OFDM系統中衰減通道的指標式估測法

Abstract

正交分頻多工技術(OFDM)是由多載波技術演變而來，並且能提供高速資料傳輸的一種多工技術。在資料傳送的過程中，由於受到通道的影響，接收端收到受到污染的資料。因此我們必須估測通道，透過通道補償的方式來重建傳送的資料。 本文探討在OFDM中利用參考訊號(pilot)做通道估測的方法，介紹在OFDM系統中，兩種通道估測的方法comb-type和block-type的優缺點。並且透過模擬的方式來驗證在comb-type的估測方法較適合在快速衰減的環境中，而對於頻率選擇衰減卻較敏感。而block-type的估測方法不受頻率選擇衰減的影響，而對於都卜勒偏移卻較敏感。除此之外¸ 我們也利用適應性訊號處理的技巧使得估測效能再提升。 最後將介紹IEEE802.16a無線都會網路實體層傳輸技術，並且針對802.16a的放置參考訊號的位置來做下傳端(downlink)的通道估測。 我們將透過模擬的方式來看系統在不同的通道延遲和都卜勒偏移之下的通道估測表現。

The channel estimation techniques for OFDM systems based on pilot arrangement are investigated. The channel estimation based on comb type pilot arrangement is studied different algorithms for estimating channel at pilot frequencies and for channel interploation. In addition, the channel estimation based on block type arrangement is performed. Block type sets pilots at every subcarriers of a symbol for specific number of following symbols sent periodically. It is shown that the comb-type arrangement is proposed in fast fading channel but is sensitive to channel delay spread. Block-type arrangement is nonsenstive to delay spread but sensitive to Doppler effect. We also discuss the relationship between symbol length and channel estimation. The comparison shows the trade-off problem among symbol length, delay spread and Doppler effect Finally, we introduce IEEE 802.16a system and its special pilot pattern. Simulation results shows the performance of channel estimation we investigated in IEEE 802.16a system