Patent Publication Number: US-2009219860-A1

Title: Base station and communication method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a base station for generating and transmitting system information to a mobile station. The present invention also relates to a communication method of communicating system information in a communication system including the base station and the mobile station. 
     2. Description of the Related Art 
     In W-CDMA (Wideband Code Division Multiple Access), a mobile station decodes broadcast channel information on a BCH (Broadcast Channel) and retrieves system information before establishing a radio link between a base station and the mobile station. The mobile station uses the system information to establish the radio link to the base station. 
     Before entering soft handover or when in intermittent reception mode for standby, the mobile station needs to detect a cell with the smallest path-loss (a cell with the second smallest path-loss when entering soft handover mode). The process for establishing the radio link in this manner is referred to as a cell search (see “W-CDMA MOBILE COMMUNICATIONS SYSTEM” edited by Keiji Tachikawa, Japan, Mar. 15, 2002, pages 35-36). 
     SUMMARY OF THE INVENTION 
     Problem(s) to be Solved by the Invention 
     In W-CDMA, however, various kinds of system information are transmitted on a single broadcast channel and the broadcast channel information increases in size (in the number of bits or in transmission time). Accordingly, the mobile station needs to receive the whole broadcast channel information, which presents a problem in that it takes considerable time for the cell search. 
     In addition, the mobile station cannot recognize whether it detects a cell until after receiving the whole broadcast channel information. Accordingly, it takes considerable time each time the mobile station repeats the cell search. Particularly when the mobile station is situated outside the coverage area, it takes considerable time to repeat the cell search, which presents a problem of increasing power consumption in the mobile station. 
     A cell search during communication in the case of handover needs a fast cell search. However, considerable time for the cell search increases time required for handover, which presents a problem of degrading communication quality such as throughput. 
     On the other hand, system information includes sector-specific (cell-specific) system information, base station-specific system information, upper node-specific system information, location registration area-specific information, and system-specific system information. When this system information is transmitted on a single broadcast channel, there is a problem in that the broadcast channel information increases in size (in the number of bits or in transmission time). 
     For example, when the mobile station changes sectors covered by the same base station and receives various kinds of system information on a single broadcast channel, the mobile station also receives base station-specific system information, which is not always needed. For example, when the mobile station is situated at a sector boundary, the mobile station only receives broadcast channel information from one sector while the same piece of mobile station-specific system information is transmitted from both sectors. Accordingly, there is a problem in that broadcast channel information from the other sector causes interference. 
     In view of the aforementioned problems in the related art, it is a general object of the present invention to reduce the amount of information received by a mobile station on a broadcast channel. 
     Means for Solving the Problem 
     In one aspect of the present invention, there is provided a base station, including: 
     a system information generating unit configured to generate system information; 
     a system information categorizing unit configured to categorize the system information into multiple pieces of system information; and 
     a multiplexing unit configured to multiplex the categorized multiple pieces of system information into multiple channels or multiple blocks. 
     In another aspect of the present invention, there is provided a communication method of communicating system information in a communication system including a base station and a mobile station, including the steps of: 
     at the base station, 
     generating system information; 
     categorizing the system information into multiple pieces of system information; 
     multiplexing the categorized multiple pieces of system information into multiple channels or multiple blocks; 
     transmitting the system information on the multiple channels or the multiple blocks; and 
     at the mobile station, 
     receiving system information included in a portion of the multiple channels or the multiple blocks. 
     EFFECT OF THE INVENTION 
     According to an embodiment of the present invention, it is possible to reduce the amount of information received by a mobile station on a broadcast channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a base station in accordance with a first or second embodiment of the present invention. 
         FIG. 2  shows a block diagram of a mobile station in accordance with the first or second embodiment of the present invention. 
         FIG. 3  shows three schemes used for transmitting multiple pieces of broadcast channel information on frequency blocks. 
         FIG. 4  shows a block diagram of a base station in accordance with a third embodiment of the present invention. 
         FIG. 5  shows a block diagram of a mobile station in accordance with the third embodiment of the present invention. 
         FIG. 6  shows a block diagram of another mobile station in accordance with the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description of Notations 
     
