United States Patent [191
`Hurst et a1.
`
`llllllliillllllllIllllIllllllllllllllllllllllllllllllllllllllllllllllllllll
`US005276905A
`[11] Patent Number:
`5,276,905
`Jan. 4, 1994
`[45] Date of Patent:
`
`[73] Assignee:
`
`[54] MOBILE RADIO TRANSMISSION SYSTEM
`USING PREFERRED HOME SERVICE AREA
`[75] Inventors: Allan F. Hurst; David J. Harrison;
`John D. Wittams, all of Cambridge,
`England
`US. Philips Corporation, New York,
`NY.
`[21] Appl. No.: 478,116
`[22] Filed:
`Feb. 7, 1990
`[30]
`Foreign Application Priority Data
`Feb. 8, 1989 [GB] United Kingdom ............... .. 8902743
`
`[51] Int. c1.s ....................... .. H_04Q 7/02;H04Q 9/02;
`HO4B17/02
`[52] us. (:1. ................................ ..455/33.1;455/54.2;
`455/62; 379/60
`[58] Field of Search ..................... .. 455/33, 34, 52, 53,
`455/54, 56, 62, 33.1, 33.2, 33.3, 34.1, 34.2, 52.1,
`53.1, 54.1, 54.2, 56.1, 62; 379/58, 59, 60, 63
`References Cited
`U.S. PATENT DOCUMENTS
`
`[56]
`
`3,898,390 8/1975 Wells ................................... .. 455/34
`
`4,392,242 7/1983 Kai . . . . . .
`4,670,905 6/1987 Sandvos et a1.
`
`. . . . .. 455/62
`.... .. 455/56
`
`4,833,701 5/1989 Comroe et a1. . . . . .
`
`. . . . .. 455/33
`
`4,850,033 7/1989 Eizenhofen et a1. . . . .
`. . . . .. 455/33
`4,876,738 10/1989 Selby ........................... .. 455/33
`4,903,320 2/1990 l-lanawa ...................... .. 455/54
`4,905,301 2/1990 Krolopp et al. .................... .. 455/34
`4,916,728 4/ 1990 Blair .................................... .. 455/33
`
`FOREIGN PATENT DOCUMENTS
`
`0126557 11/1984 European Pat. Off. .
`0283955 9/1988 European Pat. Off. .
`2069799 8/1981 United Kingdom .
`
`OTHER PUBLICATIONS
`“UK Trunking System Signalling Standard-Protocol
`Aspects”, Journal of the Institute of Electronic and
`Radio Engineers, vol. 67, No. 3, pp. 119-124 May/Jun.
`1987.
`Primary Examiner-Reinhard J. Eisenzopf
`Assistant Examiner-Lisa Charouel
`Attorney, Agent. or Firm-David R. Treacy
`[57]
`ABSTRACT
`A private mobile radio transmission system comprising
`a plurality of geographically spaced base stations (BS)
`defining a plurality of overlapping service areas (SA).
`The base stations are coupled for example by land-line
`communication links (CL) to a system controller (SC).
`Mobile stations (MS) equipped with simplex two-way
`radios are free to roam through the service areas. In
`order to be able to make a call with or via a base station
`it is necessary for the mobile station to be registered
`with the base station concerned. Details of the registra
`tion are stored by the mobile station concerned and the
`base station. In order to optimise on the usage of radio
`channels, details of a home base station are prepro
`grammed into each mobile station and when the mobile
`station wishes to register with a base station, it is in
`structed to try to register firstly with its own base sta
`tion, on the basis that the majority of calls will be to and
`from the home service area and will require only one
`pair of frequencies, secondly with a base station of any
`other service area in its region, and thirdly with a base
`station of a service area in any other region. When
`registered with a service area other than its home one,
`the mobile station checks regularly to see if it is possible
`to register with its home service area.
