Patent Publication Number: US-6904029-B2

Title: Method and apparatus for a source-initiated handoff from a source cellular wireless network to a target non-cellular wireless network

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to a co-pending application entitled “METHOD AND APPARATUS FOR A TARGET-INITIATED HANDOFF FROM A SOURCE CELLULAR WIRELESS NETWORK TO A TARGET NON-CELLULAR WIRELESS NETWORK”, filed on even date herewith, and assigned to the assignee of the instant application. 
     FIELD OF THE INVENTION 
     The present invention relates generally to wireless communication systems and, in particular, to handoff from a source cellular wireless network to a target non-cellular wireless network. 
     BACKGROUND OF THE INVENTION 
     With the growing popularity of non-cellular wireless networks, such as wireless local area networks (WLANs), a demand for integration with overlaid or adjacent cellular networks has arisen in the marketplace. A solution for the integration of WLAN and cellular networks must include the ability to perform seamless handovers at least for voice services. Current cellular systems (e.g., GSM and CDMA) allow for such mobility between cell sites, but technology does not currently exist to allow calls to be maintained across a cellular-to-WLAN border. Without this capability, a voice call would be dropped at the border of the two systems, or in an overlay situation, the call may continue but not under the control of the optimal or preferred system for that location. Therefore, a need exists for an apparatus and method for handoff from a cellular wireless network to a non-cellular wireless network. 
     An overview of some handoff prior art will support the novelty of the invention described below. Handoffs across different wireless technologies have been accomplished before, for example, between CDMA and analog cellular. CDMA to analog handoff based on DAHO (Database Assisted Handoff) is a specific example. DAHO initiates a handoff from CDMA to analog based on the existence of pilot signals and location information stored in the source cellular system. However, this is not a viable solution for a CDMA-WLAN system because the number of WLAN APs are much larger than analog base stations, thus requiring very large databases to be stored in each CDMA base site. Consequently, this approach would be cumbersome and complex. 
     Similar to CDMA-analog handoffs, UMTS-GSM handoffs are known. These handoffs are enabled by incorporating changes in the GSM and UMTS base sites to recognize each other&#39;s cell sites. This is done by modifying the existing list of neighboring cells to include cells of the other technology. Specific changes to handover signaling between the MS and the BS is also required to enable the handover. The invention described below does not involve any changes to the neighbor lists or introduce any new handover signaling between the MS and the cellular BS. 
     Inter-MSC (mobile switching center) handoffs are defined in CDMA IS-95 B and GSM systems to provide handoffs between two base sites that are controlled by distinct MSCs. The Inter-MSC handoff procedures as defined in all cellular networks are initiated by the source MSC (the MSC currently serving the serving base site). The current IS-41 and MAP procedures (the interfaces governing the handoff procedure in CDMA and GSM respectively) only provide for source initiated handoffs. This can be seen, for example, in FIG.  1 .  FIG. 1  illustrates the inter-MSC handoff procedure for IS-95 systems based on the IS-41 specifications. (MAP procedures for GSM are similar.) 
     The known handoff procedure begins with the mobile station (MS) generating a CDMA Pilot Strength Measurement Message (PSMM)  1 . The PSMM message contains the PN offsets and signal strengths (Echo) of pilots in the MS&#39;s candidate and active set. The base site (BS) determines that the PN offset sent in the PSMM does not correspond to a cell under its control. The BS generates a Handoff Required message  2  containing the Cell Identifier List (with Cell ID, and optionally more information like MSC ID, LAC, etc). The source MSC then identifies the target BS and the associated MSC. It sets up a terrestrial circuit to the target MSC, and sends an IS41_FACDIR2 message  3 . The message contains the inter-MSC circuit ID, target cell ID, and other handoff-related parameters like channel condition, etc. The target MSC then initiates a Handoff Request  4  to the appropriate target BS. The message contains parameters that are mostly obtained (directly transferred) from the FACDIR2 message. 
