GB2284725A

GB2284725A - Method for determining handoff

Info

Publication number: GB2284725A
Application number: GB9324428A
Authority: GB
Inventors: Rupinder Singh Oberoi; Neil John Dewar
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1993-11-27
Filing date: 1993-11-27
Publication date: 1995-06-14
Anticipated expiration: 2013-11-27
Also published as: GB9405539D0; FI945518A0; CA2135011A1; DE4441925A1; GB2284725B; GB2284321A; FR2713035A1; FI945518A; GB9324428D0

Description

2284725 METHOD FOR DETERMExaNG HANDOVER IN A MULTICELLULAR ENVIRONMENT

Field of the Invention

This invention relates in general to a method of determining handovers (or handoffs) in a multicellular environment, and more particularly to determining handovers in a multicellular environment based on the rate of change of a parameter of an incoming signal from at least one of the neighbouring cells.

Background to the Invention

In a cellular environment, at any one time, there is usually one serving cell defined as the cell with the base station that a mobile unit is receiving service from so that the mobile unit may receive and transmit communication and a number of cells that are neighbouring cells. The serving cell may also be referred to as the cell that the mobile unit is camped on to. In a multicellular environment, there may be cells of different sizes where a number of cells of the same size are located within one larger cell (umbrella cell). The smaller cells within the umbrella cell are called microcells. Microcells are created in a dense population of users to allow a greater capacity of users on the cellular system. The microcells facilitate the reuse of frequencies over a smaller distance. Thus, a mobile unit may be within a microcell as well as an umbrella cell.

Typically, rural areas that do not have a large number of users or do not require a large capacity only need to be divided into larger cells. As the areas grow or the cells get closer to densely populated areas, the larger cells do not have the capacity to facilitate the increased number of users. There are not enough frequencies allocated. So microcells are created within the larger cells and the larger cells become umbrella cells. This allows frequency reuse among the microcells. Such microcellular techniques improve spectral effliciency and increase the capacity of the cellular network.

Microcells have disadvantages. One disadvantage is that in microcellular areas the number of handovers increases and the time available to make handover decisions decreases. For example, having too many smaller size microcells in an area where there is a fast moving mobile, the fast moving mobile travels through a number of microcells in a short 2 amount of time causing a number of handovers to be processed. Increasing the number of handovers in a short amount of time decreases the signal quality and makes for poor communication and in extreme cases, loses calls.

Thus, a fast and reliable method of determining when to handover in a multicellular environment needs to be established.

Summga of the Invention According to the present invention, there is provided a method for determining a handover in a cellular communication system having at least one mobile unit communicating to a serving cell and a plurality of neighbouring cells where the serving cell and the neighbouring cells comprise of at least one umbrella cell and a plurality of microcells. Each cell has a respective base station. The method for determining whether to handover from the serving cell to a neighbouring cell includes the steps of measuring a rate of change of a parameter of an incoming signal to the mobile from at least one neighbouring cell base station and determining handover in dependence upon the measured rate of change.

In a preferred embodiment of the invention, the parameter is a rate of change of a received signal level.

In an alternate embodiment, the parameter is a rate of change of a power level control signal.

Brief Description of the Drawing

FIG. 1 illustrates a fast moving mobile unit in a multicellular environment.

FIG. 2 illustrates a slow moving mobile unit in the multicellular environment of FIG. 1.

FIG. 3 is a flow chart for a method for preferred embodiment of the present invention.

FIG. 4 are timing charts of signal levels for the mobile unit of FIG. 1.

FIG. 5 are timing charts of signal levels for the mobile unit of FIG. 2.

Detailed Description of the Preferred Embodiment

Referring to FIG. 1, a multicellular (or microcellular) environment is shown comprising of at least one umbrella cell 1 and a plurality of microcells 2P 3, 4, 5, 6. Each cell includes a base station typically located in the geographic centre of the cell. Not all base stations are shown in FIG. 1. A 3 base station typically determines the size and capacity of the cell. A communication system may include different sized cells as well as a mobile radio unit 20 which may be receiving service from either a base station 23 of the umbrella cell 1 or a base station 25 of one of the microcells 3. Receiving service from a particular base station in terms of being able to receive and transmit calls is also referred to as being camped on that particular base station. When a mobile radio unit enters the multicellular environment a decision should be made to determine whether to remain being served by the current cell type or handover to a new cell type. The decision may be dependent upon the speed of the mobile unit.

