Abstract:
A method for performing a handoff of an active communication connection for a mobile user station from a macrocell to one or more femtocells. In one aspect, the present invention provides an improved handoff for situations in which the mobile user station is moved into an indoor residential or enterprise environment within which the signal strength of the mobile user station with respect to the macrocell base transceiver station becomes relatively low. In one embodiment, the method efficiently performs a handoff of an active communication connection from a macrocell to one or more femtocells while minimizing the likelihood of dropping the active communication connection during the handoff.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to wireless communication systems, and more particularly, to a method for handing off an active communication connection from a macrocell to a femtocell. 
       BACKGROUND 
       [0002]    The IP Multimedia Subsystem (IMS), as defined by 3GPP and 3GPP2, merges telephony and Internet technology by providing an all-IP based architecture for the telecommunications industry. The IMS is based on the Session Initiation Protocol (SIP) and makes heavy use of the protocols defined within the Internet Engineering Task Force (IETF). The system offers a network of servers and databases that assist a user agent with the task of establishing and managing sessions. IMS uses the term “sessions,” because the connections between users are no longer limited to voice services (i.e., a telephone call). Sessions may be voice, video, text, or other services connecting two or more user agents together. 
         [0003]    Referring now to  FIG. 1 , the IMS architecture  100  includes User Equipment (UE)  125 , Call Session Control Functions (CSCF)  115 , Home Subscriber Server (HSS)  120  and Application Servers  105  and  110 . 
         [0004]    The UE includes a device that contains the SIP User Agent that will initiate or terminate sessions. 
         [0005]    The CSCFs  115  are responsible for managing the sessions, including security and interconnect. There are three types of CSCFs. First, a Proxy CSCF (P-CSCF) resides at the edge of the network and serves as an entry point for the UE into the IMS core. Second, an Interrogating CSCF (I-CSCF) serves as an entry point into the network for peering networks. The I-CSCF also acts as the lookup function for finding the appropriate serving node for a subscriber. Third, a Serving CSCF (S-CSCF) is responsible for authenticating the UE and managing ongoing sessions for the UE, including invocation of applications. The S-CSCF communicates with the HSS in order to retrieve the UE authentication information. After the user has been authenticated, the S-CSCF again communicates with the HSS  120  to retrieve the user profile which specifies the services to which a user has subscribed and which applications servers are to be invoked for those services. 
         [0006]    The HSS  120  stores the relevant user data, including authentication information and service data. As part of the user profile, initial Filter Criteria (IFC) are defined to indicate which application servers are to be invoked, based on information in the signaling plane. 
         [0007]    Applications Servers  105 ,  110  are invoked based on the iFC that are stored in the user profile. The S-CSCF will pass signaling onto an Application Server if the criteria defined in the IFC are met. Once invoked, the application server can take part in the session and provide additional capabilities. 
         [0008]    Referring now to  FIG. 2A , the architecture  200  of the converged CDMA IMS Femtocell system, as shown in  FIG. 1  of section 5.1.1 of 3GPP2-X.S0059 Femtocell Overview, is illustrated. 
         [0009]    The converged CDMA IMS Femtocell system includes a CDMA Femtocell Access Point (FAP)  238 , which is a CDMA 2000 1× access point that provides coverage in a small area, usually a private residence or a small office, and connects a mobile station (MS)  240  to an operator&#39;s network  228  via a broadband connection, such as, for example, DSL or cable. Typically, a residential FAP is a miniature base station that implements the CDMA base transceiver station function, the base station controller function, and the packet control function (PCF). A plain old telephone service (POTS) telephone can be connected to a FAP via an ATA port. A typical residential FAP has a range of approximately 15 feet. 
         [0010]    Referring also to  FIG. 2B , the converged CDMA IMS Femtocell system also includes a cluster  238  of up to n FAPs in large enterprises, where n =a maximum number of base stations supported in a soft handoff scenario in IS-95 or IS-2000. One FAP in the cluster is the Controlling FAP (CFAP)  272 , and other individual FAPs may be designated as FAP 1   270  and FAPn  274 . In a macrocell-to-femtocells active mode handoff scenario, an individual FAP other than the CFAP  272  sends voice packets to the CFAP  272 , which selects the strongest voice frame to send towards the IMS  224  via an enterprise router  276 . In the downlink direction, the CFAP  272  receives a voice frame from the IMS  224  via the enterprise router  276  and sends the same voice frame to all of the FAPs, i.e., FAP 1   270  and FAPn  274  involved in the soft handoff. Typically, an enterprise FAP is a miniature base station that implements the CDMA base transceiver station function, the base station controller function, and the packet control function (PCF). Enterprise FAPs are grouped by clusters. Each cluster has a Controlling FAP. All enterprise FAPs within a cluster communicate with each other via a local enterprise IP backbone  278 . Signaling protocols between the CFAP  272  and the individual enterprise FAPs  270 ,  274  have not yet been defined by 3GPP2. Voice paths between the CFAP  272  and the individual enterprise FAPs  270 ,  274  may be established by using the Real-Time Transport Protocol. 
         [0011]    The converged CDMA IMS Femtocell system also includes a Femtocell Management System (FMS)  230  that provides femtocell operation, administration, maintenance, and provisioning (OAM&amp;P) functions. 
         [0012]    The converged CDMA IMS Femtocell system also includes a Femtocell Authentication, Authorization, and Accounting (AAA) module  232  that provides a FAP authorization function. The AAA  232  sends authorization policy information to the Security Gateway (SeGW)  234 . The SeGW authenticates the FAPs and protects the IP core network  228  from security attacks (e.g., denial of service) initiated by FAPs. 
         [0013]    The converged CDMA IMS Femtocell system also includes IMS  224 , an IP Multimedia Network  226 , and an IP backbone network  228 . 
         [0014]    The converged CDMA IMS Femtocell system also includes a Media Gateway Control Function/Media Gateway (MGCF/MGW)  222  that connects IMS  224  to the legacy CDMA core network. 
