WO2000074419A1

WO2000074419A1 - Handover in a cellular communications system

Info

Publication number: WO2000074419A1
Application number: PCT/EP2000/005142
Authority: WO
Inventors: Peter William Dale Bishop; Kevan Hobbis
Original assignee: Motorola Limited
Priority date: 1999-06-01
Filing date: 2000-05-31
Publication date: 2000-12-07
Also published as: GB2350751B; AU5969900A; GB2350751A; GB9912533D0

Abstract

Means of performing handover in a cellular communications system; comprising determining that a mobile station is in a first coverage area served substantially entirely by a single base transceiver station and expected to have sharp coverage transition properties, preparing respective soft handover links to one or more further base transceiver stations providing coverage to locations adjoining the first coverage area irrespective of signal strength levels between the mobile station and the one or more further base transceiver stations, and using one or more of the prepared soft handover link or links to perform handover when the mobile station exits the first coverage area. The present invention is advantageous when applied to lift shafts in buildings.

Description

HANDOVER IN A CELLULAR COMMUNICATIONS SYSTEM

Field of the Invention

The present invention relates to code division multiple access (CDMA) or combined time division multiple access (TDMA)/CDMA cellular communications systems. The present invention is applicable to, but not limited to, CDMA cellular radio communications systems such as CDMA 2000 and wideband

CDMA, as envisaged to be adopted for the Universal Mobile Telecommunication System (UMTS), the latter being currently under standardisation.

Background of the Invention

One type of communications system is a cellular communications system. In a cellular communications system, service is provided on an area basis by a plurality of base transceiver stations (BTS) each of which has an antenna or antennas for transmission to and reception from user stations, normally mobile stations (MS).

In cellular communications systems handover means and processes are provided to enable the choice of BTS or BTSs which is providing service to a particular MS to be changed as the mobile station moves relative to the BTSs.

In typical cellular communications systems, handover can be accomplished as the mobile station moves from the coverage area of a particular BTS to the coverage area of another BTS as sufficient time exists for required signalling and so on to carry out the handover because the coverage area is not in practice sharply defined. In other words there will be a transition area that the mobile user is travelling through, which transition area is in fact serviceable at least to some extent or other by the BTS the MS is moving away from and also by the BTS the MS is moving towards. However, in typical cellular communications systems, a problem arises when the layout of the system gives rise to a coverage area from a particular BTS that is sharply defined, such that when the mobile station moves over the border from the coverage area to an area outside the coverage area, this happens very quickly. Such a case arises for example for a lift shaft in an in-building system. As shown in Figure 1 for example, a good quality of service can be provided to users when actually in a lift compartment 105 of the lift shaft 110, because BTS 115 is positioned within the lift shaft and provides an excellent level of coverage within the lift shaft. However when a mobile station exits the lift compartment and enters one of the floors of the building, the enclosed characteristic of the lift shaft 110 means that the signals from BTS 115 no longer reach the MSr The sharp coverage transition property of BTS 115 with respect to the MS in such a situation, that is the fact that a good level of service is provided on one side of the lift compartment door, which then falls to a very low level of service the other side of the lift compartment door when the user enters the floor of the building on exiting the lift compartment 105, is not accommodated by conventional handover methods, and consequently service to the MS may be cut.

In the case of CDMA or combined TDMA/CDMA systems, handover is generally improved by the use of a procedure that makes use of a facility called soft handover . Such a procedure follows from the aspect that in a CDMA MS there are a plurality of receivers (RAKE receivers), typically consisting of respective processors, each of which can be independently controlled by signalling which is used as part of preparation for handover. Each individual receiver of a given MS will be used to detect and receive a separate signal component from either a different BTS or from a different multi-path reflection path from a given BTS, as is well known to the skilled person, thus an MS will be receiving service from a plurality of BTSs. As the MS moves into the range of yet a further BTS, the MS will detect this further BTS normally by virtue of scanning for relevant signalling from new BTSs. The MS is then conventionally able to signal to a control station of the system, such as a Radio Network Controller (RNC) or Base Station Controller (BSC), which can then instruct the new BTS to transmit service to the MS. A soft handover link is prepared by the RNC or BSC providing suitable information to the BTS and the MS such that the BTS and the MS are more prepared for the impending service than would otherwise be the case. For example the BTS will be informed typically of the required frequency, i time offset and spreading code to use for the MS, and the MS will for example receive relevant scrambling codes for setting up its RAKE receiver. Thus the situation of the relevant BTS and the MS each having added information in preparation for communication therebetween constitutes preparation of a soft handover link.

