Patent Publication Number: US-9900820-B2

Title: Communicating data using a local wireless access network node

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. application Ser. No. 13/745,051, filed Jan. 18, 2013, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     As the number of wireless user equipments has increased, wireless access service providers are increasingly facing challenges in meeting capacity demands in regions where the density of users is relatively high. To address capacity issues, small cells have been developed. A small cell (or multiple small cells) can operate within a coverage area of a larger cell, referred to as a macro cell. A small cell has a coverage area that is smaller than the coverage area of the macro cell. 
     If small cells are deployed, then communications with user equipments (UEs) can be offloaded from the macro cell to the small cells. In this way, data communication capacity is increased to better meet data communication demands in regions of relatively high densities of UEs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments are described with respect to the following figures. 
         FIG. 1  is a schematic diagram of an example arrangement that includes macro cells and small cells, in accordance with some implementations. 
         FIG. 2  is a flow diagram of a network component process according to some implementations. 
         FIG. 3  is a schematic diagram of another example arrangement that includes various nodes of a mobile communications network, according to some implementations. 
         FIG. 4  is a block diagram of protocol layers associated with a control plane, in accordance with some implementations. 
         FIGS. 5 and 6  illustrate mappings between downlink and uplink bearers, respectively, and various channels in a macro cell and small cell, in accordance with some implementations. 
         FIGS. 7-14  illustrate various different user plane split designs according to various implementations. 
         FIG. 15  illustrates another example arrangement that includes macro wireless access network nodes and local wireless access network nodes, according to further implementations. 
         FIG. 16  is a block diagram of modules in a gateway node according to alternative implementations. 
         FIGS. 17 and 18  are block diagrams of protocol layers in a local wireless access network node, a gateway node, and core network nodes, according to further implementations. 
         FIG. 19  is a flow diagram of a mode configuration process according to further implementations. 
         FIG. 20  is a message flow diagram of a data offload process, according to some implementations. 
         FIG. 21  is a message flow diagram of a data offload reconfiguration process, according to some implementations. 
         FIGS. 22-27  are message flow diagrams of various offload procedures, according to various implementations. 
         FIGS. 28 and 29  are message flow diagrams of offload UE transfer procedures. 
         FIG. 30  is a block diagram of an example system that is capable of incorporating some implementations. 
     
    
    
     DETAILED DESCRIPTION 
     Local Wireless Access Network Nodes in Coverage Area of a Macro Cell 
       FIG. 1  illustrates an example arrangement that includes a number of network nodes that are part of a mobile communications network that supports wireless communications with user equipments (UEs). A node can refer to an active electronic device that is capable of sending, receiving, and forwarding information over a communication channel, and of performing designated tasks. A macro cell  102  corresponds to a coverage area provided by a macro wireless access network node  104 . A coverage area can refer to a region where mobile services can be provided by a network node to a target level. A wireless access network node is responsible for performing wireless transmissions and receptions with UEs. In addition, a number of small cells  106  are depicted as being within the coverage area of the macro cell  102 . Each small cell  106  corresponds to a coverage area provided by a respective local wireless access network node  108 . One of the small cells  106  is labeled as  106 - 1 , and the respective local wireless access node is labeled  108 - 1 . 
       FIG. 1  also shows a backhaul link  116  between the macro wireless access network node  104  and each of the local wireless access network nodes  108 . The backhaul link  116  can represent a logical communication link between two nodes; the backhaul link  116  can either be a direct point-to-point link or can be routed through another communication network or node. In some implementations, the backhaul link  116  is a wired link. In other implementations, the backhaul link  116  is a wireless link. 
     In the ensuing discussion, a small cell can refer to a cell whose coverage area is smaller than the coverage area provided by a larger cell, which is also referred to as a macro cell. Wireless communication in a small cell is supported by a local wireless access network node. In some examples, the frequency at which the macro wireless access network node  104  operates is lower than that of the local wireless access network node. A lower frequency allows a greater geographic reach for wireless signals transmitted by the macro wireless access network node  104 . A higher frequency, on the other hand, of wireless signals transmitted by the local wireless access nodes  108  prevents such wireless signals from propagating great distances. As a result, small cells  106  can be deployed in relatively close proximity to each other. 
     More generally, the macro cell  102  uses a frequency that is different from one or more frequencies of the small cells  106 . At least some of the small cells  106  can use different frequencies. 
     A wireless user equipment (UE)  110  within the coverage area of the small cell  106 - 1  is able to wirelessly communicate with the local wireless access network node  108 - 1 . The UE  110  is also able to wirelessly communicate with the macro wireless access network node  104 . Examples of the UE  110  can include any of the following: a smartphone, a personal digital assistant, a notebook computer, a tablet computer, or any other device that is capable of wireless communications. Although just one UE  110  is depicted in  FIG. 1 , it is noted that multiple UEs may be present in coverage areas of each of the small cells  106  as well as within the coverage area of the macro cell  102 . 
     A first wireless connection  112  is established between the UE  110  and the local wireless access network node  108 - 1 . In addition, a second wireless connection  114  can be established between the UE  110  and the macro wireless access network node  104 . The first wireless connection  112  can be used to communicate a first type of data, while the second wireless connection  114  can be used to communicate a second type of data. In some implementations, the first type of data communicated over the first wireless connection  112  between the UE  110  and the local wireless access network node  108 - 1  includes user plane data, while the second type of data communicated over the second wireless connection  114  between the UE  110  and the macro wireless access network node  104  includes control plane data. In this manner, the UE  110  has a dual connection with the macro wireless access network node and with the local wireless access network node. 
     Generally, control plane data includes control messages to perform various control tasks, such as any or some combination of the following: network attachment of a UE, authentication of the UE, setting up radio bearers for the UE, mobility management to manage mobility of the UE (mobility management includes at least determining which infrastructure network nodes will create, maintain or drop uplink and downlink connections carrying control or user plane information as a UE moves about in a geographic area), performance of a handover decision based on neighbor cell measurements sent by the UE, transmission of a paging message to the UE, broadcasting of system information, control of UE measurement reporting, and so forth. Although examples of control tasks and control messages in a control plane are listed above, it is noted that in other examples, other types of control messages and control tasks can be provided. More generally, the control plane can perform call control and connection control functions, and can provide messaging for setting up calls or connections, supervising calls or connections, and releasing calls or connections. 
     User plane data includes the bearer data (e.g. voice, user data, application data, etc.) to be communicated between the UE and a wireless access network node. User plane data can provide for transfer of bearer data, and can also include control data and/or signals between a wireless access network node and a UE associated with the communication of the bearer data, for performing flow control, error recovery, and so forth. 
     By communicating control plane data between the macro wireless access network node  104  and the UE  110  (rather than between the local wireless access network node  108 - 1  and the UE  110 ), the design of the local wireless access network node  108 - 1  can be simplified. The local wireless access network node  108 - 1  only has to communicate user plane data with the UE  110 , without having to handle and communicate control plane data with the UE  110  (in some implementations). The macro wireless access network node  104  is connected to the UE  110  for control plane traffic and optionally for user plane traffic. Simplifying the design of the local wireless access network nodes  108  results in less complex and more cost-effective local wireless access network nodes for deploying small cells. In addition, simplified local wireless access network nodes can reduce the complexity and signaling overhead in a core network of the mobile communications network. In the example of  FIG. 1 , the core network can include control nodes  118  and  120 . Additional details regarding various core network nodes are provided further below. 
     Although at least some of the local wireless access network nodes  108  may have simplified designs in which the local wireless access network nodes  108  do not have to handle and communicate control plane data with the UE  110 , it is noted that in other implementations, one or more local area network nodes may have greater functionality, such as being able to handle and communicate control plane data. 
     In some configurations, most user plane data in the macro cell  102  can be communicated with UEs through the local wireless access network nodes  108 . However, it is possible for some user plane data to be communicated with UEs over wireless connections between the macro wireless access network node  104  and UEs. 
     Generally, for the UE  110 , the macro wireless access network node  104  provides wireless coverage (by communicating control plane data), while the bulk of the data throughput is provided by the local wireless access network node  108 - 1 . In addition, the macro wireless access network node  104  can include a gateway function for the local wireless access network nodes  108  that are within the coverage area of the macro cell  102 . In its role as a gateway, the macro wireless access network node  104  can configure the operations of the local wireless access network nodes  108  in the macro cell  102 . For example, the gateway can synchronize the local wireless access network nodes  108  by sending a synchronization signal. Mobility-related radio resource control relating to a UE between small cells  106  in the macro cell  102  can be managed by the gateway. Other coordination tasks can also be managed by the gateway. 
     In some implementations, a control plane data stream (including control messages) and a user plane data stream (including data packets) for a UE can be received at a network component, such as a macro wireless access network node. An inter-cell scheduler can decide, based on a measurement report (containing radio link measurement data) from the UE, that a subset of the data plane data stream is to be sent to the UE via a local wireless access node, where the local wireless access node is connected to the network component via a backhaul link. The network component sends the subset of the data plane data stream to the local wireless access network node. The local wireless access network node sends the subset of the data plane data stream over a first wireless connection to the UE, where the first wireless connection is established between the local wireless access network node and the UE. The network component communicates the control plane data stream over a second wireless connection to the user equipment, where the second wireless connection exists simultaneously with the first wireless connection. 
       FIG. 1  also shows additional macro wireless access network nodes  132  and  114  that support respective macro cells  133  and  115 . The macro wireless access network nodes  104 ,  132 , and  114  can be coupled to each other over respective links  122 . A link  122  between two macro wireless access network nodes can be different from a link  116  between a macro wireless access network node and a local wireless access network node.  FIG. 1  further depicts control links  124  between the control nodes  118 ,  120  (part of a core network) and the corresponding macro wireless access network nodes  104 ,  132 , and  114 . 
     In some examples, there can be at least one small cell that is outside the coverage area of a macro cell (or of any macro cell). One such small cell is small cell  129 , which is supported by a local wireless access network node  128 . Control links  126  can be provided between the control nodes  118 ,  120  and the local wireless access network node  128 . 
