Reducing interference in cellular mobile communications networks

REDUCING INTERFERENCE IN CELLULAR MOBILE COMMUNICATIONS NETWORKS

The present invention relates to cellular mobile communication networks, such as, for example, Code Division Multiple Access (CDMA) cellular networks.

1 of the reference drawings shows a part of a cellular mobile communication network according to the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) standard TIA/EIA/IS-95 (hereinafter referred to as "IS95") of October 1994. do. Each of the three base transceiver stations (BTSs) 4 (BTS1, BTS2, BTS3) is connected to a base station controller (BSC) 6 via a fixed network 5, which in turn a mobile switching center (MSC) (7) is connected. The BSC 6 manages the radio resources of the connected BTSs 4 by, for example, performing hand-offs and allocating radio channels. The MSC 7 serves to provide switching functions and coordinate location registration and call delivery.

Each BTS 4 acts as 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, the mobile station transmits transmit signals of equal strength and quality from the BTSs of each of the overlapping cells. (downlink signals). Transmission signals (uplink signals) generated by the mobile station MS can also be received with equal strength and quality by these separate BTSs when the mobile station is in the SHO area 9 .

2 of the reference drawings shows a state in which the MS 10 is located within the SHO area 9 and transmits uplink signals received by a plurality of BTSs 4 . According to the IS95 standard, the BTS 4 receiving the uplink signal from the MS 10 relays the signal to the BSC 6 through a dedicated connection line of the fixed network 5 . In the BSC (6), one of the relayed signals is selected by comparing the quality of each of the received signals, and the selected signals are relayed to the MSC (7). This selection is called selection diversity.

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Similarly, FIG. 3 of the reference drawings shows a state in which the MS 10 is located within the SHO area 9 and receives downlink signals from a plurality of BTSs 4 . According to the IS95 standard, the downlink signals received by the BSC 6 from the MSC 7 are relayed to all BTSs 4 involved in soft hand-off through respective connections of the fixed network 5, It is transmitted to the MS 10 by all BTSs 4 . In MS 10, the multiple signals are synthesized using, for example, maximum ratio combination (MRC), or one of the multiple signals is synthesized based on signal strength or quality, ie in the uplink case. is selected using selection diversity.

In contrast, in a Global System for Mobile Communications (GSM) network, for example in a CDMA network, each BTS 4 transmits on the same frequency. Therefore, careful control of transmit power must be maintained to minimize interference problems.

Signals are transmitted in successive frames according to the IS95 standard. As shown in Fig. 4 of the reference figure, each frame has a duration of 20 ms and includes 16 time slots of 1.25 ms. In each time slot, several bits of user data and/or control information may be transmitted.

Transmission power control (uplink power control) from the MS 10 to the BTSs 4 in IS95 is achieved as follows. When the BTS 4 receives a signal from the MS 10, the BTS 4 responds to certain characteristics of the received signal (eg, absolute signal level, signal-to-noise ratio (SNR), signal-to-interference ratio (SIR), bit It is determined whether the 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 whether to decrease or increase the transmit power in the next time slot.

For this purpose, two bits in every time slot of the pilot channel (PCH) from the BTS 4 to the MS 10 are allocated for uplink power control (shown in FIG. 4). The two bits have the same value, and therefore will be referred to as "power control bit" (or PCB) singly hereinafter. The power control bit is assigned a value of "0" by the BTS 4 when the transmission power of the MS 10 needs to be increased by 1 dB, and "1" when the transmission power of the MS 10 needs to be decreased by 1 dB. A value is assigned. The BTS 4 cannot directly request that the MS 10 maintain the same transmit power; It is only possible to keep the transmit power at the same level by alternately transmitting "1" and "0" in the power control bit.

When the MS 10 is within the SHO area 9, the MS 10 may increase or decrease the uplink transmit power based on a number of power control bits each received from the BTSs 4 involved in the soft hand-off. have to decide whether Therefore, an OR function is performed on all power control bits. If the result by the OR function is "0", the MS 10 will increase the power for the uplink transmission, and if the result is "1", the MS 10 will decrease the power for the uplink transmission. In this way, the uplink transmit power is increased only if all BTSs 4 require an increase.

