OFDM与SC-FDE的低回馈自适应和信道估计

【作者】 李晓宁；

【导师】 赵辉；

【作者基本信息】 山东大学 ， 通信与信息系统， 2007， 硕士

【摘要】 目前,宽带无线通信受到人们的广泛关注,OFDM和SC-FDE是宽带无线通信物理层两种有效传输技术。利用自适应技术,根据信道实际状态自适应比特装载和功率控制,可以进一步提高系统传输性能。本文研究了低回馈信道状态信息的自适应OFDM和SC-FDE系统,即利用有限的信道状态信息实现自适应传输。由于回馈数据量很小,降低了回馈信道信息所需的开销;同时算法复杂度也不高。本文首先回顾了几种经典的自适应OFDM算法,之后给出了一种基于梳状导频的OFDM分组自适应比特装载与功率控制算法,该算法在考虑信道状态误差影响时,在给定数据传输速率和误比特率下,所需的最小Eb/N0与经典的Greedy算法相当,而所需运算量显著降低,且需要回馈给发射端的信息量不到Greedy算法的10%。一比特信道状态信息回馈的自适应OFDM系统,仅仅利用回馈的每子载波一个比特的信道状态信息实现自适应,可以有效提高系统的传输性能。本文研究了一比特信道信息在自适应中的利用方式,分析了自适应参数的优化问题。SC-FDE与OFDM存在时频域对偶关系,其实现复杂度与OFDM相当。但由于其符号映射和检测在时域完成,因此在SC-FDE系统中利用信道状态信息来完成自适应调制,并不像OFDM那样直接。引入最佳信号子空间概念基础,给出了同样采用一比特信道状态信息的自适应SC-FDE系统。该系统能够实现自适应子信道选择,避开深衰落的子信道,从而提高了系统的综合传输性能。与一比特信道状态信息回馈的自适应OFDM比较,自适应SC-FDE系统误码性能更好。信道状态信息通常是通过接收端利用信道估计的方法得到的,信道估计的精度直接影响到符号检测的性能,同时影响发射端自适应策略与实际信道状态的匹配关系。本文研究了OFDM和SC-FDE/FS系统的信道估计问题。给出了一种OFDM基于时频二维多项式模型的信道估计算法。该算法的突出优势在于无需任何信道的统计先验信息即可实现信道估计,且在二维多项式阶数较小时运算量不大。给出了适用于自适应SC-FDE系统的信道估计方法,先利用插入导频信息进行信道捕获,然后自适应地进行信道跟踪。这种方法由于在信道跟踪时,同时利用了多个块符号的信息,可以有效克服噪声和深衰落的影响。即使在恶劣的无线移动信道下,该跟踪方法仍然能够跟踪相当长信息符号块而无需重新捕获。减少了需要插入的导频数量,提高了系统的传输效率。更多还原

【Abstract】 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.更多还原