Mobile station, mobile communication system, and cell selection method

1. Field of the Invention

The present invention relates to a mobile station, a mobile communication system, and a cell selection method.

2. Related Background Art

Conventionally, when a mobile station selects a cell as a reselection target, the mobile station measures a received level of a signal from a base station currently under communication therewith or a base station of a cell where the mobile station is camped, and received levels from base stations of neighboring cells. The mobile station compares the received levels and selects as a target cell a cell with the maximum received level from its base station, based on the result of the comparison (e.g., reference is made to Patent Document 1 and Patent Document 2).

The conventional cell selection method will be briefly described below with reference to FIG. 1 . As shown in FIG. 1 , mobile station 10 is camped on a cell C 0 established by base station B 0 , and cells C 1 , C 2 , and C 3 exist as cells neighbor to the cell C 0 . Mobile station 10 receives broadcast information M 0 from base station B 0 to acknowledge the existence of neighboring cells C 1 , C 2 , and C 3 . The broadcast information M 0 is transmitted and received through a common broadcast channel or through a control channel individually dedicated to mobile station 10 .

Mobile station 10 measures a received level L 0 from base station B 0 , and received levels L 1 , L 2 , and L 3 from the respective base stations B 1 , B 2 , and B 3 forming the neighboring cells C 1 , C 2 , and C 3 . Thereafter, mobile station 10 compares the received level L 0 with those L 1 , L 2 , and L 3 and determines the propriety of a cell reselection. The comparison of the received levels is made, for example, using the following conditions (1) and (2).

max( Li )=max( L 1, L 2, L 3)  (1)

max( Li )> L 0 +ΔL   (2)

Here max(argument 1 , argument 2 , . . . ) represents a function that gives a maximum argument among argument 1 , argument 2 , . . . Furthermore, L 0 indicates the received level of the serving cell and ΔL a reselection hysteresis value.

When condition (2) becomes true, mobile station 10 selects a neighboring cell corresponding to max(Li) as a target cell and implements a reselection of the cell. On the other hand, when condition (2) does not become true, mobile station 10 implements no cell reselection.

[Patent Document 1] Japanese Patent No. 3233854

[Patent Document 2] Japanese Patent No. 3315869 (p2 and FIG. 4)

Cells are generally classified under indoor cells and outdoor cells, according to installation locations and surrounding environments of base stations. In general, the indoor cells have smaller radii than the outdoor cells, and the indoor environments have less disturbance factors than the outdoor environments; therefore, the indoor cells are expected to stabilize good communication quality. However, the cell selection in the aforementioned prior art involves no consideration to whether a cell as a candidate for a reselection target is an indoor cell or an outdoor cell. For this reason, the mobile station can fail to select a cell optimal for communication. For example, the cells are sometimes classified under three or more cell types like nanocells, macrocells, and microcells according to their cell radii.

An object of the present invention is, therefore, to enable a mobile station to select a cell optimal for communication as a target cell, where the cells are classified under multiple types.

In order to solve the above problem, a mobile station according to the present invention comprises measuring means for measuring received levels of a serving cell and each neighboring cell thereto; determining means for determining cell types of the current and neighboring cells; and selecting means for selecting a cell as a reselection target, based on the received levels measured by the measuring means and the cell types determined by the determining means.

A cell selection method according to the present invention comprises a measuring step wherein measuring means of a mobile station measures received levels of a serving cell and each neighboring cell thereto; a determining step wherein determining means of the mobile station determines cell types of the current and neighboring cells; and a selecting step wherein selecting means of the mobile station selects a cell as a reselection target, based on the received levels measured by the measuring means and the cell types determined by the determining means.

Here the selection of the target cell includes selection of a cell as a target candidate, of course, and also includes determination on the propriety of a reselection of the cell. Therefore, even if a cell as a target candidate is selected, where the cell is determined not to be an optimal cell as a reselection target, the mobile station can continuously camp in the current source cell without executing the candidate reselection of the cell.

According to these aspects of the invention, the received levels of the serving cell and each neighboring cell thereto are measured, the cell types of the current and neighboring cells are determined, and thereafter the target cell is selected based on the received levels and the cell types. Namely, in the selection of the target cell including the determination on the propriety of the reselection, the mobile station selects the cell while taking into account not only the received levels in the current and neighboring cells, but also attributes of the respective cells. For example, the mobile station selects as a target cell an indoor cell classified in the cell type of the high received level. Since the indoor cell has the high received level and can stabilize good communication quality at the mobile station, the reselection of the selected cell permits the mobile station to perform low-power and high-speed communication. Namely, the selection of the target cell based on the received levels and the cell types enables the mobile station to select the cell optimal for communication.

In the mobile station according to the present invention, preferably, the selecting means changes a cell reselection condition or priority for selection between cell types, according to the cell type of the serving cell determined by the determining means.

In the cell selection method according to the present invention, preferably, the selecting step is configured so that the selecting means changes a cell reselection condition, according to the cell type of the serving cell determined by the determining means.

According to these aspects of the invention, the cell reselection condition is changed according to the cell type of the serving cell. For example, where the mobile station is now camped on an indoor cell, the continuous residence in the serving cell is more likely to stabilize good communication quality, rather than where the mobile station stays in an outdoor cell. In this case, therefore, the condition for the cell reselection is set tight to restrain the reselection so that no reselection is carried out unless a better cell is present in the surrounding area. As a consequence, it becomes feasible for the mobile station to select a cell optimal for communication as a target cell.

In the mobile station according to the present invention, preferably, the selecting means changes a cell reselection condition or priority for selection between cell types, according to the cell type of the neighboring cell determined by the determining means.

In the cell selection method according to the present invention, preferably, the selecting step is configured so that the selecting means changes a cell reselection condition, according to the cell type of the neighboring cell determined by the determining means.

According to these aspects of the invention, the cell reselection condition is changed according to the cell type of the neighboring cell. For example, where the neighboring cell as a candidate for the target cell is an indoor cell, the reselection of the mobile station thereto is more likely to stabilize good communication quality, than where the neighboring cell is an outdoor cell. In this case, therefore, the condition for the reselection of the cell is loosed so as to promote the reselection of the better cell. As a consequence, it becomes feasible for the mobile station to select the cell optimal for communication as a target cell.

The mobile station according to the present invention, more preferably, comprises storing means for storing the cell types in relation with cell classes; counting means for counting the number of reselections between cells of different cell classes; and changing means for changing the relation between the cell types and the cell classes in the storing means to another when the number of reselections counted by the counting means exceeds a predetermined value.

The cell selection method according to the present invention, more preferably, comprises a counting step wherein counting means counts the number of reselections between cells of different cell classes; and a changing step wherein changing means changes a relation between the cell types and the cell classes in storing means to another when the number of reselections counted by the counting means exceeds a predetermined value.

According to these aspects of the invention, the cell types are stored in relation with the cell classes in the storing means and the number of reselections between cells of different cell classes is counted. When the number of reselections exceeds the predetermined value, the relation between cell types and cell classes is changed. These aspects of the invention are presented for clearing the concern that highly frequent reselections can cause discontinuation of data transmission to decrease the data transmission rates to the mobile station. When the number of reselections exceeds the predetermined value, it is determined that the frequency of reselections of the mobile station is too high, and the relation (correspondence) between cell types and cell classes is changed to another.

This results in, for example, changing the cell class of indoor cells having belonged heretofore to a priority cell class, into a nonpriority cell class and changing the cell class of outdoor cells having belonged heretofore to the nonpriority cell class, into the priority cell class. Namely, the cell classes are changed. In conjunction with the change of the cell classes, the conditions for the cell reselection also change to make the condition for the cell reselection tight even in the case where the mobile station is camped on an outdoor cell. Therefore, the frequency of cell reselections is restricted, thereby eliminating the concern of the decrease in the data transmission rates to the mobile station.

In the mobile station according to the present invention, more preferably, the changing means changes the relation between the cell types and the cell classes in the storing means to another when the number of reselections exceeds the predetermined value within a predetermined time from a point of a start of counting the number of reselections.

In the cell selection method according to the present invention, more preferably, the changing step is configured so that the changing means changes the relation between the cell types and the cell classes in the storing means to another when the number of reselections exceeds the predetermined value within a predetermined time from a point of a start of counting the number of reselections.

According to these aspects of the invention, the changing of the relation between cell types and cell classes is limited to cases where the number of reselections exceeds the predetermined value within the predetermined time from the point of the start of counting. Namely, where the predetermined time has elapsed after the time of initializing the number of cell reselections, the number of reselections is again initialized to 0, regardless of the number of reselections at that point. This makes the changing of the relation and, besides, the determination on the cell reselection more properly reflect the number of cell reselections (the frequency of cell reselections) within the predetermined time.

In the mobile station according to the present invention, more preferably, the changing means is configured so that, on the occasion of changing the relation between the cell types and the cell classes, the changing means brings the relation back to that before the changing after a lapse of a predetermined time from a point of the changing.

In the cell selection method according to the present invention, more preferably, the changing step is configured so that, on the occasion of changing the relation between the cell types and the cell classes, the changing means brings the relation back to that before the changing after a lapse of a predetermined time from a point of the changing.

