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Smart Antennas Nhiều tham khảo các hệ thống ăng-ten thông minh là anten thông minh, nhưng ăng-ten thực tế của mình không thông minh. Đây là khả năng xử lý tín hiệu kỹ thuật số, cùng với các ăng-ten, làm cho hệ thống thông minh. Mặc dù nó có vẻ rằng các hệ thống ăng-ten thông minh là một công nghệ mới, các nguyên tắc cơ bản mà họ được dựa không phải là mới. Trong thực tế, trong hai năm 1970 và năm 1980 các vấn đề đặc biệt của các giao dịch IEEE trên Ăng ten và Tuyên. | 33 CHAPTER 4 Smart Antennas 4.1 INTRODUCTION Many refer to smart antenna systems as smart antennas but in reality antennas by themselves are not smart. It is the digital signal processing capability along with the antennas which make the system smart. Although it may seem that smart antenna systems are a new technology the fundamental principles upon which they are based are not new. In fact in the 1970s and 1980s two special issues of the IEEE Transactions on Antennas and Propagation were devoted to adaptive antenna arrays and associated signal processing techniques 78 79 . The use of adaptive antennas in communication systems initially attracted interest in military applications 27 . Particularly the techniques have been used for many years in electronic warfare EWF as countermeasures to electronic jamming. In military radar systems similar techniques were already used during World War II 80 . However it is only because of todays advancement in powerful low-cost digital signal processors general-purpose processors and ASICs Application Specific Integrated Circuits as well as innovative software-based signal processing techniques algorithms that smart antenna systems are gradually becoming commercially available 17 59 . 4.2 NEED FOR SMART ANTENNAS Wireless communication systems as opposed to their wireline counterparts pose some unique challenges 42 i. the limited allocated spectrum results in a limit on capacity ii. the radio propagation environment and the mobility of users give rise to signal fading and spreading in time space and frequency iii. the limited battery life at the mobile device poses power constraints In addition cellular wireless communication systems have to cope with interference due to frequency reuse. Research efforts investigating effective technologies to mitigate such effects have been going on for the past twenty five years as wireless communications are experiencing rapid growth 42 . Among these methods are multiple access schemes channel
