GB2284321A

GB2284321A - Method for determining handoff candidate

Info

Publication number: GB2284321A
Application number: GB9405539A
Authority: GB
Inventors: Rupinder Singh Oberoi; Paul Crichton; Robert Ivor Davies
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1993-11-27
Filing date: 1994-03-21
Publication date: 1995-05-31
Also published as: FR2713035A1; GB9324428D0; FI945518A; FI945518A0; DE4441925A1; CA2135011A1; GB2284725A; GB2284725B; GB9405539D0

Description

2284321 METHOD FOR DETERMINING HANDOVER CANDIDATE IN A MULTICELLULAR

ENVIRONMENT

5Field of the-Invention

This invention relates in general to a method of determining possible base sites as handover candidates in a multicellular environment, and more particularly to determining handover candidates in a multicellular environment based on a step change of a parameter of an incoming signal from at least one of the neighbouring cells or serving cell.

Background to the Inventio

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 surrounding cells that are referred to as 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 may be 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 located 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 efficiency and increase the capacity of the cellular network.

2 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 amount of time causing a number of handovers to be processed. Increasing the number of handovers in a short amount of time decreases the call reliability and increases the number of breaks in communication, thus, reducing the quality of 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.

One such method has been proposed in co-pending UK Patent Application No. 9324428.3 entitled "Method for Determining Handover in a Multicellular Environment" filed on November 27, 1993 by Motorola.

Digital cellular communications systems, such as the GSM (Global System for Mobile Communications), integrate a large number of cells in a microcellular environment. It is required in GSM that a mobile station report a received signal level strength of its six strongest neighbouring cells. Current handover techniques choose a handover candidate from one of the six strongest neighbouring cells. In a microcellular environment where the signal strengths are varying rapidly, a cell may produce a strong signal level strength in one measurement report and then a weak signal level strength in a next measurement report. Thus, making a handover decision based solely on a first report may result in selecting a base site that would not be a reliable serving cell for the mobile station.

It is desired to prevent a neighbouring cell whose signal strength is varying rapidly from being considered as a handover candidate. Thus, it is desired to have a method for determining a handover candidate where a neighbouring cell is considered a handover candidate only if it would truly be a reliable handover 3 5 candidate.

3 Summary of the Invention

According to the present invention, there is provided a method for determining a handover candidate 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 determining a step change of a parameter of an incoming. signal to the mobile from at least one cell base station and determining handover in dependence upon the step change.

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

is In an alternate embodiment, the parameter is a received signal strength level from neighbouring cells measured for a time duration.

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 illustrates two buffer arrangements for a preferred embodiment of the present invention.

FIG. 4 illustrates a typical multicellular environment.

FIG. 5 illustrates received signal levels plotted for the mobile station of FIG. 4.

FIG. 6 illustrates a flow chart for a preferred embodiment of the present invention.

FIG. 7 illustrates a decision chart for the preferred embodiment of the present invention.

Detailed Description of the Preferred Embodime t

Referring to FIG. 1, a multicellular (or microcellular) environment is shown comprising of at least one umbrella cell 1 and 4 a plurality of microcells 2, 3, 4, 5, 6. Each cell includes a base station typically located in the geographic area covered by the cell. Not all base stations are shown in FIG. 1. A 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.

is 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.

When a fast moving mobile station 20 as shown in FIG. 1 enters a microcellular environment the present invention requires that the mobile station remain on the current cell type (umbrella) to alleviate the number of handovers that would be required in a short amount of time. By measuring the received signal strength level from a number of neighbouring cells for a period of time or measuring the step changes in the received signal level none of the neighbouring microcells will be reported by the mobile station as a handover candidate. The microcells will be transparent to the base station as handover candidates and any handovers required will be to umbrella cells.

