Abstract:
A connection is established between an access terminal and a first radio network controller through a first radio node. The first radio node is controllable primarily by the first radio network controller. The connection is maintained with the first radio network controller as the access terminal moves from a coverage area of the first radio node toward a coverage area of a second radio node. The second radio node is controllable primarily by a second radio network controller and controllable subordinately by the first radio network controller. A connection is also established through the second radio node. Upon a fulfillment of a predetermined criterion, the connection is transferred from the first radio network controller to the second radio network controller.

Description:
BACKGROUND 
       [0001]    This disclosure relates to active handoffs between radio networks. 
         [0002]    Cellular wireless communications systems are designed to serve many access terminals distributed in a large geographic area by dividing the area into cells, as shown in  FIG. 1 . At or near the center of each cell  102 ,  104 ,  106 , a radio network access point  108 ,  110 ,  112 , also referred to as a radio node (RN) or base transceiver station (BTS), is located to serve access terminals  114 ,  116  (e.g., cellular telephones, laptops, PDAs) located in the cell. Access terminals (AT) are sometimes referred to as mobile stations (MS) or user equipment (UE). Each cell is often further divided into sectors  102   a - c ,  104   a - c ,  106   a - c  by using multiple sectorized antennas. An RN is identified by one or more of several properties, which may include the offset of a pseudonoise pattern in its pilot signal (PN offset), a frequency, an IP address, or a SectorID. In each cell, that cell&#39;s radio network access point may serve one or more sectors and may communicate with multiple access terminals in its cell. 
         [0003]    When an access terminal moves from one sector or cell to another and control of the access terminal is transitioned between different network elements, the transfer is referred to as handoff. If the access terminal has a call in progress during handoff, the handoff is said to be active. Co-pending patent application Ser. No. 11/037,896, filed Jan. 18, 2005, and titled Radio Network Control, also assigned to Airvana, Inc., described active handoffs in partially-connected radio networks. The type of handoff described in that application is now standardized as the A 16  interface in the TIA-878-B standard. According to the standard, A 16  handoff is a hard handoff, such that during the handoff, the source radio network controller cannot add the target radio node to its active set, and the target radio network controller cannot add the source radio node to its active set. 
       SUMMARY 
       [0004]    In general, in one aspect, a connection is established between an access terminal and a first radio network controller through a first radio node. The first radio node is controllable primarily by the first radio network controller. The connection is maintained with the first radio network controller as the access terminal moves from a coverage area of the first radio node toward a coverage area of a second radio node. The second radio node is controllable primarily by a second radio network controller and controllable subordinately by the first radio network controller. A connection is also established through the second radio node. Upon a fulfillment of a predetermined criterion, the connection is transferred from the first radio network controller to the second radio network controller. 
         [0005]    Implementations may include one or more of the following features. The first and second radio network controllers are located in different subnets. The criterion is fulfilled when the first radio node is dropped from an active set of the access terminal. The first radio node is dropped from the active set of the access terminal when a strength of a signal of the first radio node falls below a minimum threshold. The criterion is fulfilled when a strength of a signal of the first radio node falls below a minimum threshold. The connection uses the Ev-DO, CDMA2000, W-CDMA, HSUPA, HSDPA, HSPA, or LTE telecommunications standard. The connection uses voice over IP (VoIP) protocol. The first radio node is also controlled by a third radio network controller. The second radio network controller continues to control the session until a predetermined criterion is fulfilled. The criterion is fulfilled when the active set of the access terminal contains only radio nodes that are controlled subordinately by the second radio network controller. The second radio node sends the second RNC&#39;s address to the first RNC. 
         [0006]    In general, in one aspect, a connection is established between an access terminal and a first radio network controller, through both a first radio node and a second radio node. The connection is maintained with the first radio network controller as the access terminal moves from a coverage area of the first radio node toward a coverage area of a second radio node. The connection is transferred to a second radio network controller. The connection is maintained with the second radio network controller as the access terminal returns toward the coverage area of the first radio node. The connection is transferred to the first radio network controller once an active set of the access terminal contains no radio nodes primarily controllable by the second radio network controller. 