However, even with such a handover procedure as ^'offered conventionally by CDMA systems, the above problem with sharp coverage transition situations, such as the lift shaft described above, is not overcome because when the MS is in the lift shaft it does not necessarily detect the signals from the BTSs of the adjoining floors. As such, no soft handover links are set up in advance of the user exiting the lift shaft, hence the aspect of dramatic fall off of signal from the lift shaft BTS is strongly compounded by the fact that otherwise available BTSs are not able to provide service to the MS because neither such BTSs nor the MS are prepared in advance for such handover.

Summary of the Invention

The present invention addresses the above problems by providing a means for service to be provided to an MS even when it suddenly leaves a coverage area with a sharp coverage transition property. The invention provides soft handover links to suitable further base stations in a manner that would not occur in conventional methods that rely on the MS itself detecting, in advance, adjoining candidate BTSs.

According to one aspect of the present invention, there is provided a method of performing handover in a cellular communications system, as claimed in Claim 1. According to another aspect of the present invention, there is provided apparatus for performing handover in a cellular communications system, as claimed in Claim 12.

Further aspects of the invention are as claimed in the dependent Claims.

Additional specific advantages are apparent from the following description and figures.

Brief Description of the Drawing

Figure 1 is a schematic illustration of an embodiment of the present invention.

Description of an Embodiment of the Invention

One particular embodiment of the invention is now described by way of example only.

Figure 1 shows an in-building cellular communications system which is operating on a CDMA basis. The in-building system provides communications coverage within building 101 as shown in Figure 1. In the present example building 101 contains a total of eight floors, namely ground floor 140 and first floor 141 through to seventh floor 147. A lift shaft 110 rises through the building as also shown in Figure 1. Within the lift shaft 110 a lift compartment 105 serves to transport users between the floors. Each of the floors 140 to 147 has a respective BTS, 120 through to 127, as shown in Figure 1. Additionally, lift shaft 110 has BTS 115, as also shown in Figure 1. All the BTSs are connected via communications link 135 to control station 130. In the present example control station 130 is physically remote from building 101 , but in other examples it can be within the building, and consists of the control portion of the BTS(s) and /or a BSC / RNC. Control station 130 is linked to the remainder of the cellular communications system via communications link 150. Details of the remainder of the system are not necessary for the understanding of the present invention.

Before describing the application of the present invention to the system shown in Figure 1 , it will assist in understanding to first summarise the conventional handover method as will be applicable to mobile stations originally in the coverage areas served by BTSs 120 to 127, in other words on floors 140 to 147. Let us consider an MS 160 originally on the third floor 143 of the building. When starting a call MS 160 will be served by BTS 123 which will have by far the strongest signal levels will respect to MS 160. Let us further assume that MS 160 has six receivers. Under conventional service and handover techniques, one receiver of MS 160 will detect arid receive tRe^~primary signal from BTS 123. The other five receivers will scan for other signals from other BTSs. In the present comparison example let us assume that leakage between the floors and transmission levels through the floor and ceiling materials are such that sufficiently high signals are received from the other floor BTSs 120 through to 127 for MS 160 to receive such signals, albeit at decreasing levels as the BTSs are respectively further away from the third floor 143. Thus under conventional techniques the other receivers will lead to soft handover links being set up to adjoining BTSs, for example in the present case BTSs 125, 124, 122, 121 and 120, from respectively floors 145, 144, 142, 141 and 140. Thus in a conventional original coverage area, the system under the control of control station 130 prepares and uses soft handover links between these other BTSs and MS 160 based on the signal strength levels between MS 160 and the other BTSs. This has two main consequences which can later be contrasted with the present invention. The first consequence is that MS 160 receives service conventionally from the other BTSs even whilst remaining in the prime coverage area of the original BTS, mainly BTS 123. The other consequence is evidenced in conventional handover technique, whereby when MS160 moves away from BTS 123 and in a direction of the other BTSs or at least some of those that are also providing service, any loss of service from the original BTS 123 is covered by effective takeover of more of the service from the other BTSs that already have this soft handover link established. To be more specific, if MS 160 leaves third floor 143 in the direction of fourth floor 144, using perhaps a staircase on which MS 160 moves relatively slowly, the staircase moreover being physically such that sufficient signal strength is also received therein from the other BTSs for the BTSs to remain detected, then even as the user moves up the building away from third floor 143, service will remain provided by use of at least BTS 124 and BTS 125, as these already have the soft handover links established.

