Patent Abstract:
Once an MS&#39;s destination area has been determined, the MS receives a frequency list of BSs belonging to the destination area from a broadcasting BS before disconnecting the current network connection, and creates and stores several Search Configurations. When performing a cell search upon arrival at the destination area, the MS connects to a broadcasting BS in the destination and receives a frequency list of the BSs belonging to the destination area. The MS compares the frequency list that it maintains with the frequency list broadcasted from the BS in the destination and performs a cell search according to the result of the comparison. This leads to achieving reliable network connection and reducing time to establish connection.

Full Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application serial no. 2014-218964, filed on Oct. 28, 2014, the content of which is hereby incorporated by reference into this application. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a wireless communication base station system, a wireless communication system, and a wireless mobile station. More particularly, the invention relates to a wireless communication base station system, a wireless communication system, and a wireless mobile station elaborated to reduce time a wireless mobile station takes to complete a cell search. 
       BACKGROUND 
       [0003]    Generally, when a wireless mobile station (hereinafter referred to as MS) initiates wireless communication, the MS performs network entry via a wireless base station (hereinafter referred to as BS). Network entry is processing that the MS must perform to establish connection to a network of a wireless communication system. In a phase before the MS performs network entry, the MS performs a cell search operation to search through radio waves emitted by its surrounding base stations. The MS searches through a search range of frequencies (from a start frequency to an end frequency) assigned to a wireless communication system in determined frequency gaps (frequency steps). The MS checks to see whether or not there is a BS whose center frequency matches a frequency to search for by the cell search. If the MS has detected a BS having a center frequency that enables communication during the search, the MS attains synchronization with the BS and then initiates network entry processing. 
         [0004]    However, an MS typically performs a search in frequency steps that are finer than frequency steps between center frequencies that BSs use. Consequently, a cell search takes much time under the assumption that an MS searches through all applicable frequencies within a search range. 
         [0005]    Accordingly, in a technique which is disclosed in Japanese Unexamined Patent Application Publication No. 2007-116561, from a mobile communication network accessed by a mobile terminal, the mobile terminal acquires information representing frequencies that are used in the mobile communication network and stores that information within it. When performing a cell search over a search range of frequencies, the mobile terminal skips frequencies that obviously do not match the center frequencies of BSs in the mobile communication network. Furthermore, based on a search result, the mobile terminal expands a range of frequencies to skip, thus reducing time for a subsequent cell search. 
       SUMMARY OF THE INVENTION 
       [0006]    When an MS performs a cell search, the MS takes a certain amount of time to complete the cell search, if a search range and frequency steps are not taken into consideration. In a wireless communication system required to have high reliability, once an MS has entered a BS area, time the MS takes to connect to a network is desired to be as short as possible. Thus, it is needed to minimize time that the MS takes to complete a cell search. 
         [0007]    In the present invention, Aeronautical Mobile Airport Communication System (AeroMACS) which is built on the basis of Mobile Worldwide Interoperability for Microwave Access (WiMAX) technology is assumed to be used. AeroMACS is one of wireless communication systems to which the present invention is intended to apply. An application scope of AeroMACS is the surface of an airport (the whole airport). AeroMACS provides a high-speed mobile communication system for airports. For AeroMACS, reliable and rapid communication is required to implement operations involved in flight operations on the surface of an airport, which are taking off and landing of aircrafts. Thus, it is required of AeroMACS to reduce time required to initiate communication with a BS and initiate communication reliably. 
         [0008]    A cell search in AeroMACS is described. In the AeroMACS specs, a start frequency is 5095 MHz and an end frequency is 5145 MHz. Because frequency steps are 250 kHz, if an MS performs a search through all applicable frequencies within a search range, the MS is to search through frequencies from 5095 MHz to 5145 MHz in 250 kHz steps. However, the width of gaps between the center frequencies of BSs adopted in AeroMACS is 5 MHz (the center frequencies are adopted from among frequencies spaced by a 5-MHz gap including 5095 MHz, 5100 MHz, etc. up to 5145 MHz). 
         [0009]    Consequently, at an MS, a cell search is to be performed in very fine frequency steps. Under the conditions mentioned above, if the center frequency of a BS selected as a target access point for an MS is 5095 MHz which is the start frequency in the AeroMACS specs, the MS will complete a cell search by one search action. But if the center frequency of the BS selected as the target access point is set to 5145 MHz which is the end frequency, the MS needs to perform search actions 201 times to complete a cell search. 