         [0015]    The converged CDMA IMS Femtocell system also includes a Femtocell Convergence Server (FCS)  210 . The FCS  210  is an IMS Application Server that provides internetworking between the FAP, the SIP environment of IMS, and the appropriate Mobile Access Protocol (MAP)  212  network elements. 
         [0016]    The converged CDMA IMS Femtocell system also includes legacy CDMA core network elements, including: the public switched telephone network (PSTN)  218  and a public safety answering point (PSAP)  220 ; a mobile switching center (MSC)  216 ; a base station controller (BSC) and a base station transceiver system (BTS)  214 , a home location register (HLR) and authentication center (AC)  202 ; a position determining entity (PDE)  204 ; a mobile positioning center (MPE)  206 ; and a message center (MC)  208 . 
         [0017]    The MSC  216  is connected to the PSTN switch  218  via a TDM trunk. The MSC  216  is connected to the MGCF/MGW  222  via a nailed-up TDM trunk. The MSC  216  interacts with the HLR  202  and the CS  210  using IS-41 signaling. The MSC  216  is connected to at least one BSC via a TI connection or an Ethernet connection. Each BSC controls at least one BTS  214  via a T 1  connection or an Ethernet connection. An MSC  216  can be either a TDM mobile switching center or a “Soft Switch” MSC. 
         [0018]    Referring now to  FIG. 2C , the MSC  216  includes a radio access control function (RACF) subsystem  262 , a call control function (CCF) subsystem  264 , and a trunking subsystem  260 . The RACF subsystem is responsible for paging, mobility management (e.g., location update), and handoff. The RACF subsystem  262  interfaces with the base station controllers  214  and  254 , typically via either a T 1  interface or an Ethernet interface. Base station controller  214  interfaces with base station transceivers  250  and  252  via T 1  or Ethernet, and base station controller  254  interfaces with base station transceivers  256  and  258  via T 1  or Ethernet. The mobile station  240  communicates with the base station transceiver having the highest signal strength at any given moment. 
         [0019]    The RACF subsystem  262  performs handoffs in coordination with the base station controllers  214  and  254 , the trunking subsystem  260 , and the convergence server  210  via respective IS-41 interfaces. 
         [0020]    The RACF subsystem  262  also interfaces with a home location register (HLR)  268  and a visitor location registry (VLR)  266 . The HLR  268  contains subscriber information for the mobile stations of a plurality of subscribers. The VLR  266  contains information relating to all mobile stations which have registered with the MSC  216 . 
         [0021]    The CCF subsystem  264  is responsible for translating and routing calls. The trunking subsystem  260  is responsible for trunk connectivity with the PSTN  218 , the MGCF/MGW  222 , and other mobile switching centers. 
         [0022]    Conventional systems include a capability to complete a handoff of an active communication connection from a femtocell to a macrocell, e.g., when a mobile user station moves from an indoor residential environment out into a generic macrocell environment. However, currently, the 3GPP2 femtocell standards and femtocell and convergence server vendors lack an efficient solution for handing off an active mobile user station (for example, a mobile user station that is on a call with another party) from a macrocell to a residential femtocell or to enterprise femtocells when the mobile user station moves into the femtocell coverage area. Consequently, a subscriber who is talking on a call via a macrocell base station and is moving to an indoor environment, such as a private residence, an enterprise, or a hotspot, will suffer from poor macrocell RF coverage in an indoor environment. Hence, there is a need for a method for efficiently performing a handoff from a macrocell to one or more femtocells that minimizes the likelihood of dropping the active communication connection during the handoff. 
       SUMMARY 
       [0023]    In one aspect, particular embodiments of the disclosed solution provide methods for performing a handoff of an active communication connection for a mobile user station from a macrocell to a femtocell. Particular exemplary embodiments of the disclosed solution contemplate that the mobile user station is moved into an indoor residential environment within which the signal strength of the mobile user station with respect to the macrocell base transceiver station becomes relatively low. Other particular embodiments of the disclosed solution contemplate an enterprise environment, including a cluster of femtocell access points, into which the mobile user station is moved. 
         [0024]    In one aspect, particular embodiments of the disclosed solution provide a method for performing a handoff of an active communication connection for a mobile user station from a macrocell to a femtocell is provided. At a mobile switching center corresponding to the macrocell, a first message is received. The first message is transmitted from a base station controller corresponding to the macrocell. The first message includes an indication that the active communication connection should be handed off to a femtocell access point corresponding to the femtocell. A second message is generated and transmitted to a target convergence server. The second message includes instructions for initiating the handoff. A third message is received. The third message is transmitted from the target convergence server. The third message includes an indication that the handoff has been successfully initiated. A fourth message is generated and transmitted to the base station controller. The fourth message includes a direction to the base station controller to commence the handoff. A fifth message is received. The fifth message is transmitted from the base station controller. The fifth message includes a handoff commencement acknowledgement. A sixth message is received from the target convergence server. The sixth message includes an indication of a completion of a successful handoff. 
         [0025]    In another aspect of the particular embodiments of the disclosed solution, the handoff may be completed without a transmission of a Session Initiation Protocol (SIP) INVITE message between the target convergence server and the femtocell access point. The receiving of the first message may occur after the base station controller determines that a signal strength corresponding to the mobile user station is less than a predetermined threshold. 
         [0026]    In some embodiments, the second message may include a femtocell identification and a Circuit Identifier Code (CIC) corresponding to a handoff trunk communication channel between the mobile switching center and a media gateway station. The media gateway station may be in communication with the femtocell access point. 
         [0027]    In some embodiments, after the second message has been transmitted, the third message may be received after the target convergence server has transmitted a request message to the femtocell access point that includes a request that the femtocell access point prepare for handoff and the femtocell access point has transmitted an acknowledgement message to the target convergence server. The third message may be received without a transmission of an additional acknowledgement message from the target convergence server to the femtocell access point. 
         [0028]    In some embodiments, after the fourth message has been transmitted, the fifth message may be received after the base station controller has transmitted a Universal Handoff Direction message to the mobile user station and the mobile user station has transmitted an acknowledgement message to the base station controller. 