The problem addressed by the present invention is suffered by MS 170 of a user in lift compartment 105. This is because in the present example although MS 170 is served very well by BTS 115, which serves all of lift shaft 110, nevertheless the physical characteristics of the lift shaft 110 , and lift compartment 105, being constructed largely of metal, are^~sucn that either no signal or very low signals only are received by MS 170 from the other BTSs in the building, likewise from any other BTSs of the wider communications system that control station 130 links to via communications link 150. Hence the other receivers of MS 170 other than that employed to receive and detect the information from BTS 115 are not used profitably. It may be the case that one or more multi-path routes signals are in fact processed by some of the further receivers of MS 170, however, in most practical situations even this would not be the case because the distance involved in lift shaft 110 will be negligible compared to most multi-path reflection problems, or at least in respect of the time offsets produced thereby. Therefore the coverage area of BTS 115, ie. the lift shaft 110, is a first coverage area served either entirely or at least substantially entirely by a single base station BTS 115. Furthermore when MS 170 exits the lift compartment 105 at one of the floors of the building, MS 170 will immediately leave the coverage area of BTS 115. This is because in the present example the lift shaft is very well defined and is of such physical characteristics that the signal from BTS 115 is strong anywhere within the lift shaft, but dramatically falls away as one moves away from the lift shaft. Thus the coverage area represented by lift shaft 110 and served by BTS 115 is an example of a coverage area that has what may be termed a sharp coverage transition property. Because in the present example MS 170, whilst in the lift shaft 110, was not able to detect BTSs 120 to 127, or alternatively only detected them at such low signal levels that they did not satisfy the criteria for being chosen for set up of a soft handover link, no soft handover links as described earlier are ready established for the case of MS 170 to these other BTSs on the occasion of MS170 exiting the lift on to one of the floors of the building. Consequently as soon as MS 170 exits the lift shaft, service fails as no BTS is both able to provide coverage and ready to provide coverage. Thus the conventional handover technique, that was described earlier above as being adequate for MS 160, is inadequate for the current example of MS 170.

In the above paragraph it was shown ^"how MS 170 being in the above described lift shaft represented an MS in a coverage area expected to have a sharp coverage transition property. The case of a lift shaft is merely one example of a physical situation that gives rise to a coverage area served substantially entirely by a single base station and expected to have sharp coverage transition properties. Other physical characteristics within a building could give rise to other similar situations. Principally any area enclosed in an electro-magnetic sense to an extent that the BTS within it is effectively the only BTS received by a mobile station fulfils the criteria. Furthermore other physical characteristics not part of an in-building system would also come under this definition. Arrangements of buildings within a street could give rise to this characteristic, for a BTS that was a conventional out-of-building BTS. As such it is to be appreciated that the invention and its advantages apply to any cellular communications system involving soft handover links such as described above for CDMA based systems, wherein a coverage area can have the characteristics described above.

The method of the present embodiment addresses the problem created by a coverage area such as the lift shaft 110 described above and comprises the following steps. The first step is that of determining that MS 170 is in a coverage area served substantially entirely by the single base station, in this case BTS 115, and is expected to have sharp coverage transition properties, which in the present embodiment is the case when exiting the lift shaft to enter the floors as described above. Different possibilities exist for the way this determining step is implemented. In the present embodiment this is carried out by means whereby MS 170 determines from signalling between itself and BTS 115 that BTS 115 has a pre-assigned category which denotes that it is a BTS serving such an area, this information is pre-programmed into the system by the controller of the system based on trial measurements during installation, commission or operational management of the system. This could alternatively be programmed in as a default programming indication merely by the fact that the installer of the system is aware that BTS 115 is being installed in a lift shaft. Alternatively the information could be ascribed by remote communications means once data from the system has been aττaιysedτTnciuding either test calls or real calls. An alternative means for determining that the coverage area is served substantially entirely by a single based station and is expected to have a sharp coverage transition property would be for MS 170 to deduce this factor from the situation that it is effectively only receiving signals from one BTS, and that when it scans for other BTSs it does not detect either any signal or at least any sufficient signal. The processing required for this could be carried entirely in MS 170 or could be carried out either fully or partly elsewhere in the system under the control of control station 130 or other processing means. Other inputs to an algorithm would be included by the skilled person according to the requirements of the system under consideration, in particular taking into account other possible reasons that similar balances of signal strengths could arise. It will also be appreciated that even more sophisticated arrangements are possible whereby a pre-assigned category of a BTS is combined with processing to verify such category or processing to refine such categorisation, the processing being carried out based on signal strength measurements.

