eNB having a good link at step 656.
[135] According to an embodiment, the MeNB 620 may select a target cell 640 suitable for the handover of the UE 610 among the cells listed in the measurement report. However, if no suitable cell is found, the MeNB 620 may prepare a second eNB for the single connectivity mode. For example, the MeNB 620 may have a good radio link while the SeNB 630 may be in the RLF state. At this time, the MeNB

may allow the UE 510 to switch from the dual connectivity with the MeNB 620 and SeNB 630 to the single connectivity with the MeNB 620. At this time, it may be necessary to transfer the context of the flow serviced by the SeNB 630 to the MeNB
620. A detailed description thereof is made later.
[136] According to an embodiment, the MeNB 620 may select one target cell 640 suitable for the handover of the UE 610 among a plurality of cells listed in the measurement report. If no suitable cell is found, the MeNB 620 may prepare an alternative cell for the handover. The alternative cell may be determined semi-statically by the MeNB 620 and/or SeNB 630. According to an embodiment, the alternative cell may be dynamically designated by a first eNB as the MeNB 620.
If no cell is configured as the alternative cell, the MeNB 620 may prepare a second eNB for the single connectivity mode. As described above, it is necessary to transfer the context of the flow serviced by the first eNB to the second eNB. A detailed description is made later.
[137] Afterward, RLF is declared at step 657. That is, the RLF may be declared when the timer T310 expires.
[138] Then the UE 610 sends the MeNB 620 an RLF message at step 658. At this time, the RLF message may be transmitted to the MeNB 620 through a second link having a good connection state.

- 20 -[139] According to an embodiment, the UE 610 may send the MeNB 620 a message including an indicator indicating that the RLF has already occurred. According to an embodiment the RLF indicator may be defined as a new indicator.
[140] According to an embodiment, the UE 610 may send the MeNB 620 a legacy RRC Connection Reestablishment message through the second link. The RRC
Connection Reestablishment message may be transmitted when the T310 expires as in the legacy 3GPP LTE. For example, an RACH code may be allocated to the UE 410 for such a purpose. The RRC Connection Reestablishment message may be handled like the RLF indicator. Alternatively, the MeNB 610 may determine that the RLF
has occurred on the first link other than the second link on which the RRC
Connection Reestablishment message is received. Accordingly, the RRC Connection Reestablishment message corresponding to the RLF indicator may be used to identify the link on which the RLF has occurred.
[141] At this time, it is necessary to maintain the L2/L3 context during a predetermined period to prepare for handover or single connectivity.
[142] If the target cell 640 suitable for the handover has been prepared at step 655 (659), the MeNB operates to perform the handover to the prepared target cell 640 at step 660. For example, the MeNB 620 may send the UE 610 a handover command. At this time, the handover command may be transmitted through a second link having a good connection state. According to an embodiment, the prepared target cell 640 may be at least one of a cell indicated in the measurement report transmitted by the UE
610, an alternative cell, and second eNBs having good links.
[143] Otherwise, if no target cell 640 suitable for handover is selected at step 661, the MeNB 620 sends the UE 610 a single connectivity switching message to operate in the single connectivity with the second eNB at step 662. At this time, the flow information changed at step 656 may also be transferred together [144] The method according to the embodiment of FIG. 6 is applicable to the RLF
scenario 1 a where the link of the SeNB 630 is good while the link of the MeNB
is in the RLF state because the radio state of the link of the MeNB 620 has priority, but it is not limited to this case.

-21 -[145] FIG. 7 is a flowchart illustrating an RLF handling method when the link of the MeNB is in the RLF state in the procedure of FIG. 6.
[146] Referring to FIG. 7, the UE determines whether the timer T310 for the MeNB
link has started at step 701. If the timer T310 for the MeNB has started, the UE
determines whether the timer T310 for the SeNB link is not running at step 703. If it is determined that the SeNB link is good, the RLF preparation timer starts at step 705.
Afterward, the UE determines whether the RLF preparation timer has expired at step 707 and, if not, waits until the RLF preparation time expires at step 709.
[147] If the RLF preparation timer has expired, the UE determines whether Ncell measurement is valid at step 711. If it is determined that the Ncell measurement is valid, the UE sends the SeNB a message including the information on the start of the T310 for the MeNB link along with the Ncell measurement report at step 713. At this time, the SeNB designates an alternative cell at step 714. Afterward, the UE
waits for a predetermined time duration at step 721. At this time, it is preferable for the UE to wait using the RLF preparation timer.
[148] Otherwise, if the Ncell measurement is not valid, the UE sends the SeNB
a message including the information on the start of the T310 for the MeNB link at step 715. Then the SeNB instructs the UE to perform the Ncell measurement at step 717.
The UE reports the measurement result to the SeNB at step 719.
[149] After step 721 or 719, the eNB determines whether a macro cell suitable for handover based on the measurement result report from the UE at step 723. If no suitable cell exists, the MeNB selects at least one macro cell preconfigured as an alternative cell or configured as a current alternative cell by the SeNB at step 725.
[150] If a suitable macro cell is selected at step 723 or if the MeNB selects the alternative cell as the macro cell at step 725, the MeNB transmits a context to the macro cell and prepares the handover at step 727. The MeNB waits for expiry of the T310 at step 729 and, if the T310 expires, instructs the UE to perform handover to the prepared macro cell at step 731. At this time, the handover instruction may be performed through the SeNB.
[151] Otherwise, if it fails to select the alternative cell as the macro cell at step 725, the MeNB transmits the context to the SeNB and makes preparations for the handover.