Transmission power control (downlink power control) from the BTS 4 to the MS 10 in IS95 is achieved as follows. When the MS 10 receives a downlink signal from the BTS 4 (or from each of multiple BTSs 4 in a soft hand-off operation) via a traffic channel (TCH), the FER of the signal is ), and the MS 10 reflects the degree to which the traffic-channel signal is corrupted, for example, by noise. This FER is then relayed by the MS 10 to the BTS 4 that has transmitted the relevant downlink signal, which BTS 4 uses to determine whether to change the downlink transmit power.

The soft hand-off system is effective in improving signal transmission between the MS 10 and the network when the MS 10 is located in an overlapping cell area adjacent to the boundary of individual cells. The signal quality in these areas when using one BTS 4 is relatively inferior, and when using more than one BTS 4, the call quality is significantly improved.

However, the IS95 soft hand-off system must transmit downlink signals carrying the same data and/or control information from all the BTSs 4 involved in the soft hand-off to the MS 10, thus signaling traffic within the cellular network. There is a drawback to increase the . This duplication of transmissions is undesirable as each transmission potentially causes interference to other transmissions within the network.

For example, the downlink power control method aims to ensure that the MS 10 receives a useful downlink signal from all BTSs 4 involved within the soft hand-off. If there is a deep fade in the downlink signal from one of the BTSs, the MS 10 will instruct the associated BTS to significantly increase its downlink transmit power. However, in this case the involved BTS will inevitably cause significant interference in its own cell and other transmissions occurring in neighboring cells. This problem is exacerbated if, as in the IS95 standard, only one PCB is commonly allocated for downlink power control to all BTSs involved in soft hand-off. In this case, not only will the BTS undergoing a severe fade significantly increase its own downlink transmit power, but each other BTS involved in soft hand-off will also increase its own downlink transmit power, thus increasing the overall cellular network. The interference in the interior is significantly increased.

Accordingly, it is desirable to reduce interference within a cellular network associated with soft hand-off operations. It is also desirable to reduce interference within a cellular network even when the mobile station is otherwise within the communication range of one or more base transceiver stations. In WO-A-97/08911, when a mobile station is able to receive downlink signals from multiple base stations in a network, at least one base transceiver station of the plurality determines not to transmit subsequent downlink signals to the mobile station. a base transceiver station determining means; and base transceiver station notifying means for notifying a plurality of base transceiver stations of a decision made by the base transceiver station determining means using one or more uplink signals transmitted by the mobile station to such base transceiver station, each said a base transceiver station comprising receiving means for receiving the uplink signal and disabling means for the base transceiver station to prevent transmission of the subsequent downlink signal in response to the received uplink signal. Disclosed is a mobile communication network. In another network disclosed in EP-A-0797367, whether the radio channels between a number of base transceivers and mobile stations are invalid (the radio condition is poor) or excessive (because there is another channel with a very good radio condition). is determined by the mobile station. In response to receiving the uplink signal comprising the determination result, the base transceiver station transmits a control signal to the base station controller, which in turn interrupts the downlink wired link transmission path from the base station controller to the base station having an invalid or excessive radio channel. make it

<Summary of the invention>

In a cellular mobile communication network embodying the first aspect of the present invention, in accordance with a decision made by the determining means using an identification system in which a plurality of base transceiver stations each have a distinct identity, the notification means is configured to provide the base transceiver station. generating selection information, and sending the same selection information not including separate identity information for each of the plurality of base transceivers in the selection notification to the plurality of base transceivers; Each of the plurality of base transceiver stations is informed of the identity in an identification system used by a notification means, separately from the selection information, according to determining that the identity needs to transmit the subsequent downlink signal. characterized by processing.

In one embodiment, the mobile station is configured to: identify a selection of at least two different candidate base transceivers, each selection designating one or more of said plurality of base transceivers available to transmit subsequent downlink signals to the mobile station; candidate base transceiver station identification means; and network interference determining means for measuring, for each candidate selection, interference in the network specified in the candidate selection and caused by base transceivers transmitting the subsequent downlink signal to the mobile station, wherein the base transceiver station comprises: The determining means determines, based on the network interference measurements, one of the candidate selections to be used for transmitting the subsequent downlink signal to the mobile station to reduce network interference resulting from the transmission of the downlink signal.

