GB2358326A

GB2358326A - Selection of soft handoff method in cellular mobile communications networks

Info

Publication number: GB2358326A
Application number: GB0000534A
Authority: GB
Inventors: Behzad Mohebbi
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 2000-01-11
Filing date: 2000-01-11
Publication date: 2001-07-18
Anticipated expiration: 2020-01-11
Also published as: GB2358326B; GB0000534D0

Abstract

In a mobile communications network, when a mobile station (MS) is to be handed off from a first base transceiver station (BTS A) of the network to a second base transceiver station of the network (BTS B), the network selects whether to use a first hand-off strategy or a second hand-off strategy. In the first hand-off strategy the hand-off involves allocating a new forward channel for communication from the second base transceiver station (BTS B) to the mobile station that is different from a forward channel allocated for communication from the first base station (BTS A) to the mobile station. This strategy preserves orthogonality in the channels used by BTS B but leads to interference because of the channel negotiation process. In the second hand-off strategy the hand-off involves using the same forward channel for communication from the second base transceiver station (BTS B) to the mobile station as for communication from the first base transceiver station (BTS A) to the mobile station. This strategy does not preserve orthogonality and therefore leads to interference but avoids the channel negotiation process. The selection of the hand-off strategy to be used is based on which strategy is expected to result in a lower level of interference in the network.

Description

2358326 -1 SOFT HAND-OFF IN CELLULAR MOBILE COMMUNICATIONS NETWORKS The

present invention relates to cellular mobile communication networks, for example Code Division Multiple Access (CDMA) cellular networks. In particular the present invention relates to soft hand off in networks having so-called I'microcells" and other cells with a small coverage area.

Figure 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 "IS9511). Each of three base transceiver stations (BTSs) 4 (ETS1, ETS2 and BTS3) is connected via a fixed network 5 to a base station controller is (BSC) 6, which is in turn connected to a mobile switching centre (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.

Figure 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 1 Diversity.

Similarly, Figure 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 11 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.

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 ETS 4 may be relatively poor, but by making use of more than one BTS 4 the quality may be substantially improved.

However, as described later in the present specification in more detail, the IS95 soft hand-off system, in common with other hand-off algorithms and techniques, is designed and optimised to operate in a so-called macrocellular environment in which the or i i i i each antenna of each cell (I'macrocell") is above the level of the average rooftop. Such a macrocell has a relatively large coverage area with generally uniform propagation characteristics across the cell or each sector of the cell.

Such macrocells are envisaged for the first phase of deployment of cellular networks, where the demand for network capacity is relatively low. However, as the demand for network capacity grows, because the network is subject to physical limits on spectrum availability, the capacity can only be increased by reduction of the cell "footprint", i.e. by cell splitting or deployment of so-called I'microcells".

In a microcell, the or each antenna is below the average rooftop. The propagation characteristics in such a microcell can be highly directional, providing, for example, propagation along individual streets.

In microcells and other small cells, because the cell coverage area is reduced, the soft hand-off system requirements become more onerous, and in particular the speed of operation of the soft hand-off system must be increased.

The problem of the small cell footprint is exacerbated by microcell propagation characteristics which effectively result in cells "appearing" and "disappearing" very quickly, for example as a mobile station travels around street corners. All cellular networks are designed to be capable of coping with significant variations in signal level as the mobile station is moving (these are sometimes referred to as "log-normal variations"), so as to enable the network to cope with radio shadows. However, in travelling around street corners in a microcellular environment, signal levels can rise or fall by as much as 30dB, which is in excess of the log-normal variations which conventional networks are designed to accommodate.

In such situations, the mobile station, which needs to measure average and report back signal strength for an active set of neighbouring cells i involved in the soft hand-off, would require processing times which are in excess of reliable operation time for successful hand-off.

When a corner is taken by a mobile station in the microcellular environment, a new cell appears almost instantaneously, and the uplink and downlink signal levels associated with the serving cell may experience severe fading which could result in the.loss of a reliable communication link for hand-off signalling, or in transmission at excessive transmit powers (20 to 30dB) for a long duration (so-called "corner effect").

When it is considered that a large number of mobile stations can be in such hand-off situations simultaneously, the loss in network capacity due to excessively powerful transmissions could be severe.

our co-pending United Kingdom patent application no. 9823736.5 discloses a soft hand-off method adapted especially for use in the microcellular environment.

In a preferred version of that method, in a call setup process a mobile station is assigned a particular downlink channel and a particular uplink channel for use throughout the call, i.e. for use in communicating with different base transceiver stations as the call progresses. In the call setup process, communication between the network and the mobile stations is carried out via a single active ("best-server") BTS but, in addition, the mobile station and at least one further BTS neighbouring the active BTS are provided by the I network with call setup information for use by the mobile station and the or each further base station to allocate respective uplink and downlink channels between the further BTS concerned and the mobile station. Upon completion of the call setup process, the or each further BTS is initially set to a dormant state in which its uplink and downlink channels allocated to the mobile stations are not in use. When, during the course of the call, it is determined that the mobile station should communicate with such a dormant BTS (which has become the new best-server BTS), the dormant BTS employs the call setup information provided to it in the call setup process to bring about change of that dormant BTS from the dormant state to the active state.

