Note: Descriptions are shown in the official language in which they were submitted.
2123G.~15 .
WO 94/07322 PCT/US93/08780
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' METHOD AND APPARATUS FOR COMMUNICATION CONTROL
~N~, RADTOTELEPHONE SYSTEM
BACKGROOND
The present invention relates to a control technique
for a radiotelephone communication system, and more
particularly, to a. control technique for a wireless
communication system.
Continuing growth in telecommunications is placing
increasing stress on the capacity of cellular systems.
The limited frequency spectrum made available for
cellular communications demands cellular systems having
increased network capacity and adaptability to various
communications traffic situations. Although the
introduction of digital cellular systems has increased
potential system capacity, these increases alone may be
insufficient to satisfy added demand for capacity and
radio coverage. Other measures to increase system
capacity, such as decreasing the size of cells in
metropolitan areas., may be necessary to meet growing
demand.
Interference between communications in cells located
near one another creates additional problems,
particularly when relatively small cells are utilized.
Thus, techniques are necessary for minimizing
interference between cells. One known technique is to
group cells into "'clusters". Within individual clusters,
communication frequencies are allocated to particular
cells in a manner which attempts to maximize the uniform
distance between cells in different clusters which use
the same communication frequencies. This distance may be
termed the "frequency reuse" distance. As this distances
increases, the interference between a cell using a
WO 94/07322 2 ~ 2 3 ~i 0 5 PCT/US93/08780
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communication frequency and a distant cell using that
same frequency is reduced.
Radio base stations are often located near the
center of each cell to provide radio coverage throughout
the area of the cell. Alternatively, a radio base '
station may be located near the center of three adjacent
"sector cel7.s" to cover those cells. The choice between
a sectorizec! and non-sectorized system is based on
various economical considerations such as equipment costs
for each baF:e station.
Localized microcells and picocells may be
established within overlying macrocells to handle areas
with relatively dense concentrations of mobile users,
sometimes referred to as "hot spots". Typically,
microcells may be established for thoroughfares such as
crossroads or streets, and a series of microcells may
provide coverage of major traffic arteries such as
highways. Microcells may also be assigned to large
buildings, a.irporta and shopping malls. Picocells are
similar to microcells, but normally cover an office
corridor or a floor of a high-rise building. The term
"microcells" is used in this application to denote both
microcells a.nd picocells, and the terra "macrocells" is
used to denote thE: outermost layer of a cellular
structure. An "umbrella cell" can be a macrocell or a
microcell as long as there is a cell underlying the
umbrella cell. Microcells allow additional communication
channels to be located in the vicinity of actual need,
thereby increasing overall system capacity while
maintaining low lEwels of interference.
The design of: future cellular systems will likely
incorporate macroc:ells, indoor microcells, outdoor
microcells, public: microcells and restricted microcells.
Macrocell umbrellas sites typically cover radii in excess
of one kilometer and serve rapidly moving users, for
example, people ir: automobiles. Microcell sites are
usually low power, small radio base stations, which
WO 94/07322 2 ~ 2 3 6 a 5 PCT/US93/08780
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primarily handle slow moving users, such as pedestrians.
Each microce:ll site can be viewed as an extended base
station which is connected to a macrocell site through
digital radio transmission or optical fibers.
In designing a microcell cluster, it is necessary to
allocate spectrum to the microcells. This can be done in
several ways: for example, microcells can reuse spectrum
from distant macrocells; a portion of the available
spectrum may be dedicated for microcell use only: or a
microcell can borz-ow spectrum from an umbrella macrocell.
In dedicating spectrum to the microcells, a portion
of the available spectrum is reserved strictly for the
microcells. Borrowing spectrum involves taking
frequencies available to the macrocell for microcell use.
Each of these channel allocation methods has
accompanying advantages and drawbacks. Reusing channels
from distant macrocells causes little reduction in
capacity of the macrocell structure. However, reuse is
not always feasible because of co-channel interference
between the microc:ells and macrocells.
By dedicating spectrum to the microcell,
interference betwEeen cell layers (microcell and
macrocell) is reduced because any co-channel interference
is between m.icrocells, not between macrocells and
microcells. When dedicating spectrum to a microcell,
that spectrum is taken from the entire macrocell system
in a certain area, for example a city. Thus, that
spectrum is not available for macrocell use. As a
result, in an area containing only a few microcells,
capacity is adversely affected because the microcells
- cover only a small portion of the area in the macrocell
area while the mac:rocell, with a reduced amount of
spectrum available, must cover a substantial area.
