CA1276681C - Mobile radio transmision system - Google Patents

Abstract

ABSTRACT
RADIO SYSTEM
A mobile radio transmission system comprises a plurality of base stations (BS) with corresponding service areas (SA) which together cover a geographical area through which mobile stations (M) can roam. Each base station is connected to a central station (CS) by means of a respective two-way communication link (CL). In order that it can be contacted without all the base stations having to put out a call, each mobile station registers in known manner with the base station in whose service area it is situated, and this is recorded by the control station. When it roams into a new service area this process is repeated. Houever, in order to preclude frequent re-registrations when the mobile station repeatedly crosses the boundary between two service areas, when it registers with a new base station and hence a new service area it retains its registration with at least one service area with which it is already registered, thereby creating its own customised enlarged registration area. Thus the transmission overhead created by the registration procedure is reduced.

Description

~ 127~681 PH~ 33299 DESCRIPTION
RA~IO SYST EM
This invention relates ~o a mob;le rad;o transmiss;on system compris;ng a plural;ty of base stat;ons s;tuated at respective geograph;cal locations to thereby define a plurality of service areas and a plurality of mob;le stat;ons capabLe of communicating by two-way radio w;th each base stat;on when in communication range of that base stat;on, the system be;ng provided with control means which ;ncludes a location record correspond;ng to each mobile station, wh;ch locat;on record is capable of storing a serv;ce area ;dent;ty~ The ;nvention also relates to a mob;le stat;on for use ;n such a system.
In kno~n such systems each mob;le station ;s arranged when operative to reg;ster 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 w;thin commun;cation 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 w;thin such range (;f any) and thereby with the correspond;ng service area. The control means responds to each registration by storing the identity of the service area corresponding to the relevant base stat;on ;n that one of its location records which corresponds to the registering mobile station. The registration procedure is carried out so that if it is desired to call a particular mobile station it is known in ~hich service area that mobile stat;on is currently located; the control means arranges that the call ;s directed to the or each base station which serves the relevant service area, which base station in turn calls the relevant mobile station. If there were no registration procedure 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.
There is, however, a disadvantage with the known registration procedures in that if a mobile stat;on is roaming near a border between two or more service areas so that it frequently moves 1~668~ PHB 33299 between areas it w;ll frequently change its registration from one service area to another, aga;n impos;ng a cons;derable overhead on the communication channel capac;ty of the system. Th;s problem can be aggravated due to local obstruct;ons, for example gas holders and large bu;ldings, which can cause received signal levels to vary m;nute by minute. It is an object of the invention to mitigate this disadvantage.
The invention provides a mobile rad;o transm;ss;on system comprising a plurality of base stat;ons ~h;ch are s;tuated at respective geograph;cal locations to thereby def;ne a plurality of service areas and a plurality of mobile stations capable of communicating by two-way radio with each base station when in communication range of that base station, the system be;ng provided with control means which ;ncludes a location record corresponding to each mobile station, which location record ;s capable of stor;ng a service area identity, characterised in that each location record is capable of storing at least two service area identities, in that each mobile station also includes a location record capable of storing at least two service area identit;es and is arranged when operative to respond to it not be;ng within communication range of any base station serving a service area the identity of which is current!y stored in the location record included therein by commun;cat;ng with a base station of which it is within such range ~if any) and storing the identity of the service area served by that base station in said location record in such manner that said ;dentity replaces the identity of another servlce area already present in said location record if the resulting number of stored identities would otherwise exceed a given value n greater than one, and in that said control means is arranged to respond to each such communication by updating the contents of that locat;on record included therein which corresponds to the communicating mobile station so that the mobile station ;s recorded as being currently located in the service area served by the base station with which the communication takes places while retaining ;n said location record the ;dentity or ;dent;ties ~;f present) of at least ~n-1) ~Z7668~ PHs 33299 other serv;ce area(s~ in wh;ch the relevant mobile station was prev;ously recorded as being located. (The replacing of the ;dentity of another serv;ce area need only be ;mpl;c;t; the sa;d ;dent;ty may cont;nue to be phys;cally present but in an ;nval;dated form, the ;nval;dity being ;ndicated, for example by means of a flag b;t).
Such a system enables each mob;le stat;on ;n effect to create its own custom;sed enlarged reg;strat;on area wh;ch consists of n serv;ce areas at most and movement w;th;n which does not require new reg;strat;ons and the transm;ssion overhead that would enta;l.
Th;s of course may be at the expense of ;ncreased transmiss;on overhead when it is desired to call a given mobile stat;on because more than one base stat;on w;th;n the enlarged area may have to be activated to locate the mob;le stat;on, but ;n many c;rcumstances th;s increased overhead will be more than offset by the reduction in the overhead required for the actual registration procedure. In general, the enlarged reg;strat;on area w;ll be opt;mum in th;s respect ;f the replaced ;dent;ty, i.e. the ident;ty of the serv;ce area which ceases to be part of the enlarged registration area when a new serv;ce area is added to the enlargPd service area, is that of the service area w;th;n wh;ch the relevant mob;le stat;on has commun;cated with a base stat;on least recently of aLl those serv;ce areas the ;dent;ties of wh;ch are stored ;n ;ts locat;on record. The commun;cat;ons taken ;nto account ;n th;s and the follow;ng respect may be, for example, only those ;n;t;ated by the relevant mob;le stat;on, or may for example also include those initiated elsewhere.
Preferably each mob;le stat;on is arranged, each t;ne it stores a service area identity in the location record included therein, to retain in said location record at least the identity (if present) of that service area with a base station serving which it has otherwise most recently communicated, and the control means is arranged, each time it carries out a said updating, to retain in the relevant location record the identity (if present) of at least that service area with a base station serving wh;ch the relevant 1~76681 PHB 33299 mobile station has other~ise most recently communicated. If this is so the problem of frequent re-registration when a mobile station ;s roaming near the border bet~een two service areas referred to hereinbefore can be successfully overcome.
When a call is requested to a given mob;le station the control means could of course act;vate all the base stations serving the relevant mobile stat;on's enlarged reg;strat;on area to call that mobile stat;on s;multaneously. However ;t may be advantageous from the po;nt of v;e~ of reduc;ng s;gnall;ng overhead ;f the base stat;ons serv;ng the respect;ve serv;ce areas are act;vated ;n success;on unt;l the mob;le stat;on ;s found. In the latter case ;t ;s preferable ;f the order of success;on ;n wh;ch the base stat;ons are act;vated ;s chosen to be such that the base stat;on(s) serv;ng the serv;ce area ;n wh;ch ;t ;s most Likely that the mob;le stat;on is located ;s/are act;vated f;rst, then ;f the mob;le stat;on does not respond, the base stat;on(s) serv;ng the serv;ce area in wh;ch the mob;le stat;on ;s next most l;kely to be located is/are act;vated next, and so on. An est;mate of the relat;ve l;kel;hoods of the mob;le station be;ng located ;n the var;ous serv;ce areas ~h;ch make up ;ts enlarged serv;ce area can be made on the bas;s of the comparative lengths of t;me wh;ch have elapsed s;nce commun;cat;ons last occurred between ;t and a base stat;on serving each of the respective said service areas, it being assumed that the mobile station is most l;kely to be located in the serv;ce area in wh;ch a commun;cation last occurred, ;t being next most likeLy to be located ;n that one of the other serv;ce areas ;n wh;ch a commun;cat;on next most recently occurred, and so on.
Preferably therefore the control means ;s arranged to ma;ntain, for each locat;on record ;ncluded there;n, a record of the comparat;ve lengths of the t;mes wh;ch have elapsed s;nce a commun;cation last occurred bet~een the relevant mobile station and a base station serv;ng each respective serv;ce area whose ;dent;ty ;s currently stored ;n that location record. Each such record of the comparat;ve lengths of the times may conveniently be in the form of a record of the absolute time at which a commun;cation last 12766~1 pHs 33299 occurred between the relevant mobile station and a base station serving each said respective service area. Again the communications taken into account in this and the following respect may be, for example, only those initiated by the relevant mob;le stat;on, or may for example also include those ;nit;ated elsewhere.
Preferably too each mobile stat;on ;s arranged to maintain a record of the comparat;ve lengths of the times ~h;ch have elapsed s;nce a communication last occurred between the relevant mobile station and a base stat;on serv;ng each respective service area whose ;dent;ty ;s currently stored ;n the location record ;ncluded therein. Each such record of the comparative lengths of the times, which again may conveniently be in the form of a record of the absolute time at which a commun;cation last occurred between the relevant mobile station and a base station serving each said respective service area, will facilitate the determination of the aforesaid identit;es of the service areas with base stations serving which the mobile stat;on has commun;cated most recently and least recently.
As mentioned above, the benefits of the customised enlarged registration area effectively created for each mobile station are offset at least to some extent by the increased signalling which may be required when a given mobile station is called, because more than one base station within the enlarged area may have to be employed in order to contact the mobile station. Preferably, therefore, the enlarged area is not too large, and its size can be tailored to some extent to the behaviour of the corresponding mobile stat;on if each mobile station is implicitly de-reg;stered with respect to a given service area should communication not occur between that mob;le station and a base stat;on serv;ng that area for a predetermined time. Thus, preferably, each mobile station and the control means is arranged to delete any service area identity stored in a said location record included therein should the time which has elapsed s;nce a communication last occurred between the relevant mobile station and a base station serving the 1276~8~ PHB 33299 relevant service area exceed a predeterm;ned value, at least ;f at least one service area ;dentity w;ll rema;n stored ;n sa;d locat;on record after the de(etion. If this is the case then, ;f desired, each mob;le stat;on and the contro~ means may also be arranged, ;f one service area identity remains stored in said location record, to also delete that identity should the t;me ~h;ch has eLapsed since a communication last occurred between the relevant mobile stat;on and a base station serv;ng the relevant serv;ce area exceed a predeterm;ned value wh;ch ;s larger than the other said predeterm;ned value(s). Th;s also enables fut;le s;gnall;ng to a mob;le station wh;ch has ;n fact roamed completely out of the overall serv;ce area of the system or has simply switched off to be reduced. ~The deletions may only be ;mpl;c;t; the relevant identities may cont;nue to be physically present but ;n an ;nval;dated form, the invalid;ty be;ng ind;cated, for example, by means of a flag b;t.) Preferably the base stations are arranged to transmit sa;d g;ven number n to each mob;le stat;on. Transm;tt;ng the g;ven number n to each mobile station enables the max;mum s;ze of the enlarged reg;stration area effectively created for each mob;le stat;on to be determined under central control, and changed ;f des;red to be opt;mum under current cond;t;ons.
An embod;ment of the invent;on ~;ll now be descr;bed, by ~ay of example, w;th reference to the accompany;ng d;agrammat;c draw;ngs, ;n ~h;ch F;gure 1 shows the overall geograph;cal layout of the embod;ment;
F;gure 2 ;s a block d;agram of relevant parts of a mob;le stat;on;
F;gures 3, 4, 5, 6, 7 and 8 are flow charts ;llustrat;ng the operation of the mob;le stat;on of F;gure 2;
F;gure 9 ;s a block d;agram of relevant parts of a control station; and F;gure 1~ ;s a flow chart illustrat;ng the operat;on of the control station of F;gure 9.

