Note: Descriptions are shown in the official language in which they were submitted.
` ~ 94/olg7~ 2 1 1 6 9 7 9 PC~/US93/05771
A METHOD AND APPARATUS FOR PE:RlFORMI~G
HA~IDOFFS IN A WIRELESS COMMUNICA,TION
SYSTEM
Field of the Inven~ion
The in~ention relates ~o communicatiorl ~yg~ms and
in specific to ccllular communicatiQn systems.
Baclcground of tbe InYention
Cellular communication syst~ms are k~own~ Such
commu~ication systems typically provide communication
services throughout ~a rela~ively large geographic arca
: 15 through a number of base sites, each havtng ~ scrvice
coverage area. Such base sites are arranged to proYidc
substantially con~inuous coverage to mobile c~mmunication
UDitS passing through :such area by ba~ding off the sourcc
*~ervice ~om one base site to another as:a con~nuni~a~ion
2 u unit passes th~ough the area.
:: : Communicadon ~ services within the ~cellular sys2em
are :p~ovided through e~change of radio frequ~ncy (R~)
SigllalS betweon: ~a~ collununicatîo~l unit and~ a senrhg base
site.: As the~ commu~îcation ~nit moves out ~ thc servîee
2:5~ coverage~ area: of ~`e s~rving base site,~ the movement îs
:`dètected by ~ ignal:~: measurements performed by tbe
o~un,unication ~uM~,~ and transferred t~ the se~ing base
:s~ o~,~ by signal measurements of: the comm~ ic~tion unit
other, nearby~ base sites~ A determinaâon of a nced to
30: handoff a communieation transacti~n, :from a serYîng base
sîte to ~a~ ew~(~et)~base site prQxîmate t~e moving
communication u nit, may be based upon companson of
measured:signal~ralues~with ~eshold values~ (The ~lobal
System for Mobile ~ommuni~atîons (C}SM), as speci~led in
': 3 5 GSM recommendations available from the European
Telecornmuni~ations~: S~andards lnstitste ~TSI) and
incorporated ~ herein~ by reference, is an example of just such
a sys~em~
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Upon determining that a communication unit needs to
be handed off from a ser~ing base site to a ~8et base site,
a communication path must be established with the
communication unit through the target base site. Under the
5 prior art, transfer is accomplished within a matrix switch
located at a mobile switching center (MSC) or a matnx
switch located at a base site controller (BSC).
The matrix switch within a cellular system provides
handover through thc availability of interconnccts within
10 the switch from~a muldtude of external signaling devices.
In the case of handoff, the matrix switch is provîdcd with
interconnects from both scrving and targct base sitcs. To
accomplish handoff a controller (e.g., the l~fSC or BSC) for
the matnx switch commands the matrix switch to
15 disconnect the path with the onginal serving cell and
reconnect to the target cell or by involcing third-party
conferencing features of the cellular infrastructure to
bridge~the cells for some period of timc. Alternativcly the
controller m?y command ;the switch to connect to both
2~0;~ target and serving ~base ~site for a prcdetermined dmc
penod under a process rcferred to as ~"soft handover". Soft
handover may~ occur ~in ~some systcms through direct control
of packet selection ~or paclcet switching by a local controllcr.
Locating the~ matnx switch at an MSC (or at the MSC
25 ~ and ~BSC) may~ require ~relatively long ~signal paths bctween
the base~ site (BS) and ~matrix switch. If the MSC controls
mul~iple BSCs,~or~the~BSCs~control multiple BSs, or the ~Ss
serve multiple~sectors, then relatively large numbers of
limited-purpose~ signal paths must be provided betwecn
30 ~ ~switch and served~dc~ricés.
