Abstract:
In a cellular mobile communications network a mobile station is capable of receiving a downlink signal from each of a plurality of base stations and transmitting an uplink signal to the plurality of base stations through a wireless channel. A transmission property of the downlink signals from the plurality of base stations to the mobile station is measured, and decided, in dependence upon the measure of the transmission property, a preferred base station transmitting the downlink signal with a preferred transmission property among the plurality of the base stations. The mobile station includes, in the uplink signal, data indicating the preferred base station(s) for transmitting the subsequent downlink signal to the mobile station. The base stations receiving the uplink signal identify from the data the preferred based station(s) and only the base station(s) identified as the preferred base station(s) transmits a subsequent downlink signal to the mobile station. Interference in such a cellular mobile telecommunications network can therefore be reduced.

Description:
This application is a divisional of application Ser. No. 09/696,431, filed Oct. 25, 2000, now pending, which is a continuation of PCT application Serial No. PCT/GB99/01344 filed on Apr. 28, 1999 and PCT application Serial No. PCT/GB99/01347 filed on Apr. 28, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cellular mobile communication networks, for example Code Division Multiple Access (CDMA) cellular networks. 
     2. Description of the Prior Art 
     FIG. 1 of the accompanying drawings shows parts of a cellular mobile telecommunication network according to the Telecommunication Industries Association (TIA)/Electronic Industries Association (EIA) Standard TIA/EIA/IS-95 of October 1994 (hereinafter “IS95”). Each of three base transceiver stations (BTSs)  4  (BTS 1 , BTS 2  and BTS 3 ) is connected via a fixed network  5  to a base station controller (BSC)  6 , which is in turn connected to a mobile switching center (MSC)  7 . The BSC  6  serves to manage the radio resources of its connected BTSs  4 , for example by performing hand-off and allocating radio channels. The MSC  7  serves to provide switching functions and coordinates location registration and call delivery. 
     Each BTS  4  serves a cell  8 . When a mobile station (MS)  10  is in a so-called “soft hand-off” (SHO) region  9  where two or more cells overlap, a mobile station can receive transmission signals (downlink signals) of comparable strength and quality from the respective BTSs of the overlapping cells. Transmission signals (uplink signals) produced by the mobile station (MS) can also be received at comparable strengths and qualities by these different BTSs when the mobile station is in the SHO region  9 . 
     FIG. 2 of the accompanying drawings shows a situation where the MS  10  is located within the SHO region  9 , and is transmitting such uplink signals that are being received by plural BTSs  4 . According to the IS95 standard, a BTS  4  that receives such an uplink signal from the MS  10  relays the signal to the BSC  6  via a dedicated connection line of the fixed network  5 . At the BSC  6 , one of the relayed signals is selected based on a comparison of the quality of each of the received signals, and the selected signal is relayed to the MSC  7 . This selection is referred to as Selection Diversity. 
     Similarly, FIG. 3 of the accompanying drawings shows a situation where the MS  10  is located within the SHO region  9  and is receiving downlink signals from plural BTSs  4 . According to the IS95 standard, downlink signals received by the BSC  6  from the MSC  7  are relayed to all BTSs  4  involved in the soft hand-off via respective connection lines of the fixed network  5 , and subsequently transmitted by all the BTSs  4  to the MS  10 . At the MS  10  the multiple signals may be combined, for example, by using maximum ratio combination (MRC), or one of them may be selected based on the signal strength or quality, i.e. using Selection Diversity as for the uplink case. 
     In contrast to, for example, Global System for Mobile Communication (GSM) networks, in CDMA networks each BTS  4  transmits at the same frequency. Consequently, careful control of transmission power must be maintained to minimize interference problems. 
     Signals are transmitted as a succession of frames according to the IS95 standard. As FIG. 4 of the accompanying drawings shows, each frame is of duration 20 ms, and comprises sixteen 1.25 ms time slots. In each time slot several bits of user data and/or control information can be transmitted. 
     Power control of transmissions from the MS  10  to the BTSs  4  (uplink power control) in IS95 is achieved as follows. When a BTS  4  receives a signal from the MS  10  it determines whether a predetermined property of the received signal (for example absolute signal level, signal to noise ratio (SNR), signal-to-interference ratio (SIR), bit error rate (BER) or frame error rate (FER)) exceeds a pre-selected threshold level. Based on this determination, the BTS  4  instructs the MS  10  either to reduce or to increase its transmission power in the next time slot. 
     For this purpose, two bits in every time slot of a pilot channel (PCH) from the BTS  4  to the MS  10  are allocated for uplink power control (see FIG.  4 ). Both bits have the same value, and accordingly will be referred to hereinafter as the “power control bit” (or PCB) in the singular. The power control bit is assigned a value of zero by the BTS  4  if the MS  10  is required to increase transmission power by 1 dB, and a value of one if the MS  10  is required to decrease transmission power by 1 dB. The BTS  4  is not able to request directly that the MS  10  maintain the same transmission power; only by alternately transmitting ones and zeros in the power control bit is the transmission power maintained at the same level. 
     When the MS  10  is in a SHO region  9 , the MS  10  is required to make a decision on whether to increase or to decrease uplink transmission power based on a plurality of power control bits received respectively from the BTSs  4  involved in the soft hand-off. Consequently, an OR function is performed on all the power control bits. If the result of this OR function is zero then the MS  10  will increase power on uplink transmissions, and if the result is one then the MS  10  will decrease power on uplink transmissions. In this way, uplink transmission power is only increased if all BTSs  4  ask for an increase. 
     Power control of transmissions from the BTS  4  to the MS  10  (downlink power control) in IS95 is achieved as follows. When the MS  10  receives a downlink signal from a BTS  4  (or from each of a plurality of BTSs  4  in soft hand-off operation) via a traffic channel (TCH), the FER of that signal is calculated by the MS  10 . The FER reflects the degree to which the traffic-channel signal has been corrupted by, for example, noise. This FER is then relayed by the MS  10  to the BTS  4  which transmitted the downlink signal concerned, and the BTS  4  uses this FER to decide whether to make any change to its downlink transmission power. 
     The soft hand-off system described above is effective in improving signal transmission between the MS  10  and the network when the MS  10  is located in regions of cell overlap near the boundaries of the individual cells. Signal quality in these regions when using a single BTS  4  may be relatively poor, but by making use of more than one BTS  4  the quality may be substantially improved. 
     However, the IS95 soft hand-off system has the disadvantage of increasing signal traffic in the cellular network since it is necessary to transmit downlink signals carrying the same data and/or control information to the MS  10  from every BTS  4  involved in the soft hand-off. This duplication of transmissions is undesirable because each transmission is potentially a source of interference to other transmissions in the network. 
