1. Field of the Invention
The present invention relates to a wireless communication system and, more particularly, to channel accessing and data stream transmission technique in a wireless local area network (WLAN) system in which a legacy station exists.
2. Discussion of the Related Art
Recently, diverse wireless communication technologies are under development in line with the advancement of information communication technology. Among them, a wireless local area network (WLAN) is a technique allowing users to wirelessly access the Internet by using mobile terminals such as personal digital assistants (PDAs), lap top computers, portable multimedia players (PMPs), and the like, at homes, offices or in a particular service providing area based on a radio frequency technology.
Since IEEE (Institute of Electrical and Electronics Engineers) 802, a standardization organization of a WLAN technique, was established in February 1980, a great deal of standardization works have been conducted.
The early WLAN technique supported the rate of 1-2 Mbps through frequency hopping, spread spectrum, infrared communications, and the like, by using a 2.4 GHz frequency based on IEEE 802.11, and recently, a maximum rate of 54 Mbps can be supported by applying orthogonal frequency division multiplex (OFDM) technology to the WLAN. Further, IEEE 802.11 are putting standards of various techniques, such as improvement of quality of service (QoS), allowing compatibility of access point (AP) protocols, achievement of security enhancement, measurement radio resource measurement, wireless access vehicular environment, ensuring fast roaming, establishing a mesh network, interworking with an external network, wireless network management, and the like, into practical use or are still developing them.
Among the IEEE 802.11, IEEE 802.11b supports a maximum of 11 Mbs communication speed while using the frequency band of 2.4 GHz. IEEE 802.11a, which has been commercialized following the IEEE 802.11b, uses the frequency band of 5 GHz, not 2.4 GHz, to reduce the influence of interference compared with the considerably congested frequency band of 2.4 GHz and has a communication speed increased up to a maximum 54 Mbps by using the OFDM technique. However, IEEE 802.11a has shortcomings in that its communication distance is shorter than that of IEEE 802.11b. Meanwhile, IEEE 802.11g uses the frequency band of 2.4 GHz, like IEEE 802.11b, to implement a communication speed of a maximum 54 Mbps and satisfies backward compatibility, and as such, IEEE 802.11g receives much attention. Also, IEEE 802.11b is superior to IEEE 802.11a, in the aspect of the communication distance.
IEEE 802.11n has been lately stipulated as a technique standard to overcome the limitation in the communication speed which has been admitted as a weak spot of the WLAN. IEEE 802.11n aims to increase the speed and reliability of a network and extend an operation distance of a wireless network.
In more detail, IEEE 802.11n supports a high throughput (HT) of more than a maximum 540 Mbps as a data processing speed, and is based on a multiple input and multiple output (MIMO) technique using multiple antennas at both ends of a transmission part and a reception part to minimize a transmission error and optimize a data rate.
Also, IEEE 802.11n standard may use orthogonal frequency division multiplex (OFDM) to increase the speed as well as using a coding scheme that transmits several duplicates to enhance data reliability.
As the WLAN is widely spreading and applications using WLAN are diversified, recently, the necessity for a new WLAN system emerges to support a higher throughput than the data processing speed supported by IEEE 802.11n.
A very high throughput (VHT) WLAN system is one of the newly proposed IEEE 802.11 WLAN systems in order to support a data processing speed of 10 Gbps or faster in a MAC service access point (SAP). The term of VHT WLAN system is arbitrary, and currently, a feasibility test is performed on a 4.times.4 MIMO (or 5.times.4 MIMO) and a system using a channel bandwidth of 80 MHz or higher to provide throughput of 1 Gbps or faster.
Aiming at satisfying an aggregated throughput of 1 Gbps, the VHT WLAN system also has the purpose of achieving a minimum 500 Mbps for one-to-one communications between terminals (e.g., user equipments (UEs)). If an offered load of VHT stations is 500 Mbps, it would be effective for the several VHT stations to simultaneously use channels to satisfy the aggregated throughput of 1 Gbps of a VHT basic service set (BSS).
The case of the VHT WLAN system has been described, but in most cases where UEs in conformity with the standards of the WLAN system and legacy UEs coexist, the legacy UEs support a narrower channel bandwidth and a smaller number of antennas than those of the UEs supporting the current standard of the WLAN system. Thus, while the legacy UEs use channels, some channel bandwidths and antennas are not in use, resulting in a waste of radio resources.