在多重路徑快速衰落通道下針對正交分頻多工系統使用錯誤更正碼輔助之疊代式通道估測

Abstract

正交分頻多工因可有效抵抗多重路徑衰減通道，而被廣泛運用於許多通訊系統中。但在多重路徑快速衰減通道中，傳輸端及接收端有相對運動時會造成無線傳輸通道產生時變性以及都普勒效應。都普勒效應造成的頻譜擴展會讓子載波間失去正交性進而導致載波間干擾，而針對無線傳輸通道產生的時變性，一般須擺放更多引示信號來觀測載波間干擾並估測通道，因而提高單一傳輸上須乘載已知訊號的比例，降低了有效信息之比例。 在此篇論文中針對具有嚴重多普勒效應之多重路徑快速衰減通道，提出降低已知信號傳輸比例之疊代式通道估測。此疊代式通道估測利用低密度奇偶檢查碼所提供的資訊來協助估測，以利降低引示信號之數目進而提有效信息之比例。論文中也探討了碼字之長度以及擺放方式對於系統效能及估測精確度的影響。

Because of the robustness against multipath fading channels, orthogonal frequency-division multiplexing (OFDM) is widely used in many communication systems. However, for multipath fast-fading channels, the relative movement between the transmitter and receiver results in time variation of the channel and Doppler spread. The Doppler spread resultant from the Doppler effect will degrade the orthogonality between subcarriers which in turn leads to inter-carrier interference (ICI). Conventionally, the time variation of channels with ICI requires substantial grouped-preambles for channel estimation which increases the overhead rate and decreases the effective transmission rate. In this work, we propose a novel iterative channel estimation scheme over multipath fast-fading channels with high Doppler spread, which employs error-correcting code to assist the channel estimation to lower the overhead rate and raise the effect transmission rate. We also study several arrangements of codewords including the locations and block lengths to find out the optimal arrangement according to the trade-off between the system performance and accuracy of the estimation.