Patent Publication Number: US-2018049085-A1

Title: Concurrent Connectivity Techniques

Description:
TECHNICAL FIELD 
     The present application relates to wireless devices, and more particularly to techniques for concurrent connectivity of a mobile device to multiple wireless base stations. 
     DESCRIPTION OF THE RELATED ART 
     Wireless communication systems are rapidly growing in usage. Further, wireless communication technology has evolved from voice-only communications to also include the transmission of data, such as Internet and multimedia content. In some networks, dual-connectivity techniques allow a mobile device to communicate using scheduled radio resources of multiple base stations during the same time interval. One of the base stations may be a master base station (referred to in LTE as a MeNB) with a connection to a mobility management entity (MME) for the mobile device. The other base station may be a secondary base station (referred to in LTE as a SeNB) that provides additional radio resources. 
     When switching to a different SeNB, the MeNB releases a current source SeNB (S-SeNB) when allocation of a new target SeNB (T-SeNB) is successful. Traditionally, it is not possible to keep resources for the mobile device on both the S-SeNB and the T-SeNB, which reduces throughput during the handover procedure. This reduction in throughput may be substantial, especially in situations with small SeNB cells that are densely deployed such that handover occurs frequently. 
     Therefore, techniques for concurrent connectivity of multiple SeNBs (e.g., after successful allocating a T-SeNB) may be desired. Further, techniques for deciding when to release an S-SeNB may be desired. 
     SUMMARY 
     Embodiments described herein relate to concurrent wireless connectivity. In some embodiments, a base station apparatus includes one or more processing elements and one or more memories having program instructions stored thereon that are executable by the one or more processing elements to perform the following operations. In some embodiments, the operations include communicating with a mobile device as a master base station during a time interval in which the mobile device is also assigned radio resources by a first secondary base station. In some embodiments, the operations include requesting that a second secondary base station allocate radio resources for the mobile device during the time interval, without releasing the first secondary base station, such that radio resources of both the first and second secondary base stations are allocated to the mobile device during the time interval. 
     In some embodiments, the request is an addition request that indicates that one or more current secondary base stations will not be released, wherein the apparatus is configured not to release the first secondary base station in response to receiving an acknowledgment to the addition request that indicates one or more current secondary base stations should not be released. 
     In some embodiments, the apparatus is configured to release the first secondary base station from connection with the mobile device in response to a signal strength measurement for the first secondary base station that is reported by the mobile device. In some embodiments, the apparatus is configured to release the first secondary base station from connection with the mobile device in response to an indication from the first secondary base station. 
     In some embodiments, the master base station is a master eNB (MeNB) that terminates at least one S1-MME connection for the mobile device and wherein the first and second secondary base stations are secondary eNBs (SeNBs) that do not maintain S1-MME connections for the mobile device. 
     In some embodiments, a method includes communicating with a mobile device as a master base station during a time interval in which the mobile device is also assigned radio resources by a first secondary base station. In some embodiments, the method further includes requesting that a second secondary base station allocate radio resources for the mobile device during the time interval, without releasing the first secondary base station, such that radio resources of both the first and second secondary base stations are allocated to the mobile device for communicating during the time interval. 
     In some embodiments, a mobile device apparatus includes one or more processing elements and one or more memories having program instructions stored thereon that are executable by the one or more processing elements to perform the following operations. In some embodiments, the operations include communicating via a master base station and a first secondary base station using radio resources allocated by both the master base station and the secondary base station during a time interval. In some embodiments, the operations include receiving a configuration message from the master base station indicating that radio resources are also available on a second secondary base station, wherein the master base station does not release the first secondary base station in response to addition of the second secondary base station. In some embodiments, the operations include communicating, during the time interval, using radio resources allocated by both the first and second secondary base stations. 
     In various embodiments, the disclosed techniques may increase wireless data throughput relative to conventional techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present disclosure can be obtained when the following detailed description of the embodiments is considered in conjunction with the following drawings. 
         FIG. 1  illustrates an example user equipment (UE) according to some embodiments. 
