應用於IEEE802.11MIMO多工正交無線傳輸系統之自動增益控制和封包偵測

Abstract

近年來，無線通訊技術蓬勃發展，其中正交分頻多工(Orthogonal Frequency Division Multiplexing，OFDM)逐漸成為主流核。由IEEE所制定的無線區域網路標準802.11ac即是多輸入多輸出的正交分頻多工系統(Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing，MIMO-OFDM)。 在這篇論文中，我們介紹了應用於OFDM系統中的自動增益控制器和封包偵測機制。由於封包在傳輸的過程中，因受到為多重路徑(Multipath)及屏蔽效應(Shadowing)的影響，能量產生劇烈的變化，同時也會受到突波(Impulse)及等幅振盪波(Continuous wave jamming)的干擾而使訊號在時域和頻域失真。為了對抗封包在傳輸過程中不可預測的能量變化，自動增益控制器(Automatic Gain Controller，AGC)必需週期性調整可變增益放大器(Variable Gain Amplifier，VGA)的增益值，使得封包訊號在經過類比數位轉換器(Analog to Digital Convertor，ADC)後的結果能夠落在線性的區間內，並且在避免飽和錯誤(Saturation Error)的前提下降低量化錯誤(Signal to Quantization Noise Error，SQNR)，讓接收端有高品質的數位訊號進行封包偵測，達到至少99百分比的封包偵測成功率。

Recently, Orthogonal Frequency Division Multiplexing (OFDM), one of the multi-carrier modulation scheme, turns out to be a suitable choice because of its high spectral efficiency and simplicity in equalization. The well-known standard, IEEE 802.11ac, is a multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) system. In this thesis, we introduce an automatic gain control (AGC) and packet detection mechanism. Due to the power variation and phase change caused by continuous wave jamming, impulse jamming, shadowing and multipath during transmitting, there should be an AGC whose main task is adjusting the variable gain amplifier (VGA) gain to keep the incoming signal amplitude in analog to digital convertor (ADC) linear working range at receiver. At the same time, maximum the signal quantization noise error (SQNR) and avoid saturation error to have high-quality ADC output for receiver to detect packet with at least up to 99% packet detect hit rate.