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According to a second aspect of the present invention, in a communication method used in a cellular mobile communication network, when a mobile station of the network can receive downlink signals from a plurality of base transceiver stations of the network, among a plurality of base transceiver stations determining by the mobile station whether the at least one base transceiver station will not transmit a subsequent downlink signal to the mobile station; a plurality of base transceivers being notified of decisions made by the mobile stations using one or more uplink signals transmitted by the mobile stations to the base transceivers; each said base transceiver station receiving uplink signals and blocking transmission of the subsequent downlink signal in response to the received uplink signal; In accordance with a decision made by the mobile station using an identification system in which each of the plurality of base transceivers has a distinct identity, the mobile station generates base transceiver selection information within the selection information, a distinct identity for each of the plurality of base transceivers transmit the same selection information without including information to a plurality of base transceiver stations; Whether it is necessary for each of a plurality of base transceiver stations to be informed of the identity in the identification system used by the mobile station, apart from the selection information, and to process the selection information according to the identity to transmit the subsequent downlink signal to judge

In one embodiment, a method of communication comprises: identifying at least two different candidate base transceiver station selections, each candidate selection designating one or more base transceiver stations of the plurality for use in transmitting a subsequent downlink signal to a mobile station; ; generating for each of the candidate selections a network interference measurement specified in the selection and caused by base transceivers transmitting the subsequent downlink signal to the mobile station; and determining, based on the network interference measurement, one of the candidate selections to be used to transmit the subsequent downlink signal to a mobile station to reduce network interference resulting from the transmission of the downlink signal. . According to a third aspect of the present invention, a base transceiver station for use in a cellular mobile communication network is capable of receiving downlink signals from a mobile station of the network from a requested base transceiver station and at least one other base transceiver station of the network. receiving means for receiving one or more uplink signals transmitted by the mobile station in case of notifying the base transceiver station that the mobile station has determined that the at least one base transceiver station does not transmit a subsequent downlink signal to the mobile station; disabling means for preventing the base transceiver station from transmitting the subsequent downlink signal in response to a received uplink signal; Receiving means includes a selection information section responsive to base transceiver station selection information generated by the mobile station using an identification system in which the base transceiver station and each other base transceiver station have distinct identities, wherein a plurality of the same selection information without including separate identity information for each base transceiver station is transmitted to the base transceiver station and each other base transceiver station requested by the mobile station; further comprising an identity information unit that responds to information supplied to the base transceiver station separately from selection information and informs the base transceiver station of the identity in an identification system used by the mobile station; The disabling means is configured to select the selection information received by the selection information unit according to the identity separately received by the identity information unit to determine whether the base transceiver station needs to transmit the subsequent downlink signal. handle

In one embodiment of the present invention, the candidate selection includes not only a selection in which only a particular BTS is designated for each of the plurality of BTSs, but also a further selection in which all of the plurality of BTSs are designated. Candidate selection, including selection to designate only one BTS, is not essential. For example, when there are 3 BTSs involved in a soft hand-off operation, the selection may be BTS1 + BTS2, BTS2 + BTS3, BTS3 + BTS1, and BTS1 + BTS2 + BTS3. Also, candidate selection, which includes a selection specifying all BTSs involved in soft hand-off, is not essential. Moreover, in a particular selection, the transmit power for the designated BTSs may be set to any suitable combination of values that may facilitate any suitable reception of the downlink signal at the target mobile station. Thus, for example, two or more candidate selections may specify the same BTSs, but different respective transmit power sets for the selection. In other words, the two candidate choices may differ only with respect to the transmit power of the (equally) designated BTSs.

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The present invention is not limited to downlink transmission selection for interference reduction. Embodiments of the present invention may be used whenever it is desirable to prevent at least one BTS within communication range of a mobile station from transmitting a downlink signal to the mobile station.

1 is a partial diagram of a cellular mobile communication network according to IS95;

Fig. 2 is a schematic diagram for explaining processing of an uplink signal in a soft hand-off operation performed by the network of Fig. 1;

3 is a schematic diagram for explaining processing of a downlink signal in the soft hand-off operation;

Fig. 4 is a diagram of a time frame format within the network of Fig. 1;

5 is a partial diagram of a mobile communication network embodying the present invention;

6 is a partial diagram of a mobile station embodying the present invention;

Fig. 7 is a detailed block diagram showing a portion of the mobile station of Fig. 6;

FIG. 8 is a flowchart for explaining the operation of the mobile station of FIG. 6;

Fig. 9 is a schematic diagram showing possible message formats transmitted by the mobile station of Fig. 6;

10 is a partial diagram of a base transceiver station embodying the present invention;

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5 shows a part of a mobile communication network embodying the present invention. In FIG. 5, the same components as those of the network components of FIG. 1 have the same reference numerals, and a description thereof is omitted.

