Abstract:
A mobile UE initiates a full search of all supported frequencies upon detecting a change in the Mobile Country Code (MCC) transmitted by a PLMN. The change in MCC indicates the user had crossed a political border, and the spectrum allocated for wireless communication may be different. In one embodiment, the mobile UE stores the current MCC. Upon obtaining an updated MCC, it is compared to the stored MCC. If the updated MCC is different from the stored MCC, the full frequency search is initiated. To prevent excessive searching, with concomitant power consumption, while traveling near a border, the UE waits a predetermined duration after an MCC-initiated full scan before conducting a subsequent full scan, even if the MCC changes within that duration.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communication systems, and in particular to triggering a PLMN frequency search upon a change in country code. 
     BACKGROUND 
     Wireless communication systems, also known as Public Land Mobile Networks (PLMN) are well known and widely deployed. A variety of protocols are in use, such as GSM, CDMA 2000, WCDMA, UMTS, and the like. These systems are operated at different frequencies by different service providers, depending on the spectrum allocated and made available by the governments of the countries in which they are deployed. In particular, many systems utilize Frequency Division Duplex (FDD), whereby downlink traffic from a network to User Equipment (UE) is transmitted on carriers that are spaced apart in frequency from uplink traffic from UE to the network. Other systems utilize Time Division Duplex (TDD), and carry both uplink and downlink transmissions in one frequency band. Frequency bands that are utilized in one geographical area may or may not be the same frequencies used in other geographical areas. For example, while most of Europe has service providers offering GMS in bands at 900 and 1800 MHz and UMTS at 2100 MHz, service providers in both Sweden and Norway also provided a CDMA 2000 system at 450 MHz. 
     Modern mobile UE often include frequency-agile transceivers, and can operate in two or more different PLMNs operating at different frequencies. On power-up, for example, these UE scan all of the frequencies in which they can operate, to locate an operative PLMN cell and register for wireless communication services. The UE may operate only within the “found” frequencies, so long as the user does not move out of the current geographical area. To detect such movement, and for other reasons, the UE periodically engages in a full linear search of all supported frequencies. This is to detect movement into a different geographical area, and also to locate a preferred or prioritized PLMN (for example, one having an economic arrangement with the user&#39;s home PLMN that reduces the cost of the preferred PLMN over other available systems), or to locate a system that provides additional services (e.g., dedicated packet data services, audio/video content broadcasts, and the like). 
     A full frequency search consumes power, as the transceiver circuits must change their operating characteristics. Since most mobile UE are battery powered, conserving power consumption is an important consideration, to provide a reasonable useful life between recharges. Accordingly, full searches of the mobile UE&#39;s supported spectrum are preferably infrequent from the viewpoint of power management. However, a longer delay between such frequency band searches prolongs the time required for the mobile UE to detect a new cell when the UE leaves the current geographical area and moves into another geographical area where other frequency bands are utilized. It would therefore be beneficial for the mobile UE to detect its movement into another geographical area, to trigger a frequency search sooner than the next scheduled routine frequency search. 
     SUMMARY 
     According to one or more embodiments of the present invention disclosed and claimed herein, a mobile UE initiates a full search of all supported frequencies upon detecting a change in the Mobile Country Code (MCC) transmitted by a PLMN. The change in MCC indicates the user had crossed a political border, and the spectrum allocated for wireless communication may be different. In one embodiment, the mobile UE stores the current MCC. Upon obtaining an updated MCC, it is compared to the stored MCC. If the updated MCC is different than the stored MCC, the full frequency search is initiated. To prevent excessive searching, with concomitant power consumption, while traveling near a border, the UE waits a predetermined duration after an MCC-initiated full scan before conducting a subsequent full scan, even if the MCC chances within that duration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of two PLMNs operating across a political border. 
         FIG. 2  is a functional block diagram of a UE. 
         FIG. 3  is a flow diagram of a method of efficiently identifying a change in MCC when searching carrier frequencies. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a mobile User Equipment (UE)  10  in wireless communication with a PLMN  100  in a first country. The PLMN  100  includes a Node B  102  providing wireless communication services within a cell  104 , under the control of a Radio Network Controller (RNC)  106 . The RNC  106  is connected to a Core Network (CN)  108 , which in turn communicates with various networks such as a Public Telephone Switching Network PTSN) the Internet  300 , or the like. As the UE  10  moves through the cell  104 , in the region of the border between the first country hosting the PLMN  100  and a second country, the UE may establish communications with a PLMN  200  in the second country. The PLMN  200  similarly includes a Node B  202  providing wireless communication services within a cell  204 , under the control of a Radio Network Controller (RNC)  206 . The RNC  206  is connected to a Core Network (CN)  208 , which in turn communicates with various networks such as the Internet  300 . 
