Method for handover in wireless communication, mobile electronic device, and wireless communication handover system

A wireless LAN device determines whether there is a necessity to search for a base station based on base station information as to a currently associated first base station, and selects all or a part of base station search conditions stored in advance to search for the base station when there is the necessity for searching the base station. When detecting at least one base station, the wireless LAN device determines whether there is a necessity to execute a handover based on base station information of each of the detected base station and the first base station. If the handover is necessary, the wireless LAN device determines the second base station from the base stations and associates the wireless LAN device with the second base station.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for performing a handover in wireless communication, a portable electronic device, and a wireless communication handover system.

2. Description of the Related Art

When a signal received from an access point (a base station), to which a wireless Local Area Network (LAN) device associates, becomes weak, the wireless LAN device becomes unable to maintain the communication. To assure that the wireless LAN link will be maintained, the wireless device switches to newly associate with another base station by performing a process by which the switching event is to take place, and which is called handover (or handoff).

A typical wireless LAN device has a single receiver unit; naturally, the wireless LAN device cannot search access points therearound while keeping the communication link via the currently serving access point. Therefore, the conventional wireless LAN devices release (dissociate) the association when the strength of the signal received from the base station with which the device currently associates is under a certain level, then scans all the available wireless LAN channels, searching for an access point from which the device receives a strong signal, and associates with the detected access point.

The wireless LAN device once releases the current association, and only then it scans all the channels to find a new access point. In this scenario, the wireless LAN device needs a substantial time to complete the handover. Moreover, the wireless LAN device will again be associated with the originally associated access point when it fails to find a new access point, rendering the series of processes end up in the waste.

On the other hand, a wireless LAN device that affords cutting off time to conduct a handover is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2005-175932. The disclosed wireless LAN device, on detecting that the level of the signal which it receives from the currently associated base station is smaller than a predetermined value, temporarily interrupts the communication with the currently associated access point, scans the channels and newly associates with a destination after detecting an appropriate channel.

This wireless device stops the communication with the currently associated access point and scans all the channels. Taking a substantial time to scan all the channels, therefore, this wireless LAN device sees large data loss in communicating with the currently associated access point.

A wireless LAN device, which requires shorter time for handover operations, is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2006-270614. This wireless LAN device, keeping association with one access point, measures the strength of the signals received from other channels, attempts to perform a handover only to a channel whose signal strength is greater than a threshold value, and stops the subsequent handover operations once it has successfully performed a handover. This wireless LAN device also sees large data loss in communicating with the currently associated access point because this wireless device, similarly to the above-mentioned devices, scans all the channels while temporarily stopping the communication with the associated channel.

Another wireless LAN device is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2006-262178, in which the wireless terminal device measures the quality of communication being performed with the adjacent systems for which an order of precedence is determined in advance, and performs handover based on the order of precedence and the measured quality of communication. In advance of the handover, this wireless device measures the quality of the communication being performed by each of the adjacent systems. Therefore, this wireless LAN device also faces the similar problem of data loss in communicating with the currently serving system.

Various mobile terminal devices, not limited to wireless LAN devices, have problems of this kind, which are commonly seen in those mobile terminal devices performing wireless communication via an access point (base station).

SUMMARY OF THE INVENTION

The present invention is made in view of the above-stated circumstances, and seeks to provide a portable electronic device that affords reduced data loss in communicating with its serving access point and affords an appropriate handover, and a method for the handover in wireless communication.

To solve the above-stated problems, the method for handover in wireless communication, is for the handover which takes place to switch a mobile electronic device associating with a first base station and including a wireless communication unit, configured for transmitting and receiving a wireless signal, to associate with and connect to a second base station, and the method comprises the steps of:

a) acquiring base station information as to the first base station;

b) determining whether there is a necessity to search a second base station that is different from the first base station, based on the acquired base station information as to the first base station;

c) selecting a part of a plurality of base station search conditions based on a predetermined criteria, when in the step b) the result of the determination is that there is the necessity to search the second base station;

d) searching for at least one base station in accordance with a selected base station search condition;

e) determining whether to execute the handover based on base station information as to a base station detected in step d) and the base station information as to the first base station; and

f) associating the mobile electronic device with the second base station by selecting the second base station from the base stations detected in step d) and switching the mobile electronic device to associate with the second base station, when in step e) it is determined that there is a necessity to execute the handover.

Further, to solve the above problems, the mobile electronic device in the present invention comprises an associator which executes the handover of the associated base station in accordance with the above method for handover in wireless communication.

Further, to solve the above problems, the wireless communication handover system comprises:

the mobile electronic device including an associator which executes the handover of the associated base station in accordance with the above method for handover in wireless communication,

a first base station with which the wireless device associates before that the associator switches the association, and

a second base station with which the wireless device associates after that the associator switches the association.

According to the method for handover, all or part of the channels can be selected as a target of base station search, by selecting a search condition on searching for a base station which is to be the destination of the handover. This produces reduction of duration time of interruption of the communication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A wireless communication system and a method for a handover in wireless communication according to the embodiment of the present invention will be described. In this embodiment, a mobile electronic device comprises a wireless Local Area Network (LAN) device1. The wireless LAN device1is capable of performing handover between the base stations.