PH8 332~9 The drawing sho~s diagrammatically the overall geograph;cal layout of a mobile radio transmission system in accordance with the invention. The system comprises a plurality of base stations 8S of which seven, BS1 ... BS7, are shown~ s;tuated at respect;~e geographical locations. Each of these base stations comprises a radio transm;tter/receiver and the respective service areas SA1 ... SA7 of these overlap, to collectively cover the whole area shown. (The service areas are in fact shown as non-overlapping hexagons, for convenience of drawing). A plurality of mobile stations M of which three, M1 ... M3, are shown, is free to roam throughout the whole area, and indeed outside it. Each of these mobile stations also comprises a radio transmittertreceiver which is capable of communicating with each base station transm;tter/
receiver when it is within satisfactory communication range of that base station transmitter/receiver, and also means for controlling various operations. The system furthermore comprises control means in the form of a control station CS provided with a two-way communication link, CLt ... CL7 respectively, shown in dashed lines, to each base station BS. Each of these communicat;on l;nks may be, for example, a dedicated land-line. The control stat;on CS
may, furthermore, be connected to the pubLic switched telephone network ~PSTN) to enable communication to take place between a mobile station M and a subscriber to that network, if desired.
~ hen a mob;le station M requires to ;nitiate a call, either to another mobile station or to a subscriber to the PSTN, the initial procedure is straightforward. The station M first radios its request to a base station 3S within communication range of which it is currently located. Thus, for example, referring to F;gure 1, mobile station M1 will radio its request to base station 3S1 whereas mobile station M2 will radio its request to base stat;on es4. Mob;le station M3 will radio its request to either base station 8S5 or base station 8S6. The receiving base station in turn transm;ts the request to the control station CS over the relevant communication link CL, and the control stat;on responds by controlling the sett;ng up of the call.