- 22 -Next, the MeNB waits for expiry of the T310 at step 735 and, if the T310 expires, instructs the UE to switch to the single connectivity with the SeNB at step 737. At this time, the instruction of switching to the single connectivity may be made through the SeNB.
[152] FIG. 8 is a flowchart illustrating an RLF handling method when the link of the SeNB is in the RLF state in the procedure of FIG. 6.
[153] Referring to FIG. 8, the UE determines whether the timer T310 for the SeNB
link has started at step 801. If the timer T310 for the SeNB has started, the UE
determines whether the timer T310 for the MeNB link is not running at step 803. If it is determined that the MeNB link is good, an RLF preparation timer starts at step 805.
Then the UE determines whether the RLF preparation timer has expired at step and, if not, waits until the timer expires at step 809.
[154] If the RLF preparation timer expires, the UE determines whether Ncell measurement is valid at step 811. If it is determined that the Ncell measurement is valid, the UE sends the MeNB a message including the information on the start of the T310 for the SeNB link along with the Ncell measurement report at step 813. At this time, the SeNB may designates an alternative cell at step 814. Then the UE
waits for a predetermined time at step 821. At this time, it is preferable for the UE to wait using the RLF preparation timer.
[155] Otherwise if the Ncell measurement is not valid, the UE sends the MeNB a message including the information on the start of the T310 for the MeNB at step 815.
Then the MeNB instructs the UE to perform the Ncell measurement at step 817.
The UE reports the Ncell measurement result to the MeNB at step 819.
11561 After step 821 or 819, the eNB determines whether a pico cell suitable for handover based on the measurement result report from the UE at step 823. If no suitable pico cell exists, the MeNB selects at least one pico cell preconfigured as an alternative cell or configured as a current alternative cell by the SeNB at step 825.
11571 If a suitable pico cell is selected at step 823 or if the MeNB selects the alternative cell as the pico cell at step 825, the MeNB transmits a context to the selected pico cell and prepares the handover at step 827. At this time, the same process may be performed at the SeNB. Then the MeNB waits for expiry of the

- 23 -at step 829 and, if the T310 expires, instructs the UE to perform handover to the prepared pico cell at step 831.
[158] Otherwise, if the MeNB fails to select the alternative cell as the pico cell at step 825, the MeNB waits until the T310 expires at step 833 and, if the T310 expires, instructs the UE to switch to the single connectivity with the SeNB at step 835.
[159] The embodiments for handling the RLF described in FIGs. 4 to 8 may be generalized by the multi-eNB configuration in which the UE connects to a plurality of eNBs of which one operates as the MeNB. For example, in the embodiment for handling the RLF described in FIG. 6 (preparing the initial RLF), when the UE
reports the RLF expected in one link connected to the MeNB, the MeNB may prepare another cell to handle flows serviced by an eNB having a link in an RLF state. An alternative cell selected by the preparation may be one cell in a set of existing cells providing a service to the UE or a new cell that is not included in the set of cells providing the service to the HE. Similarly, the embodiment for handling the RLF described in the part related to FIGS. 4 and 5 may be applied to the multi-eNB configuration.
[160] According to an embodiment, the eNB having received the RLF expected message may prepare one or more cells for the handover. This can prepare many more cells for the re-establishment and thus increase the chances that the UE
selects the prepared cells for the reestablishment, thereby increasing a success rate of the reestablishment. In another embodiment of the present invention, the RLF
expected message may be transmitted once or more than once.
[161] The RLF message in the above described embodiments may be a new layer 2 or 3 message, an information element in the existing message, or a layer 1 level signal for a faster and stronger indication. For example, an RACH code may be given to the UE for such a purpose. Alternatively, in some embodiments of the present invention, a legacy connection re-establishment request may be used for the RLF message.
The MeNB may determine which link is in the RLF state based on the link through which the connection re-establishment request is received.
[162] In the above described embodiments, it has been described that RLF is triggered based on the expiry of the timer T310. However, the above described embodiments can be equally applied to an RLF triggered by a different cause such as,