         [0029]    In some embodiments, after the fifth message has been received, the sixth message may be received after the femtocell access point (a) receives a first handoff completion message from the mobile user station and (b) transmits a second handoff completion message to the target convergence server. 
         [0030]    In another aspect, particular embodiments of the disclosed solution provide a method for performing a handoff of an active communication connection for a mobile user station from a macrocell to a femtocell. At a femtocell access point corresponding to the femtocell, a first message is received. The first message is transmitted from a convergence server. The first message includes a request to initiate a handoff from a base station controller corresponding to the macrocell. A second message is transmitted to the convergence server. The second message includes an acknowledgement of a receipt of the first message. A third message is received from the mobile user station. The third message includes a preamble message. A fourth message is transmitted to the mobile user station. The fourth message includes an acknowledgement of a receipt of the preamble message. A fifth message is received from the mobile user station. The fifth message includes an indication of a completion of the handoff. A sixth message is transmitted to the convergence server. The sixth message includes an indication of a successful completion of the handoff. 
         [0031]    In another aspect of particular embodiments of the disclosed solution, the handoff may be completed without a transmission of a Session Initiation Protocol (SIP) INVITE message between the convergence server and the femtocell access point. The receiving of the first message may occur after the base station controller determines that a signal strength corresponding to the mobile user station is less than a predetermined threshold. 
         [0032]    In some embodiments, after the second message is transmitted, the third message may be received without a transmission of an additional acknowledgement message from the target convergence server to the femtocell access point. The third message may be received after the base station controller has transmitted a Universal Handoff Direction Message to the mobile user station and the mobile user station has transmitted an acknowledgement message to the base station controller. 
         [0033]    In yet another aspect, particular embodiments of the disclosed solution provide a method for performing a handoff of an active communication connection for a mobile user station from a macrocell to a femtocell. At the mobile user station, a first message is received. The first message is transmitted from a base station controller corresponding to the macrocell. The first message includes a handoff direction message. A second message is transmitted to the base station controller. The second message includes an acknowledgement of a receipt of the handoff direction message. A third message is transmitted to a femtocell access point corresponding to the femtocell. The third message includes at least one preamble frame. A fourth message is received from the femtocell access point. The fourth message includes an acknowledgement of a receipt of the at least one preamble frame. A fifth message is transmitted to the femtocell access point. The fifth message includes an indication of a completion of the handoff. A sixth message is received from the femtocell access point. The sixth message includes an acknowledgement of a completion of the handoff. The mobile user station then communicates on the active communication connection via the femtocell access point. 
         [0034]    In another aspect of particular embodiments of the disclosed solution, the handoff may be completed without a transmission or reception of a Session Initiation Protocol (SIP) INVITE message by the femtocell access point. 
         [0035]    In some embodiments, the receiving of the first message may occur after the base station controller determines that a signal strength corresponding to the mobile user station is less than a predetermined threshold. 
         [0036]    In some embodiments, at least one null frame may be received from the femtocell access point prior to receiving the first message, wherein the receiving at least one null frame occurs after the femtocell access point has received a request to initiate the handoff. Prior to receiving the at least one null frame, the femtocell access point may receive a handoff request message from a convergence server. After receiving the at least one null frame, the femtocell access point may transmit a message to the convergence server that includes an acknowledgement of a receipt of the handoff request message without receiving a further acknowledgement message from the convergence server prior to the mobile user station receiving the first message. 
         [0037]    In some embodiments, after the sixth message is received, the femtocell access point may transmit a seventh message to a convergence server that includes an indication that the handoff has been successfully completed prior to the mobile user station communicating on the active communication connection via the femtocell access point. 
         [0038]    In yet another aspect, particular embodiments of the disclosed solution provide a mobile switching station for performing a handoff of an active communication connection for a mobile user station from a macrocell to a femtocell. The mobile switching station includes a radio access control function (RACF) subsystem; a call control function (CCF) subsystem in communication with the RACF subsystem; and a trunking subsystem in communication with the RACF subsystem and with a public switched telephone network (PSTN). The RACF subsystem is in communication with the mobile user station via a base station controller corresponding to the macrocell and in communication with a target convergence server. When the mobile user station moves to within a predetermined range with respect to the femtocell, the RACF subsystem is configured to receive a first message transmitted from the base station controller. The first message includes an indication that the active communication connection should be handed off to a femtocell access point corresponding to the femtocell. The RACF subsystem is configured to generate a second message. The second message includes instructions for initiating the handoff. The RACF subsystem is further configured to transmit the second message to the target convergence server and to receive a third message from the target convergence server. The third message includes an indication that the handoff has been successfully initiated. The RACF subsystem is further configured to generate a fourth message. The fourth message includes a direction to commence the handoff. The RACF subsystem is further configured to transmit the fourth message to the base station controller and to receive a fifth message from the base station controller. The fifth message includes a handoff commencement acknowledgement. The RACF subsystem is further configured to receive a sixth message from the target convergence server. The sixth message includes an indication of a completion of a successful handoff. Upon successful handoff completion, the trunking subsystem is configured to provide a connection between the PSTN and the femtocell access point. The RACF subsystem is further configured to transmit a seventh message to the CCF subsystem. The seventh message includes an indication that the mobile user station is communicating with the mobile switching station via a nailed-up trunk. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present disclosure and, together with the description, further serve to explain the principles of the disclosure and to enable a person skilled in the pertinent art to make and use the embodiments disclosed herein. In the drawings, like reference numbers indicate identical or functionally similar elements. 
           [0040]      FIG. 1  illustrates a conventional logical network architecture of an IP Multimedia Subsystem (IMS). 
           [0041]      FIG. 2A  illustrates a logical network architecture of a converged CDMA IMS Femtocell system. 
           [0042]      FIG. 2B  illustrates a block diagram of an enterprise femtocell access point cluster as used in the architecture of  FIG. 2A . 
