Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/24—Reselection being triggered by specific parameters
  • H04W36/32—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/14—Reselecting a network or an air interface

Definitions

• the invention relates generally to cellular communication networks and, more particularly, to a method of triggering hard handoff of a call from a cell operating on one frequency to a cell operating on another frequency.
• the served area is divided into cells, each of which may be further divided into sectors.
• Each cell is served by a single base station transceiver subsystem (“BTS”), and each base station is connected to a mobile switching center (“MSC”) via a base station controller (“BSC”) and appropriate hardware links.
• BTS base station transceiver subsystem
• a mobile unit is connected to the MSC by establishing a radio frequency (“RF”) link with a nearby BTS.
• RF radio frequency
• TDMA time division multiple access
• AMPS advanced mobile phone services
• CDMA code division multiple access
• Each sector of a CDMA network continuously outputs its own unique pilot signal.
• a mobile unit can distinguish between the sectors by the pilot signals emitted thereby and can also measure the strength of the pilot signal by measuring the carrier-to-interference ratio ("C/I") thereof.
• the strength of the pilot will indicate whether or not the sector with which it is associated can be used by the mobile unit to establish communication.
• RF radio frequency
• BTS base station transceiver subsystem
• BSC base station controller
• MSC mobile switching center
• a new communications path between the mobile unit and the MSC comprising an RF link between the mobile unit and the second BTS and hardware links between the second BTS and the MSC, is established.
• the mobile unit is directed to end communication with the first BTS and begin communication with the second BTS.
• the first and second BTSes are connected by a single MSC, it will be recognized that the BTSes may be connected by more than one MSC connected to one another via appropriate links. Moreover, the first and second BTSes may be located in two separate cellular communication networks, such as a CDMA and an AMPS network, respectively, or in CDMA networks using different operational frequencies.
• the process of a mobile unit's terminating communication with one BTS and commencing communication with another BTS is commonly referred to as "handoff.
• the specific example of a handoff set forth above is referred to as a "hard handoff, because the link between the mobile unit and the first BTS is terminated before the link between the mobile unit and the second BTS is established.
• Hard handoff is implemented in older cellular communication technologies, such as AMPS, in which each mobile unit is assigned a different radio frequency on which to communicate.
• AMPS AMPS
• each mobile unit is assigned a different radio frequency on which to communicate.
• “soft handoff in which the second communications link is established before the first communications link is terminated, can be implemented, because the same frequency band is used for all communications within the CDMA network. Additional details regarding the specifics of the CDMA environment are described in TIA/EIA/IS-95-A, Mobile Station-Base Station compatibility
• IS-95 Dual-Mode Wideband Spread Spectrum Cellular System
• handoff comprises three steps, including “trigger,” “target selection,” and “execution.”
• the handoff trigger is the event that initiates the handoff processing.
• one type of handoff trigger occurs when a new pilot channel is received by the mobile unit with a strength greater than a threshold "T_ADD.”
• Target selection refers to the process of determining which cell or sector is the most suitable server for the call.
• the handoff trigger initiates the target selection process.
• the target selection process is built into the mobile unit and automatically occurs responsive to the handoff trigger.
• Handoff execution refers to the process in which the call is actually transferred to the target by the cellular communication network and includes activities such as hardware/software resource allocation.
• the IS-95 telecommunications standard does not detail any mechanism to accomplish CDMA hard handoff. Unlike with CDMA soft handoff, with CDMA hard handoff, the mobile unit cannot provide much information to the CDMA network to facilitate the handoff. Accordingly, special intelligence must be built into the CDMA network equipment and special deployment considerations must be observed to make such hard handoffs work reliably.
• the present invention accordingly, provides a method and system for triggering hard handoff of a call from a CDMA cell operating on a first frequency (fj) to a cell operating on a second frequency (f 2 ).
• a two stage handoff trigger hereinafter referred to as a boundary sector trigger, indirectly utilizes the existing soft handoff algorithm implemented by the mobile unit to trigger the hard handoff.
• boundary cells overlaid pairs of bi-sectored cells, hereinafter referred to as "boundary cells,” are deployed along the interfrequency boundary between the two networks.
• the sectors of these boundary cells hereinafter referred to as “boundary sectors”, are oriented normal to the interfrequency boundary.
