多輸入多輸出正交分頻多工系統之量測與通道估測

Abstract

近年來，正交分頻多工系統已成為熱門及廣為使用的無線通訊系統之一。無線區域網路就是一個最典型的例子，它不但在我們的生活中隨處可見並已經成為現今生活不可缺少的一塊。隨著使用者對於影音、線上遊戲及數據通訊的需求增加，新的區域網路規格(IEEE 802.11n)因此發展出來，它修改了傳統的無線網路系統(IEEE 802.11a/g)並結合多傳送天線和多接收天線技術。不同於傳統的系統，多輸入多輸出正交分頻多工系統增加了空間的維度，因此接收端在同步方式及通道估測變得更為重要。在本論文中，我們首先會針對單天線系統的同步及通道估測做個介紹，接著介紹延伸至多天線系統的方法。 正交分頻多工系統可將寬頻通道化分成較小的窄頻通道，因此可以有效地對抗多重路徑所引起的干擾效應。因此，接收端可利用此特性配合快速傅立葉演算法使用低複雜度的一階等化器來對通道做補償。在通道估計上，一般常見的最小平方法可以藉由基於離散傅利葉法來降低雜訊的干擾量。但是實際的系統存在著一些不使用的子載波，通常這些子載波被稱為保護頻帶或虛子載波。由於這些虛子載波的影響，我們不能直接使用基於離散傅利葉法，否則會造成時域上通道響應洩漏。為了解決這問題，本論文提出一個新的通道估測方法藉由重建虛子載波上的通道響應並結合基於離散傅利葉法，使用此方法可以改善原本虛子載波所造成的效應，並透過模擬的結果可以看到通道估測的均方誤差可以逼近基於離散傅利葉法的均方誤差下界。 另一方面，針對於正交分頻多工系統，我們使用實驗室現有的儀器設備來建立一套量測系統。本篇論文中，我們分別會詳細介紹整個實驗的架構，包括傳送端及接收端的儀器特性及設定。不同於電腦上的模擬，我們利用通道產生器來模擬實際環境通道，並使用任意波形產生器將訊號經由傳輸線傳送至即時頻譜分析儀，再將量測結果加以分析及討論。此外，我們也針對實驗室的實際環境進行量測及數據分析。最後，我們所建立的量測系統可以作為多輸入多輸出正交分頻多工系統量測研究的基礎，並且可以透過這套量測系統進一步驗證所發展的演算法。

Recently, OFDM systems have gained its popularity and become widely used wireless communication systems. The WLAN system is the application which appears everywhere and becomes the part of our life. With the increasing demand of video, audio, and data communication, the new standard IEEE 802.11n is developed to obtain higher data rate and increase range performance. It amends the conventional WLAN systems (IEEE 802.11a/g) and involves the MIMO technique. Compared with the SISO-OFDM system, the synchronization and the channel estimation are more crucial for MIMO-OFDM system due to the additional spatial dimension. In this thesis, we introduce some basic conventional methods used for the synchronization and the channel estimation in SISO-OFDM system and discuss the extended methods in the MIMO-OFDM system. The OFDM system converts the wideband channel to a set of parallel narrowband subchannels. As a result, it is robust against the multipath frequency selective channel. Besides, the one-tap frequency domain equalizer implemented by fast Fourier transform can be applied at the receiver end. To improve the channel estimation, the DFT-based method is used to reduce the noise influence. In practical DFT-based OFDM systems, however, a number of sub-carriers are left unmodulated, these are referred as virtual subcarriers. The leakage phenomenon induced by the virtual subcarriers limits the performance of the DFT-based channel estimation method. To solve the problem, we propose a novel channel estimation method which recovers the channel frequency response at the virtual subcarriers. The approach is slightly suffered from the noise enhancement. The simulation results show that the mean square error is close to the lower bound of the DFT-based method. On the other hand, we establish a measurement system for MIMO-OFDM system. The setup and configuration of the transmitter (vector signal generator) and the receiver (real-time spectrum analyzer) are described in detail. Unlike the computer simulation, we use the channel emulator to emulate the practical indoor channel and take the measurement in real-time environment. This measurement system could be the basic study of the MIMO-OFDM measurement systems and used to verify an algorithm designed for the OFDM systems.