     In the ensuing discussion, reference is made to mobile communications networks that operate according to the Long-Term Evolution (LTE) standards as provided by the Third Generation Partnership Project (3GPP). The LTE standards are also referred to as the Evolved Universal Terrestrial Radio Access (E-UTRA) standards. Some LTE standards which relate to cellular communications are: 3GPP TS 36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description: Stage 2 (Release 10)”, V10.3.0 (2011-3); 3GPP TS 36.806: “Relay architectures for E-UTRA (LTE-Advanced)”; 3GPP TS 36.413: “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) (Release 11)”, v11.0.0, 2012-7; and 3GPP TR 36.912 V11.0.0 (2012-09) “Feasibility study for Further Advancements for E-UTRA (LTE-Advanced); 3GPP TS 36.423: “Evolution Universal terrestrial Radio Access (E-UTRA): X2 application protocol (X2AP) (Release 11)”, v11.2.0, 2012-9; 3GPP TS 36.331: “Evolution Universal terrestrial Radio Access (E-UTRA): Radio Resource Control (RRC), Protocol specification (Release 11)”, v11.0.0, 2012-7; 3GPP TR 36.839: “Evolution Universal terrestrial Radio Access (E-UTRA): mobility enhancements in heterogeneous networks (Release 11)”, v11.0.0, 2012-9.” For the most part, the signal acronyms and layer names described in this application are adapted from these LTE standards. Although reference is made to LTE in the ensuing discussion, it is noted that techniques or mechanisms according to some implementations can be applied to other wireless access technologies. 
     In an LTE network, a wireless access network node can be implemented as an enhanced Node B (eNB), which includes functionalities of a base station and base station controller. Thus, in an LTE network, the macro wireless access network nodes  102 ,  132 , and  114  are referred to as macro eNBs. In an LTE network, the local wireless access network nodes  108  can be referred to as local eNBs (LeNBs). The links  122  between macro eNBs  104 ,  132 , and  114  are implemented as X2 interfaces according to the LTE standards (see, for example, 3GPP TS 36.423 cited above). The backhaul links  116  between a macro eNB and an LeNB can be implemented as an X3 interface. An X3 interface is not between two peer eNBs, but rather between a macro eNB and a subordinate eNB (e.g. LeNB). The X3 interface makes the LeNB appear to be a cell of the associated macro eNB to a core network. Accordingly, the core network interacts with a macro eNB, and not with individual LeNBs. Note that the X3 interface can also be referred to as an X2e interface. 
     In an LTE network, the control nodes  118  and  120  can be implemented as mobility management entities (MMEs) that are part of the LTE core network (which also includes additional core network nodes discussed further below). An MME is a control node for performing various control tasks associated with an LTE network. For example, the MME can perform idle mode UE tracking and paging, bearer activation and deactivation, selection of a serving gateway (discussed further below) when the UE initially attaches to the LTE network, handover of the UE between macro eNBs, authentication of a user, generation and allocation of a temporary identity to a UE, and so forth. In other examples, the MME can perform other or alternative tasks. 
     The control links  124  and  126  between the MMEs  118 ,  120  and the eNBs  104 ,  132 ,  114 , and  128  can be implemented as S1 interfaces according to the LTE standards. 
       FIG. 2  is a flow diagram of a process according to some implementations. The process of  FIG. 2  can be performed by a network component, such as the macro eNB  104  (or more generally, a macro wireless access network node). The network component communicates (at  202 ) a first data unit with an LeNB (or more generally a local wireless access network node), to cause the LeNB to send the first data unit over a first wireless connection (e.g.  112  in  FIG. 1 ) between the LeNB and the UE (e.g.  110  in  FIG. 1 ). A data unit can refer to any collection of data. 
     The network component further communicates (at  204 ) a second data unit over a second wireless connection (e.g.  114  in  FIG. 1 ) between the network component and the UE. The second data unit contains data that is different from data in the first data unit; in other words, the data contained in the second data unit is not a duplicate of the data contained in the first data unit. In some examples, the first data unit can include user plane data, while the second data unit can include control plane data. 
     The second wireless connection is established simultaneously with the first wireless connection. Simultaneous wireless connections can refer to either simultaneous logical connections or simultaneous physical connections. Note that data does not have to be actively being communicated at the same time over the simultaneous wireless connections. Rather, “simultaneous” wireless connections can refer to wireless connections that are concurrently set up, but which are capable of communicating data, either concurrently or at different times. 
     In examples where the UE has multiple physical protocol stacks, the UE can establish multiple simultaneous physical wireless connections between the UE and corresponding wireless access network nodes (including an LeNB and a macro eNB). A protocol stack includes layers that provide specified procedures for performing communications. However, in other implementations, the UE may include just one physical protocol stack. In such implementations, the UE would be capable of establishing just one physical wireless connection; however, the UE can establish multiple simultaneous logical wireless connections with the corresponding wireless access network nodes (including an LeNB and a macro eNB). The simultaneous logical wireless connections are set up at the same time, and can be provided over the one physical wireless connection. The one physical wireless connection can be time-multiplexed between the LeNB and macro eNB. In a first time interval, the UE can have a physical wireless connection with the LeNB, in which case data can be communicated over the logical wireless connection between the UE and the LeNB. In a second time interval, the UE can have a physical wireless connection with the macro eNB, in which case data can be communicated over the logical wireless connection between the UE and the macro eNB. 
       FIG. 3  depicts additional components of an LTE network that includes the macro eNB  104  and the LeNB  108 - 1  as discussed above. The LTE network shown in  FIG. 3  further includes a core network  302 . The core network  302  has various core network nodes, including an MME  304 , a serving gateway (SGW)  306 , and a packet gateway (PGW)  308 . Although just one MME  304 , SGW  306 , and PGW  308  are depicted in  FIG. 3 , note that an LTE network can include multiple MMEs, SGWs, and PGWs. 
     As noted above, the MME  304  is a control node for performing various control tasks associated with an LTE network. The SGW  306  routes and forwards bearer data packets of a UE served by the SGW  306 , and can also act as a mobility anchor for the user plane during handover procedures. The PGW  308  provides connectivity between the UE (served by the PGW  308 ) and an external packet data network  310  (e.g. Internet, local area network, etc.). The PGW  308  is the entry and egress point for data communicated between a UE in the LTE network and a network element coupled to the packet data network  310 . 
     The following assumes that the small cell provided by the LeNB  108 - 1  is within the coverage area of the macro cell provided by the macro eNB  104 . The small cell is within the coverage of a macro cell if a signal from the macro eNB  104  can reach the LeNB  108 - 1  over the corresponding backhaul link  116  (which can be a wired or wireless link). 
     As noted above, user plane data can be communicated over the first wireless connection  112  between the LeNB  108 - 1  and the UE  110  (and in some cases may be communicated over the wireless connection  114  between the macro eNB  104  and the UE  110 ). However, control plane data is communicated over the second wireless connection  114  between the macro eNB  104  and the UE  110  (but is not communicated between the LeNB  108 - 1  and the UE  110  over the wireless connection  112 ). As a result, the control plane protocol can be the same as if the UE  110  is connected to the macro eNB  104  only (rather than also connected to the LeNB  108 - 1 ). 
     An S1 interface is provided between the macro eNB  104  and the MME  304 . However, note that the S1 interface does not extend between the MME  304  and the LeNB  108 - 1 , since the LeNB  108 - 1  does not communicate control plane data to the UE  110 . Maintaining the control plane within the macro eNB  104  has the benefit that the MME  304  only has to keep track of which macro cell the UE is associated with, rather than the small cell. This makes mobility handling procedures at the MME  304  simpler, where the mobility handling procedures can include handover, paging, and tracking area update. 
     For user plane data, a tunnel can be established from the SGW/PGW (that supports the UE  110 ) to the macro eNB  104 . This tunnel can be a GPRS (General Packet Radio Service) Tunneling Protocol (GTP) tunnel, in some examples. Other types of tunnels can be established in other examples. There can be one GTP tunnel per UE bearer. Note that the GTP tunnel does not extend to the LeNB  108 - 1 . In some examples, a UE bearer can refer to an Enhanced Packet Services (EPS) bearer, which can be established between the UE  110  and an Enhanced Packet Core (EPC) that includes the SGW  306  and the PGW  308 . 
     User plane data can include uplink data (sent from the UE  110  towards the core network  302 ) and downlink data (sent from the core network  302  to the UE  110 ). A downlink data packet targeted to the UE  110  is mapped to a UE bearer at the PGW  308  serving the UE  110 , and the downlink data packet is sent through the corresponding GTP tunnel to the macro eNB  104 . 
     When the macro eNB  104  receives a downlink data packet, the macro eNB  104  associates the downlink data packet in the GTP tunnel to the corresponding LeNB  108 - 1 . The macro eNB then sends the downlink data packet to the LeNB  108 - 1 , which in turn forwards the downlink data packet to the UE  110 . The macro eNB  104  sends the downlink data packet over the backhaul link  116  between the macro eNB  104  and the LeNB  108 - 1 . The LeNB  108 - 1  then transmits the downlink data packet over the wireless connection  112  between the LeNB  108 - 1  and the UE  110 . 
     An uplink data packet is transmitted in the reverse direction from the UE  110  to the LeNB  108 - 1 , and then from the LeNB  108 - 1  to the macro eNB  104 . The macro eNB  104  then sends the uplink data packet through the respective GTP tunnel for the UE bearer associated with the UE  110  to the core network  302 . 
     Note that the UE  110  can also communicate user plane data through the macro eNB  104  (in addition to communicating user plane data through the LeNB  108 - 1 ). In some examples, the UE  110  does not establish a user plane connection with the macro eNB  104  and LeNB  108 - 1  simultaneously. 
       FIG. 4  shows control plane protocol stacks in various nodes for communicating control plane data. The protocol stack in the UE  110  includes a physical (PHY) layer  402 , a medium access control (MAC) layer  404 , a radio link control (RLC) layer  406 , a Packet Data Convergence Protocol (PDCP) layer  408 , a radio resource control (RRC) layer  410 , and a non-access stratum (NAS) layer  412 . Control plane data of the upper layers (including the RRC layer  410  and the NAS layer  412 ) can be passed through the lower layers, including the PDCP layer  408 , RLC layer  406 , MAC layer  404 , and physical layer  402 , for transmission to the macro eNB  104 . In some examples, control plane data that is sent over the wireless connection between the UE  110  and the macro eNB  104  includes NAS and RLC messages. 
     In some implementations, to support dual connection of the UE  110  with both the macro eNB  104  and the LeNB  108 - 1 , at least some of the lower layers (including  408 ,  406 ,  404 , and  402 ) can be replicated as replicated lower layer(s) in the UE  110 , where the replicated layer(s) are represented as dashed box  430 . The replicated lower layer(s)  430  can be used to establish a wireless connection with the LeNB  108 - 1  for communication of user plane data with the LeNB  108 - 1 . The replicated lower layer(s)  430  for user plane data communication is (are) discussed further below. 
     Note that in other examples, replication of protocol stack layers does not have to be performed. For example, just one set of the lower layers ( 408 ,  406 ,  404 ,  402 ) can be provided in the UE  110 , such that the UE  110  can establish just a single physical wireless connection (which is connected to the macro eNB  104  and LeNB  108 - 1  at different times). However, in such implications, the UE  110  is capable of establishing multiple simultaneous logical wireless connections (provided over the physical wireless connection) to allow for communication of data with the LeNB and macro eNB  104 . 
     The protocol stack in the macro eNB  104  includes a physical layer  414 , a MAC layer  416 , an RLC layer  418 , a PDCP layer  420 , and an RRC layer  422 . In addition, the MME  304  includes a NAS layer  424  to interact with the NAS layer  412  in the UE  110 . 
     The physical layer  402  or  414  is the lowest layer in the corresponding node (UE  110  or macro eNB  104 ). The physical layer  402  or  414  can include networking hardware for transmitting signals over a wireless link. The MAC layer  404  or  416  provides addressing and channel access control mechanisms. 
     The RLC layer  406  or  418  can provide at least some of the following example functionalities, as described in 3GPP TS 36.322:
         transfer of upper layer packet data units (PDUs);   error correction, such as by using Automatic Repeat reQuest (ARQ);   concatenation, segmentation, and reassembly of RLC service data units (SDUs);   reordering of RLC data PDUs;   duplicate data detection;   discarding of an RLC SDU;   RLC re-establishment; and   protocol error detection.       