         FIG. 2  illustrates an example wireless communication system where a UE communicates with two base stations. 
         FIG. 3  is an example block diagram of a base station, according to some embodiments. 
         FIG. 4  is an example block diagram of a UE, according to some embodiments. 
         FIG. 5  illustrates an exemplary wireless communication system with concurrent connectivity, according to some embodiments. 
         FIG. 6  is a communications diagram illustrating dual connectivity, according to some embodiments. 
         FIG. 7  is a communications diagram illustrating concurrent connectivity, according to some embodiments. 
         FIG. 8  is a flow diagram illustrating an exemplary method, according to some embodiments. 
     
    
    
     While the embodiments described in this disclosure may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims. 
     This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     Various units, circuits, or other components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs the task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the “configured to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112(f) for that unit/circuit/component. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     This disclosure initially lists relevant acronyms and a glossary. It then describes, with reference to  FIGS. 1-4 , exemplary embodiments of communications between a mobile device and one or more base stations. Exemplary concurrent connectivity systems and techniques are discussed with reference to  FIGS. 4-7  while  FIG. 8  illustrates an exemplary method. 
     Acronyms 
     The following acronyms are used in the present disclosure. 
     3GPP: Third Generation Partnership Project 
     3GPP2: Third Generation Partnership Project  2   
     BER: Bit Error Rate 
     CDMA: Code Division Multiple Access 
     CPTR: Common Periodic Time Reference 
     DDR: Double Data Rate 
     eNB: evolved node B 
     EVM: Error Vector Magnitude 
     FFT: Fast Fourier Transform 
     FPGA: Field Programmable Gate Array 
     GSM: Global System for Mobile Communications 
     LTE: Long Term Evolution 
     MeNB: Master eNB 
     MIMO: Multiple Input Multiple Output 
     MRT: Maximum Radio Transmission 
     OFDM: Orthogonal Frequency-Division Multiplexing 
     PER: Packet Error Rate 
     PCIe: Peripheral Component Interconnect Express 
     PLMN: Public Land Mobile Network 
     PXIe: PCI eXtensions for Instrumentation Express 
     RAT: Radio Access Technology 
     RX: Receive 
     SDR: Software Defined Radio 
     SeNB: Seconday eNB 
     S-SeNB: Source SeNB 
     SRP: Software Radio Peripheral 
     T-SeNB: Target SeNB 
     TX: Transmit 
     UE: User Equipment 
     UMTS: Universal Mobile Telecommunications System 
     WCDMA: Wideband Code Division Multiple Access 
     ZF: Zero Forcing 
     Terms 
     The following is a glossary of terms used in the present application: 
     Memory Medium—Any of various types of memory devices or storage devices. The term “memory medium” is intended to include an installation medium, e.g., a CD-ROM, floppy disks, or tape device; a computer system memory or random access memory such as DRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash, magnetic media, e.g., a hard drive, or optical storage; registers, or other similar types of memory elements, etc. The memory medium may include other types of memory as well or combinations thereof. In addition, the memory medium may be located in a first computer system in which the programs are executed, or may be located in a second different computer system which connects to the first computer system over a network, such as the Internet. In the latter instance, the second computer system may provide program instructions to the first computer for execution. The term “memory medium” may include two or more memory mediums which may reside in different locations, e.g., in different computer systems that are connected over a network. The memory medium may store program instructions (e.g., embodied as computer programs) that may be executed by one or more processors. 
     Carrier Medium—a memory medium as described above, as well as a physical transmission medium, such as a bus, network, and/or other physical transmission medium that conveys signals such as electrical, electromagnetic, or digital signals. 
     Computer System—any of various types of computing or processing systems, including a personal computer system (PC), mainframe computer system, workstation, network appliance, Internet appliance, personal digital assistant (PDA), personal communication device, smart phone, television system, grid computing system, or other device or combinations of devices. In general, the term “computer system” can be broadly defined to encompass any device (or combination of devices) having at least one processor that executes instructions from a memory medium. 