In this way, the change of serving BTS can be carried out with a minimum amount of signalling between the mobile station and the BTSs involved in the soft hand-off operation. As indicated previously, channel negotiation signalling in the microcellular environment causes significant interference problems, firstly because of excessive transmission powers by one or both of the original best-server BTS and the new best-server BTS, and secondly because the channel negotiation signalling may take a significant amount of time. Both these factors result in a significant level of network interference, especially when the hand-off from one microcell to another is abrupt as in the "corner effect" described above.

On the other hand, the alternative soft hand-off method devised for use in microcellular environments is not itself without interference problems. In particular, because the mobile station maintains the same uplink channel and the same downlink channel throughout the duration of a call, the desired orthogonality of the channels (e.g. code sets in a CDMA network) used by different mobiles in a given cell is not guaranteed. In presently-proposed wideband CDMA (W-CDMA) systems such as the proposed European W-CDMA system (UTRA) several scrambling codes may be defined per cell (multi-identity cell). UTRA stands for UMTS Terrestrial Radio Access, and UMTS stands for Universa-1 Mobile Telecommunications System (a third generation mobile communications system). These scrambling codes can be paired with channelisation codes of the orthogonal variable spreading factor (OVSF) tree to give a large range of available code-sets for use in pico cells or microcells. Taking advantage of this large range of available code-sets, it may well be possible to perform a so-called "intra-cell" hand-off to change the code-set of an interfering mobile station after it has entered a new best-server cell following a soft hand-off operation. In this way, orthogonality can be restored between the code-sets in use in the new best-server cell.

Nonetheless, unless and until such an intra-cell hand-off is performed, non-orthogonality will exist in the SHO region, leading to multi-user interference.

It is therefore desirable to provide a soft hand off method capable of reducing interference in the microcellular environment.

According to a first aspect of the present invention there is provided a soft hand-off method, for use in a mobile communications network, comprising selecting, when a mobile station is to be handed off from a first base transceiver station of the network to a second base transceiver station of a network, whether to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from, the said first base station to the said mobile station, or to use a second hand-off strategy in which the hand off involves using the same forward channel for communication from the said second base transceiver station to the mobile station as for communication from i the said first base transceiver station to the said mobile station.

According to asecond aspect of the present invention there is provided a mobile station, for use in a mobile communications network, comprising: soft hand-off control means operable, when the mobile station is to be handed off from a base transceiver station of the network to a second base transceiver station of the network, to cooperate with the network to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base transceiver station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

According to a third aspect of the present invention there is provided a base transceiver station, foruse in a mobile communications network, comprising:

soft hand-off control means operable, when a mobile station is to be handed off from the subject base transceiver station to a further base transceiver station of the network, to cooperate with the network and the said mobile station to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said further base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the subject base transceiver station to the said mobile station, or to carry out a second hand-off strategy in which the hand- off involves using the same forward channel for communication from the said further base transceiver station to the said mobile station as for communication from the subject base transceiver station to the said mobile station.

According to a fourth aspect of the present invention there is provided a base transceiver station, for use in a mobile communications network, comprising:

soft hand-off control means operable, when a mobile station is to be handed off from a further base transceiver station of the network to the subject base transceiver station, to cooperate with the network and the mobile station to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the subject base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said further base transceiver station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the subject base transceiver station to the said mobile station as for communication from the said further base transceiver station to the said mobile station.

According to a fifth aspect of the present invention there is provided a base transceiver station controller, for connection to respective first and second base transceiver stations of a mobile communications network, comprising: soft hand-off control means operable, when a mobile station of the network is to be handed off from the first base transceiver station to the second base transceiver station, to select either to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base transceiver station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

According to a sixth aspect of the present invention there is provided a mobile communications network, comprising: a mobile station; first and second base transceiver stations; and selection means for selecting, when a mobile station is to be handed off from the said first base transceiver station to the said second base transceiver station, whether to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base transceiver station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

Reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1, discussed hereinbefore, shows parts of a cellular mobile communications network according to Is9s; 3S Figure 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 Figure 1 network; Figure 3, also discussed hereinbefore, shows a schematic view for use in explaining processing of downlink signals in such a soft hand-off operation; Figure 4 shows a schematic view of a microcellular network for use in explaining a principle underlying the present invention; Figures 5 to 7 show a sequence of steps in a soft hand-off method in a cellular communications network embodying the present invention; Figure 8 shows parts of a mobile station embodying the present invention; Figure 9 shows in detail one of the parts of the Figure 8 mobile station; Figure 10 shows parts of a base transceiver station embodying the present invention; Figure 11 shows in detail one part of the Figure base transceiver station; and Figure 12 shows parts of a base station controller embodying the present invention.

Figure 4 shows three BTSs A, B and C belonging to respective microcells. BTS A and BTS C are arranged spaced apart on opposite sides of the same street. BTS 2S B, on the other hand, is arranged around a corner from BTS A in a different street. A mobile station MS, for example in a car or carried by a pedestrian, is presently located close to BTS A and is accordingly being served by PTS A.

To illustrate a principle of the present invention, two different soft hand-off scenarios are considered, the first scenario involving a soft hand off from BTS A to BTS C as the MS moves in the direction of the arrow I, and the second scenario involving soft hand-off from BTS A to BTS B as the MS moves in the direction of the arrow II.

