Abstract:
A hybrid system and method are provided for determining the geographic location of a wireless device. A first base station subsystem is connected to the wireless device for transferring location information to and from the wireless device. A network subsystem is connected to the first base station subsystem and to a location computation entity for transferring location information between the first base station subsystem and the location computation entity. A second base station subsystem is connected to a location measurement entity and to the location computation entity for transferring location information between the location measurement entity and the location computation entity. The location computation entity generates and sends location information to the wireless device and/or location measurement entity and processes location information received from the wireless device and/or location measurement entity to obtain the geographic location of the wireless device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the determination of the location of a wireless device. More particularly, the present invention relates to the determination of the location of a wireless device with a hybrid system and method that utilizes BSS and NSS subsystems of a wireless communications network. 
     2. Description of Related Art 
     Determination of the precise geographic location of a wireless device, such as a cellular phone or personal communications services (PCS) device for example, is a capability that is now being developed and standardized for today&#39;s PCS, cellular and satellite based wireless networks. Deployment of this capability is required, for example, in the US to fulfill the FCC&#39;s phase  2  requirements for E911 calls. Known strategies to obtain a geographic location typically employ either (a) an overlay solution in which the entities that perform location measurement and location computation are external to the wireless communications network and communicate using external or externally available transmission and switching capabilities or (b) an integrated solution in which these entities form part of the wireless network and make use of communication capabilities provided within the network. 
     A typical terrestrial wireless communications network (for example cellular or PCS) consists of three major subsystems: the Base Station Subsystem (BSS), Network SubSystem (NSS) and Operations and Maintenance Subsystem (OMS). Wireless communication networks are typically categorized into “generations” according to the technology being implemented. For example, the generation of a particular wireless technology is now commonly accepted in the art with first generation corresponding to systems supporting an analog radio interface (e.g. AMPS), second generation supporting a digital radio interface with a peak bit rate for any application of around 64 kbps (e.g. GSM, PCS1900, CDMA IS-95, TDMA) and third generation supporting a peak bit rate for applications of at least 384 kbps (e.g. CDMA2000, WCDMA). 
     In many existing first and second generation wireless technologies (e.g. GSM, CDMA IS-95, TDMA, AMPS), the BSS includes logically or physically distinct entities to serve as a Base Station Controller (BSC), Base Station Transceiver Station (BTS) and a Transcoding and Rate Adaptation Unit (TRAU). Logically distinct units may be physically supported on a common hardware platform while retaining their distinct functions. The NSS contains logically or physically distinct entities to serve as a Mobile Switching Center (MSC), Visitor Location Register (VLR), Home Location Register (HLR), Authentication Center (AC), Equipment Identification Register (EIR). The functions of these different entities are known to those of ordinary skill in the art of wireless networks. In simple terms, the BSS manages the radio aspects of the network whereas the NSS manages mobility, call control and supplementary services (e.g. call forwarding, short message service). For second generation systems supporting packet data communication—e.g. General Packet Radio Service (GPRS)—other entities are included in the NSS such a Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). For third generation systems, entities are further modified and/or new ones are introduced in the NSS and BSS. The constituents of the BSS and NSS for third generation systems are subject to future developments in technology and standards. 
     Typical models of a geographic location service in a wireless network (e.g. as being deployed for FCC E911 phase  2 ) assume that some external user, sometimes termed a “client”, interacts with the wireless network to obtain the geographic location of a Wireless Mobile Subscriber (MS, or “wireless device”) either on demand or when some predetermined event occurs (e.g. the origination of an E911 call). The wireless network then instigates the procedures that will lead to a geographic location estimate for the particular MS. 
     In order to obtain an accurate geographic location for an MS, radio related measurements must generally be taken involving either (or in some cases both of) the uplink radio transmission from the MS or (and) downlink radio transmission to the MS. The resulting measurements, normally made over a short time period (e.g. a few milliseconds to a few seconds), then form the input to a computational algorithm from which a geographic position (e.g. latitude and longitude coordinates) is produced. 
