OFDM可適性調變在高速移動環境之研究

可適性調變(Adaptive Modulation)的技術主要是提高資料的傳輸量，其做法是利用目前的通道條件好壞，做為調整下一次要傳送的調變位階依據。換句話說，當通道好時，使用高位階的調變技術，例如16-QAM，當通道不好時，使用低階的調變技術，例如QPSK或是更低階的BPSK。 OFDM(Orthogonal Frequency Division Multiplexing：正交頻率多工)可以在有限的頻寬內增加資料的傳輸量，其利用多個相互正交的子載波在有限的頻寬內傳送資料。OFDM具備有抵抗多重路徑、符號之間的干擾(ISI)等優點。於是乎，將可適性調變與OFDM相互結合使用，更可以提高無線通訊的傳輸量。傳統的可適性調變應用於正交頻率多工上，只具有單方向性的傳送方式，而且還有諸多的假設性條件存在，例如接收端告知發送端訊息時的通道為一個完美通道，另一假設為包含在調變資料中的調變位階訊息不可以受到通道的影響，換句話說，接收端一定可以將調變位階訊息解調還原成正確訊息。這些假設都與現實環境有所差距。本篇論文提出一個新的方法可以讓雙方面都傳送資料，而不是局限於單方向性的傳輸。本篇論文使用的方法為利用領航字元(Pilot)的錯誤率改變下一次要傳送出去的調變位階。而且調變資料當中不需要包含調變位階的訊息，使用的方法為接收端將領航字元取出後用不同的解調器解調，再計算該錯誤率，取最低的錯誤率的調變方式就是整個OFDM的調變位階。另外也將OFDM劃分成不同的區塊，每一個區塊用不同位階的調變方式調變，這樣可以使得整體的產出量達到最大。

關鍵字

可適性調變；調變位階；正交頻率多工；抵抗多重路徑；產出量

並列摘要

The fundamental technology of adaptive modulation is to advance data throughput。The principle is receiver adjusts modulation level for next transmission depend on channel condition，i.e. when the channel condition is appropriate，it use high modulation level； when the channel is unfavorable，it use low modulation level，For example QPSK or BPSK。 OFDM (Orthogonal Frequency Division Multiplexing) can improve data throughput in the limited bandwidth。It transmits orthogonal sub-carriers in the limited bandwidth。OFDM can resist multi-path，ISI etc.. If we combine adaptive modulation and OFDM technology，we can get higher data throughput than single technology. Traditional adaptive modulation with OFDM have single transmission direction only and many assumptive conditions。For example，there is a perfect channel between transmitter and receiver。Another is modulation level in the modulated frame mustn't affect by channel，i.e. the receiver can demodulate the level of modulation accurately. However these assumptive conditions were not true in the real world。 We propose a new algorithm for adaptive modulation with OFDM。We calculate pilot's BER to adjust modulation level for next transmission，and decide which modulation level we use to demodulate it。 Other topic is to separate many blocks in a OFDM symbol。Each block has different modulation level then we can get maximum data throughput。

並列關鍵字

adaptive modulation ； modulation level ； OFDM ； resist multi-path ； throughput

參考文獻

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國際替代計量

OFDM可適性調變在高速移動環境之研究

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