Abstract:
A handover method used in a wireless communication system for handing over a mobile device from a first base station to a second base station is provided. Firstly, a first signal power between the mobile device and the first base station as well as a second signal power between the mobile device and the second base station are respectively measured by the mobile device. Next, the first signal power is compared with the second signal power, and an equal signal power is generated if the two signal powers are substantially the same. Then, the mobile device is handed over to the second base band if the signal power between the mobile device and the first base station is measured by the mobile device to have the same level with the equal signal power.

Description:
This application claims the benefit of Taiwan application Serial No. 93124889, filed Aug. 18, 2004, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to a handover method, and more particularly to a handover method used in a wireless communication system. 
   2. Description of the Related Art 
   Referring to  FIG. 1 , a diagram of a wireless communication system is shown. A wireless communication system  100 , such as a GSM system, a CDMA system or a wireless LAN, includes a cell  102  of a first base station  101  and a cell  112  of a second base station  111 . The wireless communication system  100  provides wireless Internet access or wireless communication to a mobile device  120 . The mobile device  120  can be a mobile phone or a portable device with wireless Internet access such as a mobile phone or a personal digital assistant (PDA) for instance. 
   Suppose initially, the mobile device  120  is at position A and communicates with the first base station  101 . The first base station  101  transmits a first signal, while the second base station  111  transmits a second signal. Position D is the farthest position that the first signal emitted by the first base station  101  can reach. The mobile device  120  can no longer receive the first signal from the first base station  101  beyond the position D. In other words, the signal power required for the mobile device  120  to receive the first signal from the first base station  101  at position D is a predetermined power value. Therefore, the mobile device  120  must be handed over to the second base station  111  from the first base station  101  before reaching position D, lest data might be lost. 
   The conventional handover method is exemplified below. During the movement of the mobile device  120 , the mobile device  120  compares the power of the first signal emitted by the first base station  101  and measured the power of the second signal emitted by the second base station  111 . When the mobile device  120  is at position P, the power from the first base station  101  is smaller than the power from the second base station; the mobile device  120  is handed over to perform communication via the base station  112 . 
   However, if the decision of handover is determined according to the signal power of the base station received by the mobile device  120  is larger than a fixed value, unnecessary handovers would occur and cause extra burden to the wireless communication system. For example, the received power of the first signal emitted by the first base station still suffices to maintain a good communication when the mobile device  120  may move around position P for a while. If the mobile device is handed over simply because the received power of the second signal from the second base station is slightly larger, there would be too many handovers which cause extra burden to the wireless communication system. If the mobile device  120  is handed over when the received power of the second signal emitted by the second base station is larger than a fixed value, data loss would occur because the mobile device  120  moves too fast. The mobile device  120  might have no enough time to process the handoff procedures and data transmission. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide an efficient handover method. 
   According to the object of the invention, a handover method used in a wireless communication system for handing over a mobile device from a first base station to a second base station is provided. The first base station emits a first signal, while the second base station emits a second power. Firstly, a first signal power level between the mobile device and the first base station and a second signal power level between the mobile device and the second base station are respectively measured by the mobile device. Next, the first signal power level is compared with the second signal power level, and an equal signal power level is generated if the two signal power levels are substantially of the same level. The mobile device estimates a handover power level ranging between the equivalent signal power level and the predetermined power value level. The mobile device hands the mobile device over to the second base station if the first signal power level is equal to the handover power value level. 
   Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of wireless communication system; 
       FIG. 2  is a flowchart of a handover method according to a preferred embodiment of the invention; 
       FIG. 3  is a diagram of GSM system; and 
       FIG. 4  is a diagram of wireless LAN system. 
       FIG. 5  is a flowchart of a handover method according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , a flowchart of a handover method according to a preferred embodiment of the invention is shown. Also refer to  FIG. 1 . Initially, the mobile device  120  is at position A and communicates with the first base station  101 . The first base station  101  emits a first signal and the second base station  111  emits a second signal. Position D is the farthest position that the first signal emitted by the first base station  101  can reach. The mobile device  120  can no longer receive the first signal from the first base station  101  beyond the position D. In other words, the signal power level required for the mobile device  120  to receive the first signal from the first base station  101  at position D is a minimum power value Pm. Therefore, the mobile device  120  must be handed over before reaching position D. 
