FR2717650A1 - Method for determining a candidate for transfer in a multicell environment. - Google Patents

Abstract

A method for determining a handover candidate for a mobile station (20) in a cellular telecommunications system having a serving cell and a number of neighboring cells, wherein the serving cell and neighboring cells include or are described herein. minus an umbrella cell (1) and a certain number of microcells (2 to 6). To determine the candidate that will be chosen for handover from the serving cell to a neighboring cell, a parameter of the incoming signal received by the mobile station from at least one cellular base station is measured for a certain period of time. The criterion comprises the operation which consists in determining a variation in stages of the measured parameter.

Description

The present invention relates generally to a method for

  determine possible base sites, that is to say acceptable, as candidates for a transfer in a multicellular environment and, more particularly, determine candidates for transfer in a multicellular environment as a function of the variation in stage of a parameter of a signal

  incoming from at least one of the neighboring cells or the serving cell.

  In a cellular environment, there is usually, at any time, a serving cell which is defined as the cell comprising the base station through which a mobile unit is served so that the mobile unit can receive and transmit communications, as well as a number of surrounding cells which are defined as neighboring cells. The serving cell can also be defined as the cell to which the unit

mobile is affected.

  In a multicell environment, there may be cells of different sizes, where a number of cells of the same size are placed inside a larger cell (umbrella cell). The smaller cells that are inside the umbrella cell are called microcells. Microcells are created in a dense user population to increase the processing capacity of users in the cellular system. The microcells facilitate the re-use of frequencies over a shorter distance. So a mobile unit can be placed inside a microcell as well as inside

of an umbrella cell.

  Typically, with regard to rural areas, which do not have a large number of users or which do not require a large processing capacity, it suffices that they are divided into larger cells. When the zones grow or the cells approach densely populated zones, these larger cells do not offer a processing capacity capable of facilitating the increase in the number of users. There are not enough frequencies allocated. Thus, microcells are created inside the larger cells, and these larger cells become umbrella cells. This allows the reuse of frequencies between the microcells. These microcellular techniques improve spectral efficiency and increase the processing capacity of the cellular network. Microcells have drawbacks. One is that, in microcellular areas, the number of transfers increases and the time available to make transfer decisions decreases. For example, if there are too many smaller microcells in an area where there is a fast-moving mobile, the fast-moving mobile crosses a certain number of microcells in a short time, which means that make a number of transfers. The fact that it is necessary to increase the number of transfers in a short time decreases the reliability of the communications and increases the number of interruptions occurring in the communications, which reduces the quality of the communication and, in certain extreme cases, leads to loss. of these communications. It is therefore necessary to establish a rapid and reliable process allowing

  determine when to transfer to a multi-environment

  cellular. Such a process was proposed in British patent application no. 9 324 428.3 filed on November 27, 1993 by the company Motorola under the title

  "Method for Determining Handover in a Multicellular Environment".

  Digital cellular telecommunications systems, such as the GSM (Global System for Mobile Communications) system, integrate a large number of cells in a microcell environment. In the GSM system, a mobile station is asked to report the strength of the signal level received from its six strongest neighboring cells. Common transfer techniques choose a candidate for transfer from among the six most intense neighboring cells. In a microcell environment where the signal strength varies rapidly, a cell can produce an intense signal level in a first measurement report, then a weak signal level in the next measurement report. Consequently, making a transfer decision on the basis of the first report alone may lead to the selection of a basic site which will not be a

  reliable dessert cell for the mobile station.

  It is desirable to prevent a neighboring cell whose signal strength varies rapidly from being considered a candidate for transfer. Therefore, we would like to have a method for determining a transfer candidate, in which a neighboring cell will not be considered as

  a transfer candidate only if it is really a reliable candidate for the transfer.

  According to the invention, a method is proposed for determining

  a candidate for transfer to a cellular telecommunications system

  in 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 method making it possible to determine whether or not to carry out the transfer from the serving cell to a neighboring cell comprises the operation consisting in determining the variation in stage of a parameter of an incoming signal supplied to the mobile from at least one base station of a cell and the operation consisting in determining the transfer as a function of this variation in stage. In a preferred embodiment of the invention, the parameter is the

  speed of variation of the level of a received signal.

