AU3239799A

AU3239799A - Method of administering a network of base transceiver stations

Info

Publication number: AU3239799A
Application number: AU32397/99A
Authority: AU
Inventors: Pierre Dupuy
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1998-06-04
Filing date: 1999-06-02
Publication date: 1999-12-16
Also published as: CA2273424A1; FR2779606A1; FR2779606B1; JP2000023239A; CN1245387A; EP0963131A1

Description

P/00/01i1 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT 0 *0 0 0* 0 0 0 0 *0*e 0* *000 00*0 Invention Title: Method of adm-inistering a network of base transceiver stations The following statement is a full description of this invention, including the best method of performing it known to us: FHP.9DCV.NATP()3299153" 7

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CE\99151007.1 1 Method of administering a network of base transceiver stations Field of the invention The present invention concerns a method used by a central unit to administer a network of base transceiver stations communicating with mobile telephones. The invention can be used in particular in the context of GSM mobile telephony using broadcast control channels (BCCH), as encountered in time-division multiple access (TDMA) networks. Consequently, the invention does not apply to code-division multiple access (CDMA) type networks, which have no broadcast control channel.

10 Background of the invention a.~ The operation of a prior art GSM mobile telephone network will be explained with reference to figures l a through 5. The same figures will then be used to point out the performance limitations of a network of that kind, and the solutions to these provided by the invention.

Figures la and lb are respectively symbolic representations of a GSM time frame and a frequency band allocated to a GSM network operator. In practice, in one example, a GSM time frame has a duration of approximately 4.6 milliseconds and is .divided into eight time slots having a duration equal to 577 microseconds. These time slots (TSO-TS7) are allocated to different channels. Each channel is associated with a carrier frequency. Figure lb shows the facility for an operator allocated a bandwidth B of approximately 15 MHz to use a plurality of frequency channels. In one example there are 64 channels each occupying approximately 200 kHz.

Although at a given time a transmission channel is synonymous with a time slot TSi and a carrier frequency Fi, it is associated with a frequency law Li whereby the channel frequency changes from one frame to the next. Accordingly, if a plurality of channels coexist, they can be distinguished by their time slot and/or their instantaneous carrier frequency. A planning program prevents the use for the same frame of two separate channels which are in the same time slot and have the same carrier frequency.

I CE\99151007.1 2 Figures 2a through 4 show the principle of frequency hopping from one GSM frame to the next, the diagrams being reduced to what is essential for understanding the invention. The reasons for frequency hopping are well known in the art. The aim is to distribute noise mainly occurring in one frequency band between a plurality of messages exchanged with the network so that only one message is totally compromised, namely the one in a band greatly affected by noise. This technique allows sporadic use of the same frequency Fi in neighbouring cells. It thus contributes a significant statistical gain due to the principle known as frequency diversity, as explained in "The GSM Global System for Mobile Communications" written and published by M. MOULY and M. B. PAUTET, 1992 Edition, pages 218- 233.

Figures 2a and 2b show the principle of frequency hopping in each frame. In figure o* o *2b, a base transceiver station 1 transmits a synchronisation and assignment message toward a mobile station 3 on a broadcast control channel 2 (BCCH). Figure 2a shows that the mobile station 3 is carrying a mobile telephone 201. Figure 2a shows the synchronisation and assignment message symbolically in a dashed outline frame 200.

The message 200 is transmitted on a frequency Fcc characteristic of the BCCH, for example.

A portion SY of the message 200 transmitted by the base transceiver station 1 is 20 concerned with synchronisation. This portion SY is divided into two main parts. A first part (coded on 22 bits in the GSM) represents a frame number FN. In practice the frame number represents the time since the frame number changes with time.

This is therefore a signal that varies continuously, at least during a day or over a relatively long time period.

A second part of the signal SY enables the base transceiver station to identify itself.

The base transceiver station gives its identifier, coded on around 20 bits, and its colour. The colour, here represented by the symbol a, comprises on three bits a first indication PLMN (Public Land Mobile Network) representing the code of the operator in a shortened form. This enables a mobile telephone to determine, allowing for a subscription with an operator taken out by its user, if it has to converse with and CE\99151007.1 3 to take account of information supplied by base transceiver station 1. The base transceiver station 1 is identified in a shortened form on three further bits (giving a modulo 8 differentiation possibility): these bits indicate its colour a. Experience has shown that, given the signals received at frequency FcL, possibly from more than one source, these three bits are sufficient for quickly identifying the base transceiver station transmitting on the broadcast control channel frequency.

The use of a colour enables two base transceiver stations that are relatively far apart to use the same broadcast control channel frequency Fa without mobile telephones that pick up the signals suffering from any ambiguity as to which base transceiver station they are near. After the broadcast control channel frequency Fa detected and the colour to be interpreted have been transmitted to a central unit, the central unit i' knows which base transceiver station the mobile telephone has picked up.