   Firstly, a predetermined power level, the minimum power, Pm is stored at the mobile device  120  as shown in step  210 . Next, during the course of movement, the mobile device  120  continues to receive and measure signal power level P 1  of the first signal emitted by the first base station  101  and receive and measure signal power level P 2  of the second signal emitted by the second base station  111  as shown in step  212  and step  214 . If the signal power level P 1  is measured by the mobile device  120  to be equal to the signal power level P 2  when the mobile device  120  is at position B for instance, the mobile device  120  would store and record the present signal power as an equivalent signal power Pe as shown in step  216 , the equivalent signal power level may be equal to the first power level. The mobile device  120  would estimate a handover signal power level Ph according to the equivalent signal power level Pe and the predetermined power level Pm as shown in step  218 . The handover signal power level Ph ranging between the equivalent signal power level Pe and the minimum power level Pm is obtained according to a predetermined ratio. For example, Ph=(Pe−Pm)/n+Pm, wherein n is a positive integer such as 8 for instance. 
   During the course of movement, if the power level of the first signal emitted by the first base station  101  is received and measured by the mobile device  120  to be equal to the handover signal power level Ph when the mobile device is at position C for instance, the mobile device  120  starts to be handed over to the second base station  111  as shown in step  220 . 
   The handover signal power level Ph can further be determined according to a traveling rate of the mobile device  120 . If the traveling rate is too fast, the value of the handover signal power Ph can be increased, so that the mobile device  120  is handed over when farther away from the position D, that is, the handover occurs earlier. If the traveling rate slows down, the value of the handover signal power level Ph can be decreased, so that the mobile device  120  is handed over when closer to the position D, that is, the handover occurs later. 
   When the mobile device  120  is moving away from the first base station  101 , from position A towards position D for example, the traveling rate of the mobile device  120  is positively proportional to the decay rate of the signal power. Therefore, the power decay rate Vp can be used as a reference index of the traveling rate of the mobile device  120 . In an example, as shown in  FIG. 5 , a power decay rate Vp is calculated in step  222  before the equivalent signal power Pe is generated in step  216 . For example, the received signal power level Pa of the first signal is recorded when the mobile device  120  is at position A. The received power level of the first signal is equal to Pe and the power decay rate Vp is equal to (Pa−Pe)/t when the mobile device  120  moves to position B after time t. 
   If the power decay rate Vp is larger, the value of the handover signal power level Ph can be increased, so that the handover occurs earlier. If the power decay rate Vp is smaller, the value of the handover signal power level Ph can be decreased, so that the handover occurs later when the mobile device  120  is further closer to position D. 
   The present embodiment defines the above handover timing and avoids data loss during handover. From the determination of handover to the completion of the handover would take a period of time, T 1  for instance. So, after handover is determined in step  220 , the first base station  101  can transfer the data, which are originally transferred to the mobile device  120  first, to the second base station  112  at the same time before the mobile device  120  is handed over to the second base station  112 , so that data loss can be prevented. 
   According to the power decay rate Vp, the value of the handover signal power Ph can be clearly defined as:
 
 Ph=Vp*T 1+ Pm   (1)
 
   Data loss can be prevented if the handover is performed before the time T 1 . Therefore, the signal power level before the time T 1  can be estimated according to the power decay rate Vp. Handover is performed if the signal power level between the mobile device  120  and the first base station  101  is measured by the mobile device  120  to be equal to the handover power level Ph. 
   It can be seen from equation (1) that the larger the power decay rate Vp, the larger the value of the handover signal power level Ph. This denotes that the handover would be performed earlier during the movement of the mobile device  120 . On the other hand, when the smaller the power decay rate Vp, the smaller the value of the handover signal power Ph is. This denotes that the handover would be performed later during the movement of the mobile device  120 . 
