多天線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，而多天線正交分頻多工(MIMO-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，因為其具備有小體積，低成本和高整合度等優勢。然而，此架構因為射頻(Radio Frequency)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，而I/Q失衡問題將是我們討論的重點。此論文中將同時考慮通道估測以及I/Q失衡補償兩項議題，並提出一共同最大可能性估計(Joint Maximum Likelihood Estimation)之估計和補償方法。再者，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，我們會針對兩者各自提出估計及補償的方法。另外，論文中的通道估計和I/Q失衡補償演算法不僅可應用於單天線(SISO)正交分頻多工系統，更可應用在多天線正交分頻多工系統，例如IEEE802.11n。論文內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。

Wireless communication systems with high data rate are strongly desired by a variety of applications, and MIMO-OFDM is one of the promising technologies satisfying the need for high throughput. A direct-conversion receiver (DCR) architecture becomes a trend in industry and academic world nowadays because it is small, cheap and less 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, and etc. In this thesis, the RF impairment of I/Q mismatch is addressed; moreover, we propose a joint maximum likelihood (ML) estimation of I/Q imbalance and channel for MIMO-OFDM systems, such as IEEE 802.11n. The problems of I/Q mismatch compensation and channel estimation are considered at the same time, and we offer the compensation algorithms based on not only frequency-independent but also frequency-dependent I/Q imbalances. In addition to SISO-OFDM systems, our algorithms are able to work in the MIMO-OFDM systems with tone-interleaved training sequences for MIMO channel estimation.