     A Handoff Request Ack  5  is sent by the target BS to the MSC after radio resources and terrestrial circuits are allocated, and an IS — 41_facdir2  6  is sent to the source MSC containing the parameters obtained from the Handoff Request Ack message. The Handoff Command  7  is then sent to the source BS to begin the handoff procedure, and the information in this message is used to generate an IS95_Extended Handoff Direction Message  8 , containing the new frequency channel and frame offset. The IS95_Handoff Direction Message instructs the MS to switchover to the target cell/BS and start sending preamble frames on the reverse channel. The MS acks this message by sending an IS95_Extended Handoff Direction Ack Message  9  to the source BS. The source BS then sends a Handoff Commenced message  10  to the source MSC indicating that the handoff is progress. 
     When ready, the MS sends an IS — 95 Handoff Completion message  11  to the target BS. The target BS then sends a Handoff Complete message  12  to the target MSC, and the target MSC informs the source MSC of the successful handover with an MSONCH message  13 . Finally, a Clear Command message  14  and a Clear Complete message  15  are exchanged in order to release resources between the source BS and the source MSC. 
     Two aspects of this prior art handoff messaging are particularly pertinent. First, it is the MS that identifies the handoff target to the source BS and MSC by providing the PN offset of the target. Second, it is the source MSC that initiates the handoff messaging (see  FIG. 1 , message  3 ) by translating the PN offset to a target BS/MSC. However, if the target system were a WLAN system, the handoff target would be a WLAN access point (AP), and presently there is no messaging to enable either the MS or the source MSC to identify this target WLAN AP. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a message flow diagram of prior art messaging exchanged by system components to affect a handoff. 
         FIG. 2   a  is a block diagram depiction of a communication system in accordance with an embodiment of the present invention. 
         FIG. 2   b  is a block diagram depiction of communication system components in accordance with an embodiment of the present invention. 
         FIG. 3  is a messaging flow diagram of messaging and information exchanged by system components to affect a handoff in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     To address the need for an apparatus and method for handoff from a cellular wireless network to a non-cellular wireless network (WLAN, e.g.), the present application describes an access gateway and a dual mode mobile station that enable such handoffs. Dual mode MSs can determine when a handoff to a non-cellular network is preferred and request a handin from the non-cellular network. The access gateway provides information to the MS so that it can initiate a handoff through the serving cellular network. Triggering handoffs in this manner, allows cellular networks to handle handoffs to non-cellular networks in much the same way they handle inter-MSC handoffs today, i.e., source initiated. 
     The disclosed embodiments can be more fully understood with reference to  FIGS. 2   a ,  2   b , and  3 .  FIG. 2   a  is a block diagram depiction of communication system  200  in accordance with an embodiment of the present invention. Communication system  200  comprises a known wireless local area network (WLAN), a known cellular network, and components to interface them together, the combination suitably modified to implement the present invention. The WLAN is a known wireless infrastructure such as that conforming to the IEEE 802.11 standard. The cellular network is a well-known Code Division Multiple Access (CDMA) network, based on the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) standard IS-95. (The TIA/EIA can be contacted at 2001 Pennsylvania Ave. NW, Washington, D.C. 20006). In various alternative embodiments, communication system  200  may utilize other cellular communication protocols such as, but not limited to, GSM, UMTS, IS-2000, and “IDEN.” 
     The cellular network of communication system  200  includes known radio access network (RAN) entities, such as base site (BS)  250  (comprising a base site controller and one or more base transceiver stations), mobile switching center (MSC)  251  (which interfaces with PSTN  205 ), and home location register (HLR)  252 . Communication system  200  further includes WLAN access point (AP)  210 , internet protocol (IP) network  211 , circuit gateway  212 , private branch exchange (PBX)  213 , and cellular access gateway (CAG)  214 . Both the WLAN and cellular network of system  200  support voice services. The WLAN supports voice over a pico-cellular environment, while the cellular network supports voice over the macro-cellular environment. As integrated into system  200 , these networks further support voice-session mobility from the cellular network to the WLAN. 
