FR2713035A1 - Method for determining transfers in a multicell environment. - Google Patents

Abstract

The invention relates to a method for determining the advisability of handing over a mobile radio unit (20) in a cellular telecommunications system, which includes a serving cell (1) and several neighboring cells (2 to 6). , the serving cell and the neighboring cells comprising at least one umbrella cell and several microcells, each cell having a respective base station. The method consists of measuring the rate of change of a parameter of an incoming signal supplied to the mobile from at least one base station of a neighboring cell, and determining the need for the handover as a function of this measurement . The parameter can be the received signal level, or a power level control signal.

Description

The present invention relates to a method for determining transfers (or

  commutations) in a multiclular environment and, more

  in particular, it relates to the determination of transfers in an environment

  multicellular based on the rate of change of a parameter of an incoming signal from at least one of the neighboring cells.

  In a cellular environment, at all times, there is usually

  only one serving cell, which is defined as the com-

  carrying the base station from which a mobile unit is receiving the service, whereby the mobile unit can receive and transmit

  communications with a number of cells which are neighboring cells.

  The serving cell may also trc the cell in which the mobile unit is put on hold. In a cellular environment, there may be cells of different sizes, a number of cells of the same size being placed inside a larger cell (paraplural cell). The smaller cells that

  are contained inside the umbrella cell are called microccls.

  Microcells are created in a dense population of users in order to be able to have a greater capacity of users in the cellular system. The

  microcells facilitate frequency reuse over a shorter distance.

  So a mobile unit can be inside a microcell as well

than an umbrella cell.

  Typically, nual areas which do not have a large number of users or which do not require a large capacity require only a division into larger cells. When the suras grow or the cells approach more densely populated areas, these larger cells do not have the possibility of facilitating the increase in the number of users. There are not enough frequencies allocated. Also, micro cells are created inside the larger cells, and these larger cells become umbrella cells. This allows frequency reuse between microcells. These microcllular techniques improve the spectral efficiency and

  increase the capacity of the cellular network.

  A disadvantage is that, in microcellular areas, the number of transfers increases and that the time available to take decisions of tanfcrt decreases. For example, when there are too many smaller cell sizes in an area with a fast-moving mobile, the fast-moving mobile crosses a number of micro cells in a short time, which brings the processing a number of transfers. Increasing the number of transfers in a short time decreases the signal quality, making the

  communications and, in extreme cases, results in loss of communications.

  Consequently, the realization of a fast and reliable process allowing to determine the moment to carry out a transfer in an environment

multicellular is required.

  According to the present invention, there is provided a process allowing

  determine a handover in a cellular communications system

  carrying at least one mobile unit which communicates with a serving cell and several neighboring cells, o the serving cell and the neighboring cells

  constitute at least one umbrella cell and a plurality of microcells.

  Each cell has a respective base station. The process for determining whether or not to carry out the transfer from the serving cell to a neighboring cell includes the oprati consisting in measuring the rate of variation of a parameter of an incoming signal supplied to the mobile by at least one station base of a neighboring cell and to determine the transfer as a function of the rate of

measured variation.

  In a preferred embodiment of the invention, the parameter is the

  rate of change of received signal level.

  According to another embodiment, the parameter is the rate of change

  a power level control signal.

  The following description, intended to illustrate the invention, aims

  to give a better understanding of its characteristics and advantages; it is based on the appended drawings, among which: FIG. 1 illustrates a mobile unit moving rapidly in a multicell environment; FIG. 2 illustrates a mobile unit moving slowly in the cellular environment of FIG. 1; Figure 3 is a same showing the pocdé of a preferred embodiment of the invention; FIG. 4 shows tempol diagrams of signal levels for the mobile unit of FIG. 1; and Figure 5 shows time diagrams of signal levels

for the mobile unit in Figure 2.

  Referring to FIG. 1. A multiclclular (or microcellular) reversal comprises at least one umbrella cell 1 and a plurality of microcells 2, 3, 4, 5 and 6. Each cell comprises a base station which is typically placed centrally geographic of the cell. All stations

  are not shown in Figure 1. A typical base station is

  only the size and capacity of the associated cell. A remote control system

  Information may include cells of different sizes as well as a mobile radio unit 20 which may be receiving its service from the base station 23 of the umbrella cell 1 or from the base station 25 belonging to the one of the microcells, for example the microcell 3. This causes the service to be received from a particular base station, that is to say to be able to

  receiving and placing communications, also includes the situation involved

  prior to a careful placing in this particular base station. When a mobile radio unit entered the multi-cellular environment, a decision was made to determine whether it should remain served by the current type of cell or be transferred to a new type of cell. The decision may depend on the

speed of the mobilc unit.