`
`17 Claims, 2 Drawing Sheets
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`MSl
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`SAZ
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`US. Patent
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`Jan. 4, 1994 '
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`Sheet 1 of 2
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`5,276,905
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`Fig.7.
`
`MS!
`
`Fig.2,
`T§ANSMITTER
`
`STORAGE
`STORAGE
`>20 ( MAS 212 SM 51.2 $3
`mm’ M) /
`(1-11.;
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`HUI
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`US. Patent
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`Jan. 4, 1994
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`Sheet 2 of 2
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`5,276,905
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`Fig.3.
`
`HOME
`SERVICE
`AREA\
`
`OUTSIDE
`6_z.
`
`I |
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`EXAMINE
`HUNT LIST
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`OORRESPONDS TO —-\.
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`
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`REGISTER]
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`
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`
`IMPROVEMENT
`
`I’ 80
`
`
`
`83 El‘ PREIICC NO
`RETURN T0
`\SWITCH TO LAST
`HOME CHANNEL
`
`'NREG I STER
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`1
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`MOBILE RADIO TRANSMISSION SYSTEM
`USING PREFERRED HOME SERVICE AREA
`
`5
`
`40
`
`BACKGROUND OF THE INVENTION
`The present invention relates to a mobile radio trans
`mission system and to a mobile station for use in the
`system.
`Such a system typically comprises a plurality of base
`stations situated at respective geographical locations to
`de?ne a plurality of service areas within an overall
`region and a plurality of mobile stations capable of
`communicating by two-way radio with each base sta
`tion when in communication range of that base station.
`Each base station comprises the entirety of transmitters
`and receivers operated by a trunking system controller
`(T SC) at any one site or service area. The communica
`tion link established is two-frequency simplex, that is
`one frequency. is allocated to forward transmission from
`a base station to a mobile station and a second frequency
`is allocated for return transmission from a mobile station
`to a base station. Blocks of radio channels chosen from
`an overall band of channels are allocated to each base
`station. In the case of trunked radio, the base stations of
`a region may be connected by way of land line to a
`control station (CS) or may be connected as a mesh
`with the base stations interconnected with each other.
`There may be two or more regions in which case each
`region has its own CS which controls a plurality of
`associated base stations. The CSs are connected to
`gether by land lines.
`In a known such system each mobile station is ar
`ranged when operative to register with a base station of
`which it is within communication range and thereby
`with the corresponding service area, and to respond to
`it being no longer within communication range of a base
`station serving a service area with which it is currently
`registered by registering with another base station of
`which it is within such range (if any) and thereby with
`the corresponding service area. The base station re
`sponds to the registration request by storing the identity
`of the service area corresponding to the relevant base
`station in that one of its location records which corre
`sponds to the registering mobile station. The registra
`tion procedure is carried out so that if it is desired to call
`a particular mobile station it is known in which service
`area that mobile station is currently located; the CS
`arranges that the call is directed to the or each base
`station which serves the relevant service area, which
`base station in turn calls the relevant mobile station. An
`advantage of a registration procedure is that if there was
`no provision for registration then all the base stations of
`the system would have to call the particular mobile
`station, which would impose a considerable overhead
`on the transmission capacity of the system. However, a
`disadvantage with this known registration procedure is
`that if a mobile station is roaming near a border between
`two or more service areas so that it frequently moves
`between areas it will frequently change its registration
`from one service area to another, again imposing a con
`siderable overhead on the communication channel ca
`pacity of the system.
`European Patent Speci?cation EP-A2-0260763 dis
`closes a multiple registration method aimed at reducing
`the transmission overhead associated with the above
`described system. This multiple registration method
`enables the TSC to store a list of up to n registrations for
`each mobile. The mob'iles each have storage facilities
`
`5,276,905
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`for storing the same list of registrations as are stored by
`the TSC in respect of itself. As a mobile enters a new
`service area it automatically registers with the area’s
`base station thus updating the list of registration stored
`by the mobile and its associated TSC. By storing these
`registrations an enlarged service area is created. If a
`TSC wishes to contact a mobile it refers to the list of
`registrations which it is storing for that mobile and
`initially transmits a signal to the base stations which are
`registered, either successively, beginning with say the
`most recent registration, or simultaneously. If this is not
`successful then the TSC tries the other base stations.