     Communication system  200  also includes mobile stations (MSs), such as MS  201 . MS  201  is a dual-mode phone capable of communicating with both the cellular network (e.g., BS  250 ) and the WLAN (e.g., AP  210 ).  FIG. 2   b  depicts MS  201  is greater detail. MS  201  comprises well-known entities such as processor  204 , dual-mode transmitter  202 , and dual-mode receiver  203 . Transmitters, receivers, and processors as used in MSs are all well known in the art. This common set of MS components is adapted using known telecommunications design and development techniques to implement the wireless unit aspect of the present invention. Processors typically comprise components such as microprocessors, digital signal processors, memory, and/or logic circuitry designed to implement algorithms that have been expressed as computer instructions and/or in circuitry. Given an algorithm or a logic flow, those skilled in the art are aware of the many design and development techniques available to implement a processor that performs the given logic. 
       FIG. 2   b  also depicts CAG  214  in greater detail. CAG  214  comprises a known network interface  215  and cellular interworking device  216 . Network interface  215  provides an access gateway interface to IP network  211 , while cellular interworking device  216  performs cellular mobility interworking (e.g., interworking for registration, authentication, and handoff) by interfacing with MSC  251 , HLR  252 , PBX  213 , and circuit gateway  212 . Cellular interworking device  216  also performs PSTN interworking by interfacing with PSTN  205  using landline signaling protocols such as ISDN User Part (ISUP) and/or Multi Frequency R1 (MFR1). Generally, cellular and PSTN interworking components are known in the art. These components in addition to network interface components are combined and adapted using known telecommunications design and development techniques to implement the access gateway aspect of the present invention. Given a protocol or a message flow, those skilled in the art are aware of the many design and development techniques available to implement a networking platform that performs the specified function. 
     Furthermore, those skilled in the art will recognize that  FIGS. 2   a  and  2   b  do not depict all of the network equipment and devices necessary for system  200  to operate fully but only those system blocks and logical entities particularly relevant to the description of embodiments of the present invention. Those skilled in the art are aware of the many ways the necessary devices and entities can be implemented and/or purchased from wireless networking companies and wireless communications companies such as “MOTOROLA.” 
     High-level operation of a first embodiment of the present invention occurs substantially as follows. In the first embodiment, MS  201 &#39;s dual mode functionality allows it to support voice services over the cellular network and the WLAN. Thus, MS  201  supports a standard cellular voice call model such as one specified by the GSM, CDMA, or “IDEN” technologies, for example. For the WLAN domain, MS  201  supports a voice over IP (VoIP) protocol, such as H.323, Session Initiation Protocol (SIP), or the Skinny Protocol of “CISCO.” The VoIP protocols are used between MS  201  and circuit gateway  212 . Circuit gateway  212 , when connected to PBX  213 , provides the interworking necessary for the desired PBX feature transparency to MS  201 . Also, for signaling with WLAN AP  210 , MS  201  supports IEEE 802.11 signaling in the first embodiment, but signaling types such as Bluetooth or HiperLAN 2 may additionally or alternatively be supported in other embodiments. Lastly, the dual mode capability of MS  201  allows it to measure the signal strength of the WLAN AP(s), such as AP  210 , as well as the cellular BTS(s), such as those of BS  250 . 
     Generally, in the first embodiment, CAG  214  interworks the voice call model and mobility management within the WLAN domain with the voice call model and mobility schemes of the standard macro-cellular domain. It provides the required interworking between the WLAN and cellular domain in the areas of cellular registration, authentication, and cross-technology handovers. In addition, it also interworks the cellular network with the existing voice infrastructure (i.e., PBX  213  and circuit gateway  212 ) in the WLAN domain. 
     In the first embodiment, cellular interworking device  216  provides the appearance to a GSM/“IDEN” (MAP) or a CDMA (IS-41) cellular network that the WLAN domain is another standard cellular network. Cellular interworking device  216  enforces message discrimination by sending/receiving MAP/IS-41 messaging to/from an MSC/HLR. Cellular interworking device  216  effectively emulates either an MSC or a VLR role to the far-end macro-cellular domain. 
     In the first embodiment, cellular interworking device  216  also keeps subscriber profile, supports authentication, supports registration, etc. At a minimum, cellular interworking device  216  emulates a portion of the cellular VLR. It provides higher-layer mobility support to allow CAG  214  to act like a standard MSC to the macro-cellular MSC/HLR domain. 