     User Equipment (UE) (or “UE Device”)—any of various types of computer systems devices which are mobile or portable and which performs wireless communications. Examples of UE devices include mobile telephones or smart phones (e.g., iPhone™, Android™-based phones), portable gaming devices (e.g., Nintendo DS™, PlayStation Portable™, Gameboy Advance™, iPhone™), laptops, PDAs, portable Internet devices, music players, data storage devices, other handheld devices, as well as wearable devices such as wrist-watches, headphones, pendants, earpieces, etc. In general, the term “UE” or “UE device” can be broadly defined to encompass any electronic, computing, and/or telecommunications device (or combination of devices) which is easily transported by a user and capable of wireless communication. 
     Base Station—The term “Base Station” has the full breadth of its ordinary meaning, and at least includes a wireless communication station installed at a fixed location and used to communicate as part of a wireless telephone system or radio system. 
     Processing Element—refers to various elements or combinations of elements. Processing elements include, for example, circuits such as an ASIC (Application Specific Integrated Circuit), portions or circuits of individual processor cores, entire processor cores, individual processors, programmable hardware devices such as a field programmable gate array (FPGA), and/or larger portions of systems that include multiple processors. 
     Automatically—refers to an action or operation performed by a computer system (e.g., software executed by the computer system) or device (e.g., circuitry, programmable hardware elements, ASICs, etc.), without user input directly specifying or performing the action or operation. Thus the term “automatically” is in contrast to an operation being manually performed or specified by the user, where the user provides input to directly perform the operation. An automatic procedure may be initiated by input provided by the user, but the subsequent actions that are performed “automatically” are not specified by the user, i.e., are not performed “manually”, where the user specifies each action to perform. For example, a user filling out an electronic form by selecting each field and providing input specifying information (e.g., by typing information, selecting check boxes, radio selections, etc.) is filling out the form manually, even though the computer system must update the form in response to the user actions. The form may be automatically filled out by the computer system where the computer system (e.g., software executing on the computer system) analyzes the fields of the form and fills in the form without any user input specifying the answers to the fields. As indicated above, the user may invoke the automatic filling of the form, but is not involved in the actual filling of the form (e.g., the user is not manually specifying answers to fields but rather they are being automatically completed). The present specification provides various examples of operations being automatically performed in response to actions the user has taken. 
     FIG.  1 —User Equipment 
       FIG. 1  illustrates an example user equipment (UE)  106  according to some embodiments. The term UE  106  may be any of various devices as defined above. UE device  106  may include a housing  12  which may be constructed from any of various materials. UE  106  may have a display  14 , which may be a touch screen that incorporates capacitive touch electrodes. Display  14  may be based on any of various display technologies. The housing  12  of the UE  106  may contain or comprise openings for any of various elements, such as home button  16 , speaker port  18 , and other elements (not shown), such as microphone, data port, and possibly various other types of buttons, e.g., volume buttons, ringer button, etc. 
     The UE  106  may support one or more radio access technologies (RATs). For example, UE  106  may be configured to communicate using any of various RATs such as two or more of Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access (CDMA) (e.g., CDMA2000 1XRTT or other CDMA radio access technologies), Long Term Evolution (LTE), LTE Advanced (LTE-A), and/or other RATs. For example, the UE  106  may support at least two radio access technologies such as LTE and GSM. Various different or other RATs may be supported as desired. 
     The UE  106  may comprise one or more antennas. The UE  106  may also comprise any of various radio configurations, such as various combinations of one or more transmitter chains (TX chains) and two or more receiver chains (RX chains). For example, the UE  106  may comprise two radios that may each support one or more RATs. The two radios may each comprise a single TX (transmit) chain and a single RX (receive) chain. Alternatively, the two radios may each comprise an RX chain and may share a single TX chain. UE  106  is described in further detail below with reference to  FIG. 3 . 
     FIG.  2 —Communication System 
       FIG. 2  illustrates an exemplary (and simplified) wireless communication system. It is noted that the system of  FIG. 2  is merely one example of a possible system, and embodiments may be implemented in any of various systems, as desired. 