As shown inFIG. 1, the wireless communication handover system HS of the present embodiment comprises a wireless LAN device1, a first base station2, a second base station3, and a network4.

The wireless LAN device1associates with the first base station2and transmits data to an electronic device connected via the network4.

As shown inFIG. 2, the wireless LAN device1comprises a control section11, a storage section12, a communication unit13, a display section14, an operating part15and a bus16.

The control section11comprises a Central Processing Unit (CPU) or the like. The control section11controls the entire operation of the wireless LAN device1in accordance with the program PG stored in the storage section12. For example, the control section11performs a handover to be associated with from one to another base stations, etc., in accordance with a program for executing a method for a handover in wireless communication. The control section11comprises, functionally, a timer11A, a first base station information acquirer11B, a signal level determiner11C, a search condition determiner11D, a communication interrupter11E, a base station search unit11F, a handover execution determiner11G and a handover conductor11H. These elements are implemented by a software programs configured to direct the CPU to perform their respective tasks. The details of these elements are described further below.

The storage section12comprises a Random Access Memory (RAM) or a nonvolatile memory, such as a flash memory, and serves as a main memory and a work memory for the control section11. The storage section12comprises, functionally, a program storage area121, a first base station information storage area122, a base station search condition storage area123, a base station detection results storage area124and a timer value storage area125.

The program storage area121stores a program PG for directing a control section11to perform a method for a handover in wireless communication illustrated in the flowchart ofFIG. 4.

The first base station information storage area122stores first base station information, which is acquired from the first base station by the first base station information acquirer11B. The first base station information includes information indicating the strength of a signal received from the first base station by the communication unit13.

The base station search condition storage area123stores a plurality of base station search conditions. Each of the base station search conditions, is a condition used by the base station search unit11F for searching a possible candidate for the second base station3, which is the destination of the handover.

The base station detection results storage area124stores the results of the search for base stations (results of base station search) performed by the base station search unit11F. The result of base station search is information for specifying a base station, if any, detected by the search performed by the base station search unit11F, otherwise, for indicating that no base station is detected.

The timer value storage area125stores a predetermined reference time for the timer unit11A to clock time (referred hereafter as “a timer value TM”). The timer value TM is used to keep the count of the lapsed time after that the base station search unit11F has performed the search for the base station, or to keep the count of the lapsed time after that a beacon is received. A plurality of reference times defined by the timer value TM may be provided, and those values may be different from each other. For example, a timer value TM that defines the interval for receiving the beacon (referred hereafter as beacon reception interval) may be different from that timer value TM that defines the interval for determining the necessity of performing handover (referred hereafter as “handover determination interval”).

The communication unit13comprises a network interface card, etc. based on IEEE802.11 standard or the like. The communication unit13connects the wireless LAN device1to the network4via an antenna13A. The communication unit13performs, for example, transmission and reception of a wireless signal to and from the first base station2or the second base station3.

The display section14comprises a Liquid Crystal Display (LCD) panel of dot matrix type and a driver circuit, etc. and displays any image. For example, the display section14displays a wireless channel selected by the operating part15.

The operating part15comprises various kinds of keys or buttons for inputting user instruction or data, and inputs various instructions and data to the control section11. For example, the operating part15can input information for selecting wireless channel in performing that wireless communication.

The bus16mutually transfers data between each section, that is, the control section11, the storage section12, the display section14, and the operating part15.

The first base station2represents the base station to which the wireless LAN device1currently associates. The first base station2periodically transmits a beacon for signaling the presence of itself, to the wireless LAN device1.

The second base station3periodically transmits a beacon for signaling the presence of itself to wireless communication terminal devices therearound.

Next, each component within the control section11is specifically described, with reference toFIG. 2.

The timer unit11A keeps the count of the lapsed time after a given time, referring to the timer value TM stored in a timer value storage area125. For example, the timer unit11A keeps the count of the lapsed time after the base station search unit11F has started the search for a base station. In the present embodiment, a beacon reception interval and a handover determination interval are set as proper values that are determined by the timer value TM. The beacon reception interval and the handover determination interval may have respective values different from one another.

Changing the timer value TM produces, for example, an effect of shortening the duration of interruption, which occurs on searching for the base stations, in the communication with the first base station2. This point is described later.

The first base station information acquirer11B acquires first base station information, and makes the acquired information stored in the first base station information storage area122. The first base station information includes information for specifying the first base station, and other information indicating the strength of the signal received from the first base station information. As for timing, the first base station information acquirer11B acquires the first base station information each time a time corresponding to the timer value TM passes.

The received signal level determination unit11C acquires the strength of the signal received from the currently associated base station (received signal strength) P5, compares the signal strength P5with threshold values TH1and TH2to determine which is the larger, and determines if a handover is necessary. Both of the threshold values TH1and TH2are reference strengths of the signal which the wireless LAN device1receives from the currently associated first base station2. Being greater than the threshold value TH1, the signal strength P5represents that the condition of reception is good. On the other hand, being smaller than the threshold value TH2, the signal strength P5represents that the condition of reception is poor.