           [0043]      FIG. 2C  illustrates a block diagram of a mobile switching center as used in the architecture of  FIG. 2A . 
           [0044]      FIG. 3A  illustrates a pre-handoff voice path of an active communication connection between a mobile subscriber and a PSTN subscriber in a macrocell network. 
           [0045]      FIG. 3B  illustrates a post-handoff voice path of the active communication connection of  FIG. 3A . 
           [0046]      FIG. 3C  illustrates a post-handoff voice path of an active communication connection in an enterprise femtocell scenario. 
           [0047]      FIGS. 4A and 4B  illustrate an active mode macrocell to residential femtocell handoff message sequence according to particular embodiments. 
           [0048]      FIGS. 5 ,  5 A,  5 B, and  5 C are flow charts illustrating a method for performing an active mode macrocell to residential femtocell handoff according to particular embodiments. 
           [0049]      FIGS. 6A and 6B  illustrate an active mode macrocell to enterprise femtocell handoff message sequence according to particular embodiments. 
           [0050]      FIGS. 7 ,  7 A,  7 B, and  7 C are flow charts illustrating a method for performing an active mode macrocell to enterprise femtocell handoff according to particular embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    Referring now to  FIG. 3A , a voice path  300  is illustrated for an active communication connection between a mobile subscriber and a PSTN subscriber in a macrocell network. The mobile subscriber uses a mobile user station (MUS)  308 , which is wirelessly connected over a radio traffic channel to a base transceiver station (BTS)  310 . The BTS  310  is controlled via a TDM trunk connection by a base station controller (BSC)  312  for the specific macrocell in which the MUS is geographically located. The BSC  312  is in communication via a TDM trunk connection with a mobile switching center (MSC)  306  which also corresponds to the macrocell. In an embodiment, the MUS is a CDMA mobile unit that supports IS-95/IS-200 and/or 1X EV-DO air interfaces. 
         [0052]    The MSC  306  is in communication via a TDM trunk connection with the PSTN  304 . The TDM trunk connection between the MSC  306  and the PSTN  304  may be an Integrated Service Digital Network (ISDN) User Part (ISUP) trunk or an inband signaling trunk. 
         [0053]    Referring now also to  FIG. 3B  and  FIG. 3C , as the MUS  308  moves near to a residential femtocell access point (FAP)  334 , the macrocell MSC  306  will hand the MUS  308  off to the femtocell network via IMS.  FIG. 3B  illustrates the voice path for the active communication connection between the mobile subscriber and the PSTN subscriber after the handoff from the macrocell network to the femtocell network. 
         [0054]    The MUS  308  wirelessly communicates with the FAP  334  over a radio traffic channel. The FAP  334  communicates with the media gateway (MGW)  316  using Real-time Transport Protocol (RTP) via the security gateway (SeGW)  330 . The media gateway  316  is in communication via a “nailed-up” TDM handoff trunk connection with the macrocell MSC  306 , which remains in communication via a TDM trunk connection with the PSTN  304 . 
         [0055]    In the enterprise femtocell scenario, as illustrated in  FIG. 3C , the FAP  334  includes a router  336 , a controlling FAP  338 , and additional access points FAP  1   340  and FAP n  342 . The MUS  308  wirelessly communicates with either FAP  1   340  or FAP n  342 , depending upon its physical proximity to the access points. The message communicated by MUS  308  then proceeds to the controlling FAP  338 , the router  336 , and then to MGW  316  via SeGW  330 . 
         [0056]    The operator uses the Femtocell Service Manager (FSM)  332  to provision the FAP  334  via the security gateway  330 . The FAP  334  and security gateway  330  are in communication with the femtocell convergence server (CS)  322  via IMS call session control functions (CSCF)  324 . The femtocell convergence server  322  communicates with a home location register  314  and with the macrocell mobile switching center  306  via an IS-41 connection. 
         [0057]    The CSCF  324  communicates with home subscriber server  326  and a mobile relay facility  328 . In addition, the CSCF  324  communicates with a breakout gateway control function (BGCF)  320 , which is a SIP proxy that processes requests for routing from an S-CSCF when the S-CSCF has determined that the session cannot be routed using DNS or ENUM/DNS. The BGCF  320  includes routing functionality based on telephone numbers. The BGCF  320  may also be accessed by the media gateway  316  via the media gateway control (MGC)  318 . 
         [0058]    Referring now to  FIGS. 4A and 4B , a message sequence for an active mode macrocell to residential femtocell handoff according to particular embodiments is illustrated. Prior to the start of the message sequence of  FIG. 4 , mobile subscriber “A” has established an active call with a PSTN user “B”. Double-ended arrow #1  401  illustrates a voice path between user B and macrocell mobile switching center (MSC)  306 ; double-ended arrow #2  402  illustrates a voice path between MSC  306  and macrocell base transceiver station (BTS)  310 ; and double-ended arrow #3  403  illustrates a voice path between BTS  310  and mobile user station (MUS)  308 , which is being used by mobile subscriber A. In exemplary embodiments, double-ended arrow #3  403  employs the Enhanced Variable Rate Codec (EVRC). 
         [0059]    As mobile subscriber A moves near to the RF coverage area of a femtocell access point (FAP)  334 , MUS  308  sends a “Candidate Frequency Search Report Message” (CFSRPM), i.e., message #4  404 , over the reverse traffic channel to BTS  310  to indicate the strength of the candidate pilot signal(s). The pilot signal of FAP  334  is the strongest pilot signal. 
         [0060]    The macrocell base station controller (BSC)  312  then detects that the Active Pilot Power-to-Interference Wave Power Ratio has dropped below a predetermined threshold. As a result, BTS  310  sends a “Handoff Required” message, which is message #5  405 , and which contains a “Cell Identifier List” (i.e., a FAP Cell ID) to identify FAP  334 , to MSC  306 . 
