标题: | 应用在双选择性衰减通道中之正交分频多工系统的同步技术 Synchronization Techniques for OFDM Systems in Doubly-Selective Fading Channels |
作者: | 卿文龙 Wen-Long Chin 陈绍基 Sau-Gee Chen 电子研究所 |
关键字: | 正交分频多工;同步;OFDM;synchronization |
公开日期: | 2008 |
摘要: | 在数位通讯系统中,接收讯号之同步技术是基频接收机必要的功能。在探讨同步技术之前,本论文首先推导在多个同步错误共存下的数学模型。这些同步错误包括有符元时间偏移(symbol time offset)、载波频率偏移(carrier frequency offset)及取样频率偏移(sampling clock frequency offset)。这样的分析是必需的,因为在实际的环境中,这些同步错误是不可避免且同时存在的。因此,理论之讯号对干扰杂讯比(signal-to-interference-and-noise ratio, SINR)可由此数学模型进一步求得,并界定出因同步误差所造成之效能损失。 为了改善在正交分频多工(orthogonal frequency-division multiplexing, OFDM)系统中的同步错误,尤其针对双选择性衰减通道(doubly-selective fading channels)的环境下,本论文提出四个基于不同准则之同步演算法。 第一个提出之同步演算法是基于最大或然性(maximum-likelihood) 准则。藉由接收到的频域讯号之数学模型,此方法求得两个连续符元的对数或然(log-likelihood)函数。基于此函数,本论文提出针对符元时间偏移及载波频率偏移之最大或然估测。并且,为了降低整体运算复杂度,吾人进一步由原提出之最大或然估测,提出一简化的估测。 第二个提出之符元时间偏移同步演算法是基于最小化干扰准则。这个方法利用引导(pilot)子载波的特性,置符元边界于最小干扰之取样点。再者,为了降低运算复杂度,吾人利用Parseval定理及取样理论,转换此频域之规准(metric),成为一低复杂度之时域规准。 第三个提出之同步演算法是基于最大化讯号对干扰杂讯比准则。除了干扰随着同步错误会快速增加外,有用的讯号也会随之衰减。藉由这项观察,吾人利用频域资料,以最大化讯号对干扰杂讯比准则,直觉地利用频域资料估测出符元时间偏移及载波频率偏移。再者,于此提出的最大化讯号对干扰杂讯比演算法,为无须训练符元及引导子载波之非资料辅助(non-data-aided, NDA) 方法,如此可以提升传输效率。 第四个提出之同步演算法也是基于最大化讯号对干扰杂讯比准则。与第三个方法不同之处,此方法是利用时域的循环前置(cyclic prefix, CP)。在仅须较低的复杂度之下,此方法仍有令人满意的效能。 此外,为了降低第三个方法所须运算量,本论文特别针对第三个方法,提出一追踪(tracking)技术,也就是早晚闸(early-late-gate, ELG) 技术。这个技术可用在符元时间偏移及载波频率偏移的追踪上。 综言之,除了效能外,本论文亦探讨所提方法之复杂度及实用性。本论文提出之演算法各有不同之效能与复杂度,因此,它们可以适合于不同的应用及环境中。 For digital telecommunication systems, signal synchronizations are necessary for the baseband receivers. In this dissertation, before investigating the synchronization techniques, the mathematical model due to the combined effects of the symbol time offset (STO), carrier frequency offset (CFO), and sampling clock frequency offset (SCFO) is derived. The analysis of the combined impacts of these co-existing errors is desirable due to non-ideal synchronization process in a practical environment. With the derived model, the theoretical signal-to-interference-and-noise ratio (SINR) is formulated to characterize the incurred losses owing to the synchronization errors. To cope with the synchronization errors in orthogonal frequency-division multiplexing (OFDM) systems, four synchronization methods based on various optimization metrics are proposed, which can work effectively in the environments of doubly-selective fading channels. The first proposed method is based on the maximum-likelihood (ML) criterion. Based on the derived mathematical model of the received frequency-domain (FD) data, we formulate the log-likelihood function of two consecutive symbols. With this function, the ML estimation for the STO and CFO is proposed. In addition, to reduce the overall computational complexity, a simplified estimation is also proposed based on the originally derived ML estimation. The second proposed STO technique is based on the criterion of minimizing the interference. This method locates the symbol boundary at the sampling point with the minimum interference (MinITF) utilizing the pilot subcarriers. Moreover, to reduce the computational complexity, the proposed FD minimum-interference metric is converted into a low-complexity time-domain (TD) metric by utilizing the Parseval’s theorem and the sampling theory. The third proposed technique is based on the criterion of maximizing the SINR. In addition to drastic increase of the interference at wrong synchronization points, the desired signal power also decreases accordingly. Based on this observation, both the STO and CFO are intuitively estimated by maximizing the SINR metric of FD data. Moreover, the proposed maximum-SINR (MaxSINR) method is non-data-aided (NDA) without needing training symbols and pilot subcarriers so that the transmission efficiency can be improved. The fourth proposed technique is also based on the criterion of maximizing the SINR. In contrast to the FD-MaxSINR, the fourth proposed method is developed in time-domain and utilizes the CP. With its low-complexity, the proposed time-domain Maximum-SINR (TD-MaxSINR) still has satisfactory performance. To reduce overall computational complexity of the FD-MaxSINR technique, we specifically investigate a tracking technique for the proposed FD-MaxSINR metric, i.e., the low-complexity early-late-gate (ELG) technique. The ELG technique can be employed for both the STO and CFO estimations. In summary, besides the performance, we also focus on the complexities of the proposed techniques to investigate their practicalities. The proposed techniques exhibit different performances and complexities; therefore, they can be suited for different applications and environments. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT009211837 http://hdl.handle.net/11536/67957 |
显示于类别: | Thesis |