According to these aspects of the invention, where the relation between cell types and cell classes is changed to another, the changed relation is brought back to that before the changing after a lapse of the predetermined time from the changing point. Namely, after the relation after the changing is maintained for the predetermined time from the starting point when the relation between cell types and cell classes was changed, the relation between cell types and cell classes is forcedly brought back to that before the changing. This permits the relation before the changing to be used again for the determination on the cell reselection, where the frequency of cell reselections becomes lower after the changing of the cell classes than before.

In the mobile station according to the present invention, as described previously, the cell types are, for example, information indicating that each cell is either an indoor cell or an outdoor cell.

In the cell selection method according to the present invention, the cell types are, for example, information indicating whether each cell is either an indoor cell or an outdoor cell.

A mobile communication system according to the present invention comprises a mobile station comprising measuring means for measuring received levels of a serving cell and each neighboring cell thereto, determining means for determining cell types of the current and neighboring cells, and selecting means for selecting a cell as a reselection target, based on the received levels measured by the measuring means and the cell types determined by the determining means; and a base station for notifying the mobile station of information enabling identification of a cell type of its own cell or identification of cell types of its own cell and each neighboring cell thereto.

According to the present invention, the mobile station can determine the cell types of the respective cells (e.g., whether or not an indoor cell) as to the current and neighboring cells, based on the above information notified of by the base station. The mobile station selects the target cell with reference to the determination result and the received levels, whereby it can select the cell optimal for communication.

In order to solve the above problem, another mobile station according to the present invention comprises storing means for storing information about radio channels; choosing means for choosing neighboring cells for each of which a received level is measured; measuring means for measuring received levels of a serving cell and each neighboring cell after chosen by the choosing means, out of the neighboring cells to the serving cell; determining means for determining cell types of the current and chosen neighboring cells; and selecting means for selecting a cell as a reselection target, based on the received levels measured by the measuring means and the cell types determined by the determining means.

Another cell selection method according to the present invention comprises a storing step wherein storing means of a mobile station stores information about radio channels; a choosing step wherein choosing means of the mobile station chooses neighboring cells for each of which a received level is measured; a measuring step wherein measuring means of the mobile station measures received levels of a serving cell and each neighboring cell after chosen by the choosing means, out of neighboring cells to the serving cell; a determining step wherein determining means of the mobile station determines cell types of the current and chosen neighboring cells; and a selecting step wherein selecting means of the mobile station selects a cell as a reselection target, based on the received levels measured by the measuring means and the cell types determined by the determining means.

Here the selection of the target cell includes selection of a cell as a reselection target candidate, of course, and also includes determination on the propriety of a reselection of the cell. Therefore, even if a cell as a target candidate is selected, where the cell is determined not to be an optimal cell as a reselection target, the mobile station can continuously camp in the current source cell without executing the reselection of the selected cell.

According to these aspects of the invention, the received levels of the serving cell and each neighboring cell thereto are measured, the cell types of the respective cells of the current and neighboring cells are determined, and thereafter the target cell is selected based on the received levels and the cell types. Namely, on the occasion of selecting the target cell, the mobile station selects the target cell while taking into account not only the received levels in the current and neighboring cells, but also attributes of the respective cells. For example, where the mobile station moves into a train forming a mobile cell, the mobile station selects the mobile cell as a target cell. Since better communication quality is ensured in the mobile cell than in the outdoor and indoor cells, the reselection into the selected cell makes it feasible for the mobile station to perform low-power and high-speed communication. Namely, the selection of the target cell based on the received levels and cell types permits the mobile station to select the cell optimal for communication.

In the mobile station according to the present invention, preferably, the selecting means changes a cell reselection condition or priority for selection between cell types, according to the cell type of the serving cell determined by the determining means.

In the mobile station according to the present invention, more preferably, the selecting means changes the cell reselection condition, according to the cell type of the neighboring cell determined by the determining means.

In the cell selection method according to the present invention, preferably, the selecting step is configured so that the selecting means changes a cell reselection condition or priority for selection between cell types, according to the cell type of the serving cell determined by the determining means.

In the cell selection method according to the present invention, more preferably, the selecting step is configured so that the selecting means changes the cell reselection condition, according to the cell type of the neighboring cell determined by the determining means.

In the mobile station according to the present invention, preferably, the cell reselection condition includes at least one of the following reselection conditions: a reselection condition that the target cell is determined to be a neighboring cell with the highest received level out of neighboring cells satisfying a predetermined received level when the received level of the serving cell becomes below a predetermined first threshold; a reselection condition that the target cell is determined to be a neighboring cell a difference of the received level of which from that of the serving cell exceeds a predetermined hysteresis and which has a highest received level; a reselection condition that the target cell is determined to be a neighboring cell which keeps the received level high for a predetermined time, regardless of the received level of the serving cell; and a reselection condition that, when a variation per unit time of the received level of the serving cell exceeds a predetermined second threshold, the target cell is determined to be a neighboring cell with the highest received level out of neighboring cells the received level of each of which exceeds a predetermined third threshold.

In the cell selection method according to the present invention, preferably, the cell reselection condition includes at least one of the following reselection conditions: a reselection condition that the target cell is determined to be a neighboring cell with the highest received level out of neighboring cells satisfying a predetermined received level when the received level of the serving cell becomes below a predetermined first threshold; a reselection condition that the target cell is determined to be a neighboring cell a difference of the received level of which from that of the serving cell exceeds a predetermined hysteresis and which has a highest received level; a reselection condition that the target cell is determined to be a neighboring cell which keeps the received level high for a predetermined time, regardless of the received level of the serving cell; and a reselection condition that, when a variation per unit time of the received level of the serving cell exceeds a predetermined second threshold, the target cell is determined to be a neighboring cell with the highest received level out of neighboring cells the received level of each of which exceeds a predetermined third threshold.

According to these aspects of the invention, the cell reselection condition is changed according to the types of the current and neighboring cells. For example, where the mobile station is camped on an indoor cell, the continuous residence in the serving cell is more likely to stabilize good communication quality, than where the mobile station moves into an outdoor cell. In this case, therefore, when the received level of the serving cell becomes below the predetermined threshold, the mobile station sets the appropriate reselection condition to define as a target cell an outdoor cell with the highest received level out of neighboring cells satisfying the predetermined received level. This can restrict the cell reselection so as to keep the mobile station from reselecting of outdoor cell though the communication quality of the serving cell is not deteriorated.

Furthermore, since a mobile cell moves in a cell range of an outdoor cell, the difference between the received levels of the mobile cell and the outdoor cell is not always constant, even without movement of the mobile station itself. For this reason, it is not preferable to utilize the received level difference between the serving cell (either an outdoor cell or an indoor cell) and a neighboring cell (mobile cell), as a reselection condition for a reselection of the mobile station into the mobile cell. In this case, it is preferable to employ the received level of the neighboring cell (mobile cell) as a reselection condition. Therefore, when the received level of the mobile cell is kept high over the fixed time, the mobile station determines that it is desirable to move into the mobile cell, and thus settles the mobile cell as a target cell. Alternatively, when the variation per unit time of the received level of the serving cell is not less than the predetermined value and when the received level of the mobile cell being a neighboring cell is not less than the predetermined value, the mobile station determines that it is desirable to move into the mobile cell, and thus settles the mobile cell as a target cell.

In the mobile station according to the present invention, as described previously, the cell types are, for example, information indicating that each cell is an indoor cell, an outdoor cell, or a mobile cell.

In the cell selection method according to the present invention, the cell types are, for example, information indicating that each cell is an indoor cell, an outdoor cell, or a mobile cell.

A mobile communication system according to the present invention comprises the above-stated mobile station, and a base station for notifying the mobile station of information enabling identification of a cell type of its own cell or identification of cell types of its own cell and each neighboring cell thereto. According to the present invention, the mobile station is able to determine the cell types of the respective cells (for example, whether each cell is an indoor cell, an outdoor cell, or a mobile cell) as to the current and neighboring cells, based on the above information notified of by the base station. The mobile station makes reference to the determination result and the received levels and changes the cell reselection condition in accordance with the determination result, whereby it can select the cell optimal for communication.

The mobile communication system according to the present invention, preferably, comprises the base station for notifying the mobile station of information about a physical layer of a cell.

The information about the physical layer of the cell is information to distinguish a channel in which a received level is measured, or information for distinguishing a radio communication system and a channel in which a received level is measured.

According to the present invention, the mobile station is also able to reselect a neighboring cell using a radio communication system different from that in the serving cell. For example, even in the case where the outdoor and indoor cells use the radio communication system of CDMA (Code Division Multiple Access) and where the mobile cells use the radio communication system of OFDM (Orthogonal Frequency Division Multiplexing) for communication with the mobile station in the mobile cells, it becomes feasible for the mobile station to implement an appropriate cell reselection from the serving cell to a neighboring mobile cell as long as the mobile station is able to utilize the both communication systems of CDMA and OFDM.

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

FIG. 1 is a configuration diagram for explaining the conventional cell selection method.

FIG. 2 is an illustration schematically showing the overall configuration of the mobile communication system in the first embodiment.

FIG. 3 is a block diagram showing the functional configuration of the mobile station in the first embodiment.

FIG. 4A is a diagram showing a data storage example inside the cell class table before the changing of the cell classes. FIG. 4B is a diagram showing a data storage example inside the cell class table after the changing of the cell classes.

FIG. 5 is a diagram showing a data storage example inside the reselection hysteresis value table.

FIG. 6 is a flowchart for explaining the cell reselection determination process.