`Provision is made for reordering the list on the basis
`that the channel number and frequencies of the service
`area in which a mobile last made a call are at the top of
`the list. In the event of the list of registrations not being
`updated and/or a call not being made to or received
`from a mobile after a set period then it is possible for
`registrations to be cancelled one at a time thereby re
`ducing the size of the enlarged service area. While it is
`possible for all the registrations to be cancelled, it is
`preferred that at least one registration of a service area
`remain to prevent a ?ood of registrations at the start of
`a working day. If, at the start of a working day, a mobile
`is not registered in the service area in which it is cur
`rently located it has to arrange to immediately register.
`This method is quite workable but does have a draw
`back in that it will not necessarily optimise the usage of
`radio channels.
`This drawback will be better understood by consider
`ing a hypothetical case of a circular geographical region
`of about 128 km with 7 base stations serving it. Obvi
`ously hilly areas tend to mask some mobiles from some
`base stations, and the signal strength obtained by a mo
`bile from a base station varies inversely with the dis
`tance to the base station. The propagation map allows a
`generous margin of signal strength so that most mobiles
`can “see” most base stations.
`The frequencies currently used in Great Britain for
`trunked private mobile radio systems are around 200
`MHz, which are very penetrating and give good cover
`age, especially since the mobiles are extremely sensitive.
`The power and number of radio channels ?tted per
`base station are carefully chosen to correspond to the
`expected activity local to that base station.
`The radio protocol used, for example a subset of
`MPT l327-A Signalling Standard for Trunked Private
`Land Mobile Radio Systems issued by the Department
`of Trade and Industry, London 1988, effectively re
`quires the mobiles to hold on to a registration as long as
`they can. The intention is, of course, to reduce the over
`head of registration signalling on the network, which
`supplies no calls to the user.
`Furthermore, the MPT 1327 protocol has a good
`error-correction layer, and so a signi?cant number of
`on-air bits can be lost before any particular signalling
`packet fails. In consequence a mobile will not attempt to
`re-register on another base station unless the packet
`level error rate on his present one is high. However on
`the fringe of the coverage area this can lead to a poor
`grade of service both in terms of speech signal-to-noise
`and in call set-up failure rate.
`The result of the above set of circumstances is to
`change the radio topology of the network from being a
`number of largely isolated cells with little overlap, re
`sulting in the registration map largely corresponding to
`the geographical position of the mobile, to one in which
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`the cells very greatly overlap, causing the registration
`map to be largely random. Each base station has a num
`ber of mobiles registered on it proportional to its radio
`coverage, and these mobiles are distributed over the
`part of the region in which this can be “seen”. Each
`mobile has only a small chance of being registered on
`the base station where it is most likely for its calls to be
`satis?ed by that base station alone, which base station
`may be regarded as the mobile’s home site or home
`service area.
`The effect of this is that a high percentage of calls say
`80% are intersite calls, using 2 radio traf?c channels per
`call instead of one, the system capacity is almost halved,
`and the number of calls each base station must support
`bears little relation to the carefully estimated local pop
`ulation on which its size and power were based. This
`means that a base station gets a load proportional to the
`size of its radio coverage, not the number of channels
`?tted. This has the potential of grossly overloading the
`smaller service areas. Creating more service areas
`within the region by adding more base stations actually
`makes things worse, since it means that each mobile’s
`chance of being registered on its home site is propor
`tionally reduced.
`
`20
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`10
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`4
`other regions and attempt to register with one of them
`in the event of the mobile station not being able to regis
`ter with a service area in its home region.