• Each boundary cell is equipped with two BTSes, one operating on the frequency of the CDMA network and the other operating on the frequency of the other network.
• the pilots associated with the boundary sectors are identified in a database of the BSC as boundary sector pilots.
• a first stage of the boundary sector trigger of the present invention (“Stage 1") occurs when the mobile unit's active set, which comprises the set of pilots associated with sectors with which the mobile unit is in communication, consists only of pilots marked as boundary pilots in the database.
• An active set comprising only boundary pilots is referred to as a "boundary active set.”
• the RTD measurement serves as a rough estimate of the distance between the mobile unit and the nearest BTS.
• the trigger is complete and handoff processing continues with the target selection process and handoff execution in the usual manner. It should be noted that if at anytime after Stage 2 has been entered and before handoff is initiated, the requirements for Stage 1 cease to be met (i.e., if the active set of the mobile unit ceases to be a boundary active set), the boundary sector trigger is disabled.
• the boundary sector trigger is used to trigger a hard handoff of a call from a cell in a first CDMA network operating on a first frequency to a cell in a second CDMA network operating on a second frequency.
• the boundary sector trigger is used to trigger a hard handoff of a call from a cell in a CDMA network to a cell in a non-CDMA network.
• a technical advantage of the invention is that it enables a call reliably to be handed off from a CDMA network operating on a first frequency to a cellular communication network operating on a second frequency.
• Another technical advantage of the invention is that it enables a call reliably to be handed off from a multi-carrier region of cellular communication network coverage to a single-carrier region of cellular communication network coverage.
• Yet another technical advantage of the invention is that, because the handoff takes place at a point at which both forward and reverse RF links are very robust, i.e., close to the cell site, the reliability and voice quality of the call are very high.
• Fig. 1 is a system block diagram of an interfrequency boundary region between two cellular communication systems operating on different frequencies for illustrating the operation of the boundary sector trigger of the present invention.
• Fig. 2 is a flowchart illustrating the operation of the boundary sector trigger of the present invention.
• Fig. 3 illustrates use of the boundary sector trigger of the present invention to handoff a call from a multi-carrier region to a single-carrier region.
• Description of the Preferred Embodiment Fig. 1 illustrates an interfrequency boundary region between a first cellular communication network 10a operating on a first frequency ("f ") and a second cellular communication network 10b operating on a second frequency ("f 2 ") different than that of the first cellular communication network 10a.
• the first cellular communication network 10a comprises a plurality of cells, only three of which, Cl, C2, and 03, are shown for the sake of clarity.
• Each cell Cl, 02, and 03 is served by a respective base station transceiver subsystem operating on f x ("BTS1") located at a cell site thereof; in particular, the cell Cl is served by a BTS1 12, the cell C2 is served by a BTSl 14 and the cell 03 is served by a BTSl 16.
• BTSl 12, 14, 16 is connected to a first base station controller ("BSC1"), designated in Fig. 1 by a reference numeral 18, via a hardware link 19a, 19b, 19c, respectively.
• the BSC1 18 is connected to a first mobile switching center (“MSC1"), designated in Fig. 1 by a reference numeral 20, via a hardware link.
• the BSCl 18 includes a database 21 for purposes that will be described below.
• the second cellular communication network 10b comprises a plurality of cells, only three of which, C4, 05, and 06, are shown for the sake of clarity, and two of which, specifically, cells 05 and 06, form overlaid pairs of cells with cells 02 and 03, respectively, of the first cellular communication network 10a for purposes to be described in detail below.
• Each cell 04, 05, and 06 is served by a respective base station transceiver subsystem operating on f 2 ("BTS2") located at the cell site thereof; in particular, the cell C4 is served by a BTS2 22, the cell C5 is served by a BTS2 24 and the cell 06 is served by a BTS2 26.
• BTS2 base station transceiver subsystem operating on f 2
• Each BTS2 22, 24, 26, is connected to a second BSC ("BSC2"), designated in Fig. 1 by a reference numeral 28, via a hardware link 29a, 29b, 29c, respectively.
• BSC2 is connected to a second MSC ("MSC2"), designated in Fig. 1 by a reference numeral 30, via a hardware link.
• MSC2 MSC
• the BSC2 28 includes a database 31 for purposes that will be described below.
• each of the cells 02, 03, 05, and 06 are designated as "boundary cells," due to the fact that they lie on the boundary between the cellular communication networks 10a and 10b.