Nevertheless, as t:he number of microcells increases and
the area covered by only the macrocell decreases,
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capacity problems associated with dedicating spectrum may
be reduced and a 'total net gain in overall system
capacity is achieved without introducing blocking in the
macrocells.
Borrowing channels from an umbrella macrocell, like '
reuse, presE:nts potential co-channel interference between
microcells Fu~rd macrocells. Also, capacity may be
adversely al:fectec3 because efficient spectrum allocation
is often impossible. For example, it may be difficult to
l0 address all the hot spots in a cell simultaneously when
borrowing or dedicating spectrum. An advantage of
borrowing spectrum is that the entire macrocell system is
not affected, unlike dedicating spectrum, because only
spectrum allocated to a covering macrocell is borrowed
and not specarum from the entire system. Thus, other
macrocells c:an use the same spectrum which is being
borrowed by a microcell from its covering macrocell.
Further, in cluster design, allocated spectrum must
be distributed to individual microcell sites. Known
methods employed for spectrum allocation include fixed
frequency planning, dynamic channel allocation (DCA), and
adaptive channel allocation (ACA). Further, a control
channel management: technique must be selected. One
possibility includes having each cell or sector in a
sectorized system use a unique control channel until
frequency reuse is feasible from an interference point of
view.
With th.e introduction of microcells, radio network
planning may increase in complexity. The planning
process is largely dependent upon the structure of the
microcells. For example, the size of streets, shopping
malls, and buildings are key design criteria. Microcells
suffer from a series of problems including an increased
sensitivity to traffic variations, interference between
microcells, and difficulty in anticipating traffic
intensities. Even if a fixed radiotelephone
communication system could be successfully planned, a
WO 94/07322 PCT/US93/08780
21 23~i~ 5
_ 5 _
change in system parameters such as adding a new base
station to accommadate increased traffic demand may
require replanning~ the entire system. For these reasons,
the introduction of microcells benefits from a system in
which channel assj.gnment is adaptive both to traffic
conditions and to interference conditions.
One of 'the main concerns associated with microcells
is the minimization of frequency planning in FDMA and
TDMA systems or power planning in a CDMA system. Radio
propagation 'which is dependent on environmental
considerations (e. g., terrain and land surface
irregularities) and interference are difficult to predict
in a microcelluiar environment, thereby making frequency
or power planning extremely difficult if not impossible.
One solution is ta~ use an adaptive channel allocation
(ACA) scheme which. does not require a fixed frequency
plan. According t,o one implementation of this method,
each cell site can. use any channel in the system when
assigning a :radio channel to a call. Channels are
allocated to calls in real time depending on the existing
traffic situation and the existing interference
situation. .Such a system, however, may be expensive
since more clhannel units on the average must be
installed.
Several advantages are realized with ACA. There is
almost no trunking efficiency loss since each cell can
use any channel. Thus, it is possible to employ cells
with very few channels without losing network efficiency.
Further, channel reuse is governed by average
interference conditions as opposed to the worst-case
scenario.
Several ACA schemes attempt to improve traffic
capacity and avoid the need for frequency planning.
While some s;Ystems have been moderately effective in
accomplishing these goals, it has been difficult to
achieve both goals in a system which has preassigned
control channels, i.e., a system having specified
WO 94/07322 - PCT/US93/08780
21 2361) 5
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frequencies on which a mobile station may expect a
control charnel (e. g., a 30 I~iz RF channel which contains
control signals). Systems having preassigned control
channels include AMPS (Advanced Mobile Phone Service
System), IS-~54 (Revision B) and TAGS (Total Access
Communication System). In such systems, frequency
planning is still needed for control channels. However,
frequency planning for voice channels can be avoided and
traffic capacity improved by eliminating the need to plan
a number of voice channels on each site in an area where
traffic channels are expected to be non-unifonaly
distributed.