         [0010]    A significant difference occurring in the number of search actions results in a difference in the cell search time until connection is established. When compared with the processing time required for network entry, it is undesirable that processing of a cell search takes very long for a BS whose center frequency is set to the end frequency, which is the worst-case scenario. 
         [0011]    In addition, even if an MS is able to uniquely store the center frequencies of BSs to search and a situation allows the MS to complete a cell search by one search action, airport surface communications may be affected by weather or equipment trouble and, consequently, a destination network manager may change the center frequencies of BSs or the location to which the MS is destined to move. For this reason, it may happen that the MS cannot use information representing the center frequencies of BSs which are uniquely set within it because the center frequencies have been changed just by making an MS uniquely store the center frequencies of BSs, with the result that the MS has to perform a cell search over a search range of frequencies. A time-consuming cell search over a search range of frequencies defined in the specs of a communication system should be performed exclusively for use as a final measure. 
         [0012]    To solve a problem discussed above, there is provided a wireless communication base station system including a first broadcasting wireless base station that broadcasts information on wireless base stations installed in a first location, a second broadcasting wireless base station that broadcasts information on wireless base stations installed in a second location, and a frequency management entity. The frequency management entity includes a control unit that performs message transmission and reception and a storage unit that stores information representing center frequencies which are used by wireless base stations in the second location and a coverage area. The control unit, upon receiving a message requesting information on the wireless base stations installed in the second location from a wireless mobile station which is going to move from the first location to the second location, transmits information on the wireless base stations installed in the second location, which is stored in the storage unit, to the wireless mobile station via the first broadcasting wireless base station. 
         [0013]    There is also provided a mobile wireless station that may be connected to a wireless communication base station system including a first broadcasting wireless base station that broadcasts information on wireless base stations installed in a first location, a second broadcasting wireless base station that broadcasts information on wireless base stations installed in a second location, and a frequency management entity. The mobile wireless station includes a mobile station control unit that performs message transmission and reception and compares information on wireless base stations and a mobile station storage unit. The mobile station control unit, upon receiving information on base stations from the frequency management entity via the first broadcasting wireless base station and deciding that a plurality of the second wireless base stations serve the coverage area, creates an Area Search Configuration in which the smallest and largest ones of center frequencies used by the plurality of second wireless base stations are set as search start and end frequencies for a cell search and a frequency step size to search defined in the wireless communication system is set as frequency steps, and stores the Area Search Configuration into the mobile station storage unit. The mobile station control unit, upon receiving the information on base stations and deciding that one second wireless base station serves the coverage area, creates an Area Search Configuration in which a center frequency used by the second wireless base station is set as search start and end frequencies, and stores the Area Search Configuration into the mobile station storage unit. 
         [0014]    There is further provided a wireless communication system combining the wireless communication base station system with the mobile wireless station. 
         [0015]    According to the present invention, it is possible to reduce time that an MS takes to connect to a network. Even when a target access point (BS) has been changed, a cell search for an intended frequency of the target access point can be performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which; 
           [0017]      FIG. 1  is a block diagram to explain a network structure; 
           [0018]      FIG. 2  is a block diagram to explain an MS structure; 
           [0019]      FIG. 3  is a block diagram to explain a frequency management entity structure; 
           [0020]      FIG. 4  is a sequence diagram to explain a process of updating a frequency list; 
           [0021]      FIG. 5  is a diagram to explain a home location frequency list; 
           [0022]      FIG. 6  is a diagram to explain a foreign location frequency list; 
           [0023]      FIG. 7  is a sequence diagram to explain a process in which an MS receives a frequency list; 
           [0024]      FIG. 8  is a flowchart to explain a process of creating Search Configs; 
           [0025]      FIG. 9  is a diagram to explain a search list; 
           [0026]      FIG. 10  is a flowchart to explain a process of using Search Configs; 
           [0027]      FIG. 11  is a diagram to an aircraft which is going to arrive at an airport and the coverage areas of BSs; 
           [0028]      FIG. 12  is a flowchart to explain a process of performing a cell search using Area Search Config; 
           [0029]      FIG. 13  is a flowchart to explain a process of performing a cell search using Location Search Config; 
           [0030]      FIG. 14  is a diagram to explain cell search processing using Area Search Config A; 
           [0031]      FIG. 15  is a diagram to explain cell search processing using Area Search Config C; 
           [0032]      FIG. 16  is a diagram to explain cell search processing using Location Search Config; and 
           [0033]      FIG. 17  is a diagram to explain cell search processing using Default Search Config. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In the following, modes will be described in detail using embodiments and with reference to the drawings. 
