Research On Link Adaptation Techniques In OFDM Systems

OFDM meets the modern communication demands for the high spectral efficiency, and has the resistance ability to frequency selective fading, so it is considered as one of the core technologies of the 4th Generation wireless communication systems. Furthermore, adaptive modulation techniques have been receiving more and more attentions, as they can dramatically increase the spectral efficiency and improve the transmission reliability of the wireless communication systems. Research on the adaptive modulation techniques in OFDM systems will be very valuable for the development of the future broadband wireless communication systems.The dissertation mainly focuses on the link adaptation in OFDM systems. Firstly, mathematic model and information theory of the adaptive OFDM systems are analyzed. Then, several typical adaptive bit allocation algorithms for different optimizing objects are discussed. Simultaneously, performances of these algorithms are also evaluated through computer simulation. Furthermore, the impacts of the factors including channel estimation error and feedback delay on the adaptive bit allocation algorithms are analyzed. And then, in order to improve the performance of the systems, an adaptive bit allocation algorithm is proposed in the imperfect CSI. This algorithm allocates bits based on the channel estimates with delay, and makes average BER equal to target BER. Simulation results show that channel estimation error will not be a significant problem in the adaptive OFDM systems when the MSE below can be achieved. Nevertheless, the use of multiple estimates of the channel is shown to mitigate the effect of CSI delay and improve the robustness of the system. Finally, EESM (Exponential Effective SINR Mapping) can map the instantaneous channel state set into a single effective SINR value (a scalar value) that may then be used to obtain an estimate of the block-error probability from a set of basic AWGN link-level performances. Therefore, EESM in adaptive OFDM systems predigests the information of the feedback channel and greatly reduces the signaling of the systems. 10?1.5...