The~ problem~ of~ handoff through a matrix switch is
complicated where ~ha~doff is to a~ cell that is part of a
different cellular~system, or part of a different base station
system (BSS). If the handoff target base site is served
' 35 through a different BSC tban the original serving site, theri
handoff must be~ ~performed within a matrix switch located
at the MSC. ~ Handoff is required at the MSC because neithe~
the matrix switch~ at';the BSC of the serving base site or the
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matrix switch at the BSC of ~he target base si~e has
interconnects with both serving alld target base sit¢s, The
matnx switch at the MSC, on lhe other hand, is provided
with intcrconnects from all BSCs controlled by that MSC. TQ
S achieve handoff at the MSC, the MSC: commands the MSC
matrix switch to interconncc~ to the target cell and
disconncct from~ the prior, ser,ving ccll.
If the target cell site is part of a different cellular
systcm, then handoff may require the in~olvement of the
public switch telephonc nctwork (PSTN). Altcrnativcly,
handover among differcnt cellular systems may bc
accomplished through special trunk groups maintained for
handover among~ the differcnt systems. Where different
'MSCs are involved the MSCs must exchange information on
handoff and request handoff from switching systems within
the PSTN or use special trunk groups.
While handoffs based upon matnx switches located at
the MSC or B$C have worlced well, such an arrangement is
s~ incfficient because of thc distancc and number of signal
2 0 path~s brought to centralized locadons, remotc from the
de~rices servcd. A ~need exists for a mcthod of localizL~g
handoff to the de~ices directly involYed in the handoff.
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2 5~ ' Summary of the Invention
A meth~od~ and apparatus of performing handoffs in a
celIular communication system, such method including the
step of providing~;~a switch at each point in a signal path
, 30 ~ bridging at ~least~two célls~ The mct od further includes the
steps o f deter~ining that handoff of a communica~ion unit
from a serving cdl to a target cell is indicatcd and
idendfying a switch in a signal path to the serYing cell
bridging the s~e~ving~ and target cells. The method finally
35 includes handing ~off through the bridging switch.
The apparatus, of such method of perfo~ning handof~
includes the inclusion of a switch at each signal distribution
point bridging at least two service coverage areas. Handoff
,
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within such a system may tben be initiated from a s~r~ing
base site based upon infonnation contained in a means for
configuring, and rcquesting ~hat handQff occur wîthin ~he
:: switch bridging serving and target cells.
S
Brief Descripdon of the Drawings
FIG. 1 depicts a~ block diagram of a cellula~
communication system, in accordance with one embodimen~
of the inv~ntion
FIG. 2 depicts a: prior art matrix swi~ch.
FIG. 3 depicts a base station syst~m, in accordallce
with the preferred embodiment with a switch at each
circuit branch: point.
::20 : FIG.- 4 depiGts a bl~ck diag~am of a communication
unit, in accordanco ~with the inYention.
FIG. S depicts a~:flow chart of handoff in accordance
; with: the prefemd: ombodiment of the invention.
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Dctai1ed Description of the Preferred Embodiment
e solù~don to the problem of loc~zing handoff lies,
3:0: :ccep~ually.~ in~ placing a switch at: each signal distribution
point :bridging ~more tharl one servic:e coYerage area. The
switch may b~:placed at a BS servi~g multiple sectors or
with a transcoder serving multiple BSSs. Placement of a
: `~ switch at such a:signal dis~ibution point allows handQff
35: through the lowést l~Yel of ~e system, without
involvement of other ::levels. Relegation of handoff activi~y
to low leYeL signa} braneh points beneficially reduces the
potential of cali blocking, on handover, while increasing
70 94/olg7s 2 1 1 6 9 7 9 Pcr/usg3/os77l
system capacity. Relegation of handoff to low le~cl branch
points may also allow handoff detenninations bascd upvn
signal quality'information available at low lc~el branch
point (e.g., a base site) that may not be available at highcr
5 level branch points (e.g., an MSC~.