     For example, the downlink-power control method aims at ensuring that the MS  10  receives a useful downlink signal from every one of the BTSs  4  involved in the soft hand-off. In the event that the downlink signal from one of the BTSs is undergoing a deep fade, the MS  10  will instruct the BTS concerned to increase its downlink transmission power significantly. However, in this case the BTS concerned will inevitably cause greater interference to other transmissions taking place in its cell and in neighboring cells. This problem may be exacerbated if, as in the IS95 standard, only one PCB is allocated in common for downlink power control to all of the BTSs involved in the soft hand-off. In this case, not only does the BTS that is experiencing a deep fade increase its downlink transmission power significantly, but also every other one of the BTSs involved in the soft hand-off increases its downlink transmission power, significantly increasing the interference within the cellular network as a whole. 
     Therefore, it is desirable to reduce interference in the cellular network associated with the soft hand-off operation. It is also desirable to reduce interference in cellular networks in other situations in which a mobile station is in communications range of more than one base transceiver station. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a cellular mobile communications network including: a candidate base transceiver station identifying unit operable, when a mobile station of the network is capable of receiving a downlink signal from a plurality of base transceiver stations of the network, to identify at least two different candidate base transceiver station selections. Each such selection specifying one or more base transceiver stations of the plurality for possible use in transmitting a subsequent such downlink signal to the mobile station. A network interference determining unit operable, for each of the candidate selections, to produce a measure of the network interference that would be caused by the base transceiver station(s) specified in that candidate selection transmitting the subsequent downlink signal to the mobile station. A decision unit operable, in dependence upon the network-interference measures, to decide which one of the candidate selections is to be used to transmit the subsequent downlink signal to the mobile station, so as to tend to reduce network interference arising from the transmission of that downlink signal. 
     According to a second aspect of the present invention there is provided a mobile station, for use in a cellular mobile communications network, including: a candidate base transceiver station identifying unit operable, when the mobile station is capable of receiving a downlink signal from a plurality of base transceiver stations of the network, to identify at least two different candidate base transceiver station selections. Each such candidate selection specifying one or more base transceiver stations of the plurality for possible use in transmitting a subsequent such downlink signal to the mobile station. A network interference determining unit operable for each of the candidate selections, to produce a measure of the network interference that would be caused by the base transceiver station(s) specified in that selection transmitting the subsequent downlink signal to the mobile station. A decision unit operable, in dependence upon the network-interference measures, to decide which one of the candidate selections should be used to transmit the subsequent downlink signal to the mobile station, so as to tend to reduce network interference arising from the transmission of that downlink signal. 
     According to a third aspect of the present invention there is provided a base transceiver station, for use in a cellular mobile communications network, including: A candidate base transceiver station identifying unit operable, when a mobile station of the network is capable of receiving a downlink signal from a plurality of base transceiver stations of the network including the base transceiver station, to identify at least two different candidate base transceiver station selections. Each such candidate selection specifying one or more base transceiver stations of the plurality for possible use in transmitting a subsequent such downlink signal to the mobile station. A network interference determining unit operable, for each of the candidate selections, to produce a measure of the network interference that would be caused by the base transceiver station(s) specified in that selection transmitting the subsequent downlink signal to the mobile station. A decision unit operable, in dependence upon the network-interference measures, to decide which one of the candidate selections should be used to transmit the subsequent downlink signal to the mobile station, so as to tend to reduce network interference arising from the transmission of that downlink signal. 
     According to a fourth aspect of the present invention there is provided a base station controller, for use in a cellular mobile communications network, including: A candidate base transceiver station identifying unit operable, when a mobile station of the network is capable of receiving a downlink signal from a plurality of base transceiver stations of the network, to identify at least two different candidate base transceiver station selections. Each such candidate selection specifying one or more base transceiver stations of the plurality for possible use in transmitting a subsequent such downlink signal to the mobile station. A network interference determining unit operable, for each of the candidate selections, to produce a measure of the network interference that would be caused by the base transceiver station(s) specified in that selection transmitting the subsequent downlink signal to the mobile station. A decision unit operable, in dependence upon the network-interference measures, to decide which one of the candidate selections to use to transmit the subsequent downlink signal to the mobile station, so as to tend to reduce network interference arising from the transmission of that downlink signal. 
     According to a fifth aspect of the present invention there is provided a communications method for use in a cellular mobile communications network, including: when a mobile station of the network is capable of receiving a downlink signal from a plurality of base transceiver stations of the network, identifying at least two different candidate base transceiver station selections, each such selection specifying one or more base transceiver stations of the plurality for possible use in transmitting a subsequent such downlink signal to the mobile station; producing, for each of the candidate selections, a measure of the network interference that would be caused by the base transceiver station(s) specified in that selection transmitting the subsequent downlink signal to the mobile station; and deciding, in dependence upon the network-interference measures, which one of the said candidate selections to use to transmit that subsequent downlink signal to the mobile station, so as to tend to reduce network interference arising from the transmission of that downlink signal. 
     In one embodiment of the first to fifth aspects of the invention, the candidate selections may include, for each BTS of the plurality, a selection in which just that BTS is specified, as well as a further selection in which all the BTSs of the plurality are specified. It is not essential for the candidate selections to include selections specifying only one BTS. For example, if there are three BTSs involved in a soft hand-off operation, the selections could be BTS 1 +BTS 2 , BTS 2 +BTS 3 , BTS 3 +BTS 1 , and BTS 1 +BTS 2 +BTS 3 . It is also not essential for the candidate selections to include a selection specifying all the BTSs involved in the soft hand-off. Furthermore, the transmission powers for the BTSs specified in a particular selection can be set to any suitable combination of values capable of facilitating adequate reception of the downlink signal at the subject mobile station. Thus, for example, two or more candidate selections could specify the same BTSs but specify different respective sets of transmission powers for the selections. In other words, two candidate selections could differ from one another only in respect of the transmission powers of the (same) specified BTSs. 
     According to a sixth aspect of the present invention there is provided a mobile station for use a cellular mobile communications network, including: A base transceiver station decision unit operable, when the mobile station is capable of receiving a downlink signal from a plurality of base transceiver stations of the network, to determine that at least one of the base transceiver stations of the plurality is not to transmit a subsequent such downlink signal to the mobile station; and a base transceiver station informing unit operable to inform the base transceiver stations of the plurality of the determination made by the base transceiver station decision unit using one or more uplink signals transmitted by the mobile station to such base transceiver stations. 
     According to a seventh aspect of the present invention there is provided a base transceiver station for use in a cellular mobile communications network, including: A receiver for receiving uplink signals from a mobile station of the network, one or more of which uplink signals includes, when the mobile station is capable of receiving a downlink signal from a plurality of base transceiver stations of the network including the base transceiver station, base transceiver station selection information specifying that at least one of the base transceiver stations of the plurality is not to transmit a subsequent such downlink signal to the mobile station; and a disabling unit operable to process such base transceiver station selection information and to prevent the base transceiver station from transmitting such a subsequent downlink signal if the received base transceiver station selection information specifies that the base transceiver station is not to transmit the subsequent downlink signal. 
