Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR INTER-SYSTEM
HANDOFF WITHIN A PLURAL HYPERBAND SUPPORTING
CELLULAR TELEPHONE NETVVORK
5 BACKGROUND OF THE INVENTION
Technical Field of the Invention
The present invention relates to cellular telephone networks and, in particular,to inter-system handoff of mobile stations in connection with a plural hyperbandsupporting cellular telephone network.
0 Description of Related Art
North American cellular communications have historically been implemented
solely in the 800 MHZ Cellular hyperband. The Cellular hyperband is assigned twofrequency bands (commonly l~Çelled to as the A frequency band and the B frequency
band) for carrying and controlling communications. The most recent evolution in
cellular cl mm.~nications services involves the adoption of three additional hyperbands
for use in h~ndling mobile and personal communications. Of these additional
hyperbands, only the Personal Communication Services (PCS) hyperband in the 1900MHZ frequency range has been completely defined. The PCS hyperband is specified
to include six dirre-e~-L frequency bands (A, B, C, D, E and F).
2 o Each one of frequency bands specified for the Cellular and PCS hyperbands is
allocated a plurality of voice or speech (traffic) channels, as well as access or control
channel(s). The control ch~nn~lc are used to control or supervise the operation of
mobile stations by means of information transmitted to and received from the mobile
stations. Such information may include incoming call signals, outgoing call signals,
page signals, page response signals, location registration signals, voice channel
~c~i~...,.~..l.~, m~int.,n~nce instructions, and cell selection or reselection instructions as
a mobile station travels out of the radio coverage of one cell and into the radio
coverage of another cell. The traffic channels are used to carry subscriber telephonic
communications as well as messages requesting mobile station ~si~t~nce in making3 o hand-off evaluations and controlling the hand-off operation. The control and traffic
r.h~nnr.l~ may operate in either an analog mode, a digital mode, or a combination mode.
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As the cellular telephone network has been upgraded to accommodate the
newly made available hyperbands, inst~nc~s have arisen where one cellular system area
does not support the same hyperbands as its adjacent cellular system area. Thus, for
ex~mp'e, a first cellular system area may support cellular operations in both the Cellular
5 and PCS hyperbands, while an adjacent, second cellular system area incon~ictently
supports cellular operations only in the Cellular hyperband. This presents a
communications problem when mobile stations roam between the two service areas,
as it may become necessary for a mobile station to switch between the hyperbands as
well as switch traffic çh~nnçl~ at the point of hand-off Furthermore, of perhaps even
10 more concern with respect to communications is that the included verification process,
wherein signal strength measurements are made from the target cell on the traffic
channel currently being used by the mobile station, may be improperly pe~ 1~1 Illed or the
results may be erroneous if the service area does not support the hyperband within
which the mobile station is currently operating. Accordingly, there is a need then for
15 a system and method for supporting inter-system hand-off, and in particular the
included verification process, wherein the system and method accounts for differences
and inconsistencies between system supported hyperbands.
SUMMARY OF THE INVENTION
In a cellular telephone network inrlllrline plural system areas having inconsistent
2 o hyperband support, a message sent from a first one of the system areas to a second one
ofthe system areas requ~stine the making of a verification signal strength measurement
includes not only an identification of the traffic channel currently being used by a
mobile station, but also an identification of the hyperband within which that traffic
channel exists. Responsive thereto, a measurement is made, if possible, from the2 5 second system area in the identified hyperband on the traffic channel currently being
used by the mobile station. A report on the results of the requested verification
measurement is then sent from the second system area to the first system area for
further processing. This report preferably includes an indication of the hyperband
within which the verification measurement, if any, was made. From the report, the first
3 o system area can confirm the hyperband capabilities of the second system area, and thus
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deterrnine whether the second system area would support mobile station operation in
the event of an inter-system hand-off.
In co~ ;on with the hand-offofthe mobile station from the first system area
to the second system area, the first system area requests from the second system area
the ~c!~;g~ of a traffic channel. Responsive to that request, the second system area
selects a traffic channel within a supported hyperband and reports on the selected
ch~nnel, as well as the hyperband within which that selected trafflc channel exists, to
the first system area. From that report, the first system area can confirm the hyperband
capabilities of the second system area, and thus determine whether the second system
0 area would support mobile station operation in the event of an inter-system hand-off
BRIEF DESCR~PTION OF THE DRAWINGS
A more complete unders~n~in~ of the method and a~palal~ls of the present
invention may be acquired by reference to the following Detailed Description when
taken in conjunction with the accompanying Drawings wherein:
FIGURE 1 is a cell diagram illustrating an exemplary cell configuration for a
plural hyperband sul)po~ ~;..g cellular telephone network in which the present invention
may be implem~nterl;
FIGURES 2A-2B are signal flow and network operation diagrams illustrating
operation of the network of FIGIJRE 1 in connection with a verification-type hand-off
2 0 of a mobile station from a cell within a first system area to a cell within a second system
area; and
FIGURES 3A-3B are signal flow and network operation diagrams illustrating
operation of the network of FIGURE 1 in connection with a blind-type hand-off of a
mobile station from a cell within a first system area to a cell within a second system
2 5 area.