The received signal level determination unit11C determines that there is no need of performing handover when the acquired received signal strength RSSI is greater than the threshold value TH1. If the acquired signal strength P5is smaller than the threshold value TH1and greater than the threshold value TH2, the received signal level determination unit11C determines that the reception of the signal from the currently associated first base station2is growing poor.

The search condition determiner11D determines the condition for searching the base stations, and reads out only the corresponding base station search conditions that correspond, from the base station search condition storage area123. For example, the search condition determiner11D can determine the base station search condition so that the base station search is performed in a part of the channels. Or, the search condition determiner11D can determine the base station search condition such that the base station search is performed in all the channels.

Regarding the base station search condition, a method for specifying a channel is exemplified further below, but the base station search condition is not necessarily limited to this.

The target of search can be only a part of the channels stored in the base station search condition storage area123, and thereby the time needed for searching the base stations is shortened as compared to when the search is performed over all the channels stored. Accordingly, duration of interruption of communication with the currently associated base station is shortened.

The search condition determiner11D determines from the base station search conditions so as to accord with the priority level assigned to the channels. In the present embodiment, the priority levels assigned to the channels are determined in advance. Specifically, the priority levels are assigned to the channels so that the channel number1is assigned the highest priority order, and the priority level decreases with the increasing channel number. That is, when the search is performed by the base station search unit11F on a given channel, then the channel with the next channel number is assigned a highest priority order in the next base station search.

Further, the search condition determiner11D extracts the number of target of the base station search, from the channels assigned with the priority levels. In the present embodiment, the number of channels extracted as the target of the base station search is predetermined. Specifically, three channels are read each time the base station search is performed. That is, the search condition determiner11D determines the base station search condition so that the base station search unit11F initiates the base station search from the channel1, to which the highest priority level is assigned, so as to conduct the search in three channels, in the ascending order. The search condition determiner11D reads out a search condition from the base station search condition storage area123. The maximum value of the time duration of the base station search performed for one channel each is predetermined, as a maximum base station search duration time.

Changing the methods of assigning the priority level to the channel, or changing the method of determining the number of channels, can reduce the time needed for searching a base station, to which the communication is reconnected by the handover. The details of the method of assigning the priority level to the channel and the method of determining the number of channels are described later.

The communication interruption unit11E directs the operation of the communication unit13to temporarily interrupt the communication with the currently associated first base station, for the base station search unit11F to initiate the search for the base stations.

The base station search unit11F searches for new base stations and stores a result of base station search, which includes the resultant base stations, in the base station detection results storage area124. When the base station search unit11F searches out a base station, the information of the detected base station (base station information) is stored to the base station detection results storage area124. On the other hand, when the base station search unit11F does not searched out any base station, the base station search unit11F stores information indicating that no base station is detected, in the base station detection results storage area124.

As a method for acquiring the base station information by the base station search, for example, a scanning process specified by the IEEE802.11 standard can be adopted. In this case, in order to reduce the time needed for the scanning process, the wireless LAN device1on active scan mode in which it sends a probe request to the base station, and receives a probe response from the base station. Further, for suppressing power consumption, a passive scan may be adopted, in which the wireless LAN device1does not send that probe request, and receives a beacon from the base station.

The handover execution determiner11G determines the necessity of the handover based on the result of the determination by the received signal level determination unit11C and the base station detection results which indicates the results of the detection by the base station search unit11F, and determines the presence or absence of the destination of reconnection by the handover.

When a handover execution determiner11G determines that the handover is necessary, the handover conductor11H performs the handover from the currently associated first base station2, to the second base station3that is the destination of the handover.

Referring to the illustration ofFIG. 3, the data structure of the base station detection results, which are detected by the base station search unit11F, and stored to the base station detection results storage area124, are described.

One record of the base station detection results includes the items shown in one horizontal line of the table ofFIG. 3, that is, the wireless LAN classification P1, the channel P2, the network identifier (Basic Service Set Identifier, BSSID) P3, and the logical name in the network (Service Set Identifier, SSID) P4, the signal strength (Received Signal Strength Indicator, RSSI) P5, and the elapsed time after base station search P6.

The wireless LAN classification P1indicates which protocol of11a,11band11g, etc., the LAN system searched out by the wireless LAN device1uses. In other words, the wireless LAN classification P1indicates in which frequency band the scanning is performed.

The channel P2indicates the channel in which the wireless LAN device1performed the base station search. From the wireless LAN classification P1and the channel P2, the frequency band in which the base station search unit11F preformed the base station search is specified.

The search condition determiner11D assigns a priority level to each channel P2. In the present embodiment, the 14 channels are scanned in the ascending order from 1 to 14, and the base station search is performed in three channels each time.

The network identifier (Basic Service Set Identifier, BSSID) P3is for identifying the base station detected in the channel scanned by the base station search unit11F. In the present embodiment, the network identifier P3is the same as the Media Access Control (MAC) address of the base station.