     As shown, the exemplary wireless communication system includes base stations  102 A and  102 B which communicate over a transmission medium with one or more user equipment (UE) devices, represented as UE  106 . The base stations  102  may be base transceiver stations (BTS) or cell sites, and may include hardware that enables wireless communication with the UE  106 . Each base station  102  may also be equipped to communicate with a core network  100 . For example, base station  102 A may be coupled to core network  100 A, while base station  102 B may be coupled to core network  100 B. Each core network  100  may also be coupled to one or more external networks (such as external network  108 ), which may include the Internet, a Public Switched Telephone Network (PSTN), and/or any other network. Thus, the base stations  102  may facilitate communication between the UE devices  106  and/or between the UE devices  106  and the networks  100 A,  100 B, and  108 . 
     The base stations  102  and the UEs  106  may be configured to communicate over the transmission medium using any of various RATs (also referred to as wireless communication technologies or telecommunication standards), such as LTE, W-CDMA, TD-SCDMA, and GSM, among possible others such as UMTS, LTE-A, CDMA2000 (e.g., 1xRTT, 1xEV-DO, HRPD, eHRPD), etc. 
     Base stations  102 A and  102 B and other base stations operating according to the same or different RATs or cellular communication standards may thus be provided as a network of cells, which may provide continuous or nearly continuous overlapping service to UE  106  and similar devices over a wide geographic area via one or more radio access technologies (RATs). 
     FIG.  3 —Base Station 
       FIG. 3  illustrates an exemplary block diagram of a base station  102 . It is noted that the base station of  FIG. 3  is merely one example of a possible base station. As shown, the base station  102  may include processor(s)  304  which may execute program instructions for the base station  102 . The processor(s)  304  may also be coupled to memory management unit (MMU)  340 , which may be configured to receive addresses from the processor(s)  304  and translate those addresses to locations in memory (e.g., memory  360  and read only memory (ROM)  350 ) or to other circuits or devices. 
     The base station  102  may include at least one network port  370 . The network port  370  may be configured to couple to a telephone network and provide a plurality of devices, such as UE devices  106 , access to the telephone network as described above. 
     The network port  370  (or an additional network port) may also or alternatively be configured to couple to a cellular network, e.g., a core network of a cellular service provider. The core network may provide mobility related services and/or other services to a plurality of devices, such as UE devices  106 . In some cases, the network port  370  may couple to a telephone network via the core network, and/or the core network may provide a telephone network (e.g., among other UE devices  106  serviced by the cellular service provider). 
     The base station  102  may include at least one antenna  334 . The at least one antenna  334  may be configured to operate as a wireless transceiver and may be further configured to communicate with UE devices  106  via radio  330 . The antenna  334  communicates with the radio  330  via communication chain  332 . Communication chain  332  may be a receive chain, a transmit chain or both. The radio  330  may be configured to communicate via various RATs, including, but not limited to, LTE, GSM, WCDMA, CDMA2000, etc. 
     The processor(s)  304  of the base station  102  may be configured to implement part or all of the methods described herein, e.g., by executing program instructions stored on a memory medium (e.g., a non-transitory computer-readable memory medium). Alternatively, the processor  304  may be configured as a programmable hardware element, such as an FPGA (Field Programmable Gate Array), or as an ASIC (Application Specific Integrated Circuit), or a combination thereof. 
       FIG. 4 —User Equipment (UE) 
       FIG. 4  illustrates an example simplified block diagram of a UE  106 . As shown, the UE  106  may include a system on chip (SOC)  400 , which may include portions for various purposes. The SOC  400  may be coupled to various other circuits of the UE  106 . For example, the UE  106  may include various types of memory (e.g., including NAND flash  410 ), a connector interface  420  (e.g., for coupling to a computer system, dock, charging station, etc.), the display  460 , cellular communication circuitry  430  such as for LTE, GSM, etc., and short range wireless communication circuitry  429  (e.g., Bluetooth and WLAN circuitry). The UE  106  may further comprise two or more smart cards  310  that each comprise SIM (Subscriber Identity Module) functionality, such as two or more UICC(s) (Universal Integrated Circuit Card(s))  310 . The cellular communication circuitry  430  may couple to one or more antennas, preferably two antennas  435  and  436  as shown. The short range wireless communication circuitry  429  may also couple to one or both of the antennas  435  and  436  (this connectivity is not shown for ease of illustration). 