         [0061]    MSC  306  then reserves a circuit of the nailed-up TDM handoff trunk between MSC  306  and convergence server (CS)  322 , and sends message #6  406 , which is a “FACDIR2” IS-41 message, to CS  322 . Message #6  406  directs CS  322  to initiate a “Handoff-Forward” task. Message #6  406  includes two key parameters: a “Target Cell ID,” which is the FAP Cell ID for the target femtocell, and an “InterMSCCircuit ID,” which is the Circuit Identifier Code (CIC) of the nailed-up TDM handoff trunk. 
         [0062]    CS  322  then sends message #7  407 , which is a “Reserve (CIC ID)” message, to media gateway control function (MGCF)  318 . The purpose of message #7  407  is to obtain Session Description Protocol (SDP) information of the handoff circuit. MGCF  318  interacts with media gateway (MGW)  316  using the Megaco protocol to reserve the circuit. MGW  316  then sends a response containing the SDP of the reserved circuit to MGCF  318 . 
         [0063]    MGCF  318  then sends message #8  408 , which is a “Response (SDP MGW)” message, to CS  322 . The “Response” message #8  408  includes the SDP information for the reserved circuit. The “Reserve” message #7  407  and the “Response” message #8  408  are high-level conceptual messages which may be implemented using standard Session Initiation Protocol (SIP) messages such as “INVITE/200OK/ACK,” “INFO/200OK,” “MESSAGE/200OK,” or any other proprietary mechanisms. Any implementation which enables the CIC ID and SDP MGW information to be conveyed may be used. The “Reserve” message #7  407  and the “Response” message #8  408  can be communicated directly between CS  322  to MGCF  318 , or these messages can be communicated through the IMS core via S-CSCF  324  and BGCF  320 . 
         [0064]    CS  322  then sends a multi-part “SIP MESSAGE,” which is message #9  409 , to FAP  334  to request the FAP  334  to prepare for handoff. This message encapsulates a “Handoff Request” with the FAP Cell ID and the SDP MGW information, which is the SDP information for the reserved circuit. In exemplary embodiments, message #9  409  does not include a “SIP INVITE” message, because the use of an “SIP INVITE” message increases handoff setup time by requiring the exchange of at least four messages (Le., an INVITE message, a  180  Ringing message, a 200OK message, and an ACK message) between CS  322  and FAP  334 . This, in turn, may cause a race condition to occur, which may induce timing errors and thereby cause handoff failure. After receiving message #9  409 , FAP  334  allocates necessary radio link and SDP resources (i.e., SDP FAP) for handoff, and prepares for radio channel switchover by sending forward traffic null frames (i.e., message #10  410 ) to MUS  308 . Then, in response to the handoff request message #9  409 , FAP  334  sends message #11  411  to CS  322 . Message #11  411  is a multi-part “200OK” response encapsulating a “Handoff Request Acknowledgement” and the SDP FAP information. 
         [0065]    CS  322  then sends an “Update (CIC: n, SDP FAP)” request, i.e., message #12  412 , to MGCF  318  to inform MGCF  318  and MGW  316  of the SDP FAP information. MGCF  318  then sends message #13  413 , a “Response (OK)” message, to CS  322  to acknowledge the “Update” request. The “Update” message and the “Response” message are high-level conceptual messages and can be implemented, for example, by using “SIP MESSAGE/200OK,” “SIP UPDATE/200OK,” “SIP re-INVITE/200OK/ACK,” or any other suitable mechanism which accomplishes the objective of enabling the FAP and the MGW to exchange SDP information in order to establish a Real-time Transport Protocol (RTP) voice path between the FAP and the MGW. 
         [0066]    CS  322  then sends a “FACDIR RETURN RESULT” message, i.e., message #14  414 , to the macrocell MSC  306 . This message informs the MSC that the necessary SDP information has been exchanged between the FAP and the MGW. 
         [0067]    MSC  306  sends message #15  415 , a “Handoff Command” message, to BTS  310  and BSC  312  to commence the source cell handoff procedure. Then, BTS  310  sends a “Universal Handoff Direction” message (UHDM) (i.e., message #16  416 ) to MUS  308 , thereby directing MUS  308  to hand off to the femtocell network. MUS  308  acknowledges receipt of the UHDM by sending message #17  417 , an “MS ACK” message, to BTS  310 . After receiving the acknowledgement in message #17  417 , BTS  310  sends a “Handoff Commenced” message (i.e., message #18  418 ) to MSC  306  to indicate that the handoff command has been sent to the mobile station and that an acknowledgement has been received from the mobile station. 
         [0068]    MUS  308  then switches to the new CDMA frequency channel and frame offset associated with FAP  334 . MUS  308  begins sending preamble frames, i.e., message #19  419 , to FAP  334  on a reverse traffic channel. Upon receiving preamble frames in message #19  419 , FAP  334  acknowledges receipt by sending a “BS ACK” message, i.e., message #20  420 , to MUS  308 . 
         [0069]    MUS  308  then sends message #21  421 , which is a “Handoff Completion” message, to FAP  334 , and FAP  334  acknowledges by sending a “BS ACK” message, i.e., message #22  422 , to MUS  308 . FAP  334  then sends a SIP MESSAGE encapsulating a Handoff Complete message, i.e., message #23  423 , to CS  322  to indicate that MUS  308  has successfully performed a handoff. Alternatively, message #23  423  may be a “SIP INFO” message. CS  322  then sends message #24  424 , a “200OK” message, to FAP  334  to acknowledge receipt of message #23  423 . 
         [0070]    CS  322  then sends a “MSONCH” IS-41 message, i.e., message #25  425 , to MSC  306  to indicate a successful hard handoff. MSC  306  then sends a “Clear Command” message, i.e., message #26  426 , to BTS  310  and BSC  412  to instruct BSC  412  to release all service option connections to MUS  308  and the associated dedicated resource. 
         [0071]    Mobile subscriber “A” continues to communicate on the same active call with PSTN user “B” after successful handoff completion. Double-ended arrow #27  427  illustrates a voice path between user B and MSC  306 ; double-ended arrow #28  428  illustrates a voice path between MSC  306  and MGW  316 ; and double-ended arrow #29  429  illustrates a voice path between MGW  316  and MUS  308  via FAP  334 . In exemplary embodiments, message #29  429  employs the Enhanced Variable Rate Codec (EVRC). 