A common control channel (CCCH) associated with the BCCH is used to transmit an assignment signal. This is the "Immediate Assignment" signal IA. The signal LA is also represented inside the dashed outline frame 200. It is representative of an .assignment of a communication channel to the mobile telephone 201 of the mobile station 3 for traffic it exchanges with the base transceiver station 1. The assignment message IA includes an indication TSi which is used to determine an assignment in the time frame. It also includes an indication Li of a frequency law which has three 20 main arguments. A first argument of the law Li concerns a carrier designation in the band B. For example, carriers Fl, F2 and F5 are shown here. There can be any number of carriers. The carriers are chosen from the 64 (or other number of) frequencies of the band B.

The message IA includes a Mobile Allocation Identity Offset (MAIO) as a second argument of the law Li. The message MAIO is in fact a value between 1 and n, where n is the number of carriers of the first argument.

The message IA designates a hopping sequence number (HSN) algorithm for generating pseudo-random numbers as a third argument of the law Li. The pseudorandom numbers produced by the algorithm HSN are also between 1 and n (or between 0 and n where n is the number of carriers of the first argument.

CE\99151007.1 4 The mobile telephone 201 includes an internal control circuit, in addition to control pushbuttons 4, a screen 5, a microphone 6 and a loudspeaker 7. The internal control circuit essentially includes a microprocessor 8 and a communication bus 9 linking the various units referred to above. In particular, the bus 9 enables communication with a circuit 10 running an HSN algorithm 11 for generating random numbers. The parameters of the algorithm 11 are set by the signal a, which is representative of the colour of the base transceiver station that will be managing the mobile station, and by the continuously varying signal FN representing the time.

In the simple example shown, in which there are three frequencies Fl, F2, F5, the 10 result available at the exit from the algorithm 11 is a random sequence of three *•o••symbols, for example, 0, 1 or 2, designating carrier frequencies Fl, F2, F5. In other a. words, when the mobile telephone has received the signal IA, its circuit 10 delivers at .i random the frequency Fn that must be used for each frame as the carrier frequency to a. distinguish its channel. This frequency changes in each frame because the circuit 11 receives a new signal FN via the signal SY in each frame.

Figure 3 shows the generation of the random carrier designations in accordance with a frequency law Li. A first column shows a temporal sequence of numbers 0, 1 or 2 produced by the algorithm 11. The value 0, 1 or 2 of the MAIO argument allocated o°:to the mobile stations is then added to the random number produced by the algorithm 20 11. This addition is modulo n (modulo 3 in this example). For example, for a mobile telephone MS 1 for which the MAIO allocated is 0, the number of the frequency relating to the addition is exactly the same as that delivered by the algorithm 11. For a second mobile telephone MS2 allocated an MAIO of 1, 1 is added, modulo 3, to the value of the random number. The same applies to a third mobile telephone MS3 allocated an MAIO of 2.

Given the random sequence shown, after addition, the result attributed to the mobile MS1 is 0, 2, 1, 2, 2. The corresponding allocated frequencies are then Fl, F5, F2, and so on for MS2 and MS3. This succession of carrier frequencies constitutes the law Li in accordance with which the mobile telephone 201 communicates with the base transceiver station 1. The speech signal is then CE\W99151007.1 modulated successively in a modulator 12 ald in accordance with the GSM frame numbers by a signal at a modulation frequency Fl, F5, F2, F5, F5, The modulated signal is transmitted by the antenna 13 of the mobile telephone 201.

A duplexer 14 enables the antenna 3 to be used for transmission and reception. For reception, a receiver 13 applies the same frequency law Li and after processing enables messages to be listened to on the loudspeaker 7. An additional receiver 16 centred on the broadcast control channel frequency Fa enables it to receive signals transmitted on the BCCH 2.

Although the normal traffic between the base transceiver station 1 and the mobile 10 station 3 uses a traffic channel (TCH), the Immediate Assignment signals can in practice be transmitted to it on the TCH instead of on the CCCH associated with the BCCH. If the mobile is not communicating, at the time of changes of location area, its traffic channel for communication with the base transceiver station is a standalone dedicated control channel (SDCCH) for tracking the mobile station as it moves around.

Figure 4 shows the internal structure of a base transceiver station 1 comprising eight basic circuits BTSO through BTS7 each allocated to one time slot TSO through TS7.

Each BTS includes an associated modulator in which the speech signals are modulated by a frequency FiO through Fi7 in turn dependent on the frequency law LiO through Li7 allocated to each time slot. A law Li must apply to the mobile telephone 201, to the base transceiver station 1 and for a given time slot i. There can be a different frequency law for each time slot. The frequency laws LiO through Li7 can be identical because they correspond to separate time slots.