     The PDCP layer  408  or  420  can provide at least some of the following functionalities in the user plane, as described in 3GPP TS 36.323:
         header compression and decompression;   transfer of user data;   in-sequence delivery of upper layer PDUs;   duplicate detection of lower layer SDUs;   retransmission of PDCP SDUs;   ciphering and deciphering; and   timer-based SDU discard in the uplink.       

     Example functionalities supported by the PDCP layer  408  or  420  for the control plane can include:
         ciphering and integrity protection; and   transfer of control plane data.       

     The RRC layer  410  or  422  can be used to perform at least some of the following example functionalities, as described in 3GPP TS 36.331:
         control of handover decisions based on neighbor cell measurements sent by the UE;   transmission of a page to a UE;   broadcast of system information;   control of UE measurement reporting; and   allocation of a temporary identity to a UE.       

     The NAS layer  412  or  424  can provide at least some of the following example functionalities, as described in 3GPP TS 23.060:
         network attachment of a UE;   authentication of a UE;   setting up bearers; and   mobility management.       

     Although various example functionalities are provided above for the various layers of  FIG. 4 , it is noted that in other examples, additional or alternative functionalities can be provided by respective layers. 
     In some scenarios, an LeNB is able to cause the macro eNB  104  to send an RLC message to the UE  110 . For example, if the LeNB becomes aware that a configuration, such as a radio link configuration associated with the UE  110 , is to change, then the LeNB can send update information to the macro eNB  104 . This causes the macro eNB  104  to send the respective RRC message to the UE  110  to perform the configuration change for the radio link between the LeNB and the UE  110 . Compared to an RRC message that is sent directly from an eNB to the UE  110 , the relay of RRC-related information from the LeNB to the macro eNB  104  and then to the UE  110  is associated with some amount of delay (latency). 
     From the perspective of the UE  110 , the UE  110  maintains a single RRC connection to the macro eNB  104  as long as the UE  110  is within the coverage area of the macro cell provided by the macro eNB  104 . Consequently, no hard handover is performed from a macro cell to a small cell, or between small cells. This can allow for avoidance of handover failures that may occur if hard handovers are performed between a macro cell and a small cell or between small cells. A hard handover involves breaking a wireless connection between a source cell and a UE before establishing a new wireless connection between a target cell and the UE. 
     However, as noted above, the macro eNB  104  can act as a gateway for managing soft handovers of a UE between different cells. 
     When a UE is connected to a small cell for user plane data communication, and connected to a macro cell for control plane messaging, then the mappings of signaling radio bearers (SRBs) and data radio bearers (DRBs) to respective logical, transport, and physical channels in the downlink and uplink are depicted in  FIGS. 5 and 6 , respectively. According to LTE, a signaling radio bearer (SRB) is a radio bearer (RB) that is used only for the transmission of RRC and NAS messages. Three SRBs can be provided, including SRB 0 , SRB 1 , and SRB 2 . As described in the LTE standards, SRB 0 , SRB 1 , and SRB 2  are used to carry different control plane data (RRC messages or NAS messages) under different scenarios. 
     A data radio bearer (DRB) transports bearer data between a UE and an eNB. SRBs and DRBs according to LTE are described further in 3GPP TS 36.323. 
     For downlink communications, as shown in  FIG. 5 , SRB 0 , SRB 1 , and SRB 2  can be transmitted in various downlink logical channels, downlink transport channels, and downlink physical channels in the macro cell  102 . SRB 0  is mapped to a common control channel (CCCH), while SRB 1  and SRB 2  are mapped to respective dedicated control channels (DCCHs). 
     Downlink user plane data is carried in various DRBs, including DRB 0  to DRB 7 , for example. The DRBs are mapped to respective dedicated traffic channels (DTCHs). 
     The CCCH, DCCHs, and DTCHs are downlink logical channels. The downlink logical channels in  FIG. 5  also include a paging control channel (PCCH) and a broadcast control channel (BCCH). 
     The various downlink logical channels of  FIG. 5  are mapped to respective downlink transport channels. The DTCHs are mapped to a downlink shared channel (DL-SCH) in the small cell  106 . Note that in some examples, the DTCHs can also be mapped to a DL-SCH of the macro cell  102  (for cases where downlink user plane data can be sent through the macro cell  102  in addition to or instead of being sent through the small cell  106 ). 
     The CCCH and DCCHs that carry the SRBs are mapped to a DL-SCH in the macro cell  102 . The BCCH is mapped to a broadcast channel (BCH) and the DL-SCH. The PCCH is mapped to the PCH in the macro cell  102 . 
     The various downlink transport channels of  FIG. 5  are further mapped to downlink physical channels. The DL-SCH in the small cell  106  is mapped to a physical downlink shared channel (PDSCH) in the small cell  106 . The PCH and DL-SCH in the macro cell  102  are mapped to a PDSCH in the macro cell  102 . The BCH in the macro cell  102  is mapped to a physical broadcast channel (PBCH) in the macro cell  102 . 
     The mapping between the various radio bearers, logical channels, transport channels, and physical channels describe how the respective radio bearers are carried in the corresponding channels. For example, a DRB is carried in a DTCH, which in turn is carried in a DL-SCH of the small cell  106 , which is carried in the PDSCH of the small cell  106 . Similarly, an SRB in the macro cell  102  is carried in a CCCH or DCCH of the macro cell  102 , which in turn is carried in the DL-SCH of the macro cell  102 , which further is carried in the PDSCH of the macro cell  102 . On the downlink, SRBs can utilize PCH and BCH, and DL-SCH and the BCCH are sent over the macro cell PDSCH. 
     To support the PDSCH in the macro cell  102 , a physical downlink control channel (PDCCH) is provided in the macro cell  102 , where the PDCCH carries control information for supporting communication in the PDSCH. Similarly, to support the PDSCH in the small cell  106 , a PDCCH is also provided in the small cell  106 . Although not shown, an E-PDCCH (enhanced PDCCH) can also be supported in the macro cell  102  and the small cell  106 . 
       FIG. 6  shows the mapping of the uplink radio bearers (including SRB 0 -SRB 2  and DRB 0 -DRB 7 ) to various uplink logical channels, uplink transport channels, and uplink physical channels. In the macro cell  102 , an uplink SRB 0  is mapped to the CCCH in the macro cell  102 , and an uplink SRB 1  and uplink SRB 2  are mapped to respective DCCHs in the macro cell  102 . The CCCH and DCCH in the macro cell  102  are mapped to an uplink shared channel (UL-SCH) in the macro cell  102 , which in turn is mapped to a physical uplink shared channel (PUSCH) in the macro cell  102 . In the macro cell  102 , a random access channel (RACH) is mapped to a physical random access channel (PRACH). A physical uplink control channel (PUCCH) is also defined in the macro cell  102  to support uplink transmission over the PUSCH. 
     The data bearers DRBs are mapped to respective DTCHs, which in turn are mapped to an UL-SCH in the small cell  106 . The UL-SCH is mapped to a PUSCH in the small cell  106 . In addition, the small cell  106  is also provided with an RACH that is mapped to a PRACH. In addition, a PUCCH in the small cell  106  supports uplink communications over the PUSCH in the small cell  106 . 
     The PRACH in the macro cell  102  or small cell  106  can be used to initiate synchronization with the respective macro eNB  104  or LeNB  108 - 1 . The PUCCH is used to carry various control information associated with uplink transmissions in the PUSCH in the macro cell  102  or small cell  106 . 
     On the uplink, SRBs are carried over the PUSCH of the macro cell  102 , while DRBs are carried over the PUSCH of both the macro cell  102  and the small cell  106 . To support uplink transmission, PRACH and PUCCH are defined in both the macro cell  102  and the small cell  106 . PRACH, for example, is used to obtain separate time alignment with the macro eNB  104  and the LeNB  108 , because the LeNB schedules DL-SCH independently from the macro eNB. Note also that a dual-connected UE has to synchronize with both the macro cell  102  and the small cell  106 , which can be accomplished by obtaining the separate time alignment referenced above. Also, PUCCH is also provided in the small cell  106  to carry channel feedback information and hybrid automatic repeat request (HARQ) acknowledge/negative acknowledge (ACK/NACK) associated with the LeNB. 
     In both  FIGS. 5 and 6 , note that the DRBs and DTCHs are not depicted as being part of either the small cell  106  or the macro cell  102 . The location of the DRBs and the DCHs depends upon where a split occurs in the user plane protocol stack between the macro eNB and the LeNB, as discussed further below. Depending on where the protocol stack above the MAC layer is split between the macro eNB and the LeNB, a DRB and the associated DTCH conceivably may reside in either the macro cell or the small cell. 
     Splitting of User Plane Protocol Stack 
     The user plane can include various protocol layers, including a PDCP layer, RLC layer, MAC layer, and physical layer. At least some of these protocol layers can be included in the LeNB. Which protocol layers are included in the LeNB depends on where a user plane protocol stack is split in the macro eNB. Splitting a user plane protocol stack at a given point results in multiple user plane paths, with one user plane path through the macro eNB and another user plane path through the LeNB. The splitting can be performed at one of several different points in the user plane protocol stack, as discussed in connection with split designs 1-4 below. Distribution of data along the different user plane paths can involve data distribution at the radio bearer (RB) level. Thus, for example, data of some DRBs can be communicated over the user plane path through the LeNB, while data of other DRBs can be communicated over the user plane path through the macro eNB. Communicating data of some DRBs over a user plane path that extends through an LeNB can be referred to as offloading the data of such DRBs from the macro eNB to the LeNB. 
     In terms of security, the PDCP layer provides ciphering and integrity protection for the control plane, and ciphering for the user plane. If the small cell  106  is in the coverage area of the macro cell  102 , the PDCP layer is operated as if the small cell is part of the macro cell, so that the security keys (ciphering and integrity protection) for both control plane and user plane are generated and updated with reference to the macro eNB. 
     RLC service data units (SDUs) are fragmented and/or concatenated as appropriate to fit into available transmission resources. This process is closely coordinated by the MAC layer that indicates to each RLC entity how much data the RLC entity is allowed to send as each transmission opportunity arises. Due to the close coupling between the RLC and MAC layers, it may be useful to keep the RLC and MAC layers in the same eNB, in some examples. 
     For data offload via a small cell, logical control channels associated with RLC transparent mode, such as BCCH, PCCH, and CCCH, are transmitted directly from the macro eNB. Thus the small cell does not have to handle transparent mode packets in this case. For legacy UEs, or those UEs incapable of dual connections, a small cell eNB may act as a normal eNB, and in that case, all those control channels would be supported by the small cell. 
     Split Design 1 
     While data traffic can be divided into two parallel paths (one to the macro cell, the other to the small cell) directly by the SGW, this may cause the signaling traffic through the core network to increase, especially as the number of small cells increase. 
     One option to support data offload to small cells may be to split the user plane data before the PDCP layer in the macro eNB; in this arrangement, a separate PDCP/RLC/MAC stack is deployed in the small cell while a single RRC can still be used at the macro eNB to perform functions such as mobility, paging, broadcasting, small cell activation/deactivation, security, and provision of UE measurement reports. 
     In split design 1, the user plane protocol stack can be split right above the PDCP layer  420  in the macro eNB  104 , as shown in  FIG. 7 . The split occurs at a splitting point between the PDCP layer  420  and a layer right above the PDCP layer  420  in the macro eNB  104 . The user plane data is routed to the macro cell and then split before the PDCP layer  420 . In this design, all or a subset of data radio bearers (DRBs) can be assigned to the small cell. A radio bearer (RB) level scheduler, in the form of data distribution logic  720 , may be used at the macro cell to determine if an RB is to be handled by the macro cell or small cell. Because data splitting occurs above the MAC layer, the MAC layer can support backhaul links with various latencies between the macro cell and small cell. With split design 1, only user plane data is handled by the LeNB  108 - 1  while control plane data and data routing is handled by the macro eNB  104 . Data offload occurs at the DRB level, i.e. the LeNB  108 - 1  carries different DRBs from the macro eNB  104 . With split design 1, the UE  110  is configured to have two separate MAC layers, one that communicates with the macro eNB  104  and the other that communicates with the LeNB  108 - 1 . Dynamic data scheduling is done independently in each cell since a separate MAC layer is used in the small cell. 
     The protocol layers  414 ,  416 ,  418 ,  420 , and  422  in the macro eNB  104  are the same as corresponding layers depicted in  FIG. 4 .  FIG. 7  shows a signaling path  702 , which extends through the RRC layer  422  and the lower layers  420 ,  418 ,  416 , and  414 . 
     Two user plane paths  704  and  706  (created due to the split above the PDCP layer  420 ) are also depicted in  FIG. 7 . The user plane path  704  extends through the PDCP layer  420 , RLC layer  418 , MAC layer  416 , and physical layer  414  in the macro eNB  104 . On the other hand, the user plane path  706  extends from the macro eNB  104  through the following protocol layers in the LeNB  108 - 1 : PDCP layer  714 , RLC layer  712 , MAC layer  710 , and physical layer  708 . By splitting the user plane protocol stack above the PDCP layer  420  in the macro eNB  104 , all of the PDCP, RLC, MAC, and physical layers are replicated at the LeNB  108 - 1 . In some examples, an RRC layer can also be included in the LeNB  108 - 1  to perform certain RRC functions for radio resource configuration. However, note that the control plane functions are still handled by the macro eNB  104 . 
     Deploying a protocol stack that includes all of the PDCP, RLC, MAC, and physical layers can increase the complexity of the LeNB  108 - 1 . However, splitting the user plane protocol stack above the PDCP layer  420  involves minimal change in the protocol stack deployed in the LeNB  108 - 1 . 
     Moreover, deploying both the RLC layer and MAC layer in the LeNB  108 - 1  also allows easier coordination between the RLC and MAC layers. Note that an RLC SDU can be fragmented or concatenated as appropriate to fit into available transmission resources in the MAC layer. This process can be coordinated by the MAC layer, which can indicate to the RLC layer how much data the RLC layer is allowed to send as each transmission opportunity arises. Not splitting the RLC and MAC layers between the macro eNB and LeNB makes such coordination simpler. 
     In addition, tighter interaction between hybrid automatic repeat request (HARQ) logic in the MAC layer and the RLC layer can be provided. The HARQ logic in the MAC layer can indicate failed transmissions to the RLC layer, to cause the RLC logic to perform retransmission without waiting for a negative acknowledgment (NACK) from the receiving RLC logic (in the receiving device). 
     On the other hand, compared to splitting below the RLC layer, providing the RLC layer in the LeNB  108 - 1  may not allow RLC context transfer, since the RLC layer may be reset during handover between small cells. During handover, any remaining (un-transmitted) RLC PDUs may be flushed when the protocol stack is re-established at the target cell. A central node (such as the macro eNB  104 ) may not be able to include a retransmission buffer for holding RLC PDUs for retransmission following handover to the target cell. 
     In split design 1, all or a subset of DRBs can be assigned to the LeNB  108 - 1 . In some implementations, the data distribution logic  720  steers DRBs between the two user plane paths  704  and  706 . The data distribution logic  720  can be provided in the macro eNB  104  to determine, at bearer setup, if a given DRB is to be communicated to the UE by the macro cell or small cell. In split design 1, the data distribution logic  720  can be implemented in logic above the PDCP layer  420  to decide whether the given DRB is to be passed to the PDCP layer  420  (in the macro eNB  104 ) or to the PDCP layer  714  (in the LeNB  108 - 1 ). 
     Because data splitting occurs above the MAC layer  416  in the macro eNB  104 , the backhaul link ( 116  in  FIG. 1 ) between the macro eNB  104  and the LeNB  108 - 1  can tolerate larger latencies, since coordination between different layers of the user plane protocol stack split across the macro eNB  104  and the LeNB  108 - 1  does not have to be performed. An independent MAC layer ( 710 ) at LeNB  108 - 1  for dynamic data scheduling of the user plane data through the small cell is part of some embodiments. 