When planning an antenna system, allocating spectrum
for a microcell cluster, and selecting a power level for
microcell transmitting power, several concerns must be
addressed. Sufficient radio coverage, e.g., 98%, must be
provided within the microcell area. Also, if the
spectrum allocated to the microcell cluster has been
reused from a distant macrocell, the power level of the
microcells must be: iow enough to avoid interference with
the distant ~macroc:ell from which the spectrum was reused.
Further, the power. of the control channel in the
microcell may haven to be stronger than the power of the
covering umbrella macrocell control channel if the mobile
is to lock on to t:he microcell. zn sum, the aim of such
a system is to assign as many mobiles as possible to
microcell control channels by maintaining those control
channels stronger than the control channels of the
umbrella mac:rocell. in the intended microcell area while
transmitting with a sufficiently low power to avoid
interference with the distant macrocell.
Power o:r inte:rference limitations can result in a
voice channel limited system where some of the mobiles in
the microcel:ls will receive a stronger signal from an
overlying macrocell. The number of mobiles receiving a
stronger signal from an overlying macrocell will increase
as the distance between the umbrella cell and the
CA 02123605 2003-02-24
microcell decreases. c-'.c.:~nsequently, cad>acity might not
increase since mobiles <:~r~.> Locked--~:.:an tc> the m~~crocell.
Moreover, if °nobi=:Le tr:rr.~smitting power requirements
increase, the batt=ery 1__i...fi<~ of thE> ::urrent portables would
correspondingly decreases ~o maint:ain the equi~.~alent level
of performance. Furthi:r, blockirng and intermodulation may
arise with high powere~:l mobiles loc:atecl inside the
microcell area. The h:id~n--powered mebiies are power
controlled by the umbreJ_1a macrocell and require more power
to communicate with tht~ umbrella macroc;ell than the
microcell.
S IJMMAR Y
A control. channel. management scheme implemented
according to the present_ invention rnay includa a variety of
different cells. To fa~:~.il:hate lcckincJ or camping mobile
units to the most appr~op:r:iate cell, the c:ont:rol channel of
each cell can be confa_~~.a.red to brcadca:;t. information about
other cells including tine cdharac:terist ics of the cells such
as cell type. Further, the location in frequency and tame
of other control channe_Ls may also 1;>e included among the
information broadcast o,Jer a control channel of a
particular cell. This information is then used by the
mobile to loci; to a prefer:r.ed cell. Locking is the
selecting of a cell such that a mobile reads a1.1 messages
and is prepared to receive pages and make calls. Thus,
once a mobile is locked to a particular cell., i.t may make
and receive calls.
More specifically, the present invention provides a
method for unsung control. channe;Ls in a wireless
communication:, system including a mobile station and a
plurality of <;ells, e<Yr_:h cell h<~ving a respective control
channel, the method cc:~rnprising the steps c>f in a first
cell, broadca:;ting relative information about at least one
CA 02123605 2003-02-24
7a
other cell on the firsl_ cull"s control channel, wherein the
relative infoxmation concerns ti=e charac:teris~ics of the at
least one other cell, ~rnd irl the mobile stab{m, analyzing
the relative info.rmati:;xv and loc=king ont=o the at least: one
other cell based on the relative information.
The present inven~.~on also providf~s a method for using
control channels in a w:~rele:~s communi.<:cttions system
including a me>bile stat i.on and a plura:Lit.y of cells, each
cell having a respective control channel, the method
comprising the steps of: broadcasting first control
information on a first ~::ont~rol. c=hannel assoc:fated with a
first cell, the first c:~:intro:l information including
relative infoxvmation p~>rtaining to a second cell,
broadcasting :second cr:nt: ro:1 .infc~rrr~at:ion on a second cont=rol
channel associated with a second cell, the second control
information including relative information pertaining to
the first cell_, and ire r=he mobi~'we st:atic.m, c..°.omparing the
first control informat:i;~n and tree second control
information and locking onto the second cell based on the
comparison of relative information.
The prey:Emt rover:.t;.ion also provides a method in a
mobile station for using control channei_s in a wireless
communications system including a plurality of cells, the
method comprising the steps of r_ec:eiving corutrol
information on a control channel. a~ssociat:ed with a first
cell, the coat=.rol information including relative
information concerning a second cell, wherein the relative
information concerns t:he characteristic:~ of the second
cell, and loc:)cing to c;ne of the first cell and the second
cell based on the control information.