     Examples of existing positioning methods include Time Of Arrival (TOA), Time Difference Of Arrival (TDOA), Angle of Arrival (AOA), Timing Advance (TA), Observed Time Difference (OTD), Enhanced Observed Time Difference (EOTD), Observed Time Difference Of Arrival (OTDOA), Global Positioning System (GPS). These methods can be allocated among the following categories (in some cases with different variants of the same method appearing in more than one category): 
     (a) Network Based (e.g. TOA, TDOA, TA, AOA)—measurements of uplink MS radio transmissions are made by the network with position computation in the network. 
     (b) Mobile Assisted (e.g. OTD, E-OTD, OTDOA, GPS)—measurements of downlink radio transmission (emanating from the wireless network or from other sources like GPS satellites) are made by the MS. These measurements are then provided to the network where position computation occurs. 
     (c) Mobile Based (e.g. OTD, E-OTD, OTDOA, GPS)—measurements of downlink radio transmission (emanating from the wireless network or from other sources like GPS satellites) are made by the MS. Position computation using these measurements is then also performed by the MS. 
     In order to support the above categories, two special functional entities are required in addition to those already existing in the wireless network and MS: a location measurement entity (LME) to perform radio related measurements and a location computation entity (LCE) to compute a geographic location estimate from the measurements provided by the LMEs. These terms are generic and correspond to certain more specific entities in particular wireless technologies. Thus, for example, the LCE can correspond to both the Positioning Determining Entity (PDE) used in ANSI-41 based networks (e.g. TDMA, CDMA IS-95) and the Serving Mobile Location Center (SMLC) defined for GSM and PCS1900 networks. Similarly, the LME can correspond to the radio elements of a Positioning Determining Entity (PDE) in ANSI-41 networks and to the Location Measurement Unit (LMU) defined for PCS1900 and GSM networks. Further, the LME typically may be accessed using only wireline facilities or may be accessed by wireline and wireless means (e.g. RF, microwave) including wireless access similar to or the same as that supported for normal MSs within the wireless network. 
     One known architectural solution for communicating geographic location information between the LCE and the LME and/or MS is known as an “NSS solution”. The LCE is typically attached and directly accessible only to the NSS. Disadvantageously, this architecture relies on the NSS to support signaling between the LCE and both the LME and MS. Thus, any hardware and/or software limitations and failures in the NSS can negatively impact the timely and accurate flow of information. There may also be signaling limitations in the NSS that restrict the volume of data that can be transferred between an LCE and an LME and between an LCE and an MS per unit of time, resulting in a limitation on the number of location attempts that can be performed per unit of time. This limitation may also produce increased delay for location attempts that do succeed due to the extra signaling delays in the NSS plus any queuing delay when signaling throughput limits are reached. An example of a location application for which these limitations may be significant is “home zone billing”—a service in which an MS subscriber is billed at a special (e.g. flat or reduced) rate when making or receiving calls within a predefined home zone area. To verify whether the MS actually is within the home zone area, a wireless network would need to perform at least one location determination for every incoming and outgoing call and possibly additional location determinations at periodic intervals during a call or when a call is handed over from one BSS cell site to another. Such a service implies several location determinations per subscriber per hour during peak busy periods. The ensuing signaling load in the NSS would then be heavy particularly if a network based positioning method was being used where several LMEs need to send measurements related to the location of the MS through the NSS to the LCE for each attempt to locate an MS. 
     Another known architectural solution for communicating geographic location information between the LCE and the LME and/or MS is known as a “BSS variant” or “BSS solution”. The LCE is typically attached and directly accessible only to the BSS. LME to LCE signaling (to transfer measurements from the LMEs to the LCE) and LCE to MS signaling (to support mobile based and mobile assisted position methods) are then both supported by signaling facilities in the BSS only. Disadvantageously, access by the NSS to the LCE (e.g. to initiate a location procedure and receive the resulting location estimate) needs to go through intermediate signaling facilities in the BSS—i.e. is less direct than with an NSS solution. This can negatively impact the timely and accurate flow of information that enable access to the LCE from the NSS for any location application in which the client is supported within or by the NSS. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a hybrid system and method for determining the location of a wireless device. 
     It is another object of the invention to provide a system and method for determining the location of a wireless device with fewer throughput limitations and delays than known NSS solutions. 
     It is another object of the invention to provide a system and method for determining the location of a wireless device while avoiding some throughput limitations and delays of known BSS solutions. 