The network of FIG. 5 is Wideband CDMA (W-CDMA) according to a new standard proposed for mobile telecommunications, and is called Universal Mobile Telecommunications System (UMTS) or UMTS Terrestrial Radio Access (UTRA). This is broadly similar to the IS95-standard network described above, although the details of a specific embodiment have not yet been finalized. IS95 and other details include a frame duration of 10 ms and a time slot duration of 625 μs. All bit rates 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 principle as uplink power control.

In FIG. 5 , each of three base transceiver stations (BTSs) 20 (BTS1, BTS2 and BTS3) are connected to a base station controller (BSC) 30 via a fixed network 5 and, in turn, a mobile switching center (MSC). ) is connected to (7). Each BTS 20 handles a cell 8 . A mobile station (MS) 40 is in a soft hand-off (SHO) area 9 and is capable of receiving downlink signals from all BTSs 20 involved in the soft hand-off and transmitting uplink signals. may be

The network of FIG. 5 generally corresponds to the network of FIG. 1 , but the MS 40 , BTSs 20 and BSC 30 are configured and operated differently from the components corresponding to FIG. 1 .

6 is a block diagram illustrating a portion of an MS 40 implementing the present invention. The antenna element 42 is connected to the receiving unit 44 and the transmitting unit 46 (eg, via a duplexer (not shown)). The downlink signal processing unit 48 receives, from the receiving unit 44, respective downlink signals DS1 to DSn generated by the n BTSs BTS1 to BTSn involved in the soft hand-off operation. The downlink signal processing unit 48 applies the BTS selection message (BSM) to the transmitting unit 46 .

7 shows a block diagram of the downlink signal processing unit 48 . The downlink signal processing unit 48 includes a downlink signal input unit 52 for receiving the downlink signals DS1 to DSn from the receiving unit 44 . Downlink signal processing section 48 further includes respective TX and RX power storage sections 54 and 56 respectively connected to downlink signal input section 52 . The TX power storage unit 54 receives a single power control bit (PCB) or individual power control bits PCB1 to PCBn respectively corresponding to BTSs involved in a soft hand-off operation, and the downlink signal input unit Receive TXPn at an initial transmit power TXP1 corresponding to each of the BTSs from (52).

Downlink signal input 52 also supplies RX power storage 56 with received power measurements RXP1 to RXPn corresponding to each of the BTSs, each RXP indicating that the downlink signal from the corresponding BTS is transmitted to the mobile station. Indicates the power at the point received by .

Power stores 54 and 56 each include storage areas corresponding to separate BTSs.

The downlink signal processing section 48 also includes a required RX power calculation section 58 for receiving, from the downlink signal input section 52, an additional signal measurement value (FER), indicative of the downlink frame error rate determined by the mobile station. do.

Downlink signal processing section 48 receives TXPn at transmit power TXP1 for discrete BTSs from TX power storage 54, and also receives power RXP1 at receive power RXP1 for discrete BTSs from RX power storage 56. It further includes a path loss operation unit 60 for receiving each RXPn.

The downlink signal processing unit 48 receives the required RX power RRXP from the path loss operation unit 60 and PLn from the path loss measurement values PL1 to PLn of the separate BTSs and the required RX power RRXP from the required RX power operation unit 58 . (62).

The downlink signal processing unit 48 further includes a required TX power storage unit 64 and an interference calculating unit 66 , both of which receive the first and second required transmit power sets from the required TX power calculating unit 62 . do. First set of required transmit powers P<sb>BTS1</sb>~P<sb>BTSn </sb>denotes the required transmit power of the separate BTSs when the mobile station 40 is not using maximum ratio synthesis (MRC). Second set of transmit power measurements P<sb>BTS1</sb>~P'<sb>BTSn </sb>denotes the required transmit power of each BTS when MRC is used in the MS 40 . The required TX power storage 64 has sets of first and second storage areas corresponding to two sets of transmit power measurements.

The downlink signal processing unit 48 generates interference measurements I corresponding to each of the separate BTSs (transmitting alone).<sb>BTS1</sb>~I<sb>BTSn</sb> and an interference measure (I) indicating interference when all BTSs transmit downlink signals and when MRC is performed at MS 40.<sb>MRC</sb>) further includes an interference value storage unit 68 for receiving. The interference value storage unit 68 has a storage area corresponding to each of these separate interference measurement values.