Figure 5 shows a network of base transceiver stations and is used to explain the problem solved by the inventor. The base transceiver stations shown are contiguous, have respective colours a through j and respective radio coverage areas (cells) A through J. The cells A through J can be covered by an umbrella macrocell 17 radiating at a higher power. The contour 17 of this radiation is shown in dashed outline. A base transceiver station 22 governs the macrocell 17. All the base transceiver stations are connected to a central unit 20 handling planning and CE\99151007.1 6 switching functions.

In the current state of the art, and in real life use, the broadcast control channel frequencies of the BCCH of each cell A through J are different. Consequently, a mobile station 3 which is approaching one of these cells from another macrocell 18 or from another cell can signal to the base transceiver station 19 with which it is communicating the designations and the amplitudes of the broadcast control channel frequencies of the cells it is approaching. Its additional receiver 16 scans all possible broadcast control channel frequencies in order to pick up these signals.

When handover is necessary, the central unit 20 can then impose handover from the call already set up between the mobile station 3 and the base transceiver station 19 to a call to be set up between the mobile station 3 and that of the cells A through J, or i* even the macrocell 17, from which the mobile station 3 is receiving the broadcast control channel frequencies best. Recognising this cell as the one best qualified to welcome it is immediately obvious if each welcoming cell uses a different broadcast control channel frequency. A mechanism of this kind is described in US patent application Serial No. 527,714, for example.

However, using individual broadcast control channel frequencies for each cell runs into the problem of frequency hopping previously referred to. The frequency hopping technique requires a number of free frequencies in each time slot for each cell. For example, three frequencies Fl, F2, F5 were needed to constitute the law Li.

Given the number of adjoining cells (A through H, or 10 to 15 cells for the macrocell 17, and as many for the macrocell 18, and as many again for another macrocell 21 adjoining the macrocell 17 and the macrocell 18, and so on), it can be shown that frequency congestion quickly becomes a problem.

The signals transmitted continuously on the broadcast control channel frequencies Fc are sources of interference for the useable TCH of the cells.

Planning a network of this kind, with individual broadcast control channel frequencies for each cell, therefore necessitates relatively inextricable planning which becomes more and more so as the density of the cells increases. It also requires a

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CE\99151007.1 7 physical visit to cells A through J to measure frequency channel propagation conditions. All this complexity is due to the frequency occupancy of the various broadcast control channel frequencies in the macrocells 17, 18, 21, etc. Nevertheless, this technique is the one that is used in real life.

The previously mentioned French patent and European patent application EP-A- 0 504 122 concern a method in which there is only one broadcast control channel frequency Fa in a macrocell 17. This is shown in figure 5, in which an umbrella base transceiver station 22 transmits a supervisory message on a BCCH at the broadcast control channel frequency Fa. Each base transceiver station of cells A through J of the macrocell 17 can transmit a supervisory message at the single common frequency Fa, or not (in which case they merely listen). Or again, the base transceiver station 22 **does not transmit but all the base transceiver stations of cells A through J transmit the same supervisory message. Transmission or repetition of the supervisory messages is synchronised by the central unit 20. The immediate advantage of this method is to leave unused broadcast control channel frequencies available. The available frequencies can be used as TCH for communication between the base transceiver stations and the various mobile stations.

~The documents cited above consider systems in which the various cells A through J g hand over to each other the calls of mobiles travelling from one cell to another. For o oo 20 simplicity, an HO ("handover") message, of the IA message type, is used to assign new frequency laws. The message HO relates to a handover. It is transmitted to the mobile station 3 by the base transceiver station with which the mobile station was previously communicating, before it leaves it, and on the instruction of the central unit 20. The message HO is transmitted via the TCH on which the mobile station 3 was communicating with the base transceiver station it has left. The BCCH at frequency F is therefore not used for these handovers. As the HO message is transmitted on the TCH, using a time slot once, the cell itself does not require any BCCH either.

Similarly, the above documents address the question of mobile stations that were dormant and which are solicited by an incoming call or which themselves solicit a CF\99151007.1 8 base transceiver station in order to make an outgoing call (when the mobile telephone user wishes to call another user). In this case, in a manner that is known per se, the transactions use the CCCH or paging channels (PCH) internal to the cell in which they are located.

In contrast the above documents do not consider the situation of mobile stations 3 coming from another macrocell 18 in which the broadcast control channel frequency FP is not the same as the broadcast control channel frequency F of the macrocell 17 the mobile station 3 enters.

