Patent Publication Number: US-2009239537-A1

Title: Carrier communication method, peripheral cell measurement method, basic station device, and mobile station device

Description:
TECHNICAL FIELD 
     The present invention relates to a carrier information notification method and an adjacent cell measuring method suitable for a mobile communication system in which one or more carrier frequencies are available at each of the cells in the system. 
     BACKGROUND OF THE INVENTION 
     Mobile communication is actualized making use of radio frequency carrier signals. In a conventional mobile communication system, a mobile station (or a mobile communication device) is measuring the carrier frequency of adjacent cells to appropriately carry out the handover process when it moves. For example, in widely used Universal Mobile Telecommunication System (UMTS) or Wideband Code Division Multiple Access (W-CDMA), the same carrier frequency is used throughout the cells in the system. Accordingly, a mobile station is handed over to the same carrier frequency of an adjacent cell when it moves across the cells. In this case, measurement for adjacent cells is simply carried out at the same carrier frequency during the handover process. 
     However, because frequency availability varies widely depending on countries, regions, or operators, and because further globalization is expected in the future, demand for flexible allocation of multiple carrier frequencies to radio communications is arising. Meanwhile, international standardization of Evolved-UMTS Terrestrial Radio Access (E-UTRA) is currently ongoing, and many studies and discussions are being made to standardize radio interfaces to manage and operate multiple carrier frequencies in a flexible manner. For example, in a system corresponding to E-UTRA, multiple carrier frequencies and signal bandwidths are defined so as to allow base stations or telecommunications carriers to support different carrier frequencies or different signal bandwidths. This means that the number of carrier frequencies, the center frequency of each carrier, and the signal bandwidth will vary among cells laid out by different telecommunications carriers, or among cells managed even by a single telecommunications carrier. 
     SUMMARY OF INVENTION 
     Technical Problem to be Solved 
     If the conventional handover technique is applied as it is to a mobile communication system designed to support multiple carrier frequencies, a mobile station (or mobile communication device) has to scan the entire spectrum of the frequency band used by the system for each cell to detect one or more carrier frequencies available in that cell, and measure the propagation condition for each of the detected carrier frequencies. Because a lot of time is required for detection and measurement of carrier frequencies, the workload on the mobile station and the power consumption accompanying the carrier/cell measurement will increase. It is necessary for the mobile station to complete the measurement of the adjacent cells before the reception quality of the signal from the currently connected cell has degraded in order to appropriately carry out the handover process. If the measurement of the adjacent cells takes too much time, the communication quality is degraded, and the link may be cut off depending on the situation. 
     Therefore, it is an objective of the present invention to provide a mobile communication technique that can reduce the measurement workload on a mobile station and achieve effective measurement of adjacent cells to efficiently carry out the handover procedure, with a view to a mobile communication system that supports multiple carrier frequencies. 
     TECHNICAL SOLUTION 
     To solve the above-described problem and achieve the objective, in a mobile communication system to which the present invention is applied, a base station notifies a mobile station located in the cell provided by this base station and communicating with the base station, of carrier information about carrier frequencies available in the system or adjacent cells. The notification (signaling) of the carrier information may be broadcast as a broadcast signal at constant intervals to the entire area of the cell, or it may be transmitted individually to the mobile station before the handover process is started. 
     In a preferred example, the carrier information may include the available carrier frequencies which are to be reported in a rearranged order determined from the degree of congestion of each carrier frequency. Alternatively, the carrier information may include the available carrier frequencies and the determined rank order information. 
     The mobile station does not always have to measure all carrier frequencies included in the received carrier information; instead, it may measure only a part of the carrier frequencies in the carrier information. If the propagation condition of the measured carrier frequency is less than a threshold level, no more measurement is carried out for the rest of the carrier frequencies in the carrier information. Examples of the propagation condition include a received power level, propagation loss, and the received signal-to-interference (thermal noise) power ratio (SIR). 
     In the first aspect of the invention, a carrier information notification method used in a mobile communication system that supports multiple carrier frequencies is provided. The carrier information notification method includes: 
     (a) at a base station in a cell, generating carrier information about one or more carrier frequencies available at one or more adjacent cells; and
 
(b) notifying a mobile station located in the cell of the generated carrier information.
 