         [0035]    Referring to  FIG. 1 , descriptions are provided for a structure of a communication system and AeroMACS networks. In  FIG. 1 , a communication system  500  is configured including networks in locations (AeroMACS networks)  100  and an MS  200 . Each AeroMACS network  100  is comprised of an Access Service Network (ASN)  130  and a Connectivity Service Network (CSN)  150 . 
         [0036]    The ASN  130  has a wireless connection function for communication with an MS  200 . The ASN  130  provides a function of connecting a MAC layer for management of radio resources information and a physical layer. The ASN  130  is configured including a broadcasting BS  131 , BSs  132 , and an ASN-Gateway (GW)  133 . The broadcasting BS  131  is a base station that provides a frequency list to the MS  200 . The BSs  132  are common base stations. The ASN-GW  133  ensures connectivity of the BSs  131 ,  132  with a Home Agent (HA) server  151  between the CSN  150  and the ASN  130 . The broadcasting BS  131  in a location A is denoted as the broadcasting BS  131 -A and the BSs  132  in the location are denoted as the BSs  132 -A; the same applies to BSs in a location B. 
         [0037]    The CSN  150  performs tunneling to an MS  200 . The CSN  150  provides an IP connection function including IP address distribution and communication channel processing using DHCP. The CSN is configured including an Authentication Authorization Accounting (AAA) server  152 , an HA server  151 , and a frequency management entity  300 . The AAA server  152  is a server that performs authentication, authorization, and accounting of an MS. The HA server  151  is a router that provides local communication within the CSN. The frequency management entity  300  transmits the frequencies of BSs in each location to an MS. 
         [0038]    In the communication system  500 , an MS  200  is a wireless mobile station installed in an aircraft which moves from one location to another. While staying in the location A, the MS  200  connects to the CSN  150  including the frequency management entity  300  via the broadcasting BS  131 -A or any of the BSs  132 -A and the ASN-GW  133 . When the MS moves out of the location A, the MS  200  performs disconnection processing via the BS communicating with it. When the MS arrives in its destination location B, the MS  200  performs a cell search for the center frequency of a broadcasting BS  131 -B, which is stored within it at takeoff from the location A, and detects the center frequency of the broadcasting BS  131 -B. 
         [0039]    The MS  200  attains synchronization with the broadcasting BS  131 -B and performs network entry processing. Once the MS has connected to the network, the broadcasting BS  131 -B transmits the center frequencies of BSs  132 - 1  to  132 - 3  in the location B to the MS  200  by use of the frequency management entity  300 . The MS  200  compares the center frequencies of the BSs  132 - 1  to  132 - 3  which are stored within it at takeoff from the location A with the center frequencies of the BSs  132 - 1  to  132 - 3  received from the broadcasting BS  131 -B in the location B and decides whether or not the comparison result is matched. 
         [0040]    If the comparison result is matched, the MS  200  performs a cell search using a Search Configuration (hereinafter referred to as a Search Config) stored in a search list which the MS created when it is in the location A. The MS  200  checks to see whether or not there is a BS whose center frequency matches a frequency to search for among the BSs  132 - 1  to  132 - 3  in the location B. When the MS  200  has detected a BS whose center frequency matches a frequency to search for during the cell search, the MS  200  attains synchronization with the BS  132  and performs a handover from the broadcasting BS  131 . 
         [0041]    If the comparison result is unmatched, the MS  200  creates a Search Config, using the center frequencies acquired from the broadcasting BS  131 -B, and performs a cell search. The MS  200  checks to see whether or not there is a BS having a center frequency that enables communication among the BSs  132 - 1  to  132 - 3  in the location B. When the MS  200  has detected a BS with which it can communicate during the cell search, the MS  200  attains synchronization with the BS  132  and performs handover processing from the broadcasting BS  131 . 
         [0042]    Although only one MS  200  is depicted in  FIG. 1 , plural MSs can connect to an appropriate one of the BSs. In the system structure depicted, respective frequency management entities  300  are installed in the locations A and B. However, the structure may be modified as follows: these entities are connected by the Internet which is indicated by a dotted line to provide a frequency management entity  300  that enables mutual management. If just one BS is used in an AeroMACS network due to the scale of a location, just one broadcasting BS may be provided in the network structure. 
         [0043]    Referring to  FIG. 2 , an MS structure is described. In  FIG. 2 , an MS  200  includes a baseband unit  210 , a search list creating unit  220 , a control unit  230 , antenna  260 , a storage unit  280 , and a wireless communication unit  290 . 