       FIG. 7  also shows protocol layers (within dashed box  724 ) that are used for legacy UEs or other UEs that are not capable of dual connections. The presence of the protocol layers ( 724 ) allows the LeNB  108 - 1  to appear as a normal eNB for the foregoing UEs. 
     Split Design 2 
     In split design 2, the user plane protocol stack can be split right below the PDCP layer  420 , as shown in  FIG. 8 . The split occurs at a splitting point between the PDCP layer  420  and the RLC layer  418  in the macro eNB  104 . In this split design, a single PDCP layer  420  is maintained in the macro eNB  104 , with no PDCP layer provided in the LeNB  108 - 1 . Data going to the small cell is split after the PDCP layer  420 . 
     The data distribution logic  720  (which provides inter-cell data scheduling) can be provided in the macro eNB  104  to determine if a RB is to be handled by the macro eNB  104  or the LeNB  108 - 1 . In some examples, the inter-cell data scheduling provided by the data distribution logic  720  can specify that all SRBs are handled by the macro eNB  104  and all DRBs are handled by the LeNB  108 - 1 . 
     Split design 2 preserves the tighter interaction between the MAC and RLC layers. The MAC layer can indicate failed transmissions to the RLC layer. The RLC transmitting entity can retransmit without waiting for a NACK in a status report from the receiving RLC entity. The RLC PDU size can be provided to the RLC layer internally by the MAC layer. 
     On the other hand, split design 2 may not allow for RLC context transfer, such as for RLC reset in case of data offload to a different small cell. Any remaining RLC PDUs are flushed when the protocol stack is re-established at the target small cell. To prevent packet loss, the PDCP layer  420  in the macro eNB  104  may have to implement a retransmission buffer to hold packets until the packets are successfully delivered by the RLC layer  712  in the LeNB  108 - 1 . The PDCP layer  420  can enable packets to be retransmitted to the UE (if in RLC Acknowledge Mode) following a handover to the target small cell. This may involve an indication from the RLC layer. In addition to the signaling overhead, a larger retransmission buffer in the PDCP layer may have to be provided to support higher backhaul latencies. 
       FIG. 8  depicts a user plane path  802  and a user plane path  804  split below the PDCP layer  420 . The user plane path  802  extends from the PDCP layer  420  through the RLC layer  418 , MAC layer  416 , and physical layer  414  of the macro eNB  104 . The user plane path  804  extends from the PDCP layer  420  in the macro eNB  104  through the RLC layer, MAC layer  710 , and physical layer  708  in the LeNB  108 - 1 . 
     As with split design 1, various benefits or issues associated with providing the RLC and MAC layers in the LeNB  108 - 1  may also be present with split design 2. 
       FIG. 8  also shows the data distribution logic  720  in the macro eNB  104  that can include the data distribution logic for steering of DRBs between the macro eNB  104  and the LeNB  108 - 1 . For example, one DRB may be steered by the data distribution logic  720  along user plane path  802 , such that the DRB is communicated by the macro eNB  104  to the UE. Another DRB may be steered by the data distribution logic  720  to the user plane path  804 , in which case this other DRB is routed through the LeNB  108 - 1  for transmission to the UE. 
     In split design 2, the data distribution logic  720  can be part of the PDCP  420  in the macro eNB  104 , for example. 
     Further details of the protocol layers in the macro eNB  104  for split design 2 are shown in  FIG. 9 . Radio bearers  902  (represented by respective ovals) are subject to processing at the PDCP layer  420 . The radio bearers  902  can include SRBs and DRBs. The PDCP layer  420  includes a robust header compression and decompression (ROHC) logic  904 , a security logic  906 , and an inter-cell scheduling logic (which is provided by the data distribution logic  720  depicted in  FIG. 8 , for example). 
     The data distribution logic  720  determines, at radio bearer setup, whether a radio bearer (SRB or DRB) is to be communicated by a macro cell or a small cell. In some examples, the data distribution logic  720  can determine that all SRBs are to be handled by the macro eNB  104 , while all DRBs are to be handled by the LeNB  108 - 1 . However, it is possible for DRBs to be split between the macro eNB  104  and the LeNB  108 . 
     Radio bearers (SRBs or DRBs) to be handled by the macro eNB  104  are steered by the inter-cell scheduling logic  720  along respective paths  908  to the RLC layer  418  in the macro eNB  104 . On the other hand, DRBs to be steered to the LeNB  104 - 1  are routed along respective paths  910  to the LeNB  108 - 1 . 
     The RLC layer  418  includes segmentation and ARQ logic  912 , which processes data received over paths  908  from the PDCP layer  420 . 
     Data from the RLC layer  418  is carried in respective logical channels  914  to the MAC layer  416  in the macro eNB  104 . The MAC layer  416  includes unicast scheduling and priority handling logic  916 , multiplexing logic  918 , and HARQ logic  920 . Data from the MAC layer  416  are carried in respective transport channels  922 . 
     The functionalities of the logic  904 ,  906 ,  912 ,  916 ,  918 , and  920  are described in various 3GPP Specifications. 
     Although not shown, the RLC layer  712  and MAC layer  710  ( FIG. 8 ) of the LeNB  108 - 1  has similar components as the RLC layer  418  and MAC layer  416 , respectively, shown in  FIG. 9 . 
       FIG. 10A  shows protocol layers in the UE  110 , LeNB  108 - 1 , and macro eNB  104  according to split design 2. The UE  110  includes the PDCP layer  408 , RLC layer  406 , MAC layer  404 , and physical layer  402 . The RLC layer  406 , MAC layer  404 , and physical layer  402  of the UE  110  interact with the corresponding RLC layer  712 , MAC layer  710 , and physical layer  708  of the LeNB  108 - 1 . The PDCP layer  408  in the UE  110  interacts with the PDCP layer  420  in the macro eNB  104 . 
     In addition, the LeNB  108 - 1  has lower protocol layers  1002  for interacting with corresponding lower protocol layers  1004  in the macro eNB  104 , to enable communication between the LeNB  108 - 1  and the macro eNB  104 . These lower protocol layers  1002  and  1004  can implement the X3 interface (backhaul link  116  of  FIG. 1 ) between the LeNB  108 - 1  and the macro eNB  104 . 
       FIG. 10B  provides a different view of the protocol layers in the UE  110 , LeNB  108 - 1 , and macro eNB  104 . In  FIG. 10B , the UE  110  is shown to have two sets of RLC/MAC/PHY layers to communicate with the respective LeNB  108 - 1  and macro eNB  104 . 
     Split Design 3 
     In split design 3, the user plane protocol stack is split right below the RLC layer  418  in the macro eNB  104 , as shown in  FIG. 11 . The split occurs at a splitting point between the RLC layer  418  and the MAC layer  416  in the macro eNB  104 . In this split design, a single PDCP layer  420  and RLC layer  418  are provided in the macro eNB  104 . User plane data going to the small cell is split after the RLC layer  418  in the macro eNB  104 . The data distribution logic  720  (to provide inter-cell data scheduling) is implemented to determine if the RLC packets are to be handled by the macro eNB  104  or the LeNB  108 - 1 . Split design 3 also allows a retransmitted RLC packet to be sent to a cell other than the original RLC packet, thus better leveraging the changing channel conditions between the macro cell and the small cell. 
     Split design 3, however, may not preserve the tighter interaction between the MAC and RLC layers in case of large backhaul latency between the macro cell and the small cell. The RLC PDU size is provided to the RLC layer over the backhaul interface by the MAC layer. On the other hand, split design 3 does allow RLC context transfer. RLC does not have to be reset in handover from one LeNB to another LeNB under the coverage of the same macro eNB. The transmission at the target LeNB continues from the last RLC PDU at the source LeNB when the MAC/PHY protocol stack is re-established at the target LeNB. The macro eNB is the central node that holds a retransmission buffer, where RLC packets can be retransmitted to the UE (if in RLC Acknowledge Mode) following handover to the target NodeB. 
     The split below the RLC layer  418  results in a first user plane path  1102  and a second user plane path  1104 . The first user plane path  1102  extends from the RLC layer  418  through the MAC layer  416  and physical layer  414 . The second user plane path  1104  extends from the RLC layer  418  in the macro eNB  104  through the MAC layer  710  and physical layer  7108  in the LeNB  108 - 1 . In implementations according to split design 3, the data distribution logic  720  can be provided in the RLC layer  418  to steer data between the macro cell and small cell. 
       FIG. 12  shows further details of the protocol layers of the macro eNB  104  according to split design 3. As depicted in  FIG. 12 , the inter-cell scheduling logic (provided by the data distribution logic  720 ) is included in the RLC layer  418 , rather than in the PDCP layer  420  shown in  FIG. 9  for split design 2. The inter-cell scheduling logic  720  can steer SRBs or DRBs along paths  1202  for data to be transmitted to the UE by the macro eNB  104 , or along paths  1204  for data to be routed through the LeNB  108 - 1  for transmission to the UE. 
     The remaining modules of the protocol stacks are similar to corresponding modules depicted in  FIG. 9 . 
       FIG. 13A  shows protocol stacks implemented in the UE  110 , LeNB  108 - 1 , and macro eNB  104  for split design 3. 
     The arrangement of  FIG. 13A  is similar to the arrangement depicted in  FIG. 10A  for split design 2, except in  FIG. 13A , the LeNB  108 - 1  includes just the MAC layer  710  and the physical layer  708 . Lower layers  1302  in the LeNB  108 - 1  are provided to interact with corresponding lower layers  1304  in the macro eNB  104 . 
       FIG. 13B  provides a different view of the protocol layers in the UE  110 , LeNB  108 - 1 , and macro eNB  104 . In  FIG. 13B , the UE  110  is shown to have two sets of MAC/PHY layers to communicate with the respective LeNB  108 - 1  and macro eNB  104 . 
     Since the user plane protocol stack split occurs after the RLC layer  418  in split design 3, RLC context transfer is enabled during handover between small cells. For example, the RLC layer in the macro eNB  104  can include a retransmission buffer to store RLC PDUs that are to be retransmitted in the target cell after handover. 
     Dividing the RLC layer and MAC layer between the macro eNB  104  and the LeNB  108 - 1  does not allow for simpler coordination and tighter interaction between the RLC layer and MAC layer in the LeNB  108 - 1  that is available with split designs 1 and 2. 
     Split Design 4 
     Split design 4 is shown in  FIG. 14 , in which the user plane protocol stack is split right after the MAC layer  416  in the macro eNB  104 . The split occurs at a splitting point between the MAC layer  416  and the physical layer  414  in the macro eNB  104 . Data of a radio bearer (SRB or DRB) from a UE is scheduled by the MAC at the macro eNB to be sent over either the macro cell or the small cells. With split design 4, there is only one MAC layer, which resides in the macro eNB  104 . The MAC layer  416  in the macro eNB  104  can send data selectively over one the two separate physical layers in the macro eNB and the LeNB. 
     In the MAC layer  416 , each multiplexing logic  918  is associated with two HARQ modules  1402  and  1404 . Each HARQ module  1402  provides data on a respective DL-SCH transmitted by the physical layer  414  of the macro eNB  104  over the wireless connection  114  between the macro eNB  104  and the UE. On the other hand, each HARQ module  1404  provides data on a DL-SCH  1408  that is transmitted by the physical layer  708  of the LeNB  108 - 1  over the wireless connection  112  between the LeNB  108 - 1  and the UE. Scheduling to steer data between the two paths can occur in the multiplexing logic  918 . 
     Local Wireless Access Network Nodes Outside Coverage Area of Macro Cell 
     The foregoing discussion referred to LeNBs that are within the coverage area of a macro cell. However, when an LeNB is out of the coverage area of any macro cell, the LeNB is configured to handle both control plane data and user plane data. As a result, the LeNB that is outside the coverage area of a macro cell can be configured with NAS, RRC, PDCP, RLC, MAC, and physical layers to enable the LeNB to handle control plane data and user plane data. 
       FIG. 15  illustrates an example arrangement that includes LeNBs that are outside the coverage area of any macro cell.  FIG. 15  shows macro cells  1502 ,  1504 , and  1506 , which are supported by respective macro eNBs  1508 ,  1510 , and  1512 . In addition,  FIG. 15  shows LeNBs  1514 ,  1516 , and  1518  that are outside the coverage area of any macro eNB. 
     The LeNB  1514  is an LeNB that operates in a first mode. In the first mode, the LeNB  1514  connects to core network nodes directly. In  FIG. 15 , three sets  1520 ,  1522 , and  1524  of core network nodes are shown, where each set includes an MME and an SGW (collectively referred to as “MME/SGW”). As shown in  FIG. 15 , the LeNB  1514  (that operates in the first mode) connects over S1 links with MME/SGW  1520  and MME/SGW  1522 . There is no gateway provided between the LeNB  1514  and the MME/SGW  1520  and MME/SGW  1522 . 
     The LeNBs  1516  and  1518  operate in a second mode that is different from the first mode. In the second mode, a cluster of LeNBs ( 1516  and  1518  in the example of  FIG. 15 ) is associated with a gateway  1526 , referred to as an LeNB GW in  FIG. 15 . The LeNB GW  1526  is deployed to support the cluster of LeNBs  1516  and  1518 , and the LeNB GW  1526  serves as an intermediary between the LeNBs  1516 ,  1518  and the respective core network nodes (including MMEs/SGWs  1520 ,  1522 , and  1524 ). 
     As with typical eNBs, the LeNBs  1514 ,  1516 , and  1518  can be connected with each other over X2 interfaces. 
     The LeNBs  1516  and  1518  are connected by S1 interfaces to the LeNB GW  1526 . The LeNB GW  1526  is in turn connected over an S1 interface with each MME/SGW  1520 ,  1522 , or  1524 . 
     Provision of the LeNB GW  1526  (or multiple LeNB GWs) allows the core network to support a relatively large number of LeNBs in a scalable manner (since the core network does not have to connect to LeNBs that are associated with LeNB GW(s). The LeNB GW  1526  can be connected to the core network in a way that mobility of UEs across small cells served by LeNBs associated with the LeNB GW  1526  is unlikely to involve inter-MME handovers. 
     For the control plane, the S1-MME interface (the S1 interface to an MME) from the LeNB  1516  or  1518  may be terminated at the LeNB GW  1526 . In the control plane, the LeNB GW  1526  appears to an LeNB as an MME, while the LeNB GW  1526  appears to the MME as an eNB. 
     For the user plane, the S1-U interface (the S1 interface to an SGW) from the LeNB  1516  or  1518  may be terminated at the LeNB GW  1526 . In the control plane, the LeNB GW  1526  appears to an LeNB as an SGW, while the LeNB GW  1526  appears to the SGW as an eNB. 
       FIG. 16  is a block diagram of an example arrangement that includes the LeNB GW  1526 , an LeNB  1516  or  1518 , and a core network node  1602  (e.g. MME or SGW). The arrangement of  FIG. 16  differs from a home eNB arrangement, since the small cells provided by LeNBs are deployed by a cellular operator that also deploys the other network nodes of a mobile communications network (including the core network nodes and macro eNBs). In addition, small cells provided by LeNBs are generally available to subscribers of the cellular operator, rather than just to specific home users. 
     The LeNB GW  1526  includes a protocol stack  1604  to communicate over an S1 interface to the LeNB  1516  or  1518 . The LeNB GW  1526  includes another protocol stack  1606  to communicate over an S1 interface to the core network node  1602 . In addition, the LeNB GW  1526  includes a control function  1608  that can perform various tasks as discussed further below. 
       FIG. 17  depicts protocol stacks in the LeNB  1516  or  1518 , LeNB GW  1526 , and an SGW for the user plane. The LeNB GW  1526  provides a relay function to relay user plane data between the LeNB and the SGW. In the user plane, each of the protocol stack  1604  and  1606  of the LeNB GW  1526  includes the following layers: layer 1 (L1 or physical layer), layer 2 (L2 or link layer), Internet Protocol (IP) layer, User Datagram Protocol (UDP) layer, and GTP-U layer (GTP layer for the user plane). Each of the LeNB  1516  or  1518  and the SGW also includes the same layers. These protocol layers (L1, L2, IP, UDP, GTP-U) collectively enable communication over the S1-U interface between the LeNB and LeNB GW  1526 , and the S1-U interface between the LeNB GW  1526  and the SGW. 
       FIG. 18  depicts protocol stacks in the LeNB  1516  or  1518 , LeNB GW  1526 , and an MME for the control plane. 
     The S1-MME protocol stacks with the LeNB GW is shown in  FIG. 18 . When the LeNB GW  1526  is present, the LeNB GW  1526  terminates non-UE-dedicated procedures—in other words, the MME does not see the LeNB, and the LeNB does not see the MME. The LeNB GW  1526  provides a relay function for relaying control plane data between the LeNB and the MME. 
