正交分頻多工系統之通道估計與I/Q失衡補償

Abstract

現今許多的應用需要具備高傳輸速率的無線通訊系統，而正交分頻多工(OFDM)系統正是滿足此特性的主流技術之ㄧ。在接收機架構方面，無論是在業界或是學術界，直接轉換接收機(Direct Conversion Receiver)皆被廣泛地採用，因其具備有小體積，低成本和高整合度等優勢。然而，此架構因為射頻元件不完美的緣故，無可避免地會產生一些射頻損傷，像是直流偏移(DC Offset)、頻率偏移(Frequency Offset)和I/Q失衡(I/Q Imbalance)等。此論文中將對正交分頻多工系統同時考慮通道估測以及I/Q失衡估測補償兩項議題，並在假設頻率偏移可被預先被估測出的情況下結合頻率偏移補償。I/Q失衡有兩種類型，第一種為不隨頻率變動，第二種為隨頻率變動，兩者皆在本研究範圍之內。不同於傳統方法， 我們的可能性函數(Likelihood Function)是由頻率域上所載的接收資料所組成，利用最大可能性(Maximum Likelihood)來估測I/Q失衡及通道，這個方法能使我們利用到正交分頻多工系統之特性及不同載波間的高相關性優點。論文內容也包含了最大可能性估計的效能分析與電腦模擬的結果，以驗證演算法的正確性和可行性，在訊號雜訊能量比(SNR)高到一定程度下其效能可達到Cramer-Rao下限(Lower Bounds)。

High data rate wireless communication is required by a variety of applications, and OFDM is one of the promising technologies too meet such a demand. On the other hand, direct-conversion RF receiver (DCR) architecture has become a trend in industry and academic world nowadays because it is small, cheap and low power-consuming. This kind of architecture, however, accompanies with some radio frequency (RF) imperfections such as the direct current (DC) offset, frequency offset, in-phase/quadrature (I/Q) imbalance etc. In this thesis, for OFDM system, we propose an algorithm for joint estimation of delay-spreading channel and IQ imbalance effect combined with frequency offset compensation assuming that frequency offset has been prior estimated. Both of frequency-independent and frequency-dependent IQ imbalance are covered. Different from traditional approach, our likelihood function is constructed by using frequency-domain data to estimate time-domain channel response and IQ imbalance based on ML criterion. This approach enables us to take advantage of characteristics of OFDM and smoothing property. Simulation results show that the estimated values approach their CRLBs once SNR is above certain level.