The broadcast control channel message F3 of the cell it is leaving deteriorates in 10 quality as the mobile station 3 moves away from a macrocell 18 (or from a cell). In i contrast, the quality of the broadcast control channel message Fci of the macrocell 17 Sit is approaching improves. The mobile station 3 transmits corresponding information to a base transceiver station 19. Consequently, the base transceiver station 19 warns the central unit 20 that one of the cells of another macrocell (it is not '15 yet known whether this will be the macrocell 17) must administer the call involving the mobile station. This problem is not solved in the prior art.

Although figure 5 shows the mobile station entering via cell J in macrocell 17, in 9***practice it is not known which macrocell will welcome it or which of the cells A through J of the welcoming macrocell the mobile station will detect (cell because 20 all cells A through J transmit the same supervisory signal at the broadcast control channel frequency Fa (if they transmit one at all). In particular, the adjoining cells shown in figure 5 are not representative of the real life situation. For example, a cell A, D or F may correspond to a street whereas a cell of the macrocell 18 that has been left may correspond to a corridor in the underground railway system. This corridor leads out into the open air inside cell J. In this case, cell J is highly interleaved with the adjoining welcoming cells A, D, F. Accordingly, the radio coverage areas are not so simple to identify as shown here. Thus cell J could equally well be in the middle of the network A to H. Nevertheless, the term "peripheral" is used to designate a cell J of this kind in the sense that the cell is one through which mobile stations necessarily and physically pass when they leave a macrocell 18 whose broadcast CE\99151007.1 9 control channel frequency F3 is not the same as the broadcast control channel frequency Fct in macrocell 17.

The message HO that the base transceiver station 19 transmits to the mobile station 3 before abandoning it, for it to be able to communicate with one of the cells of the macrocell 17, must designate a frequency law Li and a time slot TSi. This message HO is transmitted to the mobile station 3 simultaneously with the instruction given to all the base transceiver stations of the macrocell 17 to prepare to welcome the mobile station 3 entering their coverage. In other words, for a single incoming mobile station, it is necessary to neutralise a TCH in each cell of the macrocell 17. In other words, the cost of an incoming mobile station in terms of reservation of resource is excessively high. With TRX equipment like that shown in figure 4, a BTS circuit o* S must be reserved in each base transceiver station to receive the handover. If there are 15 cells, 15 BTS circuits will be neutralised although only one will be used.

Obviously, if the simultaneous entry of eight mobile telephones must be allowed at all times, the transmitter TRX with eight channels becomes totally unavailable for the 5*55 •calls of mobile stations already communicating. In this case the equipment from S figure 4 must be increased in size, for example doubled in size, to handle eight calls.

In normal use a base transceiver station includes only one transmitter TRX, or two in see* a few instances. This indicates the consequences in terms of cost of managing the 20 handover of calls of mobile stations entering the macrocell 17.

In other words, the problem which needs to be solved is that of increasing the rate of acceptance of mobile stations entering the macrocell 17 in which a single BCCH is chosen (transmitting on a single broadcast control channel frequency Fa) to simplify the planning of frequency hopping (the technological advantage of which is undeniable).

Broadcasting a message on a broadcast control channel frequency represents a penalty because, for cells neighbouring the broadcast control channel transmitter, which would pick up the broadcast control channel message, the broadcast control channel message is equivalent to a perfect source of interference, i.e. one that is impossible to eliminate in the frequency band of the broadcast control channel CE\99151007.1 frequency concerned. Consequently, the broadcast control channel frequency range must be neutralised for the available frequencies by frequency hopping.

Summary of the invention The invention concerns a macrocell with a common broadcast control channel frequency for a BCCH.

The invention exploits the occupancy of the time slots to solve this problem of unnecessary neutralisation of BTS circuits in all cells A through J. At a given time, given the existing traffic, it is possible that not all time slots are in use simultaneously. In most cases there is at least one free time slot, or even two or more, in a base transceiver station. The idea of the invention is to make the free time slot the same one in all the base transceiver stations. In this way the central unit can generate an instruction HO including the designation TSi of the available time eslot. All the base transceiver stations then prepare to receive from the inbound *mobile in that slot TSi. The available time slot can be any of the time slots and can change with time.

The process comprises two phases. A first phase consists in evaluating the occupancy of the time slots. If a slot TSi is already available everywhere, then that time slot TSi is assigned to the inbound mobile. If not, the evaluation includes oooo determining the time slot that is least used. Then, in a second phase, the central unit 20 20 organises migration of set up calls in the least used time slot to other free time slots in the cells concerned. Once this migration has been carried out, the central unit can send the instruction HO and impose therein the time slot that has been freed up. All the cells are then able to welcome the mobile but without allocating it a specific BTS circuit.