- 24 -for example, an RLF including RACH failure on the SeNB or RLC failure in the SeNB.
[163] According to an embodiment of the present invention, the RLF indicator may include an RLF cause value. The RLF cause may include at least one of the expiry of the timer T310, RACH failure in the SeNB, and RLC layer failure in the SeNB.
The random access may be supported by one of the cells of the SeNB.
[164] FIG. 20 is a signal flow diagram illustrating a method for transmitting an RLF
indicator including an RLF cause value according to an embodiment of the present invention.
[165] Referring to FIG. 20, a UE 2010 is dually connected to an MeNB 2020 and an SeNB 2030 at step 2051. At this time, for example, when random access fails, the SeNB 2030 cannot know of the random access failure. Similarly, when the maximum number of retransmissions is made in a cell of the SeNB 2030, it is necessary to declare RLF. RLF is declared in all scenarios such as RLF caused by the Li out-of-sync, the RACH failure of the SeNB, or the RLC failure of the SeNB during a predetermined period (e.g. T310). In order for the MeNB to perform an appropriate operation, this information is transmitted to the MeNB connected dually at step 2053.
In the case that the RLF is detected on the link of the SeNB, the UE may stop all uplink transmission such as CQI/SR/SRS as soon as possible to avoid unnecessary uplink interference.
[166] For example, an information element (IE) may be defined as follows to identify the cause of the RLF on the SeNB.
[167] SeNB-RLF-Cause ENUM {T310 Expiry, RACH Failure, RLC Failure}
[168] In an alternative embodiment, the RLF indicator may include a cause value.
The cause may include the random access (RACH) failure on the SeNB and Radio Link Control (RLC) layer failure on the SeNB.
[169] For example, an IE may be defined as follows to identify the RLF cause on the SeNB.
[170] SeNB-RLF-Cause ENUM {RACH Failure, RLC Failure}

- 25 -[171] In another alternative embodiment, if RLFs occur simultaneously, the SeNB-RLF-Cause 1E may show multiple types of RLF. For example, the IE may be defined as follows to identify the cause of the RLF on the SeNB.
[172] SeNB-RLF-Cause ENUM {T310 Expiry, RACH Failure, RLC Failure, T310 Expiry & RACH Failure, RACH Failure & RLC Failure, T310 Expiry & RLC
Failure, T310 Expiry & RACH Failure & RLC Failure}
[173] In the above case, the SeNB-RLF-Cause IE itself may be considered as the RLF indicator according to an embodiment.
[174] According to an embodiment, the IE may be included in an existing RRC
message. Also, a new RRC message may be introduce to carry the IE.
11751 FIG. 21 is a signal flow diagram illustrating a method for transmitting an RLF
indicator included in the measurement report configuration according to an embodiment of the present invention.
[176] Referring to FIG. 21, a UE 2110 is dually connected to an MeNB 2120 and an SeNB 2130 at step 2151. At this time, if the RLF is detected in a cell of the SeNB at step 2153, the UE 2110 sends the MeNB 2020 the measurement report including the RLF cause information (e.g. SeNB-RLF-Cause IE) at step 2154. Then the MeNB
2020 sends the UE 2110 a measurement configuration message to trigger report RLF
of the SeNB at step 2152.
[177] According to an embodiment, a new extended measurement report may be introduced to include the RLF cause information. In this case, if the RLF is detected in a cell of the SeNB at step 2153, the UE 2110 sends the MeNB 2120 the extended measurement report including the SeNB-RLF-Cause 1E indicating the cause of the RLF at step 2154. At this time, if the RLF is detected in the cell of the SeNB, the UE
2110 is configured by the (master) eNB to transmit the extended measurement report at step 2152.
[178] FIG. 22 is a signal flow diagram illustrating a method for transmitting an RLF
indicator included in a measurement report configuration according to another embodiment of the present invention.
[179] Referring to FIG. 22, a UE 2210 is dually connected to an MeNB 2220 and an SeNB 2230 at step 2251. In this case, if RLF is detected in a cell of the SeNB, the UE

- 26 -2210 always sends the MeNB 2220 an extended measurement report at step 2252.
In the embodiment of FIG. 22, the measurement configuration is not required, unlike the embodiment described in the part related to FIG. 21. If the RLF is detected in a cell of the SeNB 2230 at step 2252, the UE prepares the extended measurement report including the information on the cause of the RLF (e.g. SeNB-RLF-Cause IE).
According to an embodiment, the extended measurement report may include the measurement results of the serving and neighboring cells as far as possible.
The UE
2210 sends the MeNB 2220 the extended measurement report at step 2253.
[180] According to an embodiment, the RRCConnectionReestablishmentRe quest may be used for indicating the RLF on the SeNB and failure cause thereof. At this time, a spare field of the ReestablishmentCause may be used as a signal of the RLF
indicator of the SeNB. A detailed description thereof is made later. Upon receipt of the RRCConnectionReestablishmentRe quest, the MeNB determines whether the RRCConnectionReestablishmentRe quest is transmitted because of the RLF of the SeNB based on the ReestablishmentCause. The MeNB may performs an appropriate operation instead of handling the RRCConnectionReestablishmentRe quest message in the legacy manner. According to an embodiment, if the physCellID field corresponding to PUCCH corresponds to a cell of the SeNB, the MeNB may use the reestablishment request to indicate the RLF on the SeNB. In another embodiment, the SeNB-RLF-Cause IE may be included in the RRCConnectionReestablishmentRe quest in order to indicate clearly the cause of the RLF of the SeNB. The IE may be included in the RRCConnectionReestablishmentRe quest only when RRCConnectionReestablishmentRe quest is used to indicate the RLF on the SeNB.
[181] For example, the messages may be configured as shown in Table 1.
[182] Table 1 RRCConnectionReestablishmentRequest ::= SEQUENCE {
critic alExtens ions CHOICE {
rrcConnectionReestablishmentRequest-r8 RRCConnectionReestablishmentRequest-r8-IEs, critic alExtens ionsFuture SEQUENCE {}
}