         [0044]    The baseband unit  210  performs modulation and demodulation processing on radio signals. The search list creating unit  220  creates a search list. The control unit  230  performs control of all components of the MS. The control unit  230  performs comparison between the center frequencies of base stations registered in a search list which the MS creates before its departure and the center frequencies of base stations in a location in which the MS has arrived, received from a broadcasting BS in the location in which the MS has arrived. The antenna  260  transmits and receives radio signals. The storage unit  280  records data. The wireless communication unit  290  executes communication with a BS. 
         [0045]    The wireless communication unit  290  includes a wireless transmitter  291  which transmits radio signals and a wireless receiver  292  which receives radio signals. The storage unit  280  holds a search list  281  and a frequency list  282 . 
         [0046]    Referring to  FIG. 3 , a frequency management entity structure is described. In  FIG. 3 , a frequency management entity  300  includes a control unit  310 , a frequency list updating unit  340 , a communication unit  350 , and a recording unit  360 . 
         [0047]    The control unit  310  performs control of all components of the frequency management entity  300 . The frequency list updating unit  340  updates frequency lists  361 ,  362  which are contained in the recording unit  360 . The communication unit  350  executes communication with a BS. The recording unit  360  records data. 
         [0048]    The recording unit  360  contains a home location frequency list  362  and a foreign location frequency list  361 . The home location frequency list  362  is information representing the center frequencies used by BSs installed in the home location. The foreign location frequency list  361  is information representing the center frequencies used by BSs installed in a foreign location. 
         [0049]    Referring to  FIG. 4 , descriptions are provided for a process in which a frequency management entity updates a frequency list it manages. In  FIG. 4 , a frequency management entity  300  creates an update inquiry message including a home location frequency list  362  which it has read from the storage unit  360  (S 401 ). Through the communication unit, the frequency management entity  300  transmits the update inquiry message to a broadcasting BS  131  installed in the home location (S 402 ). The broadcasting BS  131  transmits the update inquiry message to a BS  132  (S 403 ). The BS  132  receives the update inquiry message and transmits an update message to the broadcasting BS  131  (S 404 ). 
         [0050]    The frequency management entity  300  transmits an inquiry message to inquire if a change has been made to the center frequency that each BS uses. If so, in an update message, a BS  132  transmits the center frequency of the BS that received the update inquiry message. The broadcasting BS  131  executes this message exchange processing to all BSs in the same location in a parallel way. 
         [0051]    Through the communication unit  350 , the control unit  310  of the frequency management entity receives the update message  402  from the broadcasting BS. The control unit  310  instructs the frequency list updating unit  340  to update the home location frequency list  362  which is contained in the storage unit  360 , according to BSID and a center frequency included in the update message. The frequency list updating unit  340  updates the home location frequency list  362 , using information representing the BSID and a new center frequency. 
         [0052]    The description with  FIG. 4  assumes the system structure in which respective frequency management functions  300  are installed in the locations A and B. However, a single frequency management entity may be provided in a framework where the frequency management functions  300  are interconnected by the Internet and updated. In that case, the Internet-connected frequency management entity  300  will transmit an update inquiry message to each of the broadcasting BSs  131  in the locations A and B and receive an update message from each of them. In this case, the Internet-connected frequency management entity collectively manages the frequency lists in the locations without discrimination between home/foreign locations and, therefore, update message transmission becomes unnecessary. Also in examples that will be described hereinafter, the frequency management entities installed in the respective locations can also be interpreted as the Internet-connected frequency management entity in an alternative framework. 
         [0053]    Referring to  FIG. 5 , descriptions are provided for a home location frequency list which is managed by the frequency management entity. The home location frequency list  362  presented in  FIG. 5  is the home location frequency list  362  (particularly,  362 -A) which is managed by the frequency management entity in the location A. The home location frequency list  362  stores information on the BSs  132 - 1  to  132 - 3  installed in the location A. 
         [0054]    The home location frequency list  362  is comprised of the following fields: BSID  371 , location  372 , BS area  373 , and center frequency  374 . 
         [0055]    BSID  371  is a BS identifier for identifying each BS. Location  372  indicates information representing a location to which an MS may move and where the BS is installed. BS area  373  indicates information representing a location to which an MS may move and which is a coverage area of the BS. Center frequency  374  indicates information representing a center frequency that the BS uses. 
