GB2359698A

GB2359698A - Multiband cell handover mechanism

Info

Publication number: GB2359698A
Application number: GB0004283A
Authority: GB
Inventors: Christopher Parker
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-02-23
Filing date: 2000-02-23
Publication date: 2001-08-29
Also published as: GB2359698A8; GB0004283D0

Abstract

A method of handing over a mobile station from a macro cell to one of a plurality of underlay cells 4 in a cellular communications system 1, the method comprising monitoring signal strengths of defined neighbours amongst the underlay cells 4, the defined neighbours being neighbours for the macro cell, setting a threshold for a minimum number of defined neighbours having signal strengths above a predetermined minimum level, and handing over a mobile station from the macro cell to one of the defined neighbours when the number of defined neighbours having a signal strength above the predetermined minimum level falls below the threshold. Such a method enables a GSM system to maximise its usage of a hot-spot macro cell without risking the possibility of dropped calls.

Description

2359698 MULTIBAND CELL HAND OVER MECHANISM The present invention relates

to cellular communications systems, and in particular to multiband/hierarchical cell hand over mechanisms in such systems.

In a cellular communications system, the area over which service is pfovi is divided into a number of smaller areas called cells. Each cell is served from a base transceiver station (BTS) which has a corresponding antenna or antennas for transmission to and reception from a user station, normally a mobile station (MS) 10 or handset.

Each cell has a number of carrier frequencies (or re-use groups) assigned to it. Typically there could be up to twelve such carrier frequencies assigned to a cell. Each frequency is divided into eight channels, one of which is assigned as a broadcast control channel (BCCH) and the remaining seven channels are used for the purposes of communication with network users. It is only necessary, however, for there to be assigned a single BCCH channel for a given cell; thus only.one frequency in each cell has such an allocation, the remaining frequencies utilising all eight channels for communication purposes.

A cellular communications system is operated according to a frequency plan. A frequency plan consists of a plurality of frequency plan settings applied to the various components of the system. Such settings typically include a specific radio frequency o r set of frequencies assigned to each respective BTS, which frequency or frequencies are supplied to the corresponding cell. Other frequency plan settings forming part of the frequency plan are typically a cell colour code which a given cell will transmit. One example of a cellular communications system is a global system for mobile communications (GSM) system.

As will be appreciated, each cell can handle a given number of users dependent upon the number of carrier frequencies assigned to the cell. However, in certain locations, such as rail stations, "hot spots" can arise where a large number of users cannot be accommodated by the local BTS. In such locations, it is known to 2 add an additional BTS to accommodate the large number of users. Clearly, however, this BTS must use different frequencies from its immediate neighbour, and this can upset the overall frequency plan of the system. Accordingly, another method is to define a macro cell overlaying the existing cell structure which uses a second frequency band. Multiband mobile phones can be used in such an environment. The macro cell can then handle a large number of the users, thereby alleviating the problems associated with the hot spot.

If the macro cells using the second frequency band provide coverage over the 10 complete area serviced by the cellular communications system, there is no problem. A frequency plan can be devised for such a. macro cell configuration. However, as is envisaged, if these macro cells are only used at hot spot sites, it is necessary for a call to be handed over from the macro cell frequency to a cell being used in the underlying cell system as a user moves away from the hot spot.

Although broadcast control channel (BCCH) signal strength and associated quality of frequencies can be reported by mobile stations, so that hand over can occur between a macro cell and its underlaying cells, a problem can arise due to the size of a macro cell. In particular, when a macro cell is significantly larger than the underlaying cells, because there is relatively less interference on the macro cell frequency, it is not usually possible for all the underlaying cells covered by the macro cell to be defined as hand over neighbour candidates for the mobile station. Failure to define all required hand over neighbour candidate cells results in dropped calls. As a result, the full potential of the macro cell cannot be utilised.

Prior solutions to force mobile stations off hot spot cells have relied on triggering the hand over based on low serving signal level from the serving hot-spot cell andlor detection of a significant level of interference in time to hand over before dropping the call, or suffering poor audio. As indicated above, neither of these triggers can determine when a mobile station served by a macro cell is about to move out of the area served by the defined neighbouring cells, since there is no correlation between them. This is where a problem arises when using a macro cell with relatively low levels of interference on the assigned frequencies.

3 When considering the aforementioned problem, there are solutions that may be considered. For example, a timing advance (distance) mechanism could be used to solve the problem by forcing a mobile station off the hot-spot macro cell before it gets too far away from the hot-spot. However, this does not use the macro cell to its full potential, and is neither efficient nor reliable, since it will be necessary to estimate the shortest timing advance at which the defined neighbour cells disappear and then to reduce it by a safety margin. This will make the hot-spot cell smaller than is truly necessary and/or there will be places where served 10 mobile stations are without defined neighbour cells resulting in dropped calls.

Other existing hand over trigger methods could also be used for varying degrees of success, including triggering based on serving cell's signal strength level, link quality (bit-error-rate) or power-budget.

