近幾年來，無線電電磁波頻帶被視為是無線通訊系統最珍貴的資源之ㄧ，越來越多的無線通訊系統共同存在在同一個空間環境中，共同分享相同的頻域，如此一來，他們將會彼此互相干擾，對系統造成相當大的負面影響。若有通訊系統是寬頻(wide-band)系統，則干擾情況又會變的更糟，為了要避免干擾，感知無線電(Cognitive Radio)是有關於感應共存無線系統、分辨系統間干擾源型態、協調系統之間談判的一種想法，在我們所做的研究內容中，我們會介紹一種演算法，叫作積極干擾消除(active interference cancellation)，利用積極干擾消除載波(AIC tone)來在特定的頻帶上避免干擾，我們利用感知無線電的精神來創造一個新的系統並消除不同使用者間的影響。
除此之外，我們更在提出的系統架構上運用了多天線系統，利用多天線系統，我們可以利用其他根傳送天線(除了積極干擾消除天線)在遭破壞的頻帶中傳送有用資料，利用的是積極干擾消除天線的保護。這樣的想法可以避免浪費而且增加了頻寬的使用效率，在多天線系統下，若要在每個地方都達成頻域凹口是不可能的，頻域凹口可能在某些特定地方或特定的直線上才能被創造出，我們會更進一步討論不同使用者間通道的狀況，而在某些狀況的假定下，我們將會發現信號到達角度估測(DOA estimation )對通道的計算是非常有幫助的，而且亦對我們提出的系統架構有助益而被納入此系統之中，在接下來的論文內容中將被詳細的探討。

In recent years, radio electromagnetic spectrum is considered as the most precious resource in wireless communication systems. More and more wireless systems co-exist in the same environment and sharing the bandwidth, consequently, they will interfere with each other. The condition is even worse as the wireless system is a wide-band system. In order to prevent interference, Cognitive Radio is an idea about sensing co-existence system, identifying the type of interference and negotiating between systems. In our work, we will introduce an algorithm called active interference cancellation to avoid interference at specific bandwidth by using AIC tones between interfering bands. We use the spirit of cognitive radio and construct a system to decrease the negative effect among users.
Furthermore, we develop a multiple-input multiple-output (MIMO) system basing on this algorithm. By exploiting MIMO, we can transmit data by other transmitting antenna (except for AIC antenna) in the interfering band under the protection of one AIC antenna. This idea achieves decreasing improvidence and increasing bandwidth efficiency. Under MIMO system structure, it’s almost impossible to cause notch at specific band everywhere. The notch could be created at specific position or specific straight lines. We will further discuss the condition of channel between users which interfere with each other and, under this assumption; we find out that the method of direction of arrival (DOA) estimation is helpful to evaluate the channel, and benefit to our proposed MIMO active interference cancellation model.

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