     In the control plane, each of the protocol stack  1604  and  1606  includes the following layers: L1, L2, IP, Stream Control Transmission Protocol (SCTP) layer (SCTP is a transport layer protocol), and an S1-Application Protocol (S1-AP) layer (which provides a signaling service between an access network and the core network). 
     These protocol layers (L1, L2, IP, SCTP, S1-AP) collectively enable communication over the S1-MME interface between the LeNB and LeNB GW  1526 , and the S1-MME interface between the LeNB GW  1526  and the MME. 
     The control function  1608  in the LeNB GW  1526  can perform various tasks, as discussed below. Certain cluster-wide signaling can be controlled by the control function  1608 —the cluster-wide signaling is transmitted by the small cells in the cluster supported by the LeNB GW  1526 . An example of cluster-wide signaling can include a System Information Block (SIB) that is broadcast by each LeNB or included in RRC messaging. An SIB can carry various system parameters, such as those for specifying a frequency-division duplex (FDD) or time-division duplex (TDD) mode of operation, component carriers for a small cell, and so forth. 
     In addition, the control function  1608  in the LeNB GW  1526  can control the provision of a synchronization signal so that all small cells in a cluster are synchronized for more efficient mobility management and interference coordination. 
     Moreover, the control function  1608  can broadcast capabilities of the LeNB GW  1526  to UEs, so that each UE can simplify cell selection/reselection procedures, cell attachment procedures, and so forth, since the UE is informed that parameters associated with the foregoing procedures are centrally determined and shared by the small cells. 
     Further, certain macro eNB tasks can be performed by the control function  1608  in the LeNB GW  1526 . For example, mobility between small cells under the same LeNB GW  1526  can be performed using an intra-eNB handover procedure. The control function  1608  in the LeNB GW  1526  can assist with small cell selection, if a link quality measurement such as a Reference Signal Received Power (RSRP) or a Reference Signal Received Quality (RSRQ) is passed to the LeNB GW  1526  to facilitate the cell selection. 
     In certain deployment scenarios, the backhaul link between LeNBs and the LeNB GW  1526  has a relatively low latency, due to the close proximity between the LeNB GW  1526  and the LeNBs. Thus a centralized dynamic or semi-dynamic data distribution/scheduling function may be included in the control function  1608  in the LeNB GW  1526  to schedule data to a UE through one or multiple LeNBs that are within the cluster of the LeNB GW  1526 . 
     Distributing or steering data through selected LeNB(s) may allow for better interference coordination among the LeNBs connected to the LeNB GW  1526 . For example, when a UE is within the coverage of two LeNBs, to avoid downlink interference, dynamic LeNB selection may be performed by the LeNB GW  1526  based on the radio link quality feedback from the UE. This can improve the data throughput of the UE. 
     Due to the central position of the LeNB GW  1526  with respect to the LeNBs in the cluster of the LeNB GW  1526 , the control function  1608  can also perform functions currently assumed by an X2 interface, thus reducing (or avoiding) the establishment of X2 links between small cells.  FIG. 15  depicts example X2 links between the LeNB GW  1526  and the LeNBs  1516  and  1518 . 
     In addition, inter-cell interference coordination (ICIC) between neighboring small cells can be handled by the control function  1608  in the LeNB GW  1526 , so that frequency domain resources can be shared between neighbor small cells without interference. In addition to ICIC signals to coordinate allocation of PDSCH resources to small cells, the control function  1608  in the LeNB GW  1526  can also assist with enhanced PDCCH (ePDCCH) resource allocation, so that interference is avoided or reduced for both control and data channels. 
     Other tasks, such as load balancing, mobility robustness, mobility optimization, and so forth, can also be centrally coordinated by the control function  1608  in the LeNB GW  1526 . With such central coordination, a UE can benefit in terms of reduced measurement and reporting, lower interference, and better mobility performance. 
     For the user plane, downlink data packets for UEs can be stored at the LeNB GW  1526 . When a UE transitions from idle mode to connected mode, or when transfers between small cells, any buffered downlink packets in the LeNB GW  1526  can be routed to the proper small cell. Similarly, uplink data packets from a UE can be collected and stored at the LeNB GW  1526  before passed to the destination. 
     Adaptation of Operation Modes 
     Small cells can be deployed under various different conditions. For example, a small cell can be deployed in a location with macro cell coverage or in a location without macro cell coverage. As another example, a small cell can be deployed in a dense or sparse region of small cells. Additionally, it may be beneficial for an LeNB to adapt over time after deployment to changing conditions. 
     In accordance with some implementations, an LeNB can be selectively configured to operate in any of multiple different modes of operation. Four example modes (A, B, C, and D) are described below. Although reference is made to four example modes, it is noted that in other examples, more modes or less modes can be used. 
     Mode A is used if an LeNB is deployed without an LeNB GW and in a location without macro cell coverage. In mode A, the LeNB is configured to have the full function of a normal eNB, and behaves as a standalone eNB. For example, the full protocol stack (including all of the PDCP, RLC, MAC, and physical layers as well as control plane protocol layers such as the RRC and NAS layers) is enabled. The full protocol stack allows the LeNB to handle both control plane and user plane without assistance from a macro eNB or an LeNB GW. In addition, in this LeNB, its protocol stack to the S1 interface to the core network is enabled such that the LeNB can communicate with the core network directly over S1 interface, without an LeNB GW or macro eNB as an intermediary. 
     Mode B is used for an LeNB deployed with an LeNB GW but without macro cell coverage. In mode B, the LeNB is configured to have the full function of a normal eNB. For example, the full protocol stack (including all of the PDCP, RLC, MAC, and physical layers as well as control plane protocol layers) is enabled so that the LeNB can handle both control plane and user plane data. However, since the LeNB GW is present, the LeNB does not connect to core network directly, but instead connects over an S1 interface to the LeNB GW that provides a relay function between the LeNB and core network. 
     For an LeNB deployed with macro cell coverage, the LeNB can either be configured with its full protocol stack enabled (mode C) or with a partial protocol stack enabled (mode D). If the full protocol stack is enabled in mode C, the LeNB can handle both control plane and user plane data. In mode C, the corresponding macro eNB can serve the function of an LeNB GW in the sense that the macro eNB provides the relay function between the LeNB and the core network for both the control plane and user plane. The interface between LeNB and the macro eNB can be an S1 interface, and the interface between LeNBs can be an X2 interface. 
     In mode D, a partial protocol stack is enabled for an LeNB deployed with macro cell coverage. The partial protocol stack causes the LeNB to have reduced functionality. Enabling a partial protocol stack can refer to enabling some of the protocol layers in the LeNB while disabling other protocol layer(s). The LeNB with the partial protocol stack can handle just user plane data communicated with a UE, and does not handle control plane data. The control plane data is handled by the macro eNB. The partial protocol stack may omit one or more of the following protocol layers: PDCP layer, RLC layer, and MAC layer, depending on which of the split designs is used as discussed further above. In this mode of operation, the interface between LeNB and the macro eNB is the X3 interface. Between LeNBs under the same macro eNB, an interface similar to the X2 interface can be established for direct handover between LeNBs. 
     To reduce deployment cost, it is desirable to allow the LeNB to operate in a variety of deployment conditions, and to operate adaptively as conditions change after deployment. In some implementations, an LeNB can be built with the full functionality of a normal eNB. This LeNB is configurable to operate in any of modes A, B, C, and D discussed above. 
     Initial configuration of the operating mode and modification of the operating mode can be achieved using operation and maintenance (O&amp;M) procedures. For example, to change from mode A to mode B, the LeNB can be instructed to switch the S1 interface from the core network towards an LeNB GW. To change from mode A to mode C, the LeNB can be instructed to switch the S1 interface from the core network towards the macro eNB. To change from mode C to mode D, the LeNB can be instructed to reconfigure its internal processing and interface functionalities. 
       FIG. 19  is a flow diagram of a mode configuration process for an LeNB, in accordance with some implementations. The LeNB scans (at  1902 ) its environment to check for certain conditions. For example, the LeNB can check for presence of a macro eNB or an LeNB GW. The LeNB can also check for other environmental conditions, such as interference levels from other small cells (which can provide some indication of sparcity and denseness of deployment of small cells). The LeNB can perform the scan periodically, or in response to a trigger event (e.g. loss of communication with a macro eNB or LeNB GW, a command from another node, etc.). 
     Based on the determined environmental conditions, the LeNB can determine (at  1904 ) whether a mode change is to be performed, and if so, which mode the local eNB is to transition to. The determination at  1904  can be performed by the local eNB. Alternatively, the local eNB can send a report of the determined environmental conditions to another node (e.g. MME, macro eNB, LeNB GW), for the other node to make the determination of whether a mode change is to be performed, and if so, which mode the local eNB is to transition to. 
     In response to determining that the mode change is to occur, the LeNB transitions (at  1906 ) to another mode. 
     In general, a method of adaptive operation of a local wireless access network node comprises scanning an environment condition, and in response to the scanned environment condition, changing a mode of operation of the local wireless access network node. 
     In some implementations, changing the mode of operation includes changing from a first mode in which a protocol stack of the local wireless access network node is fully enabled, to a second mode in which the protocol stack is partially enabled. 
     The local wireless access network node is set in the first mode in response to the environment condition indicating that the local wireless access network node is outside a coverage area of a macro wireless access network node and a gateway is unavailable. 
     Alternatively, the local wireless access network node is set in the first mode in response to the environment condition indicating that the local wireless network node is connected to the gateway. 
     As another alternative, the local wireless access network node is set in the first mode in response to the environment condition indicating that the local wireless access network node is within the coverage area of the macro wireless access network node. 
     According to further implementations, the local wireless access network node is changed to the second mode in response to the environment condition indicating that the local wireless access network node is within the coverage area of a macro wireless access network node. 
     Changing the operating mode can include selecting an operating mode from among: (1) a first mode in which the local wireless access network node is outside the coverage area of a macro wireless access network node and a gateway is unavailable; (2) a second mode in which the local wireless access network node is attached to a gateway; (3) a third mode in which the local wireless access network node is within the coverage area of a macro wireless access network node but the local wireless access network node is to be provided with full protocol stack functionality; and (4) a first mode in which the local wireless access network node is within the coverage area of a macro wireless access network node, and user plane data is to be offloaded to the local wireless access network node. 
     Brief Discussion of Various Features Described Above 
     In general, a method comprises communicating, by a network component, a first data unit with a local wireless access network node, to cause the local wireless access network node to send the first data unit over a first wireless connection between the local wireless access network node and a user equipment. The network component communicates a second data unit over a second wireless connection with the user equipment, where the second data unit contains data that is different from data in the first data unit, and where the second wireless connection is established simultaneously with the first wireless connection. 
     In some implementations, the first and second wireless connections are logical connections. 
     In some implementations, the first and second wireless connections are physical connections. 
     In some implementations, the network component is coupled to a plurality of local wireless access network nodes, and the network component coordinates operations of the plurality of local wireless access network nodes. 
     In some implementations, the first data unit contains user plane data, and the second data unit contains control plane data. 
     In some implementations, communicating the first data unit with the local wireless access network node includes communicating the first data unit over a wired backhaul link between the network component and the local wireless access network node. 
     In some implementations, the network component controls mobility operations of the user equipment between the local wireless access network node and at least another local wireless access network node. 
     In some implementations, a data distribution logic in the network component determines whether a third data unit is to flow to the user equipment over a first path that includes the local wireless access network node or a second path that includes the second wireless connection between the network component and the user equipment. The third data unit is selectively sent over one of the first path and the second path based on the determining. 
     In some implementations, the first path and second path are split at a splitting point in a protocol stack of the network component. 
     In some implementations, the splitting point is above a Packet Data Convergence Protocol (PDCP) layer in the network component. 
     In some implementations, the splitting point is between a Packet Data Convergence Protocol (PDCP) layer and a Radio Link Control (RLC) layer in the network component. 
     In some implementations, the splitting point is between a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer in the network component. 
     In some implementations, the splitting point is between a Medium Access Control (MAC) layer and a physical layer in the network component. 
     In some implementations, the local wireless access network node is within a coverage area of a cell provided by the network component. 
     In some implementations, a tunnel for carrying data of the user equipment is terminated at the network component without extending to the local wireless access network node. 
     In general, a user equipment includes a communication interface to establish simultaneous wireless connections with a macro wireless access network node and a local wireless access network node, and at least one processor to communicate a first data unit over a first of the wireless connections with the local wireless access network node, and communicate a second data unit over a second of the wireless connections with the macro wireless access network node, wherein the first data unit contains data different from data in the second data unit. 
     In some implementations, the user equipment includes protocol layers including a first layer to communicate control plane data with the macro wireless access network node without communicating control plane data with the local wireless access network node, and a second layer to communicate user plane data with the local wireless access network node. 
     In some implementations, a third layer is to communicate user plane data with the macro wireless access network node. 
     In general, a gateway node includes a first communication interface to a local wireless access network node deployed by a cellular operator and that provides a coverage area for wireless communication with a user equipment (UE), where the first interface includes protocol layers that cause the gateway node to appear as a core network node to the local wireless access network node. A second communication interface is to the core network node, where the second communication interface includes protocol layers that cause the gateway node to appear as a wireless access network node to the core network node. 
     In some implementations, each of the first and second communication interfaces is configured to communicate over a Long-Term Evolution (LTE) S1 interface. 
     In some implementations, the first communication interface is to communicate with a plurality of local wireless access network nodes that are part of a cluster associated with the gateway node, and the gateway node further includes a control function to send cluster-wide signaling to the local wireless access network nodes. 
