Abstract:
A handover method of a mobile terminal device in a wireless local area network (WLAN) includes a mobile terminal device and at least two wireless access points (APs) which communicate with the mobile terminal device using particular radio resources. The mobile terminal device communicates with a wireless AP via a first radio resource, and receives signals from wireless APs via a second radio resource. The mobile terminal device arranges and stores the signals received from the wireless APs in a certain order of size. The mobile terminal device determines an AP for the handover by sequentially searching the stored APs when intensity of the signal received via the first radio resource is smaller than a predetermined value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of 35 U.S.C. § 119(a) of Korean Patent Application No. 2004-51512 filed on Jul. 2, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to a wireless local area network (WLAN). More particularly, the present invention relates to a mobile terminal device for rapidly performing handover in a wireless local area network (WLAN) and method thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     A wireless local area network (WLAN) is a data communication system introduced to overcome disadvantages of a wired local area network (LAN) generally used in an office building or a school building. The WLAN utilizing a radio frequency, is able to communicate data through minimal number of links. Accordingly, the WLAN enables a moving user to communicate data with a simple facility, namely, the WLAN enables a mobile network.  
         [0006]     The WLAN includes at least one wireless access point (AP) which provides services in a limited geographical area (hereinafter, referred to as a cell). The WLAN should provide a mobile terminal moving from one cell to another with continuity of seamless communications. In order to respond to this requirement, handover takes place in the transition of communications from an AP to another AP. The Institute of Electrical and Electronics Engineers (IEEE) specifies an Inter-Access Point Protocol (IAPP) for the communications and the handover between the APs. The mobile terminal and the APs are required to communicate a plurality of signaling messages with each other for the sake of the handover, which results in delay and greatly affects speech quality. Thus, the need arises for an efficient handover method in a wireless communication environment.  
         [0007]      FIG. 1  is a general WLAN environment. Referring to  FIG. 1 , the WLAN includes a first AP  110 , a second AP  112 , and a mobile terminal  100  such as a notebook or a personal digital assistant (PDA) wirelessly communicating with the first AP  110 . The first AP  110  and the second AP  112  provides the point of interconnection between a wired LAN and a WLAN so that a user can freely utilize services via Internet. The first AP  110  and the second AP  112  covers a communication range of substantially 20˜30 m indoors and substantially 100˜150 m outdoors.  
         [0008]     In  FIG. 1 , the mobile terminal  100  is handed from the first AP  110  to a second AP  112 . Typically, it takes about 60 to 200 ms to handover the mobile terminal  100 . The handover is executed through scanning to seek a new AP at the mobile terminal  100 , authentication and association. The scanning phase is carried out through a passive scanning or an active scanning. In the passive scanning, the mobile terminal  100  changes to one of the channels used in the WLAN system. The mobile terminal  100  determines whether a signal for searching an AP is received by use of the changed channel. The mobile terminal  100  scans the AP using the received signal. If the signal is not received in the changed channel over a predetermined time, the mobile terminal  100  changes to another channel and performs the same procedure.  
         [0009]     In active scanning, the mobile terminal  100  requests a response from APs using the selected channel and scans the channel. To be specific, the mobile terminal  100  selects one of a plurality of channels and searches an AP using the selected channel. As compared with passive scanning, the mobile terminal  100  requests a response from the APs using the selected channel. The mobile terminal  100  determines whether the response is received within a preset time (MinChannelTime). Upon receiving the response within the preset time (MinChannelTime), the mobile terminal  100  waits for a preset time (MaxChannelTime) in order to find other APs that use the same channel. When a response to the request is not received for the preset time (MinChannelTime), the mobile terminal  100  selects another channel and performs the same procedure with respect to another channel. Such a scanning phase takes up about 90% of the total time required for the handover.  
         [0010]     IEEE 802. 11 standard does not specify that a mobile terminal can connect with two APs at the same time. The mobile terminal has to disconnect from a current AP and search a new AP for the handover. As a result, communications and connections are interrupted for a certain time during the handover.  
       SUMMARY OF THE INVENTION  
       [0011]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.  
         [0012]     The present invention has been developed in order to solve the problems discussed above and others associated with the conventional arrangement. An aspect of the present invention provides a mobile terminal device and method for reducing handover time in a wireless local area network (WLAN).  
         [0013]     Another aspect of the present invention provides a device and method for performing handover without communication interruption in a wireless local area network (WLAN).  
         [0014]     Further aspect of the present invention provides a device and method for estimating relative distance between nodes.  