lZ76681 PH~ 33299 If a call is to a mobile stat;on M, either from another mobile stat;on or ~rom a PSTN subscriber, the called mobile station has to be alerted. If ;t were not known w;th;n which part of the overall geographical area the called mobile stat;on ;s located th;s could be done by the control stat;on CS request;ng all the base stations BS to radio the relevant request~ either simultaneously or sequentially, with all the transmission overhead that would entail. To reduce this overhead, therefore, a registration strategy ;s employed. Each mobile station M communicates and thereby registers with a base station BS within which it ;s within communications range, and thereby with the correspond;ng serv;ce area SA, and the relevant base station transmits this fact to the control gtation CS which stores it, so that when a call is in;tiated to a given mobile station M control station CS knows within which part of the overall area the call has to be transmitted, enabling the transmission to be confined within this part. tIn F;gure 1 station M1 will have registered with service area SA1, station M2 with service area SA4 and stat;on M3 with service area SA5 or serv;ce area SA6). If a mob;le stat;on M
should subsequently roam into another service area SA it may eventually become no longer in commun;cation range of a base station serving a service area with wh;ch it is currently registered. Its response to this is to communicate and hence register with another base station BS of wh;ch it is then within such range (if any) and thereby with the corresponding service area SA. (The communication may or may not take the form of a speclfic reg;stration request message; any communication can in fact be arranged to serve as a registration request if desired). Thus, referring to Figure 1, mobile station M1 may roam into service area SA2 and eventually cease to be within communication range of base station BS1. ~It will, however, no~ be uithin communication range of base station BS2). Its response to th;s, if and only if it is not already also registered uith service area SA2, ;s to register with base station BS2 and hence with serv;ce area SA2, and base station BS2 transmits this fact over link CL2 to control station ~Z766~ PHB 33299 CS, which stores ;t in a location record corresponding to the relevant mobile station, so that control station CS knows the modification wh;ch has to be made to that part of the the overall area within which a call to the mobile stat;on M2 has to be transmitted~ The operat;ons descr;bed so far are ;n accordance h;th known techn;ques. However, ;n contrad;st;nct;on to the kno~n techn;ques, when mob;le stat;on M1 reg;sters w;th service area SA2 ;t rema;ns reg;stered w;th at least one other serv;ce area SA w;th ~hich it is already registered (if any). This may be, for example, the serv;ce area SA1, in which case both mobile station M1 and control stat;on ~S reta;n ;n the;r locat;on records the information that station M1 is re~istered w;th serv;ce area SA1 in addition to the information that stat;on M1 is now also reg;stered ~ith service area SA2. Now when a call ;s requested to stat;on M1 control stat;on CS e;ther instructs both base stat;on 8S1 and base station 8S2 (at least) to transm;t th;s call s;multaneously or instructs one of these stat;ons, preferably that one with which M1 most recently communicated, to transmit the call, and then the other of these stations to transmit the call if there is no response to the first. This means of course that sometimes the call may have to be transmitted by more base stations than would have to do so ;n the known systems employing reg;strat;on, but even so the number of base stat;ons transmitting the call can still be considerably less than the total, the number of service areas with which each base stat;on BS can be registered being limited (see below). However, now if mobile station M1 should roam back ;nto service area SA1 and eventually cease to be in communicat;ons range of base stat;on 3S2 ;ts response to this will be to take no action, because ;t is now ~ithin communicat;ons range of base station BS1, and ;t is st;ll registered with the correspond;ng serv;ce area SA1. Thus the transmiss;on overhead which would be ;ncurred in the kno~n systems employing registration by re-registration with serv;ce area SA1 is dispensed with; mobile station M1 has effectively created for ;tself a custom;sed enlarged registration area comprising the serv;ce areas SA1 and SA2. The effect of th;s ;s particularly 127~681 PH~ 33299 benef;cial ;f the mobile station M1 should repeatedly roam across the border between ~he serv;ce areas SA1 and SA2.
As mentioned above, the maximum number n of service areas SA
with ~hich each mobiLe station M can be registered at any g;ven time is limited. (If desired, this number may be broadcast to each mobile station by all the base stations BS, or to each mobile station each time it registers with a service area, under the control of control station CS, so that it can be changed at any given t;me if cond;t;ons warrant ;t.) Each mobile station M stores the ident;t;es of the service areas in which ;t has made reg;stration requests, and information about their chronological order, in a location record and, if registrat;on of a g;ven mob;le stat;on with a new serv;ce area SA would result ;n the number of stored ;dentit;es exceed;ng n, the "oldest" stored ;dentity ;s deleted, i.e. the mobile station is ;mplicitly (there be;ng no actual transm;ssion of this fact by the mobile station) deregistered with respect to the service area whose identity ;s deleted. The "chronological order" ~ill be basically the order in which the mobile station registers with each service area, but modified if necessary to take into account the occurrences of communications between the mob;le station and base stat;ons serv;ng the various service areas w;th which it is registered. Each time a communication occurs between the mobile station and a base station serving a particular service area the ;dent;ty of that serv;ce area ;s promoted to be treated as the most recent in the chronolog;cal order. In the present example the informat;on about the chronolog;cal order is stored ;n the form of the actual t;mes of the respective reg;strat;ons, wh;ch t;mes are where appropriate replaced by the actual t;mes at wh;ch subsequent commun;cat;ons occur. Control station CS stores in the location record corresponding to the mobile station information similar to, and ideally identical to, the information stored in the mobile station's own location record.
If there ;s no commun;cat;on between a g;ven mobile station M
and a base station ~S serving a service area SA with which the 127668~ PH~ 33299 mobile station ;s registered for a long time it can be beneficial to the system as a whole in respect of transmission overhead to assume that the mobile station is unlikely for some time to roam back into that service area. To this end, each mobile station M ;s arranged to delete from its serv;ce area registrat;on record the ;dent;ty of any service area for which no communication between the mobile stat;on and a base station serving that area has occurred for a predeterm;ned time (which predetermined time may or may not be the same for each sa;d identity), at least if at least one identity w;ll remain stored after the deletion. In other words, in such a situat;on the mob;le station is ;mplicitly (there again being no actual transmission of this fact by the mobile station) dereg;stered with respect to the relevant service area automat;caLly, thereby reduc;ng the size of its own registrat;on area. This feature can also be benef;c;al if a given mobile station roams outside the overall geographical area covered by the serv;ce areas SA for some reason. In these c;rcumstances it may eventually cease to be w;th;n commun;cations range of any of the base stations ~S (in wh;ch case ;t w;lL of course fa;l to register with a new serv;ce area SA)~ If th;s situat;on should persist it will eventually also dereg;ster ;n turn with respect to at least all but one of the serv;ce areas SA w;th ~h;ch ;t is already registered, wh;ch ;s obviously des;rable because it cannot be reached by a call anyway. (It ;s preferable although not essential to retain reg;strat;on with one serv;ce area SA, at least for a t;me which ;s substant;ally longer than the predetermined t;mes after which the ;dent;ties of the other service areas are deleted, for example to avoid a flood of reg;strat;on requests at the start of the working day). If a service area w;th respect to which the mobile stat;on deregisters ;s in fact the one in which it is currently located it will obv;ously have to be arranged to immediately re-register.
As mentioned above, control stat;on CS contains a location record correspond;ng to each of the mob;le stations M, and each time a given mobile station registers w;th a new serv;ce area the iZ7~6~1 PH~ 33299 relevant record is updated ;n response to the result;ng ;nformation transm;tted to the control stat;on by the relevant base stat;on 8S, so that ;deally at least part of each location record at the control station always corresponds to the record at the relevant mobile station. Ma;ntain;ng this correspondence is possible, at least in theory although in practice some lack of correspondance may ar;se due to incomplete communicat;ons, because ;t ;s known at the control station what the other criteria are which govern the updating of the records at the mobile stat;ons tthe number n, the chronolog;cal order, the mod;ficat;ons to the chronological order, the deletion of identities after a predetermined time at least if at least one will remain) and also ~hen each mob;le station M has communicat;ons with a particular base statipn BS. If the control stat;on deletes an identity because no communication has occurred between the relevant mob;le station and a base stat;on serving the relevant area for a predeterm;ned t;me ;t is preferably arranged that th;s occurs sl;ghtly after the correspond;ng deletion at the mobile stat;on, to enable the mobile station to reregister, if it is in fact located in the relevant service area, before the delet;on actually occurs at the control stat;on. Otherh;se there could be short period of t;me when ;t appears at the mob;le stat;on that ;t ;s reg;stered but the control stat;on ;s ;n fact fa;ling incom;ng calls for that mobile stat;on.