The logical name in the network (Service Set Identifier, SSID) P4is the logical name of a network, which is added for the sake of convenience in identifying each base station. The wireless LAN device1communicates only with a base station whose logical name in the network coincides with any of the logical names P4in the network stored in the storage section12.

The received signal strength (Received Signal Strength Indicator, RSSI) P5indicates the latest received signal strength of the signal received from the base station detected by the base station search unit11F.

The elapsed time after base station search P6indicates the lapsed time after a signal having a strength, which is indicated by the signal strength P5, is last detected.

As shown in the record of the eighth line ofFIG. 3, when no base station is detected even by the scanning performed by the base station search unit11F, the base station search unit11F stores only the wireless LAN classification P1, the channel P2, and the elapsed time after base station search P6, in the base station detection results storage area124. A record after lapse of predetermined time after being stored in the base station detection results storage area124may be deleted by the control section11.

Referring next to the flowchart ofFIG. 4, the handover of wireless LAN device1from the first base station2to the second base station3will be described.

Here, it is premised that the wireless LAN device1associates with the first base station2and periodically and normally receives a beacon. When the wireless LAN device1normally receives the beacon, the association process and the authentication process specified by the Institute of Electrical and Electronic Engineers (IEEE)802.11 standard between the wireless LAN device1and the first base station2have been completed.

Further, here, the wireless LAN device1is on the way of performing the process shown in the flowchart ofFIG. 4.

The timer11A, for determining the necessity of the handover, determines whether the lapsed time after that the first base station information transmitted by the first base station2is acquired in previous time has reached a value corresponding to the timer value TM set in advance (step A1). When the lapsed time has not reached the value corresponding to the timer value TM (No in step A1), the process returns to step A1and the counting of the lapsed time is continued.

When it is determined that the lapsed time has reached the timer value TM (Yes in step A1), the first base station information acquirer11B directs the communication unit13to receive the signal received from the currently associated base station (first base station2).

The first base station information acquirer11B derives the strength of the received signal (received signal strength) P5(step A2). The first base station information acquirer11B generates the first base station information including the signal strength P5, and stores the generated information to the first base station information storage area122.

Then, the received signal level determination unit11C acquires the signal strength P5and determines which is the larger, of the signal strength P5and the predetermined threshold values TH1and TH2(TH1>TH2) (steps A3and A4).

The received signal level determination unit11C, initially, determines whether the signal strength P5is equal to or greater than the threshold value TH1(Yes in step A3). When the signal strength P5is equal to or greater than the threshold value TH1(step A3;Yes), the received signal level determination unit11C determines that the handover process is not necessary. In this case, the control section11gets the process back to step A1, and continue the operation while associating with the first base station2.

If the signal strength P5is less than the threshold value TH1(No in step A3), the received signal level determination unit11C specifies whether the signal strength P5is greater than the threshold value TH2(step A4). When the signal strength P5is equal to or smaller than the threshold value TH2(No in step A4), the control section11gets the process back to the sequence of steps A10to A15. The details of steps A10to A15are described later.

When the signal strength P5is less than the threshold value TH1and greater than the threshold value TH (Yes in step A4), the search condition determiner11D determines (selects) the base station search condition, and reads out only the corresponding base station search condition from the base station search condition storage area123(step A5). Here, it is assumed that the search condition determiner11D selects one search condition according to which the search is performed by three channels in an ascending order of the number thereof (channel number).

The communication interruption unit11E directs the communication unit13to temporality interrupt the communication with the first base station2(step A6). Then, the base station search unit11F searches for a new base station in accordance with the search condition set in step A5(step A7). In the above-described example, the base station is searched in the order of: channel1, channel2, and channel3in accordance with the selected base station search condition.

The base station search unit11F, when finishing the base station search process, stores the base station detection results in the base station detection results storage area124in the form shown inFIG. 3(step A8).

The handover execution determiner11G, based on the base station detection results and a predetermined criteria, determines if the handover is necessary (Yes in step A9). The predetermined criteria is described later. When it is determined that the handover is not necessary (No in step A9), the control section11gets the process back to step A1, and continue the operation under the first base station2.

In the case where it is determined that the handover is necessary (step A9; Yes), handover conductor11H performs the association process to associate with the second base station3, which is selected as the appropriate destination of reconnection (step A16). In advance of the association process, the association with the first base station2, which is currently associated, may be released. After this, the wireless LAN device1operates under the second base station3, which is the destination of the handover.

On the other hand, in step A4, if the signal strength P5is equal to or less than the threshold value TH2(No in step A4), the handover execution determiner11G refers to the base station information stored in the base station detection results storage area124, and determines the presence or absence of the destination of the handover to which the communication is reconnected (step A10).

When there is any destination of the handover (Yes in step A10), the handover conductor11H performs an association process to be associated with the base station selected as the destination of the handover (step A16).

If there is no destination of the handover (No in step A10), the search condition determiner11D reads out all the base station search condition stored in the base station search condition storage area123(step A11). The communication interrupter11E directs the communication unit13to temporarily stop the communication with the first base station12(step A12). Then, the base station search unit11F attempts to detect a new base station in all the channels (step A13). The base station search unit11F, when finishing all the base station search processes on all of the base station search conditions selected in step A11, stores the base station detection results in the base station detection results storage area124(step A14).