The next step carried out in the present embodiment is that of preparing respective soft handover links to one or more further base stations which provide coverage to locations adjoining the first coverage area irrespective of the signal strength levels. In the case of the present embodiment this involves the system preparing soft handover links to a number of the BTSs 120 to 127 in the building. The crux of the invention is that this is done in response to the fact that it has been determined that the coverage area is as described previously, and the preparation of the soft handover links is done despite the fact that signal levels received by MS 170 from the other BTSs are either zero or less than would normally be required for soft handover links to be set up for standard coverage areas. Thus it can be seen that in the present embodiment the soft handover links to the other BTSs are set up irrespective of the signal strength levels between MS 170 and the respective other BTSs. It is noted that if low signal levels are received, they may in certain versions of the present invention be employed to refine certain other aspects, for example a choice of which BTSs should be set up for soft handover links, but this does in no way negate the factor that the setting up of the^off handover links is carried out in response to determination of the characteristics of the first coverage area in this case lift shaft 110 and that such setting up of soft handover links is only occurring because of the characterisation or the determination of the characterisation of the coverage area being such a lift shaft 110, and in the case of low signal levels from the other BTSs being used for any optional refinement, such low levels would for any other type of coverage area not lead to setting up of a soft handover link in the conventional handover methods. The question of which BTSs are chosen to have soft handover links prepared in the present embodiment will be discussed later below.

Although in the above it has been mentioned that multi-path reception from single base station BTS 115 will not probably be a big issue in the present lift shaft example, nevertheless in other examples as described above, the mobile station receiving service from substantially one BTS may indeed be receiving a number of multi-path reflections and would normally use a separate receiver within the MS for each multi-path reflected signal component. In such a situation a further optional improvement to the present invention is to overrule the normal use of the other receivers for the further multi-path reflections from the single BTS, and in response to having determined that the BTS is one with expected sharp coverage transition property, to consequently assign instead one or more of those receivers to instead receive soft handover link signals from other base stations, even though the signal levels will be very low. In such a method it is therefore possible to concentrate on setting up soft handover links in preparation for exiting the sharply defined coverage area, at the expense of full optimisation of the existing service from the single BTS. Such possibilities will be implemented by the skilled person in detail according to the requirements of the particular system under consideration, in particular employing usual criteria for trade off between two desirable characteristics, in the present case optimised reception from the primary BTS being traded off depending on system design or system operation with respect to the improved chance of handover success when exiting a sharply defined coverage area.

Within the scope of the present invention, the details involved in the soft handover links set up will vary according to the particular system under consideration, both in details of which types of parameters and information is set up, as well as to the degree or amount of such information. Whatever the degree or details, benefits accrue from the present invention by the aspect of the soft handover links being prepared, even if such soft handover links are not necessarily fully and absolutely ready for instantaneous handover, as even partial preparation will improve the chance of a successful handover. As such, the following details of the soft handover links set up in the present particular embodiment are to be understood as merely exemplary.

The details forming soft handover links of the present embodiment are as follows. The call parameters from MS 170 are known to serving BTS 115 as a result of the usual call processing and signalling techniques, well known to the skilled person. Typical call parameters include spreading code, power level, time offset and so on. Such call parameters are communicated to the chosen further BTSs and thus constitute part of the soft handover link established between the other BTSs and MS 170 since such data being held at the other BTSs gives them capability for taking over service for MS 170 as soon as required. Another aspect of the soft handover link of the present embodiment is that of sending to MS 170 details it will need to be ready to communicate with the other BTSs. This information is transmitted via BTS 1 15 to MS 170, since BTS 115 is the BTS in communication already with MS 170, but it should be noted that the information itself relates to the operation of the other BTSs, under the control of control station 130 in the present embodiment. Such information will include in the present embodiment the necessary information for MS 170 to configure its respective RAKE receiver settings such that it is able to decode the respective signals from the respective other BTSs. The concept of RAKE receiver techniques are well known to the skilled person, and do not need detailed description here for the purpose of understanding the present invention. The significance for the present invention is that the relevant information for processing such RAKE receiver is sent in the present example as part of the information set up forming the soft handover link. The details of the relevant information will depend on the details of the RAKE receiver methodology used in the system of the present embodiment; moreover in other embodiments employing other types of encoding the required or desired information to prepare for such processing will depend obviously on the details of the different processing systems therein. In the present embodiment the information includes scrambling codes that MS 170 will employ such that it is ready to decode the transmissions of the other BTSs when MS 170 exits the lift shaft 110. A further option is that the other BTSs can also be set up ready with respect to RAKE receiving. Another possibility is that the timing, more particularly timing offset, may be different for the other BTSs compared to original BTS 115. In reality this may not be all that likely for an in-building system such as shown in Figure 1 , but nevertheless depending on other processing factors within the system, the requirement for timing consistency may be very high, or indeed the timing will need to be considered in other embodiments where the location of the further BTSs is of a greater distance compared to the original BTS. In this case the necessary RAKE finger timing offset from the original cell can also be signalled to the MS.

