GB2364854A

GB2364854A - Determining necessity for handoff based on past experience of other mobiles

Info

Publication number: GB2364854A
Application number: GB0017114A
Authority: GB
Inventors: David Freeman
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-07-13
Filing date: 2000-07-13
Publication date: 2002-02-06
Anticipated expiration: 2020-07-13
Also published as: GB2364854B; GB0017114D0

Abstract

A communication system includes a network of base stations (33, 34, 37) in communication with at least two mobile stations (31, 32) travelling in a particular direction (12). Where the mobile station (32) passes through an area of poor performance or suffers a dropped call, but shortly thereafter camps-on to a new cell, the mobile station (32) learns that continuous communication could have been achieved if it had handed over to a new cell sooner. The mobile station (32) communicates what it has learned to the network and the network subsequently transmits a priority preference list of cells for handover to the following mobile station (31).

Description

<Desc/Clms Page number 1> MOBILE COMMUNICATION SYSTEM, MOBILE RADIO UNIT AND METHOD OF PERFORMING A HANDOVER 1. Field of the Invention This invention relates to a mobile communication system including a network of base stations and at least one mobile station and method of performing a handover between base stations in such a system.

2. Description of the Related Art Mobile communication systems, and particularly cellular communication systems, are well known in the art. In such systems, there is an overlapping array of adjacent communication cells within which a base station transceiver and plural mobile station transceivers operate in such a way that the mobile stations have limited interference with one another in the same cell, nor with other mobile stations in neighbouring cells.

Because the cells overlap one another, a mobile station is often able to communicate with several base station transceivers in neighbouring cells, and this is particularly so when the mobile station is located near inter cellular boundaries. When the mobile station is waiting to make or receive a call it is in an "idle" state and the mobile station has the ability to select one of the cell base stations that is within range of the mobile station at a particular time, the selection being based upon, for example, signal strength.

Thus, during the process of cell selection by the mobile station, the particular cell base station likely to give the best communication with the mobile station is

chosen, the network is informed of the identity of the chosen cell because the network needs to know where to send messages intended for that particular base station, and the mobile station is synchronised to the selected cell of the network so that the mobile can decode speech or data messages intended for that particular base station.

The process of a mobile station logging on to the network via a particular cell is referred to in the art as "camping-on" or being "camped-on" to a particular cell. The selection process must be periodically repeated to maintain its current cell assignment, since mobile stations often move from one cell to another. When a call is in progress this is described as "active". "Handover" refers to the situation where a mobile station passes from one cell to another whilst active and the active call is handed over from one cell to another.

Difficulties exist in maintaining communication when handing over from one cell to another and these difficulties may be caused, for example, due to minimal network design, e.g. a minimal overlap between adjacent cells, which is quite common for financial reasons, but also in situations where a base station has failed operationally. A particular difficulty arises in maintaining a radio communication link where a mobile station approaches and passes over a hill or where the mobile station passes through a tunnel. In the latter instance, a mobile station may be active on a particular cell on entering the tunnel, but on exiting from the tunnel the mobile station does not have knowledge of which cell to handover to, since the tunnel may be on a boundary between adjacent cells.

The problem of triggering a handover is overcome in analogue cellular systems such as Advanced Mobile Phone Service (AMPS), or Extended Total Access Common System (ETACS), used in the U.S.A. and U.K. respectively, where network design and geographic surveys are used to set up thresholds for triggering a handover. Such analogue systems also allow for scan receivers at the destination base station, to monitor the mobile station transmission as a check for a reasonable chance of communication between the mobile station and the base station.

A major problem with analogue systems is that the handover system set-up is established by geographic survey and the set-up is established until modified as a result of a new survey. This means that network changes, such as provision of a new base station, causes a complex change in the performance of the network requiring the system to be re-surveyed for optimum performance.