`The control means may be programmed to check the
`list of channels in its storage means and if it is not using
`the control channel assigned to its home service area, it
`checks to see if the home control channel can be re
`ceived with adequate signal quality and if so the mobile
`station attempts to register with its home service area.
`If desired, each base station may be instructed to
`transmit the number of its current control channel and
`control channels of other service areas and at the mobile
`station the control means may be programmed to check
`that the system identity codes received corresponds to
`the channel numbers stored for those service areas and
`if not it updates the numbers stored. Such a measure if
`implemented reduces the acquisition time and makes
`more accurate the mobile station's method of acquiring,
`if possible, its home site.
`Each mobile station may store a parameter indicative
`of the signal quality of the respective control channels
`in the system, and in consequence the control means of
`a mobile station unable to register with its home service
`area may be programmed to take into account the signal
`quality as indicated by the parameter when seeking to
`register with another service area.
`According to another aspect of the present invention
`a mobile station, for use with the radio transmission
`system in accordance with the ?rst aspect of the present
`invention, comprises a transmitter, a receiver, control
`means for controlling the operation of the mobile sta
`tion and means for assessing the quality of a received
`signal. The control means is pre-programmed with de
`tails identifying a predetermined service area which is
`de?ned as its home service area, and is programmed to
`register the mobile station with its home service area in
`preference to any other service area provided that the
`signal quality between the home base station and the
`mobile station is acceptable.
`The present invention will be described, by way of
`example, with reference to the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 illustrates diagrammatically the geographical
`layout of a hypothetical radio transmission system,
`FIG. 2 is a block schematic diagram of an embodi
`ment of a mobile station,
`FIG. 3 is a flow chart of a registration procedure, and
`FIG. 4 is a flow chart of an algorithm for implement
`ing the channel hunting operation.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`In the drawings the same reference numerals have
`been used to indicate corresponding features.
`Referring to FIG. 1 the system comprises a plurality
`of base stations BS of which seven, BS1 to BS7, are
`shown, situated at respective geographical locations.
`Each of these base stations comprises an entirety of
`radio transmitters and receivers operated by a trunking
`system controller. The respective service areas SA1 to
`SA7 of these base stations overlap, as shown by the
`cross-hatching, to collectively cover the whole region
`shown. A plurality of mobile stations MS are provided
`of which three, M51, M52 and M53 are shown. Each
`mobile station is free to roam throughout the whole
`region, and indeed outside it. Each of these mobile
`stations also comprises a radio transmitter/receiver
`
`SUMMARY OF THE INVENTION
`An object of the present invention is to optimise the
`usage of radio channels in a mobile radio transmission
`system.
`According to one aspect of the present invention a
`radio transmission system comprises a plurality of base
`stations situated at respective geographical locations to
`de?ne a corresponding plurality of overlapping service
`areas constituting one or more regions, and mobile sta
`tions capable of communicating by two-way radio with
`any one of the base stations. Each mobile station com
`prises a transmitter, a receiver, and control means for
`controlling the mobile station, and each mobile station
`is pre-programmed with details identifying a predeter
`mined service area which is de?ned as its home service
`area. The control means is programmed to register the
`mobile station with its home service area in preference
`to any other service area provided that the signal qual
`ity between the home base station and the mobile station
`is acceptable.
`The present invention is based on the realisation that
`most calls will be made between a mobile station and a
`despatcher who is ?xed relative to the home base sta
`tion or between two mobile stations who are registered
`with the home base station. Thus if a mobile station
`stays registered with the home base station for as long as
`possible and is instructed to return to the home base
`station whenever possible, then the likelihood of mo
`biles which are still within communication range of the
`home base station, even though they are out of its de
`?ned service area, being involved in mobile to mobile
`calls by way of two base stations and requiring 2 traf?c
`channels is reduced. Thus one traffic channel is saved
`together with the overhead of signaling between base
`stations.