     The sixth and seventh aspects of the present invention are not limited to downlink transmission selection for the purpose of interference reduction. Embodiments of these aspects of the invention can be used in any situation in which it is desired to prevent at least one BTS in communications range of a mobile station from transmitting a downlink signal to that mobile station. 
     According to an eighth aspect of the present invention there is provided a mobile station, for use in a cellular mobile communications network, including: a transmitter for transmitting uplink signals to a base transceiver station of the network and a signal information processor connected to the transmitter and operable, during a soft hand-off operation involving a plurality of such base transceiver stations of the network, to produce respective signal measures for all the base transceiver stations involved in the operation, each such signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned, and also operable to employ the produced signal measures to determine which of the involved base transceiver stations should be used to transmit a subsequent downlink signal to the mobile station, and to cause the transmitter to include, in such an uplink signal transmitted thereby, a base transceiver station selection message identifying the determined base transceiver station(s). 
     According to a ninth aspect of the present invention there is provided a base station controller, for use in a cellular mobile communications network to apply downlink signals to a plurality of base transceiver stations of the network, including: a receiver for receiving uplink signals from one or more of the base transceiver stations, at least one of which uplink signals includes, when a mobile station is engaged in a soft hand-off operation involving more than one of the base transceiver stations of the network, a base transceiver station selection message identifying which of the involved base transceiver stations should be used to transmit a subsequent one of the downlink signals to the mobile station; and a soft hand-off control unit operable to receive the uplink signal including the base transceiver station selection message and to transmit the subsequent downlink signal only to the determined base transceiver station(s) identified in the message. 
     According to a tenth aspect of the present invention there is provided a soft hand-off control method for use in a cellular mobile communications network, wherein: when a soft hand-off operation involving more than one base transceiver station of the network is being performed, a mobile station produces respective signal measures for all the base transceiver stations involved in the operation, each such signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and the produced signal measures are employed to determine which of the involved base transceiver stations should be used to transmit a subsequent downlink signal to the mobile station. 
     The signal measures can be any suitable measure of the communications-channel performance between the mobile station and the base transceiver stations, for example signal strength measures (received signal strength in terms of power or amplitude or quality measures (frame error rate, signal-to-interference ratio, etc) or a combination of both strength and quality. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will now be made, by way of example, to the accompanying drawings, in which: 
     FIG. 1, discussed hereinbefore, shows parts of a cellular mobile telecommunication network according to IS95; 
     FIG. 2, also discussed hereinbefore, shows a schematic view for use in explaining processing of uplink signals in a soft hand-off operation performed by the FIG. 1 network; 
     FIG. 3, also discussed hereinbefore, shows a schematic view for use in explaining processing of downlink signals in such a soft hand-off operation; 
     FIG. 4, also discussed hereinbefore, illustrates the format of a time frame in the FIG. 1 network; 
     FIG. 5 shows parts of a mobile telecommunication network embodying the present invention; 
     FIG. 6 shows parts of a mobile station embodying to the present invention; 
     FIG. 7 is a detailed block diagram showing parts of the FIG. 6 mobile station; 
     FIG. 8 is a flowchart for use in explaining operation of the FIG. 6 mobile station; 
     FIG. 9 is a schematic view for illustrating a possible format of a message transmitted by the FIG. 6 mobile station; 
     FIG. 10 shows parts of a base transceiver station embodying the present invention; 
     FIG. 11 shows parts of another base transceiver embodying the present invention; 
     FIG. 12 is a detailed block diagram of parts of the FIG. 11 base transceiver station; 
     FIG. 13 shows a flowchart for use in explaining operation of the FIG. 11 base transceiver station; 
     FIG. 14 shows parts of a mobile station in another embodiment of the present invention; and 
     FIG. 15 shows parts of a base station controller suitable for use with the FIG. 14 mobile station. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 shows parts of a mobile telecommunication network embodying the present invention. In FIG. 5, elements that are the same as elements of the FIG. 1 network described previously have the same reference numerals and an explanation thereof is omitted. 
     The FIG. 5 network is a wideband CDMA (W-CDMA) network for a proposed new standard for mobile telecommunications, referred to as a universal mobile telecommunications system (UMTS) or UMTS terrestrial radio access (UTRA). This is generally similar to the IS95-standard network described previously, although certain implementation details are yet to be finalized. Details that are different from IS95 include the frame duration, which is 10 ms, and the time-slot duration which is 625 μs. The overall bit rate is within the range from 8 kbits/s to 2 Mbits/s. Also downlink power control in W-CDMA is closed-loop and is based on the same principles as the uplink power control. 
     In FIG. 5, each of three base transceiver stations (BTSs)  20  (BTS 1 , BTS 2  and BTS 3 ) is connected via a fixed network  5  to a base station controller (BSC)  30 , which is in turn connected to a mobile switching center (MSC)  7 . Each BTS  20  serves a cell  8 . A mobile station (MS)  40  is in a soft hand-off (SHO) region  9  and can receive downlink signals from, and transmit uplink signals to, all the BTSs  20  involved in the soft hand-off. 
     The FIG. 5 network corresponds generally with the FIG. 1 network, but the MS  40 , BTSs  20  and BSC  30  are constructed and operate differently from the corresponding elements in FIG.  1 . 
     FIG. 6 is a block diagram showing parts of a MS  40  embodying the present invention. An antenna element  42  is connected (e.g. via a duplexer—not shown) to a receiver portion  44  and a transmitter portion  46 . A downlink signal processing portion  48  receives from the receiver portion  44  respective downlink signals DS 1  to DSn produced by n BTSs BTS 1  to BTSn (n is an integer) involved in the soft hand-off operation. The downlink signal processing portion  48  applies a BTS selection message BSM to the transmitter portion  46 . 
     FIG. 7 shows a block diagram of the downlink signal processing portion  48 . The downlink signal processing portion  48  includes a downlink signal input portion  52  which receives the downlink signals DS 1  to DSn from the receiver portion  44 . The downlink signal processing portion  48  further includes respective TX and RX power storage portions  54  and  56 , each connected to the downlink signal input portion  52 . The TX power storage portion  54  receives a single power control bit PCB, or respective power control bits PCB 1  to PCBn corresponding respectively to the BTSs involved in the soft hand-off operation, and also receives from the downlink signal input portion  52  initial transmission powers TXP 1  to TXPn corresponding respectively to those BTSs. 
     The downlink signal input portion  52  also applies to the RX power storage portion  56  received power measures RXP 1  to RXPn corresponding respectively to the BTSs, each representing the power at which a downlink signal from the corresponding BTS is received by the mobile station. 