DETAILED DESCRIPTION OF THE DRAWINGS
Reference is now made to FIGURE I wherein there is shown a cell diagram
illustrating an exemplary cell configuration for a plural hyperband supporting cellular
telephone network l0 in which the present invention may be implemented. The cellular
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telephone network 10 operates in accordance with one of a number of known air
interface types in~ ng~ for example, a digital time division multiple access (TDMA)
protocol. In a digital TDMA cellular telephone network, for example, each cell 12
operates with an ~csigned set of tr~n~mi~eion frequencies selected from one or more
5 of the available hyperbands. The set of frequencies assigned to each cell 12 includes
frequencies supporting both at least one control channel and a plurality of traffic
channels, with the control and traffic channels operable in either or both an analog
and/or a digital mode. Sets of ~signed frequencies are di~lenl for adjacent cells 12,
and such sets are not repeated for use by other cells except for those cells that are far
0 enough away from each other to minimi7e the likelihood of adjacent or co-channel
interference.
In the network 10, a base station 14 is provided for each of the cells 12. The
base stations 14 engage in sim~llt~neous comm-~niC~tions with plural mobile stations 16
operating roughly within the area of the associated cell 12. The control channel15 a~signPd to each cell 12 is used to carry system control signals between the base station
14 and proximately located mobile stations 16, and also to assist in the network with
mobile station cell reselection. Such control signals include call originations, page
signals, page ~ onse signals, location l~g,~Ll~lion signals, traffic channel ~ nmPnts,
m~int~n~nce instructions, and cell selection or re-selection instructions. The traffic
20 channels provided in each cell 12 are used to carry subscriber voice or data
communications between the base station 14 and proximately located mobile stations
16 and also to assist in the hand-offoperation.
The base stations 14 are illustrated as being positioned at or near the center of
each ofthe cells 12. However, depending on geography and other known factors, the
2 5 base stations 14 may instead be located at or near the periphery of, or otherwise away
from the centers of, each of the cells 12. In such instances, the base stations 14 may
broadcast and communicate with mobile stations 16 located within the cells 12 using
directional rather than omni-directional ~nt~nn~c. Each one of the base stations 14
in~ dec a ~ Pr, a receiver, and a base station controller (none shown) connected
3 0 to an antenna (also not shown) in a manner and with a configuration well known in the
art.
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The base stations 14 further communicate via sign~lin~ links and voice trunks
22 with a central control station, co.. ~ ly referred to as a mobile switching center 18,
which functions to control operation of the network 10. The mobile switching centers
18 are interconnected with each other and to the public switched telephone networl~
(PSTN) 20 by ~ign~ling links and voice trunks 24. The mobile switching centers 18
operate to selectively connect subscriber voice and data communications to the mobile
stations 16 through its base stations 14. Thus, the mobile switching center 18 controls
system operation through and in response to the tr~n~mi~sion of control signals over
the control cl,~; ~n~lc to set-up on the traffic ch~nnels calls that are either ori~in~ted by
or termin~ted at the mobile stations 16. The mobile switching center 18 further
controls, through and in response to control and traffic channel tr~nemi~sions, the
handoffofa subscriber comm~.nir~tion from a traffic channel of one cell 12 to a traffic
channel of another cell as the subscriber mobile station 16 roams throughout thecellular service area during an ongoing communication.
It is common within one overall cellular service area to have a plurality of
system areas 32 (differenti~ted from each other by the fact that they have different
system identifications (SIDs) and perhaps have different service providers). A
boundary 34, passing between cells 12 along the border between two system areas 32,
is shown in bold in FIGURE 1 to delimit the physical extent of each of the illustrated
2 o system areas. In this illustrated example, it will be noted that the cells 12 within two
system areas 32 are served by dirrelellt mobile switching centers 18.