-27-RRCCormectionReestablishmentRequest-r8-IEs ::= SEQUENCE {
ue-Identity ReestabUE-Identity, reestablishmentCause ReestablishmentCause, spare BIT STRING (SIZE (2)) ReestabUE-Identity ::= SEQUENCE {
c-RNTI C-RNTI, physCellId shortMAC-I
}
ReestablishmentCause ::= ENUMERATED {
reconfigurationFailure, handoverFailure, otherFailure, SeNB-RLF}

[183] In another embodiment of the present invention, the UE sends the MeNB a UE
assistance information (UEAssistanceInformation) message to notify the MeNB of the RLF situation on the SeNB. At this time, the UE assistance information message transmitted from the UE to the MeNB may include at least one of the causes of the RLF occurring at the SeNB, i.e. physical layer failure, RACH failure, and RLC
failure.
[184] The UE may perform the Radio Link Monitoring (RLM) operation to a cell of at least one SeNB to which an uplink control channel, i.e. PUCCH, is configured. The RLM operation to the SeNB cell may be performed in the same way as the RLM
operation to the MeNB cell to which the PUCCH is configured, i.e. the PCell.
At this time, parameters such as T310, N310, and N311 may be used. If the UE detects the RLF on the SeNB through the RLM operation, it stops the uplink transmission in all cells of the corresponding SeNB and sends the MeNB the UE assistance information message to notify the MeNB that the RLF has occurred on the SeNB. Here, the UE

assistance information message may include the cause of the RLF occurring on the SeNB and the message is configured as shown Table 2.
[185] Table 2

- 28 -UEAssistanceInformation message UEAssistanceInformation-r11 ::= SEQUENCE {
criticalExtensions CHOICE {
cl CHOICE {
ueAssistanceInformation-r11 UEAssistanceInformation-r11-1Es, spare3 NULL, spare2 NULL, sparel NULL
1, criticalExtensionsFuture SEQUENCE 0 }

UEAssistanceInformation-r11-IEs ::= SEQUENCE {
powerPrefindication-r11 ENUMERATED {normal, lowPowerConsumption} OPTIONAL, lateNonCriticalExtension OCTET STRING
OPTIONAL, nonCriticalExtension UEAssistanceInformation-r12-1Es OPTIONAL
}
UEAssistanceInformation-r12-IEs ::= SEQUENCE {
SCGRLFCause ENUMERATED {
t310-Expiry, randornAccessProblem, r1c-MaxNumRetx, sparel}
OPTIONAL, lateNonCriticalExtension OCTET STRING
OPTIONAL, nonCriticalExtension SEQUENCE { }
OPTIONAL
}

UEAssistanceInformation field descriptions powerPrefIndication Value lowPowerConsumption indicates that the UE prefers a configuration that is primarily optimised for power saving. Otherwise the value is set to normal.

SCGRLFCause This field is used to indicate the cause of the last radio link failure that was detected. on PScell.

- 29 -[186] FIGs. 23 to 25 are diagrams illustrating exemplary MAC CEs according to an embodiment of the present invention.
[187] According to an embodiment, the RLF indicator having an RLF cause value may be a newly defined message or MAC CE. The RLF indicator having the RLF
cause value may be added as a new information element within an existing message.
A cell identifier may be identified along with the RLF indicator. If the RLF
indicator includes no cell identifier, this may indicate that RLF is occurring on the SeNB (e.g.
PUCCH cell of the SeNB). For example, a new MAC CE may be defined as shown in FIGs. 23 and 24.
[188] Referring to FIG. 23, C7 of the new MAC CE may indicate the RLF caused by RACH failure in the PUCCH-carrying cell of the SeNB, and C6 may indicate the RLF
caused by RLC failure in the PUCCH-carrying cell of the SeNB. The other bits may be in a reserved state.
[189] Referring to FIG. 24, a MAC CE may be defined to indicate a cause of the RLF
such as RACH failure and RLC failure. At this time, C7 of the MAC CE may indicate the RLF caused by Li out-of-sync in the PUCCH-carrying cell of the SeNB, C6 the RLF caused by RACH failure in the PUCCH-carrying cell of the SeNB, and C5 the RLF caused by RLC failure in the PUCCH-carrying cell of the SeNB. The other bits may be in a reserved state.
[190] According to the embodiment of FIG. 25, the RLF may be notified to the MeNB using an existing message (VarRLF-Report). According to an embodiment, `failedPCellId' may be set to identify an MeNB or SeNB on which RLF has occurred.
For example, the failedPCellId may be set to the PCell (of MeNB) or the pSCell (as a special cell having PUCCH in the SeNB). The rlf:Cause may be set to one of the aforementioned causes. The availability of the store RLF report may be determined by the MeNB which requests to the UE for transmitting the stored RLF report. In an embodiment, the availability indicator may indicate that the RLF report is related to the SeNB. For example, the availability may be indicated by transmitting a MAC
CE.
At this time, C7, C6, and C5 of the MAC CE may carry the aforementioned information. That is, C7 indicates the RLF caused by the Li out-of-sync in the PUCCH-carrying cell of the SeNB, C6 the RLF caused by the RACH failure in the