The handover execution determiner11G, referring to the base station detection results, determines whether the base station search unit11F has detected any base station (step A15). When no base station is detected (No in step A15), the control section11gets the process back to step A1, and continue the operation under the first base station2.

When any base station is detected (Yes in step A15), the handover conductor11H performs an association process to associate with the second base station3selected as the destination of reconnection (step A16). The wireless LAN device1operates under the second base station3that is the destination of the handover.

In step A16when the association process to associate with the second base station3is not normally finished, the wireless control section11newly performs the base station search process for all the channels in accordance with all the available base station search conditions, to search for other base stations.

If, according to the determination process of the steps A3and A4, the signal strength P5is becoming weak (threshold value TH1>signal strength P5>threshold value TH2), the wireless LAN device1performs the base station search for a part of all the channels. On the other hand, when the signal strength P5is remarkably weak (threshold value TH2≧signal strength P5), the control section11performs the base station search for all the channels.

By doing so, the control section11can perform the base station search, with changing the number of the channels which are the target of the base station search, depending on the degree of the weakness of the signal strength P5.

When the signal strength P5is remarkably weak (threshold value TH2≧signal strength P5), the control section11performs the handover after determining whether there is any destination of the handover by the determination process. By doing so, the control section11can perform the handover in a short time.

Further, when no destination of the handover is detected in the determination process of step A10, the control section11performs the base station search based on all the base station search conditions and in accordance with the processes of steps A11to A15.

Accordingly, the possibility that the control section11detects a candidate base station which is to be the destination of the handover is increased.

Next, referring toFIGS. 4 and 5, a process flow until the wireless LAN device1of the present embodiment performs the handover from the currently associated first base station2to the second base station3will be described.

The process flow until the wireless LAN device1of the present embodiment completes handover from the currently associated first base station2to the second base station3is shown inFIG. 5, where the horizontal axis represents the passage time, and where the vertical axis represents the channel.

In steps S1, S3and S5the control section11periodically receives a beacon from the first base station2, and performs transmission and reception, etc. of data frames to and from the first base station2.

In step S1, during the communication with the first base station, the control section11acquires the signal strength P5(step A2), and compares the acquired signal strength P5and the threshold value TH1and TH2(steps A3and A4). If, the result of the determination is: TH1>P5>TH2(Yes in step A4), the control section11reads out a part of the search conditions from the storage section12. Here, it is assumed that the read-out search condition indicates that the scanning should be performed only for channels1to3. The control section11temporarily interrupts the communication (step A6) to search the base station, by individually and sequentially searching in the wireless LAN channels1to3in the numerical order, in accordance with the search condition (step S21to S23). The control section11stores the result of search in the storage section12, as shown inFIG. 3.

The time for the processes of steps S21to S23for base station search, is the duration time of the interruption of the communication with the first base station2. As the number of channels for which the base station search is performed increases and as the duration time of the base station search per one channel becomes longer, the duration time of the communication interruption becomes longer, while the possibility of detecting a base station, which is to be the destination of reconnection by the handover, increases.

When the base station search (steps A7and S21to S23) finishes, the control section11resumes to the condition in which it communicates with the first base station2(step S3).

Then, the control section11determines the necessity of the handover (step A9), and when it is determined that there is no need of the handover (No in step A9), returns to step A1without performing the handover.

During the communication with the first base station2in step S3, the control section11compares the signal strength P5with the threshold values TH1and TH2(steps A2to A4). If the result of the determination is: TH1>P5>TH2(Yes in step A4), the control section11reads out a part of the search conditions from the storage section. Here, it is assumed that the read-out search condition indicates that the scan should be performed only for the channels4to6. The control section11temporarily interrupts the communication (step A6), and searches the base station (steps A7and S41to S43) by individually and sequentially searching in the wireless LAN channels4to6in their numerical order (step S41to S43). In this way, by repeating the base station search operation periodically, the control section11can finish all the base station search processes on the entire base station search conditions. Further, by appropriately changing the search condition, more channels can be searched in.

When the base station search (steps A7and S41to S43) finishes, the control section11resumes to the condition in which it communicates with the first base station2(step S5).

Then, the control section11determines the necessity of the handover (step A9), and when it determines that there is no need of the handover (No in step A9), returns to step A1without performing the handover.

During the communication with the first base station2in step S5, the control section11compares the signal strength PS with the threshold values TH1and TH2(steps A2to A4). If the received signal strength of the signal from the first base station2is remarkably weak and the result of the determination by the received signal level determination unit11C is: TH2>PS (No in step A4), the control section11then determines the presence or absence of the destination of the handover (step A10). That is, the control section11refers to the base station detection results (FIG. 3) of the detection in steps S21to S23and S41to S43(and, if any, search processes performed prior to those steps).