On the other hand, when a slow moving mobile station 30, such as shown in FIG. 2, enters a microcellular environment the present invention requires that the slow moving mobile station 30 is handed over to a microcell. This ensures that the umbrella cell does not become congested and the maximum traffic is handled by the microcell. Thus, according to one embodiment of the present invention when the mobile station 30 enters a microcellular environment the six strongest cells in terms of received signal strength levels received at the mobile station are from surrounding base stations of the neighbouring microcells. Since the mobile station is slow moving some of the neighbouring cells will be reported long enough, or for a predetermined time duration, to be considered handover candidates. The present invention requires handover to the most suitable cell.

According to one embodiment of the present invention a cell does not become a handover candidate unless it has been one of the strongest cells for a given time duration Tri. A timer is associated with each neighbour and may be different for each neighbour. It may be predetermined or adaptively defined by the user of the database.

FIG. 3 shows one way in which the timer Tn may be used in a base station handover method. In FIG. 3 the mobile station reports the six strongest neighbour carriers in terms of received signal levels to the base station. The system controller of the base station puts them in a first buffer 35 and starts a timer Tn. If at the expiry of the timer Tn the cell is still one of the six strongest carriers than it is moved to a second buffer 37 as one of the handover candidates.

The handover candidate list should always contain at least one cell which is a standby handover cell. Standby handover cell may be defined as a cell that is cell barred for normal call origination but is available for any emergency handovers. It will be one of the umbrella cells (or certain timeslots in a given umbrella cell). This will ensure that in the cases that where a handover is required but there are no suitable microcells available that a call can be handed over without being dropped.

Any of the following criteria may be used to checked if a cell is suitable as a handover candidate:

A neighbouring cell has been continuously reported in the mobile measurement reports (e.g. SACCH multiframes) over the time Tn. Under this condition the neighbouring cell has the highest probability to be a good handover candidate but it suffers from the 6 disadvantage that it does not take into account fast changes in the received signal level (RXLEV) due to fading, shadowing, etc.

The neighbouring cell has been reported at least n out of m times in the mobile measurement report over time duration, Tn.

This met-hod takes into account any fast changes in received signal level in the neighbouring cell reporting and therefore has a high probability of identifying a good handover candidate. The values of n and m have to be optimised for each cell.

The average received signal level of the neighbouring cell exceeds a threshold over the time Tn. This method averages out all the peaks and troughs of received signal levels.

Signal quality and other criterion currently used in determining handovers must also be satisfied if the neighbouring cell is to be considered as a handover candidate.

is The microcellular environment is characterised by the fast rate of change of received signal levels (e.g. when turning corners, going under bridges, etc.) which makes the current handover methods sometimes inaccurate in the prediction of handover candidates.

FIG. 4 shows a typical multicellular environment. Particularly, when a mobile station 40 moves from point A to point B, the received signal level of a first neighbouring base station 44 increases to a relatively high level at point X and according to the present invention becomes one of the neighbouring cells being reported by the mobile station 40 as a strong carrier while the received signal level of the first neighbouring base station 44 remains relatively constant.

Similarly when the mobile station 40 moves from point B to C, the received signal level of a serving cell base station 42 suddenly decreases at point Y to a low level while the received signal level of the neighbouring base station 44 remains relatively constant. Due to the nature of microcells the change in the received signal level of the serving cell 42 and the neighbouring cell 44 will be a step function as shown in FIG. 5 and such characteristics may be used to predict handover candidates according to the present invention.

7 FIG. 5 shows the received signal level received by the mobile station 40 as it travels from point A to B to C in FIG. 4. From point A to B the received signal level of neighbouring cell 44 suddenly increases at point X resulting in a step change function then remains relatively constant. The received signal level of serving cell 42 remains constant.

Similarly as the mobile station travels from point B to C the received signal level of the serving cell suddenly decreases at point Y resulting in a step change function. The received signal level of neighbouring cell 44 remains constant. Points X and Y represent edges of cell coverage.