         
         
           
               10 ,  30  base station 
               101 ,  301  system information generating unit 
               103 ,  303  system information categorizing unit 
               105 ,  305  coding unit 
               107 ,  307  modulating unit 
               109 ,  309  multiplexing unit 
               111 ,  311  serial-parallel converting unit 
               113 ,  313  spreading code generating unit 
               115 ,  315  spreading code multiplying unit 
               117 ,  317  combining unit 
               20  mobile station 
               201   a  primary broadcast channel information receiving unit 
               201   b  secondary broadcast channel information receiving unit 
               203  cell search unit 
               205  secondary broadcast channel information extracting unit 
               40 ,  50  mobile station 
               401   a ,  501   a  sector-specific broadcast channel information receiving unit 
               401   b ,  501   b  base station-specific broadcast channel information receiving unit 
               401   c ,  501   c  upper node-specific broadcast channel information receiving unit 
               401   d ,  501   d  location registration area-specific broadcast channel information receiving unit 
               401   e ,  501   e  system-specific broadcast channel information receiving unit 
               403  channel configuration information storing unit 
               501  primary broadcast channel information receiving unit 
               503  channel configuration information extracting unit 
           
         
       
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     With reference to the accompanying drawings, a description is given below with regard to preferred embodiments of the present invention. 
     First Embodiment 
     In a first embodiment of the present invention, a base station transmits system information using multiple broadcast channels or multiple blocks (referred to as broadcast channel blocks) which are derived by dividing a broadcast channel.  FIG. 1  shows a block diagram of a base station  10  in accordance with the first embodiment of the present invention. 
     The base station  10  transmits system information to a mobile station on a primary broadcast channel and a secondary broadcast channel. The mobile station receives primary broadcast channel information to perform a cell search. As used herein, the “cell search” refers to procedures for detecting a cell and retrieving minimum system information. Since the mobile station needs to receive primary broadcast channel information upon establishing a connection with the base station  10 , a frequency block and timing for the primary broadcast channel have to be determined in advance. Then, the mobile station receives secondary system information to retrieve various kinds of system information. 
     The base station  10  includes a system information generating unit  101 , a system information categorizing unit  103 , coding units  105 , modulating units  107 , a multiplexing unit  109 , a serial-parallel converting unit  111 , a spreading code (scrambling code) generating unit  113 , spreading code (scrambling code) multiplying units  115 , and a combining unit  117 . 
     The system information generating unit  101  generates system information to be transmitted to the mobile station. The system information includes system information required for the cell search and system information not required for the cell search (system information other than the system information required for the cell search). 
     The system information required for the cell search includes the following information:
         SFN (System Frame Number): a radio frame number;   system bandwidth: a transmission bandwidth for the base station;   network ID: information about an operator;   cell ID: information used for identifying a cell in the base station; and   the number of transmitting antennas: the number of transmitting antennas in the base station in the case of MIMO transmission.       

     The system information not required for the cell search includes the following information:
         timer value or parameter value for the mobile station: the time interval of the cell search operations in the mobile station, the time interval of received signal levels reported from the mobile station to the base station, initial transmission power for the mobile station, or the like;   location registration area information: location registration area number, or the like;   cell search information: a threshold for the received signal level which is used for the mobile station to perform the cell search;   neighboring cell information: cell IDs of the neighboring cells, a system bandwidth, a broadcast channel bandwidth, the number of transmitting antennas, timing information (time lag among base stations), transmission power, the number of sectors, or the like;   restriction information: information about call restriction;   the amount of uplink interference: information about the amount of interference detected by the base station;   positioning information: latitude and longitude information, or the like;   channel configuration information in the cell: information about a paging channel, an L1/L2 control channel, or the like other than the broadcast channel;   core network information: information about a core network connected to the base station; and   measurement information for the mobile station: measurement information such as received power, SIR (Signal to Interference Ratio), or the like.       