         [0056]    Referring to  FIG. 6 , descriptions are provided for a foreign location frequency list which is managed by the frequency management entity. The foreign location frequency list  361  presented in  FIG. 6  is the foreign location frequency list  361  (particularly,  361 -A) which is managed by the frequency management entity  300  in the location A. The foreign location frequency list  361  stores information on BSs installed in a location to which an MS may move and which is other than the location A. Like the home location frequency list  362 , the foreign location frequency list  361  is comprised of the following fields: BSID  381 , location  382 , BS area  383 , and center frequency  384 . Here, the list stores information on the location B and a third location to which an MS may be destined to move from the location A. A maintenance person is assumed to set up the foreign location frequency list  361  beforehand. 
         [0057]    Referring to  FIG. 7 , descriptions are provided for a process in which an MS receives a frequency list. In  FIG. 7 , an MS  200  that stays in the location A receives its destination (S 411 ). The MS  200  creates a message requesting a frequency list that contains information representing the center frequencies used by a broadcasting BS and BSs installed in the destination location and transmits the message addressed to a frequency management entity  300  through the baseband unit  210  and wireless transmitter  291  (S 412 ). A broadcasting BS  131  relays the frequency list request message and transmits the frequency list request message to the frequency management entity  300  (S 413 ). 
         [0058]    The frequency management entity  300  receives the frequency list request message. The control unit  310  of the frequency management entity  300  retrieves a requested frequency list (all records relevant to the location B in this case) in the destination location from the foreign location frequency list  361  in the storage unit  360  and creates a frequency list response message from that information. Through the communication unit  350 , the control unit  310  transmits the frequency list response message S 414  to the broadcasting BS  131  (S 414 ). The broadcasting BS  131  relays the frequency list response message and transmits the frequency list response message to the MS  200  (S 415 ). 
         [0059]    The control unit  212  of the MS  200  receives the frequency list response message through the wireless receiver  292  and baseband unit  210 . The control unit  212  stores the received information into the frequency list  282  in the storage unit  280 . The frequency list  282  has the same structure as the foreign location frequency list  361  described with  FIG. 6  and includes the fields of BSID, location, BS area, and center frequency. If the destination is the location B, the frequency list holds all records with “location B” in the location field in the foreign location frequency list  361  presented in  FIG. 6 . 
         [0060]    Referring to  FIG. 8 , descriptions are provided for a process in which an MS creates Area Search Config/Location Search Config based on a frequency list. Here, Search Config is an individual search list component created from a frequency list table. A search list is a collection of Search Configs. In the present embodiment, Area Search Config which is information for performing a search on a per-BS basis, Location Search Config for performing a search for all BSs existing in a location, and Default Search Config for performing a default search are stored in the search list. 
         [0061]    The search list creating unit  220  reads in the records of BS areas covering destination locations to move to from the frequency list  282  in the storage unit  280  (S 502 ). The search list creating unit  220  determines the number of BSs from the records that it reads in and determines how to create Search Config (S 503 ). To illustrate, based on, particularly, the frequency list presented in  FIG. 6 , the search list creating unit  220  determines that one BS covers an area A in the location B and two BSs cover an area B in the location B. 
         [0062]    If plural BSs cover an area of the destination, as determined at step  503 , the search list creating unit  220  determines a start frequency, an end frequency, and a frequency step size and creates a Search Config to be used in performing a cell search (S 504 ). As the start frequency, the unit  220  sets the smallest one of the center frequencies respectively used by the plural BSs that it reads in to the start frequency. As the end frequency, the unit  220  sets the largest one of the center frequencies respectively used by the plural BSs that it reads in to the end frequency. The unit  220  sets the frequency step size to 5 MHz. 
         [0063]    On the other hand, if one BS, which is only a broadcasting BS, covers an area of the destination, as determined at step S 503 , the search list creating unit  220  determines a start frequency, an end frequency, and a frequency step size and creates a Search Config (S 505 ). As the start/end frequencies, the unit  220  sets the center frequency used by the one BS that it reads in to the start and end frequencies. However, the search list creating unit  220  does not set the frequency step size. This is because the number of BSs to search for by a cell search is only one. 
         [0064]    The search list creating unit  220  creates a Search Configs (Area Search Configs) for each of the BS areas of the destination location and maintains them in the search list  281  in the storage unit  280 . The search list will be explained later, using  FIG. 10 . 
         [0065]    The search list creating unit  220  checks whether or not it has read in all records with information in the location field matching the destination (S 506 ). If there is a record of a BS read it has not read in yet, a return is made to step  502 . When the decision is “YES” at step  506 , the search list creating unit  220  creates a Location Search Config covering the frequencies used by the BSs in all BS areas (S 507 ) and terminates the process. 