     
    
     
       DESCRIPTION 
         [0007]      FIG. 1  is a block diagram of a radio area network. 
           [0008]      FIG. 2  is a flow chart. 
           [0009]      FIG. 3  is a messaging diagram. 
       
    
    
       [0010]    Referring to  FIG. 1 , in some examples, a radio access network (RAN)  100  uses an Ev-DO protocol to transmit data packets between an access terminal, e.g., access terminals  114  and  116 , and a radio node, e.g., radio nodes  108 ,  110 ,  112 . The radio nodes are connected over a backhaul connection  118  to radio network control/packet data serving nodes (RNC/PDSN)  120 , which may be one or more physical devices at different locations. Each cell may be divided into sectors  102   a - c ,  104   a - c ,  106   a - c  by antennas as noted above. In each cell, that cell&#39;s radio network access point may serve one or more sectors and may communicate with multiple access terminals in its cell. Although this description uses terminology from Ev-DO standards, the same concepts are applicable to other communication methods, including Code Division Multiple Access 2000 (CDMA2000); Wideband Code Division Multiple Access (W-CDMA); High-Speed Uplink Packet Access (HSUPA); High-Speed Downlink Packet Access (HSDPA); High-Speed Packet Access (HSPA), Long Term Evolution (LTE), and the like. 
         [0011]    In some examples, as shown in  FIG. 2 , multihoming allows a single radio node  202  to be controlled by two or more radio network controllers  208  and  210 . Traditionally, RNs located within a subnet of a radio area network are controlled by only one RNC located within the same subnet. In contrast, a multihoming system works by associating RNs located near a subnet boundary  220  with at least two RNCs  208  and  210 . In some examples, such as that illustrated by  FIG. 2 , the RNCs  208  and  210  that jointly control the RN  202  are located in different subnets  216  and  218 . 
         [0012]    When one RN  202  is multihomed, i.e., controlled by two RNCs  208  and  210 , we describe one of the RNCs as the primary RNC, and we describe the other RNC as the secondary RNC. In the example of  FIG. 2 , the RN  202  is multihomed to the RNCs  208  and  210 . For purposes of this example, the RNC  208  is the primary RNC, shown by a solid line  205 , and the RNC  210  is the secondary RNC, shown by a dashed line  207 . We say the RN  202  is primarily homed to the primary RNC  208  and secondarily homed to the secondary RNC  210 . There may be more than one secondary RNC. 
         [0013]    In its broadest sense, a node is considered to be multihomed if that node has multiple ways of reaching a destination. In the context of mobile wireless networks and the example of  FIG. 2 , the destination is the packet data serving node (PDSN)  214 , and RNs  202  and  204  that are multihomed may communicate with a PDSN  214  through at least two different RNCs  208  and  210 . By using multihoming, the functions of the primary RNC are assumed by the secondary RNC when the primary RNC becomes unavailable (e.g., when the AT moves out of range of any RN controlled by the primary RNC), or less desirable (e.g., the AT moves closer to an RN located within the control of the secondary RNC). Uninterrupted connectivity can be achieved if at least one connection to the PDSN is maintained. 
         [0014]    When an AT  212  is in handoff from one RNC to another, the RNC  208  originally controlling the AT  212  is the serving RNC, and the RNC  210  to which control is being transferred is the target RNC. When the radio node  202  is multihomed, its primary RNC  208  may be the serving RNC for the AT  212  that is moving to the second RN  204 , which happens to be primarily homed to the first RN&#39;s secondary RNC  210 , such that the first RN&#39;s secondary RNC is the target RNC. At the same time, from that second RN  204 &#39;s point of view, it&#39;s primary RNC  210  is the target RNC for the incoming AT  212 , and the serving RNC is the secondary RNC  208 . 