FIG. 7 is a flowchart for explaining a generalized example of the cell reselection determination process.

FIG. 8 is an illustration for explaining the cell arrangement and movement path with high frequency of cell reselections by the mobile station.

FIG. 9 is a diagram showing the magnitude relation among the received levels of the respective cells, which varies with time according to locations of the mobile station.

FIG. 10 is a flowchart for explaining the cell class changing process.

FIG. 11 is an illustration schematically showing the overall configuration of the mobile communication system in the second embodiment.

FIG. 12 is a block diagram showing the functional configuration of the mobile station in the second embodiment.

FIG. 13 is an illustration conceptually showing the relation between the cell types of the serving cell and neighboring cell and the cell reselection conditions.

FIG. 14 is a diagram for explaining the cell reselection conditions and the priorities for selection among cell types in the second embodiment.

First Embodiment

The first embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 2 is a conceptual diagram showing a configuration of a mobile communication system in the present embodiment. As shown in FIG. 2 , mobile communication system 100 is comprised of mobile station 1 , base station B 10 , base stations B 11 -B 13 , and base stations B 21 -B 22 . Mobile station 1 is camped on cell C 10 established by base station B 10 . Cells C 11 , C 12 , and C 13 established by the respective base stations B 11 , B 12 , and B 13 exist as indoor cells neighbor to cell C 10 , and cells C 21 and C 22 established by the respective base stations B 21 and B 22 exist as outdoor cells neighbor to cell C 10 . In FIG. 2 , the indoor cells are indicated by dashed lines and the outdoor cells by chain lines.

Base station B 10 notifies mobile station 1 of information indicating that cells C 11 -C 13 and cells C 21 , C 22 exist as neighboring cells and, in addition thereto, information (identification information) enabling identification of each cell, either an indoor cell or an outdoor cell (corresponding to a cell type). Namely, base station B 10 notifies the mobile station 1 of broadcast information M 1 containing the identification information indicating the cell type of its own cell C 10 , the identification information indicating that the neighboring cells C 11 -C 13 are indoor cells, and the identification information indicating that the neighboring cells C 21 , C 22 are outdoor cells.

The system may also be configured as follows: base station B 10 notifies mobile station 1 of the information that cells C 11 -C 13 and cells C 21 , C 22 are present as neighboring cells, and the identification information indicating that its own cell C 10 is either an indoor cell or an outdoor cell, and the base stations forming the respective cells notify the mobile station of the identification information indicating the cell types of the respective neighboring cells C 11 -C 13 , C 21 , and C 22 .

The mobile station as a principal component of the mobile communication system according to the present invention will be detailed below. FIG. 3 is a block diagram showing a functional configuration of mobile station 1 . As shown in FIG. 3 , mobile station 1 is provided with at least broadcast information receiving part 2 , received level measuring part 3 (corresponding to the measuring means), cell class determining part 4 (corresponding to the determining means), target cell selecting part 5 (corresponding to the selecting means), cell reselection part 6 , number-of-reselections counting part 7 (corresponding to the counting means), and cell class changing part 8 (corresponding to the changing means).

Broadcast information receiving part 2 receives the broadcast information M 1 via a radio channel from base station B 10 establishing the serving cell C 10 of mobile station 1 . The broadcast information M 1 contains the identification information indicating the cell type of cell C 10 , of course, and also contains the identification information indicating the cell types of cells C 11 -C 13 , C 21 -C 22 neighbor to cell C- 10 .

Received level measuring part 3 measures the received levels of the respective cells C 10 , C 11 -C 13 , C 21 -C 22 , based on the broadcast information M 1 received by broadcast information receiving part 2 . This results in measuring the received levels of all the cells to which mobile station 1 can reselect.

Cell class determining part 4 determines the cell class of serving cell C 10 with reference to after-described cell class table 41 (corresponding to the storing means). The cell class is information as an index for mobile station 1 to determine a cell in which it is preferentially camped, and a condition for a cell reselection of mobile station 1 becomes tight where the cell class is a priority cell class than where the cell class is a nonpriority cell class.

FIG. 4A is an illustration showing a configuration example of cell class table 41 . As shown in FIG. 4A , cell class table 41 has cell type region 41 a and cell class region 41 b . Cell type region 41 a stores as “cell type, ” information indicating that the serving cell of mobile station 1 is either an indoor cell or an outdoor cell. Cell class region 41 b stores as “cell class,” information indicating that a cell of each corresponding cell type is either the priority cell class or the nonpriority cell class. Since the cell classes are correlated in one-to-one correspondence with the cell types in this way, the determination on the cell class at mobile station 1 equals indirect determination on the cell type.

The initial setting of the cell classes at a start of mobile station 1 is such that the indoor cells are classified under the priority cell class and the outdoor cells under the nonpriority cell class, but the information can be updated according to necessity. The details will be described later, but it is also possible to adopt such a configuration that when the number of reselections between cells of different cell classes exceeds a predetermined threshold (e.g., 5 or so), mobile station 1 changes the cell classes in the cell class region 41 b , for example.

Target cell selecting part 5 selects a target cell of mobile station 1 , based on the received levels of the respective cells measured by received level measuring part 3 and the cell classes of the respective cells determined by cell class determining part 4 . The selection of the target cell herein includes the determination on the propriety of the cell reselection itself, and even in the case where the target cell is selected, the reselection of that cell does not always have to be executed.

Target cell selecting part 5 performs the selection of the target cell with reference to reselection hysteresis value table 51 . FIG. 5 is an illustration showing a configuration example of reselection hysteresis value table 51 . As shown in FIG. 5 , reselection hysteresis value table 51 stores ΔL( 1 ), ΔL( 2 ), and ΔL( 3 ) as first to third reselection hysteresis values. The above reselection hysteresis values of three steps are numerals satisfying the relation of ΔL( 1 )<ΔL( 2 )<ΔL( 3 ).

ΔL( 1 ) is used for comparison of received levels between the serving cell belonging to the nonpriority cell class and a neighboring cell belonging to the priority cell class. ΔL( 1 ) is, for example, approximately −3 dB.

ΔL( 2 ) is used for comparison of received levels between the serving cell and a neighboring cell, where the serving cell and neighboring cell both belong to the same cell class. ΔL( 2 ) is, for example, approximately 6 dB. It is a matter of course that ΔL( 2 ) may take on different values according to whether the serving cell belongs to the priority cell class or not. In this case, the reselection hysteresis values are set in totally four steps.

ΔL( 3 ) is used for comparison of received levels between the serving cell belonging to the priority cell class and a neighboring cell belonging to the nonpriority cell class. ΔL( 3 ) is, for example, approximately 9 dB.

Cell reselection part 6 executes the cell reselection where the target cell is selected by target cell selecting part 5 and where the reselection of that cell is determined. The cell reselection means a change of the base station as a source of the broadcast information received by mobile station 1 . Cell reselection part 6 outputs a reselection notification indicating completion of the cell reselection, to number-of-reselections counting part 7 . This reselection notification contains information indicating whether or not the cell reselection notified of is between cells belonging to different cell classes.

Number-of-reselections counting part 7 counts the number of reselections between cells belonging to different cell classes, according to the reselection notification from cell reselection part 6 . Number-of-reselections counting part 7 retains a cell reselection threshold Nth as a criterion for determination on changing of the cell classes and always monitors the magnitude relation between the number of reselections counted and the cell reselection threshold Nth. When the number of reselections becomes larger than the cell reselection threshold Nth, the number-of-reselections counting part 7 instructs cell class changing part 8 to perform changing of the cell classes.

When cell class changing part 8 is instructed to change the cell classes by number-of-reselections counting part 7 , it accesses cell class table 41 to change the relation between the cell types and the cell classes to another. Namely, cell class changing part 8 sets the nonpriority cell class in the cell class region 41 b in which the priority cell class has been set heretofore, and also sets the priority cell class in the cell class region 41 b in which the nonpriority cell class has been set heretofore. As a consequence, as shown in FIG. 4B , the cell classes are changed in the cell class table 41 .

The operation of the mobile communication system will be described below. In conjunction therewith, each of steps in the cell selection method according to the present invention will also be described. FIG. 6 is a flowchart for explaining the cell reselection determining process executed by mobile station 1 .

First, broadcast information receiving part 2 of mobile station 1 receives the broadcast information M 1 from base station B 10 of the serving cell (S 1 ).

Received level measuring part 3 measures each of the received level L 10 from base station B 10 and the received levels L 11 -L 13 , L 21 , L 22 from base stations B 11 -B 13 , B 21 , B 22 of the respective neighboring cells, with reference to the broadcast information M 1 received from base station B 10 (S 2 ).

Subsequently, cell class determining part 4 determines to which cell class, either the priority cell class or the nonpriority cell class, the serving cell C 10 belongs, with reference to cell class table 41 (S 3 ).

When the result of the determination is that the serving cell C 10 is an outdoor cell, since it belongs to the nonpriority cell class, target cell selecting part 5 selects a target cell on the basis of the result of the comparison between the received level of the serving cell and the received levels of neighboring cells belonging to the priority cell class. Specifically, target cell selecting part 5 selects the target cell, using conditions (3), (4) below.

max( Li (1))=max( L 11, L 12, L 13)  (3)

max( Li (1))> L 0+Δ L (1)  (4)

Here max(argument 1 , argument 2 , . . . ) represents a function that gives a maximum argument out of argument 1 , argument 2 , . . . Moreover, L 0 indicates the received level of the serving cell and ΔL( 1 ) the first reselection hysteresis value as described above. In addition, Li( 1 ) indicates a received level of a neighboring cell belonging to the priority cell class.