The downlink signal processing unit 48 receives the interference measurement values I from the interference value storage unit 68 .<sb>BTS1</sb>~I<sb>BTSn </sb>and I<sb>MRC </sb>and an interference comparator 70 for receiving and calculating a comparison signal COMP supplied to the BTS selecting unit 72 . The BTS selector 72 calculates a BTS select message BSM and a power control bit PCB (or a plurality of PCBs PCB1 to PCBn) to be supplied to the transmitter 46 of the mobile station 40 .

The operation of the mobile station 40 of FIG. 7 will be described with reference to the flowchart of FIG. In this embodiment, for simplicity, it is assumed that only two BTSs are involved in the soft hand-off operation.

In the first step S1, the downlink signal input section 52 receives a downlink signal from the first of the BTSs involved in the soft hand-off operation (hereinafter referred to as BTS1), e.g., on a dedicated control channel (DCCH). Signal, ITXP1 of initial transmission power BTS1 is detected.

As above, the downlink power control method proposed for use in W-CDMA is based on the power control bits PCBs generated by the mobile station and communicates with a specific MS to adjust the transmit power of the BTSs. Currently, the proposed standard for W-CDMA specifies that a single PCB be used to control the downlink transmit power of all BTSs involved in soft hand-off operation. Thus, all BTSs involved in this case increase or decrease their transmit power according to a single PCB. However, in one embodiment of the present invention, it is possible to allocate its respective PCB to each BTS involved in the soft hand-off operation, allowing the MS to control the downlink transmit power of individual BTSs independently of each other. do. In this case (shown in parentheses in Fig. 7), the TX power storage 54 receives PCBs (PCB1 to PCBn) respectively corresponding to the separate BTSs involved in the soft hand-off operation.

In step S1, the initial transmit power ITXP1 for BTS1 is stored in a storage area assigned to BTS1 in TX power storage 54. FIG. Thereafter, each time a new PCB (PCB or PCB1 as the case may be) applicable to BTS1 is created by the MS (e.g., every time slot), and the TX power storage 54 stores the transmissions in the storage area for BTS1. Update the power TXP1, and thus, at any given time, the stored value represents the instantaneous downlink transmit power of BTS1.

In step S2, the initial transmit power ITXP2 for the second BTS (hereinafter referred to as BTS2) involved in the soft hand-off operation is detected by the downlink signal input section 52 in one of the downlink signals received from BTS2, BTS2 It is stored in the storage area of the TX power storage unit 54 allocated to . The stored transmit power (TXP2) for BTS2 is also updated every hour, and the applicable PCB (PCB or PCB2) for BTS2 is generated by the mobile station.

Next, in step S3, the downlink signal input section 52 processes the downlink signal DS1 received from the BTS1 (on the traffic channel TCH or the dedicated control channel DCCH), from which the received power of the associated downlink signal DS1 is Acquire the measured value RXP1. This measurement (eg received signal strength (RSS)) is stored in a storage area assigned to BTS1 in RX power storage 56 .

In step S4, the same operation is performed for BTS2, and the result is stored in the storage area allocated to BTS2 in the RX power storage 56. FIG. Incidentally, in steps S3 and S4, the received power RXP is computed from the DCCH downlink signal when the traffic channel TCH from the associated BTS is switched off (described below).

In step S5, the path loss calculator 60 receives the stored (and updated) transmit power TXP1 for BTS1 from the storage location for BTS1 in the TX power storage 54, and also in the RX power storage 56. It also receives received power RXP1 for BTS1 from the storage area for BTS1. The path loss calculating unit 60 subtracts the received power RXP1 from the transmission power TXP1 to determine the path loss PL1 for the BTS1. In step S6, the same operation is repeated for BTS2.

In step S7, the required RX power calculating unit 58, based on a predetermined characteristic (eg, frame error ratio (FER)) as a whole of the received downlink signals (eg, after maximum ratio synthesis (MRC)), the mobile station Determine the required RX power RRXP representing the minimum power that needs to be received to generate all downlink signals DS of this currently acceptable quality.

In step S8, the required TX power calculating unit 62 receives the path loss PL1 for BTS1 and the required RX power RRXP. Based on these inputs, the required TX power calculating section 62 calculates the required downlink transmit power P from BTS1 under the assumption that BTS1 is the only BTS allowed to transmit a downlink signal to the mobile station in the next time slot.<sb>BTS1 </sb>calculate This required transmit power P<sb>BTS1 </sb>is calculated by, for example, summing PL1 and RRXP together. Calculated required downlink transmit power (P<sb>BTS1</sb>) is then stored in the TX power storage 64 in a storage area assigned to BTS1 in a first set of storage areas (the set is relevant when maximum ratio synthesis is not performed at the mobile station).