     As shown, the SOC  400  may include processor(s)  402  which may execute program instructions for the UE  106  and display circuitry  404  which may perform graphics processing and provide display signals to the display  460 . The processor(s)  402  may also be coupled to memory management unit (MMU)  440 , which may be configured to receive addresses from the processor(s)  402  and translate those addresses to locations in memory (e.g., memory  406 , read only memory (ROM)  450 , NAND flash memory  410 ) and/or to other circuits or devices, such as the display circuitry  404 , cellular communication circuitry  430 , short range wireless communication circuitry  429 , connector I/F  420 , and/or display  460 . The MMU  440  may be configured to perform memory protection and page table translation or set up. In some embodiments, the MMU  440  may be included as a portion of the processor(s)  402 . 
     The processor  402  of the UE device  106  may be configured to implement part or all of the features described herein, e.g., by executing program instructions stored on a memory medium (e.g., a non-transitory computer-readable memory medium). Alternatively (or in addition), processor  402  may be configured as a programmable hardware element, such as an FPGA (Field Programmable Gate Array), or as an ASIC (Application Specific Integrated Circuit). Alternatively (or in addition) the processor  402  of the UE device  106 , in conjunction with one or more of the other components  400 ,  404 ,  406 ,  410 ,  420 ,  430 ,  435 ,  440 ,  450 ,  460  may be configured to implement part or all of the features described herein. In some embodiments, one or more processing elements of the UE device may be manufactured and/or sold separately from the UE device, but may be configured to perform various functionality described herein. 
     FIGS.  5 - 7 —Exemplary Concurrent Connectivity Techniques 
       FIG. 5  illustrates a system in which UE  106  is configured to be concurrently connected to MeNB  520 , S-SeNB  530 , and T-SeNB  540 , according to some embodiments. In this situation, radio resources are scheduled for UE  106  on both S-SeNB  530  and T-SeNB  540  during the same time interval. This may increase throughput, e.g., when transitioning from S-SeNB  530  to T-SeNB  540 , relative to dual-connectivity implementations in which MeNB  520  is configured to release S-SeNB  530  when handing over from S-SeNB  530 , to T-SeNB  540 , before resources are scheduled on T-SeNB  540 . 
     In various exemplary embodiments discussed herein, the term MeNB is used to describe a master base station and SeNB to describe a secondary base station. These terms are not intended to limit the scope of the disclosure to LTE embodiments, however, but are merely used for illustrative purposes. The disclosed techniques may be used in any of various types of networks in addition to or in place of LTE networks. 
     In the illustrated embodiment, the MeNB  520  terminates an S1-MME interface to the MME  510  for UE  106 . In the illustrated embodiment, the SeNBs  530  and  540  are configured to communicate with the MeNB via X2-C interfaces. In some embodiments, the base stations are configured to communicate with each other via an X2-U interface and are configured to communicate with a serving gateway (SGW) via an S1-U interface (X2-U and S1-U interfaces not shown). 
     In some embodiments, MeNB  520  is a master base station configured to service a larger coverage area than the SeNBs  530  and  540  (which may be picocell or femtocell base stations, for example). In other embodiments, the base stations may each be configured to service similarly-sized areas. In some embodiments, the base stations may operate in different modes (e.g., master at one point in time and secondary at another point in time) for a given UE, and/or may serve as a master base station for a first UE while also serving as a secondary base station for a second UE at the same time. In various embodiments, the base stations (e.g., MeNB  520 , S-SeNB  530 , and T-SeNB  540 ) may be synchronized. 