         [0066]    At step  507 , the search list creating unit  220  determines a start frequency, an end frequency, and a frequency step size and creates a Location Search Config to be used in performing a cell search. As the start frequency, the search list creating unit  220  sets the smallest one of the center frequencies used by the plural BSs that it reads in to the start frequency. As the end frequency, the search list creating unit  220  sets the largest one of the center frequencies used by the plural BSs that it reads in to the start frequency. The search list creating unit  220  sets the frequency step size to 5 MHz. 
         [0067]    The search list creating unit  220  creates Search Configs (including a Location Search Config) in the destination and maintains them in the search list in the storage unit  280 . 
         [0068]    For the frequency step size, the search list creating unit  220  sets an appropriate value according to the width of gaps between the center frequencies of BSs adopted in the wireless communication system. Because AeroMACS is assumed as the wireless communication system here, the search list creating unit  220  sets the frequency step size to 5 MHz. If there are plural wireless networks to which an MS may connect to, the search list creating unit  220  sets the frequency step size according to the width of gaps between the center frequencies of BSs prescribed in the communication system of a wireless network to which the MS is to connect to. In the present embodiment, the search list creating unit  220  of an MS sets the frequency step size to a fixed value (5 MHz) specific to the AeroMACS network. 
         [0069]    Referring to  FIG. 9 , a search list that an MS maintains is described. The search list  281  in  FIG. 9  is comprised of Area Search Configs and a Location Search Config created according to the flowchart of  FIG. 8  and a Default Search Config. The search list  281  is used when the MS performs a cell search in the destination location B. 
         [0070]    The search list  281  in  FIG. 9  includes the following fields: item  241 , location  242 , BS area  243 , start frequency  244 , end frequency  245 , and frequency step size  246 . How to use the search list will specifically be described later with reference to  FIGS. 14 to 17 . 
         [0071]    Referring to  FIG. 10 , descriptions are provided for a process of comparing a frequency list that an MS maintains and a frequency list received from the broadcasting BS in the location B. In  FIG. 10 , when an MS  200  moves to the destination location B, its control unit  212  completes network entry, using an Area Search Config identified with “Broadcasting” specified in the BS area field of the list of Area Search Configs. 
         [0072]    The control unit  212  of the MS  200  reads in a frequency list in the location B received via the broadcasting BS  131 -B (S 512 ). The control unit  212  compares the frequency list with a frequency list  282  stored at takeoff from the location A (S 513 ). If both lists match, the control unit  212  performs a cell search for a location to move to, using Area Search Configs in the currently maintained list (S 514 ). 
         [0073]    If the comparison result is unmatched at step  513 , the control unit  212  creates Area Search Configs/Location Search Config based on the frequency list received from the broadcasting BS  131 -B in the location B and performs a cell search using Area Search Configs identified with “Location B” specified in the BS area field (S 515 ). 
         [0074]    When the MS performs a cell search using Area Search Configs, if the cell search is unsuccessful, the control unit  212  performs a cell search using a Location Search Config as recovery measures. If the cell search using the Location Search Config is unsuccessful again, the control unit  212  performs a cell search using a Default Search Config as recovery measures. The Default Search Config is defined with a search range and a frequency step size prescribed in the wireless communication system and pre-stored in the search list. Because AeroMACS is assumed as the wireless communication system here, if the MS performs a cell search using the Default Search Config, the MS performs a cell search in a range from 5095 MHz to 5145 MHz in frequency steps of 250 kHz. 
         [0075]    Because the AeroMACS network is applied in the present embodiment, one Default Search Config only exists in the search list. If the MS  200  is a terminal which may connect to plural wireless communication systems, the search list includes as many Default Search Configs as the number of the wireless communication systems; in each Default Search Config, a start frequency, an end frequency, and a frequency step size prescribed in each wireless communication system are stored. 
         [0076]    Referring to  FIG. 11 , descriptions are provided for an aircraft approaching the destination location B and a coverage area α of the broadcasting BS and BSs installed in the location, where the AeroMACS network is applied. The coverage area α indicates a scope in which the broadcasting BS  131  and BSs  132 - 1  to  132 - 3  installed in the location B can connect with an MS. In particular, when an MS  200  initiates mobile communication, first, the MS is to connect to the broadcasting BS  131  and execute operations described in  FIG. 10 . Then, the MS  200  is to perform communication with one of the BSs  132 - 1  to  132 - 3 . To perform communication with the BS  131  and one of the BSs  132 , the MS installed in the aircraft  400  must enter the coverage areas of the BSs. The location B is covered by the coverage areas of the BSs and a part of the location B is covered by the coverage areas of plural BSs. 