The pre=~E~nt invention also provides a method for using
control chanroE=ls in a wireles s ~-~onununicati.on system
including a p_Lurality of cells, the method comprising the
steps of broadcasting a:resolute information about a cell on
CA 02123605 2003-02-24
7b
a control channel of tlnE:~ <:e1:1., and broadcasting relative
information about at lr:~~m;_ one ct:_hc~r cell on i.he control
channel, wherein the ab;>oLute information and the relative
information a;~ch inclu~:~ce ~-rte Leasi: one c~f cell type
information, service prc:>ft_le, control channel organization
information, equalizer :i.nformatic:m, anc.>, cell :-~el.ection and
reselection criteria.
The present= invenl_:i..on also provides an apparatus for
use with control channe:l..s in a wireless communications
system including at leap>t c.:~ne mobi Le :~t~ation and a
plurality of cells, each cell having a i:espective control
channel, the apparatus c=omprising means for broadcasting
first control information including first relative
information en a first i:ontrol channel associated with a
first cell, the first rt~.Lat.ive i.nforrnation pertaining to a
second cell, means for karoadcast:ing se~Jond control
information on a second cor~t.rol channel associated with the
second cell, t:he second control inform~~t.ion including
second relative inform;.anon pertaining to the first cell,
and means, provided in ~ mob.ile station, for' isomparing i:he
first control informati~;gin and the second control
information to determirls~ a preferred ce:Ll based on the
first and second relative information and for locking onto
the preferred cell.
Based on the control information found on a control
channel, the mobile att:empts to find the best server, i.e.,
the best cell, based on both the mobile and system
requirements. Eor example, according to one embodiment,
the best sera=er is the one which zwequir~~:; the mobile to
transmit with the lea~;t amount o.f power. In another
embodiment, t:he goal rriay be to rnirn.irnize the charge of a
call to a cusi~omer, thus the cell which provides the
CA 02123605 2003-02-24
WO 94/07322 PCT/US93/08780
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lowest cost service: to the mobile may be selected. A
variety of criteria may be used to determine a preferred
cell, and all such criteria are considered within the
scope of this invention. In general, the preferred cell
is determined by the system requirements and goals.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in more
detail with reference to preferred embodiments of the
present invention, given only by way of example, and
l0 illustrated in the accompanying drawings, in which:
Fig. 1 is a cell plan illustrating two cell clusters
in a cellular mobile radiotelephone system;
Fig. 2 illustrates a typical multi-layered cellular
system employing umbrella macrocells, microcells and
picocells;;
Fig. 3 illustrates a typical control channel: and
Fig. 4 represents an exemplary implementation of an
apparatus. for a radiotelephone system according to the
present invention.
DETAILED DESCRIPTION
Although the following description is in the context
of cellular communication systems involving portable or
mobile radio telephones and/or personal communication
networks, it will. be understood by those skilled in the
art that the present invention may be applied to other
communication systems.
Fig.. 1 illustrates a first cell cluster A and a
second cell cluster B forming part of a cellular mobile
radio telephone system in a known manner. Such a system
is described in tJ.S. Patent No. 5,230,082 entitled
"Method and Appar°atus For Enhancing Signalling
Reliability in a Cellular Mobile Radio Telephone System"
by Ghisler et al. Typically, all. frequencies available
in a syst=em are u:.--;ec~ in each cell. cluster. Within each cell
CA 02123605 2003-02-24
WO 94/.07322 PCT/US93/08780
_ g
cluster, 'the frequencies are allocated to different cells
to achieve the greatest uniform distance, known as the
frequency reuse distance, between cells in different
clusters using the same frequency. In Fig. 1, cells A~
and B~ use' a common frequency, cells A2 and B2 use a
common frequency, cells A3 and B3 use a common frequency,
etc. The radio channels in cells A~ and B~ using the same
frequency are referred to as co-ch nnels because they
share the same frequency. Although some interference
will occur between co-channels, the level of such
interference in an arrangement such as that of Fig. 1 is
normally acceptable. The cell plan of Fig. 1 allows for
a relatively simple frequency allocation and provides
reduced c:o-channe.l interference in low traffic
conditions. However, as noted above, limitations in high
traffic areas restrict the use of this cell plan. For
example, traffic in hot spots can produce blocking.