Here, it is assumed that the result of determination is that the signal strength P5of the signal received from the second base station3, which is detected in the base station search in step S42, is the largest and satisfies the condition of the handover. Then, the control section11dissociates from the first base station2, and performs a process to associate with the second base station3.

After this, the wireless LAN device communicates with the second base station.

As described above, the wireless LAN device1of the present embodiment temporarily interrupts the communication with the currently associated base station before conducting the handover, and then searches for a base station and collects information. Moreover, it does not search for base stations in all the channels in a search process of one time, but searches in a part of the channels. This only takes a short duration of interruption in the communication with the base station. Further, by performing the search with changing the search conditions, the terminal device can search in all or many of the channels.

Then, the specifics of the processes in the above-referenced flowchart ofFIG. 4will now be explained.

In the following, examples where: i) the timer value in step A1is changed; ii) the base station search condition in step A5is selected; iii) the necessity of performing the handover in step A9is determined; and iv) the presence or absence of the destination of the handover in step A10is determined.

First, an effect obtained by changing the timer value TM, which is used as a reference value when the timer unit11A counts the lapsed time in step A1will be described.

In the first example, if the signal strength P5of the currently associated base station is high, the control section11changes the timer value TM that defines the handover determination interval, depending on the signal strength P5, in order to secure a longer handover determination interval. On the other hand, if the signal strength P5is low the control section11reduces the length of the handover determination interval by changing the timer value TM. Accordingly, when the reception of the signal from the first base station is good, the number of times of execution of the base station search is reduced, therefore shortening the process time.

The timer value TM can be changed by a given method. Possible examples of those methods may be as follows: i) Storing a plurality of candidates for the timer value TM in the storage section12, and subsequently selects an appropriate one; ii) comparing the received signal strength P5with the reference value, and incrementing or decrementing the time timer value TM by a difference ΔTM in accordance with the result of the comparison; and iii) deriving a value by multiplying a count by a value obtained by deriving the difference between the signal strength P5and the reference value, and the like.

As the second example, the timer value TM is changed so that the handover determination interval is extended if in the base station detection results detected by the base station search unit11F in step A7there is any base station having a signal strength P5greater than a threshold value; otherwise, the timer value TM is changed so that the handover determination interval is shortened.

By this operation, the number of time of performance of the base station search is increased when no candidate for the handover is detected, to thereby increase the possibility of detecting a destination of the handover.

As the third example, the timer value TM is changed so that a larger handover determination interval is secured, if more than a predetermined number of base stations whose signal strengths are larger than the signal strength P5of the currently associated base station, are detected; otherwise, the timer value TM is changed so that the handover determination interval is shortened.

In this way, it becomes possible to increase the times of performance of the scanning operation when no candidate for the handover is detected, thereby suppressing the possibility that the handover process ends up in failure.

Next, an explanation is given of a method for selecting a channel to be the target of base station search in step A5ofFIG. 4, as a method for electing the base station search condition. In step A5, the wireless LAN device1selects only a part of the entire available channels. In performing the selection of the channels, first, a priority level is assigned to the each channel.

A first method for assigning a priority level to each channel is as adopted in the present embodiment wherein the order of priority level is predetermined. For example, the priority order of channels in the base station search is assigned in the ascending order from 1 to 14 or a descending order from 14 to 1, or a random order of: 1, 6, 11, 14, 2, 7, 12 . . . , etc.

The example ofFIG. 5is in the ascending order. Here, when the base station search is performed in a given channel, then the channel with the next priority is most priority subjected to the next base station search.

In the descending order, when in a given search process the channels14to12are subjected to the base station search, in the next base station search the subjects are the channels11to9.

A second method for assigning a priority level to each channel is by according to the order of date and time after being stored of the base station information, among the base station detection results stored in step A9. For example, if the base station detection results are as shown inFIG. 3, channel6of which lapsed time after the base station search is the longest (here7) is assigned a primary priority level, and channels11and40, of which lapsed time is the second longest, here5, is assigned a secondary priority level.

According to this method, the records are updated in the order of their date and time, and then comparatively new records are constantly stored as the scanning results. This narrows the gap between the base station detection results stored in the base station detection results storage area124and base station detection results to be obtained when the base station search is actually performed for all the channels. Accordingly, an appropriate destination of the handover can be detected in a short time as compared to the case where all the channel is subjected to the base station search.

Additionally, a channel whose record does not exist on the result of base station search ofFIG. 3, including a channel for which base station search is never performed and a channel whose record is deleted due to the elapse of predetermined time after the base station search, can be given the highest priority. Also in this way, the wireless LAN device1can detect an appropriate destination of the handover with shorter time than that in the case where the base station search is performed in all the channels.

The third method for assigning a priority level to each channel is by assigning a priority level to each base station in the decreasing order of the signal strength P5of the results of the base station search stored in step A8. For example, in the base station detection results ofFIG. 3, channels1and6whose received signal strengths P5are the greatest (−65) are given the primary priority, and channels11and36, whose received signal strengths P5are the second greatest (−70) are given the secondary priority. In the third method, base stations having large received signal strengths P5but included in other channels cannot be detected. However, the information about the base stations having comparatively large signal strength P5are constantly retained newest. Accordingly, the wireless LAN device1can detect a candidate for the handover in a short time as compare do the case where the base station search is performed in all the channels.

