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標題: | 寬頻無線通訊中之前端電路非理想效應估計與補償 Estimation and Compensation of Front-end Nonidealities in OFDM Systems |
作者: | Yuan-Hwui Chung 鐘元暉 |
指導教授: | 馮世邁(See-May Phoong) |
關鍵字: | 正交頻率多載波系統,實虛部非同調,載波頻率偏移,通道響應, OFDM,I/Q Imbalance,CFO,Channel Response, |
出版年 : | 2009 |
學位: | 博士 |
摘要: | 最近,直接降頻的收發器因為擁有低複雜度與低功率的優點,所以在正交頻率多載波系統上的應用越來越受到注意。但是,直接收發器會受到前置端電路的非理想效應的影響,而導致系統效能的下降。此非理想效應包括實虛部非同調與載波頻率偏移。此篇論文主要是探討此非理想效應對正交頻率多載波系統的影響,並討論在接收端如何利用通訊系統中,已知道之訓練字元,來同時估測實虛部非同調參數,載波頻率偏移與通道響應,並在接收端補償。
我們將補償的方法分為兩種: (a) 針對實虛部非同調為頻率獨立之估計與補償的方法 (b) 針對時虛部非同調為頻率相依之估計與補償的方法 我們利用一個新的概念,稱之為通道殘餘能量,來進行估測與補償的動作。我們所提出的方法可正確的估測到這些系統參數,並且只需要一個正交頻率多載波的字元,另外對於訓練字元並沒有任何限制。當傳送多重複傳送訓練字元時,我們提出一個低複雜度且效能相當不錯的方法。實驗結果顯示,我們方法提供的效能可以達到文獻中所證明的任何估計子可以達到的最低值。我們並將此一方法延伸到多天線傳輸系統中。另外針對實虛部非同調為頻率相依的情形,我們利用疊代的方法,亦可以同時估測載波頻率偏移、實虛部非同調參數與通道響應之目的 Recently, the direct-conversion transceiver for OFDM systems have received a lot of attention because of its low cost implementation and low power consumption. Nonetheless, the direct-conversion receiver may suffer serious performance loss because of the front-end mismatches, such as in-phase and quadrature phase (I/Q) imbalances, carrier frequency offset (CFO), etc. [5]. The I/Q imbalances are due to the amplitude and phase mismatches of the local oscillator, the mismatched low pass ‾lters (LPFs) and the mismatched digital-to-analog (D/C) converters or analog-to-digital (C/D) converters. CFO is due to the mismatch of carrier frequency at the transmitter and receiver. It is shown [6][7] the I/Q imbalances and CFO destroy the subcarrier orthogonality of OFDM systems, and hence may degrade the performance of OFDM systems seriously. It is an important issue to estimate and compensate these mismatches using training sequences. In this dissertation, we study the joint estimation of transmitter, receiver I/Q imbalances, CFO and channel responses using training sequences for OFDM systems. The estimation problems are divided into two types by categorizing the types of the I/Q imbalances as below: (i) Joint estimation of frequency independent I/Q, CFO and channel responses. (ii) Joint estimation of frequency selective I/Q, CFO and channel responses. To solve the two joint estimation problems, we introduce a new concept called channel residual energy (CRE). By minimizing the CRE, we can jointly estimate the I/Q and CFO parameters without knowing the channel response. Once the I/Q and CFO parameters are obtained, the channel response can be obtained by simple substitution. The proposed method needs only OFDM block for training. Moreover there is no restriction on the training data. Any modulation symbols with constant modulus can be used for training. When the I/Q imbalance is frequency independent, the problem can be solved using a non-iterative method. If two repeated blocks are available for training, we propose a low complexity two-step approach to solve the joint estimation problem. Both the proposed methods can provide a good performance close to the Cramer-Rao bound. When the I/Q imbalance is frequency selective, we propose an iterative method to solve the joint estimation problem. For both the frequency independent and frequency selective I/Q imbalances, the proposed methods can provide a BER performance close to the ideal case where all the I/Q parameters, CFO and channel responses are known at the receiver. In addition, we also extend the proposed methods to the case of multipleinput multiple-output (MIMO) OFDM systems where there are multiple antennas at the transmitter and receiver. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/42592 |
全文授權: | 有償授權 |
顯示於系所單位: | 電信工程學研究所 |
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