In the first scenario, as the MS moves in the direction of the arrow I into the coverage area of BTS C, the signals fromBTS A will still be received relatively strongly, i.e. the respective coverage areas of BTS A and BTS C overlap considerably. This means firstly that neither BTS A nor BTS C will need to raise its transmission power unduly to maintain contact with the MS in the soft hand-off region. Secondly, the amount of time available for channel negotiation signalling of the type used in the conventional soft hand-off operation is relatively large. For these reasons the amount of interference that results from use of the conventional soft hand-off method in going from BTS A to BTS C is less than the amount of interference that would be caused by using the alternative soft hand-off method described in the introduction in which the mobile station keeps the same uplink and downlink channels when handing off to the new best-server cell.

In the second scenario (hand-off from BTS A to BTS B), the corner effect means that the signal from BTS B appears abruptly and the signal from BTS A disappears abruptly. For this reason, the soft hand-off region between the two BTSs A and B is relatively small and, as the mobile station passes through the region, one or both of the two BTSs involved in the hand-off may have to raise its transmission power unduly to keep in contact with the mobile station. This results in a level of network interference that outweighs any network interference reduction that would be achieved by carrying out the conventional soft hand-off method involving a c hange of channel in the hand-off. Thus, in this situation, it is better from the network interference point of view to use the alternative soft hand-off method in which no channel negotiations are performed and the same uplink and downlink channels are maintained when handing off to the best-server cell.

In light of the differences between the two scenarios, a soft hand-off method embodying the present invention involves selecting, for a particular soft hand-off case, whether to use the conventional soft hand-off method in which the channels are changed to maintain orthogonality, or to use the alternative soft hand-off method without channel renegotiations but resulting in non-orthogonal channels (at least until an intra-cell hand-off is performed in the new best-ser-vez cell). In this way, the soft hand-off method used can be optimised for the particular soft hand-off case involved.

In order for the selection to be possible, the network needs to hold information (hand-off strategy information) for use in determining which hand-off strategy to use. This soft hand-off information can, in the simplest case, be predetermined information which simply identifies explicitly which hand-off strategy to use for each particular soft hand-off case at a particular BTS. For example, in the Figure 4 situation, for BTS A the hand-off strategy information could simply specify "conventional strategy" (hereinafter strategy 1) for hand-off from BTS A to BTS C, and "alternative strategy" (hereinafter strategy 2) for hand-off from BTS A to BTS B. It is also possible for the hand-off strategy selection to be influenced dynamically in operation of the network, for example to take account of changing traffic conditions in the network. This may be desirable because under different traffic conditions the optimum hand-off strategy may not always be the same. To enable the optimum hand-off strategy to be selected in such a case, one approach that can be used is to calculate an "interference budget" for both possible strategies, and then compare the interference i i budgets for the two strategies to select the optimum strategy. In general, the short duration of the soft hand-off operation and the complexity of the interference budget calculation will mean that it is not practicable to choose the hand-off strategy dynamically in the course of each soft hand-off operation. A more practical approach is to base the hand-off strategy selection based on a history of the interference budgets of soft hand-off operations performed previously by a given BTS.

For example, the network might initially start from the premise that for each soft hand-off operation by a given BTS the conventional strategy (strategy 1) is to be used. As each actual soft hand-off operation by the given BTS to a certain further BTS is performed, the network would record information needed to calculate the interference budget for both strategies 1 and 2. An example of the interference budget calculation will be given later in more detail, but the information includes, for example, information relating to the transmission power control (TPC) commands issued during the actual soft hand-off operation. After information has been collected for one or more such actual soft hand-off operations, the network calculates the interference budgets for the two strategies and selects, if appropriate, to change to the use of the alternative strategy (strategy 2) in future for hand off operations from the given BTS to the further BTS concerned. The correctness of the selected strategy can be reexamined from time to time as experience of the soft hand-off operation grows.

Next, one example of the overall soft hand-off procedure embodying the present invention will be explained in detail with reference to Figures 5 to 7.

In the soft hand-off procedure of Figures 5 to 7, when a call is set up for a mobile station MS (the call may be MS-originated or MS-terminated), the MS is assigned a particular set of channelisation and scrambling codes and, where possible, this code set is used for the duration of the call for communication between the MS and the network wherever the MS is located in the microcellular network. Such assignment of a particular code-set is practicable in the proposed European wideband CDMA (W-CDMA) system (UTRA) because in UTRA each BTS can have a number of scrambling codes.

The network (BSC) allocates a forward traffic channel (forward TCH) and a reverse traffic channel (reverse TCH) for use by the network and MS to communicate with one another.

Thus, in step 1 of Figure 5 it is assumed that is when the call is set up the MS is in the cell of BTS A, i.e. BTS A is the initial best-server BTS. The current.

bestserver BTS (BTS A) is informed of the allocated forward and reverse TCHs, and BTS A in turn informs the MS (step 1).

In addition, unlike in the usual call setup process, the BSC informs a preferred set of BTSs in the cells neighbouring the cell of BTS A of the allocated forward and reverse TCHs for the present MS call (step 2). The information sent to each cell of the preferred:

set includes all the information (such as the service rate, channelisation and scrambling codes etc.) needed for call set-up.

In step 3, the BSC transfers to the current best server BTS A a list (the "active cell list,') of the cells of the preferred set of neighbouring cells (i.e.

the cells which may be involved in a soft hand-off operation when the MS is handed off from BTS A) - in this example cell B only. The current best-server BTS A forwards the active cell list to the MS which receives and stores the list.

After the call has been set up in this way (step 4), only the current best-server cell BTS (13TS A) is initially in communication with the mobile station.