North American cellular communications have historically been implemented
solely in the 800 MHZ Cellular hyperband. The most recent evolution in cellular
comml.- tion~ services involves the adoption of three additional hyperbands for use
2 5 in h~n~ling mobile and personal cnmm--nications. Of these additional hyperbands, only
the Personal Communication Services (PCS) hyperband in the 1900 MHZ frequency
range has been completely defined. In the network 10 illustrated in FIGVRE 1, mobile
switching center 18(1) for system area 32(1) supports mobile station 16 operation in
both the Cellular hyperband and the PCS hyperband. The mobile switching center
3 C 18(2) for system area 32(2), however, supports mobile station 16 operation only in the
-r
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Cellular hyperband. Thus, there exists in the network 10 inconsistent hyperband
support between the plurality of system areas 32.
There are a number of mobile stations 16 shown operating within the service
area of the network 10. These mobile stations 16 may comprise Cellular hyperbandonly capable mobile stations, PCS hyperband only capable mobile stations, or dual
frequency (Cellular and PCS hyperband) capable mobile stations. With respect to the
Cellular hyperband only capable mobile stations, they may operate in an analog mode
only or in a dual (analog and digital) mode. With respect to the PCS hyperband only
capable mobile stations, they operate in a digital mode only. Finally, with respect to
o the dual frequency (Cellular and PCS hyperband) capable mobile stations, they may
operate in a digital mode only or in a dual (analog and digital) mode. It will, of course,
be understood that Cellular hyperband only capable mobile stations may operate within
the cells 12 of both system areas 32(1) and 32(2) of FIGURE 1 using any of the traffic
or control channel frequçnci~s ofthe Cellular hyperband. PCS hyperband only capable
mobile stations, however, may operate within the cells 12 of system area 32(1) using
any of the traffc or control channel frequencies of PCS hyperband. Dual frequency
(Cellular and PCS hyperband) capable mobile stations may operate within the cells 12
of both system areas 32(1) and 32(2), and use traffic or control channel frequencies of
both the Cellular and PCS hyperbands while in system area 32(1), but may only use
2 o traffic or control channel frequencies of the Cellular hyperband while within system
area 32(2).
As the mobile stations 16 move within the service area of the network 10, there
will be ;..~ çs where a mobile station will pass between two cells 12 within a single
system area 32, or from one cell in a first system area 32(1) to another cell in a second
system area 32(2). In moving between the cells 12, the mobile stations 16, in
conjunction with base station 14 information and orders exchanged with and between
the mobile switching centers 18, have an opportunity through hand-offto change the
base station through which cellular radio communications are being effectl.~ted For
example, mobile station 16(1) is shown moving in the direction of arrow 26 from
system area 32(1) into system area 32(2). Because the network 10 supports pluralhyperbands and in~l~ldçs single and multiple hyperband supporting system areas 32, by
.
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moving between cells and cl~ gil~g base stations as a result thereof, the mobile stations
] 6 may not only change the traffic or control channel frequency used for
communication, but may further change the hyperband over which communications are
being effectll~ted. It is important in such instances of inter-system hand-off that the
5 mobile switching centers 18 be informed not only of the hyperband within whichcommunications are being effected in the currently serving cell, but also of thehyperband capabilities for communication within the target cell.
Reference is now made in combination to FIGU~ES 1 and 2A-2B wherein
FIGU3~ES 2A-2B are signal flow and network operation diagram illustrating operation
0 of the network I 0 of FIGURE 1 in connection with a verification-type hand-off of a
mobile station 16(1) from a cell 12(1) within a first system area 32(1) to a cell 12(2)
within a second system area 32(2). In this instance, the mobile switching center 18(1)
for system area 32(1) supports mobile station 16 operation in both the Cellular
hyperband and the PCS hyperband, while the mobile switching center 18(2) for system
area 32(2) supports mobile station 16 operation in the Cellular hyperband only. The
mobile station 16(1), operating if capable in accordance with known mobile assisted
hand-off (MAHO) principles, periodically makes downlink signal strength
measul t~lllt;ll~S 100 on the traffic channel (of cell 12(1)) that is currently being used, and
also periodically makes downlink signal strength measurements 102 on the control2 o channels of network identified cells 12, including cell 12(2), which neighbor the cell
12(1). These signal strength measurements are reported 104 to the base station 14(1)
for the currently serving cell 12(1). The base station 14(1) concurrently makes uplink
signal strength measuleme..ls 106 on the traffic channel that is currently being used by
the mobile station 16(1).