Two cases may be defined when a mobile radio unit enters a multicellular environment or coverage area. The mobile unit may be moving at a high speed as shown by a car 20 in FIG. 1 or at a slower speed as shown by a person 30 in FIG. 2. Both FIGS. 1 and 2 assume that the serving cell is the umbrella cell 1 and a plurality of neighbouring cells are microcells 2, 3, 4 5, 6.

Referring to FIG. 3, a flow chart outlines the process in accordance with the invention of determining whether to handover to another cell for service. In a preferred embodiment, a parameter value from at least one neighbouring cell base station received at the mobile unit is passed to the serving cell base station and measured for a period of time at step 51. A rate of change for the parameter of the neighbouring cell is then calculated at the serving cell base station in step 53.

In step 55, a maximum rate of change value is chosen. The maximum value may be simply a fixed predetermined value, but would be more reliable if it were selectable and based on the variable parameters of the microcellular system itself such as capacity and size of the cells. The maximum rate of change value selected will also usually depend on the number of handovers required if the mobile were to be passed to a microcell or, if already in a microcell environment, to remain there.

The rate of change of the parameter of the neighbouring cell base station is compared against the chosen maximum rate of change value in step 57. If the rate of change is greater than the maximum rate of change value and the serving cell is a microcell then service is handed over to the umbrella cell in step 63, assuming that the umbrella cell has already been determined to have capacity. If the rate of change is less than the determined maximurn rate of change value and the serving cell is a microcell 4 then in accordance with step 65, no handover is performed. If the rate of change is greater than the maximum rate of change and the serving cell is an umbrella cell then no handover is performed and service remains on the umbrella cell in accordance with step 67. Lastly, if the rate of change value is less than the maximum rate of change value and the serving cell is an umbrella cell then service is handed over to the microcell in accordance with step 69. FIG. 3 shows only one way the method of the present invention may be implemented in a microcellular environment.

Particularly, in a preferred embodiment of the present invention a received signal level from the base station of a neighbouring cell received by the mobile unit is used as the parameter value in step 51. FIGS. 4 and 5 respectively show the comparative rates of change for the received signal level of the serving cell 1 and neighbouring microcells 2, 3, 4 for the faster moving mobile 20 and the slower moving mobile 30.

FIG. 4 plots receive signal levels from the serving umbrella cell 1 and the neighbouring microcells 2, 3, 4 as the faster mobile 20 of FIG. 1 travels through the umbrella cell 1 from tO to U. Specifically, FIG. 4(a) shows the receive signal level received by the mobile unit 20 from the base station of the umbrella cell as it travels through the umbrella cell 1 for a period of time, tO to U. The receive signal level of the serving cell 1 is kept fairly constant by power control commands. FIG. 4(b) plots the received signal level received at the mobile unit 20 from the neighbouring microcell cell 2 as the mobile unit 20 travels for the same time shown in FIG. 1, tO to U. Note that the mobile unit 20 does not travel into microcell 2 and thus receives a moderately constant signal with a small rate of change. Similarly, FIG. 4(d) plots the received signal level received at the mobile unit 20 from the neighbouring microcell cell 4 as the mobile unit 20 travels for the same time tO to U. Again, the mobile unit 20 does not travel into microcell 4 and thus receives a moderately constant signal with a small rate of change.

FIG. 4(c) plots the receive signal level at the mobile unit 20 from the microcell 3 as the mobile unit travels for period of time tO to U. In contrast to the neighbouring microcells 2, 4, the receive signal level changes dramatically indicating a higher rate of change. The mobile unit 20 has actually travelled into the microcell 3 and in a short amount of time tO to tl the received signal level received from the base station of microcell 3 has increased at a relatively fast rate indicating that the mobile unit is travelling in microcell 3 at a fast speed. The serving cell base station 23, in this case located in the umbrella cell 1, may now decide not to hand over to the microcell 3 because the mobile unit is moving at a relatively fast rate and will probably require a number of handovers in a short amount of time if served by microcells in the microcellular system. Thus, it would be more efficient to keep the fast moving mobile unit 20 served by the base station 23 in the umbrella cell 1.