The remaining BTSs of the neighbouring cells (BTS B) do not initially transmit or receive any information via S the allocated TCHs to/from the mobile station, until notified by the network that they should do so. Thus, these remaining cells may be referred to as "dormant" cells.

At this time, the MS measures the signal strength of a common control channel CCCH broadcast by the BTS in each cell specified in the active cell list transferred to the MS in step 3. Strength measurement messages, providing the results of the signal strength measurements, are transmitted (step 5) via the reverse TCH to the current best-server BTS (DTS A).

The strength measurement messages are received by the current best-server base station BTS A and forwarded to the BSC.

At some point, the BSC detects on the basis of the forwarded strength measurement messages that the signal strength of the CCCH of one of the neighbouring cells of the preferred set exceeds a predetermined threshold value T_ADD. This signifies that a soft hand-off operation should be initiated. For example, as shown in Figure 5 it is detected in step 5 that the CCCH of BTS B exceeds the predetermined threshold value.

In response to the threshold value being reached, in step 6 the BSC retrieves from a storage device thereof hand-off strategy information relating to the past experience of hand-offs from the current best server BTS (BTS A) to the prospective new best-server BTS (BTS B). Using the hand-off strategy information the BSC selects which hand-off strategy (strategy 1 or strategy 2) to use. The ESC informs both the current best-server ETS A and the respective new best-server BTS B of the selected hand-off strategy. In addition, the current best-server BTS A informs the MS of the selected hand-off strategy When strategy 1 is selected by the BSC in step 6, the soft hand-off procedure follows the steps 7 to 23 shown in Figure 6.

In step 7, the ISSC allocates a new forward TCH for! use in communication between the prospective new best server BTS B and the MS. The particular codes assigned to the new forward TCH are such as to preserve orthogonality with the remaining channels used by BTS B for communication with other mobile stations. The forward TCH used by BTS A does not change.

In step 8, BTS B begins transmitting traffic via the forward TCH to the MS. From this point onwards, all forward-direction (downlink) communications to the MS are performed by both BTS A and BTS B. Similarly, any reverse-direction (uplink) communication from the MS is received and processed by both BTS A and BTS B. The reverse traffic channel for BTS B is the same channel as the reverse traffic channel for BTS A.

Thus, in steps 9 and 10 respectively BTS A and BTS B transmit a hand-off direction message via their respective forward TCHs to the MS. In accordance with the hand-off direction message, the MS "acquires" BTS IS and uses the forward-direction signals from both BTS A and BTS 3 to receive traffic from the network. The hand-off direction message informsthe MS to enter the soft hand-off mode. The hand-off direction message contains information identifying the active set of BTSES for the soft hand-off. For example, the message contains parameters identifying the forward TCHs (i.e.

TCH "A", TCH "B", etc) assigned to the mobile station for the soft hand-off. The parameters enable the I mobile station to acquire and synchronise with the TCH i i 111311 in this case. The MS activates its circuitry for I I I receiving TCH "B".

In steps 11 and 12 respectively the MS transmits a hand-off direction message via the reverse TCH to BTS A and BTS B. This message confirms that the MS has entered the soft hand-off mode. By step 13 the MS resumes transmission and reception of data traffic.

From this point onwards, the MS is in soft hand off. During the soft hand-off period in which the MS is in communication with both ETS A and BTS B, continuous monitoring and reporting of the signal strength of the BTSs (A and B) of the active set is performed. In addition, the MS continuously measures and reports the signal strength of any remaining neighbouring cell notified to it in step 3 that is not in the active set. The reports of signal strength are sent to each BTS of the active set via the reverse TCH in the form of strength measurement messages (steps 15 and 16).

The strength measurement messages are forwarded by the receiving BTSs of the active set to the BSC which uses the messages to decide when to terminate the soft hand-off operation. When the BSC decides to terminate the soft hand-off operation (step 17) the DSC informs the BTSs of the active set that soft hand-off is to terminate. In response to this, both DTSs A and B transmit via their respective forward TCHs a hand-off direction message, for example an instruction to the MS to drop the BTS A (steps 18 and 19).

In step 20, the MS transmits via its reverse TCH a hand-off completion message. In response, BTS A ceases transmission via its forward TCH to the MS. The hand off completion message is also received by the new best-server BTS B. This is the end of the soft hand-off operation according to strategy 1. At this point, the processing resumes at step 2 in Figure 5. Step 2 is required because a new forward TCH was allocated in step 7. As it is possible that the next soft hand-off operation for the MS during its present call will be a strategy-2 hand-off operation, it is necessary to inform the neighbouring cells of the new best-server cell B of the new forward TCH. Also, as in step 3 the new best ii server cell (cell B) is informed of the active cell i list for the next soft hand-off operation (which will be a hand-off from BTS B to a neighbouring cell's BTS), and this new active cell list is forwarded by the BTS B to the MS so that it can commence signal strength measurements for the cells of the new active cell list.

Thereafter, processing proceeds according to steps 4 to 6 of Figure 5 (but with the MS being served by BTS B). i When, in step 6, the selected hand-off strategy is is strategy 2, the processing follows steps 25 to 35 in Figure 7.