Although the possible trigger mechanisms discussed above can be used to solve the problem, none will allow the mobile stations to remain on the hot-spot macro cell for the maximum period (i.e. a safety margin must be used with them, which introduces inefficiency).

In the light of the foregoing, the present invention aims to improve upon known methods, and to maximise the use which can be obtained from a hotspot macro cell.

In accordance with the foregoing, the present invention provides a method of handing over a mobile station from a macro cell to one of a plurality of underlay cells in a cellular communications system, the method comprising monitoring signal strengths of defined neighbours amongst the underlay cells, the defined neighbours being neighbours for the macro cell, setting a threshold for a minimum number of defined neighbours having signal strengths above a predetermined minimum level, and 4 handing over a mobile station from the macro cell to one of the defined neighbours when the number of defined neighbours having a signal strength above the predetermined minimum level fails below the threshold.

By using a method according to the present invention, the area covered by a macro cell can be maximised whilst retaining a hand over capability to a neighbour cell amongst the underlay cells. This is a marked improvement over other possible methods.

The present invention is particularly applicable to the situation where a macro cell is a hot-spot cell. Such a hot-spot cell could be focused on a railway station, for example.

Preferably, the macro cell uses a first frequency band and the underlay cells use a second frequency band. As a result, the macro cell can provide good coverage over a larger area, because it does not experience interference with the frequencies used by the underlay cells. Indeed, the macro cell preferably provides coverage over an area which is many times greater than an area covered by an underlay cell. In such an arrangement, the macro cell may potentially have hundreds of neighbour cells to which it can hand over a mobile station, when hand over becomes necessary.

The signal strengths of the defined neighbours are monitored using the broadcast control channel signal.

The threshold minimum number of defined neighbours having signal strength above a predetermined minimum level is preferably set at more than two defined neighbours. Clearly, however, the robustness of the system will be increased by increasing this threshold number, because it will become less likely that a call will be dropped through congestion (blocking) if more neighbour cells are available.

In a preferred embodiment, the predetermined minimum level of signal strength is above the level required for acceptable usage by a mobile station. In another embodiment, however, it would be possible to use a predetermined minimum level of signal strength which is marginally below the level required for acceptable usage by a mobile station, but in this case the threshold minimum number of defined neighbours should be higher. The system could then select the neighbour cell having the best signal strength at the time of hand over.

Although the present invention may be applied to many different systems, it is particularly appropriate for use in a global system for mobile communications (GSM).

The present invention further comprises apparatus for handing over.a mobile station from a macro cell to one of a plurality of underlay cells in a cellular communications system, the apparatus including: means for monitoring signal strengths of defined neighbours amongst the underlay cells, the defined neighbours for the macro cell, means for setting a threshold for a minimum number of defined neighbours having signal strengths above a predetermined minimum level, means for comparing the threshold with the number of defined neighbours having signal strengths above the predetermined minimum level and means for generating a handover command for handing over a mobile station from the macro cell to one of the defined neighbours when the number of defined neighbours having a signal strength above the predetermined minimum level fails below the threshold.

A list of defined meighbours may be compiled at a BTS and transmitted to the mobile station which, in turn, monitors their signal strengths. The monitored signal strengths may be reported back to a BTS.

The means for setting and comparing the threshold may be located at the mobile station or at a BTS.

The means for generating a handover command may be located at a BTS.

6 A specific embodiment of the present invention is now described, by way of example only, with reference to the accompanying drawing which shows a macro hot-spot cell overlaying a plurality of underlying cells in a GSM system.

With reference to the drawing, a typical GSM cellular communications system 1 is divided into a plurality of cells 4 each served by a base transceiver station (BTS) 3. Each cell 4 has a number of carrier frequencies assigned to it for communicating with mobile stations in its locality. When a mobile station 2 is being served by a given base transceiver station 3, it monitors the broadcast control channel (BCCH) of the serving cell as well as the BCCH's of neighbouring cells. When the mobile station (MS) 2 moves away from the given cell and finds that an improved quality of service can be provided by a neighbouring cell, a hand over occurs whereby the mobile station is served by the neighbouring cell. This is normal practice is a GSM system.

In certain places, such as railway stations, a large number of mobile stations (or handsets) can be congregated in a small area and this is known as.a hot-spot. When a hot-spot occurs, it is possible that the local base transceiver stations cannot handle all of the users and an additional base transceiver station needs to be inserted. In the specific embodiment described herein, the extra base transceiver station 5 uses a different frequency band from the underlay cells 4 served by the other base transceiver stations 3. As a result, mobile stations which use the hot-spot cell provided by the extra base transceiver station 5 experience little interference over a large area up to an outer boundary 7 surrounding the hot-spot cell. Indeed, the outer boundary 7 represents the limit of usable coverage provided by the hot-spot cell, i.e. the maximum distance from the cell at which the carrierlinterference ratio is acceptable. This area defined by the outer boundary 7 clearly includes a large number of the underlay cells 4 of the GSM system 1.