In distinction, the European digital cellular communication Global System for Mobile (GSM) communication networks-standard put the onus on to the mobile station to scan all potential radio channels from a list of channels sent by the network so that the mobile station is able to identify a valid, e.g. synchronised, and/or sufficiently strong signal, for continued communication and to report the result back to the network. The network then decides when a handover is necessary and to which channel the mobile should be assigned. A problem associated with GSM is that the mobile station makes the radio channel assessment based upon, for example, signal strength, and so the mobile station treats every performance situation as a new situation, since the mobile station does not know where, geographically it is or if it has been in that situation

(geographical location) previously. The mobile station also has no prior knowledge of the network conditions in that particular area, i.e. the mobile station does not know if there are viable handover candidates for the specific direction in which the mobile station is heading. Also, the GSM scanning algorithm which is written into the mobile station to enable the mobile station to scan cells may take a relatively long time to scan a11 available cells and, if the scan is not performed and a handover effected in a timely fashion, a loss of signal resulting in a dropped call may result. The problem is particularly great where a mobile station is travelling at high speed and it is often the situation that a valid handover may occur if the mobile station is travelling at low speed, but not if the mobile station is travelling at a high speed.

It has proved difficult to improve performance by revision of scanning algorithms in the above scenarios and it is an object of this invention to at least partially mitigate the foregoing disadvantages. SUMMARY OF THE INVENTION According to a first aspect of this invention there is provided a mobile communication including a central controller communicating with a plurality of base stations that in turn provide communications to a plurality of mobile stations including monitoring means to monitor signal strengths of transmissions from at least two of the plurality of base stations characterised by at least one first mobile station including means arranged to determine optimum operating parameters to enable continuous communication during a handover between at least first and second base stations and means for

transmitting said optimum operating parameters to said central controller or a second mobile station .

In a preferred embodiment, said means arranged to determine include further determinationJin hindsight after such handovers have taken place',an optimal handover path, and transmits said optimal handover path to said central controller or said second mobile station.

In a preferred embodiment, said further determination addresses a problem in maintaining a communication link that the at least one mobile station encountered during the handover between the at least first and second base station.

Preferably, said determining means in said mobile stations, include recognising said problem in maintaining a communication link based upon at least one of signal failure and channel error rate.

Advantageously, said optimum operating parameters include at least one of signal strength, synchronisation, and timing advance to assist said base station direct said second mobile station following substantially the same direction so as to effect continuous communication, during the handover.

Conveniently, said determining means in said mobile stations include recognising said problem in maintaining a communication link based upon one of signal failure and channel error rate.

Advantageously, said base stations include decision means arranged to make a determination based upon said problem maintaining a communication link experienced by said first mobile station whether to transmit a handover preference list to said second mobile station, said preference list being based upon one of history of signal strength of said first mobile station and timing advance

between receiving and transmission of data of said first mobile station.

Conveniently, said second mobile station includes assignment means to switch cell channels between said cells in the handover preference list.

According to a further aspect of this invention there is provided a method for providing handover in a mobile communication system including a plurality of base stations serving a plurality of mobile stations in respective communication cells, the method comprising the following steps: monitoring of signal strengths of radio transmissions from the plurality of base stations by a first mobile station as the first mobile station is handed off between a number of the plurality of base stations; characterised by determining, by the first mobile station, an alternative series of handovers between at least first and second base stations that would have maintained an acceptable communication link; selecting by the mobile station or one of the plurality of base stations a handover to said one of the plurality of base stations based on said alternative series of handovers.

Preferably, said method further includes the steps of travelling, by a second mobile station from one communication cell to another communication cell first in substantially the same direction, as said mobile station, such that identification for the optimal cell to which handover, is determined from said alternative series of handovers and performed handover for said second mobile to said optinal cell.

Preferably, the transmittal of the alternative series of handovers further includes at least one of signal strength, synchronisation, and timing advance to

assist a first base station to identify subsequent mobile stations following substantilly the same direction, whereby the first mobile station learns an optimum handover cell candidate and, communicates the optimum handover cell candidate to the second mobile station travelling along said path so as to effect continuous communication.