     The PLMN  200  may offer wireless communication services over frequency bands different than those offered by the PLMN  100 . Normally, the UE  10  will discover these services only upon performing a full search of all frequencies on which the UE  10  is capable of receiving wireless communication service. However, if the UE  10  recently concluded such a search, the next scheduled full frequency search—delayed to conserve battery power—may impose a suboptimal delay before the UE  10  discovers the full range of services offered by PLMN  200 . Since one of these different-frequency services may be preferred, the mobile UE  10  would optimally conduct the search and discover the services sooner 
     The UE  10  stores the Mobile Country Code (MCC) it receives from the PLMN  100 , which corresponds to the first country. Upon nearing or crossing the border to the second country, the UE  10  will receive a different MCC from the PLMN  200 . Upon detecting a change in the MCC, the UE  10  initiates a full frequency search, to determine the full range of wireless communication services provided by the PLMN  200 . The cost in power consumption for performing the full frequency search, perhaps relatively soon following a previous full frequency search, is offset by the ability to exploit the full services provided by the PLMN  200 . For example, a wireless communication service provided by the PLMN  200  in a frequency band different than that utilized by the PLMN  100  may incur a lower cost for use, such as due to a business arrangement between the provider or operator of the PLMN  200  and user&#39;s home wireless service provider. As another example, the PLMN  200  may provide High Speed Packet Access (HSPA) in one of the different frequency bands, allowing the UE  10  to access data services, such as web browsing, email, broadcast content, and the like. 
     If the UE  10  moves around in the area depicted in  FIG. 1 , it may cross the border several times, or receive MCCs from PLMN  100  and PLMN  200  several times. In this case, triggering a full frequency search upon every MCC change would entail excessive power consumption. To prevent this, upon a full frequency search in response to detecting an MCC change, the UE  10  waits a predetermined duration prior to performing another full search, even if the MCC changes in the meantime. The predetermined duration may be implemented in hardware as, e.g., a count-down timer, or may be implemented in software by storing a real-time clock value and monitoring when the real-time clock passes an offset. The predetermined duration may be hardwired, may be provisioned into the UE  10  at manufacture or during initialization, may be set during system configuration parameter negotiation upon registering with a PLMN  100 ,  200 , or the like. In general, the predetermined duration may be fixed or programmable. 
     The UE  10  may discover a changed MCC in a variety of ways. In one embodiment, the UE  10  may be performing a limited search of frequencies known to be operated by PLMN  100 , when it discovers PLMN  200  having a different MCC. In this case, the UE  10  would expand the search to all of its supported frequencies, to fully discover the services provided by PLMN  200  of which it may take advantage. In another embodiment, the UE  10  may exit service in PLMN  100  having a first MCC, and re-enter service in PLMN  200  having a second MCC different than the first MCC. In this case, having already searched part of its supported frequencies, the UE  10  would expand its search to cover all of its supported frequencies. In yet another embodiment, wherein the UE  10  is not camped on its home PLMN, it may periodically perform a limited search for a preferred or prioritized PLMN. If, during this limited search, a PLMN with a different MCC is discovered, the UE  10  would initiate a full search of all of its supported frequencies. In still another embodiment, the UE  10  may support multiple Radio Access Technologies (RAT). If a PLMN having a changed MCC is discovered, the UE  10  would notify all RATs of the changed MCC, and would initiate a full frequency search in each RAT, of all frequencies supported in that RAT. 
     A functional block diagram of a mobile UE  10  according to one embodiment of the present invention is depicted in  FIG. 2 . The UE  10  includes a variable frequency transceiver  12  connected to an antenna  13 . The variable frequency transceiver  12  is operative to receive and transmit wireless communications signals in at least two frequency bands. The variable frequency transceiver  12 , also known as a multi-mode transceiver, is controlled by a controller  14 , which may comprise a stored-program microprocessor or Digital Signal Processor (DSP). The controller  14  is operatively connected to memory  16 , which stores a current MCC value  18 . The variable frequency transceiver  12  is also connected to a user interface module  20 , which is also under the control of the controller  14 . The user interface  20  includes a speaker, microphone, display, keypad, and the like to enable communication with the user. The UE  10  may include numerous functions not germane to the present invention, and omitted for clarity, such as dedicated graphics, audio, and/or video processing circuits, a cryptographic engine, a camera, and the like. Those of skill in the art will recognize that the functional modules  12 ,  14 ,  20  may be implemented as dedicated or programmable hardware, software modules executed on a microprocessor or DSP, or any combination of hardware, software, and firmware known in the art. 
       FIG. 3  depicts a method  400  of efficiently identifying a change in MCC when searching carrier frequencies in a wireless communication system by a mobile UE  10 . The UE  10  obtains a current MCC from a current PLMN (block  402 ), and stores the current MCC (block  404 ). The UE  10  later obtains an updated MCC (block  406 ) and compares it to the stored MCC. If the updated MCC is different than the stored MCC (block  408 ), then the UE  10  performs a full search of all supported frequencies (block  410 ), and stores the updated MCC (block  412 ) (replacing the previously stored MCC). The UE  10  then waits a predetermined duration ( 414 ) before again obtaining an updated MCC (block  406 ) to determine if a border has again been crossed. In another embodiment, only the frequency search may be delayed by the predetermined duration, and the UE  10  may continue to receive an updated MCC during this duration. 
     Embodiments of the present invention decrease the time required to find service when moving between geographical areas with wireless communication services provided in different frequency bands. The UE minimizes the time it is not registered to its home PLMN. The reduced time to find new services and lowered power consumption due to restrictions on MCC-triggered full frequency searches enhance the user&#39;s experience. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.