Nowadays, the electromagnetic radio spectrum has become a
very precious resources in the wireless communication system. Since the spectrum band is covered by many systems, the chances that two different wireless systems colliding with each other is increased. Cognitive Radio is defined as an advanced technique for efficient spectrum utilization in wireless communication system. Cognitive radio is capable of configuring the radio system parameters and adjust the radio's settings to deliver the required quality of services subject to some user requirements, operational
limitations, and regulatory constraints.

First, we examine some traditional signal processing methods for interference avoidance. However, the problem becomes complicated if two user occupy two different length of bandwidth. Additional interference appear because of different sampling frequency.

In this thesis, we present an MIMO precoding scheme for
interference avoidance. This method allows cognitive user tradeoff between minimizing interference and maximizing own transmission performance. Cognitive user can design a spectrum notch at specific protected band. Then, we look for the possibility that secondary user transmitter could still utilize the protected band for its data transmission at the constraint of not interfering with the primary user.

在現今的無線通訊系統中，無線電波的頻譜成為了一個非常珍貴的資源。 由
於這些頻譜所在的頻域中充滿了許多不同的通訊系統， 兩個不同的無線系統使
用到同樣的頻帶，進而影響到彼此的可能性便增加了。 因此， 在無線通訊系統
中， 為了能有效率的使用頻寬的資源而發展出了感知無線電(cognitive radio)
的技術。 感知無線電技術能夠確認無線系統的相關參數並且調整系統的設定，
進而滿足一些根據使用者需求、 操作上的因素以及管理限制所要求的通訊品質。
首先，我們檢視一些現有的根據信號處理來達到避免對主使用者干擾的方
法。 然而， 我們所要處理的問題會因為兩個使用者佔據了不同長度的頻寬而變
的更加複雜。 因為不同的取樣頻率而造成一些多餘的干擾會出現。
在這篇論文中，我們介紹一個在多天線系統下的預編碼(precoding)技術的
架構， 此架構可以用來避免對主使用者產生的干擾。 這個方法允許感知無線電
的使用者可以在減少對主使用者的干擾以及對自身傳輸的最佳化的兩難中達到
想要的平衡。 感知無線電的使用者所造成的干擾在主使用者的頻帶上將會很小。
在避免了對主使用者的干擾之後， 我們接著尋求感知無線電的使用者依然能夠
在那些和主使用者重疊的頻帶上進行本身資料傳輸的可能性。

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