In step 25, the MS is in communication with the current best-server BTS A only. The MS continuously monitors (step 26) the CCCH of all of the cells in the preferred set notified to it in step 3 (i.e. cell B in this example). In each monitoring period (which may be a frame or even a timeslot within the frame) the MS selects the best-serving cell based on the measurements taken in the preceding monitoring period. In step 27, for example, the MS selects the cell A as still being the best-server cell and transmits an uplink control message (UCM) via a reverse dedicated control channel (e.g. the dedicated physical control channel DPCCH in a UTRA system). The UCM identifies the best-server cell selected by the MS.

The UCM identifying the best-server cell is preferably interleaved and/or encoded to protect the information content thereof.

Incidentally, it will be appreciated that, compared to the reporting of strength measurement messages used in steps 15 and 16 of strategy 1, the i i 1 i i amount of signalling involved to identify the new best server cell in strategy 2 is much less.

At some point during operation according to strategy 2, the MS selects one of the dormant cells of the preferred set as being the new best-server cell.

Thus, in step 29 in this example, the MS identifies the dormant cell B as being the new best-server-cell. A UCM providing the identity of cell B is then transmitted via the reverse DPCCH to the existing best server cell A.

At the existing best-server cell A, the identity of the new best-server cell provided by the UCM is compared with the cell's own identity. If it is different, the existing best-server cell sends a new best-server message (NSM) to the DSC to inform it of the identity of the new best-server cell.

In step 30, the BSC sends cell B an active status message (ASM) informing that it is the new best-server cell for the MS concerned. BTS B is therefore forwarded the next downlink frame for transmission to the MS. This frame is not forwarded to the former best-server BTS A which is now in the dormant state (not in communication with the mobile station).

Incidentally, optionally as shown in steps 31 and 32 in Figure 7, hand-off direction and completion messages may be exchanged between the network and the MS if desired to improve the reliability of the hand off.

In step 33, although the MS has dropped ETS A it continues to monitor the signal strength of the CCCH from BTS A.

In step 34, the MS is now in communication exclusively with the new best-server cell B and exchanges traffic therewith via the forward and reverse TCHs. The forward TCH used by the new best-server cell is the same as the forward TCH used by the former best- server cell A. Thus, none of the channel negotiation signalling used in strategy 1 to change the forward TCH is required in strategy 2. However, as a consequence, the forward TCH used by the MS for communication with the new best-server cell B may not he orthogonal with the traffic channels used by cell B for communication with other mobile stations served by it. Once the hand-off operation is completed, this non-orthogonality may be eliminated by performing an intra-cell hand-off within the new best-server cell B, i.e. by allocating a new orthogonal forward TCH to the MS.

In step 35 the MS reverts automatically to providing strength measurement messages to the new best-server cell B (such strength measurement messages were inhibited during the strategy-2 hand-off operation so as to reduce the amount of signalling involved in the soft hand-off operation).

More information on strategy-1 and -2 hand-off procedures can be found in our copending United Kingdom patent application no. 9823736.5, the entire content of' which is incorporated herein by reference.

Figure 8 shows parts of a mobile station for use in an embodiment of the invention. The mobile station has an antenna portion 22 connected (e.g. via a duplexer - not shown) to a receiver portion 24 and a transmitter portion 26. The mobile station 40 also includes a soft hand-off control portion 28 which receives from the receiver portion 24 a downlink signal DSi from the or each BTS with which the MS 20 is currently in communication. The soft hand-off control portion 28 also applies messages such as hand-off direction completion messages and uplink control message UCM to the transmitter portion 26.

one example of the constitution of the soft hand off control portion 28 in the Figure 8 mobile station is shown in Figure 9.

In Figure 9, the soft hand-off control portion 28 comprises a signal strength measurement section 281, a best-server selection section 282, a message section 283, an active cell list management section 284 and an active cell list storage portion 285.

In step 3 of the Figure 6 procedure, the mobile station receives from the currently-serving BTS (BTS A) the list of active cells, and their associated parameters. This information is detected, in one of the downlink signals received by the mobile station from BTS A, by the active cell list management section 284 and is stored in the active cell list storage portion 285.

Whichever hand-off strategy is selected by the BSC, the signal strength measurement section 281 is supplied by the active cell liat management section 284 with the identity of each cell in the active list stored in the active cell list storage portion. For each such active cell, the signal strength measurement section 281 performs a measurement of the received signal strength RSS of the CCCH of the active cell concerned. The measurement may be performed, for example, over a frame or over part of a frame such as a time slot.

The resulting received signal strength measure RSSi for each of the active cells (here i is the number of the cell in the active set) is supplied from the signal strength measurement section 281 to the best server selection section 282 and to the message section 283. When the BSC has selected strategy 1, the message section 283 processes the measures RSSi to form strength measurement messages (SMM) which are transmitted by the transmitter portion in the mobile station 20 to the network (step 5 in Figure 5 and step 15 in Figure 6).

When the BSC has selected strategy 2, the best- server selection section 282 compares the RSS measures for the different active cells and determines which of the active cells is currently the hest-server cell.

The identity IDBs of the determined best-server cell is then supplied by the best-server selection section 282 to the message section 283. The message section 283 formulates an uplink control message (UCM) for transmission by the transmitter portion 26 in the mobile station 20 to the current best-server cell (steps 27 and 29 in Figure 7). This uplink control message may be encoded and/or interleaved, if required, to improve data transmission integrity.

It will be appreciated that it is not essential for the selection of the best-server to be based on a RSS measure. It would be possible alternatively to base the selection on some other measure such as signal-to-interference ratio (SIR) of each active cell or on a combination of different measures (e.g. RSS and SIR).