2 5 The base station 14(1) processes the mobile station 16(1) reported 104
downlink signal strength measu~ (100 and 102), if available, and the base station
made uplink signal strength measurements (106) to determine first whether a hand-off
is necec~ry (action 108) and second, if yes, to which cell or cells the hand-offcould
and/or should preferably occur (action 110). In this example, it is assumed that the
3 o base station 14(1) determines 108 from deteriorating measured uplink and/or downlink
signal strengths that a hand-offis necessaly. It is further assumed that an identification
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110 is made of cell 12(2) in system area 32(2) as the prerel- ed target cell for hand-off.
A request 112 for hand-off inclu.ling information comprising an identification of the
currently serving cell 12(1), the traffic channel being used for communication with
mobile station 16(1) in cell 12(1), the hyperband within which that traffic channel
exists, the time s}ot (for a digital traffic channel) carrying the cellular communication,
and the target cell 12(2) for hand-off, is then sent by the base station 14(1) to the
serving mobile switching center 18(1). A hand-offif approved in this instance would
comprise an inter-system hand-offbecause the target cell 12(2) is served by a mobile
switching center 18(2) (serving system area 32(2)) di~elel.~ from the mobile swieching
o center 18(1) serving the current cell 12(1) (serving system area 32(1)). The foregoing
description is only an example of the procedure used in making the detel lnillaLion to
in~titllte a hand-off.
The currently serving mobile switching center 18(1) then signals 114 the mobile
switching center 18(2) associated with the target cell 12(2) requesting verification of
base station 14(2) comm~lnic~tions capability with the mobile station 16(1). The signal
114, like the request 112 sent by the base station 14(1), includes information
COIII~ ing an id~ntifi~ ~tion ofthe currently serving cell 12(1), the traffic channel being
used for communication with mobile station 16(1) in cell 12(1), the hyperband within
which that traffic channel exists, the time slot (for a digital traffic channel) carrying the
cellular communication, the target cell 12(2) for hand-off, and the hyperband
capabilities of the mobile station. Responsive thereto, the mobile switching center
18(2) signals 116 the base station 14(2) for the target cell 12(2) to make a verifying
signal strength measurement (action 118) on the traffic channel currently being used
by the mobile station 16(1) in the currently serving cell 12(1). This measurement
2 5 should take into account the hyperband within which that traffic channel exists, and
perhaps, if l~ecess~y, the time slot (for a digital traffic channel) carrying the cellular
communication.
The base station 14(2) then reports 120 the results of the verification signal
strength measurement to the mobile switching center 18(2), which then forwards 122
the results back to the mobile switching center 18(1). This report 120 not only
identifies whether the verification signal strength measurement was successful, but also
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identifies the particular hyperband within which the verification measurement, if any,
was made, along with an identification of the hyperband capabilities of the cell. The
results are then processed (action 124) by the mobile switching center 18(]) to
determine whether a hand-off to target cell 12(2) should be made. This determination
takes into account not only the success and strength of the verification measurement,
but also the hyperband capabilities of both the mobile station 16(1) and the target cell
12(2). If the deterrnin~tion is ~ a~ e, the mobile switching center 18(1) signals 126
the mobile switching center 18(2) requesting assignment (and reservation) of a traffic
channel (and time slot therein for a digital traffic channel) for hand-off to the target cell
0 12(2). Both the base station 14(2) and mobile switching center 18(1) are then informed
128 ofthe ~c~ignment by the mobile switching center 18(2) ofthe traffic channel (and
hyperband within which that traffic channel exists) in the target cell 12(2). The mobile
switching center 18(1) then confirms (action 130) the propriety ofthe assigned traffic
channel and hyperband in view of mobile station 16(1) communications capabilities,
and signals 132 the mobile station 16(1) via the base station 14(1) for the currently
serving cell 12(1) with a handover command directing the mobile station to switch to
the ~igned traffic channel (and time slot therein if âppl Opl iate) in the target cell 12(2) .
The mobile station 16(1) then tunes to and ~cc~s~es 134 the assigned traffic channel (in
the proper time slot). When the base station 14(2) detects the mobile station access
2 o (action 136), the mobile switching centers 18(1) and 18(2) are informed 138, and the
call is switched 140 to the mobile switching center 18(2) for further handling to
complete the hand-off procedure.
The operation ofthe network 10 in connection with the verification-type hand-
off procedure illustrated in FIGURE 2 may be better understood by reference to the
following examples.
In a first example, the mobile station 16(1) comprises a PCS hyperband only
capable mobile station which operates only in digital mode, and is currently utili7.ing
digital traffic channel number 1 of cell 12(1) in the PCS hyperband. As the mobile
station 16(1) moves, the base station 14(1) processes available signal strength
measurements (100, 102 and/or 106) and determines that a hand-offis necessary 108.