     Each of the power storage portions  54  and  56  includes storage regions corresponding respectively to the different BTSs. 
     The downlink signal processing portion  48  also includes a required RX power calculation portion  58  which receives a further signal measure FER, representing a downlink frame error rate determined by the mobile station, from the downlink signal input portion  52 . 
     The downlink signal processing portion  48  further includes a path loss calculation portion  60  which receives from the TX power storage portion  54  respective transmit powers TXP 1  to TXPn for the different BTSs and also receives respective receive powers RXP 1  to RXPn for the different BTSs from the RX power storage portion  56 . 
     The downlink signal processing portion  48  further includes a required TX power calculation portion  62  which receives respective path loss measures PL 1  to PLn for the different BTSs from the path loss calculation portion  60  and a required RX power RRXP from the required RX power calculation portion  58 . 
     The downlink signal processing portion  48  further includes a required TX power storage portion  64  and an interference calculation portion  66 , both of which receive from the required TX power calculation portion  62  first and second sets of required transmission powers. The first set of required transmission powers P BTS1  to P BTSn  represent required transmission powers of the different BTSs when the mobile station  40  is not using maximum ratio combining (MRC). The second set of transmission power measures P′ BTS1  to P′ BTSn  represent the required transmission powers of the different BTSs when MRC is employed at the MS 40 . The required TX power storage portion  64  has first and second sets of storage regions corresponding to the two sets of transmission power measures. 
     The downlink signal processing portion  48  further includes an interference storage portion  68  which receives interference measures I BTS1  to I BTSn  corresponding respectively to the different BTSs (transmitting alone), as well as a further interference measure I MRC  representing interference when all BTSs are used to transmit downlink signals and MRC is performed at the MS  40 . The interference storage portion  68  has storage regions corresponding respectively to these different interference measures. 
     The downlink signal processing portion  48  further includes an interference comparison portion  70  which receives the interference measures I BTS1  to I BTSn  and I MRC  from the interference storage portion  68  and produces a comparison signal COMP which is applied to a BTS selection portion  72 . The BTS selection portion  72  produces a BTS selection message (BSM) and a power control bit PCB (or plural PCBs PCB 1  to PCBn), which are applied to the transmitter portion  46  of the mobile station  40 . 
     Operation of the mobile station  40  of FIG. 7 will now be explained with reference to the flowchart of FIG.  8 . In this example, it will be assumed, for the sake of simplicity, that there are only two BTSs involved in the soft hand-off operation. 
     In a first step S 1  the downlink signal input portion  52  detects, in a downlink signal received from a first one (hereinafter BTS 1 ) of the BTSs involved in the soft hand-off operation, for example a signal on a dedicated control channel DCCH thereof, the initial transmit power ITXP 1  of BTS 1 . 
     As explained previously, the downlink power control method proposed for use in W-CDMA adjusts the transmission power of the BTSs in communication with a particular MS in dependence upon power control bits PCBs generated by the mobile station. At present, the proposed standard for W-CDMA specifies that a single PCB be used to control the downlink transmit powers of all of the BTSs involved in the soft hand-off operation. Thus, in this case all the involved BTSs increase or reduce their transmission powers together in accordance with the single PCB. However, it is also possible, in an embodiment of the present invention, to allocate each BTS involved in the soft hand-off operation its own individual PCB, enabling the MS to control the downlink transmission powers of the different involved BTSs independently of one another. In this case (as shown in parenthesis in FIG. 7) the TX power storage portion  54  receives PCBs PCB 1  to PCBn corresponding respectively to the different BTSs involved in the soft hand-off operation. 
     In step S 1  the initial transmission power ITXP 1  for BTS 1  is stored in the storage region allocated to BTS 1  in the TX power storage portion  54 . Thereafter, each time a new PCB (PCB or PCB 1 , as the case may be) applicable to BTS 1  is generated by the MS (for example every time slot) the TX power storage portion  54  updates the transmission power TXP 1  stored in the storage region for BTS 1  so that, at any given time, the value stored represents the instantaneous downlink transmission power of BTS 1 . 
     In step S 2  the initial transmission power ITXP 2  for the second BTS (hereinafter BTS 2 ) involved in the soft hand-off operation is detected by the downlink signal input portion  52  in one of the downlink signals received from BTS 2  and is stored in the storage region of the TX power storage portion  54  allocated to BTS 2 . The stored transmission power TXP 2 , for BTS 2  is also updated each time a PCB (PCB or PCB 2 ) applicable to BTS 2  is generated by the mobile station. 
     Next, in step S 3 , the downlink signal input portion  52  processes the downlink signal DS 1  received from BTS 1  (either on a traffic channel TCH thereof or on a control channel dedicated control channel (DCCH) thereof) and derives therefrom a measure RXP 1  of the received power of the downlink signal DS 1  concerned. This measure (for example the received signal strength RSS) is stored in the storage region allocated to BTS 1  in the RX power storage portion  56 . 
     In step S 4  the same operation is performed for BTS 2  and the result stored in the storage region allocated to BTS 2  in the RX power storage portion  56 . Incidentally, in steps S 3  and S 4 , the received power RXP may be calculated from the DCCH downlink signal in the event (as explained later) that the traffic channel TCH from the BTS concerned is switched off. 
     In step S 5  the path loss calculation portion  60  receives from the storage location for BTS 1  in the TX power storage portion  54  the stored (and updated) transmission power TXP 1  for BTS 1 , and also receives from the storage region for BTS 1  in the RX power storage portion  56  the received power RXP 1  for BTS 1 . The path loss calculation portion  60  subtracts the received power RXP 1  from the transmit power TXP 1  to determine the path loss PL 1  for BTS 1 . In step S 6  the same operations are repeated for BTS 2 . 
     In step S 7  the required RX power calculation portion  58  determines, based on a predetermined characteristic (e.g. the frame error rate FER) of the received downlink signals as a whole (e.g. after maximum ratio combining MRC), a required RX power RRXP which represents the minimum power that the mobile station presently needs to receive in order to produce an overall downlink signal DS of acceptable quality. 
     In step S 8  the required TX power calculation portion  62  receives the path loss PL 1  for BTS 1  and the required RX power RRXP. Based on these inputs, it calculates a downlink transmission power P BTS1  required from BTS 1  assuming that BTS 1  is the only BTS permitted to send the downlink signal in the next time slot to the mobile station. This required transmission power P BTS1  may be calculated, for example, by adding together PL 1  and RRXP. The calculated required downlink transmission power P BTS1  is then stored in the TX power storage portion  64  in the storage region allocated to BTS 1  in the first set of storage regions thereof (the set relating to the case in which maximum ratio combining (MRC) is not performed in the mobile station). 