Next, in step S9, the interference calculating unit 66 calculates the required downlink transmission power P calculated in step S8.<sb>BTS1</sb>), and the downlink transmit power (P<sb>BTS1</sb>) measured value I, which is the amount of network interference caused by BTS1 (alone) operating on<sb>BTS1 </sb>calculate This measured value is stored in an appropriate place in the storage area allocated to BTS1 in the interference value storage unit 68.

Next, in steps S10 and S11, the processing in steps S8 and S9 is repeated for BTS2 as well. The resulting required downlink transmit power P<sb>BTS2</sb> and network-interference measurements I<sb>BTS2 </sb>are stored in the storage areas assigned to BTS2 in components 64 and 68, respectively.

In step S12, the required TX power calculating section 62, for the BTSs BTS1 and BTS2, under the assumption that MRC is used in the mobile station, the required downlink transmission power P'<sb>BTS1 </sb>or P<sb>BTS2 </sb>calculate These results are stored in the storage areas assigned to BTS1 and BTS2 in the second set storage area of the required TX power storage 64 .

In step S13, the interference calculating unit 66 calculates the required downlink transmission power calculated in step S12.<sb></sb> P<sb>BTS1 </sb>or P<sb>BTS2 </sb>Using BTS1 P<sb>BTS1 </sb>Transmitted by BTS2, P<sb>BTS2 </sb>Determines the resulting network interference measurement under the assumption that it is transmitted from The resulting interference measure I<sb>MRC </sb>is stored in another storage area of the interference value storage unit 68 .

Next, in step S14, the interference comparison unit 70 obtains the interference measurement value I obtained from the interference value storage unit 68.<sb>BTS1 </sb>and I<sb>BTS2 </sb>compare I<sb>BTS1 </sb>this I<sb>BTS2</sb> less, the processing step proceeds to step S15,<sb>BTS1 </sb>and I<sb>MRC</sb> compare In step S15, I<sb>BTS1 </sb>this I<sb>MRC </sb>If less, in step S16, the BTS selector 72 determines that the downlink signal in the next time slot is to be transmitted by BTS1 to the mobile station, based on which minimal network interference occurs. The BTS selector 72 generates a BTS select message (BSM) specifying that BTS2 does not transmit a downlink signal in the next time slot. The BSM is passed to the transmitter 46 of the mobile station for transmission to BTS2. At the same time, the BTS selector 72 determines the power control bit (PCB) to be transmitted to BTS1 to control the downlink transmission power of BTS1 in the next time slot, and the PCB determines the P calculated in step S8.<sb>BTS1 </sb>have a value As previously described, the PCB is either a single PCB common to all BTSs involved in the soft hand-off operation, or a unique PCB for BTS1 (PCB1).

In step S14, I<sb>BTS2 </sb>go I<sb>BTS1 </sb>If less than or equal to, or in step S15, I<sb>MRC </sb>go I<sb>BTS1 </sb>In the case of less than or equal to, the processing proceeds to step S17. In step S17, the interference comparison unit 70 is I<sb>BTS2 </sb>and I<sb>MRC</sb>compare I<sb>BTS2 </sb>go I<sb>MRC </sb>If less than, the processing proceeds to step S18, where the BTS selector 72 determines that the downlink signal for the mobile station is to be transmitted by only BTS2 in the next time slot, based on which the BTS2 operating alone is at least network interference. In this case, the BTS selector 72 generates a BSM that instructs BTS1 not to transmit in the next time slot. Also, the PCB applicable to BTS2 is set by the BTS selector 72 to control the downlink transmission power of BTS2, and the required TX power P calculated in step S10.<sb>BTS2</sb> satisfy the

In step S17, the comparison result is I<sb>MRC </sb>go I<sb>BTS2 </sb>If less than or equal to, the processing operation proceeds to step S19, and the BTS selector 72 determines that BTS1 and BTS2 are used to transmit downlink signals in the next time slot, based on which minimal network interference occurs. . In this case, the BTS selector 72 generates a BSM specifying that these two BTSs do not transmit in the next time slot, and the BTSs in the next time slot, the required transmit power P' calculated in step S12.<sb>BTS1 </sb>and P`<sb>BTS2 </sb>Set the PCB (or PCBs) not to transmit a downlink signal to

