時變通道對多輸入多輸出正交分頻多工系統影響之研究

Abstract

正交分頻多工(OFDM)系統具有抵抗多重路徑干擾及頻譜使用效率高的特性，然而在高速移動的環境下，傳送訊號受到時變通道的影響，接收端所接受到的訊號會有都普勒擴展的現象，破壞了子載波間的正交性，使得子載波之間相互干擾(ICI)，造成系統效能惡化，最高接收容許速度下降。 在本篇論文中，提出在高速移動所引起載波間干擾的消除方法，並應用在二對二多輸出輸入正交分頻多工系統(MIMO)中。所提出的方法分成載波干擾估計和載波干擾消除兩部分，載波干擾估計是根據通道頻率響應(CFR)，以線性方程近似法來估計載波干擾的影響量，再根據估計值結合時空區塊編碼(STBC)來消除或降低載波干擾。 根據模擬的結果，二對二多輸入多輸出系統，在64-QAM 編碼率為3/4的條件下，系統容許的正規化都普勒頻率(Normalized Doppler Frequency)由原本的0.01提升到0.04以上。

The OFDM system exhibits outstanding characteristics in combating multi-path transmission and efficiency in spectral utilization. However, in high-mobility environment, transmitted signals pass the time-varying channel, and a loss of sub-carrier orthogonality due to Doppler spread of received signals leads to inter-carrier interference (ICI). This ICI effect deteriorates the system performance and the maximum allowed velocity of reception. This thesis presents a scheme for eliminating this ICI effect due to high-mobility reception, and applied to 2 2 MIMO-OFDM system. This scheme consists of ICI estimation and ICI cancellation. According to channel frequency response (CFR) in a symbol, the ICI estimation use linear regression to estimate the effect of ICI. From the result of ICI estimation, ICI cancellation incorporates Space-Time Block Coding (STBC) to eliminated or reduce the effect of ICI. From the simulation result, it is shown that this scheme improve the normalized Doppler frequency of system tolerant form 0.01 to 0.04 or more in 64-QAM, 3/4 code rate, and 2 2 MIMO-OFDM system.