The invention therefore consists in a method of administration by a central unit of a network of base transceiver stations communicating with mobile telephones, wherein: cells in a first macrocell are each governed by a base transceiver station and share a synchronisation

BCCH,

CE\99151007.1 11 -a signal for synchronising all base transceiver stations of the cells of the first macrocell is transmitted continuously on the BCCH at a common broadcast control channel frequency, -the base transceiver stations of the first macrocell communicate with mobile telephones on traffic channels allocated time slots and frequency laws locked onto the synchronisation signal delivered by the BCCH, a call involving a mobile telephone communicating with a base transceiver station of a second macrocell and entering the first macrocell is transferred to a base transceiver station of the first macrocell by an instruction including the assignment of a protocol for entering into communication with a base transceiver station of the first macrocell, wherein the central unit: measures a level of use of time slots, determines a least used time slot, and 15 configures the entry protocol so that it uses the least used time slot.

Brief description of the drawings *The invention will be better understood after reading the following description and examining the accompanying drawings, which are given by way of illustrative and non-limiting example only. In the drawings: figures l a through 5 are diagrams already commented on illustrating the prior art, the stated problem and the solution provided by the invention, and figure 6 is a diagram showing measuring the level of use of the time slots and how the least used time slot is determined, and figure 7 shows a most complete configuration of an entry protocol for using a time slot that has been made available.

Detailed description of the embodiments A network is synchronised when all the cells have a common timing reference, CE\99151007.1 12 possibly with an offset. A network with a common BCCH is necessarily a synchronised network. An asynchronous or non-synchronised network is one in which the frame number signal FN is dependent on each cell. A network can be synchronised even if each cell has its own BCCH with its own broadcast control channel frequency. It is sufficient for the frame signals FN to be the same and to be delivered at the same time. The invention therefore concerns synchronised networks.

When there is an arrangement of microcells within a macrocell, the macrocell is generally an umbrella macrocell radiating globally into the space of all the microcells and possibly into gaps between microcells not covered by the microcells. The antenna of a macrocell is typically above the rooftops of houses in a built-up area °whereas the antenna of the microcells is typically below roof height, for example on house fronts to constitute a cell for propagating transmissions along a street, with reflection along the building fronts. Microcells are further used to support calls at .street intersections. In this case their waves propagate in all roads leading to the intersection.

In systems with an individual broadcast control channel for each cell, the mobile telephone listens to the various signals transmitted on the various broadcast control channel frequencies and transmits to the base transceiver station with which it is communicating the various levels and qualities of the messages at the received 20 broadcast control channel frequencies. The base transceiver stations transmit the corresponding information to the central unit 20, which organises the handovers, if required.

If there is a single BCCH, with a single broadcast control channel frequency, a mobile telephone cannot tell from signals received on the broadcast control channel where they came from. In this case, the listening function is transferred to the base transceiver stations. The base transceiver stations are then responsible for listening to all the mobile stations in their environment from which they are receiving. To this end, the centre part of a voice message exchanged between any mobile telephone and its base transceiver station on a TCH includes an indication TSC (figure la) of the identifier of the mobile telephone (or of the call in progress). This identifier is CE\99151007.1 13 transmitted to the central unit 20 which, once again, allocates calls to the various base transceiver stations as required and organises the general administration of calls within the macrocell. This type of operation is exactly the same for the invention and for the prior art. In particular, the base transceiver stations receive from the central unit 20 an instruction to listen to the traffic of mobile telephones communicating with adjoining base transceiver stations. This instruction includes, for each mobile telephone to be listened to, the designation of its time slot (TSi), its frequency law (Li) and its identifier (TSC). The base transceiver stations listen to calls set up by their neighbours. They measure the quality of signals received and allocate the results to the respective mobile telephones. They then transmit the qualitative results of such listening to the central unit for the latter to administer the network accordingly.

Figure 6 shows, in the form of a table, the principles of measuring the level of use of *the time slots and determining the least used time slot. The figure 6 table contains records (represented horizontally) corresponding to each base transceiver station of the macrocell 17. Records corresponding to cell A, cell B and cell J are shown at least for macrocell 17, for example. These records each include a series of information of the same type. The information of the same type is assigned to time -o ~slots TSO through TS7. The information of the same type indicates the identifier 20 TSC of a mobile telephone which is communicating in the time slot concerned and

S..

the frequency law L it is using. Accordingly, for cell A, the base transceiver station equipment of which is typically as shown in figure 4, for time slot TSO, the record from the figure 6 table in the memories of the central unit 20 reveals that the time slot concerned is being used by a mobile station r (TSCr) complying with a law LAO.

Likewise, time slot TS I for the same cell A is being used by a mobile telephone TSCs complying with a law LA1. This record also shows that a time slot TSi for base transceiver station A is not carrying any mobile telephone traffic.

