Abstract:
A method for reducing the handover frequency by a femto gateway in femtocell networks includes the steps: receiving a measurement report from a communication device by an initial femtocell; transmitting a relocation required message to a femto gateway by the initial femtocell if the signal strength thereof is below a threshold; ignoring the relocation required message and enabling the timer by the femto gateway after receiving the relocation required message; determining whether the cell ID included in the call reestablishment request message is classified as “femtocell” or not by the femto gateway before the timer expires; if so, rebuilding the link to the femtocell in response to the cell ID by the femto gateway.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of TAIWAN patent application No. 100129636, filed Aug. 18, 2011. 
     TECHNICAL FIELD 
     The present invention relates to a communication method, and more specifically to a method for reducing the handover frequency in femtocell networks. 
     BACKGROUND OF THE RELATED ART 
     With the rapid development in communication technology, the cell phone (or mobile phone) has gradually become an indispensable necessity of life. Developed and intensive communication base station network are facilitating for popularity of mobile phones. By means of the base station, mobile phones can link to a communication networks to communicate with others. 
     In general, base station includes femtocell and macrocell. The femtocell is referred to as a home base station with small coverage which is mainly used to cover the areas without covered by the other base station and to improve data transmission rate. It is typically used for residential or small business environment. The macrocell has more coverage for service which can provide uninterrupted service to users, but with slower transmission rate. By contrast, the femtocell has less coverage for service and discontinuously, but with faster transmission rate. Therefore, within coverage of the femtocell, it is easier using to users for transmission, to facilitate improving the transmission rate, and quality of a call. As an user moves from a femtocell to another femtocell, the communication device locates within service coverage of the macrocell for a time period. In the existing mobile management mechanisms, in order to maintain uninterrupted call (conversation), the communication devices will associate with the macrocell (macro cellular base station) to establish a radio link. In general, such mechanism is referred to as handoff or handover. 
     As mentioned above, when the communication device conducts voice or data services, and moves from a femtocell to another femtocell, the communication device will locate within service coverage of the macrocell for a time period. Once, the macrocell currently has no extra radio resources, services being carried out by the communication devices will be forced to interrupt such that it substantially lowers user satisfaction. On the other hand, if the communication device can smoothly switch to the macrocell, the communication device may stay in a short time, and immediately handover to the femtocell to enhance the quality of transmission. While, this situation will make communication devices to conduct high frequency of handover. Thus, it will increase the delay time caused by switching cells, which makes a serious decrease in quality of call. 
     To sum up, the disadvantage of these traditional handover operations includes burden of processing cost, communication delay, and the communication disconnection and many other shortcomings. When the cells number increases, the frequency of handover operations of each cell will also increase, as seriously, even leading to communication networks shut down. Therefore, it is a very important key point to the development of network communications that how to design an effective handover management mechanism to reduce the probability of dropped calls, reduce the delay time as required frequency by switching cells and increase the overall capacity of network services. 
     SUMMARY 
     To address the above issues, the present invention provides a method for effectively reducing the handover frequency in communication networks. 
     One objective of the present invention is to utilize a mechanism for reducing the handover frequency such that a macrocell has extra bandwidth (radio resource) to serve more users. 
     Yet another objective of the present invention is to reduce the handover frequency in femto networks for eliminating delay time of switching cells to promote quality of call. 
     Thus, one objective of the present invention is to promote the femtocell usage frequency for further promoting data transmission rate of a communication device and quality of call. 
     To achieve the purpose of the above-mentioned, the present invention proposes a method for reducing the handover frequency in femtocell networks, comprising: receiving measurement reports from a communication device by an initial femtocell. Next, based-on the measurement reports, it is performing a step of checking whether signal strength of the initial femtocell is less than a threshold or not. Then, if it is, then sending a relocation required message to a femto gateway by the initial femtocell. Subsequently, after the femto gateway receiving the relocation required message, a relocation procedure is suspended and then starting a timer. Next, it is confirming whether the femto gateway receives a call re-establishment request message or not. Then, if it is, then checking a cell ID contained within the call re-establishment request message belongs to a femtocell or not. Finally, if it is, then re-establishing a call to the femtocell in response to the cell ID. 