     In a preferred example, the preferential order of measurement of carrier frequencies is determined for the mobile station, and information about the determined preferential order is included in the carrier information. 
     Alternatively, the preferential order of measurement of said one or more adjacent cells may be determined for the mobile station, and such preferential order information may be included in the carrier information. 
     In the second aspect of the invention, an adjacent cell measuring method used in a mobile communication system that supports multiple carrier frequencies is provided. The adjacent cell measuring method includes: 
     (a) at a mobile station, measuring a first carrier frequency supported by a first adjacent cell; and
 
(b) if the propagation state of the first carrier frequency satisfies a predetermined condition, measuring a second and subsequent carrier frequencies supported by the first adjacent cell; and if the propagation state of the first carrier frequency does not satisfies the predetermined condition, finishing the carrier measurement for the first adjacent cell and starting carrier measurement for a second adjacent cell.
 
     In the third aspect of the invention, a base station apparatus used in a mobile communication system that supports multiple carrier frequencies is provided. The base station apparatus includes: 
     (a) an information acquiring unit configured to acquire information about resource utilization state of each of one or more carrier frequencies available at a cell covered by the base station apparatus and available at adjacent cells;
 
(b) a preferential order determination unit configured to determine the preferential order of carrier measurement for measuring said one or more carrier frequencies based on the acquired information; and
 
(c) a signal generator configured to generate carrier information including the determined preferential order of the carrier measurement; and
 
(d) transmitting the carrier information to a mobile station located in the cell covered by the base station apparatus.
 
     In the fourth aspect of the invention, a mobile station used in a mobile communication system supporting multiple carrier frequencies is provided. The mobile station includes: 
     (a) a receiving unit configured to receive carrier information about one or more carrier frequencies available at each of adjacent cells from a base station that provides a cell in which the mobile station is currently located;
 
(b) a measuring unit configured to measure a first carrier frequency among said one or more carrier frequencies available at a first adjacent cell;
 
(c) a comparison and determination unit configured to compare the measurement result of the first carrier frequency with a predetermined threshold level to determine whether the first carrier frequency satisfies the condition defined by the threshold level; and
 
(d) a control unit configured to instruct the measuring unit to measure a second carrier frequency available at the first adjacent cell if the first carrier frequency satisfies the condition, and instructs the measuring unit to stop measuring for the first adjacent cell and move to measurement for a second adjacent cell when the first carrier frequency does not satisfy the condition.
 
     Advantages of the Invention 
     With the above-described structure, measurement for adjacent cells is carried out efficiently with reduced workload on the mobile station and reduced power consumption during the handover process. When the preferential order is determined for the measurement of carrier frequencies carried out at the mobile station, the efficiencies of adjacent-cell measurement and the handover procedure are further improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an example of operations of multiple carrier frequencies. 
         FIG. 2  is a schematic diagram of a mobile communication system to which the present invention can be applied. 
         FIG. 3  is a flowchart of narrowing the carrier frequencies to be measured carried out in the mobile communication system shown in  FIG. 2 . 
         FIG. 4  illustrates an example of the carrier information generated by and transmitted from the base station to the mobile station in the mobile communication system of  FIG. 2 . 
         FIG. 5  is a block diagram illustrating the first example of the base station apparatus used in the mobile communication systems shown in  FIG. 2 . 
         FIG. 6  is a block diagram illustrating the second example of the base station apparatus used in the mobile communication system shown in  FIG. 2 . 
         FIG. 7  is a block diagram illustrating an example of the mobile station used in the mobile communication system shown in  FIG. 2 . 
     
    
    
     LIST OF SYMBOLS 
     
         
           1 - 1  current cell 
           1 - 2  adjacent cell(s) 
           10 - 1 . 10 - 2  base station (base station apparatus) 
           12  transmission unit of base station 
           13  multiplexing unit of base station 
           14  transmission signal generator 
           15  preferential order determination unit 
           16  resource measuring unit 
           17  transmission buffer 
           18  interface (resource information acquiring unit and resource information reporting unit) 
           19  resource management memory 
           20  mobile station (mobile communication device) 
           24  demultiplexing unit 
           25  measurement control unit 
           26  measuring unit 
           27  comparison and judgment unit 
           28  handover determination unit 
           29  handover control signal generator 
           30  multiplexing unit of mobile station 
           50  network 
       
