補零正交分頻多工與多重存取系統中接收端設計與載波頻率偏移之估測

Abstract

正交分頻多工（orthogonal frequency division multiplexing）和正交分頻多址 （orthogonal frequency division multiple access）作為調變技術已被採用於許多無 線通訊標準中。在大多數正交分頻多工系統中，在發送端添加循環前綴（cyclic prefix）樣本，以便可以在接收端使用簡單的通道等化(channel equalization)。除 了循環前綴之外，補零（zero padding）作為傳統循環前綴的新穎替代方式也 曾被提出來以確保在回復符元時可以無視通道零點。但無論是循環前綴或是 補零，基於正交分頻多工的系統對同步的誤差都十分敏感，例如載波頻率偏移 （carrier frequency offset）。在上行(uplink)正交分頻多址系統中，於基站（base station）的接收信號是由所有各別具有載波頻率偏移的活躍用戶組成。 載波頻 率偏移引起載波間干擾（intercarrier interference）和多重存取干擾（multi-access interference），它們會嚴重地降低系統的性能。因此準確估計這些同步誤差很是重 要。 這篇論文中，我們在補零正交分頻多工(ZP-OFDM)及正交分頻多址(OFDMA) 系統針對接收端設計和載波頻率偏移估測方法進行研究。 其中考慮了訓練序 列(training sequence)、虛擬載波(virtual carrier)及盲蔽估測(blind estimation)的方 法。 在第一部分，我們為補零正交分頻多工設計了兩種接收器，它們具有性 能表現及實作複雜度之間的權衡。 然後我們會展示如何大幅度地降低現行接 收器的複雜度。 在第二部分，我們會將在循環前綴正交分頻多工中已經被 提出的載波頻率偏移的估測方法延伸到補零正交分頻多工當中，包含通道殘 餘能量(channel residual energy)、虛擬載波(virtual carrier)和頻率分析(frequency analysis)。 在頻率分析的方法中，我們推導出針對補零正交分頻多工的低複雜度 閉合解(closed-form solution)以估測載波頻率偏移。 此外，我們也提出新的零空間 法(null space method)來估測載波頻率偏移。 在第三部分，針對補零正交分頻多 址(ZP-OFDMA)設計接收端還有提出通道響應(channel response)及載波頻率偏移的 聯合估計(joint estimation)方法。 本論文各章節末附上各種情況下之模擬結果圖來 驗證所提出的方法。

Orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) have been adopted as a modulation technique in many wireless communication standards. In most OFDM systems, cyclic prefix (CP) samples are added at the transmitter so that simple channel equalization can be used at the receiver. Besides CP, zero padding (ZP) has been proposed as an appealing alternative to the traditional cyclic prefix (CP) OFDM to ensure symbol recovery regardless of the channel nulls. However, OFDM-based systems (no matter CP or ZP) are sensitive to synchronization errors, such as carrier frequency offset (CFO). In the uplink OFDMA systems, the received signal at the base station (BS) is composed of all active users with individual CFO. CFO induces both intercarrier interference (ICI) and multi-access interference (MAI), which degrade the performance of systems significantly. Therefore, accurate estimation of these synchronization errors are important. In this thesis, receiver design and CFO estimation methods in ZP-OFDM/OFDMA systems are studied. Training sequence, virtual carriers, and blind methods are considered. In the first part, two receivers are designed for ZP-OFDM system with a tradeoff between performance and implementation complexity. Then, we show how much complexity is reduced. In the second part, we extend several CFO estimation methods from CP-OFDM to ZP-OFDM. These methods include channel residual energy(CRE-based), virtual carrier(VC-based) and frequency analysis(FA-based) methods. For FA method, a low complexity closed-form solution is developed for CFO estimation in ZP-OFDM. In addition, we develop a new null-space method to estimate CFO for ZP-OFDM. In the third part, we propose receivers and a joint estimation of CFO and channel response in ZP-OFDMA systems. Numerical simulations are provided to verify the performances of the proposed methods.