     As above-mentioned, one feature of the present invention is that a timer is added into the femto gateway, and the modified algorithm of the femto gateway may be utilized to suspend the relocation procedure before the timer expires, and confirming whether exists available femtocells for serving the communication device or not. If the timer expires, then relocating, and communication services are processed by a macrocell. Therefore, the present invention can reduce effectively the handover frequency from the communication device to the macrocell, and thereby reducing bandwidth burden of the macrocell, reducing delay time caused by handovers and promoting data transmission rate and quality of call. 
     To further understand technical contents and methods of the present invention, please refer to the following detailed description and drawings related the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings; however, those skilled in the art will appreciate that these examples are not intended to limit the scope of the present invention, and various changes and modifications are possible within the spirit and scope of the present invention. 
         FIG. 1  shows an embodiment of a femto network architecture of the present invention. 
         FIG. 2  shows a preferred embodiment of a method for reducing the handover frequency in Femtocell Networks. 
         FIG. 3  shows a method for a call re-establishment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention and embodiments are now described in detail. In the diagrams and descriptions below, the same symbols are utilized to represent the same or similar elements. The possible embodiments of the present invention are described in illustrations. 
     The communication device as described below comprises mobile phone, PDA, smart phone but not limited these mobile communication devices which are compatible with various protocols, for example W-CDMA, CDMA2000, CDMA2001, TD-CDMA, TD-SCDMA, UWC0136, DECT, 3G, 4G system, etc. 
     Referring to  FIG. 1 , it shows an embodiment of a femto network architecture of the present invention. A core network comprises Home Location Register (HLR)  103 , Visitor Location Register (VLR)  104 , and Mobile Switching Center (MSC)  105 . In general, function between the Visitor Location Register (VLR)  104  and the Mobile Switching Center (MSC)  105  is inseparable. For example, the Mobile Switching Center (MSC)  105  may be used to query the Visitor Location Register (VLR)  104  about Temporary Mobile Subscriber Identity (TMSI) information of the mobile device. Therefore, the Visitor Location Register (VLR)  104  and the Mobile Switching Center (MSC)  105  are generally built together, and connecting with the Home Location Register (HLR)  103  by a network for facilitating information exchange. Moreover, a call signal may be linked to Public Switched Telephone Network (PSTN) by the Mobile Switching Center (MSC)  105 . Radio Network Controller (RNC)  106  may be connected to the Visitor Location Register (VLR)  104  and the Mobile Switching Center (MSC)  105  to manage radio resource for allocating resources to the relevant macrocell (base station)  102 . For example, when a communication device  110  requires relocation, the Radio Network Controller (RNC)  106  will request the macrocell  102  for reserving resource to provide the communication device  110  to talk. A femto gateway  107  is connected to the Visitor Location Register (VLR)  104  and the Mobile Switching Center (MSC)  105  to manage and allocate resources to a plurality of femtocells  101  but not the macrocell  102 . In some embodiments, the femto gateway  107  is connected to the femtocells  101  through a broadband network (internet)  109 . 
     One feature of the present invention is that a timer  108  is added into the femto gateway  107 , and modifying algorithm of the femto gateway  107  to suspend the occurrence of relocation mechanism. Before the timer  108  expires, searching available femtocells  101  for serving the communication device  110 . The “available” represents its signal strength higher than a predetermined threshold to provide call service. If the timer  108  expires, none available femtocells  101  found, then relocating, and communication services are processed by the macrocell  102 . 