     Then, in step S 9 , the interference calculation portion  66  receives the required downlink transmission power P BTS1  calculated in step S 8  and calculates therefrom a measure I BTS1  of the amount of network interference that would be caused by BTS 1  (alone) operating at the downlink transmission power P BTS1 . This measure is stored in an appropriate one of the storage region allocated to BTS 1  in the interference storage portion  68 . 
     Next, in steps S 10  and steps S 11  the processings of steps S 8  and S 9  are repeated for BTS 2 . The resulting required downlink transmission power P BTS2  and the network-interference measure I BTS2  are stored respectively in storage regions allocated to BTS 2  in the portions  64  and  68 . 
     In step S 12  the required TX power calculation portion  62  calculates, for each of the BTSs BTS 1  and BTS 2 , the required downlink transmission power P′ BTS1  or P′ BTS2  assuming that MRC is to be used at the mobile station. These results are stored in storage regions allocated to BTS 1  and BTS 2  in the second set of storage regions of the required TX power storage portion  64 . 
     In step S 13  the interference calculation portion  66  employs the required downlink transmission powers P′ BTS1  and P′ BTS2  calculated in step, S 12  to determine a measure of the network interference that would result assuming that BTS 1  is transmitting at P′ BTS1  and BTS 2  is transmitting at P′ BTS2 . The resulting interference measure I MRC  is stored in a further one of the storage regions of the interference storage portion  68 . 
     Next, in step S 14  the interference comparison portion  70  compares the interference measures I BTS1  and I BTS2  retrieved from the interference storage portion  68 . If I BTS1  is less than I BTS2  processing proceeds to step S 15  in which I BTS1  is compared with I MRC . If I BTS1 &lt;I MRC  in step S 15 , in step S 16  the BTS selection portion  72  determines that the downlink signal in the next time slot should be sent to the mobile station by BTS 1  alone, on the basis that this will result in the lowest network interference. The BTS selection portion  72  generates a BTS selection message (BSM) specifying that BTS 2  is not to transmit the downlink signal in the next time slot. The BSM is delivered to the transmitter portion  46  of the mobile station for transmission to BTS 2 . At the same time, the BTS selection portion  72  determines the power control bit PCB to be transmitted to BTS 1  to control the downlink transmission power of BTS 1  in the next time slot so that it has the value P BTS1  calculated in step S 8 . As noted previously, this PCB may be a single PCB common to all BTSs involved in the soft hand-off operation, or a unique PCB (PCB 1 ) for BTS 1 . 
     If, in step S 14 , I BTS2  was found to be less than or equal to I BTS1 , or if in step S 15  I MRC  was found to be less than or equal to I BTS1 , processing proceeds to step S 17 . In step S 17 , the interference comparison portion  70  compares I BTS2  with I MRC . If I BTS2  is less than I MRC  processing proceeds to step S 18  in which the BTS selection portion  72  determines that the downlink signal for the mobile station in the next time slot should be transmitted by BTS 2  alone, on the basis that BTS 2  operating alone will produce the lowest network interference. In this case, the BTS selection portion  72  generates a BSM which instructs BTS 1  not to transmit in the next time slot. Also, the PCB applicable to BTS 2  is set by the BTS selection portion  72  to control the downlink transmission power of BTS 2  to meet the required TX power P BTS2  calculated in step S 10 . 
     If in step S 17  the result of the comparison is that I MRC  is less than or equal to I BTS2 , processing proceeds to step S 19  in which the BTS selection portion  72  determines that both BTS 1  and BTS 2  should be used to transmit the downlink signal in the next time slot, on the basis that this will result in the lowest network interference. In this case, the BTS selection portion  72  generates a BSM specifying that both BTSs are to transmit in the next time slot, and sets the PCB (or PCBs) to cause the BTSs to transmit the downlink signal in the next time slot at the required transmission powers P′ BTS1  and P′ BTS2  calculated in step S 12 . 
     Thus, in the example described above it can be seen that three different candidate BTS selections are identified: a first candidate selection in which BTS 1  alone is specified for transmitting the downlink signal; a second candidate selection in which BTS 2  alone is specified for transmitting the downlink signal; and a third candidate selection in which both BTS 1  and BTS 2  are specified for transmitting the downlink signal. For each candidate selection, the required transmission power P BTS  (or P′ BTS ) of each BTS specified in the selection is calculated and a measure of the network interference that would result from the specified BTS(s) transmitting is also calculated. These network-interference measures are then employed (e.g. the lowest measure is found by comparison of the measures) to decide which of the candidate selections to use for transmission of the downlink signal, so as to tend to reduce the network interference associated with that transmission. 
     It is not essential for the candidate selections to include selections specifying only one BTS. For example, if there are three BTSs involved in a soft hand-off operation, the selections could be BTS 1 +BTS 2 , BTS 2 +BTS 3 , BTS 3 +BTS 1 , and BTS 1 +BTS 2 +BTS 3 . It is also not essential for the candidate selections to include a selection specifying all the BTSs involved in the soft hand-off. Furthermore, the transmission powers for the BTSs specified in a particular selection can be set to any suitable combination of values capable of facilitating adequate reception of the downlink signal at the subject mobile station. Thus, for example, two or more candidate selections could specify the same BTSs but specify different respective sets of transmission powers for the selections. 
     One example of the possible format of the BTS selection message BSM will now be explained with reference to FIG.  9 . 
     The BTSs involved in a soft hand-off operation are ranked in some way. For example, the ranking may be carried out in the mobile station based on a predetermined property of the respective downlink signals DS 1  to DSn that are received by the MS  40 , for example the received signal strength (RSS). Alternatively, the ranking may be on a “first-come first-served” basis, i.e. on the order in which the BTSs became involved in the soft hand-off operation. Alternatively, the ranking could be random. Once the ranking has been determined, the mobile station sends a ranking message RM, indicating the order in which the BTSs are presently ranked, via a control channel to all BTSs. 
     As shown in FIG. 9, the BSM has one bit corresponding to each rank of BTS, and these bits are arranged in the BSM in the ranking order determined by the MS. Take, for example, the case described previously with reference to FIG. 8 in which there are only two BTSs involved in the soft hand-off operation, namely BTS 1  and BTS 2 . Assume also that, in the order of ranking determined by the mobile station, BTS 2  is the highest-ranked BTS (rank {circle around ( 1 )}), and that the other BTS, BTS 1 , has rank {circle around ( 2 )}. Assume also that the outcome of the comparisons of the interference measures is the outcome shown in S 16 , namely that BTS 2  should not transmit the downlink signal in the next time slot. To communicate this result to the BTSs involved in the soft hand-off operation, the first bit (corresponding to rank {circle around ( 1 )}) in the BSM is set to 0, to indicate that BTS 2  should not transmit the downlink signal in the next time slot. The second bit of the BSM (which corresponds to the rank-{circle around ( 2 )} BTS) is set to 1, to indicate that the rank-{circle around ( 2 )} BTS, BTS 1 , should transmit the downlink signal in the next time slot. Any remaining bits of the BSM can be set to a “don&#39;t-care” state, since in this example only two BTSs are involved in the soft hand-off operation. Incidentally, the BSM in this case could consist of two bits only, of course. 