A threshold value TH3is determined in advance as a reference strength for excluding a base station, the signal strength of which is weak. Then, on the basis of the base station detection results, priority may be assigned to the channels including a base station whose signal strength P5is greater than the threshold value TH3, in the decreasing order of the signal strength P5. By doing so, the wireless LAN device1can detect a destination of handover whose reception condition is better, without performing any base station search in a channel that includes base stations whose signal strength P5is equal to or less than the threshold value TH3, which is a bad reception condition.

The fourth method of assigning a priority level to each channel is a combination of the first to third methods.

For example, of the base station detection results, a channel used by a base station whose signal strength P5is the greatest and greater than the threshold value TH3(the channels determined in accordance with the third method) is given the primary priority. Then, of the records the channel whose lapsed time after the base station search is the longest (the channel determined in accordance with the second method) is given the secondary priority. And then, the channel with the next in a predetermined priority order (the channel determined in accordance with the first method) is assigned a tertiary priority.

Any two of the first to the third methods may be combined. In any of the examples, the wireless LAN device1can detect with a shorter time, more appropriately, an appropriate destination of the handover as compared to the case where all the channels are subjected to the base station search.

Further, a method for determining the number of channels for which the base station search is performed, in step A5where the search condition determiner11D determines the base station search condition, will be described.

The first example of the methods of determining the number of channels is determining the number of channels in advance. As adopted in the present embodiment, 3 channels may be searched each time the base station search is performed, or, a fixed cycle of base station search processes with different number of search targets, may be performed, that is, the cycle of searches performed in: 4 channels, 4 channels, then 3 channels, and 3 channels, may be adopted.

Further, the number of channels of the base stations searched in the transmission and reception of the data frame may be less than the number of the channels of the base stations searched during the period in which the beacon is received. Accordingly, the wireless LAN device1can reduce the time of the base station search during the data transmission and reception. Further, in order to perform a base station search by passive scan, a waiting time for receiving the beacon from the base station becomes necessary. Therefore, the base station search relying on the passive scan might require a longer time than is required in an active scan based base station search. Therefore, in searching a base station by passive scan, the number of searched channels per one time of the base station search process may be reduced. By doing so, the wireless LAN device1can reduce the duration of interruption of communication caused in order to perform the base station search.

On the other hand, in the base station search by active scan, the time needed for the base station search can be reduced by reducing the maximum value of the base station search. Further, the maximum time length of the base station search may be different for each channel.

As a second example of the determination of the number of channels, there is a method in which the total maximum base station search duration time and the maximum base station search duration time for each channel are determined in advance and then the maximum number of channels, which can be searched within the total maximum base station search duration time, is calculated. In this method, the maximum base station search duration time for each channel is set to a predetermined value.

In a base station search relying on active scan, the total maximum base station search duration time is reduced by reducing the maximum base station search duration time for each channel. Accordingly, the duration time of the interruption of the communication with the base station is reduced even if the number of channels to be subjected to the base station search of one time is increased.

By setting the total maximum base station search duration time to be smaller than the reception interval of the periodical reception of the beacon, the wireless LAN device1can finish the base station search without interrupting receiving the beacon from the currently associated first base station2. Further, by setting the total maximum base station search duration time during the transmission and reception of the data frame so as to be shorter than the total maximum base station search duration time, the duration time of interruption of communication during the transmission and reception of data frame can be reduced.

The base station search condition in the present embodiment is determined from channel priority level information that associates the channels with their assigned priority levels, and from the search target channel number information indicating the number of channels for which the search is performed when the base station search unit11F performs a process to search the second base station3.

The base station search unit11F searches for a base station in accordance with the base station search condition determined by the search condition determiner11D and read out from the base station search condition storage area123.

The channel priority level information and the search target channel number information may be changeable by user operation. By the adoption of the configuration, the wireless LAN device1allows a user to make a change so that the wireless LAN device1increases the number of channels in which it searches for a base station when, for example, the user judged that the signal strength reviewed from the currently associated base station has become weak. Accordingly, the device of this embodiment increases the possibility to detect a base station to be a candidate for the destination of the handover.

Referring now to flowcharts shown inFIGS. 6 to 8, explanations are given of three examples of the process to determine the necessity of performing the handover, in step A9ofFIG. 4.

The first example of the process of determining the necessity of performing the handover is, as shown in the flowchart ofFIG. 6, based on the result of comparison as to which is the larger, of the extracted signal strength P5and the signal strength P5of the currently associated base station.

The control section11extracts, from the records stored in the results of base station search storage area124, a record whose elapsed time after base station search P6is shorter than the predetermined time T (step B1). Here, the predetermined time T is a reference time to historically manage the result of base station search. Any record whose elapsed time after base station search P6exceeds the predetermined time T is deleted by the control section11. If the setting designates the predetermined time T as “0”, the control section11uses only the newest result of base station search in determining whether to perform a handover.

