CA2123605C - Method and apparatus for communication control in a radiotelephone system - Google Patents
Method and apparatus for communication control in a radiotelephone system Download PDFInfo
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- CA2123605C CA2123605C CA002123605A CA2123605A CA2123605C CA 2123605 C CA2123605 C CA 2123605C CA 002123605 A CA002123605 A CA 002123605A CA 2123605 A CA2123605 A CA 2123605A CA 2123605 C CA2123605 C CA 2123605C
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W48/00—Access restriction; Network selection; Access point selection
- H04W48/20—Selecting an access point
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
- H04W16/24—Cell structures
- H04W16/32—Hierarchical cell structures
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Abstract
In a radiotelephone system, the control channel of each cell can be configured to broadcast absolute information about its cell and relative information about other cells including the characteristic of the cell. Further, the location of other control channels may also be included among the information broadcast over a control channel of a particular cell. This information is then used to lock a mobile unit to a preferred cell.
Description
2123G.~15 .
' 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 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 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, WO 94/07322 2 ~ 2 3 f 0 5 P~/US93/08780 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 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 21 2361) 5 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 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 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 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 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 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 21236n5 - to -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 2123Ei05 - 1i -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 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, 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.
WO 94/07322 21 2 3 6 0 ~ pCT/US93/08780 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 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 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, 212360~~
-1,-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 WO 94/07322 ~ ~ ~~ ~ ~ CI 5 PCT/US93/08780 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 - 1'9 -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.
WO 94/07322 Pf:.T/US93/08780 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:.
21 23so 5 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.
~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 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 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, WO 94/07322 2 ~ 2 3 f 0 5 P~/US93/08780 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 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 21 2361) 5 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 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 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 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 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 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 21236n5 - to -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 2123Ei05 - 1i -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 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, 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.
WO 94/07322 21 2 3 6 0 ~ pCT/US93/08780 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 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 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, 212360~~
-1,-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 WO 94/07322 ~ ~ ~~ ~ ~ CI 5 PCT/US93/08780 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 - 1'9 -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.
WO 94/07322 Pf:.T/US93/08780 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:.
21 23so 5 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.
Claims (14)
1. A method for using control channels a wireless communications system including a mobile station and a plurality of cells, each cell having a respective control channel, the method comprising the steps of:
in a first cell, broadcasting relative information about at least one other cell on the first cell's control channel, wherein the relative information concerns the characteristics of the at least one other cell; and in the mobile station, analyzing the relative information and locking onto the at least one other cell based on the relative information.
in a first cell, broadcasting relative information about at least one other cell on the first cell's control channel, wherein the relative information concerns the characteristics of the at least one other cell; and in the mobile station, analyzing the relative information and locking onto the at least one other cell based on the relative information.
2. The method of claim 1, wherein said relative information includes at least one of cell type information, service profile, equalizer information, time synchronization information, control channel organization information, CDMA pilot code, and cell selection and reselection criteria.
3. The method of claim 1 or 2, wherein the relative information includes a flag indicating whether the at least one other cell's control channel is time-synchronized with the first cell's control channel and information indicating a time difference between the at least one other cell's control channel and the first cell's control channel.
4. The method of claim 1, 2 or 3, wherein said relative information includes cell type information.
5. The method o claim 1, wherein said cell type information indicates that the at least one other cell is a restricted cell.
6. A method for using control channels in a wireless communications system including a mobile station and a plurality of cells, each cell having a respective control channel, the method comprising the steps of broadcasting first control information on a first control channel associated with a first cell, the first control information including relative information pertaining to a second cell;
broadcasting second control information on a second control channel associated with a second cell, the second control information including relative information pertaining to the first cell; and in the mobile station, comparing said first control information and said second control information and locking onto the second cell based on the comparison of relative information.
broadcasting second control information on a second control channel associated with a second cell, the second control information including relative information pertaining to the first cell; and in the mobile station, comparing said first control information and said second control information and locking onto the second cell based on the comparison of relative information.
7. The method of claim 6, wherein the relative information includes a flag indicating whether a second control channel in use in the second cell is time-synchronized with the first control channel and information indicating a time difference between floe second control channel and the first control channel.
8. The method of claim 6 or 7, wherein the relative information in said first control information and the relative information in said second control information include received signal strength values.
9. A method in a mobile station for using control channels in a wireless communications system including a plurality of cells, the method comprising the steps of:
receiving control information on a control channel associated with a first cell, the control information including relative information concerning a second cell, wherein the relative information concerns the characteristics of the second cell; and locking to one of the first cell and the second cell based on the control information.
receiving control information on a control channel associated with a first cell, the control information including relative information concerning a second cell, wherein the relative information concerns the characteristics of the second cell; and locking to one of the first cell and the second cell based on the control information.
10. The method of claim 9, wherein the control information includes received signal strength values for the first cell and the second cell.
11. A method for using control channels in a wireless communication system including a plurality of cells, said method comprising the steps of:
broadcasting absolute information about a cell on a control channel of said cell; and broadcasting relative information about at least one other cell on said control channel, wherein said absolute information and said relative information each include at least one of cell type information, service profile, control channel organization information, equalizer information, and cell selection and reselection criteria.
broadcasting absolute information about a cell on a control channel of said cell; and broadcasting relative information about at least one other cell on said control channel, wherein said absolute information and said relative information each include at least one of cell type information, service profile, control channel organization information, equalizer information, and cell selection and reselection criteria.