     The ranking of a BTS may periodically require updating, for several reasons. Firstly, as the MS  40  moves, a downlink signal may be received from a new BTS or an existing BTS may no longer may able to provide a detectable downlink signal. Secondly, the qualities of the signals received from the BTSs  20  may have changed, e.g. due to fading. Thus, from time to time a ranking update is required. Such an update may be carried out periodically at regular time intervals (for example every several hundred milliseconds as in GSM networks), or every frame or even every time slot. Alternatively, the ranking could be updated only when a new BTS is detected or contact with an existing one lost. 
     FIG. 10 is a block diagram showing parts of a BTS  20  embodying the present invention. This BTS  20  is specially adapted to receive and process the ranking message RM and the BTS selection message BSM sent by the MS  40  of FIG.  6 . 
     An antenna element  22  is connected (e.g. via a duplexer—not shown) to a receiver portion  24  and a transmitter portion  26 . A soft hand-off control portion  28  receives an uplink signal US from the receiver portion  24 , and in turn applies the received US (or a signal derived therefrom) to the fixed network  5  for transmission to the BSC  30 . The transmitter portion  26  receives a downlink signal DS via the connection line  5  from the BSC  30  (FIG. 5) and a disabling signal DIS from the soft hand-off control portion  28 . 
     In use of the BTS  20 , the uplink signals sent by the MS  40  when it is in the soft hand-off region  9  include, from time to time, a ranking message RM. The uplink signals US detected by the receiver portion  24  in the BTS  20  are applied to the soft hand-off control portion  28 . When the soft hand-off control portion  28  detects that a ranking message RM is included in one of the uplink signals US received thereby, it processes the ranking message concerned to determine the rank of its BTS within the ranking order determined by the MS. 
     In each time slot, the uplink signals US produced by the receiver portion  24  also include a BTS selection message BSM determined by the MS  40  as described above. 
     Operation of the soft hand-off control portion  28  in response to the presence of such a BSM in the uplink signal US produced by the receiver portion  24  will now be described. 
     It is assumed that, by the time the BSM is received, a ranking message RM has already been received and processed (as indicated above) by the soft hand-off control portion  28 . 
     The BSM is supplied by the receiver portion  24  to the soft hand-off control portion  28  where is examined. The soft hand-off control portion  28  checks the rank of its BTS based on the last-received ranking message and then examines the bit corresponding to that rank in the BSM. If the bit is 0 the soft hand-off control portion  28  applies a disabling signal DIS to the transmitter portion  26  to prevent it from transmitting the downlink signal in the next time slot. 
     The measure of network interference I BTS1 , I BTS2  or I MRC  can be calculated as follows by considering the interference that would be experienced by an imaginary mobile station, other than the subject mobile station, operating in the soft hand-off region (FIG.  5 ), as a consequence of the BTS(S) concerned transmitting at the determined required transmission power(s). In the case of I BTS1 , for example, the interference is calculated based on the required transmission power P BTS1  from BTS 1  to the subject mobile station and the associated mean path loss experienced by the imaginary mobile station (which is the same as for the subject mobile station). This mean path loss is a time-averaged path loss for which the averaging period is chosen so as to average out (or ideally eliminate) the effects of Rayleigh fading. In other words, the path loss variation due to Rayleigh fading is averaged out. 
     In the case of I MRC  the interference is calculated based on the cumulative sum of the respective carrier power levels of BTS 1  and BTS 2  at the antenna of the imaginary mobile station. Again, these carrier power levels are calculated based on the required transmission powers P′ BTS1  and P′ BTS2  for BTS 1  and BTS 2  when MRC is used and the respective mean path losses which have already been established at the subject mobile station (and are assumed to be the same for the imaginary mobile station). 
     Take, for example, a situation in which the downlink signal from BTS 2  is undergoing a deep fade. This means that PL 2  will be large relative to PL 1 . In this case, the required transmission power PBTS 2  for BTS 2  will be large as compared to the required transmission power BTS 1  for BTS 1 . Thus, I BTS2  will be large relative to I BTS1 . Also, in view of the large PL 2 , P′ BTS2  will also be large so that I MRC  will be larger than I BTS1 . Accordingly, the decision is made that BTS 2  should not transmit the downlink signal in the next time slot, so as to reduce the network interference resulting from transmission of that downlink signal. 
     In the embodiment described above, the selection of the BTS to be used to transmit the downlink signal in the next time slot is made in the mobile station  40 . However, it is not essential that this decision be made there. In another embodiment, which will be described hereinafter with reference to FIGS. 11 to  13 , each BTS includes a modified soft hand-off control portion, and these modified hand-off control portions cooperate to carry out the downlink-signal decision making. 
     Referring firstly to FIG. 11, a BTS  120  is constituted in basically the same manner as the BTS  20  described previously with reference to FIG. 9 but has a modified soft hand-off control portion  128  in place of the soft hand-off control portion  28  in the FIG. 9 BTS. 
     An example of the constitution of the modified soft hand-off control portion  128  is shown in FIG.  12 . 
     As will apparent from FIG. 12 itself, the modified soft hand-off control portion  128  in this embodiment includes the portions  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68  and  70  previously included in the downlink signal processing portion  48  of the MS  40  in the FIG. 7 embodiment. However, in place of the downlink signal processing portion  52  in the FIG. 7 embodiment, the FIG. 12 embodiment has an uplink signal input portion  152 . Also, in place of the BTS selection portion  72  in the FIG. 7 embodiment, the FIG. 12 embodiment has a decision portion  172 . 
     Operation of the FIG. 12 embodiment will now be described with reference to the flowchart of FIG.  13 . Again, in the FIG. 13 flowchart it is assumed, for the sake of simplicity, that only two BTSs, BTS 1  and BTS 2 , are involved in the soft hand-off operation. As will be apparent from FIG. 13 itself, many of the steps in the FIG. 13 flowchart are the same as (or correspond to) the steps S 1  to S 19  in the FIG. 8 flowchart relating to operation of the FIG. 7 embodiment. 