         [0077]    The MS  200  is to perform communication with the broadcasting BS  131  at its arrival. The MS  200  is to perform communication with a BS  132 - 1  when it lands and moves on ground in an area A. The MS  200  is to perform communication with either a BS  132 - 2  or a BS  132 - 3  when it lands and moves on ground in an area B. 
         [0078]    Referring to  FIG. 12 , descriptions are provided for a process in which an MS performs a cell search using Area Search Config in the location B. In  FIG. 12 , upon arrival and after connecting to the broadcasting BS  131  and executing operations described in  FIG. 10 , the control unit  212  of the MS  200  reads in Area Search Configs, a Location Search Config, and a Default Search Config in the location B from the search list  281  in the storage unit  280  to perform communication with a BS in the location B (S 522 ). 
         [0079]    To decide whether or not the MS has entered the coverage area of a BS installed in the location B, the control unit  212  executes Initial Ranging to search for a usable channel and a base station (S 523 ). When the decision is NO, the control unit  212  repeats step  523 . Once the MS  200  has entered a BS&#39;s coverage area (S 523 ; YES), the control unit  212  performs a cell search using an Area Search Config (S 524 ). The control unit  212  checks whether or not it has detected a BS using the Area Search Config (S 525 ). If no BS is detected, the control unit  212  performs a cell search using the Location Search Config (S 526 ). The control unit checks whether or not it has detected a BS using the Location Search Config (S 527 ). 
         [0080]    If no BS is detected again, the control unit  212  performs a cell search using the Default Search Config (S 528 ). The control unit  212  checks whether or not it has detected a BS using the Default Search Config (S 529 ). The control unit  212  continues to perform a cell search using the Default Search Config until it detects a BS. 
         [0081]    Upon detecting a BS with which the MS can communicate at step  525 , step  527 , or step  528 , the control unit  212  terminates cell search processing, attains synchronization with the detected BS, and then initiates network entry processing. 
         [0082]    It is preferable that the control unit  212  uses Area Search Configs only in a phase prior to network entry which is first executed after disconnection from the broadcasting BS  131 . After network entry has once been performed and the MS has connected to a BS detected, using Area Search Configs, when the MS is disconnected from the network for any reason, e.g., disconnection from the network because of blockage within the BS area, and the MS attempts to connect to a BS installed within the location B again, the control unit  212  is to perform a cell search using the Location Search Config. 
         [0083]    Referring to  FIG. 13 , descriptions are provided for a process in which the MS performs a cell search using a Location Search Config. In  FIG. 13 , in consequence of the process in  FIG. 12 , network entry is completed (S 532 ). The control unit  212  reads in a Location Search Config and a Default Search Config in the destination location from the search list  281  in the storage unit  280  (S 533 ). 
         [0084]    The control unit  212  checks whether or not the MS has been disconnected from the network after the completion of network entry (S 534 ). When the decision is NO, the control unit  212  repeats step  534 . When the decision at step  534  is YES, the control unit  212  performs a cell search using the Location Search Config (S 535 ). The control unit  212  checks whether or not it has detected a BS using the Location Search Config (S 536 ). If no BS is detected, the control unit  212  performs a cell search using the Default Search Config (S 537 ). The control unit  212  continues to perform a cell search using the Default Search Config until it detects a BS. Upon detecting a BS with which the MS can communicate at step  536  or step  538 , the control unit  212  terminates cell search processing, attains synchronization with the detected BS, and then initiates network entry processing. 
         [0085]    During a period until a next destination of the MS  200  is determined and the MS starts to move, the control unit  212  performs a cell search using the Location Search Config or Default Search Config in the location B. 
         [0086]    Descriptions are provided for a cell search that is performed at handover (i.e., the moving MS switches to another BS) after the previous connection (network entry). For a cell search to be performed at handover, the MS applies a cell search prescribed in mobile WiMAX. After completing network entry with a BS, the MS  200  receives a Mobility Neighbor Advertisement (MOB_NBR-ADV) message from the BS as broadcasting information. The NBR-ADV message includes the center frequency used by a neighboring BS as information. The mobile WiMAX prescribes that the MS should perform a cell search for only this center frequency when performing handover and switching to another BS to which it should connect. 
         [0087]    Referring to  FIG. 14 , descriptions are provided for a scheme of cell search processing with Area Search Config A in  FIG. 9 . In  FIG. 14 , the MS  200  searches for 5095 MHz only. The MS  200  performs a search only once. Here, this Search Config is used as the Area Search Config in a case where only one BS exists; i.e., a cell search for the broadcasting BS  131  is performed. 
