應用於IEEE 802.11SISO無線傳輸之自動增益控制在預防遮蔽效應影響

Abstract

在這篇論文中，我們介紹了應用在OFDM 系統中的自動增益控制器。由於訊號經過通道後其能量將會產生劇烈變化，為了對抗通道所造成的訊號能量衰竭，自動增益控制器必須週期性的對可變增益放大器作控制。我們偵測類比數位轉換器的最大可接受範圍去避免飽和錯誤和降低量化錯誤並且利用經過FFT轉換後的領航值補償通道所造成的能量衰竭，以確保類比數位轉換器的輸入訊號能保持適當且穩定。 在許多文獻中顯少針對動態遮蔽效應的影響去作對抗。在此提出了一個自動增益控制的機制，提高訊號的收斂速度與穩定度，另外一般的文獻中很少對自動增益控制所要放大訊號的參考值有所定義，尤其在經過類比數位轉換器後，其參考值也會跟著變化，因此也提出一個調整參考值的機制讓類比數位轉換器減少輸出飽和並且可以輸出一個適當的訊號振幅讓接收端可以得到良好的訊號品質去解碼，而在對抗動態遮蔽效應的能力上也能達到在一個0.25ms封包長度下可以有4π遮蔽效應。

In this thesis, we introduce automatic gain control (AGC) for OFDM system application. Due to the big variation of received signal power under path loss and shadowing effects the AGC must periodic control the VGA for whole packet with different control scheme. We are tracking the rest range of ADC to reduce quantization noise without ADC saturation and adaptive weight to defend shadowing effects and use pilot value that is the FFT converted to compensate the signal in the channel attenuation to ensure the signal stable on the analog to digital converter. In many literatures they are significantly less research on shadowing effect resistance. In this thesis it proposes an AGC mechanism to improve the convergence rate and stability of the signal. In addition they also have less discussion on the definition of reference value especially it needs to re-define when the system uses an ADC to process the signal. We also propose a mechanism of reference value alignment to allow the ADC to reduce the output saturation and output appropriate signal amplitude. Let the receiver can obtain a good signal quality to decode and the shadowing resistance ability can reach a 4πshadowing effect with whole 0.25ms frame packet.