  According to another embodiment, the parameter is the intensity level of a signal received from neighboring cells, which is measured during

a certain duration.

  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 shows a mobile unit with rapid displacement in a multicell environment; Figure 2 shows a slow moving mobile unit in the cellular environment in Figure 1; FIG. 3 illustrates two pads which can be applied to a preferred embodiment of the invention; Figure 4 shows a typical multicell environment; Figure 5 shows received signal levels which have been plotted for the mobile station of Figure 4; FIG. 6 is a flow diagram relating to a preferred embodiment of the invention; and FIG. 7 represents a decision diagram, applicable to the mode

  preferred embodiment of the invention.

  We refer to Figure 1, which shows a multi-environment

  cellular (or microcellular) comprising at least one umbrella cell 1 and a plurality of microcells 2, 3, 4, 5 and 6. Each cell comprises a station

  base which is typically located in the geographic area served by the unit.

  Not all base stations are shown in Figure 1. Typically, a base station determines the size and capacity of the cell. A telecommunications system can comprise cells of different sizes as well as a mobile radio unit 20 which can be served by the base station 23 of the umbrella cell 1 or a base station of one of the microcells, for example the radio station. base 25 of the microcell 3. The fact of being served by a particular base station with regard to the possibility of receiving and transmitting communications constitutes what is also called being assigned to this particular base station. When a mobile radio unit enters the multicell environment, a decision must be made to determine whether it should remain served by the current cell type or be transferred to a new type of cell.

  cell. The decision may depend on the speed of the mobile unit.

  Two cases can arise when a mobile radio unit enters a multicellular environment, or service area. The mobile unit can move at a high speed, as represented by the drawing of a car 20 in FIG. 1, or at a slower speed, as represented by the drawing of a pedestrian 30 in FIG. 2. For Figure 1, as for Figure 2, it is assumed that the serving cell is the umbrella cell 1, and that there is a plurality of

  neighboring cells which are microcells 2, 3, 4, 5 and 6.

  When a fast-moving mobile station 20, as shown

  felt in FIG. 1, enters a microcellular environment, the invention requires that the mobile station remains on the current cell type (umbrella) so as to reduce the number of transfers which should be carried out in a short time. Following the measurement of the signal intensity level received from a certain number of neighboring cells for a certain time or the measurement of the stage variations of the signal level received, none of the neighboring microcells will be

  designated by the mobile station as constituting a candidate for the transfer.

  For the base station, the microcells will be transparent from the point of view of the transfer application, and all transfers must be made to

umbrella cells.

  Conversely, when a slow-moving mobile station 30, as shown in FIG. 2, enters a microcell environment, the invention requires that the slow-moving mobile station 30 be transferred to a microcell. This will ensure that the umbrella cell will not be congested and that most of the traffic will be processed by the microcell. Consequently, according to an embodiment of the invention, when the mobile station 30 enters a microcellular environment, the six most intense cells, from the point of view of the intensity levels of the signals received in the station

  mobile, come from the surrounding base stations of neighboring microcells.

  Since the mobile station is moving slowly, some of the neighboring cells will be signaled long enough, or for a predetermined period, to be considered candidates for transfer. The invention

  requires that the transfer be made to the most appropriate cell.

  According to one embodiment of the invention, a cell does not become a candidate for transfer unless it has been, for a given duration Tn, one of the most intense cells. A timer is associated with each neighbor and can be different for each neighbor. It can be predetermined or defined in a way

  adaptable by the user of the database.

  FIG. 3 shows a way in which the timer intended to measure the duration Tn can be used in a transfer method to a base station. In FIG. 3, the mobile station indicates to the base station the six strongest carriers among the neighbors in terms of the levels of the signals received. The base station controller places them in a first buffer 35 and starts a timer for measuring the duration Tn. If, at the end of the duration Tn of the timer, the cell still has one of the six most intense carriers, it is then sent to a second buffer 37, as one

candidates for transfer.

  The transfer candidate list must always contain at least one cell, which will then be a backup cell for the transfer. A backup cell for transfer can be defined as a cell which is not used for normal communications, but which is available for transfers in emergencies. It will be one of the umbrella cells (or certain time slots in a given umbrella cell). Ccci ensures that, in cases where a transfer is necessary, when there is no suitable microcell available, the communication can be processed and should not be

abandoned.