     In some implementations, the first communication interface is to communicate with a plurality of local wireless access network nodes that are part of a cluster associated with the gateway node, and the gateway node further includes a control function to send a synchronization signal to the local wireless access network nodes to synchronize the local wireless access network nodes. 
     In some implementations, the first communication interface is to communicate with a plurality of local wireless access network nodes that are part of a cluster associated with the gateway node, and the gateway node further includes a control function to coordinate mobility of the UE between cells supported by the local wireless access network nodes. 
     In some implementations, the first communication interface is to communicate with a plurality of local wireless access network nodes that are part of a cluster associated with the gateway node, and the gateway node further includes a control function to perform inter-cell interference coordination among cells provided by the local wireless access network nodes. 
     User Equipment Mobility Between eNBs 
     In scenarios where LeNBs (of respective small cells) are under the coverage of a macro eNB, efficiency of UE mobility across different cells is desired. Procedures can be provided to implement UE mobility between a macro cell and a small cell or between two small cells. Efficiency can be achieved by reduced signaling to the core network as well as improved handover performance. 
     Traditionally, handover of a UE involves nodes in a core network, such as the MME and SGW in an LTE core network. As a result, signaling overhead can be increased since messages have to be exchanged with the core network nodes during a handover. 
     In accordance with some implementations, techniques or mechanisms are provided to improve mobility efficiency during transfer of a UE between a macro cell and a small cell or between small cells. Note that a UE transfer from a macro cell to a small cell in the contexts discussed herein involve a UE transfer in which the UE maintains its wireless connection (or more specifically, its radio connection at least for the control plane traffic) with the macro eNB after the UE transfer, as well as establishes another wireless connection (or more specifically, another radio connection for user plane traffic) with the LeNB of the small cell that the UE is to be transferred to. 
     UE transfer between small cells refers to a UE transfer where the UE maintains its radio connection at least for the control plane with the macro eNB, but switches its radio connection for all or part of its user plane traffic to a different small cell. 
     In accordance with some implementations, to perform a UE transfer as discussed above, a data offload feature is provided over the X3 interface (also referred to as an X2e interface) between a macro eNB and an LeNB. Various functions and associated procedures over the X3 interface are provided to improve efficiency of mobility of a UE between a macro cell and a small cell, as well as between small cells within the coverage area of the macro cell. New messages can be associated with the offload functions and associated procedures, where these new messages can be communicated over the X3 interface. 
     As discussed above, a UE under the coverage of a macro cell and a small cell can be served by both the macro cell and the small cell; in other words, the UE has dual radio connections with the respective macro eNB and LeNB. In such a scenario, the macro eNB provides the control plane functions, while the LeNB can provide the bulk of user plane functions for the dual connection capable UE. 
     A connection to a small cell can be added or removed from a UE under the control of the macro eNB. The action of adding or removing a radio connection to an LeNB for a UE can be transparent to the core network, including the MME and SGW in an LTE core network, for example. A legacy UE (a UE that is incapable of performing dual radio connections with a macro eNB and an LeNB) can be connected to either a macro cell or a small cell. To support UEs not capable of performing dual connections with a macro eNB and an LeNB, a small cell can be considered to be a normal cell that has all eNB functionalities. 
     When a dual connection capable UE moves into a small cell coverage area that is within the coverage of a serving macro cell for the UE, a dual connection can be established for the UE with both the macro cell and the small cell. As noted above, the UE maintains its radio connection with the macro eNB, and the UE establishes a second radio connection with the LeNB of the small cell. The two radio connections may or may not be simultaneous. At least a portion of the data traffic for the UE can be offloaded from the macro cell to the small cell. The establishment and teardown of the second radio connection with the LeNB can be transparent to the core network. As a result, signaling overhead in the core network due to UE mobility between a macro cell and a small cell is reduced. 
       FIG. 20  is a message flow diagram illustrating exchange of messages among the following nodes for performing data offload from the macro eNB  104  to the LeNB  108 : UE  110 , LeNB  108 , macro eNB  104 , and SGW  306 . In some implementations, a data offload to a small cell does not involve any communication with the SGW  306 . In other implementations, as discussed further below in connection with  FIG. 20 , the SGW  306  may be involved in data offload to a small cell. 
     As depicted in  FIG. 20 , the macro eNB  104  sends (at  2002 ) a small cell measurement configuration message to the UE  110 . The small cell measurement configuration message configures the UE  110  to perform measurement of radio links to one or more small cells, at the carrier frequency (or frequencies) of the respective small cell(s). In response to the small cell measurement configuration message, the UE  110  performs (at  2004 ) a small cell measurement. If the UE detects (at  2006 ) that a small cell radio link quality is greater than a specified threshold, then the UE  110  sends (at  2008 ) a small cell measurement report that contains measurement data to the macro eNB  104 . The small cell measurement report can include an indication of a strength of a radio link with a given small cell (or with multiple small cells), and can identify the small cell (or small cells). 
     Based on the small cell measurement report from the UE  110 , the macro eNB  104  can determine that the UE  110  is within the coverage area of a small cell, and thus, can initiate the offloading of at least a portion of data traffic to the small cell. Offloading at least a portion of the data traffic can include offloading data traffic associated with certain radio access bearers. A radio access bearer can refer to an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) Radio Access Bearer (E-RAB), which is used to transport data between a UE and a core network node, such as the SGW. A data radio bearer (DRB) is used to transport the data of the E-RAB between a UE and an eNB. Reference to offloading a radio access bearer can refer to either offloading a given E-RAB or the corresponding DRB. 
     The macro eNB  104  sends (at  2010 ) an Offload Request message to the LeNB  108  that is part of the small cell to which data offload is to be performed. The Offload Request message can be sent over the X3 interface between the macro eNB  104  and the LeNB  108 . The Offload Request message can include certain information, including information identifying the radio access bearer(s) to be offloaded, UE profile information (to identify the UE that is the subject of the data offload), quality-of-service (QoS) profile information (to describe the QoS associated with communications with a UE  110 ), and other information. 
     In response to the Offload Request, the LeNB  108  can send (at  2012 ) an Offload Response to the macro eNB  104 . The Offload Response can also be sent through the X3 interface. The Offload Response can indicate whether the LeNB  108  has accepted or denied the Offload Request. In situations where the Offload Response indicates that the LeNB  108  has accepted the Offload Request, the Offload Response can further identify the radio access bearer(s) that is (are) accepted by the LeNB  108 . Note that the LeNB  108  can accept just a subset of the radio access bearers identified in the Offload Request from the macro eNB  104 . Alternatively, the LeNB  108  can accept all of the radio access bearers identified in the Offload Request. 
     In situations where the Offload Response indicates that the Offload Request has been denied, the Offload Response can identify a cause of the denial. Specific messages to accept or deny an Offload Request are discussed further below. 
     In some examples, the Offload Response can also include random access information, including a dedicated preamble. The random access information, including the dedicated preamble, can be used by the UE to perform a random access procedure with the LeNB  108  to establish a radio connection with the LeNB  108 . A dedicated preamble can be used by the UE  110 , and not by other UEs, to perform the random access procedure to establish the radio connection with the LeNB  108 . 
     In further examples, the Offload Response can also include system information, including certain information included in system information blocks (SIBs) and/or a master information block (MIB). The random access information and system information is sent back in the Offload Response to the macro eNB  104  to allow the macro eNB  104  to forward the random access information and system information to the UE  110  for use by the UE  110  for establishing a radio connection with the LeNB  108 . 
     If the Offload Response indicates that the Offload Request has been accepted, the macro eNB  104  can send (at  2014 ) a Small Cell Offload message to the UE  110 , to instruct the UE  110  to start the establishment of a second radio connection with the small cell identified in the Small Cell Offload message. The Small Cell Offload message can include information about the small cell that is to be used by the UE  110  to establish the second radio connection with the small cell. For example, the information an include the random access information and the system information that was included in the Offload Response from the LeNB  108  to the macro eNB  104 . 
     In cases where the PDCP layer is implemented in the LeNB  108 , such as in the  FIG. 7  arrangement discussed above, then the macro eNB  104  can also send (at  2016 ) a sequence number (SN) Status Transfer message, which includes a sequence number (SN) of a PDCP PDU and a hyper frame number of the last PDCP PDU that is to be sent to the small cell. 
     In response to the Small Cell Offload message sent at  2014 , the UE  110  performs (at  2018 ) an attachment procedure with the LeNB  108 , for establishing a radio connection with the LeNB  108 . In the small cell attachment procedure, the UE  110  can send a PRACH with the dedicated preamble that was included in the Small Cell Offload message. 
     After sending of the Offload Response (at  2012 ) indicating acceptance of the Offload Request, the LeNB  108  is ready to receive data from and transmit data to the macro eNB  104 . After receipt of the Offload Response (at  2012 ) accepting the Offload Request, the macro eNB  104  can send (at  2020 ) downlink data for the UE  110  to the LeNB  108 . The downlink data can be sent from the macro eNB  104  to the LeNB  108  over the X3 interface. In response to the downlink data received at  2020 , the LeNB  108  can forward (at  2022 ) the downlink data to the UE  110 . In alternative implementations, the macro eNB  104  can direct the LeNB  104  to receive downlink data for the UE  110  directly from the SGW  306 . In this case, the macro eNB  104  can inform the SGW  306  that the UE  110  has two eNB connections, one with the macro eNB  104  and the other with the LeNB  108 . In such implementations, the data offload is not transparent to the SGW  306 . 
     The UE  110  can also send (at  2026 ) uplink data to the LeNB  108 . The LeNB  108  in turn forwards (at  2028 ) the uplink data to the macro eNB  104 . In alternative implementations, the LeNB  108  can be directed by the macro eNB  104  to transfer the uplink data (at  2030 ) directly to the SGW  306 . 
     In some implementations, the macro eNB can send an Offload Request to each of multiple small cells for the UE  110  if the UE  110  is in the coverage areas of the multiple small cells. In such a scenario, the macro eNB  104  may receive Offload Responses from more than one small cell accepting the Offload Request. If that is the case, then the macro eNB  104  can select one of the small cells that sent Offload Responses accepting the Offload Request. After the selection, the macro eNB  104  can send an Offload Cancel message to the un-selected small cell(s) to cancel the previously sent Offload Request at the un-selected small cell(s). When an Offload Cancel message is received by an LeNB, the LeNB can release resources that were previously reserved for the data offload. 
     In other scenarios, a target small cell that may have received an Offload Request can cancel an ongoing data offload by sending an Offload Cancel Request message to the macro eNB  104 . In response to the Offload Cancel Request message, the macro eNB  104  can send an Offload Cancel message to the requesting small cell. 
     In other examples, a macro eNB can cancel an ongoing data offload by sending an Offload Reconfiguration Request message to a small cell. Generally, an Offload Reconfiguration Request message can be used by the macro eNB  104  to add or remove one or more radio access bearers from an ongoing data offload in a small cell. The Offload Reconfiguration Request message can also terminate a data offload, such as in the case of a UE moving out of the coverage area of the small cell. 
       FIG. 21  is a message flow diagram that illustrates an offload reconfiguration procedure. In  FIG. 21 , an ongoing data offload is occurring (at  2102 ), where the data offload involves the UE  110 , the LeNB  108 , and the macro eNB  104 . 
     The UE at some point can detect (at  2104 ) that the radio link quality to the LeNB  108  has dropped below a specified threshold. If that is the case, the UE  110  sends (at  2106 ) a small cell measurement report to the macro eNB  104  indicating that the radio link quality of the small cell has dropped below the specified threshold. 
     The macro eNB  104  can then perform a reconfiguration decision (at  2108 ) to reconfigure the data offload. The macro eNB  104  then sends (at  2110 ) an Offload Reconfiguration Request message to the LeNB  108 , where the Offload Reconfiguration Request message identifies radio access bearer(s) to be removed. The Offload Reconfiguration Request message can remove all of the radio access bearers or just some of the radio access bearers that were previously offloaded from the macro eNB  104  to the LeNB  108 . 
     In response to the Offload Reconfiguration Request message, the LeNB  108  sends (at  2112 ) an Offload Reconfiguration Acknowledge message, to acknowledge the Offload Reconfiguration Request message. Any uplink data for the removed radio access bearer(s) can be sent (at  2114 ) from the LeNB  108  to the macro eNB  104 . In alternative implementations, the macro eNB  104  can configure the LeNB  108  to send (at  2116 ) the uplink data for the removed radio access bearer(s) directly to the SGW  306 . 
     In arrangements where the PDCP layer is provided at the LeNB  108 , such as in the arrangement depicted in  FIG. 7 , the LeNB  108  can also send an SN Status Transfer message to the macro eNB  104  (at  2118 ), or alternatively to the SGW  306  (at  2120 ). 
     Any downlink data for the removed radio access bearer(s) that has not yet been transferred from the LeNB  108  to the UE  110  is sent to the macro eNB  104  (at  2122 ) or to the SGW  306  (at  2124 ) for forwarding to the UE  110 . 
     In some examples, before the start of small cell data offload, it may be helpful to know the type of eNB that a given adjacent eNB is (an eNB can be of either the following types: macro eNB type and LeNB type). The given eNB may exchange transmission power information with its neighbor eNB, and based on that information, the given eNB may know whether its neighboring eNB is an LeNB or a macro eNB. 
     A type of a neighbor eNB can also be determined in the following manner. During the configuration stage of an eNB, the eNB may notify its neighboring eNBs about whether or not it can support data offload for other cells—this indicates that the eNB is an LeNB. Notification of a type of an eNB can also be accomplished using an operation and maintenance (O&amp;M) procedure. 
     Offload Functions and Procedures Over the X3 Interface 
     To support data offload using a small cell, new functions and procedures can be established over the X3 interface. One such new function is a small cell data offload function (to offload data of a macro cell to a small cell). The data offload function can be associated with respective procedures, which include an offload preparation procedure (to initiate and perform the data offload), a PDCP SN status transfer procedure (to transfer PDCP SN status information), an offload cancel procedure (to cancel an ongoing data offload), and an offload reconfiguration procedure (to modify or cancel an ongoing data offload). 
     Each procedure is accomplished by exchanging respective messages between the macro eNB and an LeNB. The procedures and their respective messages are listed below in Table 1 and Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Initiating 
                 Successful Outcome 
                 Unsuccessful Outcome 
               