When condition (4) becomes true (S 4 ; Yes), target cell selecting part 5 selects a neighboring cell corresponding to max(Li( 1 )) as a target cell and instructs cell reselection part 6 to implement the reselection of the cell. In the next process, the cell reselection part 6 thus instructed to perform the cell reselection makes mobile station 1 reselect the cell selected at S 4 (S 6 ). After completion of the reselection, the sequential reselection determining process ends.

In contrast to it, when condition (4) does not become true (S 4 ; No), the flow moves to the process of S 5 .

At S 5 , target cell selecting part 5 compares the received level of the serving cell with the received levels of the neighboring cells belonging to the same nonpriority cell class as the serving cell, and selects the target cell on the basis of the result of the comparison. Namely, target cell selecting part 5 selects the target cell, using condition s (5), (6) below.

max( Li (2))=max( L 21, L 22)  (5)

max( Li (2))> L 0+Δ L (2)  (6)

Here max(argument 1 , argument 2 , . . . ) represents a function that gives the maximum argument out of argument 1 , argument 2 , . . . Moreover, L 0 indicates the received level of the serving cell and ΔL( 2 ) the second reselection hysteresis value as described above. Furthermore, Li( 2 ) indicates a received level of a neighboring cell belonging to the nonpriority cell class.

When the result of the determination is that condition (6) becomes true (S 5 ; Yes), target cell selecting part 5 selects a neighboring cell corresponding to max(Li( 2 )) as a target cell and instructs cell reselection part 6 to make a reselection of the cell. At S 6 , cell reselection part 6 instructed to implement the cell reselection makes mobile station 1 reselect the cell selected at S 5 . After completion of the reselection, the sequential reselection determining process ends.

In contrast to it, when condition (6) does not become true (S 5 ; No), the cell reselection determining process is terminated without execution of the cell reselection.

The following will describe the processing executed when the result of the determination process at S 3 is that the serving cell belongs to the priority cell class. Target cell selecting part 5 compares the received level of the serving cell with the received levels of the neighboring cells belonging to the same priority cell class as the serving cell, and selects a target cell on the basis of the result of the comparison.

Namely, target cell selecting part 5 selects the target cell, using conditions (7), (8) below.

max( Li (1))=max( L 11, L 12, L 13)  (7)

max( Li (1))> L 0+Δ L (2)  (8)

When condition (8) becomes true (S 7 ; Yes), target cell selecting part 5 selects a neighboring cell corresponding to max(Li( 1 )) as a target cell, and instructs cell reselection part 6 to implement a reselection of the cell. Subsequently, cell reselection part 6 instructed to perform the cell reselection makes mobile station 1 reselect the cell selected at S 7 (S 6 ). After completion of the reselection, the sequential reselection determining process ends.

In contrast to it, when condition (8) does not become true (S 7 ; No), the flow moves to the process of S 8 .

At S 8 , target cell selecting part 5 compares the received level of the serving cell with the received levels of the neighboring cells belonging to the nonpriority cell class, and selects a target cell on the basis of the result of the comparison. Namely, target cell selecting part 5 selects the target cell, using conditions (9), (10) below.

max( Li (2))=max( L 21, L 22)  (9)

max( Li (2))> L 0+Δ L (3)  (10)

As described previously, ΔL( 3 ) indicates the third reselection hysteresis value.

When the result of the above determination is that condition ( 10 ) becomes true (S 8 ; Yes), target cell selecting part 5 selects a neighboring cell corresponding to max(Li( 2 )) as a target cell, and instructs cell reselection part 6 to implement a reselection of the cell. Next, cell reselection part 6 instructed to perform the cell reselection makes mobile station 1 reselect the cell selected at S 8 (S 6 ). After completion of the reselection, the sequential reselection determining process ends.

In contrast to it, when condition (10) does not become true (S 8 ; No), the cell reselection determining process is terminated without execution of the cell reselection.

In mobile communication system 100 , as described above, mobile station 1 executes the cell reselection determining process, thereby adopting the different cell reselection conditions between where the serving cell belongs to the nonpriority cell class and where the serving cell belongs to the priority cell class. Specifically, where the serving cell belongs to the nonpriority cell class, mobile station 1 is camped on an outdoor cell, and thus the lower reselection hysteresis value is used than where the serving cell belongs to the priority cell class. This looses the reselection condition from the outdoor cell and increases the percentage of mobile station 1 leaving the outdoor cell. In other words, where the serving cell belongs to the priority cell class, mobile station 1 is camped on an indoor cell, the higher reselection hysteresis value is used than where the serving cell belongs to the nonpriority cell class. This makes the reselection condition from the indoor cell tight and increases the percentage of mobile station 1 staying in the indoor cell.

Independent of to which cell class the serving cell belongs, mobile station 1 employs the different cell reselection conditions for the neighboring cells belonging to the priority cell class and for the neighboring cells belonging to the nonpriority cell class. Specifically, since a neighboring cell belonging to the priority cell class is an indoor cell, mobile station 1 uses the lower reselection hysteresis value in the selection of the target cell in that case than in the case of a neighboring cell belonging to the nonpriority cell class. This looses the reselection condition to the indoor cell and increases the percentage of mobile station 1 reselecting the indoor cell. In other words, since a neighboring cell belonging to the nonpriority cell class is an outdoor cell, mobile station 1 uses the higher reselection hysteresis value in that case than in the case of a neighboring cell belonging to the priority cell class. This makes the reselection condition to the outdoor cell tight and keeps low the percentage of mobile station 1 reselecting the outdoor cell.

Namely, mobile station 1 adaptively changes the target cell, depending upon the surrounding environment including the serving cell. This results in selecting a neighboring cell with higher communication stability as a target cell, whereby mobile station 1 can perform stable communication with the base station. As a result, mobile station 1 becomes able to perform low-power and high-speed communication.

The above cell reselection determining process was described on the assumption that the cell classes were only two types, the priority and nonpriority, and that the cell types were only two types, the indoor cells and outdoor cells. However, the target cell selection technology according to the present invention is also applicable to cases where the cell classes and cell types each are M species (M is an integer not less than 3); for example, a case where the cell classes are three species, priority 1 , priority 2 , and priority 3 and where the cell types are three species, nanocells, microcells, and macrocells. FIG. 7 is a flowchart showing the cell reselection determining process as a generalized example including such cases. The present cell reselection determining process includes steps common to the cell reselection determining process which was described with reference to FIG. 6 , and thus the corresponding steps will be denoted by the same series of step numbers (with the same last numbers), without redundant description of the same steps. Specifically, S 11 , S 12 , and S 16 in FIG. 7 are equivalent to S 1 , S 2 , and S 6 , respectively, shown in FIG. 6 .

The steps specific to the present example will be described below.

After completion of the processes of S 11 and S 12 , cell class determining part 4 makes reference to the cell class table (not shown) in which M cell types are correlated with M cell classes, and identifies a cell class k (k is an integer from 1 to M) to which the serving cell belongs, based on the cell type included in the broadcast information (S 13 ).

The class k of the serving cell is an integer not less than 1 nor more than the number M of cell classes and cell types. It is assumed herein that value 1 of k indicates the highest priority.

At S 14 , cell class determining part 4 substitutes “1” as an initial value into counter n (n is a natural number) for identifying a cell class of a neighboring cell.

At S 15 , target cell selecting part 5 performs a selection of a target cell on the basis of the received level of the serving cell and the received levels of neighboring cells. Specifically, target cell selecting part 5 selects the target cell, using conditions (11), (12) below.

max( Li ( n ))=max( Ln 1, Ln 2, . . . , Lnm )  (11)

max( Li ( n ))> L 0+Δ L ( nk )  (12)

Here Ln 1 -Lnm are cells neighbor to the serving cell C 10 , and indicate the received levels of the cells Cn 1 -Cnm the cell type of which belongs to the cell class n. The received levels used herein are the values measured at S 12 . Furthermore, max(Li(n)) is a function that gives the maximum received level value out of Ln 1 -Lnm. L 0 indicates the received level of the serving cell C 10 , and ΔL(nk) indicates a reselection hysteresis value used in the level comparison between the received level L 0 of the serving cell and the received levels Ln 1 -Lnm of the neighboring cells belonging to the cell class n, where the serving cell belongs to the cell class k. For example, when the number of cell classes is M, there exist the square of M of reselection hysteresis values like ΔL( 11 ), ΔL( 12 ), . . . , ΔL( 1 M), ΔL( 21 ), . . . , ΔL( 2 M), . . . , ΔL(MM).

For example, in the case of M=3,

ΔL(MM)=−3, 3, 6; 3, 6, 9; 6, 9, 12.

There are the nine reselection hysteresis values, and the reselection hysteresis values are set in totally (2M−1) steps. It is a matter of course that the reselection hysteresis values can be set in an arbitrary number of steps according to the configuration of cells or the like.