It would also be possible for the signal strength measurement section 281 to include a storage portion enabling it to store a past history of the RSS (and/or SIR) measures for the different BTSs in the active set.

In this case, it would be possible for the best-server selection section 282 to employ more sophisticated decision-making in relation to the best-server selection so as to avoid undesirable effects caused by temporary reception phenomena or other problems caused by too-frequent changing of the BTS selection.

Figure 10 is a block diagram showing parts of a BTS 30 for use in an embodiment of the present invention.

An antenna element 32 is connected (e.g. via a duplexer - not shown) to a receiver portion 34 and a transmitter portion 36. A soft hand-off control portion 38 receives uplink signals US from the receiver i i i portion 34, and forwards the received uplink signals US (or signals derived therefrom) to the fixed network 5 for transmission to the BSC. The soft hand-off control portion 38 also receives downlink signals DS from the BSC and selectively forwards the received downlink signals DS (or signals derived therefrom) to the transmitter portion 36 for transmission to mobile stations in the cell area covered by the BTS 30.

Figure 11 shows one example of the constitution of the soft hand-off control portion 38 in Figure 10.

The soft hand-off control portion 38 comprises a forwarding control section 381 having a downlink portion 382 and an uplink portion 383, a call setup information processing section 384, a call setup information storage section 385, an uplink control message processing section 386, a best-server comparison section 387, an active/dormant status control section 388 and a new best-server informing section 389.

As described previously, when a call is set up, not only is the current serving c ell (cell A) involved in the call setup process, but so is each neighbouring cell in the predetermined active set of cells (cell B).

Thus, if the BTS 30 is the BTS of such a neighbouring cell, in step 2 of the Figure 5 procedure the call setup information processing section 384 of the BTS receives from the BSC call setup information for the call being set up. The call setup information could be used in the dormant BTS at this stage for radio resource control purposes and other statistical purposes, but is otherwise simply stored in the call setup information storage section 385 for possible later use.

Following storage of the call setup information the active/dormant status control section 388 places the BTS in the dormant state. In this state, no transmitter or receiver resources are allocated in the BTS to the mobile station.

If the BTS 30 is the BTS of the currently-serving cell when the call is set up (cell A), in step 3 of the Figure 5 procedure the call setup information processing section 384 causes the active cell list and associated parameters, received from the BSC, to be forwarded via the forwarding control section 381 to the transmitter portion 36 for transmission to the mobile station.

When the BTS 30 is in the active state and the ESC i has selected strategy 2 the uplink control message processing section 386 monitors the reverse DCCH from the mobile station and detects when an uplink control is message UCM is included therein. When such a UCM is detected, the uplink control message processing section processes the message to derive therefrom the identity IDBS Of the best-server cell identified by the mobile station. The best-server identity ID., is compared with the BTS's own ID. The results of the comparison are transferred to the active/dormant status control section 388.

In the active/dormant status control section 388, switching between the active and dormant states is performed as follows. If the BTS is in the active state, and the derived ID,s does not match the BTS's own ID, the active/dormant status control section 388 determines that a new best-server cell has been selected by the mobile station. In this case, it switches the BTS to the dormant state and applies a control signal to the new best-server informing section 389 which transmits a new server message (NSM) to the BSC informing the BSC of the identity IDBS of the new best-server cell.

If, on the other hand, the BTS 30 is in the dormant state, its receiver portion 34 will not be in communication with the mobile station and so in this case the active/dormant status control section 388 is informed by an active state message ASM supplied by the ESC that it should enter the active state. In this case, the downlink and uplink portions 382 and 383 of the BTS are activated to forward uplink and downlink signals between the BSC and the MS concerned.

Next, an example of the process of selecting the hand-off strategy, as performed by the BSC at step 6 in Figure 5, will be explained with reference to Figure 12.

As shown in Figure 12, a BSC 40 for use in an embodiment of the present invention includes a hand-off strategy selecting section 410, a hand-off strategy information collecting section 415 and a hand-off strategy information database 420. The selecting section 410 and collecting section 415 are connected to the hand-off strategy selecting section 410 by a bus 430.

The database 420 contains a plurality of entries 440, each entry corresponding individually to a particular pair of BTSs which can be involved in a soft hand-off operation together.

For example, the first entry 440 in the database 420 relates to the pair of BTSs A and B in Figure 4 and relates to the hand-off from BTS A to BTS B. Similarly, the second entry 440 in the database 420 relates to the pair of BTSs A and C in Figure 4 and the hand-off from BTS A to BTS C. There is such an entry for every possible pair of BTSs which can be involved together in a soft hand-off operation.

Each entry contains hand-off strategy information needed by the hand-off strategy selection section 410 to select the current best strategy for hand-off for the BTSs concerned. Thus, as indicated previously, preferably the hand-off strategy information includes information from which an interference budget for both strategy 1 and strategy 2 can be calculated. This information is collected by the collecting section 415, preferably based on previously-performed actual soft hand-off operations carried out by the BTS-pair concerned. The collected information is stored by the collecting section 415 in the relevant entry of the database 420.

one example of the interference budget calculation, is given below.

In the case of strategy 1, a measure INT, of the level of network interference caused by selecting strategy 1 may be calculated using equation 1 below for the case of hand-off from ETS A to BTS B. TNEG TNEG INTI ": a A f P TxA W + L7 B f PTxB W.... (1) 0 0 In equation 1, ()A is a power-to-interference conversion factor assuming orthogonality between the I channels of BTS A, as will be the case for strategy 1.