         [0072]    Referring now to  FIG. 5 , a flowchart  500  illustrates a method for handing off an active communication connection from a macrocell to a residential femtocell, in accordance with particular embodiments. In the first step  504 , the macrocell base station controller (BSC) detects that the Active Pilot Power-to-Interference Wave Power Ratio associated with a mobile user station (MUS) on an active call has dropped below a predetermined threshold. As a result, at step  505 , the BSC controls the base station transceiver (BTS) to send a message (e.g., message #5  405 , a “Handoff Required” message) to the macrocell mobile switching center (MSC), to inform the MSC that a handoff to a femtocell is required. 
         [0073]    Upon being informed of the need for a handoff to a femtocell, at step  506 , the MSC sends a handoff initiation request (e.g., message #6  406 ) to the convergence server (CS). The handoff initiation request includes identification information to identify the femtocell to which the connection will be handed off. Then, in step  509 , the CS sends a request (e.g., message #9  409 ) to the femtocell access point (FAP) corresponding to the identified femtocell to request that the FAP prepare to receive a handoff. 
         [0074]    In step  510 , the FAP begins handoff preparation by sending null frames (e.g., message #10  410 ) to the MUS, and then, in step  511 , the FAP acknowledges the handoff preparation request by sending an acknowledgement message (e.g., message #11  411 ) to the CS. Upon receipt of this acknowledgment, the CS communicates with the media gateway (MGW) to exchange SDP information in order to establish a Real-time Transport Protocol (RTP) voice path between the FAP and the MGW. Then, in step  514 , the CS acknowledges a successful handoff initiation by sending a message (e.g., message #14  414 ) to the MSC. 
         [0075]    The MSC then directs the BSC to commence the actual handoff in step  515 . This direction may be accomplished, for example, by the transmission of message #15  415 . In response to receipt of this direction, at step  516 , the BSC controls the BTS to send a UHD message (e.g., message #16  416 ) to the MUS, and the MUS acknowledges receipt of the UHD message (e.g., by sending message #17  417 ) in step  517 . The BSC, in turn, acknowledges handoff commencement in step  518  by sending an acknowledgment message, e.g., message #18  418 , to the MSC. 
         [0076]    The MUS then sends preamble frames to the FAP (e.g., message #19  419 ) on a reverse traffic channel in step  519 , and the FAP acknowledges receipt of the preamble frames (e.g., message #20  420 ) in step  520 . Then, in step  521 , the MUS indicates that the handoff has been completed by sending a message (e.g., message #21  421 ) to the FAP, and the FAP acknowledges handoff completion to the MUS (e.g., message #22  422 ) in step  522 . 
         [0077]    At step  523 , the FAP indicates to the CS that the handoff is complete by sending a message (e.g., message #23  423 ), and then, at step  525 , the CS indicates handoff completion by sending a message (e.g., message #25  425 ) to the MSC. Finally, in step  529 , the MUS continues to conduct the active communication via the FAP. 
         [0078]    Referring now to  FIGS. 5A ,  5 B, and  5 C, each flowchart illustrates a subset of the steps performed as part of the method  500  by respective entities for handing off an active communication connection from a macrocell to a residential femtocell. In particular,  FIG. 5A  is a flowchart that includes the steps performed by the MSC  306 ;  FIG. 5B  is a flowchart that includes the steps performed by the FAP  334 ; and  FIG. 5C  is a flowchart that includes the steps performed by the MUS  308 . 
         [0079]    Referring now to  FIGS. 6A and 6B , a message sequence for an active mode macrocell-to-enterprise-femtocells handoff according to particular embodiments is illustrated. In this scenario, the enterprise femtocells include a cluster of femtocells. Prior to the start of the message sequence of  FIG. 6 , mobile subscriber “A” has established an active call with a PSTN user “B”. Double-ended arrow #1  601  illustrates a voice path between user B and macrocell mobile switching center (MSC)  306 ; double-ended arrow #2  602  illustrates a voice path between MSC  306  and macrocell base transceiver station (BTS)  310 ; and double-ended arrow #3  603  illustrates a voice path between BTS  310  and mobile user station (MUS)  308 , which is being used by mobile subscriber A. In exemplary embodiments, message #3  603  employs the Enhanced Variable Rate Codec (EVRC). 
         [0080]    As mobile subscriber A moves near to the RF coverage area of a cluster of femtocell access points (FAP)  334 , MUS  308  sends a “Candidate Frequency Search Report Message” (CFSRPM), i.e., message #4  604 , over the reverse traffic channel to BTS  310  to indicate the strength of the candidate pilot signal(s). The pilot signal of FAP  1  and FAP n are the strongest pilot signals. 
         [0081]    The macrocell base station controller (BSC)  312  then detects that the Active Pilot Power-to-Interference Wave Power Ratio has dropped below a predetermined threshold. As a result, BTS  310  sends a “Handoff Required” message, which is message #5  605 , and which contains a “Cell Identifier List” (i.e., FAP  1  Cell ID and FAP n Cell ID) to identify cluster FAP  334 , to MSC  306 . 
         [0082]    MSC  306  then reserves a circuit of the nailed-up TDM handoff trunk between MSC  306  and convergence server (CS)  322 , and sends message #6  606 , which is a “FACDIR2” IS-41 message, to CS  322 . Message #6  606  directs CS  322  to initiate a “Handoff-Forward” task. Message #6  606  includes two key parameters: a “Target Cell List,” which includes the FAP  1  Cell ID and FAP n Cell ID for the target femtocell cluster, and an “InterMSCCircuit ID,” which is the Circuit Identifier Code (CIC) of the nailed-up TDM handoff trunk. 