12. An apparatus for use with control channels in a wireless communications system including at least one mobile station and a plurality of cells, each cell having at respective control channel, said apparatus comprising:
means for broadcasting first control information including first relative information on a first control channel associated with a first cell, the first relative information pertaining to a second cell;
means for broadcasting second control information on a second control channel associated with the second cell, the second control information including second relative information pertaining to the first cell; anti means, provided in a mobile station, for comparing said first control information and said second control information to determine a preferred cell based on the first and second relative information and for locking onto the preferred cell.
means for broadcasting first control information including first relative information on a first control channel associated with a first cell, the first relative information pertaining to a second cell;
means for broadcasting second control information on a second control channel associated with the second cell, the second control information including second relative information pertaining to the first cell; anti means, provided in a mobile station, for comparing said first control information and said second control information to determine a preferred cell based on the first and second relative information and for locking onto the preferred cell.
13. The apparatus of claim 12, wherein said second control information includes absolute information pertaining to the second cell.
14. The apparatus of claim 13, wherein said absolute information and said first relative information each include at least one of cell type information, equalizer requirements, service mobile, control channel organization information, equalizer information, CDMA pilot code, and time synchronization information.
Applications Claiming Priority (5)
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| US94546892A | 1992-09-16 | 1992-09-16 | |
| US945,468 | 1992-09-16 | ||
| US07/955,591 US5353332A (en) | 1992-09-16 | 1992-10-02 | Method and apparatus for communication control in a radiotelephone system |
| US955,591 | 1992-10-02 | ||
| PCT/US1993/008780 WO1994007322A1 (en) | 1992-09-16 | 1993-09-16 | Method and apparatus for communication control in a radiotelephone system |
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| CA2123605A1 CA2123605A1 (en) | 1994-03-31 |
| CA2123605C true CA2123605C (en) | 2003-11-25 |
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| CA002123605A Expired - Lifetime CA2123605C (en) | 1992-09-16 | 1993-09-16 | Method and apparatus for communication control in a radiotelephone system |
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| US5603081A (en) * | 1993-11-01 | 1997-02-11 | Telefonaktiebolaget Lm Ericsson | Method for communicating in a wireless communication system |
| GB2303998B (en) * | 1995-08-03 | 2000-03-01 | Nokia Mobile Phones Ltd | Radio telephones and methods of operation |
| GB2320991A (en) * | 1996-12-20 | 1998-07-08 | Dsc Telecom Lp | Channel selection control in a cellular radio communications system |
| US5987322A (en) * | 1997-04-03 | 1999-11-16 | Ericsson Inc. | System specified adaptive mobile station behavior within a mobile telecommunications system |
| FI107982B (en) * | 1997-05-06 | 2001-10-31 | Nokia Mobile Phones Ltd | Cell selection based on usage profile in cellular radio system |
| FI105639B (en) | 1997-06-25 | 2000-09-15 | Nokia Mobile Phones Ltd | An improved method for cell switching |
| EP0998831B1 (en) | 1997-07-22 | 2007-08-29 | O2(Germany) GmbH & Co. OHG | Method and mobile radio telephone network for assessing charges on calls |
| GB2330484A (en) * | 1997-10-15 | 1999-04-21 | Motorola As | Mobile initiated handover during periods of communication inactivity |
| US6212405B1 (en) | 1998-08-31 | 2001-04-03 | Lucent Technologies Inc. | Extended range concentric cell base station |
| JP3196747B2 (en) * | 1998-12-18 | 2001-08-06 | 三菱マテリアル株式会社 | Mobile radio, base station radio, and recording medium thereof |
| WO2008152130A1 (en) * | 2007-06-15 | 2008-12-18 | Nokia Siemens Networks Oy | Apparatus, method and computer program product providing control of system information decoding |
| EP2345286A1 (en) * | 2008-10-31 | 2011-07-20 | Nokia Siemens Networks OY | Cell selection techniques for idle mode for wireless networks |
| EP2747490A1 (en) * | 2012-12-19 | 2014-06-25 | Koninklijke KPN N.V. | Session termination in an energy-efficient cellular wireless telecommunications system |
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| DE3200965A1 (en) * | 1982-01-14 | 1983-07-21 | Siemens AG, 1000 Berlin und 8000 München | NEARBY RADIO CELL ORGANIZATION CHANNEL REFERENCE SYSTEM |
| DE3611301C2 (en) * | 1986-04-04 | 1997-09-25 | Philips Patentverwaltung | Mobile radio system with communication channel and organization channel |
| GB2195513B (en) * | 1986-09-18 | 1990-12-19 | Philips Electronic Associated | Radio system |
| US5093926A (en) * | 1989-09-29 | 1992-03-03 | Motorola, Inc. | Trunked communication system scanning method and apparatus |
| US5239667A (en) * | 1990-01-31 | 1993-08-24 | Nec Corporation | Method of controlling handoff in cellular mobile radio communications system |
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| AU4926793A (en) | 1994-04-12 |
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| CN1085704A (en) | 1994-04-20 |
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| AU668591B2 (en) | 1996-05-09 |
| HK1006623A1 (en) | 1999-03-05 |
| GB9409631D0 (en) | 1994-07-06 |
| SE517898C2 (en) | 2002-07-30 |
| CN1051901C (en) | 2000-04-26 |
| WO1994007322A1 (en) | 1994-03-31 |
| SE9401644L (en) | 1994-07-06 |
| NZ256326A (en) | 1996-05-28 |
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