Step changes in the received signal level of neighbouring cells signify that the mobile station is on a corner or on a junction of a street. Step changes in the serving cell received signal level signifies that the mobile station has turned a corner and it is likely that a handover is required. When these step changes are recognised by the handover method of the present invention the system may move into different modes of operation accordingly. For, example the timer Tn associated with a particular neighbouring cell may be reduced so that a fast handover decision may be made. It is necessary to define an averaging method which is capable of detecting step changes in the received signal level.

FIG. 6 shows an embodiment of the present invention that brings two parameters together whereby the timer Tn associated with each neighbouring cell is changed interactively when a step change is recognised. This makes a given neighbouring cell more likely to become a handover candidate when there are step changes detected in the received signal (e.g. when a mobile is turning a corner). Furthermore, the recognition of a step change also enables the method to re-evaluate a priority of current handover candidates.

According to FIG. 6, the mobile station or the serving cell base station monitors the neighbouring cells or surrounding carriers received at the mobile station as in step 50. If no step change is determined in step 52 then it is determined if a respective timer Tn has expired and the handover criteria is met as in step 54. If so, the 8 carrier or neighbouring cell is added to the handover candidate list in step 56 and then the process begins again. If not, then it must be determined as in step 58 whether a new carrier has been identified, if not, the process starts over. If so, the respective timer Tn is started for the new carrier as in step 60 and the process starts over.

If in step 52 a step change is identified then the step change is evaluated as in step 62. It is determined if there is a handover required as in step 64. If a handover is required then it is determined whether there is a microcell as a handover candidate as in step 66. If yes, then a handover is performed to the highest priority microcell as in step 70. If there is no microcell as a handover candidate as determined in step 66 then a handover is performed to a standby cell as in step 68.

If no handover is required in step 64 then it is determined whether a timer change is required as in step 72. If a timer change is required as determined by step 72 then the respective timer is changed as in step 74.

If a no timer change is required as determined by step 72 then it is determined whether a handover list evaluation is required as in step 76, if no, then the process begins again, if yes, then the handover list priority is re-evaluated as in step 78 and then the process begins again.

Depending upon step changes in neighbouring cells and serving cells certain steps may be taken which are summarised in the table shown as FIG. 7. The top row describes possible states for a serving cell while the first column describes possible states for a neighbouring cell. If the mobile station receives a constant receive signal level from the serving cell and the mobile station receives a step change increase from a neighbouring cell then the mobile station is possibly on a junction of n cells as described in state 80. If the neighbouring cell is already a member of the second buffer then the priority is reevaluated. If the neighbour is not a member of the second buffer then its associated timer Tn is changed in the first buffer.

9 The receive signal level of the serving cell shows a step change increase and the receive signal level of the neighbouring cell shows a step change increase then the mobile station is possibly on a junction of n cells if neighbouring cell level is comparable to the serving cell as described in state 82. If the neighbouring cell is already a member of the second buffer then the priority is re evaluated. If the neighbour is not a member of the second buffer then its associated timer Tn is changed in the first buffer.

If the receive signal level from the serving cell shows a step change decrease and the receive signal level of the neighbouring cell shows a step change increase then the mobile station has probably turned a corner and a handover is possibly required as described in state 84. If the neighbouring cell is already a member of the second buffer then the priority is re-evaluated. If the neighbour is not a member of the second buffer then its associated timer Tn is changed in the first buffer. If no microcells are members of the second buffer then handover to standby handover cell.

If the receive signal level from the serving cell is constant and the receive signal level from the neighbouring cell shows a step change decrease then continue to monitor receive signal levels as in state 86. Similarly if the receive signal level from the serving cell shows a step change increase or step change decrease and the receive signal level from the neighbouring cell shows a step change decrease then the receive signal levels will continue to be monitored as in states 88 and 90.

If the receive signal level from serving cell is constant or shows a step change increase and the received signal level from the neighbouring cell is constant the receive signal levels will continued to be monitored as is states 92 and 94.