Turning to a second base transceiver station B, this carries in time slot TSO the traffic of a mobile station TSCt complying with a law LBO and in time slot TS 1 the traffic of a mobile station TSCu complying with a frequency law LB 1. The designations of CE\99151007.1 14 additional frequency channels are indicated against the records of cell B. Time slot TSO was carrying the traffic of a mobile station TSCv complying with a frequency law LB'0 and time slot TS 1 was carrying the traffic of a mobile station TSCw complying with a frequency law LB'1. Referring to figure 4, this means that the equipment of base transceiver station B includes two additional BTS circuits (over and above the eight circuits). Either these additional two circuits are allocated to time slots (TSO, TS1) permanently or the two additional BTS circuits can be allocated to time slots chosen on demand. In practice the base transceiver stations preferably comprise groups of eight BTS circuits permanently allocated to the eight 1 0 time slots.

°Each base transceiver station of macrocell 17, or of macrocell 18 or 21, transmits the occupancy status of time slots with the associated frequency laws to the central unit i 20. For completeness, the table shows for cell J that time slot TSi was carrying the 9. traffic of a mobile station TSCx complying with a frequency law LJi.

As previously mentioned, when a mobile station 3 is approaching macrocell 17 it registers the fact that the quality of the message transmitted at the broadcast control 9*99 9o*channel frequency of macrocell 18 is deteriorating. A request to hand the call over to a cell of macrocell 17 (or another macrocell) is sent to the central unit 20. This is shown in figure 5, where the quality of the signal at the broadcast control channel S 20 frequency F3 is deteriorating and the quality of the broadcast control channel signal .oo° °at frequency Fa is improving.

In accordance with the invention, the central unit 20 must then assess the level of use of the time slots. The central unit 20 already knows that it will have to concern itself with the cells of macrocell 17 because the handover request includes the information that the synchronisation messages transmitted at the broadcast control channel frequency Fa are received with good quality. It can therefore limit the assessment to the base transceiver stations of the cells of macrocell 17. As was the case for base transceiver stations A and B, none of the base transceiver stations of macrocell 17 uses the time slot TSi. In this case, the central unit 20 designates time slot TSi and allocates it a compatible frequency law Li. The channel determined in this manner CE\99151007.1 TCHi (TSi, Li) is communicated to base transceiver station 19 in conjunction with an indication of the broadcast control channel frequency F and possibly a colour cX of the base transceiver station (the umbrella base transceiver station 22 or another one) broadcasting the synchronisation message Fn.

Base transceiver station 19 then sends an HO instruction 23 to the mobile station 1.

The instruction 23 designates the broadcast control channel frequency Fa and the colour cx and indicates reservation of time slot TSi and frequency law Li. In the next frame the mobile station 3 transmits its voice signals in time slot TSi in accordance with frequency law Li locked onto the synchronisation signal it receives on the broadcast control channel at frequency Fa. Under these conditions, all cells A through H of macrocell 17 are set to wait to receive. One of them, the one receiving :traffic from the mobile station 3 with the best quality, for example base transceiver 9V i station 24 of cell J, allocates the channel TCHi corresponding to time slot TSi and frequency law Li to communication with the mobile station 3.

The problem is a different one if, as shown in figure 6, at least one cell is already using time slot TSi and as a result each time slot is occupied. It has been shown that 0~9* timeslot TSi was carrying the traffic of a mobile station TSCx complying with a frequency law LJi. Assessing the level of use of the time slots consists in adding, at 4*04 the bottom of each column of the table, the number of calls occupying the time slots.

be be be: 20 Accordingly, for the column corresponding to time slot TSO, it is necessary to count *6*4 one unit for the traffic of the mobile station TSCr in cell A. On the other hand, for cell B, it is also necessary to count one unit although this time slot is carrying the traffic of two mobile stations, TSCt and TSCv (but with two different and compatible frequency laws LBO and LB'O and with two additional BTS circuits). What must be counted is not the total number of calls in a time slot of a cell but the number of calls in a time slot of a cell which have to be made to migrate to free up at least one channel in the time slot of that cell. Thus for time slot TS7 the table shows that although a BTS circuit of transmitter TRX of cell B was occupied for communication with mobile station TSCy, the other BTS circuit was available. Consequently, for time slot TS7, base transceiver station B counts for 0. The number of calls for which CE\99151007.1 16 migration is necessary to free up a time slot is then evaluated.

For cell B, the table also shows that time slot TSi was carrying traffic of a mobile station TSCz complying with a frequency law LBi. However, another channel is available there, in time slot TSi. Thus base transceiver station B also counts for 0 for time slot TSi.

Once all the occupancy evaluations have been done, the minimum occupancy level can be evaluated. Assume that this indicates time slot TSi, for example, carrying traffic of a mobile station TSCz communicating with the base transceiver station of :°eooo cell B while another mobile station TSCx is communicating with the base transceiver 0 10 station of cell J. The idea of the invention is to bring about migration of these two calls to two neighbouring available channels, but only if necessary.