The present invention provides a first mobile station to communicate what it learns about a difficult handover location for subsequent mobile stations passing through that location. Thus, a mobile station that fails a handover and which subsequently finds a suitable cell to camp-on, or goes through an area of poor communication, communicates to the network which cell the mobile was synchronised to and its signal strength, and the solution, i.e. the adjacent cell which provided continuous communication. This information is then reported by the network to other, following, mobile stations encountering the same situation so that those following mobile stations are able to devote more of their available resources, e.g. time and processing power, to scanning and synchronising to the successful channel as quickly as possible.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows, in schematic form, a mobile station traversing a hill top, Figure 2 shows graphs of signal strength against distance for different cells,

Figure 3 shows, in block schematic form, a system in accordance with this invention, and Figure 4 shows a flow chart of the operation of a system in accordance with this invention.

In the Figures like reference numerals denote like parts.

DETAILED DESCRIPTION OF THE DRAWINGS Although the invention will now be described in relation to a GSM system, it is to be understood that the invention is not limited thereto and is applicable to any mobile radio communication system.

In Figure 1, a network of cells is each associated with a base transceiver station 1-7 and numerous of the cells will overlap with one another so that a mobile station 10 at location A, and traversing hill 11 in the direction of arrow-headed line 12, is within receiving distance of a number of cells. The mobile station 10 will normally be camped-on to cell 1, since that is the closest geographical cell, but the mobile station will be able to receive signals from cells 2, 3 and 4, and will also report those cells. The mobile station may also be able to see cells 5 and 6, but not cell 7. As the mobile station 10 moves toward the top of the hill 11 at point B, in the direction of arrow-headed line 12, the radio path signal strengths change and cells 5 and 6 will become detectable to the mobile station and, at point B, cell 7 will also become detectable. However, the mobile station 10 may be busy trying to synchronise with cells 5 and 6. As the mobile station moves from location B to location C, the mobile station will move out of the cell bounded by cell 1 and a handover to either of cells 5 or 6 will have occurred. However, the signal strength of

cells 5 and 6 will also be fading away as the mobile station moves from location B to location C. Unless the mobile station measures, synchronises to and reports cell C before arriving at location C, the network does not know that it should handover communication to cell 7. Because the mobile station does not know the local geography and cellular system topography, there is no reason for the mobile station 10 to attach priority to cell 7 rather than cells 5 or 6. If the mobile station loses a signal at point C, e.g. drops a call, the mobile station will very quickly find and camp-on cell 7 with a strong received signal. By virtue of this invention, the mobile station will know that it had not detected and synchronised to cell 7 sufficiently early, but that cell 7 was the best option. The mobile station reports the optimum cell 7 to the network along with the cell ID, channel number and most recent radio parameters such as signal strength and synchronisation status. The network is then able to provide that information to a following mobile station that passes cell 1 heading in the direction of arrow-headed line 12.

Thus, if an adaptive antenna were used then the network may advantageously instruct a base station antenna to produce a narrow or configured beam to be directed at the following mobile station to effect economic, in terms of power, and reliable handover. A following mobile station may, therefore, assign priority to cell 7 as soon as a signal from cell 7 is received by the following mobile with sufficient signal strength and the following mobile station can stop scanning other cells and devote all available resources to synchronising to cell 7 as a priority.

Such a handover may be effected even if the following mobile station discovered that handover from cell 1 to cell 7 resulted in a few seconds of poor signal quality, even though cell 5, say, was available at a better signal strength through the period of poor signal strength from cell 7. Alternatively, the following mobile station could be guided by the network to find both cells 5 and 7, with cell 7 having priority, but with the following mobile making a handover to cell 5 for a brief time. Although the foregoing has been described in relation to a hill, it will be understood by those skilled in the art that the invention has great utility in other environments such as tunnels, cuttings, etc.

Figure 2 shows an abscissa of static signal strengths at different locations L1 through L7 of an ordinate along a highway in respect of absolute radio frequency channel numbers 21 - 24 (equivalent to differing cells) on each side of the top of the hill B. At commencement, channel 21 provided the optimum signal strength which rapidly decreased as the top of the hill was approached and channel 24 initially provided the worst signal strength but, as the top of the hill@,was traversed to locations 6 and 7, so channel 24 provided the greater signal strength. At location B, there is a crossover in signal strength between channel 21 and channel 23 and so the logical candidate for handover would be from channel 21 to channel 23, but noting that within a very short distance, channel 24 provides the greatest signal strength. With this learned knowledge, the mobile station will advise the network and the network will provide a priority for a following mobile station to handover in the vicinity of location B to channel 24 with channel 23 having a lower priority.

