高車速下對於單載波分頻多重存取/正交分頻多重存取系統中減緩載波間干擾方法之研究

Abstract

正交分頻多工(Orthogonal Frequency Division Multiplexing , OFDM)系統，將傳輸頻寬切割成多個窄頻寬的子通道，並且利用多載波來傳送訊息，可以有效對抗多重路徑(Multipath)的影響，故廣泛被應用。 然而，正交分頻多工具有高峰均功率比(Peak to Average Power Ratio , PAPR)的缺點，會降低發射機功率放大器的效率，此問題對於行動用戶端更是一大考驗。3GPP-LTE(3rd Generation Partnership Project Long Term Evolution)系統為了解決高峰均功率比的問題，在上鏈路(uplink)採用了峰均功率比較低的單載波分頻多工存取(Single Carrier Frequency Division Multiple Access , SC-FDMA)系統，有利於行動用戶端電池的使用效率。 SC-FDMA系統可視為離散傅立葉轉換預編碼之OFDM系統，所以和OFDM系統有著相同的問題，亦即在高速行動的系統中，所產生都普勒效應會造成接收端子載波失去正交性，也就是載波間干擾(Inter-carrier Interference , ICI)。 在本論文中，我們針對ICI所造成系統效能降低的問題，分析兩種減緩ICI的方法，將其應用在SC-FDMA以及OFDMA系統中，接著比較兩者之間的效能好壞。並且提出了針對LTE uplink的系統架構之下的通道估測法，結合最小方差以及決策導向法估測出減緩ICI所需的通道狀態資訊，進一步利用所分析的基於最小均方誤差(MMSE)頻域等化器進行等化及資料偵測來提升系統效能。

Orthogonal frequency division multiplexing (OFDM) system divides the transmission bandwidth into several narrowband sub-channels, and uses the multicarrier to transmit signals so that it could resist the multipath effect. Therefore, it has become popularly applicable. However, OFDM reduces the efficiency of power amplifier at transmitter due to high Peak to Average Power Ratio (PAPR) and it is a challenge for mobile users. In order to solve the high PAPR problem in OFDM system, 3GPP-LTE has adopted the SC-FDMA system for the uplink which could benefit the power efficiency for the mobile users. SC-FDMA system can be regarded as a Discrete Fourier Transform (DFT) pre-coded OFDM system, so there exists the same problem that subcarriers may lose their orthogonality due to Doppler Effect in the high mobility environment that is Inter-carrier Interference (ICI). In this thesis, we focus on the problem that system performance degradation due to ICI, and analyze two different ICI mitigation methods for SC-FDMA and OFDMA systems, and compare the system performance between them. Furthermore, we proposed a channel estimation method which combined Least Square and Decision-Directed methods to find out the channel information for ICI mitigation, and then we used the MMSE-based frequency domain equalizer to equalize and detect the data for better system performance.