     The present invention is applicable to both an integrated and overlay solution. A hybrid system and method of location determination is utilized in which the location determination functionality is provided in part by the BSS and in part by the NSS. Advantageously, the division of functionality among the BSS and NSS can reduce network infrastructure impacts and improve performance compared to either a pure BSS or pure NSS approach. 
     With the hybrid architecture, LME to LCE signaling does not rely on signaling capability in the NSS and does not overburden the signaling facilities in the NSS. Although in a preferred embodiment MS to LCE signaling still relies on signaling facilities in the BSS and NSS, as in the NSS based architecture, the volume of such signaling will be substantially zero for network based positioning methods (where there is no MS to LCE signaling). With regard to the MS based or MS assisted positioning support by the hybrid architecture, while there is additional signaling through the NSS to support MS to LCE signaling, in a preferred embodiment there is no extra signaling through the BSS to enable access to the LCE from the NSS. Advantageously, software and/or hardware impacts in the BSS are not needed to support NSS access to the LCE. 
     The hybrid location services architecture can support location of MS subscribers in a wireless network and can be used in conjunction with any network based, mobile assisted and mobile based position methods. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram representing an example of LCE and LME roles for network-based position methods. 
     FIG. 2 is a diagram representing an example of LCE and LME roles for mobile-assisted position methods. 
     FIG. 3 is a diagram representing an example of LCE and LME roles for mobile-based position methods. 
     FIG. 4 is a diagram representing an example of an NSS-based location architecture. 
     FIG. 5 is a diagram representing an example of a BSS-based location architecture. 
     FIG. 6 is a diagram representing an example of a preferred embodiment for a hybrid location architecture. 
     FIG. 7 is a diagram representing an example of a preferred embodiment for a hybrid location architecture used with a GSM system. 
    
    
     DETAILED DESCRIPTION 
     For the purposes of this disclosure, the term BSS means Base Station Subsystem, which is a wireless network subsystem, including its associated, switching, transmission and processing capabilities that manages the radio interface support for mobile subscribers and the transfer of control and user signaling information over this interface. The term NSS means Network SubSystem, which includes the switching, transmission, processing and database facilities that provide support for mobility, call control, supplementary services, messaging services, subscription and access to other wireless and wireline networks. 
     With reference to FIG. 1, architecture diagram  100  illustrates the interaction of some components used in a network-based positioning system. LMEs  110 ,  115  perform measurements involving uplink radio transmissions from MS  120 . LMEs  110 ,  115  may also perform measurements not directly related to any one particular MS to obtain additional information for LCE  130 . Examples of such additional measurements include measurements of the relative timing differences between the downlink transmissions from different BSS cell sites  140 ,  145  and measurements of the correspondence of the downlink transmission timing at a particular BSS cell site  140  or  145  with some source of absolute time like GPS satellites (not shown). 
     All relevant measurements—including those for a particular MS and those independent of any MS—are supplied by LMEs  110 ,  115  to LCE  130 , where a location estimate is obtained for each MS being measured. This location estimate may then be transferred from LCE  130  to client  160  via NSS  150 . In an alternative preferred embodiment, the location is transferred from LCE  130  to client  160  directly, without going through NSS  150 . 
     With reference to FIG. 2, architecture diagram  200  illustrates the interaction of some components used in a mobile-assisted positioning system. MS  220  makes measurements of suitable downlink radio transmissions, e.g. from local BSSs  240 ,  245  or from GPS satellites (not shown) and provides these to LCE  230 . LMEs  210 ,  215  may be utilized to provide additional measurements to LCE  230  to assist with computation of a location estimate for MS  220  by LCE  230  and/or to provide information that may be sent to MS  220  to assist with the downlink radio measurements. For example, in a preferred embodiment LME  210  functioning as a GPS receiver provides LCE  230  with-GPS-related measurements and information that, following further processing in LCE  230 , are sent to MS  220  (e.g. in the form of ephemeris or almanac data) to enable faster and more accurate GPS measurements by MS  220 . Location data may be transferred from LCE  230  to client  260  via NSS  250 . In an alternative preferred embodiment, the location is transferred from LCE  230  to client  260  directly, without going through NSS  250 . 