The method of performing handover according to the present embodiment is completed by using one or more of the prepared soft handover link or links to perform the handover when the MS 170 exits the lift shaft 110. The question mentioned earlier above as to possible ways in which the question of which BTSs are selected to have soft handover links prepared therefor will now be described for the present embodiment of a lift shaft in a building. A simple method would be for the system or a control element thereof to merely select a number of BTSs by some random process. The random process could be refined by using a weighted random process whereby at least some level of coverage in certain ranges of floors of the building was included. Alternatively set lists or algorithms could be employed, according to the requirements of the particular building. However, further optional improvements are possible as will now be described. The initial floor information, by which is meant the identity of the floor on which the MS entered the lift shaft, which can be known to the system control system 130 by virtue of previous signal levels from BTSs 120 to 127, can be used to predict possible likely new floors to connect to as the lift compartment moves. For example if the mobile station is on the ground floor 140 before moving into the lift shaft it is clear that floors above will be the correct ones to set up soft handover links with. Timing characteristics can be included based on known speed of the lift or history of the lift operations stored by the communications system based on previous calls by previous MSs. An intelligent algorithm can be used to process previous call details to determine the most likely movement of a particular MS given the initial floor to which it was connected. This could be done by keeping statistics of initial floor to final floor movements to determine a most likely direction of travel. It could also be based on an MS or user identify characterisation, for example differentiating between internal users of the building and external users such as visitors, as they may have fairly well defined different characteristics, at least on a probability basis. Obviously a range of possibilities can be involved in a prediction process employed for attempting to select the most appropriate choices of BTSs in anticipation of the destination of MS 170. Whichever type of prediction process or even predetermined choices is used, the operator of the system or programmer will have another choice, in that he can choose to programme the system such that the best choices from determination of the prediction are used, on the basis of assuming such a process will be correct. In other words, for example, if MS 170 is expected to leave the lift shaft 110 at the fifth floor 145, then one possibility would be to set up BTSs 123 through to 127 as representing the best coverage should the estimation be correct. However an alternative would be to attempt to cover for any eventuality, even at the expense of optimum coverage should the prediction be correct, in which case alternate floors could be set up with soft handover links, and this should then provide at least a minimum level of service wherever the user exits the lift shaft, even if such service is not as good as optimally could be achieved at the risk of no coverage for a bad prediction.

When the above described prediction processes are being employed, a further optional refinement would be for means to be provided whereby the lift controls themselves, such as the buttons pushed by the user of MS 170 to send the lift compartment to the required floor, can be arranged such that this information is transmitted to control station 130 and included in the prediction process. Various means can be used to implement this, employing either hard wiring methods or radio communication from the lift compartment control panel to BTS 115. Obviously there is an almost limitless number of variations of algorithms that can be employed to process such information, and the details will in practice be chosen by the operator of the system according to the requirements of the particular system under consideration.

The above procedures for choosing which of the other BTSs should be set up with the desired soft handover links have been described in detail with respect to the lift shaft example of the present embodiment, where particular advantages can clearly be seen. However, such prediction processes involving estimation and calculation of which new coverage area is most likely to be reached, including the function of which coverage area the MS came from into the present single BTS coverage area and including the function of previous movement of the MS relative to the new coverage area, can all be employed separately or in combination, in other embodiments of the present invention that are not necessarily related to a lift shaft. The same applies to the possibility of using a function of a user category of the mobile station. In the above embodiment it has been described that certain control operations have been carried out at control station 130. It will be apparent that the exact means of implementing the control required for the above embodiment will vary according to the exact system requirements and the existing technology within the applicable system. Generally, the implementation can be performed by software located for example at control station 130, or by means of hardware or a combination of software and hardware.