       FIG. 6  is a communications diagram illustrating a handover technique with a release of the S-SeNB  530  while  FIG. 7  is a communications diagram illustrating techniques for concurrent connectivity of the S-SeNB  530  and T-SeNB  540 , according to some embodiments. In  FIGS. 6 and 7 , elements with the same reference numbers as used in  FIG. 5  may be configured as discussed above. In  FIGS. 6 and 7 , messages are arranged from top to bottom according to passing time, e.g., such that data transmission  632  is performed before bearer modification  622 , which is in turn performed before data transmission  636 . These communications diagrams are shown for illustrative purposes but are not intended to limit the scope of the present disclosure. Rather, various messages may be replaced, modified, rearranged in time, etc., and additional messages may be used in addition to and/or in place of various disclosed messages. 
     In  FIG. 6 , in the illustrated embodiment, UE  106  initially communicates with a serving gateway (SGW)  650  via S-SeNB  530 , as shown by data transmissions  632  and  634 . In some embodiments, UE  106  also communicates with the SGW  650  via the MeNB  520  during the same interval (data transmissions not explicitly shown), using dual-connectivity techniques. 
     In the illustrated embodiment, MeNB  520  then sends an addition request  602  to the T-SeNB  540  and receives an acknowledgement  604 . After receiving the acknowledgement, MeNB  520  sends a release request  606  to the S-SeNB  530 . This may reduce throughput, as the UE  106  may be unable to communicate via an SeNB until connection with T-SeNB  540  is fully configured. T-SeNB  540  may be configured to schedule radio resources for UE  106  subsequent to RRC reconfiguration via messages  612  and  614  and bearer modification via message  622  and  624 . At this point, the UE  106  may transfer data to and/or from SGW  650  via the T-SeNB  540  (as shown by data transmissions  636  and  638 ). 
     In  FIG. 7 , in contrast, the S-SeNB  530  is not released until after the connection to T-SeNB  540  is fully configured, and radio resources for the UE  106  may be scheduled by both the S-SeNB  530  and T-SeNB  540  during the same time interval. 
     In the illustrated embodiment, after data transmissions  732  and  734  via the S-SeNB  530 , MeNB  520  sends a new or modified SeNB addition request  702 . The request may be a new type of message that indicates (e.g., by its name or type) that the current SeNB (S-SeNB  530 ) should not be released at least until connectivity to the T-SeNB is established. In other embodiments, the request may be a modified version of a conventional SeNB Addition Request message, e.g., that indicates in a field of the modified message whether or not to release the current SeNB upon receiving a response to the message. 
     In the illustrated embodiment, T-SeNB  540  responds with a new/modified SeNB Addition Request Acknowledgement  704 . Similar to the message  702 , this may be a new type of message or may include a new field that indicates to MeNB  520  not to release the current SeNB upon receiving message  704 . Messages  702  and  704 , in some embodiments, include other information needed for the Addition Request and Addition Request Acknowledgement process. In the illustrated embodiment, MeNB  520  is configured not to release S-SeNB  530 , based on receiving the new-modified SeNB Addition Request Acknowledgement  704  (in the illustrated embodiment, S-SeNB  530  is not released until later, based on message  746 ). Various techniques for deciding when to release the S-SeNB (and/or T-SeNB) from concurrent connectivity are discussed in further detail below. 
     In the illustrated embodiment, MeNB  520  then sends an RRCConnectionReconfiguration message  712  to UE  106 . At this point, resources in both the S-SeNB and T-SeNB are valid for UE  106 , which may be indicated by message  712 . Subsequently, UE  106  responds with an RRCConnectionReconfigurationComplete message  714  and a bearer modification is performed using messages  722  and  724 . In some embodiments, the modified bearer may be a split bearer that corresponds to radio resources in the MeNB and one or more SeNBs. 
