Method and apparatus for forwarding data for small cell in wireless communication system

Abstract

A method and apparatus for forwarding data in a wireless communication system is provided. A small cell receives an indication which indicates stopping serving a small cell service from a macro eNodeB (eNB). Upon receiving the indication, the small cell starts to forward data to the macro eNB together with a sequence number (SN) status transfer message. The indication may be received via a form of a message or a form of an information element (IE) in a message.

Claims

1. A method for forwarding, by a secondary evolved NodeB (SeNB) which controls a small cell in dual connectivity, data in a wireless communication system, the method comprising: receiving, from a macro eNodeB (eNB), a SeNB release request message that includes an indication which indicates stopping serving a small cell service; and upon receiving the SeNB release message including the indication, starting to forward data to the macro eNB, wherein the indication in the SeNB release message is a downlink (DL) general packet radio services (GPRS) tunneling protocol (GTP) tunnel endpoint information element (IE) or an uplink (UL) GTP tunnel endpoint IE.

2. The method of claim 1, further comprising: transmitting a sequence number (SN) status transfer message with the data to the macro eNB.

3. The method of claim 1, further comprising: receiving an X2 end marker, generated by the macro eNB, from the macro eNB.

4. The method of claim 1, further comprising: receiving a UE X2 context release message from the macro eNB.

5. The method of claim 4, further comprising: releasing radio and control plane related resources associated to a UE context, upon receiving the UE X2 context release message.

6. The method of claim 1, wherein the SeNB release message is received before the macro eNB transmits a service request message to a second eNB.

7. The method of claim 1, wherein the SeNB release message is received after the macro eNB transmits a service request message to a second eNB.

8. A method for transmitting, by a macro eNodeB (eNB) in dual connectivity, an indication in a wireless communication system, the method comprising: transmitting, to a secondary eNB (SeNB) which controls a small cell in dual connectivity, a SeNB release message that includes an indication which indicates stopping serving a small cell service; and receiving forwarded data from the SeNB, wherein the indication in the SeNB release message is a downlink (DL) general packet radio services (GPRS) tunneling protocol (GTP) tunnel endpoint or an uplink (UL) GTP tunnel endpoint.

9. The method of claim 8, further comprising: generating an X2 end marker; and transmitting the generated X2 end marker to the SeNB.

10. The method of claim 8, further comprising: transmitting a UE X2 context release message to the SeNB.

11. The method of claim 8, further comprising: receiving a sequence number (SN) status transfer message from the SeNB.

12. The method of claim 8, wherein the SeNB release message is transmitted before transmitting a service request message to a second eNB.

13. The method of claim 8, wherein the SeNB release message is transmitting after transmitting a service request message to a second eNB.

14. The method of claim 8, further comprising: starting to buffer data packets received from a serving gateway (S-GW) right after the indication is transmitted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows LTE system architecture.

(2) FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and a typical EPC.

(3) FIG. 3 shows a block diagram of a user plane protocol stack and a control plane protocol stack of an LTE system.

(4) FIG. 4 shows an example of a physical channel structure.

(5) FIGS. 5 and 6 show an intra-MME/S-GW handover procedure.

(6) FIG. 7 shows deployment scenarios of small cells with/without macro coverage.

(7) FIG. 8 shows an example of practical deployment of a small cell.

(8) FIG. 9 shows another example of practical deployment of a small cell.

(9) FIG. 10 shows another example of practical deployment of a small cell.

(10) FIG. 11 shows another example of practical deployment of a small cell.

(11) FIG. 12 and FIG. 13 show an example of a data forwarding problem according to practical deployment of a small cell.

(12) FIG. 14 shows another example of a data forwarding problem according to practical deployment of a small cell.

(13) FIG. 15 shows another example of a data forwarding problem according to practical deployment of a small cell.

(14) FIG. 16 and FIG. 17 show an example of a method for forwarding data according to an embodiment of the present invention.

(15) FIG. 18 shows another example of a method for forwarding data according to an embodiment of the present invention.

(16) FIG. 19 shows another example of a method for forwarding data according to an embodiment of the present invention.