FIG. 5 plots the receive signal levels from the serving umbrella cell 1 and the neighbouring microcells 2, 3, 4 as the slower mobile unit 30 of FIG. 2 travels from tO to U. FIG. 5(a) plots the received signal level received from the serving cell 1 usually kept constant by power control commands. FIGS. 5(b) and (d) plot the receive signal level received by the mobile unit 30 from the base stations of neighbouring microcells 2, 4. Similarly, the rates of change are relatively small.

FIG. 5(c) plots the receive signal level of neighbouring microcell 3 as the mobile unit travels from tO to U. Since the mobile unit 30 actually travels into microcell 3 the received signal level from the base station of microcell 3 increases but not at a relatively fast rate compared to FIG. 4(c). Thus, the base station 23 of the serving umbrella cell 1 may decide to hand over service for slow moving mobile unit 30 to microcell 3. Comparing FIGS.

4(c) and 5(c) show that the rate of the receive signal level for the faster moving mobile 20 relative to the slower moving mobile 30 is higher. Thus, according to the method of the present invention it may be determined dependent upon the lower rate of change of a received signal level for a slower moving mobile to hand over from an umbrella cell to a microcell.

Similar receive signal levels will be noted if a call originates in a microcell. The majority of mobile units in a microcellular system are slow moving mobile units and thus, should be served by microcells.

A second embodiment of the present invention uses a rate of change of the power control level as the determining parameter. Similar to FIGS. 4 and 5, the rate of change of power control level signals received at a mobile unit from base stations of neighbouring cells will change more dramatically with a microcell that the mobile unit is passing through for a faster moving mobile unit. Thus, a decision to handover may be made by the base station of the serving cell dependent upon the rate of change of the power control signals received from the base stations of the neighbouring cell at the mobile unit.

6 Although the method has been described as being implemented at the base station of the serving cell, the method could actually be implemented in the mobile unit provided that the required intelligence is built into the mobile unit. The method could also be implemented at the base station of the neighbouring cell provided the proper information was passed to the base station of the neighbouring cell. As cellular systems expand, methods such as the one of the present invention may be chosen to be implemented at higher level control stations.

In conclusion, the present invention provides a method for a microcellular communication system, including microcells and umbrella cells. where slow moving mobile units utilise the microcells and the faster moving mobile units utilise the umbrella cells. Particularly, when a fast moving mobile unit enters a microcell it is required that the mobile unit remains served by the current cell type, umbrella cell, to minimise the number of handovers required. Thus, the number of handovers is significantly reduced and the microcellular environment is efficiently utilised. The present invention improves the reliability of handovers resulting in lower number of dropped cells and reduced amount of processing done by the network.

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Claims

1. A method for determining a handover of a mobile radio unit in a cellular communication system including a serving cell and a plurality of neighbouring cells where the serving cell and the neighbouring cells comprise of at least one umbrella cell and a plurality of microcells, each cell having a respective base station, the method for determining whether to handover from the serving cell to a neighbouring cell comprising the steps of..

measuring a rate of change of a parameter of an incoming signal to the mobile from at least one neighbouring cell base station (53); and determining handover in dependence upon the measured rate of change.

2. The method of claim 1 further comprising the steps of.

selecting a maximum value for the rate of change of parameter of the incoming signal (55); and handing over to the umbrella cell if the rate of change of the measured parameter is larger than the maximum value and the serving cell is a microcell (63).

3. The method of claim 1 or 2 further comprising the steps of.

selecting a maximum value for the rate of change of parameter of the incoming signal (55); and handing over to a microcell if the rate of change of the received signal level is smaller than the maximum value and the serving cell is the umbrella cell (69).

4. The method of any preceding claim wherein the parameter is a received signal level.

5. The method of claims 1, 2 or 3 wherein the parameter is a power level control signal.

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6. A method for determining a handover of a mobile unit in a cellular communication system substantially as herein described with reference to 1 FIG. 3 of the drawing.

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