     At this point, UE  106 , in the illustrated embodiment, is scheduled radio resources on both S-SeNB and T-SeNB during the same time interval, as shown by data transmissions  736 ,  738 ,  742 , and  744 . In some embodiments, these transmission may overlap in time, e.g., using different frequency resources. In other embodiments, these transmissions may overlap in frequency but be scheduled for different time slots during the time interval in which both SeNBs are allocated for UE  106 . The disclosed techniques may prevent a decrease in throughput when transitioning from S-SeNB  530  and T-SeNB  540  and may also increase throughput while both (or more) SeNBs are concurrently connected. 
     In the illustrated embodiment, MeNB  520  subsequently sends a SeNB release request  746  to release S-SeNB  530  and UE  106  proceeds with data transmissions  752  and  754  via T-SeNB  540  via T-SeNB  540  but no longer communicates via S-SeNB  530 . 
     MeNB  520 , in some embodiments, is configured to determine to release the S-SeNB  530  based on one or more of various criteria. In some embodiments, the master base station is configured to release an SeNB in response to UE  106  reporting poor radio connectivity on the SeNB cell. This may be determined, for example, based on reference signal received power (RSRP) measurements reported by UE  106 . In some embodiments, the master base station is configured to release an SeNB based on information from the SeNB itself. As a first example, this information may indicate that no downlink transmissions have been performed for a threshold time interval for UE  106  by the SeNB. As a second example, this information may indicate that a number of NACK messages for UE  106  exceeds a certain threshold. As a third example, this information may indicate that a threshold number of radio link control (RLC) retransmissions have been performed for UE  106 . As a fourth example, this information may indicate detection of physical layer problems. In other embodiments, the determination to release the SeNB may be based on one or more other criteria in place of and/or in addition to the exemplary criteria listed. 
     FIG.  8 —Exemplary Method 
       FIG. 8  shows a method for configuring concurrent connectivity for a mobile device, according to some embodiments. The method shown in  FIG. 8  may be used in conjunction with any of the computer systems, devices, elements, or components disclosed herein, among other devices. In various embodiments, some of the method elements shown may be performed concurrently, in a different order than shown, or may be omitted. Additional method elements may also be performed as desired. 
     At  810 , in the illustrated embodiment, a base station communicates with a mobile device as a master base station during a time interval in which the mobile device is also assigned radio resources by a first secondary base station. Note that a given base station may be a master base station for a mobile device at a first point in time (e.g., when it terminates an S 1 -MME interface for the mobile device) and a secondary base station at a later point in time. The first secondary base station may be a source secondary base station such as S-SeNB  530 , for example. 
     At  820 , in the illustrated embodiment, the base station requests that a second secondary base station (e.g., T-SeNB  540 ) allocate radio resources for the mobile device during the time interval. In the illustrated embodiment, the master base station makes this request and receives an acknowledgement from the second secondary base station without releasing the first secondary base station. In the illustrated embodiment, this allows radio resources of both the first and second secondary base stations to be allocated to the mobile device for communicating during the time interval. 
     In some embodiments, the request in  820  is a new or modified addition request. In some embodiments, the master base station does not release the first secondary base station in response to receiving a new or modified acknowledgement to the addition request. In various embodiments, this may avoid reductions in throughput during handover between secondary base stations. 
     In some embodiments, the master base station is configured to release the first secondary base station, after the time interval, based on information received from the mobile device or from the first secondary base station. 
     Embodiments described in this disclosure may be realized in any of various forms. For example, some embodiments may be realized as a computer-implemented method, a computer-readable memory medium, or a computer system. Other embodiments may be realized using one or more custom-designed hardware devices such as ASICs. Other embodiments may be realized using one or more programmable hardware elements such as FPGAs. 
     In some embodiments, a non-transitory computer-readable memory medium may be configured so that it stores program instructions and/or data, where the program instructions, if executed by a computer system, cause the computer system to perform a method, e.g., any of a method embodiments described herein, or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets. 
     In some embodiments, a device (e.g., a UE or a base station) may be configured to include a processor (or a set of processors) and a memory medium, where the memory medium stores program instructions, where the processor is configured to read and execute the program instructions from the memory medium, where the program instructions are executable to implement any of the various method embodiments described herein (or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets). The device may be realized in any of various forms. 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.