Abstract:
Disclosed is a method and system for transmitting/receiving a neighbor base-station information message in a broadband wireless access system. A base station sets and transmits a neighbor base-station information message, which includes a specific flag field for limiting neighbor base-station information and a reserved field and omits iterative neighbor base-station information depending on a value set in the specific flag field, thereby increasing a transmission efficiency. A mobile terminal receives the neighbor base-station information message from the base station, checks the received message, and then updates an operator ID according to the value set in the specific flag field.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. 119(a) of an application entitled “Method And System For Transmitting/Receiving Neighbor Base-Station Advertisement Message In Broadband Wireless Access System” filed in the Korean Intellectual Property Office on Oct. 26, 2004 and assigned Serial No. 2004-85915, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method and system for transmitting/receiving messages in a wireless communication system, and more particularly to a method and system for transmitting/receiving neighbor base-station advertisement messages broadcast by a base station in a broadband wireless access (BWA) system.  
         [0004]     2. Description of the Related Art  
         [0005]     Fourth generation (4G) communication systems, which are the next generation communication systems, are being designed to provide users with services having various Qualities of Service (QoS) and supporting a transmission speed of about 100 Mbps. Particularly, in current 4G communication systems, research is being undertaken to support a high-speed service by ensuring mobility and QoS in broadband wireless access communication systems, such as a local area network (LAN) system and a metropolitan area network (MAN) system. A representative communication system currently being developed is known as an IEEE (Institute of Electrical and Electronics Engineers) 802.16 communication system.  
         [0006]     Compared with conventional wireless technologies for providing voice services, the IEEE 802.16 communication system enables the transmission of data in a shorter period of time than needed if using conventional communication technologies, by providing a wider data bandwidth, and can efficiently use channels by sharing all user channels. In addition, the IEEE 802.16 communication system ensures QoS, so that users can receive services of different QoS depending on the characteristics of the services.  
         [0007]     From among the IEEE 802.16 communication system, an IEEE 802.16a communication system does not take into consideration the mobility of a terminal, while an IEEE 802.16e communication system takes into consideration the mobility of a terminal (e.g., a mobile station).  
         [0008]     Therefore, in the IEEE 802.16e communication system, a base station periodically broadcasts a MOB_NBR-ADV (mobile neighbor advertisement) message, which is information about neighbor base stations, to all mobile stations located in a corresponding cell in order to support the handover of the mobile stations, thereby notifying the mobile stations of neighbor base-station state information. That is, when a mobile station located in a cell moves into a neighbor cell, the mobile station can perform a handover because the mobile station already understands a neighbor cell&#39;s base-station information such as a network identifier and channel characteristics broadcast from the base station.  
         [0009]     Table 1 below shows the format of a conventional MOB_NBR-ADV message used to report neighbor base station information from a base station to a mobile station. These message formats are generally known in the art, so a detailed description thereof will be omitted for the sake of clarity.  
                       TABLE 1                       Syntax   Size   Notes                   MOB_NBR-ADV_Message_Format( )               {       Management Message Type = 53    8 bits       Operator ID   24 bits   Unique ID assigned to the operator       Configuration Charge Count    8 bits   Incremented each time the               information for the associated               neighbor BS has changed.       Fragmentation Index    4 bits   This field indicates the current               fragmentation index.       Total Fragmentation    4 bits   This field indicates the total number               of fragmentations       Skip-Optional-Fields Flag    1 bit   If set to ‘1’ and if a neighbor has               OFDMA PHY the BS-ID for that               neighbor is omitted in this message.               If set to ‘0’, BS-ID is not omitted for               any neighbor.       N_NEIGHBORS    8 bits       For (j=0; j&lt;N_NEIGHBORS; j++){       Length    8 bits   Length of message information               within the iteration of               N_NEIGHBOR in bytes.       PHY Profile ID    8 bits   Aggregated IDs of Co-located FA               Indicator, FA Configuration               Indicator, FFT size, Bandwidth,               Operation Mode of the starting               subchannelization of a frame, and               Channel Number       if (FA Index Indicator = = 1) {       FA Index    8 bits   This field, Frequency Assignment               Index, is present only the FA Index               Indicator in PHY Profile ID is set.               Otherwise, the neighbor BS has the               same FA Index or the center               frequency is indicated using the TLV               encoded information.       }       if (BS EIRP Indicator = = 1) {       BS EIRP    8 bits   Signed Integer from −128 to 127 in               units of dBm This field is present               only if the BS EIRP indicator is set               in PHY Profile ID. Otherwise, the               BS has the same EIRP as the serving               BS.       }       Neighbor BS-ID   24 bits   This is an optional field for OFDMA               PHY and it is omitted or skipped if               Skip Optional Fields Flag = ‘1’       Preamble Index    8 bits   The index for the PHY profile               specific preamble. Preamble Index is               PHY specific for SCa and OFDMA.               The value of Preamble Index shall be               ignored and a value of ‘0x00’ shall               be used for OFDM PHY       HO Process Optimization    8 bits   HO Process Optimization is               provided as part of this message is               indicative only. HO process               requirements may change at time of               actual HO. For each bit location, a               value of ‘0’ indicates the associated               reentry management messages shall               be required, a value of ‘1’ indicates               the reentry management message               may be omitted. Regardless of the               HO Process Optimization TLV               settings, the Target BS may send               unsolicited SBC-RSP and/or REG-               RSP management messages Bit #0:               Omit SBC-REQ/RSP management               messages during current re-entry               processing Bit #1: Omit               PKM-REQ/RSP management               message during current re-entry               processing Bit #2: Omit               REG-REQ/RSP management during               current re-entry processing Bit #3:               Omit Network Address Acquisition               management messages during               current reentry processing Bit #4:               Omit Time of Day Acquisition               management messages during               current reentry processing Bit #5:               Omit TFTP management messages               during current re-entry processing               Bit #6: Full service and operational               state transfer or sharing between               Serving BS and Target BS (ARQ,               timers, counters, MAC state               machines, etc...)       Scheduling Service Supported    4 bits   Bitmap to indicate if BS supports a               particular scheduling service. ‘1’               indicates support, ‘0’ indicates not               support: bit 0: Unsolicited Grant               Service (UGS) bit 1: Real-time               Polling Service (rtPS) bit 2: Non-               real-time Polling service (nrtPS)               bit 3: Best Effort value of ‘1111’               indicates no information on service               available         Available Radio Resource    4 bits   Percentage of reported average               available subchannels and symbols               resources per frame 0000: 0% 0001:               20% 0010: 40% 0011: 60% 0100:               80% 0101: 100% 0110-1110:               reserved 0110-1110: reserved value               of ‘1111’ indicates no information on               service available        Hand Off Neighbor Preference    2 bits   00 Normal 01 Preferred 10 Non-               Preferred 11 Reserved       DCD Configuration Change Count    4 bits   This represents the Neighbor BS               current DCD configuration change               count       UCD Configuration Change Count    4 bits   This represents the Neighbor BS               current UCD configuration change               count       TLV Encoded Neighbor information   Variable   TLV specific          }          }                  
 
         [0010]     The following Table 2 illustrates information included in a TLV Encoded Neighbor information field of the MOB_NBR-ADV message, which includes a downlink channel descriptor setting (DCD_settings) field and an uplink channel descriptor setting (UCD_settings) field.  
                           TABLE 2                       Name   Type (1 byte)   Length (1 byte)   Value (Variable-length)                   DCD_settings       Variable   The DCD_settings is a compound TLV                   value that encapsulates a DCD message                   that may be transmitted in the advertised                   BS downlink channel. This information is                   intended to enable fast synchronization of                   the SS with the advertised BS downlink.                   The DCD settings fields shall contain only                   neighbor&#39;s DCD TLV values which are                   different from the Serving BS                   corresponding values. For values that are                   not included, the MSS shall assume they                   are identical to the serving BSs                   corresponding values.       UCD_settings       Variable   The UCD_settings is a compound TLV                   value that encapsulates a UCD message                   that may be transmitted in the advertised                   BS downlink channel. This information is                   intended to enable fast synchronization of                   the SS with the advertised BS uplink. The                   UCD settings fields shall contain only                   neighbor&#39;s UCD TLV values which are                   different from the Serving BS&#39;s                   corresponding values. For values that are                   not included, the MSS shall assume they                   are identical to the serving BS&#39;s                   corresponding values.                  