Shown in ~;IG. 1 is a cdlular communication system,
generally (10), in accordance with one embodiment of thc
invention. Included within such a system (10) are MSCs
#1-j (10 and 11). Each MSC of MSCs #lj (e.g., MSC #1) may
10 have associated with it ~anscoders *1-k (12 and 13). Each
transcoder (e.g., transcodcr #1), o~ transcodcrs #l-lc (12
and 13), in turn, services'BSCs #l-m (15 and 16). Each BSC
' ~ (c.g., I~SC #l) of BSCs #l-m con~ols BSs #l-n (18 and 19).
Each BS (e.g., BS #1 ~18)) of BSs #l-n (18 and 19) provide
15 communication services~ within a number of sectors (cells)
(22-24) .
Shown in FIG. 2 is a means for switcbing (34~ that,
under the preferred - embodiment, is included within ~SCs
(10 and 11), transcodcrs (l2-14)" BSC:s ~15-173 and BSs
20~ (18-21~. The means~for switching (switch) may be a circuit
witch ~matrix switch3;~o~ pacl~et s~,vitch. The switch (34)
(as known in the an) is ~capable of providing a path (witbin
;the~switch (34)) botween~any two cxternal connections (1-
*.~ If~ ~e~ switch (34)'~ is~ a packet switch, thcn the path may
2~5~ be~created~ by referonce ~to a header within a transn~itted
data~ ~ packet or created ~ under external contr~ol ~fast pacl~et
sw'itching).~ If the switch (34) ~is a mdrix switch, then tbe
~ùit'path is created unde~ control of an extemal
essor. ~ Inclusion' of tbo switch (34) within cach MSC (10
30' ;~a~d~ 11), t:ranscoder ~(12-14), BSC (15-17) and BS (18-21)
; benc~lcially allows bandoff to occur at thc lowest possiblc
Ie~el~within the ~system~ (lû). ;~Becausc of ~arying needs and
switching applications ~ within the ~ ~system (lû~ pacl,cet
switcbing and circuit switching may cxist simultaneously
35 within different levels of the same system ~10).
Allowing handoff to occur at the lowest possible level
within a system (10) reduces~function-speciac interconnect
wiring within such a ~ system (10) by placing a switch (34)
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2116979 .
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at each branch point associated wilh two or more scrvicc
coverage arcas. Locating a switch (34) at each branch point
allows trunking to occur to the lowest possible levol
thereby increasing overall system efficiency. Locating the
S switch at such branch points may also localize eontrol
traffic to the branch point in cases where the handoff
controller (call controller~ is also locatcd a~ thc branch
point.
Shown in FIG. S is a flow chart of handoff in
10 accordance with thc preferred embodimcnt of the
invcndon. Referencc will be made to the flow chart as
appropriate to understanding the invention.
By way of examplc a communication unit (50, ~7IG 3)
is exchanging a communicated signal with a public switch
15 telephone network (PSTN) ~ subscriber ~not shown) on an
allocated frequency fl. During call set-up, the switch (34)
within the BS (18) is commanded to establish a signal path
between te~ninals 1 ~and; p~3. The switch (34) within the
call controller (BSC 15~ is~ commanded to cstablish a signal
20 path between tern~iDals~ and p+2. The switches (34)
within the transcoder~ (l2~ and MSC (10) (not shown) are
similarly configured. ~
During thc cxchange the communication unit (50)
moves out of cell 22 and into cell 23. Dcterminadon of a
25~ need~ for handoff~within such~a system ~10~ may be
determined, in part,~by~ comparison of measured signal
~values with thr~eshold~ values. Under GSM, a scanner (43)
within the communication unit (50, ~IG. 4) measures ( IOQ)
a~signal~quality factor~;(such as signal strength) of control
30~;signals of surrounding~cells (23 and 24) and transfers (101)
such measurements to~the call contro11er (BSC 15) for a
determinaeion o f handoff. Within other systems a scanner
~28) located at a~ ~base;~;~site (l8 or 19) measu~es (100) signal
quality factors of the communication unit (50) within cells
35 (22-27) surrounding ;the serving cell and transfers such
measurements to ~the~ call controller (BSC 15).