  To check whether a cell is suitable as a transfer candidate, we

  can use any of the following criteria.

  A nearby cell has been repeatedly reported in reports

  measurement of the mobile (for example SACCH multi-frames) during the duration Tn.

  Under these conditions, the neighboring cell has the maximum probability of being a good transfer candidate, but it suffers from the drawback resulting from the fact that rapid variations in the received signal level (RXLEV) have not been taken into account. ) due to fading, masking, etc. The neighboring cell was cited at least n times among m in the mobile measurement report during the duration Tn. This process takes into account rapid variations in the level of signal received and therefore has a high probability of identifying a good transfer candidate. The values of n and m must be optimized for

each cell.

  The average signal level received from the neighboring cell exceeds a certain threshold over the duration Tn. This process averages all of the peaks and dips in the received signal levels. The signal quality and other criteria commonly used to determine transfers must also be met in order to be able to

  consider the neighboring cell as a candidate for transfer.

  The microcellular environment is specially characterized by the high speed of variation of the levels of received signals (for example when the user turns the corner of a street, passes under a bridge, etc.), which makes the transfer processes common. sometimes imprecise with regard to the prediction of

transfer candidates.

  Figure 4 shows a typical multicell environment. In particular, when a mobile station 40 passes from point A to point B, the signal level received from a first neighboring base station 44 increases to a relatively high level, at point X, so that , according to the invention, it becomes one of the levels of the neighboring cells signaled by the mobile station 40 as intense carrier levels, while the level of signal received from the first

  neighboring base station 44 remains relatively constant.

  Similarly, when the mobile station 40 goes from point B to point C, the signal level received from the base station of the serving cell 42 abruptly decreases at point Y to go to a low level, while the level signal received from neighboring base station 44 remains relatively constant. Due to the nature of the microcclules, the variation of the signal level received from the serving cell 42 and that of the neighboring cell 44 will be a stage function, as shown in FIG. 5, and these characteristics can be used to

  predict candidates for transfer according to the invention.

  FIG. 5 shows the signal level received by the mobile station 40 when it moves from point A to point B, then to point C in FIG. 4. From point A to point B, the signal level received from the cell neighbor 44 increases sharply at point X, which results in a stage variation function, then remains relatively constant. The signal level received from the serving cell 42

remains constant.

  Similarly, when the mobile station passes from point B to point C, the signal level received from the serving cell suddenly decreases at point Y, which results in a stage variation function. The signal level received from the neighboring cell 44 remains constant. The points X and Y represent the edges

cell coverage.

  Floor variations in the signal level received from neighboring cells mean that the mobile station is at a street corner or at a junction of streets. Variations in stage of the signal level received from the serving cell mean that the mobile station has turned the corner and that a transfer is likely to be necessary. When these variations in stage have been recognized by the process of

  transfer according to the invention, the system can pass into operating modes-

  significantly different. For example, it is possible to reduce the time Tn of the timer associated with a particular neighboring cell so that a quick transfer decision can be made. It is necessary to define an averaging process which is able to detect changes in stage in

the signal level received.

  FIG. 6 represents an embodiment of the invention, which associates two parameters, so that the timer time Tn associated with each neighboring cell is modified interactively when a variation in stage is recognized. This makes a given neighboring cell more likely to become a transfer candidate when floor variations are detected in the signal.

  received (i.e. when a mobile turns a street corner). In addition, the recognition

  birth of a variation in stage also allows the process a reassessment

  priorities of current transfer candidates.

  According to FIG. 6, the mobile station or the base station of the serving cell controls the neighboring cells, or the surrounding carriers which are received by the mobile station, in step 50. If no variation in stage is determined in step 52, it is then checked whether a respective timer time Tn has expired and if the transfer criterion is satisfied, in step 54. If this is the case, the carrier or the neighboring cell come s 'Add to the transfer candidate list in step 56, then the process starts again. If this is not the case, it is then necessary to determine, in step 58, if a new carrier has been identified, and, if this is not the case, the process starts again. If this is the case, the respective timer Tn

  starts for the new carrier, in step 60, and the process resumes.

