FIG.  1 is a flow chart illustrating a network entering process when a mobile station (MS) for a broadband wireless access system is initiated. 
(1) If the mobile station (MS) is initially powered on, it searches for a downlink channel, and acquires downlink synchronization from a base station.  In this case, the mobile station (MS) receives a downlink-MAP (DL-MAP) message, an uplink-MAP (UL-MAP) message, a downlink channel descriptor (DCD) message, and an uplink channel descriptor (UCD) message, such that it acquires UL/DL channel parameters, and can establish UL/DL synchronization using UL/DL channel parameters. 
(2) The mobile station (MS) performs ranging with the base station (BS) to adjust uplink transmission parameters, and receives a basic management CID and a primary management CID from the base station (BS). 
(3) The mobile station (MS) negotiates with the base station (BS) about the basic performance. 
(4) The base station (BS) authenticates the mobile station (MS). 
(5) The mobile station (MS) is registered in the base station (BS).  The mobile station (MS) managed by an Internet Protocol (IP) receives a secondary management CID from the base station (BS). 
(6) IP connection is established. 
(7) A current date and a current time are established. 
(8) Configuration files of the mobile station (MS) are downloaded from a TFTP server. 
(9) Connection for a prepared service is established. 
In a network entering process during the above-mentioned MS initiation, if the base station (BS) supports at least two communication systems, there is needed a method for entering one communication system, and then entering the other communication systems through the one communication system. 
Disclosure
Technical Problem
Accordingly, the present invention is directed to a method for receiving system information that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
An object of the present invention is to provide a method for allowing a base station (BS) which supports at least two communication systems to provide system information. 
Technical Solution
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for receiving system information from a base station (BS) which supports a first communication system and a second communication system comprising: a) searching for a frequency band of the first communication system, and receiving a frame control header (FCH) including specific information which indicates whether the second communication system is supported or not; and b) upon receiving the specific information indicating that the second communication system is supported, receiving system information of the second communication system. 
Preferably, the step b) of receiving the system information includes: receiving the system information via a downlink MAP message. 
Preferably, the FCH includes count information indicating which one of downlink MAP messages includes the system information of the second communication system. 
Preferably, the count information is firstly set to an initial value, such that it is reduced by ‘1’ whenever a downlink MAP message is transmitted, and if the count information is set to ‘0’, it is reset to the initial value at the next frame of a frame at which the FCH is received. 
Preferably, the initial value is associated with a transmission (Tx) period of the system information associated with the second communication system. 
Preferably, the method further comprises, if the ‘Count’ value is set to ‘0’, receiving the system information via a downlink MAP message of a frame at which the FCH is received. 
Preferably, the step b) for receiving the system information includes: receiving the system information via a downlink channel descriptor (DCD) message. 
Preferably, the downlink channel descriptor (DCD) message includes second information indicating that the system information of the second communication system has been changed. 
Preferably, the step b) for receiving the system information includes: receiving the system information via at least one of a ranging response message, a basic capability response message, a system information response message, and an authentication message. 
Preferably, the system information includes at least one of downlink center frequency information, base bandwidth information, information indicating the number of base bandwidths, FFT size information, cyclic prefix information, frame duration code information, which are used in the second communication system. 
Preferably, the step b) for receiving the system information includes: receiving, by the base station (BS), the system information using at least one of a broadcast scheme, a multicast scheme, and a unicast scheme. 
In another aspect of the present invention, there is provided a method for allowing a mobile station (MS) to receive system information from a base station (BS) which supports a first communication system and a second communication system comprising: searching for, by the mobile station (MS), a frequency band of the first communication system, and receiving a frame control header (FCH) including first information which indicates whether the second communication system is supported or not; upon receiving the first information indicating that the second communication system is supported, transmitting, by the mobile station (MS), a specific message including second information indicating that the second communication system is supported; and receiving system information of the second communication system as a response to the specific message. 
Preferably, the specific message includes at least one of a ranging request message, a basic capability request message, a system information request message, and an authentication message. 
Preferably, the method further comprises, if the system information of the second communication system is changed, receiving a system-information change message including the changed system information. 
Preferably, the system information includes at least one of downlink center frequency information, base bandwidth information, information indicating the number of base bandwidths, FFT size information, cyclic prefix information, frame duration code information, which are used in the second communication system. 
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
Advantageous Effects
The above-mentioned embodiment of the present invention has the following effects. 
The present invention can initially enter a communication system.  The present invention can acquire system information from other communication systems via the entrance communication system.  Therefore, the present invention may also enter other communication systems via the acquired system information.