         [0015]     To achieve the above aspects of the present invention, there is provided a handover method for a mobile terminal device in a wireless local area network (WLAN) comprising the mobile terminal device and at least two wireless access points (APs) which communicate with the mobile terminal device using particular radio resources. The handover method comprises the steps of sending and receiving a signal to and from the wireless APs over a first radio resource, and receiving signals from the wireless APs over second radio resources in sequence; arranging and storing the signals received from the wireless APs over the second radio resources in a certain order of size; and determining an AP for the handover by sequentially searching the stored APs when intensity of the signal received via the first radio resource is smaller than a predetermined value.  
         [0016]     The signals from the wireless APs are received by sequentially searching the particular radio resources used by the wireless APs.  
         [0017]     The mobile terminal device operates by changing into an active mode to receive the signals from the wireless APs and a sleep mode to stop the signal reception from the wireless APs in an alternative manner at predetermined time intervals.  
         [0018]     The determining of the wireless AP for the handover comprises the sub-step of comparing a reception status of the signals received via the second radio resource with a reception status of the first radio resource, and selecting a wireless AP which uses the second radio resource having the reception status better than the reception status of the first radio resource.  
         [0019]     The handover method further comprising the sub-step of determining an AP for the handover among APs not being stored when the AP for the handover is not selected from the stored APs.  
         [0020]     The first radio resource is a data channel, and the second radio resource is a control channel.  
         [0021]     Consistent with the above aspects of the present invention, a mobile terminal device for communicating with at least two wireless access points (APs) by use of particular radio resources, comprises a first receiver for receiving a signal from a wireless AP via a first radio resource; a second receiver for receiving signals from wireless APs via second radio resources in sequence; a memory for arranging and storing the signals received from the wireless APs over the second radio resources in a certain order of size; a controller for controlling to determine an AP for the handover by sequentially searching the stored APs when intensity of the signal received via the first radio resource is smaller than a predetermined value.  
         [0022]     The second receiver changes a frequency of the second radio resources so as to sequentially receive radio resources used by the wireless APs.  
         [0023]     The controller controls the second receiver to change into an active mode to receive the signals from the wireless APs and a sleep mode to stop the signal reception from the wireless APs in an alternative manner at predetermined time intervals.  
         [0024]     The mobile terminal device further comprises a comparator for comparing a reception status of the first radio resource with a reception status of the second radio resources respectively used by the stored APs, in sequence under the control of the controller.  
         [0025]     The controller carries out the handover to a wireless AP using the second radio resource having the reception status better than the reception status of the first radio resource.  
         [0026]     The controller determines an AP for the handover among APs not being stored when the AP for the handover is not selected from the stored APs. The first radio resource is a data channel, and the second radio resource is a control channel.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0028]      FIG. 1  is a general wireless local area network (WLAN);  
         [0029]      FIG. 2  is a block diagram of a mobile terminal device according to an embodiment of the present invention;  
         [0030]      FIG. 3  is operation modes of the mobile terminal device of  FIG. 2 ;  
         [0031]      FIG. 4  illustrates the mobile terminal device classifies APs based on intensity of received signals;  
         [0032]      FIG. 5  illustrates the mobile terminal device performs a fast search;  
         [0033]      FIG. 6  illustrates the mobile terminal device performs the handover according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.  
         [0035]     According to an embodiment of the present invention, a mobile terminal device includes a first receiver to receive data from a current access point (AP), and a second receiver to receive signals (signals to search an AP) from neighbor APs. Thus, the mobile terminal device transceives only data in the current channel (data channel) and receives a signal to search an AP in a new channel (control channel).  
         [0036]      FIG. 2  is a block diagram of the mobile terminal device according to an embodiment of the present invention. In  FIG. 2 , the mobile terminal device includes a first receiver  210 , a second receiver  220 , a first signal processors  212 , a second signal processor  222 , a third signal processor  230 , a comparator  214 , a memory  224 , a controller  200 , and a sender  232 . It is to be understood that the mobile terminal device can further include other components in addition to the above components.  
         [0037]     The first receiver  210  receives data from a current AP in a data channel. The second receiver  220  receives signals to search APs from neighbor APs in a control channel. A conventional mobile terminal device receives data and signals to search APs in the data channel. That is, in order to receive the signals for the AP search, the conventional mobile terminal stops the data reception and receives the signals in the data channel. According to an embodiment of the present invention, the mobile terminal device includes two receivers  210  and  220 , and thus receives the data and the signals for the AP search to the respective receivers  210  and  220 .  