When a call ;s requested to a given mob;le stat;on M, the control station CS may, for example, control each base station ~S
wh;ch serves an area SA with which the mobile station M ;s currently registered to call the mobile station simultaneously.
This will obviously minimise the time taken to actually locate the relevant mob;le station. ~owever it may be advantageous from the po;nt of v;ew of m;nimising the s;gnall;ng overhead to ;nstead arrange that the control stat;on controls these base stat;ons to call in succession unt;l the mob;le station responds. In the latter case the succession preferably corresponds to the reverse of the chronoLogical order, the base stationts) serving the area SA in which the relevant mobile station has last communicated with a base lZ7668~ PH~ 33299 station being controlled to call first.
Although each serv;ce area ;n the draw;ng is shown as being served by a single base station 8S this ;s not necessarily the case. Each serv;ce area may be served by more than one such base station, which base stations may or may not transm;t identical information.
Each mobile station M may determine whether or not it is within communication range of a base station by, for example, monitor;ng the siynal strength of the transm;ssions from that base station to determ;ne whether or not ;t l;es above a predeterm;ned threshold. The mon;tored transmiss;ons may be, for example, control s;gnals wh;ch ;n many systems w;ll be transmitted over ded;cated control channels and consist of command messages, acknowledgement messages etc. for the var;ous mob;le stations and conta;n ;nformat;on ;dent;fy;ng the transm;tt;ng base stat;ons. If the nature of the mon;tored transm;ss;ons allow ;t an alternative or additional cr;terion on which such a determ;nat;on can be based is the rate at which errors occur in the transm;ss;ons as received.
Although the system described includes control means in the form of a single control station CS ~h;ch ;s separate from the var;ous base stations and communicates w;th them v;a respect;ve direct two-way l;nks CL th;s ;s not essent;al. The control means funct;ons descr;bed may be d;stributed over a plurality of control stat;ons which commun;cate w;th one another and w;th the base stat;ons e;ther d;rectly or v;a base stations and/or control stations. Moreover a control station can be co-located or even co;ncident with a base station ;f desired.
Some elements of an example of a system as descr;bed with reference to F;gure 1 will now be descr;bed in more detail with reference to Figures 2 to 10 of the drawings. For convenience it will be assumed that the system operates in accordance with a signalling protocol as descr;bed in a paper by P.J. Mabey, D.M.
3all and D.J. Harrison ent;tled "UK Trunking System S;gnalling Standard - Protocol Aspects in the Journal of the IERE, May~une 1987, pages 119-124, although th;s ;s not necessar;ly the case~ In lZ76~ PHB 33299 accordance with this protocol the base stations BS of Figure 1 each transmit on a substantiaLly cont;nuous bas;s a succession of data code~ords on a "forward" (radio) control channel, these code~ords normally be;ng so-called control channel system code~ords and other codewords, transm;tted alternately. In the system of the present example a bit field of each control channel system codeword transmitted by each base station BS constitutes an identifier AREA
of the particular service area S~ served by the relevant base station. Thus, if a mobile station M tunes ;ts radio receiver to the control channel correspond;ng to a base station BS ~;th ~h;ch ;t is with;n communicat;ons range it w;ll receive repeatedly a code indicative of the particular service area SA served by that base station. The "other" code~ords, wh;ch each contain an address f;eld, may constitute messages addressed to all, part;cular ones, or groups of the mobile stations M, each such mobile station being assigned a unique address wh;ch ;s storeJ there;n for reference.
Moreover, many of the codewords transmitted by each base stat;on constitute an invitation to a(l, particular ones, or groups of mobile stations within communication range to transm;t to the base stat;on a message codeword on a "return" control channel w;th;n a randomly chosen one of a number of ;mmed;ately succeed;ng t;me slots, wh;ch number ;s conta;ned ;n a further b;t f;eld of the ;nv;tat;on codeword. Th;s commun;cat;on techn;que ;s descr;bed, for example, ;n GB-B-2 069 799 ~PHN 9693). The message codeword transm;tted by a g;ven mobile station M contains the un;que address of that stat;on and may const;tute, for example, a request for serv;ce. The s;gnall;ng on the forward and return control channels (the s;gnalling on the forward channel being under the control of the control station CS) may be employed, for example, to set up a commun;cat;ons channel between a pair of mobile stat;ons v;a the base station(s) with which they are within communications range.
F;gure 2 is a block d;agram ot those port;ons of a poss;ble construct;on for each mob;le station M; wh;ch are relevant to the present ;nvent;on. The station Mi comprises a transmitter portion 1, a rece;ver portion 2 and a digital computer system 3. The ~276681 PHB 33299 system 3 ;ncludes, in add;tion to the conventional processing un;t(s) 4 and program store 5, data storage means 6, 7, 8, 9, 24 and 10, t;mers 11 and 55, and a real-time clock 105. A serial data output port 12 of the system 3 ;s connected to a modulat;on s;gnal input 13 of the transm;tter port;on 1 and a demodulated s;gnal output 14 of the rece;ver port;on 2 is connected to a ser;al data ;nput 15 of the system 3. A parallel data output port 16 of the system 3 ;s connected to tun;ng control inputs 17 and 18 of the transmitter and receiver portions 1 and 2 respectively. The output 19 of a received signal-level threshold detector 20 ;ncluded ;n the rece;ver portion 2 is connected to a further signal input 21 of the system 3.
Storage means 6 stores a list of all the rad;o channels employed in the communications system, each "for~ard" channel being stored in conjunction ~;th the corresponding "return" channel~
Storage means 7 stores the address of the relevant mobile stat;on Mi. Storage means 8, 24 and 9 store a parameter m, a parameter p and the aforesa;d parameter n, respectively. Storage means 10 constitutes the aforesaid locat;on record, service area ;dent;t;es be;ng stored ;n the respect;ve f;elds 221, ..~ 22;. Fields 231, 232 and 233 of each f;eld 22 store the actual serv;ce area ;dent;ty, a control channel number and a t;me respect;vely.
System 3 is programmed to decode the digital ~ords applied by rece;ver port;on 2 to ;ts ;nput 15 and su;tably react ;f appropriate, and also to format digital words and apply them to its output 12 for transmission by the transmitter port;on 1 ~hen appropr;ate. (The manner ;n ~h;ch th;s decod;ng and formatt;ng ;s carried out is convent;onal, and w;ll not be elaborated upon here as it is ;rrelevant to the present ;nvent;on.) ~t ;s also programmed to apply to ;ts parallel output 16 a d;g;tal ~ord for tun;ng the transm;tter and rece;ver port;ons to the return and forward channel frequenc;es respectively corresponding to any of the channels stored ;n store 6. (The program may ;n fact cons;st of a plural;ty of ;nteract;ng and concurrently runn;ng cyclic programs for a corresponding number of process;ng units). The ~Z7~6~1 PHB 33299 program or one of the concurrently runn;ng programs includes the steps shown ;n the flow chart of Figure 3 ;n wh;ch the var;ous blocks have the following sign;f;cances.
25 - START.
26 - Initialise, ;ncluding set parameter n in storage locat;on 9 to unity and tune transmitter 1 and receiver 2 to the frequencies of the channel stored ;n the f;rst locat;on of store 6.
27 - Does the signal at output 19 of detector 20 ;nd;cate that the rece;ved s;gnal strength is above the threshold?
28 - Call control channel search subrout;ne (Figure 4).
29 - Has a new n-value-determ;n;ng ;nstruction been rece;ved?
30 - Call wr;te subroutine for storage location 9 (Figure 5).
31 - Is the difference between the t;me stored ;n the f;eld 233 of a val;d serv;ce area registrat;on or null record present ;n a f;eld 22 of store 10 and the current time ;ndicated by clock 105 greater than a first predetermined value?
32 - Call expired registration or null record servicing subroutine (Figure 6).
106 - Is the differnce bet~een the time stored in the f;eld 233 of a val;d record present in a field 22 of store 10 and the current time ;ndicated by clock 105 greater than a second predetermined value greater than the f;rst~
107 - Invalidate the relevant record.
33 - Has a new registration renewal response (see below) been received?
51 - Call reg;strat;on renewal subroutine ~Figure 7).
52 - Has registration renewaL t;mer (see below) newLy expired.
53 - Call registration renewal timer expiry subroutine (Figure 8).
54 - Other operations (if desired).
Thus, after start up of the mobile station, the transm;tter and rece;ver are tuned to one of the control channels of the system and the parameter n is set to a default value of unity to a~a;t a message from a base station ind;cating what it should be. If the control channel signal strength is unsatisfactory a search is made to find another control channel for wh;ch this is not the case.