The base station 14(1) further determines that cell 12(1), among perhaps other
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-10--
neighboring cells 12, co.l.p.ises a c~ntlid~te target cell for that hand-of~. The base
station 14(1) then requests 112 a hand-offfrom the mobile switching center 18(1).
This request inf.hldçs an identification of not only traffic channel number 1, but also
that traffic channel number 1 is within the PCS hyperband. This is important because
the Cellular hyperband may also include a traffic channel number 1, and absent an
identification of the hyperband, the traffic channel number identification provided may
be ambiguous with respect to a system area 32 supporting plural hyperbands.
The mobile switching center 18(1) then requests (with signals 114 and 116) that
base station 14(2) for the target cell 12(2) perform a verifying signal strengtho measurement (action 118) within the PCS hyperband on the traffic channel number 1
currently being used by the mobile station 16(1). Because the service area 32(2)supports cellular operations in the Cellular hyperband only, the base station 14(2)
cannot make the requested verification signal strength measurement in the PCS
hyperband. Accordingly, the base station 14(2) may report 120 and 122 that it cannot
make the requested verification signal strength measurement. Alternatively, the base
station 14(2) may mistakenly make the verification signal sllen~ measurement on
traffic channel number l of the Cellular hyperband, and the report 120 and 122 will
include an indication that the measurement was made (albeit improperly) in the Cellular
hyperband. In either case, the processing 124 ofthe report confirms the Cellular only
20 hyperband capabilities oftarget cell 12(2), and determines that a hand-offto target cell
12(2) is not possible with respect to PCS hyperband mobile station 16(1). That cell is
then discarded as a viable target cell for hand-off. Accordingly, for PCS hyperband
only capable mobile station 16(1)~ no attempt is made to hand-offto a cell in a service
area 32(2) that supports only Cellular hyperband operations.
2 5 In a second example, the mobile station 16(1) comprises a Cellular hyperband
only capable mobile station which operates in either an analog only mode or a dual
analog/digital mode, and is currently utilizing traffic channel number 2 in the Cellular
hyperband. As the mobile station 16(1) moves, the base station 14(1) processes
available signal strength measurements (100~ 102 and/or 106) and determines that a
3 0 hand-off is neces~ry 108. The base station 14(1) further determines that cell 12(1),
among perhaps other neighboring cells 12, comprises a candidate target cell for that
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hand-off. The base station 14(1) then requests 112 a hand-off from the mobile
switching center 18(1). This request includes an identification of not only traffic
channel number 2, but also that traffic channel number 2 iS within the Cellular
hyperband. This is important because the PCS hyperband may also include a traffic
5 channel number 2, and absent an identification of the hyperband, the traffic channel
idçntifiç~tion provided may be ambiguous with respect to a system area 32 supporting
plural hyperbands.
The mobile switching center 18(1) then requests (with signals 114 and 116) that
base station 14(2) for the target cell 12(2) perform a verifying signal strengtho measurement (action 118) within the Cellular hyperband on the traffic channel number
2 currently being used by the mobile station 16(1). Because the service area 32(2)
supports cellular operations in the Cellular hyperband only, the base station 14(2)
should be capable of making the requested verification signal strength measurement in
the Cellular hyperband. The measurement may then be made but be ~lncl~cces~fill,15 it~ ;"g iml~ffici~nt signal strength for mobile station 16(1) communication with base
station 14(2). The measurement may also fail to be made. perhaps because the base
station 14(2) iS not functioning properly Alternatively, the measurement may be made
and be sllcces~fi~l, indicating sufficient signal strength for mobile station 16(1)
commlmic~tiQn with base station 14(2). In any of the foregoing cases, the report 120
and 122 from the base station 14(2) includes an indication conr.. ,l-g that a Cellular
hyperband measurement was to be made.
If the measurement is made and is successful, or if the measurement fails to be
made, the processing 124 confirms the Cellular only hyperband capabilities of target
cell 12(2), and may then result in the selection and approval of the target cell 12(2) for
hand-of~: The mobile switching center 18(2) iS then cign~led 126 to request ~ignment
(and reservation) of a traffic channel for hand-off to the target cell 12(2). In order to
maximize the chances of a succç.s~ful hand-off~ the assigned traffic channel typically
comprises an analog traffic channel within the Cellular hyperband. Following
confirmation 130 ofthe selected channel. a handover command 132 is broadcast to the
mobile station 16(1) via the base station 14(1). The ~e~igned traffic channel is then
accessed 134 by the mobile station 16(1), and the call is switched 140 to the mobile
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switching center 18(2) for further h~ntlline to complete the hand-off procedure.Alternatively, if the measurement is made but is unsuccessful (indicating insufficient
signal strength), the processing 124 again confirms the Cellular only hyperband
capabilities oftarget cell 12(2), and d~te~ nes that a hand-off is not possible. That cell
5 12(2) is then discarded as a viable target cell for hand-off, and no attempt is made to
hand-offto that cell.