*o* For example, it appears necessary, given that cell J has only one BTS circuit in time slot TSi, to bring about migration 25 or 26 of the call involving mobile station TSCx.

The migration of the call involving mobile station TSCx is preferably organised in the same cell J. It will therefore be necessary for cell J to have at least one available :.**time slot. It has time slot TSO available, for example.

If this is not the case, if all time slots of the base transceiver station of cell J are occupied, the call could be caused to migrate to a time slot of a neighbouring cell, for example cell F for migration 27. Cell F is recognised as a candidate for this migration by the base transceiver station of cell F measuring calls exchanged within cell J and showing that cell F is capable of handling the call involving at least one of the mobile stations communicating with cell J. Either cell F also has a time slot available to welcome mobile station TSCx directly, in which case the migration 27 is organised, or this is not so, in which case a call involving another mobile station communicating in cell F, and for which this is possible, is caused to migrate at the same time as the call involving the mobile station TSCx is caused to migrate to the space freed up in cell J by the migration of that mobile station.

With regard to base transceiver station B, it has been determined that it left a channel available to time slot TSi. However, this condition may not be sufficient to authorise CE\99151007.1 17 installation of the mobile station 3 entering macrocell 17 in time slot TSi. The mobile station 3 must also be told a frequency law Li. It must then be verified that the law Li allocated to the inbound mobile is different to the law LBi with which the mobile station TSCz is complying. Where applicable, the call in time slot TSi involving mobile station TSCz will have to be caused to migrate (28) first to another time slot TS7.

Figure 6 therefore includes a summation line in which are permanently stored the measured levels of use m, n, 1, o of the time slots. From what has already S"••been explained, it follows that the level of use of time slot TSi would be 1 at a given if 10 time, subject to verification that the frequency law Li allocated is different to the frequency law LBi. This is relatively improbable given that the frequency law itself can easily be differentiated by allocating the inbound mobile an MAOI different to that of the mobile station TSCz.

It is therefore possible for the central unit 20 to transmit the instruction 23 to the mobile station 3.

Figure 7 shows the operations leading to acceptance of the inbound mobile station 3.

*o In a step 29 the mobile station 3 indicates the reception quality for transmissions it is picking up from base transceiver station 19 and from other base transceiver stations.

Registering the deterioration in the quality of communication between it and the mobile station, the base transceiver station 19 then transmits a handover request to the central unit 20 (step 30). Step 30 takes about 10 ms, for example. During a subsequent step 31 the central unit 20 performs the selection explained with reference to figure 6. Given the ongoing advance evaluation carried out by the central unit 20, this selection of an available time slot is very quick, taking approximately 20 ms.

In a subsequent step 32 the central unit 20 activates time slot TSi in all base transceiver stations that are able to receive from mobile station 3. Thus base transceiver stations B and J are also involved, which are already handling a call in time slot TSi. This activation consists in having the base transceiver stations listen to the mobile station 3. During this step 32 the central unit 20 also indicates to the base CE\99151007.1 18 transceiver station 19 the radio definition adopted (TSi, Li).

During a subsequent step 33, the base transceiver station 19 sends an HO instruction to the mobile station 23 together with the defined indications (TSi, Li). During a subsequent step 34 the mobile station 3 which has received the HO instruction sends "Handover Access" messages HA. The messages HA are received by all the base transceiver stations in its environment, including base transceiver stations B and J.

The messages HA indicate that time slot TSi and law Li have already been set aside for the mobile station 3 for future communication with another base transceiver i~ station. During another subsequent step 35, the base transceiver stations, including 10 stations B and J, transmit the level and quality at which they receive the message HA.

Stations B and J also transmit occupancy information to the central unit 20. The occupancy information indicates that time slot TSi is totally (or partly) occupied for them. The sequence of operations 32 through 35 takes approximately 50 ms.

Accordingly, the central unit 20, which has already specified the use of time slot TSi 15 (because it was the least used one), brings about the freeing up of time slot TSi, but only for stations B and J which have indicated on the one hand that they are valid candidates for welcoming the mobile station 3 and on the other hand that time slot S. TSi was used in them.

*o This avoids having to bring about migration of calls involving mobile stations that were using time slot TSi to communicate with another base transceiver station that is not a valid candidate for welcoming the mobile station 3 (essentially because the level at which they receive transmissions from the mobile station 3 is too low).

Similarly, if time slot TSi is free everywhere from the outset, this approach enables the mobile station 3 to be welcomed in that time slot without difficulty. If, as already explained, time slot TSi is not available, the central unit 20 then transmits one or more handover injunctions in a step 36. Base transceiver stations B and J receive these injunctions and simultaneously transmit the handover instructions HO previously discussed to the mobile stations TSCz and TSCx, respectively. At the end of step 36 it is certain that time slot TSi (with frequency law Li) is available in all base transceiver stations of the macrocell which are involved in welcoming the CE\99151007.1 19 mobile station 3.