It is also noted that the present invention can be applied to systems other than that shown by reference to Figure 1. Even in the case of the Figure 1 system, the details of the larger communications system to which the building is connected via communications link 150 will vary whilst remaining applicable to the present invention. In that sense the in-building communication system shown in Figure 1 may represent a pico-cellular system existing in a layered sense within a more general cellular communications system of which a further BTS, not shown in Figure 1 , may also be a candidate for providing service to MTS 170 on exiting the lift shaft. In such a case this further general BTS will possibly be employed as a candidate for soft handover links according to the present invention depending on the requirements of the system under consideration, consequently also benefiting from the present invention.

Claims

1. A method of performing handover in a cellular communications system; the method comprising the steps of: determining that a mobile station is in a first coverage area served substantially entirely by a single base transceiver station and expected to have sharp coverage transition properties; preparing respective soft handover links to one or more further base transceiver stations providing coverage to locations adjoining the first coverage area irrespective of signal strength levels between the mobile station and the one or more further base transceiver stations; and using one or more of the prepared soft handover link or links to perform handover when the mobile station exits the first coverage area.

A method according to claim 1 , wherein the step of determining is performed by means of determining a pre-assigned category of the single base transceiver station.

3. A method according to claim 1 , wherein the step of determining is performed by means of comparing the signal strength received by the mobile station from the single base transceiver station with the respective signal strengths received by the mobile station from the one or more further base transceiver stations.

4. A method according to claim 3, wherein one or more receivers of the mobile station that would normally be assigned to receiving multi-path signals from the single base transceiver station are assigned instead to receiving signals from the one or more further base transceiver stations.

5. A method according to any preceding claim, wherein the step of preparing respective soft handover links comprises the step of communicating one or more call parameters of the mobile station from the single base transceiver station to the one or more further base transceiver stations.

6. A method according to any preceding claim, further comprising the step of communicating information related to RAKE processing settings to the mobile station and/or the one or more further base transceiver stations.

7. A method according to any preceding claim, wherein the first coverage area is a lift shaft.

8. A method according to claim 7, wherein the one or more further base transceiver stations are selected on the basis of being ones serving floors of a building containing the lift shaft that are predicted by a prediction process to be floors where the mobile station will exit the lift shaft.

9. A method according to claim 8, wherein the prediction process is a function of which floor the mobile station entered the lift on.

10. A method according to claim 8 or 9, wherein the prediction process is a function of previous movement of the mobile station relative to the lift and/or is a function of a user category for the mobile station.

1 1. A method according to any of claims 8-10, wherein the prediction process includes information transmitted from user controls of the lift.

1 2. Apparatus for performing handover in a cellular communications system; the apparatus comprising:

means for determining that a mobile station is in a first coverage area served substantially entirely by a single base transceiver station and expected to have sharp coverage transition properties; means for preparing respective soft handover links to one or more further base transceiver stations providing coverage to locations adjoining the first coverage area irrespective of signal strength levels between the mobile station and the one or more further base transceiver stations; and means for using one or more of the prepared soft handover link or links to perform handover when the mobile station exits the first coverage area.

13. Apparatus according to claim 12, wherein the step of determining is performed by means of determining a pre-assigned category of the single base transceiver station.

14. Apparatus according to claim 12, wherein the step of determining is performed by means of comparing the signal strength received by the mobile station from the single base transceiver station with the respective signal strengths received by the mobile station from the one or more further base transceiver stations.

15. Apparatus according to claim 14, wherein one or more receivers of the mobile station that would normally be assigned to receiving multi-path signals from the single base transceiver station are assigned instead to receiving signals from the one or more further base transceiver stations.

16. Apparatus according to any of claims 12-15 wherein the step of preparing respective soft handover links comprises the step of communicating one or more call parameters of the mobile station from the single base transceiver station to the one or more further base transceiver stations.

17. Apparatus according to any of claims 12-16 further comparing the step of communicating information related to RAKE processing settings to the mobile station and/or the one or more further base transceiver stations.

18. Apparatus according to any of claims 12-17 wherein the first coverage area is a lift shaft.

19. Apparatus according to claim 18, wherein the one or more further base transceiver stations are selected on the basis of being ones serving floors of a building containing the lift shaft that are predicted by a prediction process to be floors where the mobile station will exit the lift shaft.

20. Apparatus according to claim 19, wherein the prediction process is a function of which floor the mobile station entered the lift on.

21. Apparatus according to claim 19 or 20, wherein the prediction process is a function of previous movement of the mobile station relative to the lift and/or is a function of a user category for the mobile station.

22. Apparatus according to any of claims 19-21 , wherein the prediction process includes information transmitted from user controls of the lift.