(17) FIG. 20 shows another example of a method for forwarding data according to an embodiment of the present invention.

(18) FIG. 21 shows a wireless communication system to implement an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(19) The technology described below can be used in various wireless communication systems such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), etc. The CDMA can be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA-2000. The TDMA can be implemented with a radio technology such as global system for mobile communications (GSM)/general packet ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc. IEEE 802.16m is an evolution of IEEE 802.16e, and provides backward compatibility with an IEEE 802.16-based system. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in downlink and uses the SC-FDMA in uplink. LTE-advance (LTE-A) is an evolution of the 3GPP LTE.

(20) For clarity, the following description will focus on the LTE-A. However, technical features of the present invention are not limited thereto.

(21) Handover (HO) is described. It may be referred to Section 10.1.2.1 of 3GPP TS 36.300 V11.4.0 (2012-12).

(22) The intra E-UTRAN HO of a UE in RRC_CONNECTED state is a UE-assisted network-controlled HO, with HO preparation signaling in E-UTRAN: Part of the HO command comes from the target eNB and is transparently forwarded to the UE by the source eNB; To prepare the HO, the source eNB passes all necessary information to the target eNB (e.g., E-UTRAN radio access bearer (E-RAB) attributes and RRC context): When carrier aggregation (CA) is configured and to enable secondary cell (SCell) selection in the target eNB, the source eNB can provide in decreasing order of radio quality a list of the best cells and optionally measurement result of the cells. Both the source eNB and UE keep some context (e.g., C-RNTI) to enable the return of the UE in case of HO failure; UE accesses the target cell via RACH following a contention-free procedure using a dedicated RACH preamble or following a contention-based procedure if dedicated RACH preambles are not available: the UE uses the dedicated preamble until the handover procedure is finished (successfully or unsuccessfully); If the RACH procedure towards the target cell is not successful within a certain time, the UE initiates radio link failure recovery using the best cell; No robust header compression (ROHC) context is transferred at handover.

(23) The preparation and execution phase of the HO procedure is performed without EPC involvement, i.e., preparation messages are directly exchanged between the eNBs. The release of the resources at the source side during the HO completion phase is triggered by the eNB. In case an RN is involved, its donor eNB (DeNB) relays the appropriate S1 messages between the RN and the MME (S1-based handover) and X2 messages between the RN and target eNB (X2-based handover); the DeNB is explicitly aware of a UE attached to the RN due to the S1 proxy and X2 proxy functionality.

(24) FIGS. 5 and 6 show an intra-MME/S-GW handover procedure.

(25) 0. The UE context within the source eNB contains information regarding roaming restrictions which were provided either at connection establishment or at the last TA update.

(26) 1. The source eNB configures the UE measurement procedures according to the area restriction information. Measurements provided by the source eNB may assist the function controlling the UE's connection mobility.

(27) 2. The UE is triggered to send measurement reports by the rules set by i.e., system information, specification, etc.

(28) 3. The source eNB makes decision based on measurement reports and radio resource management (RRM) information to hand off the UE.

(29) 4. The source eNB issues a handover request message to the target eNB passing necessary information to prepare the HO at the target side (UE X2 signalling context reference at source eNB, UE S1 EPC signalling context reference, target cell identifier (ID), K.sub.eNB*, RRC context including the cell radio network temporary identifier (C-RNTI) of the UE in the source eNB, AS-configuration, E-RAB context and physical layer ID of the source cell+short MAC-I for possible radio link failure (RLF) recovery). UE X2/UE S1 signalling references enable the target eNB to address the source eNB and the EPC. The E-RAB context includes necessary radio network layer (RNL) and transport network layer (TNL) addressing information, and quality of service (QoS) profiles of the E-RABs.

(30) 5. Admission Control may be performed by the target eNB dependent on the received E-RAB QoS information to increase the likelihood of a successful HO, if the resources can be granted by target eNB. The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a RACH preamble. The AS-configuration to be used in the target cell can either be specified independently (i.e., an establishment) or as a delta compared to the AS-configuration used in the source cell (i.e., a reconfiguration).

