多重模式數位系統之頻率偏移估計

Abstract

由於航空交通量大幅增加，傳統飛航裝備與系統漸不符合民航界需求，特高頻的飛機通訊定址與回報系統 ( ACARS )雖然目前廣泛被使用，但其位元傳輸率太低，通訊容量小已不敷航空通訊應用內容與數量之用。因此 國際民航組織(ICAO)等組織便制訂了一套新的航空通訊系統，特高頻數位鏈結 (VDL)，其數據傳輸率高且通訊容量大將取代 ACARS，成為下一代陸空通訊的主流。 在航空系統中，一定都會存有都卜勒效應，而造成頻率偏移的現象，因此頻率偏移的估計需特別加以考慮，否則會提高符元的錯誤率，讓通訊的可靠度降低，本篇論文的目的是在研究頻率偏移估計的方法並且提出適合VDL Mode 2系統頻率偏移估計及補償的演算法，本論文中所提出的方法將頻率估計分兩階段，在開始有訓練訊號時是採用批次方式估計，在資料傳輸時則用適應性方式求取，並配合相位/頻率追蹤迴路，將頻率偏移加以修正。模擬結果顯示我們的做法可以有效的估計並補償在Mode 2系統之頻率偏移。

Due to the rapid growth of air traffic, conventional navigation equipment and air traffic management system have not conformed to the need of international civil aviation. The currently used air-ground communication system is the Aircraft Communication Addressing and Reporting System (ACARS). Due to its low data transmission rate and small communication capacity, ACARS cannot cope with the need of high throughput in aeronautical communication. Thus, ICAO embarked on definition of a new standard for aeronautical communication. This is the VHF digital link (VDL); it has higher transmission rate and large capacity. The VDL will gradually replace the ACARS and become the key VHF data link system for the next generation air-ground data communication. Aeronautical communications always have the Doppler effect, which causes the frequency offset in receivers. Frequency offset will increase the detection error rate and decrease the reliability of communications. The goal of this thesis is to investigate frequency estimation methods and propose proper frequency estimation and compensation methods for the VDL mode 2 system. Our method consists of two phases. In the training mode, we proposed to use a block processing scheme while in the data mode, to use an adaptive processing one. We develop new adaptive frequency estimation methods and several phase/frequency tracking loops. Simulation results show that our methods can effectively estimate the frequency offset in the VDL mode 2 system.