 
         [0011]     The DCD_settings field in Table 2 is configured using the TLV value of a DCD message, which may be expressed in the format of Table 3. Such a DCD_settings field is generally known in the art, so a detailed description thereof will be omitted for the sake of clarity.  
                                                     TABLE 3                           Type                   Name   (1 byte)   Length   Value (variable length)   PHY scope                                Downlink_Burst_Profile   1       May appear more than once   All                   (see 6.3.2.3.1). The length is                   the number of bytes in the                   overall object, including                   embedded TLV items       BS EIRP   2   2   Signed in units of 1 dBM.   All       Frame duration   3   4   The number of PSs contained   SC                   in a Burst FDD or TDD                   frame. Required only for                   framed downlinks       PHY Type   4   1   The PHY Type to be used.   SC       Power adjustment rule   5   1   0 = Preserve Peak Power   SC, SCa                   1 = Preserve Mean Power                   Describes the power                   adjustment rule when                   performing a transition from                   one burst profile to another.       Channel Nr   6   1   Downlink channel number as   SCa,                   defined in 8.5. Used for   OFDM,                   license-exempt operation   OFDMA                   only.       TTG   7   1   TTG (in PSs)   SCa,                       OFDM,                       OFDMA       RTG   8   1   RTG (in PSs)   SCa,                       OFDM,                       OFDMA       RSSIR,max   9   2   Initial Ranging Max.   All                   Received Signal Strength at                   BS Signed in units of 1 dBm       Channel Switch Frame   10   3   Channel switch frame   SCa,       Number           number as defined in 6.3.15.7,   OFDM,                   Used for license-exempt   OFDMA                   operation only       Frequency   12   4   Downlink center frequency   All                   (kHz).       BS ID   13   6   Base Station ID   SCa,                       OFDM,                       OFDMA       Frame Duration Code   14   1   The duration of the frame.   OFDM                   The frame duration code                   values are specified in Table                   230.       Frame Number   15   3   The number of the frame   OFDM                   containing the DCD message.                  
 
         [0012]     The conventional base station periodically broadcasts a MOB_NBR-ADV message including a TLV encoded neighbor information field as shown in Table 1 in order to notify a mobile station of neighbor base station information. Meanwhile, information related to a base station identifier (BS_ID) field included in the DCD_settings is an iteration of information related to neighbor base-station identifier (neighbor BS_ID) of the MOB_NBR-ADV message. That is, the BS_ID information of the DCD_settings can be recognized by using a neighbor BS_ID and an operator identifier of the MOB_NBR-ADV message. Therefore, the conventional system repeatedly transmits the TLV information of the MOB_NBR-ADV message corresponding to the number of neighbor base stations, and incurs a signaling load due to iterative transmission of the BS_ID of the DCD_settings field.  
         [0013]     In addition, an operator identifier (ID) included in the MOB_NBR-ADV message in order to report information about a network operator as shown in Table 1 does not change while a mobile station is using the same network operator. Therefore, when operator ID information is included in every transmitted MOB_NBR-ADV message, overhead may increase, degrading transmission efficiency.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a method and system for transmitting/receiving a neighbor base-station advertisement message to be transmitted to a mobile station in a broadband wireless access system.  
         [0015]     Another object of the present invention is to provide a method and system for transmitting/receiving a neighbor base-station message in a broadband wireless access system, capable of omitting an operator ID, which must be transmitted whenever a neighbor base-station advertisement message is transmitted, according to necessity and reducing an overhead caused by iterative transmission of base station identifier information.  
         [0016]     To accomplish these objects, in accordance with one aspect of the present invention, there is provided a method for receiving a neighbor base-station advertisement message from a base station using a mobile station communicating with the base station in a broadband wireless access system, the method including receiving a neighbor base-station advertisement message, including a predetermined flag field for indicating the presence or absence of operator identifier information; and determining that operator identifier information is current operator identifier information when the predetermined flag field indicates the absence of the operator identifier information is omitted.  