Thus, it can be seen that in the above embodiment, three distinct candidate BTS selections are identified: a first candidate selection in which only BTS1 is designated for transmitting the downlink signal; a second candidate selection in which only BTS2 is designated to transmit the downlink signal; A third candidate selection in which both BTS1 and BTS2 are designated to transmit downlink signals. In each candidate selection, the required transmit power P of each BTS specified in the selection<sb>BTS</sb> (or P<sb>BTS</sb>) is computed, and a measure of network interference resulting from BTSs designated to transmit is also computed. These network-interference measurements are used to determine a candidate selection used for the downlink signal transmission (eg, a lowest value is found compared to the measurements), thereby reducing network interference associated with the transmission.

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It is not essential to include a selection specifying only one BTS in the candidate selection. For example, if three BTSs are involved in the soft hand-off operation, the choices may be BTS1 + BTS2, BTS2 + BTS3, BTS3 + BTS1, and BTS1 + BTS2 + BTS3. It is also not necessary to include a selection specifying all BTSs involved in soft hand-off in the candidate selection. Moreover, in a particular selection, the transmit power for the designated BTSs may be set to an appropriate combination of values that facilitate proper reception of the downlink signal at the target mobile station. Thus, for example, two or more candidate selections may specify the same BTSs, but may specify different respective transmit power sets for the selection.

An example of a possible format of the BTS Select Message BSM will be described with reference to FIG. 9 .

The BTSs involved in the soft hand-off operation are ranked in a certain way. For example, the ranking is performed within the mobile station based on a predetermined characteristic of each of the downlink signals DS1 to DSn received by the MS 40, such as the received signal strength (RSS). Alternatively, the ranking may be performed "first-come first-served", ie in the order in which the BTSs are involved in a soft hand-off operation. Alternatively, the ranking may be performed randomly. Once the ranking is determined, the mobile station sends a ranking message RM to all BTSs over the control channel indicating the order in which the BTSs are currently ranked.

As shown in FIG. 9, the BSM has one bit corresponding to each BTS ranking, and these bits are ranked in the BSM in the order of ranking determined by the MS. For the example above with reference to FIG. 8, only two BTSs, BTS1 and BTS2, are involved in the soft hand-off operation. Assume that, in the ranking order determined by the mobile station, BTS2 is the highest ranked BTS (grade 1), and another BTS, BTS1, has grade 2). Also, suppose that the comparison result of the interference measurement values is the result shown in step S16, that is, the result that BTS2 should not transmit a downlink signal in the next time slot. In order to notify the BTSs involved in the soft hand-off operation of this result, the first bit in the BSM (corresponding to class 1) is set to "0" so that BTS2 does not transmit a downlink signal in the next time slot. should instruct The second bit of the BSM (corresponding to the class 2 BTS) is set to "1", indicating that the class 2 BTS, BTS1 will not transmit a downlink signal in the next time slot. Since only two BTSs are involved in the soft hand-off operation in this embodiment, the remaining bits of the BSM can be set to a "don't-care" state. Incidentally, the BSM in this case may, of course, consist of only two bits.

The rankings of BTS require periodic updates for various reasons. First, as the MS 40 moves, a downlink signal is received from a new BTS, and the old BTS is no longer able to provide a detectable downlink signal. Second, the quality of the signal received from the BTSs 20 may have changed, for example due to fading. Therefore, it is necessary to update the ranking from time to time. The update is performed periodically at regular time intervals (eg, every few hundred milliseconds as in a GSM network), or every frame or even every time slot. Alternatively, the ranking may be updated only when a new BTS is detected or contacted with an existing BTS.

10 is a block diagram illustrating a portion of a BTS 20 implementing the present invention. The BTS 20 is specially adapted to receive and process the ranking message RM, and the ranking message RM and BTS selection message BSM are transmitted by the MS 40 of FIG.

The antenna element 22 is connected to the receiving unit 24 and the transmitting unit 26 (eg, via a duplexer (not shown)). The soft hand-off control unit 28 receives the uplink signal US from the receiving unit 24 , and then supplies the received US (or a signal derived therefrom) to the fixed network 5 for transmission to the BSC 30 . do. The transmission unit 26 receives a downlink signal DS from the BSC 30 (FIG. 5) and a prohibition signal DIS from the soft hand-off control unit 28 through the connection line 5 .