If cell class 1 is a cell class with the highest priority and if the following relations exist among the reselection hysteresis values:

Δ(1 k )<Δ(2 k )< . . . <Δ( Mk )  (13)

Δ( n 1)<Δ( n 2)< . . . <Δ( nM )  (14),

the reselection condition is loosed more to the cell belonging to the cell class with the higher priority, and the reselection condition is made tight to the cell belonging to the cell class with the lower priority.

For example, in the case of k=2 and n=1, the conditions equivalent to aforementioned conditions (3), (4) are used and the process equivalent to S 4 in FIG. 6 is executed.

At S 15 , when condition (12) becomes true (S 15 ; Yes), target cell selecting part 5 selects a neighboring cell corresponding to max(Li(n)) as a target cell, and instructs cell reselection part 6 to implement a reselection of the cell. In the next process, cell reselection part 6 instructed to perform the cell reselection makes mobile station 1 reselect the neighboring cell selected at S 15 (S 16 ). After completion of the reselection, the sequential reselection determining process is terminated.

In contrast to it, when condition (12) does not become true (S 15 ; No), the flow proceeds to the process of S 17 described below.

At S 17 , cell class determining part 4 determines whether n=M, i.e., whether the counter n for distinguishing the cell class of the neighboring cells used in the reselection determination reaches the number M of cell classes. When the result of the determination is that n reaches M (S 17 ; Yes), mobile station 1 terminates the sequential reselection determination process without execution of the cell reselection from the serving cell C 10 .

On the other hand, when the result of the determination at S 17 is that n is still below M, i.e., n<M (S 17 ; No), the flow shifts to S 18 . At S 18 , cell class determining part 4 adds 1 to the current counter n, to set n=n+1. After completion of this process, the flow returns to S 15 and the processes at and after S 15 are again executed.

Namely, since the previous process of S 15 resulted in selecting no target cell from the neighboring cells Cn 1 -Cnm belonging to the cell class n, S 15 at this time is carried out again to attempt to select a target cell out of neighboring cells C(n+1)1−C(n+1)m belonging to the cell class (n+1) with the priority lower than that of the cell class n.

As the sequential processes of S 15 -S 18 are repeatedly executed, the reselection hysteresis value ΔL(nk) used for the selection of the target cell by mobile station 1 is successively updated in the named order of ΔL( 1 k), ΔL( 2 k), . . . , ΔL(Mk) until the counter n for distinguishing the cell class of the neighboring cell reaches the number M of cell classes and cell types. Even if the counter n is equal, the reselection hysteresis value ΔL(nk) is set at values different depending upon the cell class k to which the serving cell C 10 belongs. Therefore, mobile station 1 can properly change the cell reselection condition according to the cell class k of the serving cell C 10 . As described previously, the cell classes k are set based on the cell types. Accordingly, in the case where the cell classes are classified in three or more species as in the present example, it also becomes feasible to change the cell reselection condition according to the cell type of the serving cell C 10 .

In the cell reselection determination process described with reference to FIG. 6 , mobile station 1 was configured to implement the cell reselection with the higher priority on the indoor cells belonging to the priority cell class than on the outdoor cells belonging to the nonpriority cell class, and the cell reselection determination process of this type involves the concern that frequent cell reselections occur between cells belonging to different cell classes (for example, between indoor cells and outdoor cells). Such concern becomes prominent, particularly, in the case where mobile station 1 moves at high speed in the vicinity of the marginal territory of the serving cell, for example. Let us consider a situation in which the frequency of cell reselections becomes excessively high in the case where the indoor cells are classified under the priority cell class and the outdoor cells under the nonpriority cell class, and a solution thereto will be described below.

FIG. 8 is an illustration schematically showing a configuration of a mobile communication system wherein mobile station 1 is camped on a cell different from that in the above example. As shown in FIG. 8 , base station B 31 establishes an outdoor cell C 31 , and base stations B 41 , B 42 , B 43 their respective indoor cells C 41 , C 42 , C 43 . In FIG. 8 , the indoor cells are represented by dashed lines, and the outdoor cell by a chain line. In the present embodiment, mobile station 1 is camped on at least the outdoor cell C 31 , and near the marginal territory of the outdoor cell C 31 , it moves back and forth between location A and location B through a path indicated by solid arrows Y 1 .

FIG. 9 shows a state in which the received levels from the respective base stations vary according to locations of mobile station 1 under the above situation. In FIG. 9 , the locations of mobile station 1 are defined on the horizontal axis, and the received levels from the base stations B 31 , B 41 -B 43 according to the locations of mobile station 1 on the vertical axis. The solid lines indicated by L 31 , L 41 -L 43 show respective states in which the received levels from the base stations B 31 , B 41 -B 43 vary with displacement of mobile station 1 . Mobile station 1 always stays in the outdoor cell C 31 , while entering and leaving each of the indoor cells C 41 -C 43 . Therefore, the range of variation of the received level L 31 is small, and those of L 41 -L 43 are large.

Therefore, if mobile station 1 executes the foregoing cell reselection determination process under the condition that the indoor cells belong to the priority cell class and the outdoor cell to the nonpriority cell class, cell reselections will be carried out at a total of six positions, P 1 -P 6 as shown in FIGS. 8 and 9 . Such highly frequent reselections can induce degradation of quality of links and increase in the control load on the system and possibly impede stable communication between the mobile station and the base station. It is thus desirable to restrain the frequency of cell reselections by some means according to necessity.

With reference to FIG. 9 , it is anticipated that cell reselections will occur because of the frequent change in the magnitude relation between the received level L 31 and the received levels L 41 -L 43 . For this reason, an effective means for suppressing the frequent cell reselections is to increase the threshold of the received level L 31 as a criterion to determine the propriety of cell reselections. By defining the received level of L 31 +ΔL( 3 ) as a threshold as indicated by the dashed line, the threshold always becomes greater than the received levels L 41 -L 43 , regardless of the rapid variation of the received levels L 41 -L 43 . In this case, no cell reselection occurs during the process in which mobile station 1 moves between the locations A, B.

In order to increase the received level as the above threshold, a conceivable method is, for example, a technique of changing the cell classes. Namely, the indoor cells are classified under the nonpriority cell class and the outdoor cells under the priority cell class, as in the cell class table 41 shown in FIG. 4B . This results in changing the reselection hysteresis value, which is used in the determination on the reselection from cell C 31 into one of cells C 41 -C 43 , from ΔL( 1 ) to ΔL( 3 ). Since ΔL( 3 )>ΔL( 1 ) as described previously, L 31 +ΔL( 3 ) becomes larger than L 31 +ΔL( 1 ), so as to decrease the frequency of cell reselections.

The cell class changing process executed by mobile station 1 will be described below as an example of the processing for restraining the highly frequent cell reselections, with reference to FIG. 10 .

The operation will be described on the premise that the cell class changing process is a process independent of the aforementioned cell reselection determination process and can also be executed in parallel with the cell reselection determination process, of course. The cell class changing process is initiated at the timing of a start of mobile station 1 .

First, number-of-reselections counting part 7 substitutes “0” as an initial value into the number N of cell reselections of initialize and retain the number of cell reselections (T 1 ).

Number-of-reselections counting part 7 always monitors input of a reselection notification. When detecting input of a reselection notification from cell reselection part 6 , it determines whether mobile station 1 has reselected between cells belonging to different cell classes, based on the reselection notification (T 2 ).

When the result of the determination is that the cell reselection was carried out (T 2 ; Yes), number-of-reselections counting part 7 adds an increment of “1” to the number of cell reselections, to set N=N+1 (T 3 ).

At T 4 , number-of-reselections counting part 7 compares the number N of cell reselections at the present time with the reselection threshold Nth to determine the magnitude relation. When the result of the comparison satisfies the relation of N>Nth (T 4 ; Yes), the flow shifts to T 5 and cell class changing part 8 changes the cell classes presently stored in cell class region 41 b . Specifically, the nonpriority cell class is set as a cell class corresponding to the cell type of indoor cells, and the priority cell class as a cell class corresponding to the cell type of outdoor cells.

On the other hand, when the result of the comparison at T 4 does not satisfy the relation of N>Nth (T 4 ; No), it is determined that the frequency of cell reselections is still not too high to require the changing of cell classes, and the flow returns to T 2 . Then number-of-reselections counting part 7 repeatedly executes the processes of T 2 -T 4 before N exceeds Nth.

When the number N of cell reselections exceeds the reselection threshold Nth and when the changing of the cell classes at T 5 is completed, the flow moves to T 1 to again initialize the number N of cell reselections and then repeatedly execute the processes at and after T 1 .

The changing operation of the cell classes to which the respective cells C 31 , C 41 -C 43 belong, results in making the serving cell C 31 of mobile station 1 belong to the priority cell class and making the neighboring cells C 41 -C 43 to mobile station 1 belong to the nonpriority cell class. Therefore, the cell reselection determination process is executed using ΔL( 3 ) as a reselection hysteresis value in the comparison of the received levels at mobile station 1 in FIG. 8 (cf. S 8 in FIG. 6 ). If the change of the cell classes is not carried out, ΔL( 1 ) will be continuously used. Since ΔL( 3 )>ΔL( 1 ), the change of the cell classes makes the cell reselection condition tight, and decreases the frequency of cell reselections due to the temporal variation of the received levels of the neighboring cells C 41 -C 43 .