(31) 6. The target eNB prepares HO with L1/L2 and sends the handover request acknowledge to the source eNB. The handover request acknowledge message includes a transparent container to be sent to the UE as an RRC message to perform the handover. The container includes a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms, may include a dedicated RACH preamble, and possibly some other parameters, i.e., access parameters, SIBs, etc. The handover request acknowledge message may also include RNL/TNL information for the forwarding tunnels, if necessary.

(32) As soon as the source eNB receives the handover request acknowledge, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated.

(33) Steps 7 to 16 in FIGS. 5 and 6 provide means to avoid data loss during HO.

(34) 7. The target eNB generates the RRC message to perform the handover, i.e., RRCConnectionReconfiguration message including the mobilityControlInformation, to be sent by the source eNB towards the UE. The source eNB performs the necessary integrity protection and ciphering of the message. The UE receives the RRCConnectionReconfiguration message with necessary parameters (i.e. new C-RNTI, target eNB security algorithm identifiers, and optionally dedicated RACH preamble, target eNB SIBs, etc.) and is commanded by the source eNB to perform the HO. The UE does not need to delay the handover execution for delivering the HARQ/ARQ responses to source eNB.

(35) 8. The source eNB sends the sequence number (SN) status transfer message to the target eNB to convey the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of E-RABs for which PDCP status preservation applies (i.e., for RLC AM). The uplink PDCP SN receiver status includes at least the PDCP SN of the first missing UL service data unit (SDU) and may include a bit map of the receive status of the out of sequence UL SDUs that the UE needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB shall assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the E-RABs of the UE shall be treated with PDCP status preservation.

(36) 9. After receiving the RRCConnectionReconfiguration message including the mobilityControlInformation, UE performs synchronization to target eNB and accesses the target cell via RACH, following a contention-free procedure if a dedicated RACH preamble was indicated in the mobilityControlInformation, or following a contention-based procedure if no dedicated preamble was indicated. UE derives target eNB specific keys and configures the selected security algorithms to be used in the target cell.

(37) 10. The target eNB responds with UL allocation and timing advance.

(38) 11. When the UE has successfully accessed the target cell, the UE sends the RRCConnectionReconfigurationComplete message (C-RNTI) to confirm the handover, along with an uplink buffer status report, whenever possible, to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB verifies the C-RNTI sent in the RRCConnectionReconfigurationComplete message. The target eNB can now begin sending data to the UE.

(39) 12. The target eNB sends a path switch request message to MME to inform that the UE has changed cell.

(40) 13. The MME sends a modify bearer request message to the serving gateway.

(41) 14. The serving gateway switches the downlink data path to the target side. The Serving gateway sends one or more end marker packets on the old path to the source eNB and then can release any U-plane/TNL resources towards the source eNB.

(42) 15. The serving gateway sends a modify bearer response message to MME.

(43) 16. The MME confirms the path switch request message with the path switch request acknowledge message.

(44) 17. By sending the UE context release message, the target eNB informs success of HO to source eNB and triggers the release of resources by the source eNB. The target eNB sends this message after the path switch request acknowledge message is received from the MME.

(45) 18. Upon reception of the UE context release message, the source eNB can release radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue.

(46) Small cell enhancement is described. It may be referred to 3GPP TR 36.932 V12.0.0 (2012-12).

(47) FIG. 7 shows deployment scenarios of small cells with/without macro coverage. Small cell enhancement should target both with and without macro coverage, both outdoor and indoor small cell deployments and both ideal and non-ideal backhaul. Both sparse and dense small cell deployments should be considered.

(48) Referring to FIG. 7, small cell enhancement should target the deployment scenario in which small cell nodes are deployed under the coverage of one or more than one overlaid E-UTRAN macro-cell layer(s) in order to boost the capacity of already deployed cellular network. Two scenarios can be considered: where the UE is in coverage of both the macro cell and the small cell simultaneously where the UE is not in coverage of both the macro cell and the small cell simultaneously.

(49) Also, the deployment scenario where small cell nodes are not deployed under the coverage of one or more overlaid E-UTRAN macro-cell layer(s) may be considered.

