- 學位論文
正交分頻多工系統在頻率平坦通道中之頻率同步和通道估測研究利用最大概似率與最小平方估測法之研究
Frequency Synchronization and Channel Estimation for OFDM Systems over Frequency-Flat Channels: ML and LS Approaches.
摘要
正交分頻多工系統是新的調變技術用來抵抗多路經衰減效應。IEEE802.11a將OFDM定為標準規格。OFDM系統的一些優點：如降低計算複雜度、對抗窄頻干擾的能力、頻率分級的利用等等。在這篇論文中，我們考慮聯合頻率偏移和通道估測，但是我們先假定時間同步化的工作已經完成了。OFDM系統是在一個頻率平坦的環境下所做的。在這裡我們提出使用最大概似（ML）估計演算法來做聯合頻率偏移和通道估測。模擬的步驟：我們利用梯度圖來求得頻率偏移估測，再根據頻率偏移估測來求得通道頻率反應。然後在矯正頻率偏移後，我們再以合理假設運用最小平方法(least squares)求得通道估測做一比較。最後再將理論值跟模擬作比較。
並列摘要
Orthogonal frequency division multiplexing (OFDM) is a new generation modulation technique. OFDM system is robust against the fading effects of multipath propagation. The IEEE 802.11a applies OFDM for wireless local area networks (WLAN) as standard specification. OFDM has several advantages ：reduced computational complexity、exploitation of frequency diversity、and robust against narrowband interference. In this paper, we consider joint frequency offset and channel estimation assuming timing synchronization has been completed .Flat fading channels are considered. We use a maximum-likelihood (ML) algorithm to jointly estimate the frequency offset and the channel frequency response (CFR). By first obtaining the frequency offset estimation using gradient plot, then the CFR estimation is obtained from the frequency offset estimation result. Further, an alternative approach using a least squares formulation for channel estimation is also presented based on completion of the frequency offset correction. Simulations are performed to compare with theoretical predictions.