In a consecutive step 37 the central unit 20 finally sends an instruction to accept the new mobile telephone 3. This acceptance is sent to base transceiver station 19 and to stations B and J. The mobile station then terminates its connection as in the prior art.

The standard imposes a maximum time of approximately 320 ms for welcoming the inbound mobile 3. The foregoing description indicates that the method of the invention achieves this.

The handover instructions HO are preferably transmitted to the mobile stations via the TCH with which they are communicating with the previous base transceiver 10 station, before leaving that channel for another one. Thus the base transceiver station 19 sends a switching message to the mobile station 3. The mobile station 3 switches as soon as it receives the message. The only switching that the mobile station 3 must o: perform consists in modifying its frequency law and its time slot. The base transceiver station 19 that the mobile station 3 has left ceases to provide the link with the mobile station. In contrast, base transceiver stations A-H have been warned by the central unit 20 in step 32 that they must monitor the link with the mobile station .3.

In drawing up the figure 6 list, all cells of macrocell 17 can be considered. For managing inbound mobile stations, it is preferable for only the peripheral cells to be taken. Only cells A, B, C, J, F, G and H are taken, for example, and not cells D and E which are geographically within macrocell 17. More generally, the only cells taken are those which have a chance of being the first to enter into communication with a mobile station in macrocell 17.

Similarly, a time slot can be excluded when determining the least used time slot, for example because it is allocated to a priority mobile station (fire department, police), and so there must be no risk of interfering with a call, or because it is temporarily indispensable because of temporary processing being carried out in a macrocell.

Similarly, the list can be either a global list or a list allocated to macrocell 17 (by virtue of correspondence with the broadcast control channel frequency Fa). In the CE\9915 1007.1 latter case, there will be as many lists as there are macrocells.

S SS 5.55

S

S. S S 55

Claims

1. A method of administration by a central unit of a network of base transceiver stations communicating with mobile telephones, wherein: -cells in a first macrocell are each governed by a base transceiver station and share a synchronisation BCCH, a signal for synchronising all base transceiver stations of the cells of the first macrocell is transmitted continuously on the BCCH at a common broadcast control channel frequency, .l -the base transceiver stations of the first macrocell communicate with mobile telephones on traffic channels allocated time slots and frequency laws locked onto the synchronisation signal delivered by the BCCH, and -a call involving a mobile telephone communicating with a base transceiver S•station of a second macrocell and entering the first macrocell is transferred to a base transceiver station of the first macrocell by an instruction including the assignment of 15 a protocol for entering into communication with a base transceiver station of the first macrocell, wherein the central unit: S measures a level of use of time slots, determines a least used time slot, and configures the entry protocol so that it uses the least used time slot.

2. A method according to claim 1 wherein, for configuring the entry protocol, calls involving mobile telephones already communicating with an allocated base transceiver station are handed over to the least used time slot.

3. A method according to claim 2 wherein, for handing over calls, the calls are allocated to the same allocated base transceiver station or to a base transceiver station other than that with which they are already communicating using a different time slot and possibly a different frequency law. CE\99151007.1 22

4. A method according to any one of claims 1 to 3 wherein the measurement or determination procedure takes into account only predetermined base transceiver stations in a list of peripheral base transceiver stations.

A method according to any one of claims 1 to 4 wherein the measurement or determination procedure eliminates a time slot for which a call involving a mobile telephone using it cannot be handed over to a free time slot of the base transceiver station with which it is communicating or of another base transceiver station.

6. A method according to any one of claims 1 to 5 wherein the least used time slot is measured and/or determined periodically. 10

7. A method according to any one of claims 1 to 6 wherein: the base transceiver stations receive from the central unit an instruction to listen to the traffic of mobile telephones communicating with neighbouring base transceiver stations, the instruction designates the time slot, frequency law and identifier for each 15 mobile telephone to be listened to, the base transceiver stations listen to calls set up by their neighbours, the base transceiver stations measure the quality of the signals received, allocate these results to the respective mobile telephones, and the base transceiver stations transmit the qualitative results of listening to the central unit for the latter to administer the network accordingly.

8. A method according to any one of claims 1 to 7 wherein a first base transceiver station transmits on the traffic channel it is using to communicate with a mobile telephone an assignment of a protocol for that mobile telephone to use to communicate with a second base transceiver station to allow for the movement of the mobile telephone relative to the first base transceiver station. CE\99151007.1 23

9. A method substantially as hereinbefore described with reference to figures 6 and 7 of the accompanying drawings. Dated this 2nd day of June 1999 Alcatel by its attorneys Freehills Patent Attorneys a a a