(50) Small cell enhancement should target both outdoor and indoor small cell deployments. The small cell nodes could be deployed indoors or outdoors, and in either case could provide service to indoor or outdoor UEs.

(51) For indoor UE, only low UE speed (0-3 km/h) is targeted. For outdoor, not only low UE speed, but also medium UE speed (up to 30 km/h and potentially higher speeds) is targeted.

(52) Both throughput and mobility/connectivity shall be used as performance metric for both low and medium mobility. Cell edge performance (e.g., 5%-tile cumulative distribution function (CDF) point for user throughput) and power efficiency (of both network and UE) are also used as metrics.

(53) Both ideal backhaul (i.e., very high throughput and very low latency backhaul such as dedicated point-to-point connection using optical fiber, line-of-sight (LOS) microwave) and non-ideal backhaul (i.e., typical backhaul widely used in the market such as xDSL, non-LOS (NLOS) microwave, and other backhauls like relaying) should be studied. The performance-cost trade-off should be taken into account.

(54) A categorization of non-ideal backhaul based on operator inputs is listed in Table 1 below.

(55) TABLE-US-00001 TABLE 1 Backhaul Latency Priority (1 is the Technology (One way) Throughput highest) Fiber Access 1 10-30 ms 10M-10 Gbps 1 Fiber Access 2 5-10 ms 100-1000 Mbps 2 DSL Access 15-60 ms 10-100 Mbps 1 Cable 25-35 ms 10-100 Mbps 2 Wireless Backhaul 5-35 ms 10 Mbps-100 Mbps 1 typical, maybe up to Gbps range

(56) A categorization of good to ideal backhaul based on operator inputs is listed in Table 2 below.

(57) TABLE-US-00002 TABLE 2 Backhaul Latency Priority (1 is the Technology (One way) Throughput highest) Fiber 2-5 ms 50M-10 Gbps 1

(58) For interfaces between macro and small cell, as well as between small cells, the studies should first identify which kind of information is needed or beneficial to be exchanged between nodes in order to get the desired improvements before the actual type of interface is determined. And if direct interface should be assumed between macro and small cell, as well as between small cell and small cell, X2 interface can be used as a starting point.

(59) Small cell enhancement should consider sparse and dense small cell deployments. In some scenarios (e.g., hotspot indoor/outdoor places, etc.), single or a few small cell node(s) are sparsely deployed, e.g., to cover the hotspot(s). Meanwhile, in some scenarios (e.g., dense urban, large shopping mall, etc), a lot of small cell nodes are densely deployed to support huge traffic over a relatively wide area covered by the small cell nodes. The coverage of the small cell layer is generally discontinuous between different hotspot areas. Each hotspot area can be covered by a group of small cells, i.e., a small cell cluster.

(60) Furthermore, smooth future extension/scalability (e.g., from sparse to dense, from small-area dense to large-area dense, or from normal-dense to super-dense) should be considered. For mobility/connectivity performance, both sparse and dense deployments should be considered with equal priority.

(61) Both synchronized and un-synchronized scenarios should be considered between small cells as well as between small cells and macro cell(s). For specific operations, e.g., interference coordination, carrier aggregation and inter-eNB coordinated multipoint (COMP), small cell enhancement can benefit from synchronized deployments with respect to small cell search/measurements and interference/resource management. Therefore time synchronized deployments of small cell clusters are prioritized in the study and new means to achieve such synchronization shall be considered.

(62) Small cell enhancement should address the deployment scenario in which different frequency bands are separately assigned to macro layer and small cell layer, respectively, where F1 and F2 in FIG. 7 correspond to different carriers in different frequency bands.

(63) Small cell enhancement should be applicable to all existing and as well as future cellular bands, with special focus on higher frequency bands, e.g., the 3.5 GHz band, to enjoy the more available spectrum and wider bandwidth.

(64) Small cell enhancement should also take into account the possibility for frequency bands that, at least locally, are only used for small cell deployments.

(65) Co-channel deployment scenarios between macro layer and small cell layer should be considered as well.