The handover execution determiner11G extracts a record corresponding to a base station whose signal strength P5is the greatest of the extracted records (step B2). The base station corresponding to the record extracted in step B2is a candidate for the destination of the handover. The handover execution determiner11G compares the signal strength P5of the signal from the candidate base station with the signal strength P5of the signal from the currently associated first base station2(step B3).

If the signal strength P5of the extracted base station is the greater, the handover execution determiner11G determines the handover to the base station as necessary (Yes in step B3). Then, the control section11goes from the current operation to step A16ofFIG. 4to execute handover. If the signal strength P5of the currently associated base station is the larger (No in step B3), the handover execution determiner11G determines that the execution of the handover is not necessary. Then the control section11proceeds with the process to step A1ofFIG. 4, and continues operating under the first base station2.

According to the above, this device affords reduction of time to conduct handover, to thereby reduce the time duration of interruption in communication with the currently associated base station is shortened.

The second example of the process of determining whether to perform the handover is, as shown in the flowchart ofFIG. 7, the process with the same totality asFIG. 6with an exception of step C3that replace with step B3inFIG. 6. The same processes inFIG. 7andFIG. 6are affixed with common reference numerals.

In step C3, the signal strength P5of the base station acquired in step B2is compared with a sum of signal strength P5of the currently associated first base station2and a predetermined threshold value TH4.

If the signal strength P5of the base station extracted from the results of base station search is the larger (Yes in step C3), the handover execution determiner11G determines the execution of handover as necessary, and the control section11proceed with the operation to step A16ofFIG. 4to perform handover. If the sum of signal strength P5of the first base station2and the predetermined threshold value TH4is the greater (No in step C3), the handover execution determiner11G determines that the handover is not necessary, and the control section11continues to operate under the first base station2.

According to the above, the time duration of interruption of communication with the currently associated first base station2is shortened. Further, the handover can be performed in which a base station with sufficient signal strength P5, effecting good reception, is the destination of reconnection.

The third example of process for determining whether to execute the handover is, as shown in the flowchart ofFIG. 8, the process with the same totality asFIG. 6with the exception of determination step D0added as the step preceding to step B1and not appearing inFIG. 6. Similarly to the above, the same processes inFIG. 8andFIG. 6are affixed with common reference numerals.

In step D0ofFIG. 8, the handover execution determiner11G compares the signal strength P5of the currently associated first base station2with the predetermined threshold value TH5. If the signal strength P5of the first base station2is the greater (Yes in step D0), the handover execution determiner11G determines that the handover is not necessary, and the control section11continues to operate under the first base station2. On the other hand, if the threshold value TH5is the greater (No in step D0), the control section11executes the determination process of steps B1to B3, similarly toFIG. 6.

According to the above, the time duration of interruption of communication with the currently associated first base station2is shortened.

On the other hand, if the signal strength P5of the currently associated first base station2is sufficient, the control section11does not execute handover, thus reducing unnecessary execution of handover.

The process for determining whether to execute the handover may be a combination of the processes ofFIGS. 7 and 8. In this case, the control section11can reduce the duration time of interruption in the communication with the currently associated first base station2, and execute the handover so that a base station with sufficient signal strength P5, effecting good reception, is the destination of reconnection. Moreover, the control section11can reduce unnecessary execution of the handover.

Lastly, the process of determining the presence or absence of the destination of the handover in step A10ofFIG. 4will be described with reference toFIG. 9. The same processes inFIG. 9andFIG. 6are affixed with common reference numerals.

Step A10is a process of determination executed when the handover execution determiner11G determines in step A4that the signal strength P5is smaller than the threshold value TH2.

When the process of flowchart ofFIG. 9is activated, similarly to the steps B1and B2ofFIGS. 6 to 8, the control section11extracts a record whose lapsed time after the finish of the base station search is shorter than the predetermined length, from the results of base station search stored in the base station detection results storage area124(step B1). Then, the control section11extracts one whose signal strength P5is the greatest (step B2).

The handover execution determiner11H compares the signal strength P5of the base station indicated by the record extracted in step B2with the threshold value TH6(step E3). If the signal strength P5of the base station is the greater (Yes in step E3), the handover execution determiner11H determines the base station as the destination of reconnection in the handover, and the control section11proceed with the process to step A16inFIG. 4to execute the handover. On the other hand, if the threshold value TH6is the greater (No in step E3), the handover execution determiner11H determines that there is no destination of reconnection of the handover, and the control section11execute the base station search based on all the base station search conditions, in accordance with the processes of steps A11to A15.

In step A15, the base station search process may be terminated when a base station whose signal strength P5is greater than the predetermined threshold value is detected before that the base station search processes in all of the base station search conditions selected by the search condition determiner11D are finished. This enhance the effect of reducing the time needed for the base station search time.

The present invention can widely be applied to electronic devices provided with the wireless communication function, for example, cellular phones, PDAs, electronic cameras, electronic wrist watches, music recorders and players, and the like.

This application is based on Japanese Patent Application No. 2006-310100 filed on Nov. 16, 2006 and including specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety.