Ⅰ. INTRODUCTION
In recent year, BWA (Broadband Wireless Access)
occupies the important position in access networks for its good
performance of wide coverage, high band width and its vast
future prospects. BWA is a kind of communication and
information technology which provides user with broad band
services such as audio, video, data and multimedia service by
radio access. BWA can be divided into four parts according to
its coverage range: WPAN (Wireless Personal Area Network)
including Bluetooth, IEEE802.15 and UWB, WLAN (Wireless
Local Area Network) including IEEE802.11b/a and Wi-Fi
(Wireless Fidelity), WMAN (Wirelesss Metropolitan Area
Network) including MMDS (Multichannel Multipoint
Distribution System), LMDS (Local Multipoint Distribution
System) and IEEE802.16d/e and WWAN (Wireless Wide Area
Network) including IEEE802.20. Among many BWA
technologies, WiMAX (Worldwide Interoperability for
Microwave Access) as a WMAN technology attracts more
attentions for its advantages such as high data rate, low
investment ,quick construction etc.