In the mobile communication system 100 , as described above, mobile station 1 is preferably configured to count the number of changes in the cell class of the serving cell due to cell reselections on a historical basis and change the relation between the cell types and the cell classes when it determines that reselections between cells of different cell classes are too frequent. By autonomously changing the classifications of the cell classes according to the activity and the surrounding environment of the mobile station 1 in this way, it becomes feasible to increase the received level threshold as a criterion to determine the propriety of cell reselections. This restrains the frequency of cell reselections. As a consequence, it becomes feasible to decrease or suppress the discontinuation of data transmission due to the cell reselections and thus increase the throughput of the system, while decreasing the control load due to the cell reselections.

It is noted that the modes described in the present embodiment are just the preferred examples of the mobile communication system according to the present invention and that the present invention is by no means intended to be limited to the above modes.

For example, a potential configuration is such that a timer T 01 (not shown) is started at the same time as the initialization of the number of cell reselections (cf. T 1 in FIG. 10 ) and when the elapsed time indicated by the timer T 01 reaches a predetermined time (e.g., about ten minutes), the reselection counter N is initialized, regardless of the value of the reselection counter N at that point. This process can limit the condition for the change of the cell classes to cases where the number of cell reselections between cells belonging to different cell classes exceeds the threshold Nth within the above predetermined time. Accordingly, it becomes feasible to execute the cell reselection determination based on the frequency of cell reselections calculated more precisely. Such condition setting is effective particularly in application to cases where mobile station 1 moves back and forth in the same path at high speed.

A further potential configuration is such that a timer T 02 (not shown) is started at a point where the relation between the cell types and the cell classes is changed from the initial setting (cf. T 5 in FIG. 10 ) and if the relation different from the initial setting is continued until the elapsed time of the timer T 02 reaches a predetermined time (e.g., about five minutes), the relation is forcedly brought back to the initial setting. This configuration permits the relation of the initial setting to be used again for the determination on cell reselections in the case where the cell classes are once changed and thereafter the frequency of cell reselections becomes lower than before. As a consequence, it is feasible to prevent the relation different from the initial setting from being continuously used even during periods in which the degree of necessity for use of the relation different from the initial setting is low.

Second Embodiment

Subsequently, the second embodiment of the present invention will be described with reference to FIGS. 11 to 14 . Since the mobile communication system in the second embodiment includes the components and processes common to those in the mobile communication system in the first embodiment, only differences from the first embodiment described above will be detailed below.

Specifically, the cell selection technology in the first embodiment involved no consideration to existence of mobile cells as one of the cell forms. A mobile cell is constructed by a moving object such as a train, a bus, or the like, and is a cell having a mechanism capable of maintaining communication between a mobile station in the cell and the mobile communication system while keeping the configuration even with movement in the mobile communication system. A mobile base station communicates through a radio link with external base stations (outdoor cells and indoor cells) and a mobile station in the mobile cell is connected through a relay node installed on the mobile cell, to the mobile communication system.

This configuration of making the mobile station connected through the use of the mobile cell to an external base station without being directly connected thereto, has the advantage of achieving low power consumption and stabilize communication quality. It is thus contemplated that where the serving cell or the neighboring cell is a mobile cell, it is necessary to adopt another cell selection method different from that for the indoor and outdoor cells in the first embodiment, from the characteristics of the mobile cells described above. The mobile communication system in the second embodiment implements it.

The detailed system configuration will be described later, but the principal features of the present embodiment are to change the cell reselection condition according to the types of the serving cell of the mobile station and a neighboring cell thereto and to change selection priorities among the cell types according to the cell type of the serving cell. Namely, the present mobile communication system variably determines the cell reselection condition, depending upon whether the serving cell is either an indoor cell, an outdoor cell, or a mobile cell. The cell reselection condition is also variably determined depending upon whether the neighboring cell is either an indoor cell, an outdoor cell, or a mobile cell. Furthermore, the present system determines for which cell type a process for a determination on a reselection of a neighboring cell should be preferentially executed, according to the type of the serving cell of the mobile station.

FIG. 11 is a conceptual diagram showing a configuration of the mobile communication system in the second embodiment. As shown in FIG. 11 , mobile communication system 200 is comprised of mobile station 11 , base stations B 51 , B 52 , base stations B 61 -B 63 , and mobile base stations B 71 -B 72 . The outdoor cells are those C 51 , C 52 established by the respective outdoor base stations B 51 , B 52 , and the indoor cells are those C 61 -C 63 established by the respective indoor base stations B 61 -B 63 .

The mobile base stations B 71 , B 72 are provided with their respective radio devices M 71 , M 72 for connection to the external base stations B 51 , B 52 , B 61 -B 63 . The mobile base stations B 71 , B 72 are also provided with their respective radio devices W 71 , W 72 for establishing the mobile cells C 71 , C 72 . In FIG. 11 , the mobile station 11 and mobile base station B 71 are camped on the outdoor cell C 51 established by base station B 51 .

It is assumed in the present embodiment that the mobile base stations B 71 , B 72 are trains and that routes R 1 , R 2 are railroads. Mobile base station B 71 moves on the route R 1 and mobile base station B 72 on the route R 2 . Base station B 51 notifies mobile station 11 of information (identification information) enabling identification of each cell, either an indoor cell, an outdoor cell, or a mobile cell (corresponding to a cell type), in addition to information indicating that the outdoor cell C 52 , indoor cells C 61 , C 62 , and mobile cell C 71 exist as neighboring cells.

Namely, base station B 51 notifies mobile station 11 of broadcast information D 1 containing the identification information indicating the cell type of its own cell C 51 , the identification information indicating that the neighboring cell C 52 is an outdoor cell, the identification information indicating that the neighboring cells C 61 , C 62 are indoor cells, and the identification information indicating that the neighboring cell C 71 is a mobile cell.

The neighboring cells herein include both cells overlapping in part with a cell in question and cells overlapping completely with a cell in question. Namely, in FIG. 11 , the neighboring cells to outdoor cell C 51 encompass cells C 62 , C 71 as well as cells C 52 , C 61 . The neighboring cells to indoor cell C 62 encompass cell C 51 as well as cell C 61 .

The system may be modified so that base station B 51 notifies mobile station 11 of the identification information indicating whether its own cell C 51 is either an indoor cell, an outdoor cell, or a mobile cell and the base stations forming the respective cells notify the mobile station of the identification information indicating the cell types of the respective neighboring cells C 52 , C 61 , C 62 , C 71 .

The mobile station, which is a principal component of the mobile communication system in the present embodiment, will be detailed below. FIG. 12 is a block diagram showing a functional configuration of mobile station 11 . As shown in FIG. 12 , mobile station 11 is provided with at least detected channel choosing part 12 (corresponding to the choosing means), channel detecting part 13 , broadcast information receiving part 14 (corresponding to the determining means), received level measuring part 15 (corresponding to the measuring means), reselection condition selecting part 16 , target cell selecting part 17 (corresponding to the selecting means), and cell reselection part 18 . Channel detecting part 13 internally has channel information storage 131 (corresponding to the storing means).

Broadcast information receiving part 14 receives the broadcast information D 1 via a radio channel and channel detecting part 13 from base station B 51 establishing the serving cell C 51 of mobile station 11 . The broadcast information D 1 contains the identification information indicating the cell type of cell C 51 , of course, and also contains the identification information indicating the cell types of cells C 52 , C 61 , C 62 , C 71 neighbor to the cell C 51 . Broadcast information receiving part 14 determines the cell types of the respective cells, based on these identification information. Broadcast information receiving part 14 may be configured to determine the cell types of cells C 51 , C 52 , C 61 , C 62 , C 71 on the basis of information preliminarily stored in channel information storage 131 , without depending upon the broadcast information D 1 .

Channel information storage 131 of channel detecting part 13 stores information about a plurality of radio channels. Channel detecting part 13 detects a channel on the basis of the information stored in channel information storage 131 . In a potential configuration, detected channel choosing part 12 chooses channels (detected channels) as targets for measurement of the received level and notifies channel detecting part 13 of there result thereof. This enables channel detecting part 13 to detect only one channel or a limited number of channels.

Received level measuring part 15 measures the received levels of the respective cells C 51 , C 61 , C 62 , C 73 , which were detected by channel detecting part 13 . This permits mobile station 11 to measure the received levels about all the cells to which the mobile station itself can reselect.

Reselection condition selecting part 16 selects a reselection condition on the basis of the cell types of the serving cell C 51 and neighboring cells C 61 , C 62 , C 73 notified of by broadcast information receiving part 14 , and notifies target cell selecting part 17 of the reselection condition thus selected.

Target cell selecting part 17 selects a target cell optimal for mobile station 11 , based on the received levels of the respective cells measured by received level measuring part 15 and the reselection condition selected by reselection condition selecting part 16 . The selection of the target cell herein includes in the determination on the propriety of the cell reselection itself and, even in the case where the target cell is selected, the reselection of the selected cell does not always have to be executed.

An example of the relation between cell types and cell reselection conditions in the present embodiment is presented in FIG. 13 . In FIG. 13 , solid arrows E 11 -E 33 represent reselection conditions from the serving cell to a neighboring cell. A starting point of each arrow indicates a type of the serving cell, and an end point of each arrow a type of a neighboring cell. The reselection conditions E 11 -E 33 are held in reselection condition selecting part 16 , and a reselection condition is selected based on the types of the serving cell and neighboring cells notified of by broadcast information receiving part 14 . Target cell selecting part 17 is notified of the reselection condition thus selected.