     The system information categorizing unit  103  categorizes the system information into the system information required for the cell search (information to be transmitted on the primary broadcast channel) and the system information not required for the cell search (information to be transmitted on the secondary broadcast channel). The system information categorizing unit  103  may incorporate channel information (a frequency block and timing) about the secondary broadcast channel into information to be transmitted on the primary broadcast channel. 
     The system information required for the cell search is encoded by the coding unit  105   a  and then modulated by the modulating unit  107   a . In the same manner, the system information not required for the cell search is encoded by the coding unit  105   b  and then modulated by the modulating unit  107   b.    
     The modulated system information is multiplexed by the multiplexing unit  109  into the primary broadcast channel and the secondary broadcast channel, and then converted by the serial-parallel converting unit  111  into N information symbol sequences on the frequency axis. The converted N information symbol sequences are arranged on the frequency axis. 
     The symbol sequences with a sequence length N are multiplied by N spreading code multiplying units  115  with spreading codes (scrambling codes) supplied by a spreading code generating unit  113  in the frequency direction, and then the multiplied symbol sequences are supplied to a combining unit  117 . 
     The combining unit  117  multiplexes the multiplied symbol sequences with the sequence length N into a certain subcarrier among N subcarriers corresponding to the synchronization signal. An IFFT (Inverse Fast Fourier Transform) unit transforms N symbols into orthogonal multicarrier signals. A cyclic prefix (CP) adding unit inserts CPs into the multicarrier signals for each Fourier target time period. Then, the base station  10  generates radio signals from the multicarrier signals supplied by the CP adding unit and transmits them to the air. 
     The base station  10  may transmit primary broadcast channel information in a shorter time interval and transmit secondary broadcast channel information in a longer time interval. Since the mobile station first needs to receive primary broadcast channel information, it is preferable for the base station  10  to transmit the primary broadcast channel information in a shorter time interval, in order to reduce time for receiving system information. It should be noted that transmitting primary broadcast channel information in a shorter time interval may not have significant effects on overhead, because the amount of primary broadcast channel information is smaller than the amount of secondary broadcast channel information. Also, transmitting secondary broadcast channel information in a longer time interval can reduce overhead, because the mobile station need not quickly receive secondary broadcast channel information. 
     When a common spreading code (scrambling code) is used within the base station  10 , signals from adjacent sectors are not subjected to interference but are combined when signals transmitted from each sector are received (soft-combining). Soft-combining may be used for both the primary broadcast channel and the secondary broadcast channel. Using soft-combining for both the channels can increase the effect of combining received signals. Alternatively, soft-combining may be used only for the primary broadcast channel. Using soft-combining for the primary broadcast channel can efficiently increase the effect of combining received signals, because primary broadcast channel information is important and includes various kinds of common information for the base station. Alternatively, soft-combining may be used only for the secondary broadcast channel. Using soft-combining for the secondary broadcast channel can efficiently increase the effect of combining received signals, when various kinds of common information for the base station is included in the secondary broadcast channel information. 
     The primary broadcast channel information may be categorized into a portion to be soft-combined and the other portion not to be soft-combined. For example, common information for the base station may be soft-combined using the common spreading code. 
       FIG. 2  shows a block diagram of a mobile station  20  in accordance with the first embodiment of the present invention. 
     The mobile station  20  includes a primary broadcast channel information receiving unit  201   a , a secondary broadcast channel information receiving unit  201   b , a cell search unit  203 , and a secondary broadcast channel information extracting unit  205 . 
     The primary broadcast channel receiving unit  201   a  receives primary broadcast channel information transmitted at a predetermined frequency block and timing. 
     The cell search unit  203  extracts system information required for the cell search which is included in the primary broadcast channel information and then performs the cell search. When the primary broadcast channel information includes channel information about the secondary broadcast channel, the secondary broadcast channel information extracting unit  205  extracts channel information (a frequency block and timing) about the secondary broadcast channel. 