         [0017]     In accordance with another aspect of the present invention, there is provided a method for transmitting a neighbor base-station advertisement message from a base station to a mobile station communicating with the base station in a broadband wireless access system, the method including setting a value of a predetermined flag field indicating the presence or absence of operator identifier information to indicate the absence of the operator identifier information; and transmitting a neighbor base-station advertisement message including the predetermined flag field to the mobile station.  
         [0018]     In accordance with still another aspect of the present invention, there is provided a broadband wireless access system for transmitting/receiving a neighbor base-station advertisement message, the system including a base station for transmitting a neighbor base-station advertisement message, including a predetermined flag field for indicating the presence or absence of operator identifier information; and a mobile station for receiving the neighbor base-station advertisement message and determining whether the predetermined operator identifier information is current operator identifier information when the predetermined flag field that the absence of the operator identifier information. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0020]      FIG. 1  is a diagram illustrating the configuration of cells configured by base stations for supporting an IEEE 802.16e system according to an embodiment of the present invention; and  
         [0021]      FIG. 2  is a flowchart illustrating a handover procedure of a mobile station in a mobile communication system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description of the embodiments of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention. It should be noted that the MOB_ADV message will hereinafter refer to the MOB_ADV message according to an embodiment of the present invention unless indicated otherwise.  
         [0023]     The present invention proposes a method and system for minimizing the size of a neighbor base-station advertisement (MOB_NBR-ADV) message to be transmitted in a broadband wireless access system according to the present invention. In detail, the present invention proposes a method for omitting an operator identifier(ID) field of the MOB_NBR-ADV message, and a method for omitting a base station identifier (BS ID) of a downlink channel descriptor (DCD) setting field from among a type/length/value (TLV) encoded neighbor information field of the MOB_NBR-ADV message. The following description according to the present invention can be applied to the IEEE 802.16e system.  
         [0024]      FIG. 1  is a diagram schematically illustrating the configuration of an IEEE 802.16e communication system according to an embodiment of the present invention.  
         [0025]     A mobile station  10  having received the service of a first base station (BS  1 )  21  moves to the cell of a second base station (BS  2 )  22  (as depicted by the arrow), which is a neighbor base station. In this case, the mobile station  10  must have information about the neighbor base station in advance in order to receive the service of the second base station  22  and not experience a service discontinuity.  
         [0026]     The mobile station  10  can check neighbor base station&#39;s network operator information, base station information, scheduling information, available resource information, whether or not a message is fragmented, etc., by receiving and using the MOB_NBR-ADV message. The format of the MOB_NBR-ADV message according to the embodiment of the present invention is shown in Table 4 below.  
                       TABLE 4                       Syntax   Size   Notes                   MOB_NBR-ADV_Message_Format( )               {       Management Message Type = 53    8 bits       Operator ID Flag    1 bit   If set to ‘1’, Operator ID will be               omitted       Reserved    2 bits       Configuration Change Count    8 bits   Incremented each time the               information for the associated               neighbor BS has changed.       Fragmentation Index    4 bits   This field indicates the current               fragmentation index.       Total Fragmentation    4 bits   This field indicates the total number               of fragmentations       Skip-Optional-Fields Flag    1 bit   If set to ‘1’ and if a neighbor has               OFDMA PHY the BS-ID for that               neighbor is omitted in this message.               If set to ‘0’, BS-ID is not omitted for               any neighbor.       If ( Operator ID Flag = = 0)        Operator ID   24 bits   Unique ID assigned to the operator       }       N_NEIGHBORS    8 bits       For (j=0; j&lt;N_NEIGHBORS; j++){       Length    8 bits   Length of message information               within the iteration of               N_NEIGHBOR in bytes.       PHY Profile ID    8 bits   Aggregated IDs of Co-located FA               Indicator, FA Configuration               Indicator, FFT size, Bandwidth,               Operation Mode of the starting               subchannelization of a frame, and               Channel Number       if (FA Index Indicator = = 1) {       FA Index    8 bits   This field, Frequency Assignment               Index, is present only the FA Index               Indicator in PHY Profile ID is set.               