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爲了克服ISI的問題，在OFDM symbol前端加入一保護區間（Guard Interval）。 爲了對抗訊號因通道延遲的影響，Guard interval（Tg）長度要大於最大的Delay spread，即Tg>delay spread time。

By adding guard interval (GI) between OFDM symbols, OFDM WLAN system can be against multipath delay spread. Signals can be detected up on the reception of just one training sequence of two OFDM symbols.

GI是在OFDM符号之间插入的一段空白时间，用于消除多径传播引起的符号间干扰（ISI）。 通过插入GI，可以确保前一个符号的多径延迟不会影响到当前符号。

In OFDM, the beginning of each symbol is preceded by a guard interval. As long as the echoes fall within this interval, they will not affect the receiver's ability to safely decode the actual data, as data is only interpreted outside the guard interva

The Guard Interval is the ratio of the Cyclic Prefix " CP" time to the inverse FFT time "T (IFFT)." The guard interval is used to eliminate inter-symbol and inter-carrier interference.

這一章將會介紹 OFDM 傳輸技術的原理架構,並針對外加防護區間(Guard interval)以降低多 重路徑造成的符元間干擾(Inter-symbol interference, ISI)及次載波正交性作 介紹,最後分析時變通道對載波間干擾(Inter-carrier interference, ICI)的影響。

Recently, an orthogonal frequency division multiplexing (OFDM) technique for the infinite impulse response (IIR) channel (IIR-OFDM) was proposed, which carries the dedicated guard interval to maintain the circular convolution of the received signal and ch