     With reference to FIG. 3, architecture diagram  300  illustrates the interaction of some components used in a mobile based positioning system. With MS based position methods, MS  320  makes measurements of suitable downlink radio transmissions as with MS assisted methods illustrated in FIG.  2 . In a preferred embodiment, LMEs  310 ,  315  may provide additional information to MS  320  by making additional measurements and transferring these to LCE  330  for possible further processing and subsequent transfer to MS  320 . MS  320  then computes its own location using its own measurements and any additional information supplied by LCE  330  and provides the computed location to LCE  330 . This computed location may then be transferred to client  360  via NSS  350 . In an alternative preferred embodiment, the location is transferred from LCE  330  to client  360  directly, without going through NSS  350 . 
     With each of the three types of position methods illustrated in FIGS. 1-3, signaling paths are required between LCE ( 130 ;  230 ;  330  respectively) and LMEs ( 110 ,  115 ;  210 ,  215 ;  310 ,  315  respectively) and between the LCE ( 130 ;  230 ;  330  respectively) and MS ( 120 ;  220 ;  320  respectively). Signaling paths are also normally required between NSS ( 150 ;  250 ;  350  respectively) and LCE ( 130 ;  230 ;  330  respectively) and between BSS ( 140 ,  145 ;  240 ,  245 ;  340 ,  345  respectively) and LCE ( 130 ;  230 ;  330  respectively) in order to instigate location, return a location estimate to the client and enable LCE ( 130 ;  230 ;  330  respectively) to obtain relevant data when needed from NSS ( 150 ;  250 ;  350  respectively) or BSS ( 140 ,  145 ;  240 ,  245 ;  340 ,  345  respectively). With an overlay architectural solution, these signaling paths are supported either by facilities external to the wireless network or by existing facilities of the wireless network (e.g. short message service) whose usage for positioning determination is invisible to the wireless network. With an integrated architectural solution, the signaling paths are supported by facilities within the wireless network that are at least in part specially created to support positioning determination. 
     With either an integrated or overlay solution, two types of architectural variant are utilized (e.g. defined in relevant standards for GSM, TDMA, CDMA IS-95). An architecture diagram  400  of one variant, an NSS solution, is shown in FIG.  4 . Here LCE  430  is attached directly to NSS  450 . With this variant, LME  410  to LCE  430  signaling (to transfer measurements from LME  410  to LCE  430 ) is supported by signaling facilities in both NSS  450  and BSS  440 . LCE  430  to MS  420  signaling to support mobile based and mobile assisted position methods is supported by signaling facilities in both BSS  445  and NSS  450 . In diagram  400 , LME  410  and MS  420  are shown as associated with different BSSs  440  and  445 , respectively. Those versed in the art will recognize that in some situations, BSSs  440  and  445  can be the same BSS. Similar merger of the different BSSs shown in the subsequently described diagrams  500 ,  600  and  700  is also possible. 
     An architecture diagram  500  of another variant, the BSS variant, is shown in FIG.  5 . Here LCE  530  is attached directly to BSSs  540 ,  545 . With this variant, LME  510  to LCE  530  signaling (to transfer measurements from LME  510  to LCE  530 ) and LCE  530  to MS  520  signaling (to support mobile based and mobile assisted position methods) are supported by signaling facilities in BSSs  540  and  545 , respectively, and not by facilities in NSS  550 . Access by NSS  550  to LCE  530  (e.g. to initiate a location procedure and receive the resulting location estimate) is supported by intermediate signaling facilities in BSSs  540  and/or  545  (e.g. to initiate location of MS  520 , NSS  550  would normally use BSS  545  since this is the BSS currently serving MS  520 ). With this variant, signaling between LCE  530  and any one of MS  520 , LME  510  or NSS  550  must pass through BSS  540  or BSS  545 . 
     With reference to FIG. 6, architecture diagram  600  illustrates the interaction of some components in a preferred embodiment of a “hybrid architecture” that provides support for location services with either an integrated or overlay location solution. At least one difference between a hybrid architecture and other architectures is that LCE  630  is connected to both NSS  650  and to BSS  640  and BSS  645 . 
     In a preferred embodiment, LME  610  to LCE  630  signaling is supported by signaling facilities located in BSS  640  without the need to go through NSS  650 , while MS  620  to LCE  630  signaling is supported by signaling facilities located in both NSS  650  and BSS  645 . Since the NSS  650  has direct access to LCE  630 , NSS  650  to LCE  630  signaling does not need facilities in BSSs  640 ,  645 . 