   The conventional handover method performs handover as long as the position B is passed by as shown in position P of  FIG. 1 . The method according to the invention does not perform handover until the mobile device  120  is very close to position D, hence reducing unnecessary occurrences of handover and reducing the burden of the wireless communication system. Moreover, the handover timing can be adjusted according to the traveling rate of the mobile device  120 . 
   The handover method of the present embodiment can be used in a wireless communication system such as a GSM system and a wireless LAN system for instance. The application of the handover method of the present embodiment in the GSM system is disclosed below.  FIG. 3  is a diagram of GSM system. The GSM system  310  includes a first base station  301 , a second base station  311  and a control center  310 . Initially, the mobile phone  320  is at position A′ and performs communication via the first base station  301 . The first base station  301  transfers the signal emitted by the mobile phone  320  to the control center  310 . The control center  310  transfers the signal to mobile phone  320  via the first base station  301 . The minimum power level for the power of the first signal emitted by the first base station  301  to be received by the mobile phone  320  must be Pm′. 
   During the course of movement, the mobile phone  320  continues to receive and measure the signal power level P 1 ′ of the first signal emitted by the first base station  301  and receive and measure the signal power level P 2 ′ of the second signal emitted by the second base station  311 . If the signal power level P 1 ′ is measured by the mobile phone  320  to be equal to the signal power level P 2 ′ when the mobile device  320  is at position B′ for instance, the mobile phone  320  would store and record the present signal power level as an equivalent signal power Pe′. The mobile phone  320  or the control center  310  estimates the handover signal power level Ph according to the equivalent signal power level Pe and the minimum power level Pm. During the course of movement, if the signal power level between the mobile phone  320  and the first base station  301  is measured by the mobile phone  320  to be equal to the handover signal power level Ph when the mobile phone  320  is at position C′ for instance, the control center  310  starts to transfer the subsequent data to the second base station  311 , meanwhile, the mobile phone  320  is handed over to the second base station  311 . 
   The application of the present embodiment handover method in a wireless LAN system is exemplified below.  FIG. 4  is a diagram of wireless LAN system. A wireless LAN system  400  includes a first base station  401 , a second base station  411  and a router  430 . Both the first base station  401  and the second base station  402  are a wireless access point (AP). The mobile device is exemplified by a personal digital assistant (PDA)  420 . Initially, the PDA  420  is at position A′ and is connected to the Internet via the first base station  401  and the router  430 . The first base station  401  and the second base station  402  are together coupled to the router  430 . The minimum power level for the power of the first signal emitted by the first base station  401  to be received by the PDA  420  must be Pm″. 
   During the course of movement, the PDA  420  continues to receive and measure the signal power level P 1 ″ of the first signal emitted by the first base station  401  and to receive and measure the signal power level P 2 ″ of the second signal emitted by the second base station  411 . If the signal power level P 1 ″ is measured by the PDA  420  to be equal to the signal power level P 2 ″ when the PDA  420  is at position B″ for instance, the PDA  420  would store and record the present signal power level as an equivalent signal power level Pe″. The PDA  420  or control center  310  obtains the handover signal power Ph according to the equivalent signal power Pe″ and the minimum power value Pm″. During the course of movement, if the power of the first signal emitted by the first base station  401  is received and measured by the PDA  420  to be equal to the handover signal power level Ph″ when the PDA  420  is at position C″ for instance, the PDA  420  is handed over to the second base station  411 . 
   Before the PDA  420  is handed over to the second base station  411 , a copy of the packet originally to be transferred to the PDA  420  is made and transferred to the second base station  411 . According to a common practice, the packet is transferred to the second base station  411  by the first base station  401 . According to another practice, the packet originally to be transferred to the PDA  420  is transmitted to the first base station  401  the second base station  411  via the network by the router  430  transfers. 
   The handover method disclosed in above embodiment of the invention reduces unnecessary occurrences of handover. Moreover, the handover timing can be adjusted according to the traveling rate of the mobile device so as to effectively reduce the burden of the wireless communication system. 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.