Study of methods for channel estimation with interpolation in OFDM systems

[[abstract]]在本篇論文中，我們研究於正交分頻多工(Orthogonal Frequency Division Multiplexing, OFDM)系統下，使用低通濾波器內插法(Low-pass interpolation with Kaiser window)的二維分離內插估測方式。在我們系統中，首先在時域和頻域插入領航子載波(pilot subcarrier)，接著我們使用最小平方(Least Square, LS)估測法來得到領航子載波的通道響應，最後我們再使用內插法估測出資料子載波的通道響應，以便重建系統完整的通道響應。為了互相比較，我們也研究其他二維分離內插估測方式，包含線性內插法(linear interpolation)、二階內插法(second-order interpolation)、三次板條樣內插法(cubic spline interpolation)、轉換域處理(transform-domain processing)內插法、串級結合轉換域處理和二階線性內插法(quadratic linear interpolation)以及最小均方誤差(Minimum Mean Square Error, MMSE)內插法作其模擬。透過模擬結果我們發現在通道估測錯誤率上，使用低通濾波器內插法其估測效能在位元錯誤率(Bit Error Rate, BER)上比其他內插法還好，且使用此種內插方式，在通道狀態不快速變化情況下可解決其他內插法在高SNR時會出現的錯誤溢位(error floor)問題。[[abstract]]In this thesis, we study the channel estimation with two dimensional (2-D) separable low-pass interpolation and Kaiser window for the OFDM system. In the studied system, the pilot subcarriers are inserted in the time domain and the frequency domain, and we use the least square (LS) algorithm to estimate the channel response of the pilot subcarriers. In order to reconstruct the complete channel response, we use the interpolation to estimate the channel response of the data subcarriers. For comparison, we also investigate other 2-D separable interpolation methods, including linear interpolation, second-order interpolation, cubic spline interpolation, transform-domain processing interpolation, MMSE interpolation, and cascade of transform-domain processing interpolation and quadratic linear interpolation. We found from simulation results that the low-pass interpolation with Kaiser window results in less channel estimation error than other interpolation methods. Also, we found that the low-pass interpolation with Kaiser window method can eliminate the error floor problem at high SNR when the channel state does not change rapidly.[[note]]碩