         [0088]    Referring to  FIG. 15 , descriptions are provided for a scheme of cell search processing with Area Search Config C in  FIG. 9 . In  FIG. 15 , the MS  200  performs a search in a range from 5105 MHz to 5110 MHz in frequency steps of 5 MHz. The MS  200  performs a search twice from the start frequency of 5105 MHz. 
         [0089]    Referring to  FIG. 16 , descriptions are provided for a scheme of cell search processing with Location Search Config in  FIG. 9 . In  FIG. 16 , the MS  200  performs a search in a range from 5095 MHz to 5110 MHz in frequency steps of 5 MHz. The MS  200  performs a search four times from the start frequency of 5095 MHz. 
         [0090]    Referring to  FIG. 17 , descriptions are provided for a scheme of cell search processing with Default Search Config in  FIG. 9 . In  FIG. 17 , the MS  200  performs a search in a range from 5095 MHz to 5415 MHz in frequency steps of 250 kHz. The MS  200  performs a search by 201 times at a maximum if the center frequency of a BS  200  with which the MS is to attain synchronization is set to 5145 MHz. 
         [0091]    A cell search using Default Search Config should be performed as recovery measures against the failure of cell searches using Area Search Config and Location Search Config. Except for a cell search for the broadcasting BS  131 , the MS  200  should perform a cell search using Area Search Config only once and a cell search using Location Search Config only once. After the failure of all these searches which should be performed once, the MS  200  is to perform a cell search using Default Search Config. The MS  200  should perform a cell search using Default Search Config repeatedly until it detects a BS with which it can communicate. 
         [0092]    In the illustrations of cell searches in the AeroMACS communication system, presented in  FIGS. 14 to 16 , the number of cell search actions required for a cell search using Area Search Config or Location Search Config is reduced in comparison with a cell search using Default Search Config. Accordingly, time for a cell search is reduced, and additionally, it is possible to succeed a cell search reliably because the MS is provided with cell search schemes in three stages. 
         [0093]    According to the present embodiment, before an MS performs a cell search in its destination (more specifically, at takeoff), the MS is provided in advance with information representing the center frequencies used by BSs for which a cell search will be performed. After moving to the destination, by comparing a frequency list that the MS maintains with a frequency list received from the broadcasting BS in the destination location, it can be checked whether or not the frequency list maintained is applicable. Accordingly, time that the MS takes to complete a cell search is reduced and a cell search can be performed reliably. 
         [0094]    Further, it would be easy to response to an unexpected change in a destination network after moving to the destination. By executing the process illustrated in  FIG. 10  by the MS, an adaptive operation can be carried out in a case where the destination network has intentionally changed a frequency list and the MS has to perform a cell search for an arbitrary frequency. More efficient use of BS resources can be achieved. 
         [0095]    Because the MS has a function of changing the frequency step size as a function of creating a search list, the present example is applicable to an MS which may connect to plural wireless communication systems where the width of gaps between the center frequencies used by BSs differs. 
         [0096]    While the present embodiment concerns the wireless airport surface communication system, the present embodiment can also be applied to a system where an MS moves from a location to another and moves to another network and another wireless communication system, other than the airport surface communication system. In that case, once a destination location has been determined, the MS receives a frequency list of BSs covering areas in the destination location before disconnection from the network. According to the contents of the frequency list, the MS calculates a search range and a frequency step size and creates and stores Search Configs which are used in performing a search. 
         [0097]    When performing a cell search in the destination location, the MS connects to a broadcasting BS using Search Config created in advance, based on the frequency list in the destination location, and compares the pre-acquired frequency list before it moves with a frequency list in the destination location received from the broadcasting BS. According to the result of the comparison, the MS performs a cell search for a BS using Search Config created in advance before it moves or Search Config created based on the frequency list received from the broadcasting BS in the destination. Thereby, it is possible to reduce time for a cell search and perform a cell search reliably. If no BS is detected by searching in a search range defined in the Search Config, the MS performs a cell search by searching through all applicable frequencies that are used in the destination wireless communication network as recovery measures. If no BS is detected yet, as further recovery measures, the MS is to perform a cell search for a BS by continuing to search in a search range in fine frequency steps, defined in the wireless communication system that the MS supports. 
         [0098]    As the application scope of the technique set forth in the present specification, the technique is applicable to mobile wireless communication systems other than AeroMACS. The technique is applicable to wireless communication systems which are currently put in practical use, including, in particular, Long Term Evolution (LTE) and WiMAX, and wireless communication systems which will be put into practical use in the future.

Technology Classification (CPC): 7