離散多頻系統與離散小波多頻系統在噪音通道下其性能之比較

Abstract

我們知道離散小波多頻系統其子通道頻譜限制較離散多頻系統要好，亦即落在頻帶外的信號量很小，因此受到傳輸通道衰減時其所受到的連鎖影響會較小。 而離散小波多頻系統其濾波器長度設計越長，其子通道頻譜越能集中在一定範圍內，且旁波帶的能量會越小，但是系統會更複雜，需要更多的運算單元及記憶體。 本論文利用很多種不同的通道模型來模擬實際傳輸的通道，並比較當相同信號分別經由離散多頻系統與不同濾波器長度的離散小波多頻系統，受到相同的這些通道干擾後，在接收端解調回信號時，會有多大的錯誤率。 由模擬結果顯示，即使濾波器設計得再好，旁波帶設計得更低，仍無法降低受白高斯分布雜訊的影響，但卻可以對多路徑通道造成的衰減現象造成較好的保護作用。 另由模擬結果顯示，如果我們能好好的設計濾波器，即使長度只有兩倍子通道數，在對抗通道衰減上也能達到非常不錯的效能。 故在實際應用上，或許我們可以最簡單的離散小波多頻系統 (濾波器長度等於兩倍子通道數) 來代替以離散傅立葉轉換為調變濾波器組的離散多頻系統，如此不但不會增加太多系統的複雜度，又能減低受通道衰減的影響。

We have known that DWMT system has a better spectral containment than DMT system, and thus it is more robust in fading channel. In DWMT system we can design a filter bank, whose spectrum has very low side-lobes, with a very long filter length, but the system will become more complicated, and need more calculators and memories. We try to use many channel models to simulate the real communication channels and compare in those channel models the performances of DMT system with the performances of ELT-based DWMT of different filter length systems. From our simulations, we can find that though DWMT system has better spectral containment, it can not have a better protection to white Gaussian noise, but it is really more robust in fading channel. Another result shows that we could design a filter bank with filter length only equal twice the number of subchannels and it could also have a very good performance in fading channel. Thus, we can use the simplest DWMT system (filter length equal twice the number of subchannels) to substitute the DMT system. We will not have a very complicated system, and it could reduce the channel effect.