(66) Some example spectrum configurations are: Carrier aggregation on the macro layer with bands X and Y, and only band X on the small cell layer Small cells supporting carrier aggregation bands that are co-channel with the macro layer Small cells supporting carrier aggregation bands that are not co-channel with the macro layer

(67) One potential co-channel deployment scenario is dense outdoor co-channel small cells deployment, considering low mobility UEs and non ideal backhaul. All small cells are under the Macro coverage.

(68) Small cell enhancement should be supported irrespective of duplex schemes (FDD/TDD) for the frequency bands for macro layer and small cell layer. Air interface and solutions for small cell enhancement should be band-independent, and aggregated bandwidth per small cell should be no more than 100 MHz, at least for 3GPP rel-12.

(69) In a small cell deployment, it is likely that the traffic is fluctuating greatly since the number of users per small cell node is typically not so large due to small coverage.

(70) In a small cell deployment, it is likely that the user distribution is very fluctuating between the small cell nodes. It is also expected that the traffic could be highly asymmetrical, either downlink or uplink centric.

(71) Both uniform and non-uniform traffic load distribution in time-domain and spatial-domain should be considered. Non-full buffer and full buffer traffic are both included, and non-full buffer traffic is prioritized to verify the practical cases.

(72) Backward compatibility, i.e., the possibility for legacy (pre-rel-12) UEs to access a small-cell node/carrier, is desirable for small cell deployments.

(73) The introduction of non-backwards compatible features should be justified by sufficient gains.

(74) For one feature of small cell enhancement, dual connectivity has been discussed. Dual connectivity is an operation where a given UE consumes radio resources provided by at least two different network points (master eNB (MeNB) and secondary eNB (SeNB)) connected with non-ideal backhaul while in RRC_CONNECTED. Furthermore, each eNB involved in dual connectivity for a UE may assume different roles. Those roles do not necessarily depend on the eNB's power class and can vary among UEs.

(75) Practical deployment for small cells and handover in small cell deployment are described.

(76) FIG. 8 shows an example of practical deployment of a small cell. The example described in FIG. 8 corresponds to a case of an X2 handover to other macro eNB with different small cells. Referring to FIG. 8, the UE receives two kinds of services by dual connectivity. The UE is connected to the macro eNB 1, and receives a service 1 from the macro eNB 1 directly. The UE is also connected to the small cell 1 which is controlled by the macro eNB 1, and receives a service 2 from the small cell 1. In a certain situation, for example, in the macro eNB coverage edge, X2 or S1 handover may happen. That is, the UE has to be handed over from the macro eNB 1 to another macro eNB, i.e., macro eNB 2. After handover, the UE is connected to the macro eNB 2, and receives the service 1 from the macro eNB 2 directly. The UE is also connected to the small cell 2 which is controlled by the macro eNB 2, and receives the service 2 from the small cell 2.

(77) FIG. 9 shows another example of practical deployment of a small cell. The example described in FIG. 9 corresponds to a case of an X2 handover to other macro eNB with a common small cell. An example described in FIG. 9 is a special case of an example described in FIG. 8. Referring to FIG. 9, the UE receives two kinds of services by dual connectivity. The UE is connected to the macro eNB 1, and receives a service 1 from the macro eNB 1 directly. The UE is also connected to the common small cell which is shared by the macro eNB 1 and macro eNB 2, and receives a service 2 from the common small cell. After handover, the UE is connected to the macro eNB 2, and receives the service 1 from the macro eNB 2 directly. The UE is still connected to the common small cell, and receives the service 2 from the common small cell.

(78) FIG. 10 shows another example of practical deployment of a small cell. The example described in FIG. 10 corresponds to a case of moving a service of a small cell to other small cell. Referring to FIG. 10, the UE receives two kinds of services by dual connectivity. The UE is connected to the macro eNB 1, and receives a service 1 from the macro eNB 1 directly. The UE is also connected to the small cell 1 which is controlled by the macro eNB 1, and receives a service 2 from the small cell 1. In a certain situation, especially when a large number of small cells are deployed within macro eNB coverage area, a handover-like behavior may happen. That is, the service 2, provided by the small cell 1, has to be moved to other small cell, while the service 1 is still provided by the macro eNB1. After handover-like procedure, the UE is still connected to the macro eNB 1, and receives the service 1 from the macro eNB 1 directly. The UE is also connected to the small cell 2 which is controlled by the macro eNB 1, and receives the service 2 from the small cell 2.