         [0038]     The first signal processor  212  processes the data input from the first receiver  210  and provides the processed data to the controller  200 . The second signal processor  222  processes the data input from the second receiver  200  and stores the processed result in the memory  224 . The information stored in the memory  224  will be explained below. The second signal processor  222  provides the processed result to the controller  200 .  
         [0039]     The controller  200  analyzes status of a wireless channel by use of the data provided from the first signal processor  212 . If the status of the wireless channel is normal, the controller  200  controls to continuously receive the data from the current AP. If the status of the wireless channel is abnormal, the controller  200  starts to perform a handover procedure. The controller  200  determines a new AP for the handover by means of the information (AP list) stored in the memory  224 .  
         [0040]     The comparator  214  compares the wireless channel status of the current AP with those of APs stored in the memory  224 . If there is any AP having a better channel status than the current AP, information relating to the new AP is transferred to the controller  200 .  
         [0041]     The third signal processor  230  processes the data (data to be transferred to the new AP) received from the controller  200 . The processing of the third signal processor  230  is reverse to that of the first signal processor  212 . The sender  232  wirelessly transmits the signal processed at the third signal processor  230 .  
         [0042]      FIG. 3  illustrates operations of the second receiver  220  of the mobile terminal device. The second receiver  200  includes two modes as shown in  FIG. 3 . That is, the second receiver  200  includes an active mode to scan the neighbor APs and a sleep mode to stop the scanning. The active mode is divided into a fast search mode and a full search mode. The mobile terminal device scans specific APs in the fast search mode, and scans the entire neighbor APs in the full search mode. In  FIG. 3 , the mobile terminal device changes the mode between the two modes of the active mode and the sleep mode.  
         [0043]     The following describes the information stored in the memory  224 . The second receiver  220  of the mobile terminal device receives the signal for the AP search from the neighbor APs. The wireless channel status of the neighbor APs are obtained using the received signal, and the neighbor APs are classified depending on their wireless channel status. Generally, the mobile terminal device classifies the neighbor APs into three categories, that is, a candidate AP, a monitored AP and a detected AP. The candidate AP can perform the handover at any time, and the monitored AP is an AP of which signal state is acquired to some extent. The detected AP is an AP of which a signal is only detected. Table 1 shows an example of an AP list stored in the memory  224  of the mobile terminal device.  
                           TABLE 1                                       Candidate AP   AP1, AP3           Monitored AP   AP4           Detected AP   AP5                      
 
         [0044]     In Table 1, the candidate AP is AP 1  and AP 3 , the monitored AP is AP 4 , and the detected AP is AP 5 . The mobile terminal device can allocate priority to the candidate APs based on a certain order, specifically, based on the wireless channel status (intensity of the received signal).  
         [0045]     The mobile terminal device updates Table 1 at predetermined time intervals. The mobile terminal device can update Table 1 each time an event occurs such as handover and turn-on of the mobile terminal device. The user can adjust the predetermined time intervals. It is noted that power consumption of the mobile terminal device increases when the predetermined time intervals are set shorter.  
         [0046]     Referring back to  FIG. 3 , the second receiver  220  maintains the sleep mode. The second receiver  220  changes to the active mode at the predetermined time intervals. Generally, the second receiver  220  changes to the full search mode of the active mode. The second receiver  220  scans the neighbor APs in the full search mode and updates Table 1 using the scanning result.  
         [0047]      FIG. 4  depicts a criterion for the classification of the neighbor APs, which is stored in the memory  224 . Referring to  FIG. 4 , the user sets a first threshold and a second threshold. As for the signal intensity exceeding the first threshold, the channel status of the candidate AP is indicated. As for the signal intensity below the second threshold, the channel status of the detected AP is indicated. As for the signal intensity between the first threshold and the second threshold, the channel status of the monitored AP is indicated.  
         [0048]     The status of the neighbor APs changes as the time passes by as shown in  FIG. 4 . As the time passes, AP 1  changes from the candidate AP to the monitored AP and then the detected AP, and AP 2  changes from the detected AP to the monitored AP and then the candidate AP. The AP 3  maintains the detected AP. As the second receiver  220  classifies the neighbor APs at T 1 , AP 1  and AP 2  becomes the candidate AP and AP 3  becomes the detected AP.  
         [0049]      FIG. 5  depicts that the mobile terminal device determines whether to perform the handover. The mobile terminal device analyzes the channel status of the signal received from the first receiver  210 . The channel status is obtained using an intensity or a power of the received signal. The mobile terminal device compares the intensity of the received signal with a third threshold. When the signal intensity is below the third threshold, the mobile terminal device performs the handover. The mobile terminal device scans the APs defined in Table 1 without having to scan all of the neighbor APs. In order to reduce the scanning phase, the mobile terminal device may store detailed information relating to the APs. Table 2 shows another example of the information stored in the memory  224 .  