A system in accordance with this invention shown in Figure 3 has mobile stations 31, 32 in which mobile station 31 is travelling from a cell provided by a base station transceiver (BST) 33 to a cell provided by a second base station transceiver (BST) 34. Handover is controlled by base station controller (BSC) 35. The base station transceivers 33 and 34 and base station controller 35 are within a base station subsystem 36. An adjoining network having a base station subsystem (not shown) includes a base station transceiver 37 having a base station controller 38, the mobile station 32 being in communication with the cell associated with base station transceiver 37.

The base station controllers 35, 38 are in communication with a mobile switching centre (MSC) 39 associated with an operation and maintenance centre (OMC) 40. The operation of the mobile switching centre is to effect communication between differing networks of base station subsystems.

Figure 3, as thus far described, is conventional. It will be assumed that mobile stations 31 and 32 are both travelling in the direction in a geographical sense, of arrow-headed line 12, that the mobile station 32 has travelled from the cell associated with base station transceiver 33, a handover was effected to base station transceiver 34, and that the mobile station 32 is now in the cell associated with base station transceiver 37.

The operation of the invention will now also be described in relation to the flow diagram of Figure 4. Having traversed from the cell associated with BST 33 to BST 34, the mobile station 32 makes a hindsight decision whether handover could have been improved if the

mobile station 32 had knowledge of a better alternative. The decision is based upon, for example, signal failure due to poor signal strength but shortly thereafter the mobile station finds a strong signal to camp-on, or communication between the mobile station and BST becomes poor, i.e. the channel error rate increases but it is found that there is a strong signal from another cell with which to synchronise. Having made a handover decision at step 41, the mobile station reports information concerning optimum operation parameters and the preferred cell identification (ID), or signal strength, or synchronisation, or timing advance, at step 42 to the base station 33 and the cell in which the problem occurred will also be reported. Although it is envisaged that the report will be provided to the base station transceiver 34, the report could, in fact, be submitted via the subsequent BST 37.

At step 43, the base station (or mobile switching centre, or operation and maintenance centre in GSM) decides on particular other mobile stations to send the optimum operation parameters, e.g. mobile station 31. The base station makes the decision based upon the history of signal strength of the mobile station 32 and/or timing advance between the reception and transmission of signals by the mobile station 32. The base station then transmits a handover preference list of channels at step 44 to the following mobile station 31, the preference list being determined in terms of channel/cell priority. In the situation envisaged in Figure 3 with mobile station 31, say traversing the brow of a hill between BST 33 and 34, it is envisaged that the handover preference list will be transmitted by BST 33 to enable continuous communication by the mobile station 31

from BST 33 to BST 34. The mobile station 31 then assigns priority at step 45 to the base station transceiver 34 in accordance with the preference list.

It will be appreciated that, by use of the present invention, continuous communication may be achieved by a mobile station having experienced difficulty submitting information concerning what that mobile station has learned of the communication network to a base station for use by a subsequent, following, mobile station. The invention may be used by a manufacturer of mobile stations or a network operator, to obtain better performance, reduced number of non-optimal handovers, or to provide a potential cost saving in reducing the number of cell sites that are needed for a specific dropped call performance.

It is to be understood that other modifications could be made and that all such modifications falling within the spirit and scope of the appended claims are intended to be included in the present invention.

