IEEE 802.11 OFDM 無線區域網路之自動增益控制系統

Abstract

近年來，由於高傳輸速率的無線區域網路(WLAN)通訊之應用與需求遽增，因而能實現高傳輸速率的正交分頻多工技術(Orthogonal Frequency Division Multiplexing , OFDM)被廣泛的研究，並應用數位訊號處理(Digital Signal Processing , DSP)與超大型積體電路(Very Large Scale Integrated Circuit , VLSI)的技術，成功的發展出符合IEEE 802.11 OFDM WLAN 標準規範之高傳輸速率的無線區域網路消費性電子產品，如今隨時隨地無線上網已不再是口號，而是現代忙碌生活中的一個寫照。 然而要設計一個接收品質良好的無線區域網路通訊系統，必須先要有準確且快速的自動增益控制系統(Automatic Gain Control , AGC)，才能夠準確判別是封包信號(Package Signal)或是雜訊(Noise)，進而正確估算出信號能量，快速有效地設定放大系統之增益，如此頻率偏移(Frequency Offset)之粗調估算(Coarse Estimation)、通道(Channel)之估算、頻率偏移之微調(Fine)估算及信號時間(Timing)之同步(Synchronize)才能正確的進行。 在本論文中，吾人針對IEEE 802.11 OFDM WLAN 標準規範，製作射頻收發模組(RF Transceiver Module)、類比/數位(AD/DA)及數位/類比(DA/AD)轉換電路，並提出一套快速有效之自動增益控制系統，經由電腦模擬分析後，藉由FPGA 來作硬體實現。並經實驗測試驗證，本自動增益控制系統在不同的調變(Modulation)及不同的資料傳輸速率(Data RATE)下，針對所有動態範圍(Dynamic Range)內之不同強度的信號，均能在二微秒至五微秒內準確的完成收斂動作。自動增益控制是IEEE 802.11 標準規範中，在偵測到封包訊號後，第一件要做的動作，若自動增益控制沒做好，則後面的頻率偏移估算調整、通道估算、信號時間同步根本無法進行。所以本論文中，吾人針對自動增益控制系統加以研究並提出一套快速且有效並可運用在實際IEEE 802.11 OFDM WLAN的方法，更進一步藉由硬體電路實現來加以驗證。

In recent years, due to the dramatic increasing applications and needs in high transmission rate of Wireless Local Area Network (WLAN), the high transmission rate--- Orthogonal Frequency Division Multiplexing (OFDM) have been being researched extensively. With the application of Digital Signal Processing (DSP) and Very Large Scale Integrated Circuit (VLSI), high transmission rate WLAN commercialized electronic products which meet IEEE 802.11 OFDM WLAN standards have been successfully developed. Today surfing the Internet wirelessly anytime and anywhere is no longer a slogan but rather a vivid portray of modern busy life.

To design a good quality reception condition in WLAN system, however, an accurate and quick Automatic Gain Control (AGC) is needed so that we might precisely judge Package Signal from Noise and then correctly estimate the energy of signals and efficiently and effectively set gain of the enlarging system. The Coarse Estimation of Frequency Offset, the estimation of Channel, the fine estimation of Frequency Offset and the Synchronize of Timing could thus proceed without errors.

In the thesis, based on 1EEE 802.11 OFDM WLAN standards, a RF Transceiver Module, AD/DA and DA/AD exchange circuit is made and a quick and effective Automatic Gain Control System has been presented. The system is put into hardware practice by FPGA after being simulated analyzed by computer. Verified with experiments and tests, the system, in different Modulations and different Data RATE, could complete precisely convergence for all signals in Dynamic Range in 2 microseconds to 5 microseconds. Automatic Gain Control system is the first action when package signals are detected in IEEE 802.11 standards. The frequency offset estimation adjustment, channel estimation and timing on the later stages cannot carry on if the automatic gain control fails. The thesis explores and researches automatic gain control system and proposes a fast and effective method which is applicable in practical IEEE 802.11 OFDM WLAN. The method has been also proved useful by hardware circuit.