In a third example, the mobile station 16(1) comprises a dual frequency
(Cellular and PCS hyperband) capable mobile station which operates in either a digital
only mode or a dual analog/digital mode, and is currently utili7:ine traffic channel
0 number 3 in the Cellular hyperband. As the mobile station 16(1) moves, the base
station 14(1) processes available signal strength measurements (100, 102 and/or 106)
and determines that a hand-off is necess~ry 108. The base station 14(1) further
determines that cell 12(1), among perhaps other neighboring cells 12, comprises a
candidate target cell for that hand-off. The base station 14(1) then requests 112 a
15 hand-off from the mobile switching center 18(1). This request includes an
id~ntifil~tion of not only trafiic channel number 3, but also that traffic channel number
3 is within the Cellular hyperband. This is important because the PCS hyperband may
also include a traffic channel number 3, and absent an identification of the hyperband,
the traffic channel identification provided may be ambiguous with respect to a system
2 o area 32 supporting plural hyperbands.
The mobile switching center 18(1) then requests (with signals 114 and 116) that
base station 14(2) for the target cell 12(2) pelrol.,l a verifying signai strength
measurement (action 118) within the Cellular hyperband on the traffic channel number
3 currently being used by the mobile station 16(1). Because the service area 32(2)
25 supports cellular operations in the Cellular hyperband only, the base station 14(2)
should be capable of making the reqllested verification signal strength measurement in
the Cellular hyperband. The measurement may then be made but be unsuccessfi~l,
in-lir.~tine in~llffi~ie.nt signal strength for mobile station 16(1) communication with base
station 14(2). The measurement may also fail to be made, perhaps because the base
3 0 station 14(2) is not functioning properly. Alternatively, the measurement may be made
and be succes~fill~ indicating sufficient signal strength for mobile station 16(1)
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comm~lniç~tion with base station 14(2). In any ofthe foregoing cases, the report 120
and 122 from the base station 14(2) includes an indication co~lr~lllling that a Cellular
hyperband measurement was to be made.
If the measurement is made and is succes.~ful, or if the measurement fails to bemade, the proces~ing 124 confirms the Cellular only hyperband capabilities of target
cell 12(2) and may then result in the selection and approval of the target cell 12(2) for
hand-of~ The mobile switching center 18(2) is then signaled 126 to request a.csi~nmçnt
(and reservation) of a traffic channel for hand-off to the target cell 12(2). In order to
maximize the chances of a s~lccessful hand-off, the ~c~igned traffic channel typically
0 comprises an analog traffic channel in the Cellular hyperband. No change in the
hyperband being used by the mobile station 16(1) is necessary. Following conr~""~tion
130 ofthe selected channel, a handover command 132 is then broadcast to the mobile
station 16(1) via the base station 14(1). The assigned traffic channel is then acce.csed
134 by the mobile station 16(1), and the call is switched 140 to the mobile switching
center 18(2) for further h~ndling to complete the hand-off procedure. Alternatively,
if the measurement is made but is unsuccessful (indicating insufficient signal strength),
the processing 124 again COIlr~ S the Cellular only hyperband capabilities of target cell
12(2), and determines that a hand-o~is not possible. That cell 12(2) is then discarded
as a viable target cell for hand-off, and no attempt is made to hand-offto that cell.
In a fourth example, the mobile station 16(1) comprises a dual frequency
(Cellular and PCS hyperband) capable mobile station which operates in either an analog
only mode or a dual analog/digital mode~ and is currently utili7.ing traffic channel
number 4 in the PCS hyperband. As the mobile station 16(1) moves, the base station
14(1) processes available signal strength measurements (100, 102 and/or 106) anddetermines that a hand-offis n~cç~ y 108. The base station 14(1) further determines
that cell 12(1), among perhaps other neigl-bo~"~g cells 12, comprises a candidate target
cell for that hand-off. The base station 14(1) then requests 112 a hand-offfrom the
mobile switching center 18(1). This request in~illdP,s an identification of not only traffic
channel number 4, but also that traffic channel number 4 is within the PCS hyperband.
3 o This is important because the Cellular hyperband may also include a traffic channel
-T . ~.
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-14-
number 4, and absent an identification of the hyperband, the traffic channel
identification provided may be ambiguous.