(79) FIG. 11 shows another example of practical deployment of a small cell. The example described in FIG. 11 corresponds to a case of moving a service of a small cell to a macro eNB. An example described in FIG. 11 is also a special case of an example described in FIG. 8. Referring to FIG. 11, the UE receives two kinds of services by dual connectivity. The UE is connected to the macro eNB 1, and receives a service 1 from the macro eNB 1 directly. The UE is also connected to the small cell 1 which is controlled by the macro eNB 1, and receives a service 2 from the small cell 1. In a certain situation, for example when the UE is out of small cell coverage area, a handover-like behavior may happen. That is, the service 2, provided by the small cell 1, has to be moved back to the macro eNB 1, while the service 1 is still provided by the macro eNB1. After handover-like procedure, the UE is still connected to the macro eNB 1, and receives the service 1 and service 2 from the macro eNB 1 directly.

(80) Data forwarding problems, which may happen according to practical deployment of small cells described in FIG. 8 to FIG. 11 above, are described.

(81) FIG. 12 and FIG. 13 show an example of a data forwarding problem according to practical deployment of a small cell. FIG. 12 and FIG. 13 show an X2 handover corresponding to a case described in FIG. 8 and FIG. 9, where the X2 handover procedure to other macro eNB is performed with different small cells or a common small cell. Referring to FIG. 12 and FIG. 13, the X2 handover procedure for small cell deployment is similar to the handover procedure described in FIG. 5 and FIG. 6 above, except that the small cell is deployed.

(82) Comparing FIG. 12 with FIG. 5, since the small cell is deployed, packet data is exchanged between the UE and small cell, and between the small cell and macro eNB 1 (i.e., source eNB). Further, the small cell, not the macro eNB 1, delivers buffered and in transit packets to the macro eNB 2 (i.e., target eNB). Comparing FIG. 13 with FIG. 6, since the small cell is deployed, DL data packet is transmitted from the S-GW to the small cell via the macro eNB 1. Further, the end marker is transmitted from the S-GW to the small cell directly after the macro eNB 2 transmits a path switch request message to the MME. The macro eNB 1 forwards the UE context release message to the small cell, and upon receiving the UE context release message, the small cell flushes DL buffer, and continues delivering in-transit packets.

(83) Data back and forth forwarding problem may happen since the end marker is transmitted from the S-GW to the small cell, only after the path switch request message is transmitted. By the data forwarding problem, redundant transmission of DL packet, between the macro eNB 1 and the small cell, may increase. This may be a waste of resources and may also increase data latency. It can be serious since the small cell may provide very high speed data service. The data packets which are forwarded back and forth would be very huge amount.

(84) FIG. 14 shows another example of a data forwarding problem according to practical deployment of a small cell. FIG. 14 shows moving a service of a small cell to other small cell, which corresponds to a case described in FIG. 10. Referring to FIG. 14, the macro eNB 1 decides to move the service of the small cell 1 to the small cell 2, and transmits a service request, which indicates handover of a partial service, to the small cell 2. The small cell 2 transmits a service request acknowledge to the macro eNB 1, if necessary. However, the end marker is not transmitted from the S-GW to the macro eNB 1 directly. Therefore, the data forwarding problem may happen.

(85) FIG. 15 shows another example of a data forwarding problem according to practical deployment of a small cell. FIG. 15 shows moving a service of a small cell to a macro eNB, which corresponds to a case described in FIG. 11. Referring to FIG. 15, the macro eNB 1 decides to move the service of the small cell back to the macro eNB 1, and transmits a service deactivation, which indicates stopping small cell service, to the small cell. The small cell 2 transmits a service deactivation acknowledge to the macro eNB 1, if necessary. However, the end marker does not transmitted from the S-GW to the macro eNB 1 directly. Therefore, the same data forwarding problem as described in FIG. 14 may happen.