`If desired, the control means of each mobile station
`may be programmed to hunt through the other service
`areas in its home region and attempt to register with one
`of them in the event of the mobile station not being able
`to register with its home service area.
`.
`In the event of a mobile being unable to register in its
`home region, the control means of a mobile station may
`be programmed to hunt through the service areas in
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`which is capable of communicating with a base station
`transmitter/receiver when it is within satisfactory com
`munication range of that base station transmitter/
`receiver, and also comprises means for controlling vari
`ous operations. The system may furthermore comprise a
`system controller SC provided with a two-way commu
`nication link, CLl to CL7 respectively, to each base
`station BS1 to BS7. Each of these communication links
`may be, for example, a dedicated land-line. The system
`controller SC may, furthermore, be connected to the
`public switched telephone network (PSTN) to enable
`communication to take place between a mobile station
`M81 and a subscriber to that network, if desired. Alter
`natively the base stations BS1 to BS7 may be intercon
`nected by a mesh network.
`For convenience it will be assumed that the system
`operates in accordance with a signalling protocol as
`described in a paper by P. J. Mabey, D. M. Ball and D.
`J. Harrison entitled “UK Trunking System Signalling
`Standard-—Protocol Aspects” in the Journal of the
`IERE, May/June 1987, pages 119-124, although this is
`not necessarily the case. In accordance with this proto
`col the base stations BS each transmit on a substantially
`continuous basis a succession of data codewords on a
`“forward” (radio) control channel, these codewords
`normally being so-called control channel system code
`words (CCSC) and other codewords, transmitted alter
`nately. In the system of the present example a bit ?eld of
`each control channel system codeword transmitted by
`each base station BS constitutes an identi?er AREA of
`the particular service area SA served by the relevant
`base station. Thus, if a mobile station MS tunes its radio
`receiver to the control channel corresponding to a base
`station BS with which it is within communications
`range it will receive repeatedly a code indicative of the
`particular service area SA served by that base station.
`The “other” codewords, which each contain an address
`?eld, ‘may constitute messages addressed to all, particu
`lar ones, or groups of the mobile stations MS, each such
`mobile station being assigned a unique address which is
`stored therein for reference. Moreover, many of the
`codewords transmitted by each base station constitute
`an invitation to all, particular ones, or groups of mobile
`stations within communication range to transmit to the
`base station a message codeword on a “return” control
`45
`channel within a randomly chosen one of a number of
`immediately succeeding time slots, which number is
`contained in a further bit ?eld of the invitation code
`word. This number may be determined dynamically in
`accordance with a method described, for example, in
`GB-B-2 069 799. The message codeword transmitted by
`a given mobile station MS contains the unique address
`of that station and may constitute, for example, a re-.
`quest for, service. The signalling on the forward and
`return control channels may be employed, for example,
`55
`to set-up a communications channel between a pair of
`mobile stations via the base station(s) with which they
`are within communications range. The signalling on the
`forward channel is under the control of the system
`controller SC.
`In accordance with the present invention, each mo
`bile station MS is assigned a base station BS. The assign
`ment of a mobile station to a base station is done on the
`basis that it is anticipated that the majority of the calls to
`and from the mobile station will originate from within
`that service area. Accordingly the base station will be
`treated as the home base station and details of the con
`trol channel for that service area will be programmed
`
`6
`into non-volatile storage means in the mobile station
`and in an appropriate storage location of the system
`controller SC. Once this has been done then the mobile
`station is said to be registered. In the illustrated example
`the mobile stations M81, M82 and M53 will be assumed
`to be registered with “home” base stations BS1, BS4
`and BS6, respectively. Details of the control channels of
`all the service areas in the region are stored in the sys
`tem controller SC together with the corresponding
`details of service areas in other regions.