     The secondary broadcast channel receiving unit  201   b  uses the extracted channel information about the secondary broadcast channel to receive secondary broadcast channel information and to extract system information not required for the cell search. 
     According to the first embodiment of the present invention, the mobile station may only receive primary broadcast channel information upon the cell search, because the base station transmits system information required for the cell search on the primary broadcast channel. Therefore, it is possible to reduce the amount of information received by the mobile station on the primary broadcast channel and to reduce time for the cell search. 
     Although broadcast channel information is categorized into primary broadcast channel information and secondary broadcast channel information in the first embodiment, system information to be transmitted on the secondary broadcast channel may be further categorized into secondary broadcast channel information and shared data channel information. 
     In addition, the broadcast channel information may be categorized into multiple blocks (primary broadcast channel block and secondary broadcast channel block). Even though the single broadcast channel is used, the mobile station can receive system information included in a primary broadcast channel block. This approach can achieve the effect similar to the first embodiment. 
     Second Embodiment 
     In a second embodiment of the present invention, a base station uses multiple frequency blocks to transmit broadcast channel information including primary broadcast channel information and secondary broadcast channel information. The base station in accordance with the second embodiment also has the same structure as shown in  FIG. 1 . The combining unit  117  multiplexes symbol sequences into multiple frequency blocks as shown in  FIG. 3 .  FIG. 3  shows three schemes used for transmitting multiple pieces of broadcast channel information on multiple frequency blocks. 
     In 3GPP Evolved UTRA and UTRAN, the system bandwidth can be between 1.25 MHz and 20 MHz. In the second embodiment, the whole system bandwidth is divided into multiple frequency blocks and broadcast channel information is transmitted on frequency blocks with some space therebetween, in order to improve reception quality due to the frequency diversity effect. 
     In order to achieve the frequency diversity effect, a localized transmission scheme, a distributed transmission scheme, or a generalized distributed transmission scheme may be used. The localized transmission scheme uses a portion of the system bandwidth. The distributed transmission scheme uses comb-shaped frequency blocks. The generalized distributed transmission scheme uses frequency blocks with some space therebetween. 
     According to the second embodiment of the present invention, it is possible to reduce the amount of information received by the mobile station on the primary broadcast channel and to reduce time for the cell search. In addition, it is possible to improve reception quality by means of the frequency diversity effect. 
     Third Embodiment 
     In a third embodiment of the present invention, a base station transmits system information using multiple broadcast channels or multiple blocks (referred to as broadcast channel blocks) which are derived by dividing a broadcast channel.  FIG. 4  shows a block diagram of a base station  30  in accordance with the third embodiment of the present invention. 
     The base station  30  categorizes system information into sector-specific (cell-specific) information, base station-specific information, upper node-specific information, location registration area-specific information, and system-specific information. Then, the base station  30  transmits the system information to a mobile station on multiple broadcast channels or multiple broadcast channel blocks. The mobile station only receives necessary information on the multiple broadcast channels or multiple broadcast channel blocks. For example, when the mobile station changes sectors covered by the same base station, the mobile station only receives sector-specific information without receiving base-station specific information. 
     The base station  30  includes a system information generating unit  301 , a system information categorizing unit  303 , coding units  305 , modulating units  307 , a multiplexing unit  309 , a serial-parallel converting unit  311 , a spreading code generating unit  313 , spreading code multiplying units  315 , and a combining unit  317 . 
     The system information generating unit  301  generates system information to be transmitted to the mobile station. The system information can be categorized into multiple pieces of system information, each of which is specific to each component in the radio network configuration. For example, the system information can be categorized into sector-specific information, base station-specific information, upper node-specific information, location registration area-specific information, and system-specific information, as shown in Table 1. As used herein, “an upper node” refers to an apparatus installed at an upper level of the base station, such as an access router or a radio network controller. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example categories of system information 
               