         [0015]    In some examples, as the AT moves from the coverage area of one RN to the coverage area of another RN, it sends a RouteUpdate message to its serving RNC. The RouteUpdate message indicates the identification and strength of any compatible radio nodes&#39; pilot signals that the AT can detect at its current location. The serving RNC uses the RouteUpdate message to trigger and perform normal soft handoffs, in which the AT is transitioned to another RN controlled by the same RNC without dropping any active calls. Through these RouteUpdate messages, the serving RNC obtains a fairly accurate assessment of where the AT is located. The RNC uses this information to trigger an active RNC handoff. 
         [0016]    An AT&#39;s “active set” refers to the set of available RNs whose pilot signals are received by the AT and are sufficiently strong to remain in communication with the AT. Thus, if an AT were being served by both RN 1  and RN 2 , its active set would be the pilot signals for both RN 1  and RN 2 , which we represent in the parenthetical form (RN 1 , RN 2 ). An RN is dropped from an AT&#39;s active set when its pilot falls below a certain minimum strength threshold. In traditional wireless networks, all the RNs in an ATs active set must be controlled by the same RNC. 
         [0017]    In traditional wireless networks using the TIA-878-B standard, an AT crossing the boundary between two RNCs needs to end communication with the serving RNC in order to establish a communication link with the target RNC. These A 16  handoffs are “hard handoffs” because while the AT is controlled by the source RNC, it cannot add a target RN controlled only by the target RNC to its active set, and once controlled by the target RNC, the AT could not keep the source RN controlled only by the source RNC in its active set. Because of the lack of overlap in the active sets before and after handoff, a connection cannot be maintained. 
         [0018]    In some examples, enhancements to the handoff methods in multi-homed networks allow an A 16  handoff to be converted into a soft handoff, rather than a hard handoff. This allows more robust handoffs to take place between radio nodes that otherwise would be controlled by separate radio network controllers, requiring hard handoff, and for active calls to be handed off without being dropped. 
         [0019]    To provide a multi-homed handoff, each RN associates a single traffic channel with the two (or more) RNCs that are controlling it. In some examples, one or two layers of RNs of each subnet, generally those near the boundary  220  between subnets  216  and  218 , are secondarily homed to the RNC controlling the RNs on the other subnet, which will be the target RNC when an AT is handed off across the boundary. With the bordering RNs homed to both source and target RNCs, the AT is able to add the pilots signals of target RNs to its active set when it is located near a boundary, and to keep the source RNs in its active set during the handoff. The primary RNC knows that the AT is near the subnet boundary when the AT begins to report pilots from both RNs in the RouteUpdate message. 
         [0020]    When the AT  212  is in transition range (e.g., near a subnet boundary  220 , shown as position t 1  in  FIG. 2 ), all the RNs in the active set are homed to both RNCs  208  and  210 , such that the active set is the same immediately before and after handoff between the two RNCs. Before the AT is in range (e.g., where the AT  212  is at position t 0 ), its active set includes RNs of only the original serving RNC. After the AT moves past the transition range (e.g., where AT  212  is at position t 2 ), as discussed below, the RNs of the original serving RNC  208  are removed, so that the active set includes RNs of only the target RNC  210 . In both cases, the transition to and from the joint active set is done with standard soft handoff. While the active set is (RN 1 , RN 2 ), the AT  212  may begin using the RN 2   204 . Thus, when the AT moves from the RN 1   202  toward the RN 2   204 , with the RNC subnet boundary  220  between the two, the active set transitions as: (RN 1 )→(RN 1 , RN 2 )→(RN 2 ). Because communication with at least one RN is preserved throughout the handoff, each handoff is soft and connectivity is maintained without interruption. 
         [0021]    Handoff is triggered when a predetermined criterion is fulfilled. For example, as the AT  212  moves away from the first subnet  216 , the strength of the pilot signal of the RN  202  will decrease until the RN is dropped form the AT&#39;s active set. The target RNC is then selected based on the strongest pilot in the AT&#39;s active set. When all the pilots in the AT&#39;s active set are only secondarily homed to the serving RNC, control of the AT, including any initiated connections, is transferred to the target RNC. Referring to  FIG. 2  as an example, when the AT reaches position t 2  and drops RN 1  from its actives set, the only RN left in the active set is RN 2 , which is primarily homed to the target RNC 2   210  and only secondarily homed to the source RNC 1   208 . By triggering handoff at this point, rather than waiting until the AT moves so far into the subnet  218  that the serving RNC  208  may not be homed to any RNs, both the serving RNC  208  and the target RNC  210  can access the RNs in the active set during handoff. 