Otherwise, the neighbor BS has the               same FA Index or the center               frequency is indicated using the TLV               encoded information.       }       if (BS EIRP Indicator = = 1) {       BS EIRP    8 bits   Signed Integer from −128 to 127 in               units of dBm This field is present               only if the BS EIRP indicator is set               in PHY Profile ID. Otherwise, the               BS has the same EIRP as the serving               BS.       }       Neighbor BS_ID   24 bits   This is an optional field for OFDMA               PHY and it is omitted or skipped if               Skip Optional Fields Flag = ‘1’       Preamble Index    8 bits   The index for the PHY profile               specific preamble. Preamble Index is               PHY specific for SCa and OFDMA.               The value of Preamble Index shall be               ignored and a value of ‘0x00’ shall               be used for OFDM PHY       HO Process Optimization    8 bits   HO Process Optimization is               provided as part of this message is               indicative only. HO process               requirements may change at time of               actual HO. For each Bit location, a               value of ‘0’ indicates the associated               reentry management messages shall               be required, a value of ‘1’ indicates               the reentry management message               may be omitted. Regardless of the               HO Process Optimization TLV               settings, the Target BS may send               unsolicited SBC-RSP and/or REG-               RSP management messages Bit #0:               Omit SBC-REQ/RSP management               messages during current re-entry               processing Bit #1: Omit               PKM-REQ/RSP management               message during current re-entry               processing Bit #2: Omit               REG-REQ/RSP management during               current re-entry processing Bit #3:               Omit Network Address Acquisition               management messages during               current reentry processing Bit #4:               Omit Time of Day Acquisition               management messages during               current reentry processing Bit #5:               Omit TFTP management messages               during current re-entry processing               Bit #6: Full service and operational               state transfer or sharing between               Serving BS and Target BS (ARQ,               timers, counters, MAC state               machines, etc...)       Scheduling Service Supported    4 bits   Bitmap to indicate if BS supports a               particular scheduling service. ‘1’               indicates support, ‘0’ indicates not               support: bit 0: Unsolicited Grant               Service (UGS) bit 1: Real-time               Polling Service (rtPS) bit 2: Non-               real-time Polling service (nrtPS)               bit 3: Best Effort value of ‘1111’               indicates no information on service               available       Available Radio Resource    4 bits   Percentage of reported average               available subchannels and symbols               resources per frame 0000: 0% 0001:               20% 0010: 40% 0011: 60% 0100:               80% 0101: 100% 0110-1110:               reserved 0110-1110: reserved value               of ‘1111’ indicates no information on               service available       Hand Off Neighbor Preference    2 bits   00: Normal; 01: Preferred 10: Non-               Preferred; 11: Reserved       DCD Configuration Change Count    4 bits   This represents the Neighbor BS               current DCD configuration change               count       UCD Configuration Change Count    4 bits   This represents the Neighbor BS               current UCD configuration change               count       TLV Encoded Neighbor information   Variable   TLV specific       }       }                  
 
         [0027]     The MOB_NBR-ADV message shown in Table 4 further includes a 2-bit reserved field in order to set the length of the conventional MOB_NBR-ADV message shown in Table 1 using byte units. In the MOB_N BR-ADV message according to the present invention, an operator ID flag field is newly defined. The operator ID flag field is used to report whether a MOB_NBR-ADV message includes an operator ID field. For example, when the operator ID flag field is set to “1”, it indicates that the MOB_NBR-ADV message dose not include an operator ID field. In contrast, when the operator ID flag field is set to “0”, the mobile station refers to an operator ID field.  
         [0028]     Since the operator ID field contains information about a network operator, it is unnecessary to periodically update the operator ID field while a mobile station is using the same operator&#39;s network. Therefore, while a mobile station is continuously located in the same operator&#39;s network, the operator ID flag field is set to “1” and the operator ID field can be deleted. As a result, the total size of the MOB_NBR-ADV message can be reduced by 3 bytes.  
         [0029]     Hereinafter, the other fields of the MOB_NBR-ADV message shown in Table 4 will be described.  