Since there is a restriction on the number of hand over neighbour cell candidates which can be defined for the hot-spot macro cell provided by base transceiver station 5 in the network, these hand over neighbour cells are arranged around and 7 closest to the base transceiver station 5. These hand over neighbour cells are shown in the drawing within the inner circle 9. When a mobile station is to be handed over from the hot-spot macro cell defined by base transceiver station 5 to an underlay cell defined by a base transceiver station 3, it will be to one of the cells defined by the base transceiver stations 3 within the inner ring 9.

If hand over between the hot-spot macro cell and one of the underlay cells were to be left until a mobile station has travelled as far as the outer boundary 7, it is possible that the hand over could fail due to low neighbour signal strengths, thereby dropping a call. This is clearly unacceptable. Alternatively, if hand over occurs only whilst a mobile station is positioned within the inner circle 9, the use of the hot-spot macro cell area coverage is not maximised. This is inefficient. Hence, the present invention has been devised to improve upon both of these options.

When a mobile station served by the hot-spot macro cell defined by the base transceiver station 5 moves out of and away from the inner ring 9, it continues to monitor signal strengths of the defined neighbour cells within the inner ring 9 using the signals transmitted on the broadcast control channels. As the mobile station moves away from the inner ring 9, the number of neighbour cells having a signal strength above a predetermined minimum level will decrease and the mobile station will monitor this decrease in number. Further, when the number of neighbour cells having a signal strength above a predetermined minimum level fails below a threshold number, which may be greater than two, a decision to hand over the mobile station from the hot-spot macro cell to an underlay cell is taken. Hand over then occurs. In this way, a reliable hand over can be achieved without fear of failure and dropping of a call.

With reference to the drawing, the dotted ring 11 may indicate roughly where hand 30 over is likely to occur. More particularly, hand over will obviously occur outside the inner ring 9 and inside the outer boundary 7 defined by the hot-spot macro cell. The hand over position 11 may, however, be nearer to the inner ring 9 or to the outer boundary 7, depending upon system characteristics and location in general.

1 8 As will be appreciated, the present invention provides a more efficient use of system capability and allows a mobile station to remain on a hot- spot macro cell for the maximum amount of time. Further, the system and method of the present invention ensures that a hand over will be triggered for a mobile station before the call is dropped due to running out of coverage when moving in a hot-spot macro cell.

The present invention is superior over other methods in that a hand over is triggered based on the potential for poor qualityla dropped call. The quality of a call does not have to be poor before a hand over is triggered. This can be defined by setting the required signal strength levels and threshold for number of defined neighbours. Additionally, the present invention is extremely simple and requires no further action on the part of the network operator after setting the threshold number of reported neighbours required to force hand over. This threshold is likely to be the same for all hot-spots cells in a city, for example.

It will of course be understood that the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention.

9

Claims

1. A method of handing over a mobile station from a macro cell to one of a plurality of underlay cells in a cellular communications system, the method comprising monitoring signal strengths of defined neighbours amongst the underlay cells, the defined neighbours being neighbours for the macro cell, setting a threshold for a minimum number of defined neighbours having signal strengths above a predetermined minimum level, and handing over a mobile station from the macro cell to one of the defined neighbours when the number of defined neighbours having a signal strength above the predetermined minimum level falls below the threshold.

2. A method as claimed in claim 1, wherein the macro cell is a hot-spot cell.

3. A method as claimed in claim 1 or claim 2, wherein the macro cell uses a first frequency band and the underlay cells use a second frequency band.

4. A method as claimed in any preceding claim, wherein the macro cell can provide coverage over an area more than fifty times greater than an area covered by an underlay cell.

5. A method as claimed in any preceding claim, wherein the signal strengths of defined neighbours are monitored using the broadcast control channel signal.

6. A method as claimed in any preceding claim, wherein the threshold minimum number of defined neighbours having signal strengths above a predetermined minimum level is set at two or more defined neighbours.

7. A method as claimed in any preceding claim, wherein the predetermined minimum level of signal strength is above the level required for acceptable usage by a mobile station.

8. A method as claimed in any preceding claim, wherein the cellular communications system is a global system for mobile communications (GSM).

9. A method of handing over a mobile station from a macro cell to one of a plurality of underlay cells in a cellular communications system, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

10. Apparatus for handing over a mobile station from a macro cell to one of a plurality of underlay cells in a cellular communications system, the apparatus including:

means for monitoring signal strengths of defined neighbours amongst the underlay cells, the defined neighbours being neighbours for the macro cell, means for setting a threshold for a minimum number of defined neighbours having signal 15 strengths above a predetermined minimum level, means for comparing the threshold with the number of defined neighbours having signal strengths above the predetermined minimum level, and means for generating a handover command for handing over a mobile station from the macro cell to one of the defined neighbours when the number of defined neighbours having a signal strength above the predetermined minimum level fails below the threshold.

11. Apparatus for handing over a mobile station from a macro cell to one of the plurality of underlay cells in a cellular communications system, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.