127~681 PHB 33299 After this the parameter n is set to the correct value ;f an appropriate instruct;on has been rece;ved, suitable action is taken if a reg;strat;on record has exp;red, the sole rema;n;ng reg;strat;on record is invalidated ;f it is suff;ciently "old" and any necessary action is taken ;f a registration renewal s;tuat;on is present, after which the program returns to the s;gnal strength test, possibly with the interposition of further processing steps.
The flo~ chart of Figure 4 indicates the control channel search subroutine 28 performed if it is determined in test 27 that the signal strength of the current channel is unsatisfactory~ In Figure 4 the various blocks have the follow;ng significances.

35 - Set parameters m and p to 1.
36 - Are there at least m val;d service area ;dentit;es stored in location record 10?
37 - Tune transm;tter 1 and rece;ver 2 to the frequenc;es of the channel stored in the fieLd Z32 f the mth record 22m ;n storage means 10.
38 - Does the signal at output 19 of detector 20 ind;cate that the received s;gnal strength is above the threshoLd?
39 - Are control channel system codewords being received in which the identif;er AREA corresponds to that stored ;n a field 23 of a valid one of the records 22 ;n storage means 10?
40 - If the channel stored in the field 231 of the relevant one of the records 22 is d;fferent from that to which the transmitter and rece;ver portions 1 and 2 are currently tuned replace the former by the latter.