FIG. 14 is a diagram showing the detailed contents of the reselection conditions according to the cell types, selection priorities among cell types, and relations among them. As shown in FIG. 14 , there selection conditions E 11 -E 32 are correlated with three types of reselection conditions, that is, the reselection conditions E 21 , E 31 , E 32 are set to be a condition that “when the received level of the serving cell becomes below a predetermined threshold, a neighboring cell with the highest received level is selected as a target cell out of neighboring cells satisfying a predetermined received level.” The reselection conditions E 11 , E 22 are set to be a condition that “a neighboring cell a difference of the received level of which from that of the serving cell exceeds a predetermined hysteresis and which has the highest received level is selected as a target cell.” Furthermore, the reselection conditions E 12 , E 13 , E 23 are set to be a condition that “a neighboring cell the received level of which is continuously kept high for a predetermined time is selected as a target cell, regardless of the received level of the serving cell.”

The reselection condition of E 33 is set to be a condition that “no cell reselection is carried out between mobile cells.” This is implemented by a first or second technique described below. The first technique is a method in which where mobile station 11 is camped on mobile cell C 71 in FIG. 11 and even if the mobile cell C 72 becomes a neighboring cell to mobile cell C 71 because of movement of mobile cell C 72 into the range of outdoor cell C 51 , the radio device W 71 forming the mobile cell C 71 does not notify mobile station 11 that the mobile cell C 72 is a neighboring cell. The second technique is a method in which mobile station 11 camped on mobile cell C 71 does not select the mobile cell C 72 as a cell of a channel detection target.

Subsequently, the following will describe the reasons why mobile station 11 becomes able to select a cell optimal for communication, by the setting of the above reselection condition according to the cell type.

(1) The reason why, in the case of the serving cell being an outdoor cell and the neighboring cell being an outdoor cell, the reselection condition E 11 is set to be “the reselection condition that the target cell is defined as a neighboring cell the difference of the received level of which from that of the serving cell exceeds the predetermined hysteresis and which has the highest received level” is as follows. In this case, the cell type of the neighboring cell is the same as that of the serving cell, which is nothing different from the form of the conventional mobile communication systems. Therefore, the present embodiment can also simply apply the cell reselection condition commonly used in the conventional mobile communication systems. Namely, target cell selecting part 17 of mobile station 11 selects as a target cell a cell with the highest received level out of outdoor cells the received level difference of which from that of the current outdoor cell exceeds the predetermined hysteresis (e.g., 6 dB) and which are neighbor to the current outdoor cell.

(2) The reason why, in the case of the serving cell being an outdoor cell and the neighboring cell being an indoor cell, the reselection condition E 12 is set to be “the reselection condition that the target cell is defined as a neighboring cell the received level of which is kept high for the fixed time, regardless of the received level of the serving cell” is as follows. Namely, since a base station of an indoor cell has a smaller transmission power than a base station of an outdoor cell, the cell radius of the indoor cell is expected to be smaller than that of the outdoor cell. In addition, the indoor cell provides high-speed communication than the outdoor cell. For this reason, regardless of the quality of the current outdoor cell, faster communication can be expected by performing the reselection of the indoor cell and communication therein as long as the quality of the indoor cell is kept at or above the fixed value. It is, however, probable that mobile station 11 reselecting while camping in the outdoor cell detects a neighboring cell with a high received level but passes the range of the neighboring cell in moment. Therefore, target cell selecting part 17 of mobile station 11 selects as a target cell a neighboring indoor cell the received level of which is kept high (e.g., the signal to interference noise ratio of not less than 3 dB) for the predetermined time (e.g., 5 seconds), regardless of the level difference between the current outdoor cell and the neighboring indoor cell. This permits mobile station 11 to reselect the indoor cell only if it stays in the range of the indoor cell for the fixed time, whereby it is feasible to prevent the mobile station from frequently reselection between the outdoor cell and the indoor cell. As a consequence, it is feasible to prevent the discontinuation of data transmission due to highly frequent cell reselections and thus prevent the decrease of data transmission rates.

(3) The reason why, in the case of the serving cell being an outdoor cell and the neighboring cell being a mobile cell, the reselection condition E 13 is set to be “the reselection condition that the target cell is defined as a neighboring cell the received level of which is kept high for the fixed time, regardless of the received level of the serving cell” is as follows. First, a mobile cell is not always fixed at a specific site, in view of the property thereof. It is, therefore, probable that while mobile station 11 is stationary in a current outdoor or indoor cell, a mobile cell can pass in its neighborhood so that the received level of the mobile cell becomes instantaneously higher than that of the serving cell. The level difference between the outdoor cell and the mobile cell at this time differs depending upon the location of mobile station 11 . A conceivable situation suitable for the reselection of mobile station 11 to the mobile cell is, for example, that a man (mobile station 11 ) waits for a train (mobile cell) on the platform, the desired train arrives at the platform, and the man steps onto the train. Then target cell selecting part 17 of mobile station 11 selects as a target cell the mobile cell the received level of which is kept high (e.g., the signal to interference noise ratio of not less than 3 dB) for the predetermined time (e.g., 10 seconds), regardless of the level difference between the current outdoor cell and the neighboring mobile cell. This causes mobile station 11 to reselect the mobile cell only if it continuously stays in the mobile cell during a period of not less than the predetermined time. It is thus feasible to prevent mobile station 11 from reselecting the mobile cell merely passing by mobile station 11 . As a result, it is feasible to prevent the discontinuation of data transmission due to highly frequent cell reselections and prevent the decrease of data transmission rates.

(4) The reason why, in the case of the serving cell being an indoor cell and the neighboring cell being an outdoor cell, the reselection condition E 21 is set to be “the reselection condition that the target cell is defined as a neighboring cell with the highest received level out of neighboring cells satisfying the predetermined received level, when the received level of the serving cell becomes below the predetermined value” is as follows. Namely, because the radius of the indoor cell is smaller than that of the outdoor cell and because the indoor environment includes less disturbance factors than the outdoor environment, it is considered that good communication quality can be stabilized in the indoor cell. Therefore, target cell selecting part 17 of mobile station 11 selects as a target cell a cell with the highest received level out of neighboring outdoor cells satisfying the predetermined received level (e.g., the signal to interference noise ratio of 5 dB) only when the received level of the current indoor cell becomes below the predetermined threshold (e.g., the signal to interference noise ratio of 3 dB). By setting the tight reselection condition than the after-described reselection condition E 22 (in the case of the serving cell and neighboring cell both being indoor cells) in this way, it becomes feasible to prevent frequent reselections from the indoor cell to the outdoor cell.

(5) The reason why, in the case of the serving cell being an indoor cell and the neighboring cell being an indoor cell, the reselection condition E 22 is set to be “the reselection condition that the target cell is defined as a neighboring cell the difference of the received level of which from that of the serving cell exceeds the predetermined hysteresis and which has the highest received level” is as follows. In this case, the cell type of the neighboring cell is the same as that of the serving cell, which is nothing different from the form of the conventional mobile communication systems. Therefore, the present embodiment can also suitably apply the cell reselection condition commonly used in the conventional mobile communication systems. Namely, target cell selecting part 17 of mobile station 11 selects as a target cell a cell with the highest received level out of indoor cells the received level difference of which from that of the current indoor cell exceeds the predetermined hysteresis (e.g., 6 dB) and which are neighbor to the current indoor cell.

(6) The reason why, in the case of the serving cell being an indoor cell and the neighboring cell being a mobile cell, the reselection condition E 23 is set to be “the reselection condition that the target cell is defined as a neighboring cell the received level of which is kept high for the fixed time, regardless of the received level of the serving cell” is as follows. First, a mobile cell is not always fixed at a specific site, in view of the property thereof. It is, therefore, probable that while mobile station 11 is stationary in a current outdoor or indoor cell, a mobile cell can pass in its neighborhood so that the received level of the mobile cell becomes instantaneously higher than that of the serving cell. The level difference between the indoor cell and the mobile cell at this time differs depending upon the location of mobile station 11 . A conceivable situation suitable for the reselection of mobile station 11 to the mobile cell is, for example, that a man (mobile station 11 ) waits for a train (mobile cell) on the platform (e.g., where an outdoor base station cannot establish a cell as on the subway platform, the platform is in a cell range of an indoor cell), the desired train arrives at the platform, and the man steps onto the train. Then target cell selecting part 17 of mobile station 11 selects as a target cell the mobile cell the received level of which is kept high (e.g., the signal to interference noise ratio of not less than 3 dB) for the predetermined time (e.g., 10 seconds), regardless of the level difference between the current indoor cell and the neighboring mobile cell. This permits mobile station 11 to reselect the mobile cell only if it stays in the mobile cell during a period of not less than the predetermined time. It is thus feasible to prevent mobile station 11 from reselecting the mobile cell merely passing by mobile station 11 . As a result, it is feasible to prevent the discontinuation of data transmission due to excessively high frequency of cell reselections and, besides, prevent the decrease of data transmission rates.