(86) In order to solve the data forwarding problem described above, the present invention provides a data forwarding method for small cell enhancement. According to embodiments of the present invention, a method for transmitting an indication is described. Hereinafter, various solutions corresponding to cases described in FIG. 12 to FIG. 15 are described.

(87) FIG. 16 and FIG. 17 show an example of a method for forwarding data according to an embodiment of the present invention. FIG. 16 and FIG. 17 show a solution for the data forwarding problem of a case described in FIG. 12 and FIG. 13.

(88) Referring to FIG. 16, after the macro eNB 1 receives the handover request acknowledge message from the macro eNB 2, the macro eNB 1 may send an indication, which indicates stopping small cell service, to the small cell. The indication may be transmitted via the following possible ways. Service deactivation message: The indication may be transmitted via the service deactivation message. Upon receiving the service deactivation message, the small cell may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. One information element (IE) of the service deactivation message: The indication may have transmitted via an IE in the service deactivation message. The IE may be a downlink GPRS tunneling protocol (GTP) tunnel endpoint or an uplink GTP tunnel endpoint, which is generated by the macro eNB. Upon receiving the IE in the service deactivation message, the small cell may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. Independent message or IE in other message: The indication may be transmitted via an independent message, e.g., data forwarding request message, or may have a form of an IE in other message. Upon receiving the independent message or IE in other message, the small cell may know that the data forwarding can be started. X2 end marker generated by the macro eNB 1 (User plane): The X2 end marker is used for notifying the small cell of stopping small cell service. The X2 end marker may also take the role of the end marker, which means that the macro eNB 1 can know the end of data forwarding from the small cell.

(89) The X2 end maker generated by the macro eNB 1 may also be necessary when the indication is transmitted via a message or an IE in the message. In this case, the X2 end marker may take the role of original end marker. Thus, upon receiving the X2 end marker back, the macro eNB 1 can know the end of data forwarding from the small cell. If end marker is not transmitted, a timer in the macro eNB 1 may be necessary to give a time duration for data forwarding in X2 interface.

(90) The X2 end marker may be transmitted right after the handover request acknowledge message is received from the macro eNB 2. Further, the X2 end marker may be transmitted before or after the indication is transmitted to the small cell.

(91) The macro eNB 1 may start to buffer the data packets receiving from the S-GW right after the X2 end marker is transmitted, or the indication is transmitted. The data forwarding may start from the small cell after the small cell receives the indication, with an SN status transfer message.

(92) Referring to FIG. 17, the macro eNB 1 may transmit a UE X2 context release message to the small cell. By receiving the UE X2 context release message, the small cell can release radio and control plane related resources associated to the UE context.

(93) FIG. 18 shows another example of a method for forwarding data according to an embodiment of the present invention. FIG. 18 shows a brief procedure of the procedure described in FIG. 16 and FIG. 17.

(94) In step S100, the macro eNB transmits an indication which indicates stopping serving a small cell service to the small cell. The indication may be received via a service deactivation message or an SeNB release message. Alternatively, the indication may be received via an IE in a service deactivation message or in an SeNB release message. The IE may be a downlink GTP tunnel endpoint or an uplink GTP tunnel endpoint, which is generated by the macro eNB. Alternatively, the indication may be received via an X2 end marker.

(95) In step S110, upon receiving the indication, the small cell transmits an SN status transfer message to the macro eNB. In step S111, the small cell transmits forwards data to the macro eNB to the macro eNB together with the SN status transfer message.

(96) In step S120, the macro eNB transmits a UE X2 context release message to the small cell.

(97) FIG. 19 shows another example of a method for forwarding data according to an embodiment of the present invention. FIG. 19 shows a solution for the data forwarding problem of a case described in FIG. 14.