            
           
           
               
               
               
            
               
                   
                 system information 
                 category 
               
               
                   
                   
               
               
                   
                 SFN (System Frame Number) 
                 sector-specific or base 
               
               
                   
                   
                 station-specific 
               
               
                   
                   
                 information 
               
               
                   
                 system bandwidth (overall 
                 base station-specific 
               
               
                   
                 transmission bandwidth) 
                 information 
               
               
                   
                 network ID (PLMN 
                 system-specific 
               
               
                   
                 identities of neighboring 
                 information 
               
               
                   
                 cells) 
               
               
                   
                 timer value or parameter 
                 system-specific 
               
               
                   
                 value for a mobile 
                 information 
               
               
                   
                 station (UE timers and 
               
               
                   
                 constraints in an idle 
               
               
                   
                 mode) 
               
               
                   
                 location registration 
                 location registration 
               
               
                   
                 area information 
                 area-specific information 
               
               
                   
                 sector (cell) ID (cell 
                 sector-specific 
               
               
                   
                 identity) 
                 information 
               
               
                   
                 cell selection 
                 sector-specific or base 
               
               
                   
                 information (parameters 
                 station-specific 
               
               
                   
                 for cell selection and 
                 information 
               
               
                   
                 re-selection) 
               
               
                   
                 restriction information 
                 sector-specific 
               
               
                   
                 (cell access restriction) 
                 information 
               
               
                   
                 channel configuration 
                 sector-specific or base 
               
               
                   
                 information in the cell 
                 station-specific 
               
               
                   
                 (configuration of common 
                 information 
               
               
                   
                 physical channels in the 
               
               
                   
                 cell) 
               
               
                   
                 amount of uplink 
                 sector-specific 
               
               
                   
                 interference 
                 information 
               
               
                   
                 measurement information 
                 sector-specific or base 
               
               
                   
                 for the mobile station 
                 station-specific 
               
               
                   
                 (measurement control 
                 information 
               
               
                   
                 system information) 
               
               
                   
                 core network information 
                 base station-specific, 
               
               
                   
                   
                 system-specific, or upper 
               
               
                   
                   
                 node-specific information 
               
               
                   
                 positioning information 
                 sector-specific or mobile 
               
               
                   
                 (UE positioning 
                 station-specific 
               
               
                   
                 information) 
                 information 
               
               
                   
                   
               
            
           
         
       