    
     BEST MODE FOR IMPLEMENTING THE INVENTION 
     The preferred embodiments of the invention are now described in detail in conjunction with the attached drawings. 
       FIG. 1  illustrates an example of utilization of multiple carrier frequencies. In this example, three different frequency bands, the 80 MHz band, the 2 GHz band, and the 5 GHz band, are available in the system. In each of the frequency bands, multiple carrier frequencies can be used. The band widths of the carrier frequencies in the same band may be the same or different from one another. 
     Since in the conventional technique carrier frequencies available for a system are fixed and limited within a certain band, it is unnecessary for a base station to notify the mobile stations of which carrier frequencies to be used. The mobile stations can specify the carrier frequencies in advance when measuring adjacent cells. However, in the future flexible utilization and sharing of carrier frequencies, the mobile station will have to scan the entire spectrum of each of the frequency bands to discover the carrier frequencies of the adjacent cells. 
     To avoid the overload, the base station of each cell notifies the mobile stations of carrier information about the adjacent cells in a mobile communication system to which the present invention is applied. For a spread spectrum system, the carrier information may be supplied to the mobile stations together with the spread code required for measurement. 
       FIG. 2  is a schematic diagram of a mobile communication system according to an embodiment of the invention. A mobile station  20  is currently located in cell  1 - 1  and communicating with a base station  10 - 1 . The mobile station  20  is moving toward another cell, and accordingly, carrier frequencies of adjacent cells (cell  1 - 2 , for example) have to be measured. Although only one adjacent cell is illustrated in  FIG. 2  for simplification of the figure, there are actually several cells  1 - 2  existing adjacent to the current cell  1 - 1 . 
     The base station  10 - 1  notifies the mobile station  20  that, among the carrier frequencies supported by the system, carrier frequencies A, B and C are available at adjacent cell  1 - 2 , to the mobile station  20 . Currently, the degree of congestion of the carrier frequencies available in cell  1 - 2  is highest at carrier C, followed by carriers A and B in this order. In this case, the base station  10 - 1  may notify the mobile station  20  of carrier frequencies B, A and C in this order, beginning at the least congested carrier frequency. If the system employs a code spreading scheme, the spreading code required for the carrier measurement is also reported, together with the carrier information, to the mobile station  20 . 
     Upon receiving the carrier information, the mobile station  20  carries out carrier measurement only for the carrier frequencies from the notification by base station  10 - 1 , without scanning the entire spectra of the system frequency bands. At this stage, the carrier frequencies to be measured for the adjacent cell are narrowed to some extent. When the preferential order of the carrier frequencies to be measured for the adjacent cell  1 - 2  is included in the carrier information supplied from base station  10 - 1 , the carrier frequencies to be measured can be further narrowed according to the preferential order. 
       FIG. 3  is a flowchart of the carrier frequency narrowing operations carried out at the mobile station  20 . Based on the carrier information supplied from the base station  10 - 1 , measurement of carrier frequency B currently at the lowest congestion level in the adjacent cell  1 - 2  (in other words, at the highest preferential order of carrier measurement) is started at the mobile station  20  for the handover process (S 101 ). Then, based on the measurement result, it is determined whether the propagation condition of carrier frequency B satisfies a predetermined condition (S 103 ). If the predetermined condition is the power level or the signal to interference (thermal noise) ratio of the received signal, it is determined whether the measured level or value is at or above a predetermined threshold level or value. If the predetermined condition is the propagation loss, it is determined whether the measured value is at or below a predetermined threshold value. 
     If the propagation condition of carrier frequency B is good, satisfying the predetermined condition (YES in S 103 ), the rest of the carrier frequencies A and C are measured consecutively, to determine the optimum carrier frequency (or the optimum combination of carrier frequencies) for telecommunications (S 107 ). For example, when the carrier frequencies A and B satisfy the predetermined condition of the received power level or the received signal to interference (thermal noise) ratio, while carrier frequency C does not satisfy the condition, then carrier frequencies A and B are selected as being available in the adjacent cell. 
     If the most preferential carrier frequency B does not satisfy the predetermined condition and is not in good condition (NO in S 103 ), the measurement for the cell  1 - 2  is terminated without conducting further measurement at carrier frequencies A and C, and measurement for the next adjacent cell is started (S 105 ). 
