在第四代通訊技術中長期演進技術(Long Term Evolution，LTE)具有重要一席之地，本論文採用的下行傳輸實體層根據3GPP Release 12版定義之標準規格。傳輸的頻譜被分為控制區域(Control region)與數據區域(Data region)，控制區域中實體下行控制通道(Physical Downlink Control Channel)乘載下行控制訊息(Downlink Control Information)，最重要功能其中之一便是做資源分配，所有使用者必須先解出DCI才有辦法找到自己資料位於數據區域中哪一區塊，從而還原資料。而使用者在收到PDCCH時卻幾乎沒有太多資訊，在這樣的情況下，要進行盲檢(Blind Decoder)。此外本論文會介紹LTE下行鏈路系統其中的所使用正交分頻多工(Orthogonal Frequency Division Multiplexing，OFDM)技術、下行鏈路的規格以及調變原理。
而本篇論文以軟體定義無線電的方式，實現LTE下行鏈路的發射機。透過搭載實務軟體定義無線電平台，可以實際即時的在特定的頻段發設與接收空中的訊號，也可以改變發射及接收增益來觀察訊號在經過空氣後發生的變化，以及探討訊號的品質評斷系統優劣度等等。

LTE(Long Term Evolution) plays an important role in fourth generation of mobile phone and mobile communication technonlogy. In this paper, LTE downlink physical layer which is mentioned according to the standard specifications defined by 3GPP Release 12. Transmitted Frequency Spectrum is divided into Control Region and Data Region by different bandwidth. One of the most important application about Downlink Control Information, carried by Physical Downlink Control Channel, is Resource assignment. In order to get the where is data in Data Region, All of Users must demodulate Downlink Control Information , Before receiving data. However, there is not too much information when Users receiving Physical Downlink Control Channel. Blind decoding is applied in this situation. Otherwise, this paper introduces for the LTE downlink system,including the use of OFDM(Orthogonal Frequency Division Multiplexing) technology, specifications and modulation principle, etc.

In this thesis, we used Software-define-radio to achieve LTE downlink transmitter. By the Software-define-radio platform, we can immediatly transmit and receiver signal in the air on some particular band. Also we can change transmit and receive gain to observe the transformation of the signal in the air, discussion on the quality of the signal and system, etc

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