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Method and apparatus for inter-system active handoff of a hybrid subscriber unitRelated Patent Categories: Multiplex Communications, Communication Over Free Space, Having A Plurality Of Contiguous Regions Served By Respective Fixed Stations, Channel Assignment, Hand-off ControlMethod and apparatus for inter-system active handoff of a hybrid subscriber unit description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060109819, Method and apparatus for inter-system active handoff of a hybrid subscriber unit. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATION [0001] The present application claims priority from provisional application Ser. No. 60/629,960, entitled "METHOD AND APPARATUS FOR INTER-SYSTEM ACTIVE HANDOFF OF A HYBRID SUBSCRIBER UNIT," filed Nov. 22, 2004, which is commonly owned and incorporated herein by reference in its entirety. FIELD OF THE INVENTION [0002] The present invention relates generally to wireless communication systems, and more specifically to handoff of a hybrid communication device between a Voice over Internet Protocol (VoIP) communication session and a conventional cellular communication session. BACKGROUND OF THE INVENTION [0003] The evolution of cellular communications has resulted in a proliferation of networks of different technologies and corresponding different air interfaces. As a result, during the course of a single voice call, a wireless subscriber unit may roam among multiple networks, wherein each such network implements a different technology than the other networks of the multiple networks. Among the different network technologies are packet switched CDMA (Code Division Multiple Access) technologies, such as CDMA 2000 1XEV-DO (1X Evolution Data Only) or packet switched CDMA 1XRTT (1X Radio Transmission Technology), that are capable of providing Voice over Internet Protocol (VoIP) communication services, and conventional, or legacy, CDMA cellular communication technologies, such as a CDMA 1X, that provide circuit switched voice communication systems. [0004] As the subscriber unit roams among a packet switched CDMA communication network and a circuit switched CDMA communication network, it may be beneficial to system performance to handoff the subscriber unit from the former network to the latter network or from the latter network to the former network. For example, the channel conditions associated with one such network may be more favorable than the channel conditions associated with the other such network due to such factors as fading, adjacent and co-channel interference, and available power at a serving base station (BS) or radio access network (RAN). By way of another example, an operator of both a packet switched CDMA network and a circuit switched CDMA network may desire to move the subscriber unit from one such network to the other such network for purposes of balancing system loading. [0005] Currently, the only defined method for executing a handoff between a packet switched CDMA or WLAN network and a circuit switched CDMA network is an execution of a hard handoff, wherein a subscriber unit must drop a radio resource of a network of a first CDMA technology prior to acquiring a radio resource of a network of a second CDMA technology. A result is a brief period of time during which the subscriber unit is not actively engaged in a communication session with either network. Further, when executing a hard handoff there is no linkage between the two networks as the subscriber unit must drop the first network and acquire the second network without any assistance from the BS or RAN of either network. As a result, voice traffic may be lost during the handoff, resulting in poor system performance and efficiency and disgruntled end users. [0006] Therefore, a need exists for a method and apparatus for an active handoff of a voice call between a packet switched CDMA or WLAN network and a circuit switched CDMA network that assures that the subscriber unit is actively engaged in a communication session with at least one network at all times. BRIEF DESCRIPTION OF THE DRAWINGS [0007] FIG. 1 is a block diagram of a wireless communication system in accordance with an embodiment of the present invention. [0008] FIG. 2 is a block diagram of a packet data control switch of FIG. 1 in accordance with an embodiment of the present invention. [0009] FIG. 3 is a block diagram of a subscriber unit of FIG. 1 in accordance with an embodiment of the present invention. [0010] FIG. 4 is a block diagram of a subscriber unit of FIG. 1 in accordance with another embodiment of the present invention. [0011] FIG. 5 is a signal flow diagram illustrating a handoff of a voice call from a packet-based network of FIG. 1 to a circuit switched network of FIG. 1 in accordance with an embodiment of the present invention. [0012] FIG. 6 is a diagram of an exemplary SIP REFER message as modified in accordance with an embodiment of the present invention. [0013] FIG. 7 is a signal flow diagram illustrating a handoff of a voice call from a circuit switched network of FIG. 1 to a packet-based network of FIG. 1 in accordance with another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0014] To address the need for a method and apparatus for an active handoff of a voice call between a packet switched CDMA or WLAN network and a circuit switched CDMA network that assures that the subscriber unit is actively engaged in a communication session with at least one network at all times, a communication system is provided that provides for an active handoff of a voice call between a packet switched network, such as a CDMA 2000 1XRTT network, a CDMA 2000 1XEV-DO network, or a WLAN network, and a circuit switched network, preferably a CDMA 1X network, by transmitting the call over both networks (herein referred to as multicasting) during the course of the handoffs. An active handoff from the packet switched network to the circuit switched network is accomplished by multicasting the call over forward links of both networks during the handoff. An active handoff from the circuit switched network to the packet switched network is accomplished by multicasting the call over reverse links of both networks during the handoff. The former handoff further may be facilitated by maintaining a packet data control switch of the packet switched network in the call even when the call is routed through the circuit switched network, and the latter handoff further may be facilitated by maintaining a mobile switching center of the circuit switched network in the call even when the call is routed through the packet switched network. In order for a hybrid subscriber unit to operate concurrently in both networks, the subscriber unit may comprise multiple transceivers. However, in another embodiment of the present invention, the hybrid subscriber unit may comprise a single transceiver that is rapidly switched between the networks to give the appearance of concurrent operation. In addition, when switching between networks, a vocoder in the single transceiver subscriber unit may be instructed by the processor to use a reduced rate that allows for the time for the rapid switching without severely degrading voice quality. [0015] Generally, an embodiment of the present invention encompasses a method for