Similarly, a,, is a power- to- interference conversion factor for ETS B, again assuming orthogonality of the channels of BTS B. In each case, a is a function of the number of mobile stations currently being served by the BTS concerned as well as the respective transmission powers to those mobile stations.

PTIA(t) represents the instantaneous transmission power of the signals exchanged between the BTS A and the MS as part of the channel negotiations in strategy 1 (steps 9 to 20 in Figure 6). This channel negotiation signalling takes place over a channel negotiation time period TNE which is measured by the G BSC. Information about the transmission powers over this period is available to the ESC based on a TPC i i i history of the downlink and/or uplink transmission powers of the signals involved in the channel negotiation, as measured during an actual handoff operation from BTS A to BTS B using strategy 1. The same information is also available for BTS B. For each BTS, the transmission powers are integrated over the channel negotiation period TWEG and the resulting integrated power is multiplied by the power-to interference conversion factor to arrive at an interference contribution for each BTS A and B. The interference contributions for the two BTSs are then summed to determine the interference budget INT, for strategy 1.

In the case of strategy 2, an interference budget is INT2 may be calculated using equation 2 below for the case of handoff from BTS A to BTS B. TSH02 TSH01 TSH03 INT2-:"'fiB fPW(O+aA fPTkAW+aB fPTxB(t)... (2) TsHol 0 TSH02 In equation 2 0,, denotes the power- to - interference conversion factor in the case of non-orthogonality between the TCH of the MS and the respective TCHs of other users in the new best-server BTS (BTS B). Again, 0,, is dependent on the number of other users served by BTS B and their respective transmission powers.

Generally, the factor 0, will be greater than each factor a in equation 1, reflecting the implication of non-orthogonality of the TCHs.

In equation 2 the time period from 0 to Ts,(l represents the time for which the original best-server BTS (BTS A) is transmitting whilst the prospective new best-server BTS (BTS B) is in the active cell list.

The time period from TSH01 to TSH02 represents the time taken by BTS B to do an intra-cell handoff. The time period from TSH02 to TSH03 represents the time for which BTS A remains in the active cell list whilst the BTS B is transmitting.

Equations 1 and 2 are only examples of the form of interference calculation contemplated by the present invention.

After producing the respective interference measures INT, and INT2 for the two strategies, the selecting section 410 selects the strategy to be used for the next hand-off between the pair of BTSs by comparing the two measures. For example, when INT,:! INT2, strategy 1 is selected and when INT, > INT2 strategy 2 is selected. In this way, the strategy that'.

is expected to lead to the lower level of network interference is selected.

The interference measures INT, and INT2 themselves may also be stored in the collecting section 420 instead of or as well as the other parameters mentioned above. Alternatively, to save memory space it would be possible only to store the difference INT, - INT2 or the ratio INTI/INT2. Storage of the parameters needed to calculate the measures INT, and INT2 does, however, give greater flexibility to use and reuse sophisticated algorithms to calculate the measures.

As mentioned above the information in the entry for each hand-off operation in the database 420 is preferably derived by the collecting section 415 from experience gained by the BSC from actual hand-off operations performed by the network between the two BTSs concerned. For example, the network may initially always select hand-off strategy 1 and perform a number of preliminary hand-off operations according to strategy 1 whilst the collecting section 41S gathers i i information such as the parameters needed to calculate the interference budgets according to equations 1 and 2. After a sufficient number of preliminary operations have been performed, the hand-off strategy selecting section 410 will be in a position to select a current "best" strategy for the particular BTS pair concerned.

This current best strategy may, for instance, be stored in the entry for the pair concerned, as shown in Figure 12.

It will be appreciated that the best strategy for a particular BTS pair may in fact change according to the traffic conditions in the network. For example, during rush hour in an urban environment a different strategy may be appropriate from that used in off-peak hours. Thus, the strategy may vary according to the time of day and/or according to the day of the week.

The BSC may therefore be configured to collect hand-off strategy information relating to different times of the day and different days of the week and may store different best strategy selections for the different times and days.

one possible implementation of the hand-off strategy selecting section 410 and collecting section 415 would be in the form of a neural network which can "learn" from the experience of previous hand-off operations and then make the best strategy selection in a highly sophisticated manner.

In any event, it will be desirable for the hand off strategy selecting section 410 to review the best strategy for each particular BTS from time to time to make sure that the selected strategy is still appropriate.

Claims

CLAIMS:

1. A soft hand-off method, for use in a mobile communications network, comprising selecting, when a mobile station is to be handed off from a first base transceiver station of the network to a second base transceiver station of the network, whether to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the said-second base transceiver station to the mobile station as for communication from the said first base transceiver station to the said mobile station.

2. A method as claimed in claim 1, wherein the I selection of the hand-off strategy to be used is based on which strategy is expected to result in a lower level of interference in the said network.

3. A method as claimed in claim 2, wherein for the said first strategy a level of interference is assessed based on one or more of the following parameters:

a channel negotiation time period required for allocating the said new forward channel; a transmission power of one or both of the said first and second base transceiver stations during the said period; a number of other mobile stations in communication with one or both of the said first and second base transceiver stations during the said period; and the respective transmission powers of the said first and/or second base transceiver station(s) when communicating with those other mobile stations during the said period.