               
                 Procedure 
                 Message 
                 Response message 
                 Response message 
               
               
                   
               
             
            
               
                 Offload 
                 Offload 
                 Offload Request 
                 Offload Request 
               
               
                 Preparation 
                 Request 
                 Acknowledge 
                 Failure 
               
               
                 Offload 
                 Offload 
                 Offload 
                 Offload 
               
               
                 Reconfig- 
                 Reconfig- 
                 Reconfig- 
                 Reconfig- 
               
               
                 uration 
                 uration 
                 uration 
                 uration 
               
               
                   
                 Request 
                 Acknowledge 
                 Failure 
               
               
                 Offload 
                 Offload 
                 Offload 
               
               
                 Cancel 
                 Cancel 
                 Cancel 
               
               
                   
                 Request 
                 Acknowledge 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Procedure 
                 Initiating Message 
               
               
                   
                   
               
             
            
               
                   
                 Offload Preparation Cancel 
                 Offload Cancel 
               
               
                   
                 SN Status Transfer 
                 SN Status Transfer 
               
               
                   
                   
               
            
           
         
       
     
     The following describes a successful operation of an offload preparation procedure. As shown in  FIG. 22 , the macro eNB  104  initiates the offload preparation procedure by sending (at  2202 ) an Offload Request message to the target LeNB  108 . The message includes a list of radio access bearers (e.g. E-RABs) and the associated QoS parameters that the macro eNB  104  would like to offload to the target small cell. If at least one of the requested E-RABs is accepted by the small cell, the target LeNB reserves the respective resources for the data offload, and sends (at  2204 ) an Offload Request Acknowledge message back to the macro eNB  104 . 
     The macro eNB  104  can abort the offload preparation procedure if the macro eNB  104  does not receive a response from the target LeNB  108  after a specified time period. The timer implemented for this purpose may take into account the actual backhaul link delay between the macro eNB  104  and target LeNB  108 . For example, the timer can choose from a range of time limits, with the range defined between a lower time limit and an upper time limit. A lower time limit can be selected for a relative small delay backhaul link, such as one implemented with an optical cable, while a higher time limit can be selected for a relatively long-delay backhaul link, such as one implemented with a wireless link. 
     For each E-RAB for which the macro eNB  104  is requesting an offload of the corresponding downlink data, the macro eNB  104  includes a DL Forwarding information element (IE) within an E-RABs To be Setup Item IE of the Offload Request message. 
     For each E-RAB in the E-RABs To be Setup Item IE for which the macro eNB  104  is requesting acceptance of uplink data transfer from the target LeNB, the macro eNB  104  can include a UL GTP Tunnel Endpoint IE to indicate that the macro eNB  104  is requesting data forwarding, by the LeNB, of uplink data received by the LeNB associated with the respective E-RAB to the macro eNB. 
     The target LeNB can include the E-RABs for which resources have been prepared at the target LeNB in an E-RABs Admitted List IE for both downlink data and uplink data. The target LeNB can include the E-RABs that have not been admitted in the E-RABs Not Admitted List IE for both downlink and uplink data, with a respective cause value. 
     For each E-RAB that the target LeNB has decided to admit, the target LeNB may include a DL GTP Tunnel Endpoint IE within the E-RABs Admitted Item IE of the Offload Request Acknowledge message to indicate that the target LeNB accepts the proposed offload of downlink data for this E-RAB and the corresponding address at the LeNB for the macro eNB to forward DL data associated to the E-RAB. 
     The allocation of resources according to values of an Allocation and Retention Priority IE included in an E-RAB Level QoS Parameters IE can follow the principles described for an E-RAB Setup procedure in 3GPP TS 36.413. 
     Upon reception of an Offload Request Acknowledge message, the macro eNB  104  can terminate the offload preparation procedure. The macro eNB  104  is then considered to have a prepared data offload. 
     The Offload Request Acknowledge message may include the dedicated preamble for the UE, other random access (RA) procedure information, and even partial information from an MIB and some SIBs (e.g. SIB1 and SIB2). The Offload Request Acknowledge message may also include other UE-specific radio link configuration information, such as PUCCH information, Sounding Reference Signal (SRS) information, PUSCH information, and so forth. 
     Although reference is made to data offloading E-RABs in the present discussion, it is noted that data offloading can also be applied to data at other protocol levels. 
     An unsuccessful operation of an offload preparation procedure initiated by an Offload Request message (sent at  2302 ) is depicted in  FIG. 23 . If the target LeNB  108  rejects all the requested radio access bearers, or a failure occurs during the offload preparation procedure, the target LeNB  108  can send (at  2304 ) an Offload Request Failure message to the macro eNB  104 . The Offload Request Failure message can include a Cause IE that has a respective value to indicate the cause of the denial. 
     A successful operation of an offload reconfiguration procedure is depicted in  FIG. 24 . The offload reconfiguration procedure can be used to modify an ongoing data offload, by adding and/or removing radio access bearers. This procedure can also be used to cancel an ongoing data offload if the list of radio access bearers includes all radio access bearers offloaded to the target LeNB and there is no new radio access bearer to be added. 
     The macro eNB  104  initiates the offload reconfiguration procedure by sending (at  2402 ) the Offload Reconfiguration Request message to the target LeNB  108 . After the macro eNB  104  sends the Offload Reconfiguration Request message, the macro eNB  104  aborts the offload reconfiguration procedure if the macro eNB  104  does not receive a response from the target LeNB after a specified time period. 
     For each new E-RAB for which the macro eNB  104  is requesting to do offload of downlink data, the macro eNB  104  can include the DL Forwarding IE within the E-RABs To be Setup Item IE of the Offload Reconfiguration Request message. 
     For each new radio access bearer in the E-RABs To be Setup Item IE for which the macro eNB  104  is requesting to accept forwarding of uplink data, the macro eNB  104  can include the UL GTP Tunnel Endpoint IE to indicate that the macro eNB  104  is requesting data forwarding of uplink data received by the LeNB for that E-RAB to the macro eNB. 
     For each E-RAB for which the macro eNB  104  is requesting to remove from offload, the macro eNB  104  can include the DL GTP Tunnel Endpoint IE to indicate that the macro eNB  104  is requesting the small cell eNB to forward any unsent DL data of the E-RAB back to the macro eNB. 
     The target LeNB sends (at  2404 ) the Offload Reconfiguration Acknowledge message back to the macro eNB  104  regardless whether any requested new E-RABs are admitted or not. The target LeNB  108  can include the E-RABs for which resources have been prepared at the target LeNB  108  in the E-RABs Admitted List IE. The target LeNB  108  can also include the E-RABs that have not been admitted in the E-RABs Not Admitted List IE with an appropriate Cause value. 
     For each E-RAB that it has decided to admit, the target LeNB  108  can include the DL GTP Tunnel Endpoint IE within the E-RABs Admitted Item IE of the Offload Reconfiguration Acknowledge message to indicate that target LeNB  108  has accepted the proposed forwarding of downlink data for this E-RAB from the macro eNB to the LeNB. 
     If none of the new E-RABs requested is admitted or there is no new E-RAB is requested, the target LeNB  108  sends the Offload Reconfiguration Acknowledge message back to the macro eNB  104  to indicate the successful removal of the E-RABs requested. 
     Upon reception of the Offload Reconfiguration Acknowledge message, the macro eNB  104  terminates the offload reconfiguration procedure. 
       FIG. 25  illustrates an unsuccessful operation of an offload reconfiguration procedure. If a failure occurs during the offload reconfiguration procedure initiated by an Offload Reconfiguration Request message sent (at  2502 ), the target LeNB  108  sends (at  2504 ) an Offload Reconfiguration Failure message to the macro eNB  104 , where the message contains a Cause IE with a respective value to indicate the cause of the failure. 
       FIG. 26  illustrates an offload cancel procedure. The target LeNB  108  initiates the procedure by sending (at  2602 ) an Offload Cancel Request message to the macro eNB  104 . In response, the macro eNB  104  sends (at  2604 ) an Offload Cancel Acknowledge message to the target LeNB  108 . If the macro eNB  104  wants the target LeNB  108  to transfer any downlink data back to the macro eNB  104 , the macro eNB  104  can include the list of E-RABs and the corresponding DL GTP tunnel points. After receiving the Offload Cancel Acknowledge message, the target LeNB can transfer all uplink data (and downlink data if requested) to the macro eNB  104  and releases all resources associated with the data offload. 
     Upon receiving the Offload Cancel REQUEST message, the macro eNB can trigger the UE  110  to perform measurement. The macro eNB  104  makes the final decision on where the UE  110  is to be transferred to another small cell. 
       FIG. 27  illustrates an example offload preparation cancel procedure. The macro eNB  104  initiates the procedure by sending (at  2702 ) the Offload Cancel message to the target LeNB  108 . The macro eNB  104  can indicate the reason for cancelling the data offload by including an appropriate Cause value. The reason for the cancellation may be that the macro eNB  104  has initiated an offload preparation procedure with multiple small cells and only one small cell is selected, so the macro eNB  104  has to cancel the offload preparation procedure for the un-selected small cells. 
     At the reception of the Offload Cancel message, the target LeNB  108  can remove any reference to, and release, any resources previously reserved for the data offload. 
     The purpose of the SN status transfer procedure is to transfer the downlink PDCP sequence number and hyper frame number transmitter status between eNBs (macro eNB to LeNB or vice versa) for each respective E-RAB for which PDCP sequence number and hyper frame number status preservation applies. The procedure is only applicable when PDCP SDUs are sent to a small cell and the PDCP layer is implemented in the small cell. The procedure is the same as that in the normal handover case. 
     When the data offload is carried out at lower portions of the protocol stack (below the PDCP layer), the SN status transfer does not have to be a separate step. Instead, the SN status transfer information can be carried as part of another message. Also the sequence numbers transferred can be a PDCP sequence number or an RLC sequence number, depending on the type of packets being offloaded. 
     Messages to Support Offload Procedures 
     In some examples, the Offload Request message can include the following IEs:
         Message Type (to identify a type of the message);   Macro eNB UE X3 ID (identifier allocated to the UE over the X3 interface at the macro eNB);   SmallCell Cell ID (to identify the small cell that is the subject of the data offload);   UE Context Information.       

     The UE Context Information of the Offload Request message includes the following IEs:
         UE Aggregate Maximum Bit Rate (to indicate the maximum bit rate that may be supported for the UE);   E-RABs To Be Setup List, which includes:
           E-RABs To Be Setup Item, which in turn includes:
               E-RAB ID (to identify a specific E-RAB);   E-RAB Level QoS Parameters (to indicate QoS for the E-RAB);   DL Forwarding (to indicate forwarding of downlink data to the LeNB);   UL GTP Tunnel Endpoint (to identify the X3 transport bearer used for forwarding of uplink data);   
               
           Some RRC Context including one or more of the following
           UE ID (i.e. radio network temporary identifier (RNTI));   UE radio capability information.   
               

     In case of data distribution before the PDCP layer, the following additional information is included in the Offload Request message as part of the RRC context:
         PDCP configuration   UE security capability information;   Security context to be used in the LeNB.       

     In case that the UE is only capable of a single radio connection in any given subframe, the macro cell and small cell can schedule data to the UE in different subframes. In this case, the macro eNB may also suggest a subframe pattern to the target small cell to use for scheduling data to the UE. Thus a subframe pattern may also be included in the Offload Request message. 
     The Offload Request Acknowledge message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID (identifier allocated to the UE over the X3 interface at the macro eNB);   SmallCell eNB UE X3 ID (identifier allocated to the UE over the X3 interface at the target LeNB);   E-RABs Admitted List, which includes:
           E-RABs Admitted Item, which in turn includes:
               E-RAB ID (to identify an E-RAB that has been admitted);   DL GTP Tunnel Endpoint (to identify the X3 transport bearer used for forwarding downlink data to the LeNB);   
               
           E-RABs Not Admitted List (includes one or more E-RAB IDs of E-RAB(s) that has or have not been admitted);   Common and UE specific radio configurations for the UE in the small cell, which are to be relayed to the UE by the macro-eNB. It can include the following information
           UE ID assigned in the small cell;   Small cell radio resource configurations for the UE including:
               DRB reconfiguration;   MAC layer configuration;   Physical channel configurations.   
               