handoff of a voice call from a packet switched network having a packet data control switch to a circuit switched network. The method includes conveying forward link voice traffic to, and receiving reverse link voice traffic from, a subscriber unit via the packet switched network and receiving, by the packet switched network, a notification to hand off the call to the circuit switched network. The method further includes, in response to receiving the handoff notification, conveying forward link voice traffic to the subscriber unit via the packet switched network and via the packet data control switch and the circuit switched network, and concurrently receiving reverse link voice traffic only via the packet switched network. The method further includes, when conveying forward link voice traffic to the subscriber unit via each of the packet switched network and the circuit switched network, switching from the packet switched network to the packet data control switch and the circuit switched network for reception of reverse link voice traffic and, in response to switching from the packet switched network to the circuit switched network for reception of reverse link voice traffic, ceasing conveying forward link voice traffic to the subscriber unit via the packet data control switch and the packet switched network while continuing to convey forward link voice traffic to the subscriber unit via the packet data control switch and the circuit switched network. [0016] Another embodiment of the present invention encompasses a packet data control switch having a processor configured to, in association with a subscriber unit call, route forward link voice traffic and receive reverse link voice traffic via a packet switched network, receive a notification to hand off the call from the packet switched network to a circuit switched network, in response to receiving the handoff notification, route forward link voice traffic to the subscriber unit via both the packet switched network and the circuit switched network and concurrently receive reverse link voice traffic only via the packet switched network, when conveying forward link voice traffic to the subscriber unit via each of the packet switched network and the circuit switched network, switch reception of reverse link voice traffic from the packet switched network to the circuit switched network, and in response to switching reception of reverse link voice traffic from the packet switched network to the circuit switched network, cease routing forward link voice traffic to the subscriber unit via the packet switched network while continuing to route forward link voice traffic to the subscriber unit via the circuit switched network. [0017] Yet another embodiment of the present invention encompasses a method for handoff of a voice call from a circuit switched network to a packet switched network. The method includes conveying forward link voice traffic to, and receiving reverse link voice traffic from, a subscriber unit via the circuit switched network and determining, by the circuit switched network, to hand off the call. The method further includes notifying the packet switched network, by the circuit switched network, of the need for a handoff and, in response to receiving the handoff notification, conveying, by the packet switched network of the subscriber unit, a request to perform a handoff and establishing a communication session between the packet switched network and the subscriber unit. The method further includes receiving reverse link voice traffic from the subscriber unit via the circuit switched network and via the packet switched network, and concurrently conveying forward link voice traffic to the subscriber unit only via the circuit switched network and, when receiving reverse link voice traffic via the packet switched network and the circuit switched network, switching from the circuit switched network to the packet switched network for conveyance of forward link voice traffic to the subscriber unit. The method further includes, in response to switching forward link voice traffic from the packet switched network to the circuit switched network, ceasing reception of reverse link voice traffic via the circuit switched network while continuing to receive reverse link voice traffic via the packet switched network. [0018] Still another embodiment of the present invention encompasses a communication system for inter-network handoff of a voice call from a circuit switched network to a packet switched network. The communication system includes a base station that transmits forward link voice traffic to a subscriber unit via a first forward link, receives reverse link voice traffic from the subscriber unit via a first reverse link from, and that determines to hand off the call. The communication system further includes a mobile switching center in communication with the base station that conveys the forward link voice traffic to the base station, receives the reverse link voice traffic from the base station, and that, in response to the determination to hand off the call, conveys a notification of the need for a handoff, and a packet data control switch that receives the handoff notification from the mobile switching center and, in response to receiving the handoff notification, requests that the subscriber unit perform a handoff. The communication system further includes an access network in communication with the packet data control switch that, in response to the request to perform a handoff, establishes a second forward link and a second reverse link with the subscriber unit. In response to the establishment of the second forward link and the second reverse link with the subscriber unit, each of the base station and the access network receives reverse link voice traffic from the subscriber unit while only the base station conveys forward link voice traffic to the subscriber unit. After each of the base station and the access network begins receiving reverse link voice traffic from the subscriber unit, conveyance of the forward link voice traffic to the subscriber unit is switched from the base station and the first forward link to the access network and the second forward link. In response to switching conveyance of forward link voice traffic from the base station and the first forward link to the access network and the second forward link, the base station ceases receiving reverse link voice traffic while the access network continues to receive reverse link voice traffic. [0019] Yet another embodiment of the present invention encompasses a method for handoff of a voice call from a packet switched network having a packet data control switch to a circuit switched network. The method includes communicating voice traffic via the packet switched network, receiving, by the packet switched network, a notification to hand off the call, wherein the notification to hand off identifies the circuit switched network as the network to hand off the call to, and in response to receiving the handoff notification, establishing communication via the circuit switched network and terminating communication via the packet switched network. Continue reading about Method and apparatus for inter-system active handoff of a hybrid subscriber unit... 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