4. A method as claimed in claim 2 or 3, wherein for the said secondstrategy a level of interference is assessed based on one or more of the following parameters:

a soft hand-off time period during which the said mobile station is in a soft hand-off region of the first and second base transceiver stations; a transmission power of the said first base transceiver station during the said soft-hand off period; a number of other mobile stations in communication with the said second base transceiver station during the said soft hand-off period; and the respective transmission powers of the said second base transceiver station when communicating with those other mobile stations during the said soft hand off period.

5. A method as claimed in any preceding claim, further comprising the step of collecting hand-off strategy information relating to previously-performed hand-off operations from the said first base transceiver station to the said second base transceiver station; and using the collected hand-off strategy information to select the strategy to be used for a next hand-off operation from the said first base transceiver station to the said second base transceiver station.

6. A method as claimed in claim 5, wherein the collected hand-off strategy information is reviewed from time to time, and a new strategy is selected, if appropriate, in dependence upon the results of the review.

7. A method as claimed in claim 5 or 6, further comprising the step of:

carrying out one or more preliminary hand-off operations using a predetermined one of the first and second hand-off strategies so as to collect the said hand-off strategy information; and after the said preliminary hand-off operation(s) has (have) been completed, selecting the strategy to be used for subsequent hand-off operations based on the collected information.

8. A method as claimed in any preceding claim, wherein the selected strategy is changed in dependence upon changes in traffic conditions prevailing in respective cells associated with the said first and/or second base transceiver stations.

9. A method as claimed in any preceding claim, wherein a different strategy is selected at different times of the day and/or different days of the week.

10. A method as claimed in any preceding claim, wherein the network is a code-division multiple-access network, and the channel allocation in the said first strategy involves assigning to the said new forward channel for communication from the said second base transceiver station to the said mobile station a code or code-set that is orthogonal to the code or code-set of the forward channel allocated for communication between the said second base transceiver station and each other mobile station served by the said second base transceiver station.

11. A mobile station, for use in a mobile communications network, comprising:

soft hand-off control means operable, when the mobile station is to be handed off from a first base transceiver station of the network to a second base transceiver station of the network, to cooperate with the network to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said i i i 1 1 second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base transceiver station to the said mobile station, or to carry out a second hand-off strategy in which the hand off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

12. A base transceiver station, for use in a mobile communications network, comprising:

soft hand-off control means operable, when a mobile station is to be handed off from the claimed base transceiver station to a further base transceiver station of the network, to cooperate with the network and the said mobile station to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said further base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the claimed base transceiver station to the said mobile station, or to carry out a second hand-off strategy in which the hand off involves using the same forward channel for communication from the said further base transceiver station to the said mobile station as for communication from the claimed transceiver station to the said mobile station.

13. A base transceiver station, for use in a mobile communications network, comprising:

soft hand-off control means operable, when a mobile station is to be handed off from a further base transceiver station of the network to the claimed base transceiver station, to cooperate with the network and the mobile station to carry out selectively a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the claimed base transceiver station to the said mobilE' station that is different from a forward channel allocated for communication from the said further base transceiver station to the said mobile station, or to carry out a second hand-off strategy in which the hand off involves using the same forward channel for communication from the claimed transceiver station to the said mobile station as for communication from the said further base transceiver station to the said mobile station.

14. A base transceiver station controller, for connection to respective first and second base transceiver stations of a mobile communications network, comprising:

soft hand-off control means operable, when a mobile station of the network is to be handed off from the first base transceiver station to the second base transceiver station, to select either to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel 2S allocated for communication from the said first base transceiver station to the said mobile station, or to use a second h and-off strategy in which the hand-off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

15. A base transceiver station controller as claimed in claim 14, wherein the said soft hand-off control means include:

hand-off strategy information collecting means for collecting hand-off strategy information relating to previously-performed hand-off operations from the said first base transceiver station to the said second base transceiver station; and selection means for using the collected hand-off strategy information to select the strategy to be used for a next hand-off operation from the said first base transceiver station to the said second base transceiver station.

16. A base transceiver station controller as claimed in claim 15, wherein the soft hand-off control means further include hand-off strategy information storing means for storing the collected hand-off strategy information for use by the selection means.

17. A mobile communications network, comprising:

a mobile station; first and second base transceiver stations; and selection means for selecting, when a mobile station is to be handed off from the said first base transceiver station to the said second base transceiver station, whether to use a first hand-off strategy, in which the hand-off involves allocating a new forward channel for communication from the said second base transceiver station to the said mobile station that is different from a forward channel allocated for communication from the said first base transceiver station to the said mobile station, or to use a second hand-off strategy in which the hand-off involves using the same forward channel for communication from the said second base transceiver station to the said mobile station as for communication from the said first base transceiver station to the said mobile station.

18. A soft hand-off method substantially as hereinbefore described with reference to Figures 4 to 12 of the accompanying drawings.

19. A mobile station for use in a mobile communications network substantially as hereinbefore described with reference to Figures 4 to 12 of the accompanying drawings.

20. A base transceiver station for use in a mobile communications network substantially as hereinbefore described with reference to Figures 4 to 12 of the accompanying drawings.

21. A base transceiver station controller for use in a mobile communications network substantially as hereinbefore described with reference to Figures 4 to 12 of the accompanying drawings.

22. A mobile communications network substantially as hereinbefore described with reference to Figures 4 to 12 of the accompanying drawings.