42 - Generate at output 12 a reg;stration request message for transmission by transmitter 1 to the relevant base stat;on.
~rite a null record (see below~ contain;ng the time at which the message is actually transmitted (read from clock 105) into a field 22 of storage means 10 which does not conta;n a valid record or, if th;s would result in a number of valid records lZ766~ PH8 33299 in storage means 10 ~hich exceeds the parameter n currently contained in storage means 9, overwrite that val;d record the contents of the f;eld 233 of ~hich contain the least recent time. Start timer 11.
43 - Has a "registration accepted" message been received?
44 - Replace the null record written in step 42 by a registration record conta;ning the relevant AREA code, the relevant channel number, and the time wr;tten when the null record was ~r;tten.

46 - Has timer 11 expired?
47 - Is parameter p greater then the number of channels (pairs of forward and return) stored in storage means 6?
48 - Tune transmitter 1 and receiver 2 to the frequencies of the pth channel stored in storage means 6.
49 - Energise indicator to user that registration attempt has failed.

The effect of the subrout;ne of F;gure ~ is therefore to test various channels in turn for sat;sfactory s;gnal strength, starting with valid ones of those stored in storage means 10 and cont;nuing, if necessary, w;th those stored ;n storage means 6. If such a channel is found then, ;f the rece;ved AREA ;dentification corresponds to a valid identification stored in any f;eld 231 f storage means 10, a return is made at 41, changing the channel stored in the relevant field 232 to the one to wh;ch the mobile stat;on ;s currently tuned, if necessary. On the other hand, if such a channel is found but the rece;ved AREA ;dentification does not correspond to a vaLid identification stored in any field 231 of storage means 10, an attempt is made in step 42 to register in the new area. If this attempt is successful ("yes" output Y from test 43) a return is made at 45 after the null record entered in storage means 10 in step 42 has been converted to an actual registration record for the new area. (The "null records" are dumm;es created to assist ;n ensuring that all registration records at each mob;le station have, at any given time, counterparts at the control 12766~1 PH~ 3329 stat;on CS, it being possib~e, for example, for a registration request to be rece;ved at the base station but ~he resulting "registration accepted" message sent by the base station not be;ng received by the relevant mobile seatiOn.) If the registration attempt faiLs, and timer 11 times out before a registration message is rece;ved (result r from test 46) the transmitter and receiver are tuned to the next control channel. If all channels are tried, without one being found of sufficient signal strength and for ~hich the relevant and valid AREA code is stored in storage means 10 or on which a registration attempt has succeeded, the user ;s alerted in step 49 and a return is made at 50. ~hether or not a f;eld 22 contains a val;d record may be indicated, for example, by means of a stored flag bit corresponding to each field. Alternatively, an invalid record may be positively deleted rather than being merely implicitly deleted by changing the flag.
The fLow chart of Figure 5 indicates the ~rite subroutine 30 for storage location 9 which is performed when it is determined in test 29 that a new n-determining instruction has been received over the control channel to which the mobile station is currently tuned. Such an instruction may be broadcast by the base stations ~S, under the control of the control station CS, to all mobile stations M when it is required to change the value. In Figure 5 the various blocks have the following significances.
56 - START.
57 - Over~rite current contents of storage location 9 with received value of n.
58 - Is the number of valid records in the fields 22 of storage means greater than the number n in location 9?
59 - RETURN.
60 - Invalidate a suff;c;ent number of the valid records so that only n remain, the inval;dated records being those for which the field 233 contains the least recene t;me.
61 - RETURN.
Thus the current contents of locat;on 9 are replaced by the newly received value for n (the max;mum value of ~hich may be, for example, three). If this results ;n storage means 10 containing more than n val;d records a suffic;ent number of the "oLdest" are invalidated, ;.e. ;mpl;citly deleted to ensure that exactly n rema;ns.
The fLo~ chart of Figure 6 indicates the subrout;ne 32 which is performed when it is determined in test 31 that an otherwise valid record in storage means 10 has expired, i.e. when it is determined that the difference between the time stored in the f;eld 233 of the relevant record and the current time exceeds a first predetermined value, for example 30 minutes. In Figure 5 the various blocks have the following significances.
62 - Does storage location 10 contain a valid and unexpired registration record?
63 - RETURN.
64 - Does the AREA code ;n the f;eld 231 of the newly expired registration or null record correspond to that currently being received?
65 - Invalidate newly expired registration record.
66 - RETURN.
67 - Generate at output 12 a registration request message for transmission by transmitter 1 on the current controL channel.
Write a null record conta;ning the time at uhich the message is actually transmitted into the field 22 of store 1~ which currently contains the newly expired record. Start timer 11.
68 - Has a "registration accepted" message been received?
69 - Replace the null record written in step o7 by a reg1stration record containing the relevant AREA code, the relevant channel number, and the time which was included in the null record.
70 - RETURN.
71 - Has timer 11 expired.
7Z - Call the control channel search subroutine 28 tFigure ~.
73 - RETUR~.
Thus, when it is determined that the time in the field 233 of a registration or null record contained in store 1U has not been changed for a predetermined time, i.e. the record has been present ~Z7~8~ PHB 33299 for the predetermined time ~ithout being renewed, the relevant record is processed provided that at least one valid and unexpired registration is also present in the store. The registration is ;nval;dated if it ;s for an area which does not correspond to that ;n wh;ch the mobile stat;on is currently situated. If it does so correspond an attempt is made to re-register in the current area.
Such an attempt may or may not succeed. If it does not succeed a search operation is carried out for a new control channel.
In principle, the time in the field 233 of a given valid registration record is updated each time a commun;cation occurs between the mob;le station and a base station serving the relevant area. (The commun;cat;ons wh;ch give r;se to the updat;ng may be, for example, only those ;n;t;ated by the mob;le stat;on, or both those ;nit;ated by the mobile stat;on and those ;nitiated by the base stat;on). In the present embod;ment the updat;ng is triggered by the sending by the mobile station of a particular message (other than a reg;stration request) which demands a response within a predetermined time (timed by timer 55), this message being chosen to be one which is always sent in the course of a communication initiated by the mobile station, for example a request for service message. Computer system 3 is programmed to, each time such a message is sent, start timer 55, create a null record in store 10 (the field 233 of the null record containing the time at wh;ch the message was actually sent) and invaL;date the oldest record ;n store 10 ;f store 10 would otherw;se conta;n more than n val;d records. The tests 33 and 52 of F;gure 3 then determ;ne whether the demanded response is actually received.
F;gure 7 ;s a flow chart of the reg;stration renewal subrout;ne 51 wh;ch is performed when test 33 determines that a "renewal response" message has been neuly rece;ved. In F;gure 7 the various blocks have the follow;ng sign;f;cances.
74 - START.
75 - Is t;mer 55 running?
76 - Replace the time in the f;eld 233 of the registration record of the area ;n ~h;ch the mob;le ;s currently operat;ng by the 7~;~i8~
P~B 33299 time ;n the null record created when the response-demanding message was sent. Invalidate the null record created ~hen the response-demand;ng message was sent and revalidate the record (if any~ wh;ch was ;nval;dated at that t;me.
77 - RETURN.
F;gure 8 ;s a flow chart of the subrout;ne 53 ~hich ;s performed when test 52 determ;nes that t;mer 55 has newly exp;red.
In F;gure 8 the var;ous blocks have the follow;ng s;gnf;cances.
78 - START.
79 - ~as a reg;strat;on renewal response rece;ved wh;le the t;mer was running?
80 - RETURN.
81 - Does the AREA code ;n the control channel system code~ords currently be;ng rece;ved correspond to that ;n that one of the fields 22 of store 10 which conta;ns the most recent t;me in ;ts f;eld 233?
82 - RETURN.
83 - Inval;date the null record created when the response-demand;ng message was sent and reval;date the record (;f any) wh;ch was ;nvalidated at that time.
84 - RErURN.
It will be seen from F;gures 7 and 8 that, if a renewal response message ;s received within the correct t;me, the contents of store 10 are restored to what they were before the demand message was sent, w;th the except;on that the time in the relevant reg;stration record ;s updated. lf such a response message is not rece;ved then, ;f the AREA code currently be;ng rece;ved corresponds to that ;n the registration record wh;ch conta;ns the most recent t;me the contents of store 10 are restored to what they were before the demand message was sent, whereas ;f ;t does not so correspond, the r,ull record ;s ma;ntained.
Although the var;ous tests of F;gure 3 are shown as be;ng ;ncluded ;n the same cyclic program, it ~ill be evident that this ;s not necessar;ly the case. Each of these tests, or groups of these tests, may be included ;n respective ones of concurrently ~Z7~68~ PH~ 33299 runn;ng cyclic programs.
Figure 9 is a block diagram of a poss;ble construction for the control station CS of Figure 1, only those parts being sho~n ~h;ch are relevant to the present invention (It will be assumed for s;mpl;city that each base station BS acts merely as a transmitter for information received from the control station CS over the corresponding link CL and as a rece;ver for information for the control station, the latter being sent by the base station over the correspond;ng link. In practice, ho~ever, some of the requ;s;te data processing will normally be carr;ed out at each base station BS). The control station CS comprises a computer system hav;ng a data input~output 86 corresponding to and coupled to each l;nk CL
;n Figure 1. In addition to the conventional processing unitts) and program store(s) 87 and 88, system 85 comprises a real-t;me clock 89, a storage location 90 and further data storage means 91.
A respective field 92 of storage means 91 is assigned to each mobile station M and is capable of storage a plurality of registration records. The storage locat;on for each record comprises a subfield 93 for an AREA code and a subf;eld 94 for a t;me.
In add;t;on to despatching the aforesaid alternate control channel system codewords and other code words along each l;nk CL, system 85 ;s programmed to react to the recept;on of any registrat;on request or registration renewing message received at an input/output 86 by storing this message together ~ith the t;me of ;ts rece;pt Sread from clock 89) and a code ;nd;cat;ng at wh;ch ;nput/output 86 ;t was rece;ved. It ;s furthermore programmed ~o perform the steps ;nd;cated ;n the flow chart of F;gure 1û, ;n ~hich the var;ous blocks have the follow;ng s;gnif;cances.