     In a system implementing the hybrid architecture, LME  610  to LCE  630  signaling does not rely on signaling facilities in NSS  650 . In a preferred embodiment MS  620  to LCE  630  signaling still relies on signaling facilities in BSS  645  and NSS  650 ; however the volume of such signaling will be substantially zero for network based positioning methods (where there is no MS  620  to LCE  630  signaling). With regard to MS based or MS assisted positioning support by the hybrid architecture, while there is additional signaling through NSS  650  to support MS  620  to LCE  630  signaling, in a preferred embodiment there is no extra signaling through BSS  640  or BSS  645  to enable access to LCE  630  from NSS  650 . Software and/or hardware impacts in BSS  640  or BSS  645  are therefore not needed to support NSS  650  access to LCE  630 . 
     The connectivity in the hybrid architecture shown in architecture diagram  600  suggests that communication between MS  620  and LCE  630  might be more efficiently supported by eliminating NSS  650  from the communications path and using only BSS  645 . However, in a preferred embodiment NSS plays an important role in the hybrid architecture. When NSS  650  is eliminated from the MS  620  to LCE  630  communications path, LCE  630  would have to be told by NSS  650  (e.g. when NSS  650  first sends a location request for MS  620  to LCE  630 ) that communication between LCE  630  and MS  620  should be transferred through BSS  645  and not, for example, through BSS  640  or NSS  650 . LCE  630  would also have to be provided with information from NSS  650  to identify to BSS  645  that the intended recipient of any communication sent by LCE  630  to MS  620  through BSS  645  is MS  620 . MS  620  might have to provide similar information to BSS  645  related to LCE  630  when sending communication to BSS  645  for transfer to LCE  630 . In addition, if MS  620  were to move to a new cell site controlled by another BSS (e.g. BSS  640 ), additional information might have to be provided by NSS  650  to LCE  630  to enable communication between LCE  630  and MS  620  via the new BSS. These additional requirements may complicate communication between LCE  630  and MS  620  and thus this method is not a part of the preferred embodiment hybrid architecture described previously, although it is allowed as a special variant in an alternative preferred embodiment. In the following description, the special variant hybrid architecture is not considered. 
     In a preferred embodiment, LCE  630  is an entity that is separate from both NSS  650  and BSSs  640  and  645 , though in an alternative preferred embodiment LCE  630  is a logical function supported within an existing entity in NSS  650 , BSS  640 , or BSS  645 . 
     In a preferred embodiment, LCE  630  to NSS  650  signaling is transparent to BSS  640  and BSS  645 , while LCE  630  to BSS  640  or BSS  645  signaling is transparent to NSS  650 . The signals from LCE  630  to NSS  650  either do not pass through BSS  640  or BSS  645  or do not require special support if they do pass through BSS  640  or BSS  645 . Similarly, signals from LCE  630  to BSS  640  or BSS  645  either do not pass through NSS  650  or do not require special support if they do pass through NSS  650 . For example, a physical 64 Kbit/sec channel that is routed through a circuit based switch (not shown) in BSS  640  or BSS  645  or NSS  650  would generally be transparent to that switch in terms of its usage to support a location service and thus would not count as special support. The same would be true of an X.25 connection or Signaling System 7 (SS7) Message Transfer Part (MTP) based association that was routed through a packet or message based switch in BSS  640  or BSS  645  or NSS  650  where the switch already existed and possessed the necessary X.25 or SS7 MTP switching capabilities to support services other than a location service. Signaling information that is routed through a switch in BSS  640  or BSS  645  or NSS  650  that required the switch to support any protocol or any capability not already needed for another service would not be considered transparent to the switch in the context of the description herein and would thus be considered as special support. 
     In a preferred embodiment, signals between LCE  630  and LME  610  only travel through BSS  640  and either do not travel through NSS  650  or travel transparently through NSS  650  with no special support. LCE  630  may have one or more signaling connections to each of BSS  640 ,  645 . 
     In a preferred embodiment, signals between LCE  630  and MS  620  travel through both BSS  645  and NSS  650 , while signals between LCE  630  and NSS  650  use a signaling connection between LCE  630  and NSS  650 . LCE  630  may have one or more signaling connections to NSS  650 . 