The mobile switching center 18(1) then requests (with signals 114 and 116) that
base station 14(2) for the target cell 12(2) perform a verifying signal strength5 measurement (action 118) within the PCS hyperband on the traffic channel number 4
currently being used by the mobile station 16(1). Because the service area 32(2)supports cellular operations in the Cellular hyperband only, the base station 14(2)
cannot make the requested verification signal strength measurement in the PCS
hyperband. Accordingly, the base station 14(2) may report 120 and 122 that it cannot
o make the requested verification signal strength measurement. Alternatively, the base
station 14(2) may mistakenly make the verification signal strength measurement on
traffic channel number 4 of the Cellular hyperband, and the report 120 and 122 will
include an indication that the measurement was made (albeit improperly) in the Cellular
hyperband. In either case, the processing 124 of the report confirms the Cellular only
5 hyperband capabilities oftarget cell 12(2), and accordingly may result in a discarding
of cell 12(2) as a viable target cell for hand-off
Alternatively, the cell 12(2) may be retained as the best available option for
hand-offin spite ofthe inability to make a verification signal strength measurement on
traffic channel number 4 of the PCS hyperband. This is because the mobile station
20 16(1) is dual frequency (Cellular and PCS hyperband) capable. Through processing
124, the target cell 12(2) is then selected and approved for hand-off. The signal 126
is then sent to mobile switching center 18(2) to request ~.cignment (and reservation)
of a traffic channel for hand-offto the target cell 12(2). In order to maximize the
chances of a successful hand-off, the assigned traffic channel typically comprises an
2 5 analog traffic channel in the Cellular hyperband. Following confirmation 130 of the
s~lected channel, a handover CO~ n(l 132 is broadcast to the mobile station 16(1) via
the base station 14(1). A hyperband change from PCS to Cellular is effect~l~ted. and
the assigned traffic channel is ~ccç~sed 134 by the mobile station 16(1). The call is then
switched 140 to the mobile switching center 18(2) for further handling to complete the
3 0 hand-off procedure.
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Reference is now made in combination to FIGURES I and 3A-3B wherein
FIGURES 3A-3B and signal flow and network operation diagrams illustrating
operation of the network of FIGI~RE I in connection with a blind-type hand-off of a
mobile station 16(1) from a cell 12(1) within a first system area 32(1) to a cell 12(2)
within a second system area 32(2). The blind-type hand-off differs from the
verification-type hand-off of FIGURES 2A-2B, as will be seen, primarily with respect
to the point in time at which the verification signal strength measurement is made.
The mobile switching center 18(1) for system area 32(1) supports mobile
station 16 operation in both the Cellular hyperband and the PCS hyperband, while the
0 mobile switching center 18(2) for system area 32(2) supports mobile station 16
operation in the Cellular hyperband only. The mobile station 16(1), operating inaccordance with known mobile assisted hand-off (MAHO) principles, periodically
makes downlink signal strength measule~ 200 on the traffic channel (of cell 12(1))
that is currently being used, and also periodically makes downlink signal strength
measu,e,l,t;.,ls 202 on the control t~ nn~l~ of network identified cells 12, including cell
12(2), which nei~llbol the cell 12(1). These signal strength measurements are reported
204 to the base station 14(1) for the currently serving cell 12(1). The base station
14(1) concurrently makes uplink signal strength measurements 206 on the traffic
channel that is currently being used by the mobile station 16(1).
The base station 14(1) processes the mobile station 16(1) reported 204
downlink signal strength measu,~",e"l~ (200 and 202), if available. and the base station
made uplink signal strength measurements (206) to determine first whether a hand-off
is necP.c~.y (action 208), and second, if yes, to which cell or cells the hand-off should
pl~ bly occur (action 210). In this example, it is assumed that the base station 14(1)
2 5 determines 208 from deteriorating measured uplink and downlink signal strengths that
a hand-offis l-~cç~,y. It is further a~s.lmed that an identification 210 is made of cell
12(2) in system area 32(2) as the target cell for hand-off. A request 212 for hand-off
inr.lutling i"Çul,.,alion comprising an identification ofthe currently serving cell 12(1),
the traffic channel being used for communication with mobile station 16(1) in cell
3 o 12(1), the hyperband within which that traffic channel exists, the time slot (for a digital
traffic channel) carrying the cellular communication, and the target cell 12(2) for hand-
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WO 98/03031 PCT/SE97/01246
off, is then sent by the base station 14(1) to the serving mobile switching center 18(1).