     In addition, in the first embodiment, cellular interworking device  216  provides service logic similar to call processing, but not a complete set. The distinction typically is between service/feature “control” and service/feature “execution.” There are only a few scenarios (e.g. handoff from cellular to WLAN) where cellular interworking device  216  provides full call processing, allowing the connection to be made (i.e., control) and setting up the bearer connection through CAG  214  (i.e., basic execution). Since CAG  214  is only involved in inter-domain session establishment and handoffs, these scenarios require functionality to maintain the basic state of the subscriber&#39;s session. In most other scenarios, like a PSTN to WLAN session establishment, PBX  213  provides all call processing. 
     In the first embodiment, the general role of PBX  213  is to terminate circuit voice calls and provide call processing with access to voice features as if MS  201  were a typical wired telephone in the enterprise domain. In addition, the general purpose of circuit gateway  212  is to interwork the voice call models in the WLAN-IP domain and the typical circuit (i.e., PBX) domain. This requires both bearer and control interworking. The voice bearer and signaling from dual mode MS  201  and WLAN APs connect over IP and may use IP telephony call model conventions. Since the IP telephony conventions do not work with the typical wired PBX, circuit gateway  212  provides this important interworking to PBX  213 . 
     Messaging-focused operation of the first embodiment of the present invention occurs substantially as follows.  FIG. 3  is a messaging flow diagram  300  of messaging and information exchanged by system components to affect a handoff from a cellular wireless network to a non-cellular wireless network (e.g., a WLAN) in accordance with the first embodiment of the present invention. Already involved in a call, MS  201  receives call information ( 301 ) via serving BS  250  and associated (i.e., serving) MSC  251 . This call information refers to real-time call content such as voice or video-telephony. 
     As MS  201  moves within the coverage area of WLAN AP  210 , MS  201  performs signal strength measurements and establishes contact with AP  210 . Establishing contact typically involves obtaining an IP address for itself (MS  201 ) and for an access gateway (CAG  214 , in the first embodiment). At some point, MS  201  determines that a handoff from serving BS  250  to AP  210  is preferred. MS  201  may determine this based on criteria such as the relative signal strength of BS  250  and AP  210 , the relative cost of wireless service, and/or user indications of preference. For example, the user may set an MS option to switch to WLAN service whenever signal conditions allow or whenever the WLAN service is determine to be cheaper. 
     Having determined that a handoff is preferred, processor  204  sends a handin request ( 302 ) to CAG  214 . The request is sent to CAG  214  via transmitter  202 , WLAN AP  210 , and IP network  211 . Thus, the handin request is sent using an IP packet addressed to CAG  214 . The handin request contains an indication of from which cellular wireless network MS  201  is attempting to handoff, i.e., which MSC is serving MS  201 . The indication takes the form of a serving cell identifier which CAG  214  can use to determine the corresponding serving MSC. In the first embodiment, this serving cell identifier is the PN offset of MS  201 &#39;s serving cell within BS  250 , while in an alternative GSM embodiment, the serving cell identifier may be the Base Transceiver Station Identity Code (BSIC) of MS  201 &#39;s serving cell. 
     Cellular interworking device  216  of CAG  214  receives the IP-packetized handin request from MS  201  via network interface  215 . In response to MS  201 &#39;s handin request, cellular interworking device  216  sends a handin request acknowledgment ( 304 ) to MS  201 . This handin request acknowledgment is sent via network interface  215 , IP network  211 , and WLAN AP  210 . Importantly, the acknowledgment contains a handoff-target identifier, such as a cell identifier. In the first embodiment, this handoff-target identifier is a value that is predefined to trigger an automatic handoff determination by the cellular wireless network from which the MS is attempting to handoff. In other words, it could be either a “spoof” value or a valid cell identifier that will be recognized (i.e., the cellular network has been preprogrammed to recognize) as a trigger for handoff to this non-cellular network. In an alternative embodiment, the handoff-target identifier may simply be a valid cell identifier for the non-cellular network that will not be specially recognized. 