- 30 -PUCCH-carrying cell of the SeNB, and C7 the RLF caused by the RLC failure in the PUCCH-carrying cell of the SeNB. Although not shown, C7 may indicate the RLF
caused by the RACH failure in the PUCCH-carrying cell, and C6 may indicate the RLF caused by the RLF failure in the PUCCH-carrying cell of the SeNB.
Meanwhile, Co may indicate the availability of the RLF report. The eNB may use the legacy HE
Information Request and UE Information Response messages to acquire the report from the UE.
[1911 In another embodiment, a Physical (PHY) layer signal may be used to indicate the RLF on the SeNB and a cause thereof. For example, an RACH code may be preconfigured to specify the RLF cause.
[192] In an embodiment, a timer T310 for detecting RLF may be set to a different value per eNB.
[193] One or more RLF causes may be applied to one or more cells served by the SeNB. For example, since the RACH is likely to be supported in only one cell of the SeNB (called pSCell), the RACH failure may occur in the pSCell.
[194] According to an embodiment, the RLF message may include link identity information of the link on which the RLF has occurred. The link identity information may indicate whether the link is an MeNB link or an SeNB link. For example, the link identity information may be a 1-bit identifier in the dual connectivity. The link identity information may be extended for use in a multi-BS system.
[195] FIG. 9 is a signal flow diagram illustrating an early RLF indication procedure in a single connectivity system.
[196] Referring to FIG. 9, the early RLF indication before expiry of the RLF
timer may be used in the single connectivity system. A UE 910 expects RLF on a first link at step 951. For example, if the RLF timer T310 starts, it may be possible to expect RLF. Also, if a predetermined time elapses after the start of the RLF timer T310, it may be possible to expect RLF. Detailed descriptions thereof have been made above so as to be omitted herein.
[197] Then the UE 910 sends to a source eNB 920 an RLF expected message at step 952. If the source eNB 920 receives the RLF expected message successfully, it may prepare a target cell 930 early enough for reestablishment. Since the operations at

-31-steps 952 to 955 are similar to those described in the above embodiments, detailed descriptions thereof are omitted herein.
[198] Since the new cell prepared for the reestablishment (having the UE
context) has a high probability of being connected(*i_11 ,c1) when the UE 910 attempts the reestablishment procedure to the new cell because of the real occurrence of the RLF at step 956, the probability of successful reestablishment increases.
[199] The RLF handling method for use in the dual or single connectivity has been described above.
[200] A description is made of the switching between the single connectivity and the dual connectivity in detail hereinafter.
[201] FIG. 10 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to an embodiment of the present invention.
[202] Referring to FIG. 10, a UE 1010 may be connected to an MeNB 1020 and an SeNB 1030 in the dual connectivity mode. It may operate to switch to the single connectivity with the MeNB 1020. The UE 1010 determines switching to the single connectivity mode at step 1051 and sends the MeNB 1020 a single connectivity switching request message to indicate the need of switching to the single connectivity mode at step 1052. At this time, the single connectivity switching request message may include the identifier (ID) of the MeNB 1020.
[203] Then the MeNB 1020 may fetch the information on the flows (e.g. QoS) serviced by the SeNB 1030. In detail, the MeNB 1020 determines to acquire the information on the flows of another eNB (e.g. SeNB 1030) which the MeNB does not have at step 1053. Next, the MeNB 1020 sends the SeNB 1030 a message requesting for the information on the flow serviced by the SeNB 1030 at step 1054. In reply, the SeNB 1030 sends the MeNB 1020 a response message including the information on the flow which it serves at step 1055.
[204] The MeNB 1020 controls to accept the flows based on the received flow information at step 1056. If it cannot support the flows, the MeNB 1020 downgrades or terminates the flows at step 1056-1.

- 32 -[205] In order to check the necessity of serving the UE 1010 in the single connectivity mode, the MeNB sends the SeNB 1030 a single connectivity indication message at step 1057. Upon receipt of this message, the SeNB deletes the UE
context at step 1058. According to an embodiment, the SeNB 1030 may delete the UE
context after a predetermined time elapses.
[206] Next, the MeNB 1020 triggers switching to the single connectivity mode at step 1059 and sends the UE 1010 a single connectivity switching response message along with the changed flow information in order for the UE to operate in the single connectivity mode at step 1060. Then the UE 1010 deletes the context of SeNB

and transmits the changed flows to the MeNB 1020 to switch to the single connectivity mode with the MeNB at step 1061.
[207] FIG. 11 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to another embodiment of the present invention.
[208] Referring to FIG. 11, a UE 1110 may be currently connected to an MeNB

and an SeNB 1130 in the dual connectivity mode. Then the UE may operate to switch to the single connectivity with the SeNB 1130. The UE 1110 determines switching to the single connectivity mode at step 1151 and sends a single connectivity switching request message to indicate the need of switching to the single connectivity mode at step 1152. At this time, the single connectivity switching request message may include the identifier (ID) of the SeNB 1130.
[209] Then the MeNB 1120 determines to provide the SeNB 1130 with the information on the flows (e.g. QoS) which it is servicing at step 1153. The MeNB
1120 sends the SeNB 1130 a single connectivity switching request message at step 1154. At this time, the MeNB 1120 may transmit the information on the flows which it is servicing.
[210] The SeNB 1130 controls to accept the flows based on the received flow information at step 1155. If it cannot support the flows, the SeNB 1130 downgrades or terminates the flows at step 1156.
[211] Afterward, in order to confirm the necessity of serving the UE 1110 in the single connectivity mode, the SeNB 1130 sends the MeNB 1120 a single connectivity