Future systems may nat require the type of frequency
planning associated with the cell structure of Fig. 1.
For example, a CDMA (code division multiple access]
system may have very different assignment techniques and
may not require frequency planning. However, transmitter
power planning may be a concern instead. Also, in a CDMA
system channels may not need to be reused as described
with reference to Fig. 1. CDMA systems are shown in U.S.
Patents Wo. 5,151.,919 and No. 5,218,619, both entitled
"CDMA Subtractive Demodulation". In non-CDMA systems,
techniques such as ACA may be used so that strict
frequency planning :is not necessary, in particular pre-
3o planning., i.e., planning without knowledge of
instantaneous conditions including traffic patterns and
interference distribution.
Fic~. 2 is an exemplary mufti-layered cellular
system. An umbrella macrocell to represented by a
hexagonz~l shape makes up an overlying cellular structure.
Each umt~rella cell may contain an underlying microcell
WO 94/07322 PCT/US93/08780
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structure. The umbrella cell 10 includes microcell 20'
represented by the area enclosed within the dotted line
and microcel:L 30 represented by the area enclosed within
the dashed lane corresponding to areas along city .
streets, and microcells 40, 50, and 60, which cover '
individual f7loors .of a building. The intersection of the
two city strE:ets covered by the microcells 30 and 40 may
be an area o1: dense traffic concentration, and thus might
represent a hot spot.
l0 Briefly,. cont:col channels are used for setting up
calls, informing the base stations about locations and
parameters associated with mobile stations, and infonaing
the mobile st:ations about locations and parameters
associated with the base stations. The microcell base
stations listen for call access requests by mobile
stations and the mobile stations in turn listen for
paging messagres. Once a call access message has been
received, it must be determined which cell will be
responsible for the call.
Future systems will employ additional cells. For
example, new systems'inay include any combination of
macrocells, indoor microcells, outdoor microcells, public
microcells and restricted or private microcells. New
systems therefore will likely be designed to incorporate
an increasing' number of control channels. Currently,
there are approximately twenty-one control channels
available for a cluster in a typical system employed in
the United States.
According to t:he present invention, each control
channel in each cell is configured to broadcast
information about the presence, if any, of other cells
and the characteristics of those cells including minimum
quality criteria, power requirements, etc. Typically,
information about t:he presence of other cells is
broadcast about neighboring cells. For instance, a
neighboring cell may be adjacent to, overlapping, or non-
contiguous with the: broadcasting cell. A mobile
WO 94/07322 PCT/US93/08780
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periodically scans during idle mode the control channe3s
in the coverage area that the mobile is located in to
determine wh:fch cell it should be locked to. Thus, a
mobile may continuously select cells to be locked to
based on the existing location of the mobile and guality
criteria (e. g., received signal strength) associated with
the cells. '.the cell to-which the mobile may be locked is
the cell in which the mobile satisfies the quality
criteria associated with the cell. For example, the most
l0 underlying cell based on capacity considerations may be
preferred by the mobile.
Two types of information are broadcast over the
control chamael according to the present invention:
"absolute in:Eormation" and "relative information".
Absolute information includes information about the
particular cell corresponding to the control channel on
which the in:Eormation is being broadcast. This
information ~aight include the service profile of that
cell, the control channel organization, and/or the type
of cell (for example restricted or unrestricted). An
unrestricted cell is a cell which is available to all
users at all times and a restricted cell is the opposite.
Relative information is generally the same kind of
information as absolute. information, but is information
concerning tine characteristics of other cells.
It is i~aportant for the mobile constantly to be
locked to a preferred cell. Specifically, the mobile may
be paged at any time, therefore the mobile must be locked
to a particular cell in a location area so that the
mobile may receive the page. For example, if the mobile
has moved out of the location area of a first cell to
which the molbile was locked to a second cell in a
different location area, a paging request for the mobile
will not be lheard or received because the mobile
switching center, or MSC, will page the mobile over a
paging channel available to the location area in which
the mobile is registered. Thus, a paging request would
WO 94/07322 PCT/US93/08780
21 2360 5 _ 12 -
not be received by the mobile in the distant location
area if it is not registered in that location area.