  Two cases can be defined when a mobile radio unit enters an environment, or area of desserts, multicellular. The mobile unit can be moved at high speed, as shown in Figure 1 by a car 20, or

  well at a slower speed, as indicated by a personaf 30 in figure 2.

  In Figures 1 and 2, we assume that the dcsscert cell is the parapluic cell 1

  and that the plurality of neighboring cells is formed by microclules 2, 3, 4,5 and 6.

  We refer to FIG. 3. An om presents the process according to the invention allowing to determine if it was to be carried out or not to transfer to another cell. In a preferred embodiment, a pammeter value coming from at least one base station of a neighboring cell received by the mobile unit cst transmitted to the base station of the serving cell cc is measured for a certain duration, as indicated in step 51. The rate of variation of the parameter in the neighboring cell is then calculated in the base station of the cell

service point, step 53.

  In step 55, a maximum variain rate value is chosen. The maximum value can simply be a fixed valCU p1 _teiD, but it would be more reliable if it were chosen according to the variable parameters of the

  microcellular system itself, for example the capacity and size of cells.

  The maximum value of the rate of change thus chosen naturally also exceeded

  the number of transfers required in the event that the mobile should go to a micro-

  cell or, if it is already in a microcellular environment, in the case where

he would stay there.

  In step 57, the rate of change of the base station parameter of

  the neighboring cell is compared with the maximum value of rate of variation chosen.

  If the rate of change is greater than the maximum value of the rate of change and the serving cell is a microcell (step 59), then the serving is transferred to the umbrella cell, as shown in step 63, if the 'it is assumed that the umbrella cell has already been determined to have the capacity. If the rate of change is less than the determined maximum rate of change value and the serving cell is a microcell (step 61), then, according to step 65, no transfer is made. If the rate of change is greater than the maximum rate of change and the serving cell is an umbrella cell (step 59), then no transfer is made and the service remains the responsibility of the umbrella cell, depending on the step 67. Finally, if the rate of change is lower than the maximum value of rate of change and the serving cell is an umbrella cell, then the service is transferred to the microcell, according to step 69. Figure 3 only shows

  that a way of implementing the method of the invention in an environment-

microcellular.

  In particular, according to a preferred embodiment of the invention, the signal level from the base station of a neighboring cell which is received by the unit is used as the parameter value of step 51 mobile. FIGS. 4 and 5 respectively show the comparative rates of variation of the signal level received from the serving cell 1 and the neighboring microcells 2, 3, 4 in the case of the

  fast moving mobile 20 and in the case of slow moving mobile 30.

  FIG. 4 shows the signal levels received from the serving umbrella cell 1 and neighboring microcells 2, 3 and 4 when the rapid mobile 20 of FIG. 1 moves in the umbrella cell 1 between the time t0 and the time tl. More specifically, part "a" of FIG. 4 shows the level of signal received by the mobile unit 20 from the base station of the umbrella cell

  when it crosses the umbrella cell 1 for the period between t0 and tl.

  The signal level received from the serving cell 1 is kept fairly constant by power adjustment instructions. Part “b” of FIG. 4 represents the level of signal received by the mobile unit 20 from the neighboring microccll 2 when the mobile unit 20 moves during the same time, indicated in FIG. 1, namely from tO to tl. Note that the mobile unit 20 does not enter the microcell 2 and therefore receives a moderately constant signal with a small rate of change. Likewise, the part "d" of FIG. 4 represents the level of signal received by the mobile unit 20 from the neighboring microcell 4 when the mobile unit 20 moves for the same duration, from t0 to tl. Again, the mobile unit 20 does not enter the microcell 4 and, therefore, receives a moderately constant signal having a small rate of change. Part (c) of FIG. 4 represents the level of signal received by the mobile unit 20 from the microcell 3 when the mobile unit moves during the duration from t0 to tl. Unlike neighboring microcells 2 and 4, the level of signal received varies significantly, indicating a higher rate of change. The mobile unit 20 actually entered the microcell 3 and, during the short time from t0 to tl, the level of signal received from the base station of the microcell 3 increased at a relatively rapid rate, which indicates that the mobile unit moves in microccllule 3 with a high speed. The base station 23 of the serving cell, which is in this case placed in the umbrella cell 1, can then decide not to transfer to the microcell 3, since the mobile unit is moving with a relatively high speed and will probably ask a number of transfers in a short time to

  that it can be served by microcells of the microcellular system.