     The FIG. 13 flowchart relates to processing performed at BTS 1  during the soft hand-off operation. Accordingly, the step S 1  used in the FIG. 8 flowchart is not required in FIG. 13, as the soft hand-off control portion  128  in BTS 1  already knows the instantaneous downlink transmission power of BTS 1  (this is stored in the storage region allocated to BTS 1  in the TX power storage portion  54 ). However, BTS 1  does need to know the downlink transmission power of the other BTS, BTS 2 , involved in the soft hand-off operation. Accordingly, in step S 2 , the initial transmission power ITXP 2  for BTS 2  is received (in one of the uplink signals US) from the mobile station. The mobile station can include this information for example in the ranking message RM which it transmits periodically or whenever a new BTS becomes involved in the soft hand-off operation. The received initial transmission power ITXP 2  for BTS 2  is stored in the storage region allocated to BTS 2  in the TX power storage portion  54 . 
     Incidentally, the downlink power control in this embodiment is performed in the same way as in the FIG. 7 embodiment. Thus, the mobile station may either use a single PCB in common to control the downlink transmission powers of all of the BTSs involved in the soft hand-off operation, or alternatively the mobile station may allocate each involved BTS its own PCB. In any event, the TX power storage portion  54  needs to receive the PCBs applicable to all of the BTSs involved in the soft hand-off operation. If there is a single PCB allocated to all the BTSs, then this single PCB will be available to the soft hand-off control portion  128  from one of the uplink signals US received from the mobile station. If, on the other hand, each involved BTS is allocated its own PCB by the mobile station, then some mechanism must be provided for enabling each involved BTS to receive the respective PCBs of all the other involved BTSs. One suitable mechanism for achievings this is described in co-pending PCT patent publication No. WO 99/59367, the entire content of which is incorporated herein by reference. In this proposed mechanism, the mobile station includes, in an uplink signal transmitted thereby to each involved BTS, a power control message (PCM) made up, in the order of ranking of the involved BTSs determined by the mobile station, the respective PCBs of all the involved BTSs. Thus, this PCM would have a format similar to that of the BSM shown in FIG. 9, except that in this case each bit would be the PCB of the BTS concerned. 
     Thus, in the FIG. 12 embodiment, any PCB (or PCM as the case may be) included in an uplink signal US received from the mobile station is detected by the uplink signal input portion  152  and supplied to the TX power storage portion  54  so as to enable the TX power storage portion  54  to update the transmission power TXP for each of the BTSs involved in the soft hand-off operation. 
     After the step S 2 , processing proceeds to a step S 3 ′. This step S 3 ′ corresponds generally to the step S 3  in the FIG. 8 flowchart. In this step S 3 ′, the uplink signal input portion  152  detects, in one of the uplink signals US received from the mobile station, a transmission power control (TPC) signal representing the power RXP 1  at which the downlink signal from BTS 1  was received by the mobile station. This received power RXP 1  for BTS 1  is stored in the storage region allocated to BTS 1  in the RX power storage portion  56 . In step S 4 ′ the same operation is repeated for BTS 2 . 
     Then, in steps S 5  and S 6 , the path loss calculation portion  60  calculates the respective path losses PL 1  and PL 2  for the downlink signals sent to the mobile station by BTS 1  and BTS 2 . 
     In step S 7 ′, which corresponds to the step S 7  in the FIG. 8 flowchart, the required RX power calculation portion  58  determines a required receive power RRXP for the mobile station. This may be achieved, for example, by the mobile station including, in one of the uplink signals US transmitted thereby, a measure of the downlink channel performance, for example the frame error rate (FER) of the downlink signal received by the mobile station. When such a communications-channel measure (FER) in a received uplink signal US is detected by the uplink signal input portion  152  it supplies this measure to the required RX power calculation portion  58  for use thereby in generating the RRXP. 
     The steps S 8  to S 15  and S 17  in FIG. 13 are then the same as the corresponding steps in the FIG. 8 flowchart. 
     In step S 16 ′, which corresponds to the step S 16  in the FIG. 8 flowchart, the decision portion  172  in the soft hand-off control portion  128  of BTS 1  determines that BTS 1  (alone) should transmit the downlink signal DS in the next time slot to the mobile station on the basis that this will result in the lowest network interference. The decision portion  172  then generates suitable power control information (for example a PCB) so as to adjust the downlink transmission power to the value P BTS1  determined in step S 8 . 
     Rather than a PCB, this power control information may simply be the explicit required transmission power P BTS1  in this case. 
     If the determination in step S 17  is that I BTS2  is less than I MRC , the decision portion  172  determines in step S 18 ′ that BTS 1  should not transmit the downlink signal in the next time slot. Thus, the decision portion  172  applies the disabling signal DIS to the transmission portion  26  in its BTS (BTS 1 ). 
     If, on the other hand, in step S 17  it is determined that I MRC  is less than or equal to I BTS2 , then in step S 19 ′ the decision portion  172  determines that both BTS 1  and BTS 2  should be used in the next time slot to transmit the downlink signal. In this case, it sends appropriate power control information (a PCB or possibly the explicit downlink transmission power P′ BTS1 ) to the transmission portion  26 . 
     It will be appreciated that the processing shown in FIG. 13 is also carried out independently in the other BTS, BTS 2 , involved in the soft hand-off operation (in that case, of course, in step S 2 , the received initial transmission power that is received and stored is ITXP 1  relating to BTS 1 ). 
     Naturally, the decision-making embodied in steps S 14  to S 19 ′ in FIG. 13 must be made consistent in each different BTS involved in the soft hand-off operation so that there will always be at least one BTS which transmits the downlink signal to the mobile station in the next time slot. 
     In the embodiments described above, the TX power storage portion  54  receives the initial downlink transmission powers of the involved BTSs and then updates these as necessary on receipt of the power control bits PCBs for the different BTSs. However, it would also be possible for the instantaneous downlink transmission powers TXP themselves to be supplied directly to the TX power storage portion  54  in each time slot in place of the PCBs. 
     It will also be appreciated that it would also be possible for the decision as to which BTS is to transmit the downlink signal in the time slot to be made in the BSC  30  instead of in each involved BTS. In this case, the elements  54  to  70 ,  152  and  172  shown in FIG. 11 would be provided in the BSC instead of in each BTS. 
     It will also be understood that the way in which the transmission powers TXP (or IXTP+ΣPCB) and receive powers RXP are made available to the decision-making entity (be it MS, BTS or BSC) is not critical to the invention. For example, it is not necessary for the MS to rank the BTSs. All that is necessary is that each BTS is able to identify to which BTS a particular received value (e.g. ITXP or RXP) relates. Such identification could be carried out in many different ways other than ranking. 
     It will also be understood that it is not necessary for the processing shown in FIGS. 8 and 13 to take place every time slot. It would be possible for the signals such as RXP and PCM to be transmitted only once per frame, in which case the decision-making would be made on a frame-by-frame basis. 
     Next, another example of downlink processing in the FIG. 5 network will be described with reference to FIGS. 14 and 15. In such downlink processing, if macro-diversity based on maximum ratio combining (MRC) is required at the MS during the soft hand-off operation, all of the BTSs involved in the soft hand-off operation must transmit the same information to the MS. However, if MRC is not required at the MS in the soft hand-off region, downlink macro-diversity can be based on selection (or switched) diversity at the BSC  30 , in accordance with another embodiment of the present invention. 
