正交頻分多址系統中載波頻率偏移之估測

Estimation of Multiuser CFOs in OFDMA Systems

李世聖(Shih-Sheng Li)

指導教授 ： 馮世邁

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802277

摘要

正交頻分多址在近期的無線通訊標準中被廣泛的應用，然而，正交頻分多址的系統易受頻域上的同步錯誤影響，其中載波頻率偏移將使子載波間失去正交性。在上行的正交頻分多址系統中，基地台接收到的訊號是由不同的使用者受到個別的載波頻率偏移所構成，而通常將這頻率同步的錯誤分成探測階段與追蹤階段。載波頻率偏移將誘發內載波干擾與多重進接干擾，這將大大降地的系統效能。因此，對於上行的正交頻分多址系統中頻率上同步錯誤的估測與補償是一個重要且富挑戰性的議題。

　　多數存在的載波頻率偏移方法受限於特定的載波分配法，在這論文中，我們將介紹多種可以適用於一般性載波分配法的方法，而這些方法包含了盲估測以及引示估測。在第一個部分，我們首先回顧了常數係數的方法，接著，提出了多種估測頻率載波偏移的方法，包含了虛擬載波、虛擬載波併常數係數以及峰度的估測法，同時，我們在載波頻率偏移的追蹤階段找到了一個近似的方法也在載波頻率偏移的探測階段提出了兩階段的估測方法，這兩種方法降低了不少在估測方面的運算複雜度。在第二部分為引示協助下的估測，我們提出了一個同時估測載波頻率偏移與通道的方法，除此之外，我們合併了虛擬載波的想法讓載波頻率偏移的估測更加精確，而這方法可以應用於近似方法與兩階段估測。最後，模擬圖放置於各章節的後面，從模擬結果可以印證我們的想法，亦可與過去方法進行比較。

關鍵字

載波頻率偏移 ； 正交頻分多址 ； 估測 ； 多用戶 ； 上行傳輸 ； 載波分配方案

並列摘要

Recently, orthogonal frequency-division multiple access (OFDMA) has been widely adopted in many wireless communication standards. However, the OFDMA-based systems are sensitive to the frequency synchronization error, i.e. carrier frequency offset (CFO), which destroys the orthogonality among the subcarriers. In the uplink OFDMA systems, the received signal at the base station (BS) is composed of all active users with individual CFO. This frequency synchronization error is normally partitioned into an acquisition stage followed by a tracking stage at the BS. CFO induces both intercarrier interference (ICI) and multiple-access interference (MAI), which degrade the performance of systems significantly. Therefore, the estimation and the correction of the frequency synchronization error is a crucial and challenging topic in uplink OFDMA systems.

Most existing CFO estimation methods are derived under the certain carrier assignment scheme (CAS). In the thesis, we introduce several new estimation methods that can be applied to generalized-CAS (GCAS). Both blind and pilot-aided methods are considered. In the first part, we first review on estimator using the constant-modulus (CM) constellation and then propose several blind estimations of multiple CFOs, such as virtual carrier based (VC-based), VC-CM based and kurtosis-based (KT-based) estimators. In addition, we develop a closed form solution for CFO tracking and obtain a 2-stage algorithm for the VC-based estimator.
The complexity of these closed form solutions are very low. In the second part, for pilot-aided scheme, we propose a new algorithm for the joint estimation of CFO and channel response. We also incorporate the VC in the CFO estimation to further the estimation accuracy. Low complexity closed form solutions are also derived. Moreover, numerical simulations are provided to show the effectiveness of the proposed methods and to compare the performances with other existing methods.

並列關鍵字

carrier frequency offset (CFO) ； orthogonal frequency division multiple access (OFDMA) ； estimation ； multiuser ； uplink transmission ； carrier assignment scheme (CAS)

參考文獻

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