-33-response message at step 1157. At this time, the changed flow information may be transmitted, along with the single connectivity response message, to the MeNB
1120.
[212] Upon receipt of this, the MeNB deletes the UE context at step 1158.
According to an embodiment, the MeNB may delete the UE context after a predetermined time elapses.
[213] Next, the MeNB 1120 triggers switching to the single connectivity at step 1159 and sends the UE 1110 a single connectivity switching response message along with the changed flow information in order for the UE 1110 to operate to switch to the single connectivity at step 1160. Then the UE 1110 deletes the context of the MeNB
1120 at step 1161 and sends the SeNB 1130 the changed flows in order to switch to the single connectivity mode with the SeNB 1130.
[214] FIG. 12 is a signal flow diagram illustrating a method for switching from the dual connectivity to the signal connectivity according to another embodiment of the present invention.
[215] Referring to FIG. 12, a UE 1210 may be connected to an MeNB 1220 and an SeNB 1230 in the dual connectivity mode. Then the UE may operate to switch to the single connectivity with the MeNB 1220. The MeNB 1220 makes a switching decision for the UE 1210 to switch to the single connectivity mode therewith at step 1251.
[216] Afterward, the MeNB 1220 may fetch the information on the flows (e.g.
QoS) serviced by the SeNB 1230. In detail, the MeNB 1220 determines to acquire the information on the flows of another eNB (e.g. SeNB 1230) which the MeNB does not have at step 1252. Then the MeNB 1220 sends the SeNB 1230 a flow information request message to request for the information on the flows serviced by the SeNB
1230. The SeNB 1230 sends the MeNB 1220 an information response message including the information on the flows which it serves at step 1254.
[217] The MeNB 1220 controls to accept the flows based on the received flow information at step 1255. If it cannot support the flows, the MeNB 1220 downgrades or terminates the flows at step 1256.
[218] In order to check the necessity of serving the UE 1210 in the single connectivity mode, the MeNB 1220 sends the SeNB 1230 a single connectivity

- 34 -indication message at step 1257. Upon receipt of this message, the SeNB 1230 deletes the UE context at step 1258. According to an embodiment, the SeNB 1230 may delete the UE context after a predetermined time elapses.
[219] Next, the MeNB 1220 triggers switching to the single connectivity mode at step 1259 and sends the UE 1210 a single connectivity switching response message along with the changed flow information in order for the UE to operate in the single connectivity mode at step 1260. Then the UE 1210 deletes the context of SeNB

and transmits the changed flows to the MeNB 1220 to switch to the single connectivity mode with the MeNB at step 1261.
[220] FIG. 13 is a signal flow diagram illustrating a method for switching from the dual connectivity to the single connectivity according to another embodiment of the present invention.
[221] Referring to FIG. 13, the UE 1310 may be currently connected to an MeNB
1120 and an SeNB 1130 in the dual connectivity mode. Then the UE may operate to switch to the single connectivity with the SeNB 1330. The UE 1310 determines switching to the single connectivity mode with the SeNB 1330 at step 1351.
[222] Then the MeNB 1320 determines to provide the SeNB 1330 with the information on the flows (e.g. QoS) which it is servicing at step 1352. The MeNB
1320 sends the SeNB 1330 a single connectivity switching request message at step 1353. At this time, the MeNB 1320 may transmit the information on the flows which it is servicing.
[223] The SeNB 1330 controls to accept the flows based on the received flow information at step 1354. If it cannot support the flows, the SeNB 1330 downgrades or terminates the flows at step 1355.
[224] Afterward, in order to confirm the necessity of serving the UE 1310 in the single connectivity mode, the SeNB 1330 sends the MeNB 1320 a single connectivity response message at step 1357. At this time, the changed flow information may be transmitted, along with the single connectivity response message, to the MeNB
1320.
[225] Next, the MeNB 1320 triggers switching to the single connectivity at step 1357 and sends the UE 1310 a single connectivity switching response message including