               

     The Offload Request Failure message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   Cause;   Criticality Diagnostics (containing diagnostic information).       

     The Offload Reconfiguration Request message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   Cause;   SmallCell Cell ID;   E-RABs To Be Setup List, which includes:
           E-RABs To Be Setup Item, which in turn includes:
               E-RAB ID;   E-RAB Level QoS Parameters;   
               
           E-RABs To Be Removed List, which includes:
           E-RABs To Be Removed Item, which in turn includes:
               E-RAB ID;   DL GTP Tunnel Endpoint.   
               
               

     When an E-RAB is removed, the macro eNB  104  may also request the small cell to forward downlink data associated with the E-RAB to the macro eNB  104 . For received uplink data, the small cell can forward all of the uplink data to the macro eNB  104 . 
     The Offload Reconfiguration Acknowledge message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   Cause;   SmallCell Cell ID;   E-RABs Admitted List, which includes:
           E-RABs Admitted Item, which in turn includes:
               E-RAB ID.   
               
               

     The Offload Reconfiguration Failure message can include the following IEs:
         Message Type;   Macro eNB UE ID   Cause.       

     The Offload Cancel Request message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   SmallCell eNB UE X3 ID.       

     The Offload Cancel Acknowledge message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   SmallCell eNB UE X3 ID;   E-RABs List, which includes:
           E-RABs Item, which in turn includes:
               E-RAB ID;   DL GTP Tunnel Endpoint   
               
               

     The Offload Cancel message can include the following IEs:
         Message Type;   Macro eNB UE X3 ID;   SmallCell eNB UE X3 ID;   Cause.       

     Data Forwarding Between Small Cells 
     After data offloading from a macro cell to a first small cell has occurred, such as according to the procedure depicted in  FIG. 21 , there can be scenarios where the UE  110  is to be transferred from the first small cell to a second small cell. In this case, the macro eNB  104  can manage UE transfer of the UE  110  from the first small cell to the second small cell, while maintaining a radio connection between the UE  110  and the macro eNB  104 . UE transfer of the UE  110  from the first small cell to the second small cell causes the data offload to be transferred from the first small cell to the second small cell; in other words, handling of at least some radio access bearer(s) is transferred from the first small cell to the second small cell. 
     The UE transfer of the UE  110  from the first small cell to the second small cell can be initiated by (1) the macro eNB  104  in response to a measurement report from the UE  110  to the macro eNB  104  indicating that the UE  110  should no longer be served by the first small cell; or (2) the LeNB of the first small cell, which can occur due to various reasons, such as to achieve load balancing at the LeNB of the first small cell or for some other reason. The first small cell can initiate a UE transfer by sending an Offload Cancel Request message over the X3 interface to the macro eNB  104 . 
     Once UE transfer is performed from the first small cell to the second small cell, data forwarding can occur. Data forwarding refers to transferring data for the UE  110  from the first small cell to the second small cell, where the transferred data can include either downlink data or uplink data that has not yet been communicated by the first small cell to the appropriate next destination (UE  110  for downlink data and the macro eNB  104  for uplink data). Data forwarding can occur either directly from the first small cell to the second small cell, or indirectly from the first small cell to the second small cell via a shared macro eNB. If the first and second small cells do not share the same macro eNB, then data forwarding may be accomplished indirectly via multiple respective macro eNBs. 
     Alternatively, instead of performing data forwarding as part of the transfer of the data offload from the first small cell to the second small cell, the UE can first establish a radio link with the second small cell, and then after establishing the radio link with the second small cell, can terminate the radio link with the current small cell. In this way, data forwarding between the first and second small cells does not have to be performed. 
       FIG. 28  is a message flow diagram illustrating UE transfer of a UE  110  (more specifically, transfer of the data offload for the UE  110 ) from a first small cell to a second small cell. The nodes involved in such a procedure include the UE  110 , a first LeNB  108 - 1  of the first small cell, a macro eNB  104 , and a second LeNB  108 - 2  of the second small cell. As shown in  FIG. 28 , an ongoing data offload has been established (at  2802 ) between the macro eNB  104  and the first LeNB  108 - 1 , such as by using the process depicted in  FIG. 20 . 
     At some later point in time, the UE  110  can detect (at  2804 ) that the radio link quality to the second small cell exceeds a specified threshold. Although not shown, the UE  110  may also detect that the radio link quality to the first small cell has deteriorated. In response to such a determination at  2804 , the UE  110  sends (at  2806 ) a small cell measurement report to the macro eNB  104 . The small cell measurement report identifies the second small cell and contains an indication that the radio link to the second small cell exceeds the specified threshold (and possibly an indication of the quality of the radio link to the first small cell). 
     The macro eNB  104  can make a decision (at  2808 ) to perform data offload to the second small cell (effectively transferring the data offload from the first small cell to the second small cell). The macro eNB  104  sends (at  2810 ) an Offload Request message to the second LeNB  108 - 2 , which responds (at  2812 ) with an Offload Request Acknowledge message (to accept the request) or Offload Request Failure message (to deny the request). 
     Assuming that the LeNB  108 - 2  has accepted the Offload Request, the macro eNB  104  can send (at  2814 ) an Offload Reconfiguration Request message to the first LeNB  108 - 1 . The Offload Reconfiguration Request message can identify the radio access bearer(s) that is (are) to be removed from the first LeNB  108 - 1 . The first LeNB  108 - 1  responds (at  2816 ) with an Offload Reconfiguration Acknowledge message. The first LeNB  108 - 1  also sends uplink data associated with the removed radio access bearer(s) either to the macro eNB  104  (at  2818 ) or to the second LeNB  108 - 2  (at  2820 ). 
     In implementations where the PDCP layer is also included in the LeNBs (such as according to the arrangement of  FIG. 7 ), the first LeNB  108 - 1  can also send a PDCP SN Status Transfer message to the macro eNB  104  (at  2822 ) or to the second LeNB  108 - 2  (at  2824 ). 
     Similarly, the first LeNB  108 - 1  can also send any downlink data for the removed radio access bearer(s) to the macro eNB  104  (at  2826 ) or to the second LeNB  108 - 2  (at  2828 ). 
     Once all uplink data and downlink data for the removed radio access bearer(s) have been transferred from the first LeNB  108 - 1  to the second LeNB  108 - 2  (either directly between the first LeNB  108 - 1  and second LeNB  108 - 2  or indirectly through the macro eNB  104 ), the first LeNB  108 - 1  can release (at  2830 ) offload resources. 
     In response to receipt of the Offload Reconfiguration Acknowledge message from the second LeNB  108 - 2 , the macro eNB  104  sends (at  2832 ) a Small Cell Offload message to the UE  110 , where this Small Cell Offload message contains information for the second LeNB  108 - 2 . In response, using the information (e.g. random access information and system information) in the Small Cell Offload message, the UE  110  performs (at  2834 ) an attachment procedure with the second LeNB  108 - 2 . 
     Downlink data for the UE  110  can subsequently be sent (at  2836 ) from the macro eNB  104  to the second LeNB  108 - 2 . The second LeNB  108 - 2  then forwards (at  2838 ) the downlink data to the UE  110 . In the uplink direction, the UE  110  can send (at  2840 ) uplink data to the second LeNB  108 - 2 . The second LeNB  108 - 2  then forwards (at  2842 ) the uplink data to the macro eNB  104 . 
     In alternative implementations, if macro eNB  104  is a relay point for data, say at the PDCP layer, the macro eNB  104  can store local copies of PDCP PDUs. An LeNB will notify the macro eNB  104  about the delivery status of each PDCP PDU. Unless a PDCP PDU is delivered successfully, the macro eNB  104  may not remove its local copy of the PDCP PDU. The delivery status of each PDCP PDU is thus fully available in the macro eNB  104 . In this case, when the UE transfer from the first small cell to the second small cell occurs, the macro eNB  104  is able to fully control the data forwarding. The operation of the first and second small cells can be simplified since no data is to be forwarded from the first small cell to the second small cell or macro cell. The macro cell will forward any undelivered PDCP PDU to the second small cell. 
     In some implementations, the availability of a direct forwarding path from the first small cell to the second small cell is determined in the first LeNB  108 - 1 , and can be indicated to the macro eNB  102 . If X2 connectivity is available between the LeNBs  108 - 1  and  108 - 2 , then a direct forwarding path may be available. The direct forwarding path may also be available during the configuration stage of a small cell or during the UE transfer initialization stage. If a direct forwarding path is not available, indirect forwarding may be used. The macro eNB  104  uses the indication (of whether a direct forwarding path is available) from the first LeNB  108 - 1  to determine whether to apply indirect forwarding, and if another macro eNB should be contacted for the indirect forwarding. 
     As an example, the first LeNB  108 - 1  may provide a list of target LeNBs that the first LeNB  108 - 1  has an X2 connection with. This list may be carried in a new information and included in one of the messages sent from the first LeNB  108 - 1  to the macro eNB  104 . As an example, an information element called “Direct Forwarding Path Availability” can be defined. This information element is optional over the X3 interface, where its absence can be interpreted as “direct path not available.” 
       FIG. 28  depicts an example process in which UE transfer from the first small cell to the second small cell is initiated by the macro eNB  104 . In other examples, a UE transfer may be initiated by the first LeNB  108 - 1 , which can cause the macro eNB  104  to decide to transfer the UE  110  to the second LeNB  108 - 2  attached to the same macro eNB  104 . A process for performing such UE transfer is depicted in  FIG. 29 . 
     An ongoing data offload has been established (at  2902 ) between the macro eNB  104  and the first LeNB  108 - 1 , such as by using the process depicted in  FIG. 20 . To initiate a UE transfer for any reason, the first LeNB  108 - 1  sends (at  2904 ) an Offload Cancel Request message over the X3 interface to the macro eNB  104 . In response, the macro eNB  104  can configure the UE  110  to initiate a measurement procedure if not configured earlier, or the macro eNB  104  can trigger the UE  110  to start the measurement procedure with new measurement configurations. To perform either of the foregoing, the macro eNB  104  sends (at  2906 ) a small cell measurement configuration message to the UE  110 . The UE  110  may send (at  2908 ) a measurement report, and based on the measurement data in the measurement report, the macro eNB  104  can make a decision (at  2910 ) to offload to the second LeNB from a set of target LeNBs. The macro eNB  104  can also simply choose the second LeNB from prior measurement reports if they are not out-of-date. 
     The macro eNB  104  next sends (at  2912 ) an Offload Request message to the second LeNB  108 - 2  with information identifying radio access bearer(s) to be offloaded to the second small cell. If the second LeNB  108 - 2  accepts the Offload Request, the second LeNB  108 - 2  responds (at  2914 ) with an Offload Request Acknowledge message to the macro eNB  104 . 
     The macro eNB  104  then sends (at  2916 ) an Offload Cancel message to the first LeNB  108 - 1 . The remaining process is similar to tasks  2830 - 2842  depicted in  FIG. 28 . 
     In another example, in a UE transfer initiated by the macro eNB  104 , the macro eNB  104  can decide to move all of the UE&#39;s radio access bearers or a subset of the UE&#39;s radio access bearers back to the macro eNB  104  itself. The corresponding process is outlined as follows. 
     The macro eNB  104  sends an Offload Reconfiguration Request message to the first LeNB  108 - 1 . Within the Offload Reconfiguration Request message, the macro eNB  104  provides a list of radio access bearers that are to be removed. If the list contains all the radio access bearers of the UE that are handled by the first LeNB  108 - 1 , then the first LeNB  108 - 1  can consider the UE as being completely transferred over to the macro eNB  104 . The first LeNB  108 - 1  responds with an Offload Reconfiguration Acknowledge message back to the macro eNB  104 . 
     Alternatively, the macro eNB  104  can send an Offload Cancel message to the first LeNB  108 - 1 , instead of the Offload Reconfiguration Request message. 
     System Architecture 
       FIG. 30  is a block diagram of an example system  3000  that can be any of various nodes described above, include a UE, LeNB, macro eNB, and LeNB GW. The system  2000  includes machine-readable instructions  3002  that can perform tasks of any of the foregoing entities as discussed above. The machine-readable instructions  3002  are executable on one or multiple processors  3004 . A processor can include a microprocessor, microcontroller, processor module or subsystem, programmable integrated circuit, programmable gate array, or another control or computing device. 
     The processor(s)  3004  can be coupled to one or multiple communication interfaces  3006 , which enable communication between the system  3000  and one or more other nodes. Each communication interface  3006  includes network interface hardware in addition to firmware or software for implementing higher layers (including those protocol layers discussed above). The system  3000  also includes a storage medium (or storage media)  3008  to store data and instructions. 
     The storage medium (or storage media)  2008  can be implemented as one or more computer-readable or machine-readable storage media. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; optical media such as compact disks (CDs) or digital video disks (DVDs); or other types of storage devices. Note that the instructions discussed above can be provided on one computer-readable or machine-readable storage medium, or alternatively, can be provided on multiple computer-readable or machine-readable storage media distributed in a large system having possibly plural nodes. Such computer-readable or machine-readable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components. The storage medium or media can be located either in the machine running the machine-readable instructions, or located at a remote site from which machine-readable instructions can be downloaded over a network for execution. 
     In the foregoing description, numerous details are set forth to provide an understanding of the subject disclosed herein. However, implementations may be practiced without some or all of these details. Other implementations may include modifications and variations from the details discussed above. It is intended that the appended claims cover such modifications and variations.