If the received signal level of the serving cell shows a step decrease and the receive signal level from the neighbouring cell is constant then the mobile station has probably turned a corner and a handover is possibly required as described in state 96. If the neighbouring cell is already a member of the second buffer then the priority is re-evaluated. If the neighbour is not a member of the second buffer then its associated timer Tn is changed in the first buffer. If no microcells are members of the second buffer then handover to standby handover cell.

Thus, the present invention provides a method that determines handover candidates reliably lessening the possibility of loss calls. There are four important scenarios that may be analysed according to the method of the present invention.

First, a mobile station is camped on an umbrella cell and the mobile station is moving at a high speed. According to the method of the present invention as described in reference to FIG. 1 none of the microcells will be reported by the mobile station for a long enough time to be considered as handover candidates. Thus, any handovers required will be to umbrella cells.

Second, a mobile station is camped on an umbrella cell and moving at a slow speed. As described in reference to FIG. 2, the mobile station will report the six strongest cells as neighbouring cells and some of which will be reported for long enough to become handover candidates. Thus, handover to a microcell will result.

Third, the mobile station is camped on a microcell and moving at a fast speed. The mobile station will originate a call on a microcell and the call will be handed over to an umbrella cell to minimise the number of handovers required. With every handover performed there is a probability that a call may be dropped. Thus, minimising the number of handovers reduces the probability of dropped calls.

If the mobile station is moving fast through the microcellular coverage area, the list of neighbouring microcells being reported by the mobile station will keep changing and none of the microcells will become a handover candidate. However one of the cells being reported back by the mobile station will be an umbrella cell which will be a handover candidate. This umbrella cell may be the standby handover cell. If a handover is required and no suitable microcells are available then the mobile station will be handed over to the umbrella cell.

Fourth, if the mobile station is moving slow through the microcellular coverage area, some microcells will be reported by the mobile station for long enough time duration to be considered as 11 handover candidates. If handover is required the mobile station will be handed over to the most suitable microcell. A measured (or detected) step change in the neighbouring and serving cells receive signal levels as described in FIG. 7 may be used to predict the cell to 5 which the mobile station will be handed over to.

Particularly, received signal levels have been described as being used for determining step changes and handover candidates. The received signal levels received from monitored cells are usually kept constant by power control commands.

An alternative embodiment of the present invention may use a step change of the power control level as the determining parameter. Thus, a decision to handover may be made by the base station of the serving cell dependent upon a step change of the power control signals received from the base stations of the cells being monitored at the mobile unit.

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 implemented elsewhere in the infrastructure of the system.

The parameters defined in this method may be used to predict the most suitable cell to which handover should be made. This prediction is based on monitoring any step changes in the received signal level of the serving and neighbouring cells or may be based on the number of step changes detected in a particular period of time. For example, if there are a number of step changes made in a relatively short period of time it may make sense to handover to an umbrella cell to avoid a high number of microcell handovers.

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 12 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 cellular environment is efficiently utilised. By efficiently utilising the cellular environment, capacity of the cellular communication system may be increased. The present invention improves the reliability of handovers resulting in lower number of dropped cells and reduced amount of processing done by the 10 network.

13

Claims

1. A method for determining a handover candidate in a microcellular 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 a handover candidate comprising the steps of.

measuring received signal parameters received from the serving cell and each of the plurality of neighbouring cells; detecting step changes in the received signal parameters received from the serving cell and each of the plurality of neighbouring cells; and is determining a handover candidate based on the step changes detected in the received signal parameter of the serving cell and each of the plurality of neighbouring cells.

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

measuring a received signal parameter from the serving cell and a plurality of neighbouring cells for a time duration; and determining a handover candidate based on the measured received signal parameter after the time duration and any step changes detected in the received signal parameter of the serving cell and each of the plurality of neighbouring cells.

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

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

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