Study Of Adaptive OFDM Technique In The Physical Layer Of WiMAX

WiMAX is a new broadband wireless access standard based on theIEEE 802.16 standard which is also known as WirelessMAN. It defines theair interfaces for the microwave and millimeter-wave frequencies. It cannot only achieve long distance access, but also can provide high-speedtransmission without the limitation of "last mile" choke point, large systemcapacity, wide multimedia communication services and high security levelservices. WiMAX supports both fixed access and mobile access.Corresponding standards are IEEE 802.16d and IEEE 802.16e. There arefour frameworks available in WiMAX physical layer: WirelessMAN-SCa,WirelessMAN-OFDM, WirelessMAN-OFDMA, WirelessMAN-HUMAN.OFDM technique is both adopted in WirelessMAN-OFDM andWirelessMAN-OFDMA. OFDM technique improves the channelutilization ratio effectively and makes wireless broadband access possible.OFDMA offers the scheme of multi-user access, and makes mobile accesspossible.In this paper, the WirelessMAN-OFDM framework is studiedcombined with the IEEE 802.16d standard. The author goes deep into theadaptive OFDM technique including channel estimation and adaptive bitsand power allocation method. Two channel estimation methods (block-pilot based channel estimation method & comb-pilot based channelestimation method) are studied and some of their simulation results arepresented. Some typical adaptive bits and power allocation methods arestudied and some of their simulation results are presented. In addition, animproved bits and power allocation method is proposed. The simulationresults prove that it can make the throughput of the OFDM system maximal and the error rate of the transmitted signal stable. In the end ofthis paper, we carry out some modules of a MODEM based on WiMAXstandard.