96 - Supply an n-value-determining instruction message to the outputs 86 for transmission by the base stations. ~rite the relevant value of n ;nto location 90. If the ne~ value of n results ;n any f;eld 92 conta;n;ng more than n val;d reg;strat;on records ;nval;date a sufficient number of these 1~76~8i PHB 33299 records to ensure that exactly n rema;n, the invalidated records be;ng those wh;ch conta;n the least recent times in their subfields 94. (This block is shown ;n broken lines because in practice ;t will only be performed ;nfrequently, for example in response to activation by an operator).
97 - Has a reg;stration request message been newly received?
105 - Write the AREA code corresponding to the input~output 86 on which the message was received ;nto a free subfield 93 of that f;eld 92 of storage means 91 wh;ch corresponds to the mob;le station M from wh;ch the message was rece;ved, and the time the message was rece;ved ;nto the corresponding subfield 94. ~A "free" subfield is one which does not contain a valid registration record). Supply a "registrat;on accepted"
message to the relevant input/output 86 for transmiss;on by the relevant base station ~S.
106 - Does the number of valid records in the relevant field 92 exceed the value of n in location 90?
107 - Invalidate that record in the relevant field 92 which conta;ns the earliest time in its subfield 94.
99 - Has a registration-renewing message been newly rece;ved?
100 - Write the t;me the message was rece;ved into a subf;eld 94 of the field 92 assigned to the mobile station M from wh;ch the message was received, this subfield being that for which the contents of the assoc;ated subf;eld 93 correspond to the AREA
code for the particular input/output 86 on wh;ch the message was received. Supply the appropriate response message to the relevant input/output 86.
101 - Does the d;fference between the time ;n the subfield 94 of any valid reg;strat;on record in store 85 and the current t;me ;n t;mer 89 exceed the aforesaid first predetermined value?
102 - Does the relevant field 92 contain more than one valid registrat;on record?
98 - Invalidate the relevant registration record.
103 - Does the difference between the time ;n the subfield 94 of lZ7668~