- 35 -the ID of the SeNB 1320 and the changed flow information in order for the UE

to operate to switch to the single connectivity at step 1358.
[226] Afterward, the MeNB 1320 deletes the UE context at step 1359. According to an embodiment, the MeNB 1320 may delete the UE context after a predetermined time elapses.
[227] The UE 1310 deletes the context of the MeNB 1320 at step 1360 and transmits the changed flows to the SeNB 1330 to switch to the single connectivity mode with the SeNB 1330.
[228] FIG. 14 is a signal flow diagram illustrating a method for switching from the single connectivity to the dual connectivity according to an embodiment of the present invention.
[229] Referring to FIG. 14, the UE 1410 may be connected to an MeNB 1420 in the single connectivity mode. The UE 1410 determines to switch from the single connectivity to the dual connectivity at step 1451 and sends the MeNB 1420 a dual connectivity switching request message indicating the necessity of switching to the dual connectivity mode at step 1452. At this time, the dual connectivity switching request message may include an identifier of the SeNB 1430.
[230] The MeNB 1420 identifies the dual connectivity with the SeNB 1430 at step 1453. For example, the dual connectivity identification may be performed by identifying a flow acceptance control and a pair of the MeNB and SeNB for supporting the dual connectivity, but it is not limited thereto.
[231] Afterward, the MeNB 1420 sends the SeNB 1430 a dual connectivity request message. At this time, the dual connectivity request message includes the flow information. The MeNB may control to accept the flows based on the received flow information at step 1455. If it cannot support the flows, the SeNB 1430 downgrades or terminates the flows at step 1456.
[232] The SeNB 1430 sends the MeNB 1420 a dual connectivity response message to confirm the acceptance of the dual connectivity at step 1457. Then the MeNB

triggers switching to the dual connectivity at step 1458. The MeNB sends the UE
1410 a dual connectivity switching response message in order for the UE 1410 to

- 36 -switch to the dual connectivity at step 1459. Next, the UE 1410 switches to the dual connectivity mode with the MeNB 1420 and SeNB 1430.
[233] FIG. 15 is a signal flow diagram illustrating a method for switching from the single connectivity to the dual connectivity according to another embodiment of the present invention.
[234] Referring to FIG. 15, the UE 1510 may be connected to an MeNB 1520 in the single connectivity mode. The MeNB 1520 determines to switch from the single connectivity to the dual connectivity at step 1551. At this time, the MeNB
1520 may identify the dual connectivity with the SeNB 1530. For example, the dual connectivity identification may be performed by identifying a flow acceptance control and a pair of the MeNB and SeNB for supporting the dual connectivity, but it is not limited thereto.
[235] Afterward, the MeNB 1520 sends the SeNB 1530 a dual connectivity request message at step 1552. At this time, the dual connectivity request message includes the flow information.
[236] The SeNB 1530 may control to accept the flows based on the received flow information at step 1553. If it cannot support the flows, the SeNB 1530 downgrades or terminates the flows at step 1555.
[237] The SeNB 1530 sends the MeNB 1520 a dual connectivity response message to confirm the acceptance of the dual connectivity at step 1554. Then the MeNB

triggers switching to the dual connectivity at step 1556. The MeNB 1520 sends the UE 1510 a dual connectivity switching response message in order for the UE
1510 to switch to the dual connectivity at step 1557. Next, the UE 1510 switches to the dual connectivity mode with the MeNB 1420 and SeNB 1430 at step 1558. Then the UE
1510 sends the MeNB 1520 a dual connectivity response message.
[238] FIG. 16 is a block diagram illustrating the UE according to an embodiment of the present invention.
[239] Referring to FIG. 16, a control unit 1610 controls the UE to perform at least one of the operations described in the above embodiments. For example, the control unit 1610 may control transmitting an RLF expected message related to a first link to an eNB and an RLF message related to the first link through a second link to the eNB

- 37 -through a second link and receiving a handover command for handover to the target cell selected by the eNB based on the RLF expected message.
[240] A communication unit 1620 transmits/receives signals through at least one of the operations described in the above embodiments. For example, the communication unit 1620 may transmit the RLF message and RLF expected message to the MeNB.
[241] FIG. 17 is a block diagram illustrating the eNB according to an embodiment of the present invention.
[242] Referring to FIG. 17, a control unit 1710 controls the eNB to operate at least one of the operations described in the above embodiments. For example, the control unit 1710 may control receiving an RLF expected message related to a first link, searching for a target cell for handover of the UE based on the RLF expected message, selecting a found cell as the target cell, receiving an RLF message related to the first link from the UE through a second link, and transmitting a handover command to the UE in order for the UE to perform handover to the selected target cell.
[243] The communication unit 1720 transmits/receives signals through at least one of the operations described in the above embodiments. For example, the communication unit 1720 may receive the RLF message and RLF expected message from the UE.
[244] Although various embodiments of the present invention have been described using specific terms, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense in order to help understand the present invention. It is obvious to those skilled in the art that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention.
[245] Although preferred embodiments of the invention have been described using specific terms, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense in order to help understand the present invention. It is obvious to those skilled in the art that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention.

