CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/325,387 filed on Apr. 19, 2010 and entitled “Method and Apparatus for load balancing in connection establishment in a wireless communication system”, the contents of which are incorporated herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The application relates to a method used in a wireless communication system and related communication device, and more particularly, to a method for load balancing in a wireless communication system and related communication device. 
         [0004]    2. Description of the Prior Art 
         [0005]    A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as to user equipments (UEs). 
         [0006]    LTE technology supports packet based services only. However 3GPP does specify fallback for circuit switched (CS) services as well. To achieve this, LTE architecture and network nodes require additional functionality. CS Fallback defines a mechanism for using a CS legacy network (e.g. Global System for Mobile communications (GSM) or Universal Mobile Telecommunications System (UMTS)) to provide voice services along side of an LTE network. CS fallback provides voice and traditional CS-domain services (e.g. voice call, video call, short message service (SMS), Unstructured Supplementary Service Data (USSD), supplementary service). To provide these CS services, LTE reuses CS infrastructure when the UE is served by E-UTRAN. 
         [0007]    A UE with multi-modes supports multiple radio access technologies (RATs) , such as GSM, UMTS, and LTE systems. A connection established in LTE can be handover to GSM or UMTS system and vice versa. Since the LTE system supports packet based services only, the aforementioned CS services in the GSM or UMTS systems are not supported in the LTE system. To make the CS services available to UEs camped on the LTE network, a CS fallback procedure is designed. When the UE initiates a CS service or receives a paging of the CS services in the LTE system, the UE is forced to connect to the GSM or UMTS system using “inter-RAT handover to GSM or UMTS”, “cell change order (CCO) to GSM or UMTS”, or “RRC connection release with redirection to GSM or UTMS”. The “inter-RAT handover”, “CCO” and “RRC connection release” procedures are specified in 3GPP TS 36.331 v9.1.0. 
         [0008]    The UE in radio resource control (RRC) connected mode performs PS data transmission and reception (e.g. ftp) in LTE network. The UE initiates a CS call or a Mobile Originated Location Request (MO-LR) by using CS fallback procedure. The network sends a RRC connection release message or a Mobility from EUTRA message to redirect the UE to UTRAN. When the UE camps on UTRAN, the UE initiates a RRC connection procedure. The UE performs random access and transmits a RRC Connection Request message including establishment cause “Originating Background Call”, triggered by PS data transmission (e.g. ftp) to the UTRAN. However, the UTRAN is designed to redirect UEs supporting LTE to the network for PS services for load balancing so the UTRAN redirects the UE to the LTE network. Therefore the UE fails to initiate the CS call. If the UE tries again, the UE is redirected back and forth between the UTRAN and LTE network. 
       SUMMARY OF THE INVENTION 
       [0009]    A method of load balancing for a network in a wireless communication system and a related communication device are provided. 
         [0010]    A method of load balancing for a first network in a wireless communication system is disclosed. The method comprises receiving a radio resource control (RRC) connection request message from a mobile device, wherein the RRC message includes a specific cause; and not redirecting the mobile device to a second network according to the specific cause. 
         [0011]    A method of load balancing for a mobile device in a wireless communication system is disclosed. The method comprises including a circuit-switched (CS) establishment cause in a radio resource control (RRC) connection request message when having a CS service and a packet-switched (PS) service; and sending the RRC connection request message to a network for RRC connection establishment. 
         [0012]    A method of loading balancing for a mobile device in a wireless communication system is disclosed. The method comprises moving from a first network to a second network; and including a cause in a radio resource control (RRC) message according to mobility of the mobile device. 
         [0013]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of an exemplary wireless communication system. 
           [0015]      FIG. 2  is a schematic diagram of an exemplary communication device. 
           [0016]      FIG. 3  illustrates the program code in  FIG. 2 . 
           [0017]      FIG. 4  is a flow chart of an exemplary process. 