         [0022]    Delaying handoff until all RNs primarily homed to the source RNC are dropped from the active set helps prevent handoff ping-pong, where an AT that stays near the subnet border is continually handed off, back and forth between the two subnets. If the AT  212  moves back toward the subnet boundary  220 , it will add the RN 1   202  back into the active set, but it will not handoff back to the RNC  208  unless it moves so far into the subnet  216  that the RN 2   204  is dropped. As long as the AT stays near the boundary  220 , the active set will include RNs primarily homed to both RNCs and handoff will not take place. Delaying handoff also reduces the amount of A 16  session transfer latency and the number of dropped calls, as the active set remains the same during the A 16  handoff. 
         [0023]    To facilitate this type of handoff, the target RN  204  sends its primary RNC&#39;s  210  address (e.g., an IP address if an IP backhaul network is used to connect the RNCs and RNs) to its secondary RNC  208 . With this information, the serving (secondary to RN  204 ) RNC  208  can find the target (primary as to RN  204 ) RNC  210  automatically rather than through operator configuration. 
         [0024]    Relevant elements of the flow of messaging and control to implement multihomed soft handoff is shown in  FIG. 3 . The A 16  handoff is triggered  300  when RN 1  is dropped from the AT&#39;s active set and the AT informs the source RNC 1  about the change. The session configuration is locked  302  to avoid session configuration change in the middle of a handoff. Next, the source RNC 1  sends  304  an A 16  transfer request to the target RNC 2 . The RN 2  associates  306  the existing traffic channel over which it serving the AT with the target RNC 2 . This will allow the active connection to be transferred to RNC 2  without dropping. Once the traffic channel is associated with RNC 2 , RNC 2  and RNC 1  acknowledge completion of the transfer by sending  308  A 16  transfer response and A 16  transfer complete messages, respectively. 
         [0025]    The RNC 2  next sets up  310  an A 10  connection to the PDSN and resets  312  the RLP flows with the AT. The former A 10  connection from the RNC 1  to the PDSN is torn down  314 , and the target RNC 2  assigns  316  a new Unicast Access Terminal Identifier (UATI) to the AT. A 16  Release and Release acknowledge signals are exchanged  318  between the RNC 1   208  and RNC 2   210  to let the source RNC release the session. The session configuration is then unlocked  320 . Some time later, the connection between the AT  212  and the target RN 2   204  is closed  322  and A 13  release request and response messages are exchanged  324  between the RNCs. The A 13  release request is sent to the source RNC which requests that the source RNC release the UATI assigned by said source RNC, finally completing the handoff. 
         [0026]    The techniques described herein can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The techniques can be implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
         [0027]    Method steps of the techniques described herein can be performed by one or more programmable processors executing a computer program to perform functions of the invention by operating on input data and generating output. Method steps can also be performed by, and apparatus of the invention can be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). Modules can refer to portions of the computer program and/or the processor/special circuitry that implements that functionality. 
         [0028]    Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry. 
         [0029]    To provide for interaction with a user, the techniques described herein can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer (e.g., interact with a user interface element, for example, by clicking a button on such a pointing device). Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. 
         [0030]    The techniques described herein can be implemented in a distributed computing system that includes a back-end component, e.g., as a data server, and/or a middleware component, e.g., an application server, and/or a front-end component, e.g., a client computer having a graphical user interface and/or a Web browser through which a user can interact with an implementation of the invention, or any combination of such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet, and include both wired and wireless networks. 
         [0031]    The computing system can include clients and servers. A client and server are generally remote from each other and typically interact over a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
         [0032]    Other embodiments are within the scope of the following claims and other claims to which the applicant may be entitled. The following are examples for illustration only and do not limit the alternatives in any way.