A New Adaptive Modulation Algorithm Of OFDM

It is no doubt that Wireless communication technology is the most active force at present in the telecommunications. On the one hand, public mobile communication keep uptrend particularly in China and some countries or regions, and it is showing a strong growth trend. On the other hand, in addition to the traditional mobile communications, global broadband wireless access and wireless transmission is very active in recent years on the research and application. Hotspot emerged, which includes MIMO technology, HSDPA technology, OFDM technology, UWB technology and etc. It's a flourishing situation. The emergence and development of these technologies have brought vitality for the development of the entire wireless communications industry. However, as more and more wireless applications are required, wireless spectrum resources become more and more strained. Thus, a wide range of high spectral efficiency of communications technology has been developed. OFDM (Orthogonal Frequency Division Multiplexing) is the maximum utilization of the spectrum in the known communication system.The main idea of OFDM technology is that a given channel in the frequency domain is divided into many orthogonal subchannels. In each subchannel, using one sub-carrier to modulate one signal, all subchannels transmit those signals in parallel. OFDM actual channel is divided into a number of subchannels. One of its advantages is that the bits can be allocated flexibily based on different CSI (Channel state Information), and then adaptive modulation is made. OFDM adaptive modulation technology basically derived from the xDSL adaptive modulation technologies which take into account the complexity of algorithm. Therefore, if the local SNR of some subchannel are lower than modulation threshold, close those subchannel which still have remained capacities. It will lose some transmission capacity due to close that subcarrier.Based on researched into the features of wireless channel and OFDM channel model, this thesis put forward a new adaptive modulation algorithm of OFDM. The algorithm can make full use of bad subchannels' transmission capacity, but in some other adptive modulation algorithm, those bad subchannels will be closed. When some subcarriers' local SNR is lower than the modulation threshold, those subcarriers are divided into groups by OFDM subcarrier-diversity technology, so each group-SNR can reach the required threshold, and then adaptive modulation is done. Through theoretical analysis and simulation results show that this adaptive modulation algorithm can improve OFDM system performance.