     This is because if the most preferential carrier frequency does not satisfy the predetermined condition, it is unlikely that the rest of the carrier frequencies satisfy the required level. This arrangement is applicable to the case in which the preferential order information is not supplied from the base station  10 - 1 , as long as the carrier frequencies A, B and C are in the same frequency band, because the propagation loss is expected to be substantially the same among these carrier frequencies within the same band. By measuring one of the carrier frequencies, it can be speculated that the other carrier frequencies are also in similar propagation conditions. Accordingly, the mobile station  20  measures an adjacent cell focusing on one of the carrier frequencies included in the carrier information supplied from the base station  10 - 1 . If the propagation condition of this carrier frequency is unsatisfactory (without reaching the threshold level), no more measurement at the rest of the carrier frequencies is conducted and the measurement for the focused-on cell is terminated. With this arrangement, the quantity of measurement workload can be reduced. 
     As an alternative, if a carrier frequency that does not satisfy the predetermined threshold level is detected in step S 107 , the measurement for this adjacent cell may be terminated and the process may be returned to step S 105  to move to measurement of the next adjacent cell. 
       FIG. 4  illustrates an example of the carrier information (which example includes adjacent-cell information as well as the preferential order information) supplied from the base station  10 - 1  to the mobile station  20 . In this example, the available carrier frequencies included in the carrier information are rearranged according to the preferential order determined by the base station  10 - 1 . For the mobile station  20 , the cell with the code identification (ID) number  15  is the most preferential cell to access. This cell supports three carrier frequencies, among which carrier frequency B is the most preferential, then carrier frequencies A and C follow in this order. The next most preferential cell is a cell of code identification (ID) number  27 , which cell supports only carrier frequency A. Then, the cell with the code ID number  8  is listed, which cell supports carrier frequencies C and D. 
     The mobile station  20  acquires the carrier information from the currently connected base station  10 - 1 , and carries out measurement of carrier frequencies in the descending order of preference for each of the adjacent cells, while narrowing the number of carrier frequencies to be measure as illustrated in  FIG. 3 . When the measurements are finished, the cell to be accessing in the handover process and the carrier frequency used in that cell are selected based on the measurement result. 
       FIG. 5  is a schematic block diagram of the base station apparatus  10 A which corresponds to the base station  10 - 1  used in the mobile communication system shown in  FIG. 2 . The base station  10 - 1  includes an antenna  11 , a transmission unit  12 , a multiplexing unit  13 , a transmission signal generator  14 , a preferential order determination unit  15 , a resource measuring unit  16 , a transmission buffer  17 , an interface  18 , and a resource management memory  19 . 
     The transmission unit  12  encodes and modulates a signal to be transmitted, converts the encoded and modulated signal to a radio frequency (RF) signal, and excites the antenna  11  to transmit the RF signal. 
     The multiplexing unit  13  multiplexes notification information generated by the transmission signal generator  14  and transmission data. The notification information generated by the transmission signal generator  14  includes carrier information, and it may be referred to as carrier notification information. The carrier information may include system information and/or adjacent cell information. 
     The preferential order determination unit  15  acquires information about the resource utilization state (or the degree of congestion) of each of the carrier frequencies available in the corresponding cell  1 - 1  and the adjacent cell  1 - 2  from the resource management memory  19  via the interface  18  to determine the preferential order of the carrier frequencies. 
     The resource measuring unit  16  acquires information about the transmission power level, the number of connected users, the amount of buffered data and so on from the transmission unit  12  and the transmission buffer  17 , and estimates the resource utilization state (or the degree of congestion) of each of the carrier frequencies. The transmission buffer  17  buffers transmission data received via the interface from the network. The interface  18  transmits and receives information to and from other base stations (apparatuses)  10 - 2  or the network. The interface  18  also reports the resource utilization state of the corresponding cell  1 - 1  to the base station (apparatuses)  10 - 2  of the adjacent cells  1 - 2  via the network, and receives the resource utilization state information from the adjacent cells  1 - 2  via the network. 
     The resource management memory  19  stores the resource utilization state information of the base station  10 - 1 , as well as the resource utilization state information of the base station  10 - 2  of the adjacent cell  1 - 2  acquired via the interface  18  from the network. 