     Processor  204  of MS  201  receives the handin request acknowledgment via WLAN AP  210  and receiver  203 . After receiving the acknowledgment, processor  204  sends a signal strength message ( 306 ) via transmitter  202  to serving BS  250 . This signal strength message comprises values intended to trigger a handoff determination. Specifically, in the first embodiment, the signal strength message is a CDMA PSMM containing the handoff-target identifier from the handin request acknowledgment. Thus, the PSMM is sent in order to trigger a handoff to the WLAN, as identified by the handoff-target identifier. Alternatively, the PSMM could contain a regular cell identifier for the WLAN but with an artificial signal strength value associated with the cell identifier, which is intended to trigger a handoff to the WLAN identified by the cell identifier. In an alternative GSM embodiment, the signal strength message could instead be either a MEAS_RES (Measurement Result) message or a MEAS_REP (Measurement Report) message. 
     Thus, it is the handoff source (i.e., the serving cellular network) that initiates the handoff of MS  201  from the cellular network to the WLAN. However, for this to occur, handoff-target information is sent to the MS by the target network (i.e., the WLAN). This information is then used by the MS to trigger the handoff procedures. Note, that the cellular network needs to be able to recognize the handoff-target identifier that it receives in the PSMM, so some sort of agreement that addresses this between the network operators of the WLAN and cellular network is envisioned. 
     BS  250  receives the PSMM and determines that a handoff for MS  201  should be initiated. BS  250  sends a handoff required message ( 308 ) to MSC  251 , and serving MSC 251  then sets up the necessary circuits and sends a FACDIR2 message. CAG  214  receives the MAP FACDIR2 message ( 310 ) from serving MSC  251  and sends a MAP facdir2 message ( 312 ) back in response. 
     Serving MSC  251  then sends an initiate handoff message ( 314 ) to serving BS  250 . In the first embodiment, this initiate handoff message would be a Clear Command signaling serving BS  250  to clear its wireless resources supporting MS  201 . Release channel messaging particular to the cellular network (e.g., IS-95 or GSM messaging) is then exchanged ( 316 ) between MS  201  and BS  250 . For example, processor  204  of MS  201  receives a handoff release indication from BS  250  via receiver  203 . In the first embodiment, this indication would be a CDMA Handoff Direction Message, while in an alternative GSM embodiment this indication may be a HND_CMD (handoff command) message. 
     After completing channel release messaging, processor  204  of MS  201  sends a handoff complete indication ( 318 ) to CAG  214  via transmitter  202 , WLAN AP  210 , and IP network  211 . Thus, the handoff complete indication is sent using an IP packet addressed to CAG  214 . Cellular interworking device  216  of CAG  214  receives the IP-packetized handoff complete indication from MS  201  via network interface- 215 . In response to this indication, cellular interworking device  216  sends an indication to MSC  251  that the MS is on channel ( 320 ). Specifically, this indication is a MAP MSONCH message. 
     MSC  251  then switches the MS  201  call information to CAG  214 . CAG  214  receives the call information (via DS 0  signaling, e.g.) and routes ( 321 ) it to MS  201  via IP network  211  and WLAN AP  210 . Thus, MS  201  completes a handoff from the cellular network to the WLAN, continuing to receive its call information via MSC  251 , CAG  214 , and WLAN AP  210 . 
     In the foregoing specification, the present invention has been described with reference to specific embodiments. However, one of ordinary skill in the art will appreciate that various modifications and changes may be made without departing from the spirit and scope of the present invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. In addition, those of ordinary skill in the art will appreciate that the elements in the drawings are illustrated for simplicity and clarity, and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help improve an understanding of the various embodiments of the present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments of the present invention. However, the benefits, advantages, solutions to problems, and any element(s) that may cause or result in such benefits, advantages, or solutions, or cause such benefits, advantages, or solutions to become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein and in the appended claims, the term “comprises,” “comprising,” or any other variation thereof is intended to refer to a non-exclusive inclusion, such that a process, method, article of manufacture, or apparatus that comprises a list of elements does not include only those elements in the list, but may include other elements not expressly listed or inherent to such process, method, article of manufacture, or apparatus. 
     The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. The term program, as used herein, is defined as a sequence of instructions designed for execution on a computer system. A program, or computer program, may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.