         [0083]    CS  322  then sends message #7  607 , which is a “Reserve (CIC ID)” message, to media gateway control function (MGCF)  318 . The purpose of message #7  607  is to obtain Session Description Protocol (SDP) information of the handoff circuit. MGCF  318  interacts with media gateway (MGW)  316  using the Megaco protocol to reserve the circuit. MGW  316  then sends a response containing the SDP of the reserved circuit to MGCF  318 . 
         [0084]    MGCF  318  then sends message #8  608 , which is a “Response (SDP MGW)” message, to CS  322 . The “Response” message #8  608  includes the SDP information for the reserved circuit. The “Reserve” message #7  607  and the “Response” message #8  608  are high-level conceptual messages which may be implemented using standard Session Initiation Protocol (SIP) messages such as “INVITE/200OK/ACK,” “INFO/200OK,” “MESSAGE/200OK,” or any other proprietary mechanisms. Any implementation which enables the CIC ID and SDP MGW information to be conveyed may be used. The “Reserve” message #7  607  and the “Response” message #8  608  can be communicated directly between CS  322  to MGCF  318 , or these messages can be communicated through the IMS core via S-CSCF  324  and BGCF  320 . 
         [0085]    CS  322  accesses its cell database to determine that FAP  1  and FAP n are located in a cluster FAP  334  which is controlled by a Controlling FAP. CS  322  then sends a multi-part “SIP MESSAGE,” which is message #9  609 , to the Controlling FAP within FAP  334  to request the Controlling FAP to prepare for handoff. This message encapsulates a “Handoff Request” with the FAP  1  Cell ID, the FAP n Cell ID, and the SDP MGW information, which is the SDP information for the reserved circuit. In exemplary embodiments, message #9  609  does not include a “SIP INVITE” message, because the use of an “SIP INVITE” message may cause a race condition to occur, which may induce timing errors and thereby cause handoff failure. After receiving message #9  609 , the Controlling FAP of FAP  334  allocates necessary radio link and SDP resources (i.e., SDP FAP) for handoff, and sends an “Allocate_Channel” message, i.e., message #10  610 , to FAP  1  over a local enterprise IP backbone to request that FAP  1  allocate radio resources. 
         [0086]    FAP  1  prepares for radio channel switchover by sending forward traffic null frames (i.e., message #11  611 ) to MUS  308 . Then, in response to the “Allocate_Channel” message #10  610 , FAP  1  sends an acknowledgement message, i.e., message #12  612 , to the Controlling FAP to indicate a successful radio resource allocation. The Controlling FAP then interacts with FAP n similarly as with FAP  1  by sending an “Allocate_Channel” message, i.e., message #13  613 , to FAP n over a local enterprise IP backbone to request that FAP n allocate radio resources. FAP n prepares for radio channel switchover by sending forward traffic null frames (i.e., message #14  614 ) to MUS  308 . Then, in response to the “Allocate_Channel” message #13  613 , FAP n sends an acknowledgement message, i.e., message #15  615 , to the Controlling FAP to indicate a successful radio resource allocation. 
         [0087]    Upon receiving message #12  612  and message #15  615  indicating successful radio resource allocations from FAP  1  and FAP n, respectively, and in response to handoff request message #9  609 , the Controlling FAP sends message #16  616  to CS  322 . Message #16  616  is a multi-part “200OK” response encapsulating a “Handoff Request Acknowledgement” and the SDP CFAP information. 
         [0088]    CS  322  then sends an “Update (CIC: n, SDP FAP)” request, i.e., message #17  617 , to MGCF  318  to inform MGCF  318  and MGW  316  of the SDP FAP information. MGCF  318  then sends message #18  618 , a “Response (OK)” message, to CS  322  to acknowledge the “Update” request. The “Update” message and the “Response” message are high-level conceptual messages and can be implemented, for example, by using “SIP MESSAGE/200OK,” “SIP UPDATE/200OK,” “SIP re-INVITE/200OK/ACK,” or any other suitable mechanism which accomplishes the objective of enabling the FAP and the MGW to exchange SDP information in order to establish a Real-time Transport Protocol (RTP) voice path between the FAP and the MGW. 
         [0089]    CS  322  then sends a “FACDIR RETURN RESULT” message, i.e., message #19  619 , to the macrocell MSC  306 . This message informs the MSC that the necessary SDP information has been exchanged between the FAP and the MGW. 
         [0090]    MSC  306  sends message #20  620 , a “Handoff Command” message, to BTS  310  and BSC  312  to commence the source cell handoff procedure. Then, BTS  310  sends a “Universal Handoff Direction” message (UHDM) (i.e., message #21  621 ) to MUS  308 , thereby directing MUS  308  to hand off to the femtocell network. MUS  308  acknowledges receipt of the UHDM by sending message #22  622 , an “MS ACK” message, to BTS  310 . After receiving the acknowledgement in message #22  622 , BTS  310  sends a “Handoff Commenced” message (i.e., message #23  623 ) to MSC  306  to indicate that the handoff command has been sent to the mobile station and that an acknowledgement has been received from the mobile station. 
         [0091]    Then, MUS  308  actually switches to the new CDMA frequency channel and frame offset associated with FAP  1  within cluster FAP  334 . MUS  308  begins sending preamble frames to FAP  1  on a reverse traffic channel. Upon receiving preamble frames, FAP  1  acknowledges receipt by sending a “BS ACK” message to MUS  308 . MUS  308  then sends message #24  624 , which is a “Handoff Completion” message, to FAP  1 , and FAP  1  informs the Controlling FAP that the handoff has been completed by sending message #25  625 . . MUS  308  also sends message #26  626 , another “Handoff Completion” message, to FAP n, and FAP n informs the Controlling FAP that the handoff has been completed by sending message #27  627 . 
         [0092]    The Controlling FAP then sends a SIP MESSAGE encapsulating a Handoff Complete message, i.e., message #28  628 , to CS  322  to indicate that MUS  308  has successfully performed a handoff to the enterprise FAP cluster  334 . Alternatively, message #28  628  may be a “SIP INFO” message. CS  322  then sends message #29  629 , a “200OK” message, to the Controlling FAP to acknowledge receipt of message #28  628 . 