           [0018]      FIG. 5  is a flow chart of an exemplary process. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Please refer to  FIG. 1 , which simply illustrates an exemplary wireless communication system  10 . In  FIG. 1 , a serving network  12  and a target network  14  employ different radio access technologies (RATs), and the mobile device supporting both of the RATs is served by the serving network  12 . The serving network  12  supports a single service domain and may be referred as to a LTE (long-term evolution) or a HSPA+(High Speed Packet Access Plus) system network only supporting a PS (Packet Switched) service domain. The target network  14  supports multiple service domains and may be referred as to a network of a UMTS (Universal Mobile Telecommunications System) system supporting both PS (Packet Switched) and CS (Circuit Switched) service domains. In the UMTS system, the network is referred as a Universal Terrestrial Radio Access Network (UTRAN) comprising a radio network controller (RNC) and a plurality of NBs (Node Bs); In the LTE system, the network is referred as a evolved-UTRAN (E-UTRAN) comprising a plurality of eNBs (evolved-Node Bs). The mobile device is referred as to a user equipment (UEs) or a mobile station (MS) supporting the abovementioned RATs and may be a device such as a mobile phone, a computer system, etc. Besides, the networks  12  and  14  and the mobile device can be seen as a transmitter or receiver according to transmission direction, e.g., for uplink (UL), the mobile device is the transmitter and the networks  12  and  14  are the receivers, and for downlink (DL), the networks  12  and  14  are the transmitters and the mobile device is the receiver. When a subscriber intends to have CS services (e.g. voice call, video call, short message service (SMS), Unstructured Supplementary Service Data (USSD), supplementary service), the mobile device makes a service request to the serving network  12 , performing CS fallback to redirect the UE to the target network  14 . To provide these services, CS fallback allows the serving network  12  reuses CS infrastructure of the target network  14  when the mobile device is served by the serving network  12 . 
         [0020]    Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram of an exemplary communication device  20 . The communication device  20  can be the mobile device or the networks  14  or  12  shown in  FIG. 1  and may include a processing means  200  such as a microprocessor or ASIC, a memory unit  210 , and a communication interfacing unit  220 . The memory unit  210  may be any data storage device that can store program code  214  for access by the processing means  200 . Examples of the memory unit  210  include but are not limited to a subscriber identity module (SIM), read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storage devices. The communication interfacing unit  220  is preferably a radio transceiver for wirelessly communicating with the network according to processing results of the processing means  200 . 
         [0021]    Please refer to  FIG. 3 , which illustrates the program code  214  in  FIG. 2 . The program code  214  includes program code of multiple communications protocol layers, which from top to bottom are a radio resource control (RRC) layer  300 , a packet data convergence protocol (PDCP) layer  310 , a radio link control (RLC) layer  320 , a medium access control (MAC) layer  330  and a physical (PHY) layer  340 . 
         [0022]    Please refer to  FIG. 4 , which is a flow chart of an exemplary process  40 . The process  40  is used for load balancing for a wireless communication system. The wireless communication system could be the wireless communication system  10  and include a UE, networks NT 1  and NT 2 . The network NT 1  could be the serving network  12 . The UE was having a PS data transmission (e.g. ftp) in the network NT 1  and initiating a CS service via a CS fallback procedure. The UE received a RRC connection release message or a Mobility from EUTRA message to redirect the UE to a current network NT 2 . The network NT 2  could be the target network  14  in the wireless communication system  10 , which supports both of CS domain and PS domain. The UE camps on the network NT 2  and initiates a RRC connection procedure for the CS service. The process  40  can be compiled into the program code  214  and include the following steps: 
         [0023]    Step  400 : Start. 
         [0024]    Step  402 : The UE includes a CS establishment cause in a RRC connection request message M 1  when having the CS service and the PS data transmission in the network NT 2  (e.g. UTRAN). 
         [0025]    Step  404 : The UE sends the RRC connection request message M 1  to the network NT 2  for RRC connection establishment. 
         [0026]    Step  406 : The network NT 2  receives the RRC connection request message M 1  from the UE. 
         [0027]    Step  408 : The network NT 2  does not redirect the UE to the network NT 1  (e.g. E-UTRAN) according to the CS establishment cause. 
         [0028]    Step  410 : End. 
         [0029]    According to the process  40 , when the UE which has been having the PS data transmission initiates the RRC connection procedure for the CS service in the network NT 2 , the UE includes the CS establishment cause in the RRC connection request message M 1 . The CS service may include a voice call, video call, short message service (SMS), Unstructured Supplementary Service Data (USSD), supplementary service. The PS data transmission is initiated or ongoing. The UE sends the RRC connection request message M 1  to the network NT 2  for RRC connection establishment. The network NT 2  receives the RRC connection request message M 1  from the UE. According to the CS establishment cause, the network NT 2  does not redirect the UE the network NT 1 . In other words, the UE perform RRC connection establishment with the network NT 2  instead of redirecting the UE to the network NT 1  when the CS establishment cause is found in the RRC connection message M 1 . Thus, the UE is not redirected back and forth between the network NT 1  and the network NT 2 . 