- 38 -

Claims (20)

[CLAIMS]
1. [Claim 1]
   A multi-base station connectivity communication method of a base station, the method comprising:
   receiving a Radio Link Failure (RLF) expected message related to a first link from a terminal;
   searching for a target cell for handover of the terminal based on the RLF
   expected message;
   selecting, when a target cell exists as a search result, the target cell;
   receiving an RLF message related to the first link from the terminal through a second link; and transmitting a handover command to the terminal for handover to the selected target cell.
2. [Claim 2]
   The method of claim 1, further comprising:
   selecting, when no target sell exists as a search result, a base station for single connectivity to the second link; and transmitting a single connectivity switching message to the terminal for single connectivity with the selected base station.
3. [Claim 3]
   A multi-base station connectivity communication method of a terminal, the method comprising:
   transmitting a Radio Link Failure (RLF) expected message related to a first link to a base station;
   transmitting an RLF message related to the first link to the base station through a second link; and receiving a handover command for handover to the selected target cell based on the RLF expected message.
4. [Claim 4]
   The method of claim 3, further comprising:
   
   receiving a signal connectivity switching message from the base station through the second link; and switching to a single connectivity mode a single base station based on the received signal connectivity switching message.
5. [Claim 5]
   A multi-base station connectivity communication method of a base station, the method comprising:
   receiving a Radio Link Failure (RLF) message related to a first link from a terminal through a second link;
   searching for a target cell for handover of the terminal based on the RLF
   message;
   selecting, when a target cell exists as a search result, the target cell; and transmitting a handover command to the terminal for handover to the selected target cell.
6. [Claim 6]
   The method of claim 5, further comprising:
   selecting, when no target sell exists as a search result, the base station for single connectivity through the second link; and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
7. [Claim 7]
   A multi-base station connectivity communication method of a terminal, the method comprising:
   transmitting a Radio Link Failure (RLF) message related to a first link to a base station through a second link; and receiving a handover command for handover to a target cell selected by the base station based on the RLF message.
8. [Claim 8]
   The method of claim 7, further comprising:
   
   receiving a single connectivity switching message from the base station through the second link; and switching to a single connectivity mode with a single base station based on the received single connectivity switching message.
9. [Claim 9]
   A multi-base station connectivity communication method of a base station, the method comprising:
   receiving a Radio Link Failure (RLF) message related to a first link from a terminal through a second link;
   selecting a base station for single connectivity through the second link based on the RLF message; and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
10. [Claim 10]
    A multi-base station connectivity communication method of a terminal, the method comprising:
    transmitting a Radio Link Failure (RLF) message related to a first link to a base station through a second link;
    receiving a single connectivity switching message- from the base station through the second link; and switching to a single connectivity mode with a single base station based on the received signal connectivity switching message.
11. [Claim 11]
    A base station of performing a multi-base station communication, the base station comprising:
    a communication unit which communicates with other base stations and terminals; and a control unit which controls receiving a Radio Link Failure (RLF) expected message related to a first link from a terminal, searching for a target cell for handover of the terminal based on the RLF expected message, selecting, when a target cell exists as a search result, the target cell, receiving an RLF message related to the first link from the terminal through a second link, and transmitting a handover command to the terminal for handover to the selected target cell.
12. [Claim 12]
    The base station of claim 11, wherein the control unit controls selecting, when no target sell exists as a search result, a base station for single connectivity to the second link and transmitting a single connectivity switching message to the terminal for single connectivity with the selected base station.
13. [Claim 13]
    A terminal of performing a multi-base station communication, the terminal comprising:
    a communication unit which communicates with base stations; and a control unit which controls transmitting a Radio Link Failure (RLF) expected message related to a first link to a base station, transmitting an RLF message related to the first link to the base station through a second link, and receiving a handover command for handover to the selected target cell based on the RLF
    expected message.
14. [Claim 14]
    The terminal of claim 13, wherein the control unit controls receiving a signal connectivity switching message from the base station through the second link and switching to a single connectivity mode a single base station based on the received signal connectivity switching message.
15. [Claim 15]
    A base station of performing multi-base station connectivity communication, the base station comprising:
    a communication unit which communicates with other base stations and terminals; and a control unit which controls receiving a Radio Link Failure (RLF) message related to a first link from a terminal through a second link, searching for a target cell for handover of the terminal based on the RLF message, selecting, when a target cell exists as a search result, the target cell, and transmitting a handover command to the terminal for handover to the selected target cell.
16. [Claim 16]
    The base station of claim 15, wherein the control unit controls selecting, when no target sell exists as a search result, the base station for single connectivity through the second link and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
17. [Claim 17]
    A terminal of performing multi-base station communication, the terminal comprising:
    a communication unit which communicates with base stations; and a control unit which controls transmitting a Radio Link Failure (RLF) message related to a first link to a base station through a second link and receiving a handover command for handover to a target cell selected by the base station based on the RLF
    message.
18. [Claim 18]
    The terminal of claim 17, wherein the control unit controls receiving a single connectivity switching message from the base station through the second link and switching to a single connectivity mode with a single base station based on the received single connectivity switching message.
19. [Claim 19]
    A base station of performing multi-base station communication, the base station comprising:
    a communication unit which communicates with other base stations and terminals; and a control unit which controls receiving a Radio Link Failure (RLF) message related to a first link from a terminal through a second link, selecting a base station for single connectivity through the second link based on the RLF message, and transmitting a single connectivity switching message to the terminal for the single connectivity with the selected base station.
20. [Claim 20]
    A terminal of performing multi-base station communication, the terminal comprising:
    a communication unit which communicates with base stations; and a control unit which controls transmitting a Radio Link Failure (RLF) message related to a first link to a base station through a second link, receiving a single connectivity switching message from the base station through the second link, and switching to a single connectivity mode with a single base station based on the received signal connectivity switching message.