A standard for a wireless local area network (WLAN) technology has been developed as IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard. IEEE 802.11a and IEEE 802.11b use an unlicensed band in 2.4 GHz or 5 GHz, IEEE 802.11b provides transmission speed of 11 Mbps and IEEE 802.11a provides transmission speed of 54 Mbps. IEEE 802.11g provides transmission speed of 54 Mbps in a manner of applying Orthogonal Frequency Division Multiplexing (OFDM) in 2.4 GHz. IEEE 802.11n provides transmission speed of 300 Mbps for 4 spatial streams in a manner of applying Multiple Input Multiple Output—OFDM (MIMO-OFDM). IEEE802.11n supports a channel bandwidth up to 40 MHz. In this case, IEEE802.11n provides transmission speed of 600 Mbps.
Currently, ongoing effort to develop IEEE 802.11af standard has been performed. IEEE 802.11af standard is a standard set to regulate an operation of an unlicensed device in a TV whitespace (TVWS) band.
The TVWS band is a frequency band assigned for a TV broadcast and includes a Very High Frequency (VHF) band and an Ultra High Frequency (UHF) band. The TVWS means a frequency band permitted to an unlicensed device to use under a condition that the unlicensed device does not impede a communication of a licensed device operating in a corresponding frequency band. In particular, a device operating as the unlicensed device can use an available channel or an available frequency band, which is not used by the licensed device, in the corresponding frequency band. In this case, the licensed device can include a TV, a wireless microphone, and the like. The licensed device may be called a licensed user, an incumbent user, a primary user or the like.
The unlicensed device wishing to use the TV WS band should provide a protection function for a licensed device. Hence, the unlicensed device should check whether the licensed device occupies a corresponding band before starting signal transmission in the TV WS band to obtain information on an available channel.
To this end, the unlicensed device can check whether a corresponding band is used by the licensed device by performing spectrum sensing. An energy detection scheme (a scheme of judging a use of an incumbent user if a strength of a reception signal is greater than a prescribed value), a feature detection scheme (a scheme of judging a use of an incumbent user if a digital TV preamble is detected), and the like can be utilized as the spectrum sensing mechanism. If strength of a signal received on a specific channel is greater than a prescribed value or a DTV (digital TV) preamble is detected, the unlicensed device can determine it as the specific channel is in use by the licensed device. If it is determined that the licensed device uses a channel directly adjacent to a channel currently used by the unlicensed device, transmit power of the unlicensed device should be lowered according to the extent of an amount of interference radiated to an adjacent band in some cases.
Yet, if the unlicensed device depends on frequency sensing only in the TV WS band to obtain information on an available channel, a burden of sensing of the unlicensed device to operate in the TV WS band grows and a procedure may be delayed. Hence, the unlicensed device may obtain channel list information available for a corresponding region in a manner of accessing a geo-location database via the Internet or a dedicated network. The geo-location database is a database storing and managing channel usage information, which is dynamically changing according to information of registered licensed devices, geographical locations of the licensed devices and channel usage time.