  If, in step 52, a variation in stage has been identified, then this variation in stage is evaluated in step 62. It is determined whether a transfer is necessary, in step 64. If a transfer is necessary, we then determines whether there is a microcell which is a candidate for transfer, in step 66. If this is the case, a transfer is then carried out to the microcell having the highest priority, in step 70. S ' there is no microcell which is a candidate for transfer, as determined in step 66, then the transfer is carried out to a backup cell,

in step 68.

  If no transfer is necessary (step 64), it is then determined whether a timer change is necessary, in step 72. If a timer change is necessary, as determined by step 72, then the timer

  is changed in step 74.

  If no timer change is required, as determined in step 72, then it is determined whether an evaluation of the transfer list is required, in step 76, and, if not, the process starts again, and, if so, the priorities of the transfer list are reassessed, in step 78, and then the

process begins again.

  Depending on the floor variations appearing in the neighboring cells and the serving cells, certain operations can be undertaken, which are summarized in the table presented in FIG. 7. The upper row describes the possible states of a serving cell, while that the first column describes the possible states of a neighboring cell. If the mobile station receives a constant received signal level from the serving cell and the mobile station receives a floor variation in the form of an increase from a neighboring cell, then the mobile station may be at the junction of n cells, as indicated in state 80. If the neighboring cell is already present in the second buffer, then the priorities are reassessed. If the neighboring cell does not belong to the second buffer, we modify the timer which

  is associated (Tn) in the first buffer.

  If the signal level received from the serving cell exhibits a variation in stage of the increase type and the signal level received from the neighboring cell exhibits a variation in stage of the increase type, then the mobile station may be at the junction of n cells in the case where the level of the neighboring cell is comparable to that of the serving cell, as described in state 82. If the neighboring cell already belongs to the second buffer, then the priorities are reassessed. If the neighboring cell is not in the second buffer, the associated timer (Tn) in the first buffer is then modified. If the signal level received from the serving cell shows a variation in floor of the decreasing type and the signal level received from the neighboring cell shows a variation in floor of the increasing type, then the mobile station has probably turned a street corner and a transfer is possibly necessary, as described in state 84. If the neighboring cell belongs to the second buffer, then the priorities are reassessed. If the neighboring cell is not in the second buffer, the associated timer (Tin) in the first buffer is then modified. If there are no microcells in the second buffer, then the

  transfer is made to a rescue cell.

  If the level of signal received from the serving cell is constant and the level of signal received from the neighboring cell shows a variation in stage of the decreasing type, then the levels of the signals received are continued to be monitored, as indicated in the state 86. Likewise, if the signal level received from the serving cell shows a variation in stage of the increase type or a variation in stage of the decrease type and the level of signal received from the neighboring cell presents a variation in stage of the decrease type, then we continue to

  check the levels of the signals received, as indicated in states 88 and 90.

  If the signal level received from the serving cell is constant or exhibits an increase in stage of the increase type and the signal level received from the neighboring cell is constant, the levels of the signals are continued to be monitored.

  signals received, as indicated in states 92 and 94.

  If the signal level received from the serving cell has a decrease in floor and the signal level received from the neighboring cell is constant, then the mobile station has probably turned a corner and a transfer may be necessary, such as indicated in state 96. If the neighboring cell already belongs to the second buffer, then the priorities are reassessed. If the neighboring cell is not in the second buffer, then the associated timer (TCn) in the first buffer is modified. If there are no microcells in the second

  buffer, it must then be transferred to a backup cell.

  Thus, the invention provides a method which determines transfer candidates by reliably reducing the possibility that communications are lost. According to the method of the invention, there are four important scenarios,

that we can analyze.

  First, a mobile station is assigned to an umbrella cell and the mobile station travels at high speed. According to the method of the invention, as described in connection with FIG. 1, none of the microcells is signaled by the mobile station for a time long enough for it to be considered as a candidate for transfer. Therefore, all transfers

  necessary will go to umbrella cells.

