Ofdm And Sc-fde, The Low-feedback Adaptive Channel Estimation,

Broadband wireless communication technology has been drawing great attention recent years. OFDM and SC-FDE are two kinds of promising transmission techniques on physical layer. The performance can be improved more by using adaptive bit loading and power control to match the current channel state. This thesis focuses on adaptive OFDM and adaptive SC-FDE with low feedback channel state information (CSI), that is, how to carry out adaptive transmission by using limited CSI. It benefits from lower CSI feedback spending and low algorithm complexity.Several classical adaptive algorithms in OFDM are reviewed. Then we introduce a grouped adaptive bit loading and power control method based on comb-pilots in OFDM. The needed minimum E_b/N₀ is almost the same with the classical "Greedy" algorithm under the fixed data rate and BER when imperfect CSI is considered, while the proposed method has lower algorithm complexity and less 10% CSI feedback. The transmitter of adaptive OFDM system with one bit per subcarrier CSI feedback implements adaptive transmission using only one bit per subcarrier CSI. It is showed that one bit CSI feedback can enhance the system performance effectively and the motheds of parameters optimization is studied too.The duality of OFDM signal and SC-FDE signal on time-frequency domains decides that SC-FDE has the same computational complexity with OFDM. But SC-FDE cannot finish adaptive modulation utilizing CSI directly like OFDM due to it has to make symbol mapping and detecting in time domain. Based on optimal signal subspace (OSS) a adaptive SC-FDE system utilizing the same one bit per subcarrier CSI like OFDM is conceived. This system uses those frequency domain sub-channels with higher gain to transmit information and avoids the sub-channels with lower gain. With this method, the performance of SC-FDE is improved obviously. The performance of BER of the proposed SC-FDE system overcomes adaptive OFDM system with one bit per subcarrier CSI feedback in the same conditions.CSI is obtained by channel estimation generally at the receiver and its precision directly affects the performance of symbol detecting and decides if the used adaptive policy is matching the current channel state well. This thesis studied the channel estimation methods of OFDM and adaptive SC-FDE. We propose a channel estimation algorithm based on a time-frequency polynomial model of the fading multipath channels. The best merit of this method is that there is no statistical information of the channel needed in advance when channel estimation can be finished. The computational complexity is limited when the order of the time-frequency polynomial is small. A channel estimation algorithm for adaptive SC-FDE is designed which consists of channel acquisition and channel tracking. The system uses inserted pilots to accomplish the channel acquisition before adaptive channel tracking. This adaptive channel tracking method can overcome deep attenuation and AWGN noises effectively due to several blocks of symbol are considered simultaneously. Even in the badly wireless channel environment the proposed tracking method can track considerable blocks of symbol and there is no need for channel acquisition again. The transmission efficiency is improved due to less pilots data needed.