       FIG. 6  is a schematic diagram illustrating a second structural example of the base station apparatus  10 B corresponding to base station  10 - 1 . In the second example, the resource management memory is not included in the base station  10 - 1 , and instead, it is furnished in the resource management server  40  provided on the network  50 . In this case, the resource utilization states of arbitrary cells  1 - n  (including cells  1 - 1  and  1 - 2 ) are collectively controlled and managed by the resource management server  40 . Accordingly, the base station  10 - 1  acquires the information about the resource utilization state of each of the carrier frequencies of the adjacent cell  1 - 2  from the resource management server  40  via the network  50 . The other components of the base station apparatus  10 B are the same as those of the base station apparatus  10 A as indicated by the same symbols, and the overlapping explanation for them is omitted. 
       FIG. 7  is a schematic diagram of the mobile station  20  used in the mobile communication system shown in  FIG. 2 . The mobile station  20  includes at least an antenna  21 , a circulator  22 , a receiving unit  23 , a demultiplexing unit  24 , a measurement control unit  25 , a measuring unit  26 , a comparison and judgment unit  27 , a handover determination unit  28 , a handover control signal generator  29 , a multiplexing unit  30 , and a transmission unit  31 . 
     The receiving unit  23  demodulates and decodes a received signal (containing carrier notification information) supplied from the base station  10 - 1  of the current cell  1 - 1 . The demultiplexing unit  24  demultiplexes the received signal into a data component and a control signal component. 
     The measurement control unit  25  designates the spreading code and the carrier frequencies to be measured first at the measuring unit  26  based on the notification information received from the base station  10 - 1 . The measurement control unit  25  also selects the spreading code and the carrier frequencies to be measured next at the measuring unit  26  based on the determination result of the comparison and judgment unit  27 . In addition, the measurement control unit  25  designates a threshold value set in the comparison and judgment unit  27 . If the threshold value is included in the carrier notification information, then that threshold value may be used. Alternatively, a value set in the measurement control unit  25  may be used as the threshold value. 
     The measuring unit  26  measures the propagation condition for the combination of the spreading code and each of the carrier frequencies designated by the measurement control unit  25 . 
     The comparison and judgment unit  27  compares the carrier measurement result output from the measuring unit  26  with the threshold value designated by the measurement control unit  25 , and returns the comparison/judging result to the measurement control unit  25 . The comparison and judgment unit  27  and the measurement control unit  25  cooperate to perform the operations shown in  FIG. 3  to narrow the number of carrier frequencies to be measured. If the measurement result satisfies the condition defined by the threshold value, and if there is another carrier frequency supported by the currently measured adjacent cell  1 - 2 , then measurement of the next carrier frequency for the same cell is instructed to be performed. If the measurement result does not satisfy the condition given by the threshold value, then the measurement control unit  25  does not instruct further measurement for the currently measured cell, and instructs moving to measurement for the next adjacent cell. 
     The handover determination unit  28  determines implementation of handover. The handover control signal generator  29  generates a control signal for controlling the handover when it is determined by the handover determination unit  28  that handover can be implemented. 
     The multiplexing unit  30  multiplexes the handover control signal and the transmission data. The transmission unit  31  encodes and modulates the multiplexed signal, and converts the encoded and modulated signal to a radio frequency (RF) signal suitable for transmission from the antenna  21 . The circulator  22  allows the antenna  21  to be used in common between signal receiving and signal transmission. The circulator  22  supplies the signal received at the antenna  21  to the receiving unit  23 , and supplies the transmission signal output from the transmission unit  31  to the antenna  21 . 
     With the above-described structure, the measurement load on the mobile station  20  can be reduced in the multi-carrier or multi-band environment in which multiple carrier frequencies are used in the same frequency band or across different frequency bands. This arrangement leads to prompt measurement of adjacent cells and reduced power consumption during the measurement. 
     When the carrier frequencies to be measured are rearranged in the preferential order, the efficiency of the measurement of adjacent cells and the handover process can be further improved. 
     Although the invention has been described based on the preferred examples, the invention is not limited to these examples, and many substitutions and alternations are possible for a person with an ordinary skill in the art. 
     This international patent application claims the benefit of the earlier filing date of Japanese Patent Application No. 2005-321540 filed Nov. 4, 2005, and the entire contents of which is incorporated herein by reference.