         [0030]    In some examples, the network NT 2  may receive a RRC connection request message M 2  from the UE. The RRC connection request message M 2  does not include the CS establishment cause. Instead, the RRC connection request message M 2  may include a PS establishment cause. In this situation, the network NT 2  redirects the UE to the network NT 1  according to the PS establishment cause. 
         [0031]    Please refer to  FIG. 5 , which is a flow chart of another exemplary process  50 . The process  50  is used for load balancing for a wireless communication system. The wireless communication system could be the wireless communication system  10  and include a UE, networks NT 3  and NT 4 . The network NT 3  could be the serving network  12  or a GSM (Global System for Mobile communications) network. A UE was having a PS data transmission (e.g. ftp) in the network NT 3  and initiating a CS service via a CS fallback procedure. The UE received a RRC connection release message or a Mobility from EUTRA message to redirect the UE to a current network NT 4 . The network NT 3  could be the target network  14  in the wireless communication system  10 , which supports both of CS domain and PS domain. The UE camps on the network NT 4  and initiates a RRC connection procedure for the CS service. The process  50  can be compiled into the program code  214  and include the following steps: 
         [0032]    Step  500 : Start. 
         [0033]    Step  502 : The UE includes a specific cause in a RRC connection request message M 3  when moving from the network NT 3  (e.g. E-UTRAN or GSM) to the network NT 4  (e.g. UTRAN). 
         [0034]    Step  504 : The UE sends the RRC connection request message M 3  to the network NT 4  for RRC connection establishment. 
         [0035]    Step  506 : The network NT 4  receives the RRC connection request message M 3  from the UE. 
         [0036]    Step  508 : The network NT 4  does not redirect the UE to the network NT 3  according to the specific cause. 
         [0037]    Step  510 : End. 
         [0038]    According to the process  50 , when the UE is redirected from the network NT 3  to the network NT 4  the UE includes the specific cause in the RRC connection request message M 3 . Then, the UE send the RRC connection request message M 3  to the network NT 4  for RRC connection establishment. The network NT 4  receives the RRC connection request message M 3  from the UE. According to the specific cause, the network NT 4  does not redirect the UE the network NT 3 . In other words, the UE perform RRC connection establishment with the network NT 4  instead of redirecting the UE to the network NT 3  when the specific cause is found in the RRC connection message M 3 . Thus, the UE is not redirected back and forth between the network NT 3  and the network NT 4 . 
         [0039]    Preferably, the specific cause is Inter-RAT cell changed order or designed for redirection trigged by a RRC connection release or a RRC connection reject. 
         [0040]    Please note that the abovementioned steps including suggested steps can be realized by means that could be hardware, firmware known as a combination of a hardware device and computer instructions and data that reside as read-only software on the hardware device, or an electronic system. Examples of hardware can include analog, digital and mixed circuits known as microcircuit, microchip, or silicon chip. Examples of the electronic system can include system on chip (SOC), system in package (Sip), computer on module (COM), and the communication device  20  in which the processing means 200 processes the program code  214  related to the abovementioned processes and the processed results can handle load balancing in the wireless communications system  10 . 
         [0041]    To sum up, when the UE has both CS service and PS data transmission, the UE include a CS establishment cause in an RRC connection request message and send the RRC connection request message to UTRAN. When UTRAN receives the RRC connection request message, UTRAN does not redirect the UE to E-UTRAN according to the CS establishment cause. In another example, the UE includes a specific cause in the RRC connection request message when the UE moves from E-UTRAB/GSM to UTRAN. When UTRAN receives the RRC connection request message, UTRAN does not redirect the UE to E-UTRAN according to the specific cause. According examples of present disclosures, the UE is not redirected back and forth between E-UTRAN and UTRAN. 
         [0042]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
A method of load balancing for a first network in a wireless communication system is disclosed. The method comprises receiving a radio resource control (RRC) connection request message from a mobile device, wherein the RRC message includes a specific cause; and not redirecting the mobile device to a second network according to the CS establishment cause.