     
     The system information categorizing unit  303  categorizes the system information into multiple pieces of system information, each of which is specific to each component in the radio network configuration. The coding units  305  through the CP adding units operate in the same manner as the first embodiment shown in  FIG. 1 . 
     The base station  30  may categorize the system information based on predetermined configuration information about the broadcast channel which is stored in both the base station and the mobile station. For example, channel information (a frequency block and timing) about the broadcast channel for sector-specific information may be determined in advance. A mobile station  40  corresponding to this base station  30  is shown in  FIG. 5 . The mobile station  40  includes a sector-specific broadcast channel information receiving unit  401   a , a base station-specific broadcast channel information receiving unit  401   b , an upper node-specific broadcast channel information receiving unit  401   c , a location registration area-specific broadcast channel information receiving unit  401   d , a system-specific broadcast channel information receiving unit  401   e , and a channel configuration information storing unit  403 . Each of the broadcast channel information receiving units  401   a - 401   e  refers to the channel configuration information storing unit  403  as needed and receives necessary broadcast channel information. For example, when the mobile station  40  changes sectors covered by the same base station, the mobile station  40  only receives sector-specific broadcast channel information. 
     Alternatively, the base station  30  may add configuration information about the broadcast channel to the primary broadcast channel information (system information to be transmitted on the primary broadcast channel) as described in the first embodiment. A mobile station  50  corresponding to this base station  30  is shown in  FIG. 6 . The mobile station  50  includes a primary broadcast channel information receiving unit  501 , a sector-specific broadcast channel information receiving unit  501   a , a base station-specific broadcast channel information receiving unit  501   b , an upper node-specific broadcast channel information receiving unit  501   c , a location registration area-specific broadcast channel information receiving unit  501   d , a system-specific broadcast channel information receiving unit  501   e , and a channel configuration information extracting unit  503 . The primary broadcast channel information receiving unit  501  receives primary broadcast channel information. Then, the channel configuration information extracting unit  503  extracts configuration information about the broadcast channel from the primary broadcast channel information. Each of the broadcast channel information receiving units  501   a - 501   e  refers to the configuration information about the broadcast channel as needed and receives broadcast channel information. 
     Alternatively, the configuration information about the broadcast channel may specify sector-specific system information, base station-specific system information, or the like. In this case, the specified system information may be determined in advance or transmitted on the primary broadcast channel. 
     The spreading code generating unit  313  generates a common spreading code for the same piece of system information. For example, the spreading code generating unit  313  generates a common spreading code for base station-specific information, because the base station-specific information is common to multiple sectors covered by the base station  30 . In this manner, different spreading codes among sectors are used for sector-specific information and a common spreading code within the base station  30  is used for base station-specific information which does not include sector-specific information. When a common spreading code is used within the base station  30 , signals from adjacent sectors are not subjected to interference but are combined when signals transmitted from each sector are received (soft-combining). 
     When soft-combining is used, a pilot channel corresponding to radio resources to be soft-combined may be common to multiple sectors. For example, a pilot channel controlling unit may be provided to the base station  30 , in order to generate the pilot channel which is common to multiple sectors. Providing the pilot channel controlling unit can increase a gain in soft-combining. In the case where the pilot channel is generated which is common to multiple sectors, some portions of frequency blocks for the pilot channel may be common to multiple sectors, while the other portions may be independent. Providing both the pilot channel which is common to multiple sectors and the pilot channel which is independent among multiple sectors allows the mobile station to distinguish processes which are common to multiple sectors and processes which are independent among multiple sectors. For example, the processes which are independent among multiple sectors include reception measurement at the mobile station, reception of L1/L2 control channels, and so on. 
     Alternatively, the pilot channels may be orthogonalized among multiple sectors. The mobile station can receive the orthogonalized pilot channels and combine them. 
     When soft-combining is used, it is preferable that radio resources (frequency blocks) used in each sector should be common. 
     In addition, a CDD (Cyclic Delay Diversity) scheme can be applied to radio resources to be soft-combined. For example, a CDD unit may be provided to the base station  30 , in order to apply CDD to the radio resources to be soft-combined. Providing the CDD unit can achieve a higher diversity effect. 
     In the same manner, a common spreading code may be used for upper-node specific information among multiple base stations belonging to the same upper node. This enables soft-combining among multiple base stations. 
     When soft-combining is used, the same spreading code may not be always used among multiple sectors or multiple base stations, but different spreading codes may be used. When different spreading codes are used, the mobile station can independently receive multiple pieces of broadcast channel information with the same content and combine them. This approach can achieve the effect similar to the use of soft-combining (precisely, the combining effect of this approach is smaller compared to soft-combining). 
     According to the third embodiment of the present invention, the mobile station may only receive necessary broadcast channel information. Therefore, it is possible to reduce the amount of information received by the mobile station on the broadcast channel, to reduce power consumption, and to achieve fast cell selection. For example, the mobile station can determine whether it changes sectors based on a cell ID and neighboring cell information detected during the cell search. 
     In addition, soft-combining can improve reception quality, because signals transmitted from multiple sectors can be combined (without interference). In other words, the base station can reduce transmission power or radio resource blocks (time intervals or frequency blocks) in order to achieve the same quality level. 
     Although broadcast channel information is classified into multiple pieces of broadcast channel information in the third embodiment, the broadcast channel information may be categorized into multiple blocks (sector-specific broadcast channel block, base-station specific broadcast channel block, and so on). Even though the single broadcast channel is used, the mobile station can receive system information included in a predetermined broadcast channel block. This approach can achieve the effect similar to the third embodiment. 
     According to an embodiment of the present invention, it is possible to reduce the amount of information received by a mobile station on a broadcast channel or a primary broadcast channel. 
     This international patent application is based on Japanese Priority Application No. 2006-127992 filed on May  1 ,  2006  and Japanese Priority Application No. 2006-169445 filed on Jun. 19, 2006, the entire contents of which are incorporated herein by reference.