(98) Referring to FIG. 19, after the macro eNB 1 makes a decision to move a small cell service from the small cell 1 to the small cell 2, the macro eNB 1 may send an indication, which indicates stopping the small cell service, to the small cell 1. The indication may be transmitted after or before the service request message sent to the small cell 2. The indication may be transmitted via the following possible ways. Service deactivation message: The indication may be transmitted via the service deactivation message. Upon receiving the service deactivation message, the small cell 1 may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. One IE of the service deactivation message: The indication may have transmitted via an IE in the service deactivation message. Upon receiving the IE in the service deactivation message, the small cell 1 may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. Independent message or IE in other message: The indication may be transmitted via an independent message, e.g., data forwarding request message, or may have a form of an IE in other message. Upon receiving the independent message or IE in other message, the small cell 1 may know that the data forwarding can be started. X2 end marker generated by the macro eNB 1 (User plane): The X2 end marker is used for notifying the small cell 1 of stopping the small cell service. The X2 end marker may also take the role of the end marker, which means that the macro eNB 1 can know the end of data forwarding from the small cell 1.

(99) The X2 end maker generated by the macro eNB 1 may also be necessary when the indication is transmitted via a message or an IE in the message. In this case, the X2 end marker may take the role of original end marker. Thus, upon receiving the X2 end marker back, the macro eNB 1 can know the end of data forwarding from the small cell 1.

(100) The X2 end marker may be transmitted just after the macro eNB 1 makes decision of moving the small cell service, or before or after the service request message is transmitted to the small cell 2.

(101) The macro eNB 1 may start to buffer the data packets receiving from the S-GW right after the X2 end marker is transmitted, or the indication is transmitted.

(102) FIG. 20 shows another example of a method for forwarding data according to an embodiment of the present invention. FIG. 20 shows a solution for the data forwarding problem of a case described in FIG. 15.

(103) Referring to FIG. 20, after the macro eNB 1 makes a decision to move a small cell service from the small cell back to the macro eNB 1, the macro eNB 1 may send an indication, which indicates stopping the small cell service, to the small cell. The indication may be transmitted via the following possible ways. Service deactivation message: The indication may be transmitted via the service deactivation message. Upon receiving the service deactivation message, the small cell may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. One IE of the service deactivation message: The indication may have transmitted via an IE in the service deactivation message. Upon receiving the IE in the service deactivation message, the small cell may know that the data forwarding can be started. The service deactivation message may be an SeNB release message. Independent message or IE in other message: The indication may be transmitted via an independent message, e.g., data forwarding request message, or may have a form of an IE in other message. Upon receiving the independent message or IE in other message, the small cell may know that the data forwarding can be started. X2 end marker generated by the macro eNB 1 (User plane): The X2 end marker is used for notifying the small cell of stopping the small cell service. The X2 end marker may also take the role of the end marker, which means that the macro eNB 1 can know the end of data forwarding from the small cell.

(104) The X2 end maker generated by the macro eNB 1 may also be necessary when the indication is transmitted via a message or an IE in the message. In this case, the X2 end marker may take the role of original end marker. Thus, upon receiving the X2 end marker back, the macro eNB 1 can know the end of data forwarding from the small cell.

(105) The X2 end marker may be transmitted before or after transmitting the service deactivation message, or after moving the small cell service back to the macro eNB 1 is approved.

(106) The macro eNB 1 may start to buffer the data packets receiving from the S-GW right after the X2 end marker is transmitted, or the indication is transmitted.

(107) FIG. 21 shows a wireless communication system to implement an embodiment of the present invention.

(108) An MeNB 800 includes a processor 810, a memory 820, and a radio frequency (RF) unit 830. The processor 810 may be configured to implement proposed functions, procedures, and/or methods in this description. Layers of the radio interface protocol may be implemented in the processor 810. The memory 820 is operatively coupled with the processor 810 and stores a variety of information to operate the processor 810. The RF unit 830 is operatively coupled with the processor 810, and transmits and/or receives a radio signal.

(109) An SeNB or a UE 900 includes a processor 910, a memory 920 and an RF unit 930. The processor 910 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 910. The memory 920 is operatively coupled with the processor 910 and stores a variety of information to operate the processor 910. The RF unit 930 is operatively coupled with the processor 910, and transmits and/or receives a radio signal.

(110) The processors 810, 910 may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The memories 820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units 830, 930 may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories 820, 920 and executed by processors 810, 910. The memories 820, 920 can be implemented within the processors 810, 910 or external to the processors 810, 910 in which case those can be communicatively coupled to the processors 810, 910 via various means as is known in the art.

(111) In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.