Research On The Adaptive Modulation Techniques In OFDM System

With the development of communication technology and economy, the demand for higher data rate and higher quality in wireless communication systems has seen unprecedented growth. However, one of the most limiting factors in wireless communications is the scarcity of spectrum. In addition, the multi-path propagation and time varying nature of wireless channel give some impairment to the signals transmitted in it. In recent years, link adaptation techniques have received more and more attentions, as they can dramatically increase the spectral efficiency and improve the transmission reliability of wireless communication systems. In this dissertation, the adaptive modulation technique in orthogonal frequency division multiplexing (OFDM) system is studied.Knowledge of wireless channel and OFDM system is critical in designing and assessing the performance of adaptive modulation in OFDM system. Therefore, wireless channel model and basic principles of OFDM are presented at first. As the basis of adaptive modulation, information theory is also discussed.Adaptive modulation in the OFDM system, sometimes called bit loading, is to choose different modulation scheme on each subcarrier with the channel state information accordingly, which realizes link adaptation in both frequency domain and time domain. Several adaptive modulation algorithms are discussed in this dissertation. Performance of these algorithms is also evaluated through computer simulation. Furthermore, the impacts of the factors including frequency group, time interval, channel estimation error and feedback delay on adaptive modulation are analyzed.The combination of multi-input and multi-output (MIMO) and OFDM is called MIMO-OFDM. This kind of technique not only can increase the...