Therefore, t:he mobile should register with a new base
station when entering a new location area. Location
areas typically include a large group of adjacent cells.
It would be inefficient and impractical to instruct all
location areas to page the mobile.
A mobile may register with one base station in a
location area. A location area is typically a group of
l0 contiguous cells which do not necessarily have the same
area of radio coverage. For example, with respect to
Fig. 2, microcells 20, 30, 40, 50 and 60 may define one
location area.
In an exemplary system including a public umbrella
cell, public: microcell, and a private microcell, i.e., a
microcell accessible to a closed user group at all times
like a home base station or a campus system, all may
provide sufficient radio coverage for the mobile to
access or for the mobile to receive a page. A mobile may
be locked to an appropriate cell based on both
information found on an umbrella cell control channel
which contains information about the underlying cells and
information on each microceli control channel about any
overlying cells. In sum, information can be broadcast
over control. channels to the mobiles about the presence
of other ove:rlyinc~ or underlying cell structures, the
attributes of those cell structures and where to find
those cell structures.
When selecting the appropriate cell, a mobile can
lock on to a: candidate control channel and find all the
absolute information about the cell associated with that
control charnel to determine if the cell is appropriate.
Alternatively, the mobile can find the same information
in the form of relative information by reading messages
present on a:ny nearby cells and their control channels.
If a mobile determines that more than one cell meets
minimum reqL:irements, for example signal link quality,
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21 2360 5
- 13 -
access restriction, service profiles, etc., the mobile
can scan all or a subset of all control channels and then
make the best selection. For example, the mobile may
choose to lock on to a cell which does not require an
equalizer to be used, or a private cell like a home base
station. Typically, small cells do not require an
equalizer. Radio propagation and bit rate conditions
determine whether an equalizer is needed.
Table 1 lists. some of the attributes of a cell which
may be included ire the information on a particular
control channel. One cell may have more than one control
channel, in 'which case the absolute and relative
information on the: several control channels of the cell
typically are the same. Some or all information about a
control channel can be transmitted on other control
channels on cells that are "close" to the control channel
of interest. Typically, "close" will include contiguous
cells which 'may have the same area of radio coverage.
For example, with reference to Fig. 2, a control channel
of microcell 30 may broadcast information concerning
cells 10, 20, 40, 50 and 60.
In Table 1, t:he first column represents the type of
information about a cell and a control channel which can
be broadcast on a control channel. A "Y" in column 2
indicates that they information may be useful if it is
about the present control channel to which a mobile is
tuned and also may be useful if t:he information concerns
other cells and their respective control channels,
absolute and relative information. A "Z" in column 2
indicates the information may be useful if about other
cells, relative information.
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TABLE 1
INFORMATION WH ERE PRESENT
Where to find and decode a control Z
channel: e.,g., frequency, time slot I
identifier,. DS-CDMA code, CDMA pilot
code, frequency hopping sequence
Emergency calls only Y
Minimum mobile station transmission Y
power
Maximum mobile station transmission Y
power
Equalizer needed,/not needed (may Y
save power)
"City phonE: system", may differ from Y
back-bone :system regarding power,
services , r. ates , etc .
Test cell !:or operations; may or may Y
not accept emergency calls: special
mobiles may access
Absolute location area Y
Relative location area (in respect Z
to present cell)
Campus system (clased user group): Y
full or abbreviated ID may be sent
on other cell
Home base station Y
Moving cell. (e.g., on buses, trains) Y
Rescue cell. for call re- Z
establishmE:nt or to quickly find a
new cell to lock onto if needed
Service profile (e.g., data only, Y
voice, etc.)
Air-interface specification revision Y
System owner ( fu:l1 ID) Y
System owner (re:lative to present Z
cell)
Time synchronized control channels Z
(time slot alignment, spreading code
alignment, super or hyper frame
alignment); one cell (or site) may
carry more than one control channel
WO 94/07322 21 2 3 s o ~ PCT/US93/08780
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Barred cell call set up Y
possible
Non-identical cell types present in Y
this radio coverage area (e. g.,
public/private)
Control channel organization, e.g., Y
where to find different paging
channels, packet data, etc.
i
Cell selection and re-selection Y
l0 parameters/criteria (e. g., to ensure
sufficient radio link quality)
A control channel may be formatted having an
"overhead part" and an "other part" as shown in Fig. 3.