any valid reg;strat;on record ;n store 85 and the current t;me in t;mer 89 exceed the aforesa;d second predetermined value?
104 - Inval;date the relevant registration record.
Thus the controL stat;on reacts to the rece;pt of reg;strat;on request;ng messages and registration renew;ng messages ;n an appropriate way to ensure that, ideaLly, the val;d records ;n each f;eld 92 will correspond to those in the relevant mobile station, t;me-out of the var;ous records taking place on the same criter;a ;n both the mob;le stat;ons and the control stat;on.
When a request is rece;ved at the control station for a communication to one of the mobile stat;ons it is programmed to refer to the field 92 assigned to the relevant mob;le station and determine therefrom that val;d AREA code for which the time in the corresponding subf;eld 94 ;s the most recent~ It then suppl;es a response-demand;ng message, addressed to the relevant mobile stat;on, to that one of the inputs~outputs 86 wh;ch corresponds to the AREA code so determined, and waits for a response. If no response is received it refers once again to the relevant field 92 and determ;nes therefrom that AREA code for which the time in the corresponding subfield 94 ;s the next most recent, and supplies a su;tably addressed response-demand;ng message to the correspond;ng one of the inputs/outputs 8~, and so on, unt;l a response is eventually rece;ved or until all the areas validly recorded in the relevant field 92 have been tried.
From reading the present disclosure, various modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already kno~n in the design, manufacture and use of systems and component parts thereof and which may be used ;nstead of or ;n add;t;on to features already descr;bed herein. Although claims have been formulated ;n this application to particular comb;nat;ons of features, it should be understood that the scope of the d;sclosure of the present application also includes any novel feature or any novel comb;nat;on of features d;sclosed herein either expLicitly or ;mplic;tLy or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give not;ce that new claims may be formulated to such features and~or comb;nations of such features during the prosecut;on of the present appl;cation or of any further application derived therefromA

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A mobile radio transmission system comprising a plurality of base stations which are situated at respective geographical locations to thereby define a plurality of service areas and a plurality of mobile stations capable of communicating by two-way radio with each base station when in communication range of that base station, the system being provided with control means which includes a location record corresponding to each mobile station, which location record is capable of storing a service area identity, characterised in that each location record is capable of storing at least two service area identities, in that each mobile station also includes a location record capable of storing at least two service area identities and is arranged when operative to respond to it not being within communication range of any base station serving a service area the identity of which is currently stored in the location record included therein by communicating with a base station of which it is within such range (if any) and storing the identity of the service area served by that base station in said location record in such manner that said identity replaces the identity of another service area already present in said location record if the resulting number of stored identities would otherwise exceed a given value n greater than one, and 1 that said control means is arranged to respond to each such communication by updating the contents of that location record included therein which corresponds to the communicating mobile station so that the mobile station is recorded as being currently located in the service area served by the base station with which the communication takes places while retaining in said location record the identity or identities (if present) of at least (n-1) other service area(s) in which the relevant mobile station gas previously recorded as being located.

2. A system as claimed in Claim 1, wherein each mobile station is arranged, each time it stores a service area identity in the location record included therein, to retain in said location record at least the identity (if present) of that service area with a base station serving which it has otherwise most recently com-municated, and the control means is arranged, each time it carries out a said updating, to retain in the relevant location record the identity (if present) of at least that service area with a base station serving which the relevant mobile station has otherwise most recently communicated.

3. A system as claimed claim 1, wherein the replaced iden-tity is of that service area within which the relevant mobile station has communicated with a base station least recently of all those service areas the identities of which are stored in its location record.

4. A system as claimed in claim 1, wherein the control means is arranged to maintain, for each location record included therein, a record of the comparative lengths of the times which have elapsed since a communication last occurred between the rele-vant mobile station and a base station serving each respective service area whose identity is currently stored in that location record.

5. A system as claimed in Claim 4, wherein the record of the comparative lengths of the times is in the form of a record of the absolute time at which a communication last occurred between the relevant mobile station and a base station serving each said respective service area.

6. A system as claimed in claim 5, wherein each mobile station is arranged to maintain a record of the comparative lengths of the times which have elapsed since a communication last occurred between the relevant mobile station and a base station serving each respective service area whose identity is currently stored in the location record included therein.

7. A system as claimed in Claim 6, wherein the record of the comparative lengths of the times is in the form of a record of the absolute time at which a communication last occurred between the relevant mobile station and a base station serving each said respective service area.

8. A system as claimed in Claim 7, wherein each mobile station and the control means is arranged to delete any service area identity stored in a said location record included therein should the time which has elapsed since a communication last occurred between the relevant mobile station and a base station serving the relevant service area exceed a predetermined value, at least if at least one service area identity will remain stored in said location record after the deletion.

9. A system as claimed in Claim 8, wherein each mobile station and the control means is arranged, if one service area identity remains stored in said location record, to also delete that identity should the time which has elapsed since a communica-tion last occurred between the relevant mobile station and a base - 29a - 20104-8375 station serving the relevant service area exceed a predetermined value which is larger than the other said predetermined value(s).

10. A system as claimed in claim 1, 2 or 3, wherein the base stations are arranged to transmit said given number n to each mobile station.

11. A mobile station for use in a system as claimed in claim 1, 2 or 3, which station includes a location record capable of storing at least two service area identities and is arranged when operative to respond to it not being within communication range of a base station serving a service area the identity of which is currently stored in said location record by communicating with a base station of which it is within such range (if any) and storing the identity of the service area served by that base station in said location record in such manner that said identity replaces the identity of another service area already present in said loca-tion record if the resulting number of stored identities would otherwise exceed a given value n greater than one.

12. In a mobile radio transmission system comprising a plurality of base stations which are situated at respective geo-graphical locations to thereby define a plurality of service areas and a plurality of mobile stations capable of communicating by two-way radio with each base station when in communication range of that base station, a method by which the location of a mobile station is recorded, in which method the mobile station responds to it not being within communication range of any base station serving a service area the identity of which is currently stored in a location record included therein by communicating with a base station of which it is within such range (if any) and storing the identity of the service area served by that base station in said location record in such manner that said identity replaces the identity of another service area already present in said location record if the resulting number of stored identities would otherwise exceed a given value n greater than one, and a control means with which the system is provided responds to each such communication by updating the contents of a location record included therein which corresponds to the communicating mobile station so that the mobile station is recorded as being currently located in the service area served by the base station with which the communication takes places while retaining in said location record the identity or identities (if present) of at (east (n-1) other service area(s) in which the mobile station was previously recorded as being located.