     Various embodiments of the invention are adapted for use with specific internal architectures of the NSS and BSS subsystems. For example, preferred embodiments may be used in conjunction with known Global System for Mobile Communications (GSM) systems and 3GPP (3 rd  Generation Partnership Project) WCDMA (Wideband Code Division Multiple Access) systems (which may be, for example, of the time division duplexing or frequency division duplexing variety), wherein the LCE corresponds to an entity known as a Serving Mobile Location Center (SMLC) that is resident in a GSM BSS or 3GPP WCDMA UTRAN (UMTS Terrestrial Radio Access Network). In these preferred embodiments, the LME corresponds to an entity known as a Location Measurement Unit (LMU) that may be either part of a GSM BSS or 3GPP UTRAN or a separate entity accessed via radio frequency signaling from a GSM BSS or 3GPP UTRAN in the same way as a normal MS. In currently standardized GSM and 3GPP WCDMA systems, the location service employs the BSS based variant. In order to transform these existing systems to conform to an alternative preferred embodiment of the hybrid architecture described herein, dedicated signaling connections could be added between the SMLC and certain switching entities in the NSS such as the MSC and SGSN. Such connections could be supported, for example, using dedicated physical links employing SS7, IP or X.25 based signaling between the SMLC and the MSC or SGSN. These would transport messages between the SMLC and the MSC or SGSN as well as relay messages through the MSC or SGSN between the SMLC and an MS. In a more radical transformation, the SMLC could be physically moved out of the BSS and into the NSS—e.g. by connection to an MSC and/or SGSN or integration within an MSC or SGSN. In this more radical case, additional signaling connections are implemented from the SMLC to major entities in the BSS like the GSM BSC or 3GPP WCDMA Radio Network Controller (RNC). In a preferred embodiment, these signaling connections are routed through existing entities (e.g. MSC, SGSN) and use existing transmission capabilities to allow usage to be transparent—e.g. by using 64 Kbit/sec circuit mode connections or SS7 MTP signaling. 
     With reference to FIG. 7, architecture diagram  700  illustrates the interaction of some components in a preferred embodiment of a hybrid architecture used in conjunction with a GSM system. SMLC  730  exists as an entity separate from BSSs  740 ,  745 , and is connected to MSC  750  by one or more 64 Kbit/sec signaling links. SMLC  730  is also indirectly connected to BSSs  740 ,  745 . MSC  750  acts as an MTP Signaling Transfer Point (STP) to form the connection between SMLC  730  and each of BSS  740 ,  745 , which connection is shown in FIG. 7 with dashed lines. BSSs  740 ,  745  can send location related messages to SMLC  730  by encapsulating each message inside an MTP message whose header contains the MTP signaling point code (SPC) of SMLC  730 . When such a message is transmitted by BSS  740  or BSS  745 , it is received first by MSC  750 . The SPC of SMLC  730  inside the message then tells MSC  750  that the message is intended for SMLC  730 . MSC  750  uses this information to send the message to the proper recipient, SMLC  730 . Those familiar with the art will recognize this existing capability and appreciate that a similar process can be employed by SMLC  730  to send messages to BSS  740  or BSS  745 . Since MSC  750  is acting only as an STP, it is not aware of and need not support the location-related signaling between SMLC  730  and BSS  740  or BSS  745 . 
     Signaling between SMLC  730  and MS  720 , which is compatible with GSM standards, can utilize a signal path through BSS  745  and MSC  750 . The signaling can be made transparent to BSS  745  by employing message containers between MS  720  and MSC  750  that are already supported by BSS  745 . In an alternative preferred embodiment, new message types can be created for both BSS  745  and MSC  750  that makes the SMLC  730  to MS  720  signaling visible to both MSC  750  and BSS  745 . 
     Signaling between SMLC  730  and LMU  710  utilizes a signal path through BSS  740  and MSC  750 . In a preferred embodiment, BSS  740  has been provided with dedicated MTP based signaling associations with SMLC  730  (using MSC  750  as STP) and the ensuing LMU  710  to SMLC  730  signaling is transparent to MSC  750 . Thus, LMU  710  is supported in the same fashion as if SMLC  730  were solely BSS based while MS  720  is supported in the same fashion as if SMLC  730  were solely NSS based—resulting in a hybrid approach. 
     While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.