The overhead part may contain general information about
the system 1:~ke the parameter necessary to be read before
a mobile makes an access. Also, the overhead part may
contain the :~nformation identifying where a particular
mobile station will find its paging channel. The "other
part" may contain the paging channels and other types of
channels. An exemplary control channel organization can
be found in 1J. S. Fatent No. 5,081,704 to Umeda et al.,
entitled "Method a~f Arranging Radio Control Channels in
Mobile Communications". Thus, control channel
organization information may include information such as
where in a particular control channel message that
overhead information can be found and where a paging
channel may lbe found.
According to an exemplary embodiment of the present
invention, a flag indicating that a respective control
channel is time synchronized may be added to the location
information ~of other control channels to aid cell
reselection 'when the mobile is in idle mode. The
synchronized infoz~mation may contain time difference
information. Time: difference information may be in
several forms. The actual time difference between the
present control channel and the control channel of
another candidate cell may be included on the present
control channel. For practical purposes, however, the
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time difference int:ormation may include a single bit of
information which indicates whether the time difference
is zero.
If the two control channels are aligned, for example
on a slot, frame, super frame (a collection of TDMA
frames), or hyper frame (a collection of super frames),
or the time difference is known to the mobile, the mobile
can rapidly lock to the candidate control channel (cell)
assuming information about where to find and decode that
control channel is included on the present control
channel. The present invention is not limited to TDMA
systems and can be applied to other systems including
CDMA. Even if only a limited amount of relative
information is broadcast on the control channel for
capacity reasons, the time difference information makes
the cell selection and reselection process faster which
improves system performance. For example, the mobile
station may be blind for pages a shorter amount of time.
Thus, "dead time" associated with receiving pages while
searching is reduced.
Direct Sequence (DS) CDMA codes and CDMA pilot codes
may be included as relative information. The DS CDMA
code is the P.N sequence code for communicating with a
particular base station. The pilot code is a short code
which is used for synchronizing, and provides information
about how to find the PN sequence, i.e., the appropriate
phasing and start tame.
Some of the information items.which can be placed on
a control channel are mutually similar, for example, low
mobile station power and maximum mobile transmitter
power.
Cell selection and reselection criteria may include
a path loss criterion parameter which is the difference
between the received signal strength and a minimum access
threshold signal level. The GSM specification discusses
such criteria, but GSM does not employ cell selection and
reselection criteria as relative information. Typically,
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if the path :loss criterion parameter is positive then the
cell is of acceptable coverage quality. A second
parameter can be used in conjunction with the path loss
criterion. ',The second parameter determines priorities in
the reselect:ion of suitable cells and is a combination of
the path loss criterion and network controlled
parameters. The network parameters control the cell
selection fo:r a hierarchical cell structure, for example
a microcell and macrocell structure where the network
operator wants to~assign mobiles to a cell other than the
cell from which the mobile receives the strongest signal.
The above cell selection and reselection criteria
may involve measurement of received signal strengths on
respective control channels with absolute or relative
parameters sent on the control channels. By comparing
the set of measurements and parameters of different
control charunels, the mobile can select the most
appropriate cell. Further, the service profile of a
candidate cell may also determine the selection of
control chamnels (cells). Service profile information
might include information such as whether a half rate or
full rate speech coder is being used: the data bit
transmission rate: and the type of data the cell can
handle, i.e., data, voice, and data and voice.
To determine the received signal strength of a
particular cell, the mobile must go to a control channel
of the particular cell in order to measure the received
signal stren~~th. Thus, received signal strength is an
absolute parameter and cannot be determined relatively.
Consequently, if received signal strength is an essential
parameter, tlne mobile must make a final check of the
received signal strength on the candidate control channel
before locking to a control channel. Relative
information can be used to screen the control channels
and reduce tlhe list of possible suitable control
channels. T3nus, the mobile with some type of relative
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information determines a group of candidate cells which
satisfy a particular criterion. Then, the mobile
accesses at :least one candidate control channel to find a
control channel with an acceptable signal strength. Some
parameters associated with signal link requirements may
be sent as relative information, but some form of
absolute information may be necessary.
Control channels often correspond to a cell.