  Second, a mobile station is assigned to a parallel cell.

  rain and moves slowly. As described in connection with Figure 2, the mobile station signals the six most intense cells as neighboring cells, and some of them will be flagged long enough to become transfer candidates. There will therefore be a transfer to a microcell. Third, the mobile station is assigned to a microcell and moves quickly. The mobile station starts a communication on a microcell and the communication must be transferred to an umbrella cell to decrease the necessary number of transfers. With each transfer made, there is a probability that a communication may be lost. Therefore, by minimizing the number of transfers, we reduce the probability that the communication

be lost.

  If the mobile station moves rapidly in the microcellular coverage area, the list of neighboring microcells which is reported by the mobile station is constantly changing and none of the microcells will become a candidate for transfer. However, one of the cells thus signaled by the mobile station is an umbrella cell, which will become a candidate for transfer. This umbrella cell can be the backup cell. If a transfer is necessary and no suitable microcell is available, then the mobile station will be transferred to the umbrella cell. Fourth, if the mobile station moves slowly in the microcellular service area, certain microcells will be indicated by the mobile station for a sufficient time to be considered as candidates for transfer. If a transfer is necessary, the mobile station is transferred to the most suitable microcell. Measured (or detected) stage variations can be used in the levels of signals received from neighboring cells and from the serving cell, as described in conjunction with FIG. 7, for

  predict the cell to which the mobile station will be transferred.

  In particular, it has been said that the levels of signals received are used to determine variations in stage and candidates for transfer. Signal levels received from monitored cells are usually maintained

  constant by power control instructions.

  Another embodiment of the invention can use the variation in

  stage of the power control level under the determination parameter.

  Thus, a transfer decision can be made by the base station of the serving cell as a function of a variation in stage of the power control signals received from the base stations of the cells which are monitored by

the mobile unit.

  While the method has been described as being implemented in the base station of the serving cell, it could actually be implemented in the mobile unit as long as the desired "intelligence" has been incorporated into the unit mobile. The method could also be implemented in the base station of the neighboring cell, as long as the correct information has been transmitted to the base station of the neighboring cell. With the extension of cellular systems, methods such as that of the invention can be implemented anywhere

in the system infrastructure.

  The parameters defined in this process can be used to predict the most suitable cell to which the transfer should be performed. This prediction is based on the control of all the variations in stage appearing in the level of signal received from the serving cell and the neighboring cells or can rest on the number of variations in stage which is detected in a particular period of time. For example, if a certain number of floor variations are carried out in a relatively short period of time, it may be advantageous to transfer to a

  umbrella cell to avoid this high number of transfer to microcells.

  In conclusion, the invention provides a method for a microcellular telecommunications system, which includes microcells and umbrella cells, where the slowly moving units use the microcells and the fast moving units use the umbrella cells. In particular, when a rapidly moving mobile unit enters a microcell, it is necessary that the mobile unit remains served by the current cell type, namely an umbrella cell, to minimize the number of transfers required. Thus, the number of transfers is considerably reduced and the cellular environment is used efficiently. By making efficient use of the cellular environment, the processing capacity of the cellular telecommunications system can be increased. The invention improves the reliability of transfers, which reduces the number of lost telecommunications, and reduces the amount of

  processing performed 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 (4)

  1. Method for determining a transfer candidate in a microcellular telecommunications system comprising a serving cell and a plurality of neighboring cells, where 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 being characterized in that it comprises the following operations: measuring a received signal parameter which is received from the serving cell and from each cell of the plurality of neighboring cells; detecting stage variations in the received signal parameter which is received, u from the serving cell and from each cell of the plurality of neighboring cells; and determining a transfer candidate based on the stage variations detected in the received signal parameters which is received from the   service and each cell of the plurality of neighboring cells.   2. Method according to claim 1, characterized in that it further comprises the following operations: measuring a received signal parameter which is received from the serving cell and from a plurality of neighboring cells for a certain duration ; and determining a transfer candidate as a function of the received signal parameter which has been measured at the end of this period and as a function of all the stage variations which have been detected in the received signal parameter coming from   the serving cell and each cell of the plurality of neighboring cells.   3. Method according to claim 1 or 2, characterized in that the   parameter is the signal level received.   4. Method according to claim 1 or 2, characterized in that the   parameter is the power level control signal.