• the cells 02, 03, C5, and 06 are bi-sectored cells oriented normal to an interfrequency boundary 34 between the cellular communication networks 10a and 10b such that the boundary between the each pair of sectors of the boundary cells 02, 03, 05, and C6 lies substantially along the same line as the interfrequency boundary 34.
• the cell C2 comprises sectors 401 and 40r
• the cell 03 comprises sectors 421 and 42r
• the cell 05 comprises sectors 441 and 44r
• the cell 06 comprises sectors 461 and 46r.
• the pilots of the sectors 40r and 42r are identified in the database 21 as boundary sector pilots, while the pilots of the sectors 441 and 461 are identified in the database 31 as boundary sector pilots.
• the non-boundary cells C l and 04 are shown as being unidirectional cells, although use of any other type of cell configuration (e.g., three- or six-sector) is supported.
• a mobile unit 50 initially located within the cell Cl and in communication with the BTSl 12 moves in a direction indicated by an arrow A, it enters soft handoff with the cell Cl and the sector 42r of the cell C3, meaning that the cell C 1 (a non-boundary cell) and the sector 42r (a boundary sector) are in the active set of the mobile unit 50; hence, the active set is not a boundary active set.
• the mobile unit 50 continues to travel in the direction indicated by the arrow A, it eventually reaches a point at which it is in communication only with the sector 42r, and/or any number of other boundary sectors. Accordingly, the active set of the mobile unit 50 is now a boundary
• the RTD of communications between the mobile unit 50 and the nearest BTS, in this case, the BTSl 16 is monitored.
• the RTD exceeds a predetermined threshold, the condition for Stage 2 of the boundary sector handoff trigger is met and target selection processing begins.
• the cell deployment strategy is such that the target selection process will result in the selection of the sector 46r, served by the BTS2 26.
• a target i.e., the sector 46r
• the BSC1 18 then initiates hard handoff from the sector 42r in the f j coverage region to the sector 46r in the f 2 coverage region.
• the mobile unit 50 continues traveling in the direction indicated by the arrow A and enters soft handoff with the cell C4 on f 2 .
• the second cellular communication network 10b may be either a CDMA or a non-CDMA.
• step 200 a determination is made whether the active set of the mobile unit 50 on the active traffic channel is a boundary active set. If not, execution returns to step 200; otherwise, execution proceeds to step 202.
• step 202 the RTD of communications between the mobile unit 50 and the BTS to which it is closest is measured.
• step 204 a determination is made whether the RTD measured in step 202 is greater than a predetermined threshold value.
• Fig. 1 illustrates a situation in which a call is handed off from one single- carrier region (the first cellular communication 10a) to another single-carrier region (the second cellular communication network 10b). It will be recognized, however, that there may be situations in which a call will need to be handed off between a multi-carrier region and a single-carrier region or to an AMPS network. For example, as illustrated in Fig. 3, in areas of high traffic, it may be beneficial to deploy a second CDMA network operating on a second frequency (f 2 ) over a first CDMA network operating on a first frequency (f ). As shown in Fig.
• a coverage map of the region would show single carrier (f ⁇ ) coverage by the first CDMA network, as represented in Fig. 3 by single-carrier cells CIO, CH, and C12, and a core of dual-carrier coverage (fj and f 2 ) by the combination of the first and second CDMA networks, as represented by dual- carrier cells C13, C14, C15, and C16.
• each of the single-carrier cells CIO, CH, and C12 is served by a single BTSl
• each of the dual-carrier cells C13, C14, 015, and C16 is served by two BTSes, including a BTSl and a BTS2.
• the dual-carrier cells C13, 014, C15, and 016 may be served by a single BTS with multiple carriers.
• the dual-carrier cells bordering single-carrier cells in this case, dual-carrier cells 013, 015, and 016, are designated as boundary cells.
• the boundary cells C13, 015, and C16 are bi-sectored cells oriented normal to the single-carrier/multi-carrier boundary, although it will be recognized that the cells 013, 015, and 016 may be omni-, three-, or six-sectored cells as needed.
• the boundary sector trigger operates in the same manner as described above, with dual-carrier cells 013, 015, and 016 being designated as boundary cells, as mentioned above. It will be recognized that in this alternative embodiment that no additional hardware is required, as each of the boundary cells 013, 015, and C16 already includes appropriate hardware for serving both frequencies, i.e., a BTSl and a BTS2.

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• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)

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• 1998
  • 1998-05-05 EP EP98920236A patent/EP0983694A4/de not_active Withdrawn
  • 1998-05-05 AU AU72855/98A patent/AU735575B2/en not_active Ceased
  • 1998-05-05 WO PCT/US1998/009132 patent/WO1998053620A1/en not_active Ceased
  • 1998-05-05 CN CN98805293.8A patent/CN1265253A/zh active Pending
  • 1998-05-05 CA CA002290517A patent/CA2290517A1/en not_active Abandoned

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