However, one cell may have more than one control channel
l0 thus the elimination of a control channel as being
unsuitable does not necessarily eliminate the cell as a
candidate ce7Ll.
Location area is another parameter which may be
broadcast ovE:r the control channel. Location areas are
constructed t:o determine where the mobile is located for
paging. Typically,, a mobile registers with a new base
station when it changes location area.
A variety of cell type information can be provided
on a control channel. to aid the mobile to lock to a most
appropriate cell. For example, some cells are available
for particular situations like cells for emergency calls
only; moving cells where the moving entity defines the
cell like a plane or bus; test cells which are cells an
operator may want to test (e. g., because they are being
added to the system) and thus are limited to specific
mobiles: a home base station which is assigned and
accessible to. a closed user group, e.g., members of a
particular household: rescue cells which are used when
the mobile may immediately need a cell, for example to
reestablish a call; barred cells where no call set up is
possible because of, for example, imminent cell shut down
for maintenance purposes: and other public and private
cell types. Cell type information can take the form of
cell system information. Exemplary cell systems may
include a campus system which may be limited to a certain
area and a certain user group, and a city phone system
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which may have different services, costs, and power
requirements.
Important considerations when broadcasting the
information on the control channel are the system
limitations concerning the amount of information which
can be broadcast in a message over the control channel,
viz., the transmission capacity of the control channel.
Consequently, it :is: generally not feasible or necessary
to broadcast all relevant information. Therefore, a
:10 tradeoff must be made between the necessary relative
information and the control channel capacity. At one
extreme, no relative information is sent and the mobile
must lock: on to all candidate control channels to
discover the information for initial cell selection and
cell rese;lection. At the other extreme, a mobile may be
able to tune to a single control channel and receive all
information necessary to determine a preferred cell. In
this scenario, the mobile compares all the information
about the: nearby cells and selects a preferred cell.
According to the present invention, base stations
may be equipped to handle several voice channels and
typically at least one control channel. It is, however,
conceivable that a cell may not have a control channel,
bast is used only for handoff purposes. Thus, a call is
not originated on a cell without a control channel. Fig.
4 represE:nts a block diagram of an exemplary cellular
mobile radiotelephone system according to one embodiment
of the present invention. The system shows an exemplary
base station 110 and a mobile 120. The base station
includes a control and processing unit 130 which is
connected to the MSC 140 which in turn is connected to
the public switche~3 telephone network (not shown).
General aspects of such cellular radiotelephone systems
are known in the art. An exemplary system can be found
in U.S. 1latent No. 5,175,867 entitled "Neighbor-Assisted
Iiandoff in a Cellular Communication System" by Wejke et
al.
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The base: station 110 for a cell includes a plurality
of voice charnels handled by voice channel transceiver
150 which is controlled by the control and processing
unit 130. Also, each base station includes a control
channel transceiver 160 which may be capable of handling
more than one: control channel. The control channel
transceiver 1.60 is controlled by the control and
processing unit 130. The control channel transceiver 160
broadcasts control information over the control channel
of the base sctation or cell to mobiles locked to that
control channel.
When the: mobile 120 is in idle mode, it periodically
scans the control channels of base stations like base
station 110 t.o determine which cell to lock on or camp
to. The mobile 120 receives the absolute and relative
information broadcast on a control channel at its voice
and control c:hanne7. transceiver 170. Then,' the
processing unit 1130 evaluates the received control
channel information which includes the characteristics of
the candidate cellf: and determines which cell the mobile
should lock t.o. The received control channel information
not only includes absolute information concerning the
cell with which it is associated, but also contains
relative information concerning other cells proximate to
the cell with which the control channel is associated.
The types of information which may be present on the
control channel are shown in previously discussed Table
1.
Existing mobile units can limit the flexibility in
designing control channel schemes. For example, the
practical power capacity of mobile units are currently
limited by the size of the units and the characteristics
of the energy sources. It is expected that the present
invention will be an important part of future mobile
units which will easily implement many more of the
invention's aspects:.
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While particular embodiments of the present
invention hare been described and illustrated, it should
be understood that the invention is not limited thereto
since modific:ations may be made by persons skilled in the
art. The preasent application contemplates any and all
modifications that fall within the spirit and scope of
the underlying invention disclosed and claimed herein.