     Referring to  FIG. 2 , it shows a preferred embodiment of a method for reducing the handover frequency in femtocell networks. The steps of the method are described as below. Firstly, in step  201 , an initial femtocell periodically receives measurement reports from a communication device. The “initial” femtocell indicates a cell presently used by the communication device to differentiate other femtocells. The measurement reports include signal strength values detected for surrounding cells by the communication device. Next, in step  202 , utilizing the femtocell, based-on the measurement reports, it is performing to check whether signal strength of the initial femtocell is less than a threshold or not. If it is, then proceeding to step  203 , sending a relocation required message to a femto GW by the initial femtocell; if not, then back to the step  201 , periodically receiving measurement reports by the initial femtocell. In step  204 , by the step  203 , after the femto gateway receives the relocation required message, relocation procedure is suspended based-on modified algorithm of the femto gateway and then starts a timer. Meanwhile, the communication (mobile) device occurs radio link failure because the femto gateway suspends the relocation required message, and then sending a call re-establishment request message to re-establish call. Subsequently, in step  205 , it confirms whether the femto GW receives the call re-establishment request message or not. If not receiving the call re-establishment request message, then proceeding to step  211 , it performs a process of releasing resource linked and reserved by Radio Resource Control by the communication device and the initial femtocell, stop to call. If receiving the call re-establishment request message, then proceeding to step  206 , it performs a process of checking a cell ID (identification) contained within the call re-establishment request message, and followed by the step  207 , determining whether the cell ID belongs to the femtocell or not by the femto gateway. 
     If the cell ID does not belong to the femtocell, then proceeding to step  209 , determining (checking) whether the timer expires or not by the femto gateway. If the timer expires, then proceeding to step  210 , it performs a process of re-establishing call to a macrocell and then sending a call re-establishment confirm message to the communication device for facilitating the communication device calling with the macrocell. In other words, if the timer expires, then proceeding a relocation mechanism for re-establishing call to the macrocell. 
     If the timer does not expire, then back to the step  205 , the femto gateway receives other call re-establishment request message to confirm (check) whether its cell ID belongs to the femtocell or not. If the cell ID belongs to a femtocell, then proceeding to step  208 , utilizing the femtocell, it is performing to re-establish a call to the femtocell in response to the cell ID, and then sending a call re-establishment confirm message to the communication device for facilitating the communication device calling with the femtocell. It should be noted that the femtocell in response to the cell ID may be the initial femtocell or others femtocell, which depends on signal strength at location of the communication device. If signal strength of the initial femtocell is more than that of other cells, then the communication device is served by the initial femtocell. Relatively, if signal strength of the initial femtocell is less than that of other cell, then the communication device is served by the other cell. In other words, in the present invention, if the communication device temporarily leaves service range of the initial femtocell and back to the service range prior to expiration of the timer, then the initial femtocell continuously serving without any relocation mechanism, and therefore effectively eliminating delay time of handover and reducing bandwidth burden of the macrocell. 
     Referring to  FIG. 3 , it shows a method for a call re-establishment of the present invention. The steps of the method are described as follows. Firstly, in step  301 , it is performing a process of cell re-selection procedure by the communication device to search a better cell with the best signal. Next, in step  302 , sending a cell update message to the better cell by utilizing the communication device is performed as the communication device has found the better cell. In order to facilitate concise description, the better cell is referred to as a new cell and the initial femtocell is referred to as an old cell, in the following. Subsequently, in step  303 , it performs a process of sending the cell update message to the old (initial) cell by the new cell. For example, the new cell searched by the communication device may send an Uplink Signaling Transfer Indication with the cell update message to the old (initial) cell. Next, in step  304 , the new cell is requested for reserving resource to serve the communication device as the old (initial) cell receives the cell update message. Subsequently, in step  305 , it is performing a process of checking whether the new cell has enough resource providing for the communication device or not as the new cell receives the request. If it is “yes”, then proceeding to step  306 , sending reserve resource confirm message to the old (initial) cell after the new cell has reserved resource. Then, in step  307 , after the old (initial) cell receives the reserve resource confirm message, it performs a process of releasing radio resource by the old (initial) cell and then sending an uplink signaling transfer indication confirm message to the new cell. Finally, in step  308 , it performing a process of sending the cell update confirm message to the communication device by the new cell to accomplish the call re-establishment procedure. 
     As above-mentioned will be understood by persons skilled in the art, the foregoing preferred embodiment of the present invention illustrates the present invention rather than limiting the present invention. Having described the invention in connection with a preferred embodiment, modifications will be suggested to those skilled in the art. Thus, the invention is not to be limited to this embodiment, but rather the invention is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation, thereby encompassing all such modifications and similar structures. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made without departing from the spirit and scope of the invention.