Claims

CLAIMS: 1. A mobile communication system including a central controller communicating with a plurality of base stations (33, 34) that in turn provide communications to a plurality of mobile stations including monitoring means to monitor signal strengths of transmissions from at least two of the plurality of base stations characterised by at least one first mobile station including means (41) arranged to determine optimum operating parameters to enable continuous communication during a handover between at least first and second base stations (33, 34) and means (42) for transmitting said optimum operating parameters to said central controller or a second mobile station.
2. A mobile communication system in accordance with Claim 1 wherein the means arranged to determine optimum operating parameters further determine in hindsight after such handovers have taken place an optimal handover path, and transmits said optimal handover path to said central controller or said second mobile station.
3. A mobile communication system in accordance with Claim 2 wherein such further determination addresses a problem in maintaining a communication link that the at least one mobile station encountered during the handover between the at least first and second base stations.
4. A system as claimed in Claim 3, wherein said determining means in said mobile stations include recognising said problem in maintaining a communication

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link based upon at least one of signal failure and channel error rate.
5. A mobile communication system as claimed in any preceding Claim, wherein said central controller includes routing means (35, 38, 39) for routing said optimum operating parameters to a base station of the plurality of base station and said base station includes transceiver means arranged to retransmit said optimum operating parameters to a second mobile station (31).
6. A system as claimed Claim 5, wherein said first and second mobile stations are travelling in substantially the same direction, and said optimum operating parameters include at least one of cell identification (ID) and Absolute Radio Frequency Channel Number (ARFCN) for the cell to which handover for the second mobile station would be an optimum.
7. A system as claimed in any preceding Claim, wherein said optimum operating parameters include at least one of signal strength, synchronisation, and timing advance to assist said base station (33) directing said second mobile station (31) following substantially the same direction.
8. A system as claimed in any one of claims 3-7, wherein said base stations include decision (43) means arranged to make a determination based upon said problem maintaining a communication link experienced by said first mobile station (32) whether to transmit a handover preference list to said second mobile station (31), said preference list being based upon one of history of signal

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strength of said first mobile station and timing advance between receiving and transmission of data of said first mobile station.
9. A system as claimed in any preceding Claim, wherein said second mobile station (31) includes assignment means to switch cell channels between said cells in the handover preference list.
10. A system as claimed in any preceding Claim wherein said first mobile station transmission of said optimum operating parameters is used by the mobile communication system for subsequent system re-planning.
11. A method for performing mobile handovers in a mobile communication system including a plurality of base stations serving a plurality of mobile stations in respective communication cells, the method comprising the following steps: monitoring of signal strengths of radio transmissions from the plurality of base stations by a first mobile station as the first mobile station is handed off between a number of the plurality of base stations; characterised by determining, by the first mobile station, an alternative series of handovers between at least first and second base stations that would have maintained an acceptable communication link; selecting by the mobile station or one of the plurality of base stations a handover to said one of the plurality of base stations based on said alternative series of handovers, and transmitting the alternative

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series of hand-offs to at least one of the plurality of base stations.
12. A method as claimed in Claim 11, the method further comprising the steps of travelling, by a second mobile station from one communication cell to another communication cell in substantially the same direction as said first mobile station such that identification for the optimal cell to which handover is determined from said alternative series of handovers and performing handover for said second mobile station to said optimal cell.
13. A method as claimed in Claims 11 or 12, wherein transmittal of the alternative series of handovers further includes at least one of signal strength, synchronisation, and timing advance to assist a first base station to identify subsequent mobile stations following substantially the same direction, whereby the first mobile station learns an optimum handover cell candidate and, communicates the optimum handover cell candidate to the second mobile station travelling along said path so as to effect continuous communication.
14. A method as claimed in any of Claims 11 to 13, wherein said first mobile station makes a decision based upon one of signal failure and channel error rate whether to advise transmitting means in said first mobile station to transmit said optimum handover cell candidate and/or alternative series of handovers.
15. A method as claimed in any of Claims 11 to 14, wherein said base station makes a determination based

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upon difficulty experienced by said first mobile station whether to transmit a handover preference list to said second mobile station, said preference list being based upon one of history of signal strength of said first mobile station and timing advance between receiving and transmission of data of said first mobile station.
16. A mobile station operating in the mobile communication system of any one of Claims 1 to 10.
17. A central controller operating in the mobile communication system of any one of Claims 1 to 10.
18. A mobile communication system substantially as hereinbefore described with reference to, and/or as illustrated by figure 3 of the drawings.
19. A method of performing handovers in a mobile communication system substantially as hereinbefore described with reference to and/or as illustrated by Figure 4 of-the drawings.