                               TABLE 2                                       Candidate AP   AP1   Channel 2               AP3   Channel 4           Monitored AP   AP4   Channel 3           Detected AP   AP5   Channel 5                        
         [0050]     Table 2 shows the neighbor APs and information relating to channels used by the neighbor APs. The memory  224  may store the intensity of the signal received from each AP together with the channel information. Thus, the mobile terminal device can reduce the handover time.  
         [0051]      FIG. 6  is a flowchart of a handover operation of the mobile terminal device according to an embodiment of the present invention, to be explained in detail.  
         [0052]     The mobile terminal device performs the handover when the intensity of the signal received from the first receiver  210  is below the third threshold. The mobile terminal device determines the presence or absence of a candidate AP in Table 1 stored in the memory  224  (S 600 ). If so, the mobile terminal device proceeds to S 602 , or if not, the mobile terminal device proceeds to S 606 . The mobile terminal device compares the signal intensity of the candidate AP (intensity of the signal received from the second receiver  220 ) with that of the current channel (intensity of the signal received from the first receiver  210 ) (S 602 ). When the signal intensity of the candidate AP is greater than that of the current channel, the mobile terminal device proceeds to S 608 . Otherwise, the mobile terminal device proceeds to S 604 .  
         [0053]     It is illustrated that the signal intensity of the candidate AP is compared with that of the current channel (S 602 ) by way of example, but not limited to this comparison. In order to prevent a ping-pong phenomenon, the signal intensity of the candidate AP can be compared with the signal intensity +α.  
         [0054]     The mobile terminal device determines whether there is any candidate AP not being compared (S 604 ). If so, the mobile terminal device proceeds to S 602 , or if not, the mobile terminal device proceeds to S 606 .  
         [0055]     The mobile terminal device determines whether there is a monitored AP in Table 1 stored in the memory  224 (S 606 ). If so, the mobile terminal device proceeds to S 610 , or if not, the mobile terminal device proceeds to S 614 . The mobile terminal device compares the signal intensity of the monitored AP with that of the current channel (S 610 ). When the signal intensity of the monitored AP is greater than that of the current channel, the mobile terminal device proceeds to S 608 . When the signal intensity of the monitored AP is not greater than that of the current channel, the mobile terminal device proceeds to S 612 . To prevent the ping-pong phenomenon, the signal intensity of the monitored AP can be compared with the signal intensity +β of the current channel (S 610 ).  
         [0056]     The mobile terminal device determines whether there is any monitored AP not being compared (S 612 ). If so, the mobile terminal device proceeds to S 610 , or if not, the mobile terminal device proceeds to S 614 .  
         [0057]     The mobile terminal device determines whether there is any detected AP in Table 1 stored in the memory  224  (S 614 ). If so, the mobile terminal device proceeds to S 616 , or if not, the mobile terminal device proceeds to S 620 . The mobile terminal device compares the signal intensity of the detected AP with that of the current channel (S 616 ). When the signal intensity of the detected AP is greater than that of the current channel, the mobile terminal device proceeds to S 608 . When the signal intensity of the detected AP is not greater than that of the current channel, the mobile terminal device proceeds to S 618 . To prevent the ping-pong phenomenon, the signal intensity of the detected AP can be compared with the signal intensity +δ of the current channel (S 616 ).  
         [0058]     The mobile terminal device determines whether there is any detected AP not being compared (S 618 ). If so, the mobile terminal device proceeds to S 616 , or if not, the mobile terminal device proceeds to S 620 .  
         [0059]     The mobile terminal device performs the handover (S 608 ), and performs the full search (S 620 ). According to an embodiment of the present invention, the fast search is carried out prior to the handover. If an AP for the handover is not found through the fast search, the full search is carried out to discover an AP for the handover.  
         [0060]     The mobile terminal device searches the neighbor APs through the full search, and determines an AP for the handover among the searched APs.  
         [0061]     In light of the foregoing as explained above, the conventional method performs the handover using only one channel, and thus causes the communication interruption. According to an embodiment of the present invention, an additional channel is used for the handover so that the communication interruption is prevented. Furthermore, an AP is selected from APs allowing the handover and the handover to the selected AP is performed. As a result, the handover time can be reduced.  
         [0062]     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.