The BSC ~15) upon receipt of signal quality
measurements determines a need to handoff and a handoff
QYO 94Jo1975 2 1 1 6 9 7 9 Pcr/usg3/os771
target (102) based upon the transfcned signal quality
measurements. Upon detcrmination of a need for handoff,
~he BSC (15) commands the communlca~ion unit ~50) to
handoff to the target on the same or another frequcncy f2-
5 The BSC ~15) also determines ~103) by rcfcrcnce to a meansfor configuring (36) (e.g., a loolc-up table or algorithm) that
the lowest-le~el signal~ branchîng point, senring both
serving (22) and target (23) cells, is the switch (34) located
at the BS (18). Upon~such detcrminadon the BSC (15)
10 commands thc switch::(34) at the BS (18) to handoff (104)
:: ~the communication unit~ ~50): by discontinuing the signal
path betwccn terminals l:: and: p~3 and to re-establish a
voice path through ~tcrmmals l and p~2.
: As the communication unit passes through cell 23 into
15 cell 24 the process ropeats :with the call controller (BSC 15)
commanding the ~switch~ (34) within thc BS (18) to
discondnue the path through terminals 1 and p+2 and re-
estaUish the path ~rough~tcrminaIs I and p+l. The BSC
(15),~in such case, :again ~:identi~tes thc switch ~34) within
20 ~e~ BS~ (18) as the lowcst-lcvcl switch (34) by referencc to
the::mams~for::configunng~:(36~ within thc BSC (15).
As:the communicadon:~un1t (50) passes into cdl 25,
the~B:SC: (:15) by~reference to thc means for: con~lguring (36)
now~ detennines that~the~ lowest-level branch point in the
2~5 :~ signal~ dis~ibution system~:is the s~tch (34~ Iocated at the
BSC~ 15;).~: Upon~such~dctermiDation the controller ~35)
~n the BSC ~15)~ commands the switch (34) within the
`BSC~ 15i~ to discontinue: the signa path betwcen terminals 1
and ~p+2 ~and~ re-establish~ the signal path: through lcnninals
: 3:0: :: 1 and ~p~l::. Ibe BSC:~:(15~ also co~ands the: switch (34)
within~the~target~BS~:(l9)~:to establish a signal path bctween
~: terminals 1 and p+3.:
after passing out of cell 25, the communication unit
: (50)~ were: to pass :into~a;~cell: ~not shovm) controlled by BSC
~; ~ 3 5 16:then the ser~ing~::BSC~:(15)~ by reference to the means for
configuring (36)~ would determine that the switch ~34) (not
~: shown) within the~: transcoder ~(l2) is the lowest-level
branching point serving both ~ serving ~24) and target cells.
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2116979
Upon such a determination the servi~g BSC ~153 would
~ansmit a request to the transcoder (12) requestiDg that
the transcoder (12? handoff the commurlication unit (50~ to
the target cell.
S As a further examplet a first communicatian unit (50,
FIG. 3), wîthin cell 22, ~ansmits an access request to th~
serving BSC (15) requesting access to a second
communication unit ~51) in cell 23. The BSC ~15~, using
prior art paging techniques, identifies the target
communicadon unit ~51) as being in cell 23. Upon a
determination of the loca~ion o~ both first ~SQ) and second
(51) communication units, the BSC (15), in accordance with
the preferred embodiment of the invention and by
reference to tbe means ~for configuring (36)~ dctennines
that the lowest level branchin~ point senring both cells is
the switch (34) located at BS 18. Upon such detenninad~n
the call controller ~BSC 15) transmits a command ~o ~e
:: switch (34) establishing a voice path between tenninals p~3
~;: and p+2. The BSC (15) also transmits an access grant to the
:~ ~ 20 f~rst communication unit: ~50) on a first frequcllcy (fl) and
an access grant t~ the second comnnunication unit ~51) on a
second frequency (f2~ :The f;rst (50) and second (51)
: communication unit, UpOIl tuning to the proper frequencies
;begin~ exchanging a communicated signal on their resp~ctive
: ~ :25 freauencies
During t3-e exchange the second communication unit
51~moves out of cell::23 and into cell 24. The mo~rcmerlt
:~: of the communication unit (51) is de~ected by signal
strength measuremznts`~ of control ~ansmission of
3 0 su~ounding cells (~2-27) performed by the communication
unit (51) and transferred t9 the BSC (15~ through the BS
(1 8).
Upon detennination of a need for handoff ~he call
controller (BSC 15) commands the communica~on unit (51)
to handoff to the handoff target cell :(24) on the same or
another channel (f3). The call controller (BSC 1~) also
commands the matrix switch (34~ to discontinue the signal
~wo 94tolg75 2 1 1 6 9 7 ~ Pc~JlJss3~o~77l
path between terminals p~3 and p~2 and re-establish a
si~nal path between tenninals p~3 and p+l.
As a further example, and continuing the abovc
example, the second communication unit (5 l ) may continue
S to move through cell 24 a~d into cell 25. As abov~, such
moYemcnt is detected by signal strength rn~asuremçnts
performed by the communication unit (S l ) and tran~ferred
to the ser~ing BSC (lS~.
The BSC (15) now detcrmirlcs that handoff to cell 25
is appropriate. De~rmination of the identity of the switch
(34) to handoff from cell 24 to cell 25, under the prefer~ed
embodiment, is determined: by refercnce to the means for
configuring (36) within; the BSC (lS~.
Upon reference to the means for con~lguring (36) the
call controller (BSC 15) detem~ines that the ma~ix switch
(34~ within the BSC (15) is the lowest level ma~ix switch
bridging the two se~vice coYerage ~eas (24 and 25). Upor
reaching such a determination the con~oller (35) within
the BSC (IS) commands the switch ~34~ within BS 18 to
:: 20 establish a circuit path between te~ninals p+3 and 1. The
controller (35) also: conunands the matrix switch (34)
:: within BSC 15 to configure a path betwecn p+2 and p+l. As
a final: step the con~oller (35) commands the ma~ix switch
within BS 19 to configure a path between tem~inals 1 and
2 5: p*3~
: ~: : If the communication ur~ 5 1 ) should continue to
move into :another serYice coverage area ser~ed by anothe
BSC (16, FIG. 1):then ~he process would cont~ue. The
:co~nication unit (51) would again measure signal values
;30 as a~:means of detecdng:~a handoff t~get. The serving BSC
(15): would identify the lowest level signal dis~ibution
point ~transcoder (1233 serving both service coverage areas
by refere~ce to the means for configurin% (36). Upon
identifying the lowest leve} branch between BSCs (15 and
:~ ~ 3~ 16) the serving BSC ~15) would forward a han~off request
to the transcoder (t2).
Upon receiving ~e handoff request the MSC (10)
would transmit routing requests to the switch (34) within
WO 94/0197~; PCI/US~3~0~7~
2116979
the transcoder (12) (not shown) and ~o the BSC (163 of the
target base site (not showJI). The handoff target BSC (16)
would~ in turn, transmit routing ins~uctions ~o swi~ches
(34) (not shown) of the BSC (16~ and h~ndoff target BS (not
5 shown).
Inclusion of a swi~h at cach signal branching point
provides the benefit of signal handoff at the lowest levcl,
that in the case where the call con~oller is the BS of both
ser~ing and target cells~ invol~es ~he fewest con~ollers.
10 The use of switches at each signal branching point allows
for greater ~unking efficiency by allowing ~un~ed use of
interconnect wiring that otherwise would be dedica~ed to a
less flexible, specific function.
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