         [0093]    Then, CS  322  sends a “MSONCH” IS-41 message, i.e., message #30  630 , to MSC  306  to indicate a successful hard handoff. MSC  306  then sends a “Clear Command” message, i.e., message #31  631 , to BTS  310  and BSC  412  to instruct BSC  412  to release all service option connections to MUS  308  and the associated dedicated resource. 
         [0094]    Mobile subscriber “A” continues to communicate on the same active call with PSTN user “B” after successful handoff completion. Double-ended arrow #32  632  illustrates a voice path between user B and MSC  306 ; double-ended arrow #33  633  illustrates a voice path between MSC  306  and MGW  316 ; and double-ended arrow #34  634  illustrates a voice path between MGW  316  and MUS  308  via FAP  334 . Double-ended arrow #35  635  illustrates a voice path between MUS  308  and FAP  1 , and double-ended arrow #36  626  illustrates a voice path between MUS  308  and FAP n. 
         [0095]    Referring now to  FIG. 7 , a flowchart  700  illustrates a method for handing off an active communication connection from a macrocell to a enterprise femtocell cluster, in accordance with particular embodiments. In the first step  704 , the macrocell base station controller (BSC) detects that the Active Pilot Power-to-Interference Wave Power Ratio associated with a mobile user station (MUS) on an active call has dropped below a predetermined threshold. As a result, at step  705 , the BSC controls the base station transceiver (BTS) to send a message (e.g., message #5  605 , a “Handoff Required” message) to the macrocell mobile switching center (MSC), to inform the MSC that a handoff to a femtocell is required. 
         [0096]    Upon being informed of the need for a handoff to a femtocell, at step  706 , the MSC sends a handoff initiation request (e.g., message #6  606 ) to the convergence server (CS). The handoff initiation request includes identification information to identify the individual femtocell access points (FAPs) within the enterprise femtocell cluster to which the connection will be handed off. The identified individual femtocell access points may or may not include the controlling femtocell access point corresponding to the enterprise femtocell cluster. The CS uses the list of identified individual FAPs in conjunction with its cell database to determine the controlling FAP of the cluster. In step  709 , the CS sends a request (e.g., message #9  609 ) to the controlling FAP to request that the controlling FAP prepare to receive a handoff. 
         [0097]    In step  710 , the controlling FAP begins handoff preparation by sending channel assignments to individual FAPs within the cluster (e.g., message #10  610  and message #13  613 ). In step  711 , the respective individual FAPs send null frames (e.g., message #11  611  and message #14  614 ) to the MUS, and then, in step  712 , the respective individual FAPs send acknowledgement messages (e.g., message #12  612  and message #15  615 ) to the controlling FAP to acknowledge the channel assignments. Then, in step  716 , the controlling FAP acknowledges the handoff preparation request by sending an acknowledgement message (e.g., message #16  616 ) to the CS. This acknowledgement message includes Session Description Protocol (SDP) information pertaining to the controlling FAP, including IP address and port number of the controlling FAP. 
         [0098]    Upon receipt of this acknowledgment, the CS communicates with the media gateway (MGW) to exchange SDP information (e.g., message #17  617  and message #18  618 ) in order to establish a Real-time Transport Protocol (RTP) voice path between the FAP and the MGW. Then, in step  519 , the CS acknowledges a successful handoff initiation by sending a message (e.g., message #19  619 ) to the MSC. 
         [0099]    The MSC then directs the BSC to commence the actual handoff in step  720 . This direction may be accomplished, for example, by the transmission of message #20  620 . In response to receipt of this direction, at step  721 , the BSC controls the BTS to send a UHD message (e.g., message #21  621 ) to the MUS, and the MUS acknowledges receipt of the UHD message (e.g., by sending message #22  622 ) in step  722 . The BSC, in turn, acknowledges handoff commencement in step  723  by sending an acknowledgment message, e.g., message #23  623 , to the MSC. 
         [0100]    The MUS then sends preamble frames to the individual FAPs within the enterprise femtocell cluster on a reverse traffic channel, and the individual FAPs respectively acknowledge receipt of the preamble frames. Then, in step  724 , the MUS indicates that the handoff has been completed by sending messages (e.g., message #24  624  and message #26  626 ) to the respective individual FAPs, and the individual FAPs respectively acknowledge handoff completion to the controlling FAP (e.g., message #25  625  and message #27  627 ) in step  725 . 
         [0101]    At step  728 , the controlling FAP indicates to the CS that the handoff is complete by sending a message (e.g., message #28  628 ), and then, at step  730  the CS indicates handoff completion by sending a message (e.g., message #30  630 ) to the MSC. Finally, in step  733 , the MUS continues to conduct the active communication via the enterprise femtocell cluster. In the uplink direction, a voice frame “x” transmitted by the MUS is received by FAP  1  and FAP n, which then forward voice frame “x” to the controlling FAP. The controlling FAP selects the strongest received voice frame and forwards that voice frame to the MGW  316 . In the downlink direction, the MGW transmits a voice frame “y” to the controlling FAP, which then forwards that same voice frame “y” to both FAP  1  and FAP n, each of which forward voice frame “y” to the MUS. 
         [0102]    Referring now to  FIGS. 7A ,  7 B, and  7 C, each flowchart illustrates a subset of the steps performed as part of the method  700  by respective entities for handing off an active communication connection from a macrocell to a enterprise femtocell cluster. In particular,  FIG. 7A  is a flowchart that includes the steps performed by the MSC  306 ;  FIG. 7B  is a flowchart that includes the steps performed by the enterprise FAP  334 ; and  FIG. 7C  is a flowchart that includes the steps performed by the MUS  308 . 
         [0103]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. For example, although an exemplary embodiment that relates to voice communication has been described, the disclosure may be used for any type of communication, including voice, data, video, text, or any other communication service connecting two or more user agents. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context. 
         [0104]    Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re- arranged, and some steps may be performed in parallel.