了解和實現MIMO射頻鏈路

　　多年來，作為一種理論方法和學術課題，多輸入多輸出（MIMO）收發系統正受到關注和支持組件的重大設計。該技術已經通過了最新的4G一代和下一代LTE（長期演進）蜂窩標準，提供吞吐量和性能的需要，以及在Wi-Fi的IEEE 802.11n標準。

　　MIMO方法的原理是在基站甚至用戶手持機上使用多個半獨立天線和發送/接收信道，以克服不可避免的信號路徑問題，例如噪聲、衰落和多徑。實際上，對這些問題多“柔術”翻轉；采用先進的硬件和軟件將他們從以前被認為是無線的弱點轉化為顯著的長處和優點。

　　MIMO versus diversity systems

　　MIMO is much more sophisticated and complex than diversity antenna systems. Using multiple antennas for better signal coverage is a well-established technique; usually this is done at the receiver but it can also be done at the transmitter， which may not be co-located. （Note that to avoid confusion， the industry is now referring to basic single-antenna systems as single input， single output， or SISO.）

　　In a basic antenna diversity-receiver design， two or more antennas are used， spaced at least one-quarter wavelength apart. Each antenna‘s signal goes to an RF low-noise amplifier （LNA）， and then the signals are combined. This combining can be done is different ways， with an increasing scale in complexity， cost， and benefit.