`Considering mobile station M81 as an example, when
`it roams around its home service area SAl, calls to and
`from other mobiles also registered with the base station
`BS1 as their home base station make use of one channel
`pair of frequencies because the communications link is a
`simplex one. However a call which is set-up between
`say mobile stations M51 and M82 requires the alloca
`tion of one traffic channel pair of frequencies for com
`munication between mobile station M81 and base sta
`tion BS1 and a second traffic channel pair of frequencies
`for communication between mobile station M82 and
`base station BS4.
`When the mobile station MSl roams out of its service
`area SAl in which it is registered, instead of automati
`cally registering with the service area it has entered, it
`maintains communication with its home base station
`until for example the packet-level error rate on its pres
`ent channel pair is unacceptably high or some other
`measure of signal quality becomes unacceptable. Thus
`for example it is able to roam over an enlarged service
`area of radius R without having to register with another
`service area.
`e
`Once the signal quality becomes unacceptable, for
`example because the packet-level error rate is unaccept
`ably high, then it is necessary for the mobile station to
`register with another service area. In order to reduce
`the hunt for another service area, the mobile station also
`has in its non-volatile memory an ordered list of fre
`quencies and their associated service area identities. The
`first entry is the home base station and this is followed
`by all the other service areas in the home region, fol
`lowed by those service areas inother regions which
`have been acquired in the mobile station’s travels. The
`mobile station also knows the frequency and service
`area identity of the control channel which it is at pres
`ent using. All the information in this list, apart from the
`identity of the home base station is acquired dynami
`cally from the network.
`Referring to FIG. 2, the mobile station MS comprises
`a transmitter portion 10, a receiver portion 12 and a
`digital computer system 14. The computer system 14
`includes, in addition to the conventional processing
`unit(s) 16 and program store 18, data storage means 20,
`22 and 24, timers 26 and 28 and a real-time clock 30. A
`serial data output port 32 of the computer system 14 is
`connected to a modulation signal input 34 of the trans
`mitter portion 10 and a demodulated signal output 36 of
`the receiver portion 12 is connected to a serial data
`input 30 of the computer system 14. A parallel data
`output port 40 of the computer system 14 is connected
`to tuning control inputs 42 and 44 of the transmitter and
`receiver portions 10 and 12 respectively. The output 46
`of a received signal-level threshold detector 48 included
`in the receiver portion 12 is connected to a further
`signal input 50 of the system 14.
`Storage means 20 stores a list of all the radio channels
`employed in the communications system, each “for
`ward" channel being stored in conjunction with the
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`secondly-—~any service area in the home region, op
`corresponding “return” channel. Storage means 22
`tionally the service area with the best signal, and
`stores the address of the relevant mobile station MS.
`thirdly-any other service area, optionally the ser
`Storage means 24 constitutes a location record, service
`vice area with the best signal.
`area identities being stored in the respective ?elds 521 to
`This is summarised by FIG. 3 in which the blocks 60,
`52;. Fields 541, 542 and 543 of each ?eld 52 store the
`62 and 64 respectively represent the operations regis
`actual service area identity, a control channel number
`tered on the home service area, registered in home
`and a time respectively.
`region and registered outside the home region. The
`The computer system 14 is programmed to decode
`arrow 66 represents the state of the home service area
`the digital words applied by receiver portion 12 to its
`signal being unusable and a mobile station trying to
`input 38 and suitably react if appropriate, and also to
`register with another service area in the home region.
`format digital words and apply them to its output 32 for
`The arrows 67, 68 both represent the situation of the
`transmission by the transmitter portion 10 when appro
`mobile station not being able to register in any home
`priate. (The manner in which this decoding and format
`region service area and trying to register with a service
`ting is carried out is conventional, and will not be elabo
`area outside the home region.
`rated upon here as it is irrelevant to the present inven
`The arrows 69 and 70 represent the mobile station
`tion.) It is also programmed to apply to its parallel out
`checking to see if the home service area is usable, this
`put 40 a digital word for tuning the transmitter and
`checking taking place at regular intervals, say every 3
`receiver portions to the return and forward channel
`minutes, or when the present control channel becomes
`frequencies respectively corresponding to any of the
`unusuable. Finally the arrow 71 represents the situation
`channels stored in store 20.