     FIG. 14 is a block diagram showing parts of a MS  40  in this embodiment of the present invention. An antenna element  42  is connected (e.g via a duplexer—not shown) to a receiver portion  44  and a transmitter portion  46 . A signal selection information processing portion  48  from the receiver portion  44  respective downlink signals DS 1  to DS 3  produced by the three BTSs BTS 1  to BTS 3  involved in the soft hand-off operation. The signal selection information processing portion  48  applies a ranking message RM and a power control message PCM to the transmitter portion  46 . 
     The signal selection information processing portion  48  processes the respective downlink signals DS 1  to DS 3  received from the BTSs (BTS 1  to BTS 3 ) involved in the soft hand-off operation, and compares these downlink signals according to a predetermined property. In a preferred embodiment, the predetermined property is the received signal strength (RSS), possibly together with the signal-to-interference ratio (SIR). These performance measures are determined for the downlink DCCH. 
     The signal selection information processing portion  48  employs the performance measures to select which of the BTSs involved in the soft hand-off operation is to be used to transmit the downlink signal to the MS in the next time slot. 
     The signal selection information processing portion  48  may select the BTS that is to transmit the downlink signal in the next time slot based on the following cases. 
     Case 1: If the RSS (and/or SIR) of a single BTS is higher than each other BTS, that single BTS is selected to transmit the downlink signal in the next time slot. 
     Case 2: If two or more BTSs have comparably-good RSS (and/or SIR), one of them is selected based on an order of ranking (e.g. order of involvement in the soft hand-off operation or random). 
     Case 3: If all the BTSs involved in the soft hand-off operation fail to meet a prescribed RSS (and/or SIR) threshold, all the BTSs are selected to transmit the downlink signal in the next time slot, so that a MRC operation can be performed at the MS  40  to give the best chance of obtaining a useful signal. 
     After determining which BTS(s) is/are to be used, the signal selection information processing portion  48  transmits a BTS selection message (BSM), identifying the BTS(s) to be used, to all of the BTSs on a control channel. 
     For example, using two bits to provide the BSM, the BSM may be set to “01” to designate BTS 1 ; “10” to designate BTS 2 ; and “11” to designate BTS 3 . “00” denotes that all the BTSs should be used to transmit the downlink signal in the next time slot. 
     Each BTS receives the BSM via the control channel from the MS  40 . One or more of the BTSs then forward the BSM to the BSC  30 . All BTSs could forward the BSM to the BSC. 
     FIG. 15 shows part of a BSC adapted to perform downlink processing with the FIG. 14 mobile station. The BSC  30  includes a control portion  32  and a selector portion  34 . 
     In this example, it is assumed that the connection lines  5   1  to  5   3  linking each BTS to the BSC  30  are duplex lines which carry respective uplink and downlink signals US and DS between the BTS concerned and the BSC. For example, a first connection line  5   1  carries respective uplink and downlink signals US 1  and DS 1  between the BTS 1  and the BSC  30 . 
     The selector portion  34  receives at its input a downlink signal DS supplied by the MSC ( 7  in FIG.  5 ). The selector portion  34  has three outputs connected respectively to the connection lines  5   1  to  5   3 . 
     The selector portion  34  also has a control input which receives a selection signal SEL. In response to the SEL selection signal the selector portion  34  connects its input to one, or all, of its three outputs. 
     The control portion  32  also has three inputs connected respectively to the connection lines  5   1  to  5   3  for receiving the uplink signals US 1  to US 3  from BTS 1  to BTS 3  respectively. The control portion applies the selection signal SEL to the selector portion  34 . As can be appreciated the selector portion  34  may be part of the BTSs, such that the selection signal SEL selects a BTS(s) for transmission of the downlink signal. 
     In operation of the BSC shown in FIG. 15, in each time slot of the uplink signal the control portion  32  receives one or more of the three uplink signals US 1  to US 3  from the BTSs involved in the soft hand-off operation. When the BSM supplied by the MS  40  is detected within a received uplink signal US 1 , US 2  or US 3 , the control portion  32  examines the BSM and determines therefrom which of the BTSs is to be used to transmit the downlink signal in the next time slot to the MS  40 . 
     If the BSM designates a single BTS, the control portion  32  sets the selection signal SEL such that the selector portion  34  supplies the downlink signal DS just to that one of the connection lines  5   1  to  5   3  connecting the BSC  30  to the designated BTS. If, on the other hand, all BTSs are designated by the BSM, the selection signal SEL is set so that the downlink signal DS received from the MSC  7  is supplied to all of the connection lines  5   1  to  5   3 . 
     It will be appreciated that it is not necessary for the downlink processing to be performed on a time slot-by-time slot basis. It could be performed on a frame-by-frame basis or the BTS selection could be made at some other suitable time interval. 
     It would also be possible for the signal selection information processing portion  48  (FIG. 14) to include its own storage portion enabling it to store a past history of the RSS (and/or SIR) measures for the different BTSs currently involved in the soft hand-off operation. In this case, it would be possible for the MS to employ more sophisticated decision-making in relation to the BTS selection so as to avoid undesirable effects caused by temporary reception phenomena or other problems caused by too frequent-changing of the BTS selection. 
     It is not necessary for the mobile station to carry out the comparison of the signal measures for the different downlink signals and make the determination of the BTS to be used to transmit the downlink signal. The comparison and BTS determination could be carried out in the BSC; in this case instead of transmitting the BSM to the BTSs involved in the soft hand-off operation, the mobile station could transmit the downlink signal measures themselves (in some suitable form). These measures would then be delivered in the usual way to the BSC, enabling it to compare them and then make the BTS determination. 
     In the embodiment of FIGS. 6 to  8  the processing is carried out mainly in the mobile station, whereas in the embodiment of FIGS. 11 to  13  the processing is carried out mainly in the BTSs. However, the present invention is not limited to these possibilities. For example, the processing could be carried out mainly in the base station controller or in the mobile switching center. It would also be possible for the processing to be distributed amongst any two or more or these network elements. 
     Furthermore, it would be possible for the decisions to be made at time intervals other than frames or time slots, for example based on a time interval consistent with the fading characteristics of the RF channels in the network. 
     Although the present invention has been described above in relation to the proposed European wideband CDMA system (UTRA) it will be appreciated that it can also be applied to a system otherwise in accordance with the IS95 standard. It would also be possible to apply the invention in other cellular networks not using CDMA, for example networks using one or more of the following: multiple-access techniques: time-division multiple access (TDMA), wavelength-division multiple access (WDMA),frequency-division multiple access (FDMA) and space-division multiple access (SDMA).