  Thus, it would be more efficient to maintain the service of the fast mobile unit 20 at

  using the base station 23 located in the umbrella cell 1.

  FIG. 5 represents the signal levels received from the serving umbrella cell 1 and neighboring microcells 2, 3, 4, when the slower mobile unit 30, appearing in FIG. 2, moves between the instants t0 and tl. The part "a" in FIG. 5 represents the level of signal received from the

  serving cell 1, which is usually kept constant by instruc-

  power adjustment. Parts "b" and "d" of FIG. 5 represent the signal levels received by the mobile unit 30 from the base stations of neighboring microcells 2 and 4. Similarly, the rates of change are

relatively small.

  The part "c" of FIG. 5 represents the level of signal received from the neighboring microcell 3 when the mobile unit moves between t0 and tl. Since the mobile unit 30 actually moves in the microcell 3, the level of signal received from the base station of the microcell 3 increases, but not

  at a relatively high speed, compared to part 'c' in Figure 4.

  Thus, the base station 23 of the serving umbrella cell 1 can decide to transfer the service from the slow-moving mobile unit 30 to the microcell 3. The comparison of parts "c" in FIGS. 4 and 5 shows that the rate variation in the signal level received for the fast moving mobile 20 is higher than that of the slow moving mobile 30. Consequently, according to the method of the invention, it is possible to determine, as a function of the slow rate of change of the level of signal received from a slow moving mobile, to effect the transfer from the umbrella cell to a microcell. Analog received signal levels will be noted in the case where a communication comes from a microcell. The majority of mobile units in a microcell system are slow moving mobile units and therefore these must be served by

microcells.

  A second embodiment of the invention uses the rate of change of the power control level as a parameter used for the determination. As in Figures 4 and 5, the rate of change of power control level signals that are received by a mobile unit from neighboring cell base stations will vary more in a microcell than the mobile unit is crossing, if it is a fast moving mobile unit. Thus, the decision to effect the transfer can be made by the base station of the serving cell as a function of the rate of change of the power control signals which are received from the

  from the base stations of neighboring cells by the mobile unit.

  While the method has been described as being implemented in the base station of the serving cell, it could in fact be implemented in the mobile unit provided that this has been carried out with the level of intelligence required. The method could also be implemented in the base station of the neighboring cell as long as the appropriate information has been transmitted to the

  base station of the neighboring cell. When cellular systems develop

  loppent, methods such as that of the invention can be chosen to be

  implemented in higher level control stations.

  In conclusion, the invention proposes a method intended for a microcellular telecommunications system, which comprises microcells and umbrella cells, where mobile units with slow displacement use the microcells and

  fast moving mobile units use umbrella cells. In part-

  In particular, when a fast-moving mobile unit enters a microcell, the mobile unit must remain served by the current type of cell, namely the

  umbrella cell, so as to minimize the number of transfers required.

  Thus, there is a significant reduction in the number of transfers and an efficient use of the microcellular environment. The invention improves the reliability of transfers and leads to a reduction in the number of lost communications.

  as well as a reduction in the work done by the network.

  Of course, those skilled in the art will be able to imagine, from the

  process whose description has just been given by way of illustration only and

  in no way limiting, various variants and modifications not departing from the scope of the invention.

Claims (5)

  1. Method for determining the transfer of a mobile radio unit (20, 30) in a cellular telecommunications system comprising a serving cell (1) and a plurality of neighboring cells (2 to 6), o the serving cell and the neighboring cells comprise at least one umbrella cell and a plurality of microcells, each cell having a respective base station, the method for determining whether or not a transfer should be made from the serving cell to a neighboring cell being characterized in that it comprises the following operations: measuring the rate of variation of a parameter of an incoming signal supplied to the mobile from at least one base station (25) of a neighboring cell; and   determine the transfer as a function of the measured rate of change.   2. Method according to claim 1, characterized in that it further comprises the following operations: selecting a maximum value for the rate of change of the parameter of the incoming signal; and carry out the transfer to the umbrella cell if the rate of change of the measured parameter is greater than the maximum value and the serving is a microcell.   3. Method according to claim 1 or 2, characterized in that it further comprises the following operations: selecting a maximum value for the rate of change of the parameter of the incoming signal; and transfer to a microcell if the rate of change of the measured parameter is less than the maximum value and the serving cell is the umbrella cell.   4. Method according to any one of claims 1 to 3, characterized   in that the parameter is a received signal level.   5. Method according to any one of claims 1 to 3, characterized   in that the parameter is a power level control signal.