`when the mobile station has found that the home service
`As mentioned above the base station of each service
`area is unusuable but is checking to see if another ser
`area broadcasts on a regular basis on its control channel
`vice area in the home region is available. Such checking
`all the service area identities of its region with their
`is carried out at regular intervals, say every 3 minutes or
`associated control channel frequencies, plus those of at
`when the present control channel becomes unusable.
`least some of the adjacent regions. Since each of these
`An algorithm relating to an implementation of at least
`messages contains only one service area and control
`some of the operations described with reference to FIG.
`channel number, many such messages will have to be
`3, will now be described with reference to FIG. 4. FIG.
`sent, though the repetition rate can be low, perhaps by
`4 assumes that the system is using the MPT 1327 proto
`cycling through a list of ten or twenty messages over a
`col mentioned in the preamble of this speci?cation.
`period of a half hour. Any mobile station on reception
`This algorithm can be summarised by saying that if
`of one of these messages will use it to update the appro
`the home service area is available and is acceptable for
`priate entry in its list of site identities and control chan
`use when the mobile station uses it in preference to
`nel frequencies.
`others. When the home service area is not available,
`Whenever a mobile station is not engaged in a call it
`then additional criteria to be added to the mobile sta
`will periodically, perhaps every 3 minutes, check its list.
`tion’s reasons for leaving a control channel so that
`If its present control channel is not the one belonging to
`within certain limits, the service area with the strongest
`the mobile station’s home service area, it will check to
`base station may be acquired.
`see if it can receive its home service area’s control chan
`The channel number relating to the home service
`nel, and if it can and the signal quality is satisfactory,
`area will be stored in the mobile station’s non-volatile
`then it will attempt to register on it. If the mobile station
`memory whenever it registers on a channel where the ‘
`is currently on its home service area’s control and the
`control channel system codeword (CCSC) corresponds
`signal is satisfactory, it will take no action.
`to its home service area data.
`Whenever a roaming mobile station is at present reg
`A ?eld, relating to the home data within the CCSC is
`istered with a service area outside its home region, it
`stored as a customisable parameter within the mobile
`will attempt to register within its home region even if it
`station’s read only memory. The control channel relin
`cannot receive signals from its home service area. It
`quishing criteria will be split into that required for the
`may do this by selecting the list of control channels
`home service area and that for any other service area.
`associated with the service areas forming the home
`With respect to the home service area these criteria will
`region and cycle through this list in an attempt to find a
`be as currently employed, for example bad signalling or
`usable control channel amongst them.
`excessive error rate.
`In order to reduce the risk of missing an incoming call
`The algorithm in FIG. 4 assumes that the mobile
`a mobile station may optionally wait for a withdrawn
`station is registered on a service area other than the
`slot in the present control channel’s signalling stream to
`home service area, the mobile station, following every
`look at the home service area. The network may option
`20 seconds of operator inactivity and no involvement in
`ally use the “vote now” message to withdraw slots for
`signalling, will momentarily switch to one channel
`this purpose.
`(channel “n”) within its normal hunt list and examine
`If the present control channel being used by a mobile
`that channel as a possible new control channel, opera
`station is unsatisfactory (poor error rate, low signal
`tion 72.
`level) the mobile station will attempt to register on
`The field strength is monitored, operation 73, and if
`another control channel in the order of preference
`the ?eld strength is less than —107 dBm the mobile
`station will reject the channel, operation 74, and return
`given in its list. The mobile station may have as a param
`eter in the list a measure of the quality of the control
`to the control channel it ?rst left. In this case, following
`another 20 sec. period, the mobile station will then try
`channels and may use this measure in order to bias
`registration attempts towards a good quality signal.
`the next channel in the normal hunt list (