(7) The reason why, in the case of the serving cell being a mobile cell and the neighboring cell being an outdoor cell, the reselection condition E 31 is set to be “the reselection condition that the target cell is defined as a neighboring cell with the highest received level out of neighboring cells satisfying the predetermined received level, when the received level of the serving cell becomes below the predetermined first threshold” is as follows. Namely, a mobile cell is not always fixed at a specific site, in terms of the property thereof. For this reason, as the mobile cell moves, the received levels of signals from the neighboring cells received at mobile station 11 in the mobile cell vary with time. It is practically quite unlikely that mobile station 11 in the mobile cell moves at high speed inside the mobile cell, and it is expected that there is little change in the received level of the signal from the serving cell received at mobile station 11 in the mobile cell. It is, therefore, contemplated that the desirable timing of the cell reselection of mobile station 11 in the mobile cell is a point of degradation of the received level in the current mobile cell, i.e., a time of movement from the interior of the mobile cell to the outside of the mobile cell (e.g., when the man gets off the train). Therefore, target cell selecting part 17 of mobile station 11 selects as a target cell a cell with the highest received level out of neighboring outdoor cells satisfying the predetermined received level (e.g., the signal to interference noise ratio of 5 dB) only if the received level of the current mobile cell becomes below the predetermined threshold (e.g., the signal to interference noise ratio of 3 dB). This can prevent mobile station 11 in the mobile cell from reselecting neighboring cells even if the received levels of signals from neighboring indoor and outdoor cells vary with time during movement of the mobile cell.

(8) The reason why, in the case of the serving cell being a mobile cell and the neighboring cell being an indoor cell, the reselection condition E 32 is set to be “the reselection condition that the target cell is defined as a neighboring cell with the highest received level out of neighboring cells satisfying the predetermined received level, when the received level of the serving cell becomes below the predetermined first threshold” is as follows. Namely, a mobile cell is not always fixed at a specific site, in terms of the property thereof. For this reason, as the mobile cell moves, the received levels of signals from the neighboring cells received at mobile station 11 in the mobile cell vary with time. It is practically quite unlikely that mobile station 11 in the mobile cell moves at high speed inside the mobile cell, and it is expected that there is little change in the received level of the signal from the serving cell received at mobile station 11 in the mobile cell. It is, therefore, contemplated that the desirable timing of the cell reselection of mobile station 11 in the mobile cell is a point of degradation of the received level in the current mobile cell, i.e., a time of movement from the interior of the mobile cell to the outside of the mobile cell (e.g., when the man gets off the train). Therefore, target cell selecting part 17 of mobile station 11 selects as a target cell a cell with the highest received level out of neighboring indoor cells satisfying the predetermined received level (e.g., the signal to interference noise ratio of 5 dB) only if the received level of the current mobile cell becomes below the predetermined threshold (e.g., the signal to interference noise ratio of 3 dB). This can prevent mobile station 11 in the mobile cell from reselecting neighboring cells even if the received levels of signals from neighboring indoor and outdoor cells vary with time during movement of the mobile cell.

(9) The reason why, in the case of the serving cell being a mobile cell and the neighboring cell being a mobile cell, the reselection condition E 33 is set to be “no cell reselection is conducted between mobile cells” is as follows. The reason for the setting of the reselection condition E 33 is as follows. There can arise such a situation that mobile station 11 (man) moves from a mobile cell (train) to another mobile cell (train), for example, when the man changes the trains at the platform. However, a reselection of mobile station 11 between mobile cells passing each other is unlikely to occur, and it is thus undesirable to perform the cell reselection operation on the basis of the received level difference between mobile cells. Therefore, no cell reselection is conducted between mobile cells in such cases, as described above. This can be implemented by a method of keeping the mobile cell from announcing the information about the other mobile cell of the neighboring cell to mobile station 11 or a method of keeping mobile station 11 camped on the mobile cell from identifying another mobile cell as a target for detection of channel.

Furthermore, with reference to FIG. 14 , the priorities, which are ranks for execution of the cell reselection determination process, are different among types of neighboring cells. According to FIG. 14 , for example, where the serving cell is an outdoor cell, mobile station 11 first performs a determination on a reselection of an indoor cell with the priority of 1. When the determination on the reselection of the indoor cell results in finding no cell as a candidate, mobile station 11 executes the reselection determination process on a reselection of a mobile cell with the priority of 2. Furthermore, when the determination on the reselection of the mobile cell results in finding no cell as a candidate, mobile station 11 executes the reselection determination process on a reselection of a neighboring outdoor cell, as the last reselection determination process. In the case where the serving cell is an indoor cell, the cell reselection determination is also carried out similarly in the order of the priorities.

In the mobile communication system 200 of the second embodiment, as described above, mobile station 11 properly changes the cell reselection condition according to the types of the serving cell and neighboring cell and properly changes the priorities of the target neighboring cells according to the type of the serving cell. This permits the mobile station 11 to select a cell optimal for communication of its own, as a target cell.

The mode described in the present embodiment, particularly, the relation among the cell types, reselection conditions, and priorities, is just a preferred example of the mobile communication system according to the present invention, and it is noted that the present invention is by no means intended to be limited to the above mode.

For example, the above second embodiment was described based on the mode in which the same radio communication system was used in all the cells, but the cell selection technology according to the present invention is also applicable to cases where different radio communication systems are used depending upon cells. Namely, the cell selection technology according to the present invention is also applicable to a case where a radio communication system P (e.g., CDMA) is used in outdoor cells C 51 , C 52 and indoor cells C 61 , C 62 , C 63 and another radio communication system Q (e.g., OFDM) in mobile cells C 71 , C 72 .

A process specific to such a mode will be described below, again referring to FIGS. 11 and 12 . Base station B 51 transmits to mobile station 11 the information (identification information) enabling identification of each cell, either an indoor cell, an outdoor cell, or a mobile cell (corresponding to the cell type) and information about the physical layer (physical layer information) indicating which radio communication system is applied, in addition to the information that outdoor cell C 52 , indoor cells C 61 , C 62 , and mobile cell C 71 exist as neighboring cells.

Namely, base station B 51 notifies mobile station 11 of the broadcast information D 1 which collectively contains the identification information indicating the cell type of its own cell C 51 , the identification information indicating that the neighboring cell C 52 is an outdoor cell, the identification information indicating that the neighboring cells C 61 , C 62 are indoor cells, the identification information indicating that the neighboring cell C 71 is a mobile cell, the physical layer information indicating that the radio communication system used in the neighboring cells C 52 , C 61 , C 62 is the radio communication system P, and the physical layer information indicating that the radio communication system used in the neighboring cell C 71 is the radio communication system Q.

Mobile station 11 analyzes the broadcast information D 1 at broadcast information receiving part 14 to find out that the radio communication system used in the neighboring cells C 52 , C 61 , C 62 is the radio communication system P and that the radio communication system used in the neighboring cell C 71 is the radio communication system Q, and notifies the channel detecting part 13 and reselection condition selecting part 16 of the acknowledged information. Channel detecting part 13 detects the neighboring cells C 52 , C 61 , C 62 by the radio communication system P and detects the neighboring cell C 31 by the radio communication system Q.

On the other hand, reselection condition selecting part 16 selects an optimal cell reselection condition out of the cell reselection conditions E 11 -E 33 according to the above procedure, while taking into account the use of the radio communication system P in the outdoor cell C 52 and indoor cells C 61 , C 62 and the use of the radio communication system Q in the neighboring cell C 71 . This permits mobile station 11 to select a cell optimal for communication as a target cell even in the case where the different radio communication systems are used according to the cell types.

Here the indoor cells applicable encompass cells formed by compact transceivers. The cells formed by compact transceivers are expected to increase in the future in conjunction with multiplexing and layering of cells by application of microcells in the fourth generation technology, wireless LAN, home networks, active radio tag networks, and so on. This provides the prospect of increase of indoor cells. In this case, however, mobile station 11 can be configured to measure the received levels of only cells judged as necessary by detected channel choosing part 12 , without measuring the received levels of all the cells, and thus can efficiently perform the cell selecting process.

The second embodiment described above can also be modified in a modification form as described below. Namely, the reselection conditions E 13 , E 23 , E 31 , E 32 are set to “a reselection condition that a neighboring cell is selected as a target cell if a variation per unit time of the received level of the serving cell exceeds a predetermined value and if the received level of the neighboring cell is not less than a predetermined value.”

The following will describe the reason why the reselection conditions E 13 , E 23 , E 31 , E 32 are set to the above reselection condition in the modification.

For example, the user of the mobile station moves from the platform (an indoor cell or an outdoor cell) onto the train, and as the train starts moving, the received level of the indoor cell or outdoor cell where the mobile station is camped, varies rapidly. In contrast to it, the received level of the cell established in the train with the user aboard (a mobile cell as a neighboring cell at this point) is expected to be stable at predetermined value. Therefore, the favorable timing of the cell reselection of the mobile station from the indoor cell or outdoor cell to the mobile cell is the time when the received level of the serving cell starts varying rapidly, i.e., the time when the train with the user aboard starts moving.

Immediately after the mobile station moves from the interior of the mobile cell to the outside of the mobile cell (e.g., as the user gets off the train onto the platform and then the train again starts moving), the received level of the mobile cell where the mobile station is camped, varies rapidly, whereas the received level of the cell on the platform where the user is standing (an outdoor cell or indoor cell being a neighboring cell at this point) is expected to be relatively stable. Therefore, the favorable timing of the cell reselection of the mobile station from the mobile cell to the indoor cell or outdoor cell is the time when the received level of the serving cell starts varying rapidly, i.e., the time when the train again starts moving.

From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.