         [0030]     The MOB_NBR-ADV message includes an 8-bit “Management Message Type=53” field; a “Configuration Change Count” field for representing whether a corresponding message changes, by increasing by “1” whenever configuration changes; a 4-bit “Fragmentation Index” field for representing an order of a currently transmitted message when the MOB_NBR-ADV message is fragmented and therefore transmitted using a plurality of message fragments a 4-bit “Total Fragmentation” field for representing the total number of fragmented messages; and a 1-bit “Skip-Operational-Flag” field. In addition, the MOB_NBR-ADV message includes a 24-bit “Operator ID” field when the Operator ID Flag field is set to “0”.  
         [0031]     The other fields except for the above-mentioned fields in the Table 4 are related to the length, BS ID and handoff of neighbor base stations, which are generally known in the art, so a detailed description thereof will be omitted for the sake of clarity.  
         [0032]     The operation for performing a handover of a mobile station by using the above-mentioned MOB_NBR-ADV message will be described in detail with reference to the accompanying drawings.  
         [0033]      FIG. 2  is a flowchart illustrating a handover procedure of a mobile station in a mobile communication system according to an embodiment of the present invention.  
         [0034]     In order to perform a handover, a mobile station  10  must receive information about a neighbor base station in advance of the handover procedure by receiving a MOB_NBR-ADV message from a first base station  21 , from which the mobile station  10  is currently receiving service. Therefore, the first base station  21  communicating with the mobile station  10  transmits a MOB_NBR-ADV message according to an embodiment of the present invention as shown in Table 4 in order to reduce iterative transmission of the TLV information of a neighbor base-station message.  
         [0035]     Then, as shown in  FIG. 2 , the mobile station  10  receives the MOB_NBR-ADV message from the first base station  21  in step  201 .  
         [0036]     In step  202 , the mobile station  10  checks an Operator ID Flag field from among the MOB_NBR-ADV message received from the first base station  21 . As a result of the determination, if Operator ID Flag field is set to “0”, the mobile station  10  updates the value of the operator ID in step  203 , and then proceeds to step  204 .  
         [0037]     Alternatively, in step  202 , if it is determined that the Operator ID Flag field is set to “1”, the mobile station  10  directly proceeds to step  204 . In step  204 , the mobile station  10  scans a base station  22 , to which the mobile station  10  moves, and then performs a handover procedure in step  205 .  
         [0038]     Meanwhile, the MOB_NBR-ADV message set to reduce iterative transmission of the TLV information of a neighbor base-station message will be described with reference to Table 4.  
         [0039]     The MOB_NBR-ADV message includes a 3-byte “Operator ID” field and a 3-byte “Neighbor BS_ID” field. Meanwhile, a 6-byte BS ID field included in the DCD_setting is created by combination of the 3-byte Operator ID and 3-byte Neighbor BS_ID. Therefore, when the MOB_NBR-ADV message includes the Operator ID and Neighbor BS_ID, the 6-byte BS ID field of the DCD_setting message can be omitted.  
         [0040]     In the prior art, since the TLV information of the MOB_NBR-ADV message must be repeatedly transmitted by all neighbor base stations, overhead increases proportionately to the number of iterative transmissions of the 6-byte BS ID of the DCD_setting field. In this case, the BS ID field of the DCD_setting TLV has a size of 6 bytes and includes a type of 1 byte and a length of 1 byte, so that the total size for representing the BS ID becomes 8 bytes. However, according to an embodiment of the present invention, the BS ID field from among the format of the DCD_setting is omitted, thereby reducing overhead “R” as expressed in Equation 1 below. 
 
 R=N× 8 bytes  Equation 1 
 
         [0041]     That is, when a base station broadcasts neighbor base-station information message for 10 neighbor base stations to a mobile station, an overhead of (N×8 bytes=10×8 bytes) 80 bytes can be reduced by omitting the BS ID field included in the DCD_setting message.  
         [0042]     As described above, according to the present invention, when a base station transmits neighbor base station information messages to a mobile station in a broadband wireless access system, information either not required to be repeatedly transmitted for every message or iteratively transmitted is omitted, and an indication field (Operator ID Flag) for updating such information is added, so that it is possible to reduce overhead and improve a transmission efficiency.  
         [0043]     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.