A hand-off if approved in this in~t~nce would comprise an inter-system hand-off
because the target cell 12(2) is served by a mobile switching center 18(2) (serving
system area 32(2)) dilI~relll from the mobile switching center 18(1) serving the current
cell 12(1) (serving system area 32(1)). The roregoh~g description is only an example
of the procedure used in making the determination to institute a hand-off.
The currently serving mobile switching center 18(1) then signals 214 the mobile
switching center 18(2) associated with the target cell 12(2) requesting ~Csignment (and
reservation) of a traffic channel (and time slot therein for a digital trafflc channel) for
o hand-offto the target cell 12(2). The signal 214, like the re~uest 212 sent by the base
station 14(1), includes i~ ion cO.ll~Jl;s.llg an identification of the currently serving
cell 12(1), the traffic channel being used for communication with mobile station 16(1)
in cell 12(1), the hyperband within which that traffic channel exists, the time slot (for
a digital traffic channel) carrying the cellular communication, the target cell 12(2) for
hand-off, and the hyperband capabililies of the mobile station. Responsive thereto. the
mobile switching center 18(2) signals 216 the base station 14(2) for the target cell
12(2) to make a verifying signal ~ n~ measurement (action 218) on the traffic
channel currently being used by the mobile station 16(1) in the currently serving cell
12(1). This measurement should take into account the hyperband within which thattraffic channel exists, and perhaps, if necessary, the time slot (for a digital traffic
channel) carrying the cellular communication. The base station 14(2) then reports 220
the results of the verification signal strength measurement to the mobile switching
center 18(2). If the verification was S~lCCe~fill, both the base station 14(2) and mobile
switching center 18(1) are then inro,l"ed 222 ofthe ~.cignmP.nt by the mobile switching
center 18(2) of the traffic channel (and hyperband within which that traffic channel
exists) in the target cell 12(2). This differs from the process of FIGURE~ 2A-2Bwhere verifi~.~tion was pe.r~llned prior to requesting ac~ignmPnt of the traffic channel.
The mobile switching center 18(2) then forwards 226 the results of the
verific~tion and ~;g~.,..l,..l back to the mobile switching center 18(1). This report 226
3 o not only identifies whether the verification signal strength measurement was successful,
but also ide.ntifiPc the particular hyperband within which the measurement, if any, was
.. . .
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made, along with an idPntific~tion of the hyperband capabilities of the cell. The results
are then processed (action 228) by the mobi}e switching center 18(1) to determine
whether a hand-offto target cell 12(2) should be made. This detel -l.ina~ion takes into
account not only the success and strength of the verification measurement, but also the
hyperband c~p~hi'itie.c of both the mobile station 16(1) and the target cell 12(2). If the
determination is alllll.la~ e, the mobile switching center 18(1) then confirms (action
230) the proprietary of the ~.csigned channel and hyperband in view of mobile station
16(1) comm~mic~tionc r~p~hilitiec and signals 232 the mobile station 16(1) via the base
station 14(1) for the currently serving cell 12(1) with a handover con~ dlld directing
the mobile station to switch to the assigned traffic channel (and time slot therein if
appropriate) in the target cell 12(2). The mobile station 16(1) then tunes to and
~ccesses 234 the assigned traffic channel (in the proper time slot). When the base
station 14(2) detects the mobile station access (action 236), the mobile switching
centers 18(1) and 18(2) are informed 238, and the call is switched 240 to the mobile
switching center 18(2) for further h~n~lling to complete the hand-offprocedure.
The operation of the networlc 10 in connection with the blind-type hand-off
procedure illustrated in FIGURES 3A-3B may be better understood by reference again
to the examples discussed above with respect to FIGIJRES 2A-2B. The primary
difference with respect to FIGURES 2A-2B and its examples comprises the point in2 o time at which the mobile switching center 18(1) requests that base station 14(2) for the
target cell 12(2) perform verifying signal strength measurement within the design~ted
hyperband on the decien~ted traffic channel currently being used by the mobile station
16(1). In FIGURES 3A-3B with respect to each of those four examples, the
verification signal strength measurement is not made until after the request for25 a~;g~"~l of a traffic channel in the target cell 12(2) has been made. Otherwise, the
hand-offprocess and results remain the same.
Although pl~--ed embodiments ofthe method and apparatus ofthe present
invention have been illustrated in the accompanying Drawings and described in the
roregoing Detailed Description, it will be understood that the invention is not limited
3 0 to the embodiments disclosed, but is capable of numerous rearrangements,
, . .
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-18-
modifications and substitutions without departing from the spirit of the invention as set
forth and defined by the following claims.