In using the BTS 20, the uplink signals transmitted by the MS 40 when in the soft hand-off region 9 sometimes include a ranking message RM. The uplink signals US detected by the receiving unit 24 in the BTS 20 are fed to a soft hand-off control unit 28 . When the soft hand-off control section 28 detects that a ranking message RM is included in one of the uplink signals US received thereby, the soft hand-off control section 28 processes the related ranking message, and the MS Determines the ranking of the BTS within the ranking order determined by . In each time slot, the uplink signals US generated by the receiver 24 also include the BTS selection message BSM determined by the MS 40 as above. The operation of the soft hand-off control unit 28 according to the above BSM in the uplink signal US generated by the receiving unit 24 will be described next.

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By the time the BSM is received, it is set that the ranking message RM has already been received and processed by the soft hand-off control 28 (as above).

The BSM is supplied to the soft hand-off control unit 28 by the receiving unit 24 and is irradiated. The soft hand-off control section 28 checks the ranking of the BTS based on the last-received ranking message, and then checks the bit corresponding to the ranking in the BSM. If the bit is "0", the soft hand-off control unit 28 applies the inhibit signal DIS to the transmission unit 26, and prevents the transmission unit 26 from transmitting a downlink signal in the next time slot. .

Network Interference Value I<sb>BTS1</sb>, I<sb>BTS2</sb>, or I<sb>MRC </sb>, taking into account the interference experienced by the hypothetical mobile station, other than the target mobile station, operating within the soft hand-off region (FIG. 5), since the BTSs involved transmit at a given required power, is computed as follows. For example, I<sb>BTS1</sb>In the case of , the interference is the required transmit power P from BTS1 to the target mobile station.<sb>BTS1</sb> and the associated average path loss experienced by the hypothetical mobile station (same as the target mobile station). The average path loss is a time-averaged path loss, and the average duration is chosen to average (or ideally eliminate) the effect of Rayleigh fading. In other words, the change in path loss due to ray ray fading is averaged.

I<sb>MRC</sb> In the case of , the interference is computed based on the accumulated sum of the carrier power levels of each of BTS1 and BTS2 at the antenna of the hypothetical mobile station. Also, these carrier power levels are the required transmit power P for BTS1 and BTS2 when MRC is used.<sb>BTS1</sb> and P<sb>BTS2</sb> and based on the respective average path losses predetermined (and assumed to be equal to the virtual mobile station) at the target mobile station.

Let's take the case where the downlink signal from BTS2 undergoes a severe fade. This means that PL2 is larger than PL1. In this case, the required transmit power P for BTS2<sb>BTS2 </sb>is the required transmit power P for BTS1<sb>BTS1 </sb>will be larger than Accordingly, IBTS2 becomes larger than IBTS1. Also, considering the large PL2, P'<sb>BTS2</sb>also gets bigger, I<sb>MRC</sb>is I<sb>BTS1</sb>gets bigger Accordingly, it is determined that BTS2 does not transmit a downlink signal in the next time slot, and network interference due to the transmission of the downlink signal is reduced.

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In the above embodiment, the TX power store 54 receives the initial downlink transmit power of the involved BTSs, and then usefully updates them as it receives the power control bits PCBs for the separate BTSs. However, it is also possible that the instantaneous downlink transmit power TXP itself is supplied directly to the TX power storage 54 in each time slot instead of the PCBs.

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It can also be seen that the manner in which the transmit power TXP (or IXTP + PCB) and the receive power RXP can be used for the MS is not critical to the present invention. For example, it is not necessary to rank the BTSs relative to the MS. All that is needed is for each BTS to be able to identify to which BTS a particular received value (eg, ITXP or RXP) relates. Such identification may be performed in many different ways besides ranking.

It will also be appreciated that the processing operation shown in FIG. 8 need not occur in every time slot. It is possible that signals such as RXP and PCM are transmitted only once per frame, in which case the decision-processing is performed frame by frame.

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Furthermore, it is possible for the determination to be performed in a time interval other than frames or time slots, for example based on a time interval consistent with the fading characteristics of 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 may also be applied to other systems that do not conform to the IS95 standard. The present invention relates to other cellular networks that do not use CDMA, e.g., one or more of the following techniques: multiple-access techniques: time-division multiple access (TDMA), wavelength-division multiple access (WDMA), frequency-division multiple access It is also applicable to networks using (FDMA) and space-division multiple accesses (SDMA).