近年來隨著無線多媒體影音的蓬勃發展，使得無線通訊系統必須在有限的頻寬下傳輸大量的資料，為能有效利用頻寬資源，新一代無線通訊標準普遍採用正交分頻多工(Orthogonal Frequency Division Multiplexing，OFDM)為主的調變技術。LTE(Long Term Evolution)系統是由3GPP(3rd Generation Partnership Project)所提出的第四代行動通訊系統。LTE下行鏈路採用了OFDMA(Orthogonal Frequency Division Multiple Access)系統，它是根據OFDM(Orthogonal Frequency Division Multiplexing)系統演進而來，上行鏈路則採用單載波分頻多工(Single-Carrier Frequency DivisionMultiple Access，SC-FDMA)系統。SC-FDMA 是由OFDMA 演進，也被叫做線性預編碼OFDMA技術，因此LTE上行鏈(UL)與下行鏈(DL)技術十分類似，但SC-FDMA的特徵是輸出單載頻發射信號。由於SC-FDMA 是將每個子載波上的調變符號，在相同時間做線性合併，故每個子載波上皆有各個訊號的分量，使得SC-FDMA系統比起 OFDMA 系統具有較低的峰值/平均功率比 (Peak-to-Average Power Ratio，PAPR)。如此一來，就可以提高移動終端的功率發射效率，節省電池的消耗，降低終端成本。然而，對下行鏈路而言，預失真就顯出其重要性了。 另外，I/Q Imbalance的影響也會讓硬體干擾訊號的品質，所以I/Q補償對於硬體來說也是很重要的。 ;In recent years, development of wireless multimedia is rapid. To make wireless communication systems must transfer large amount of data in a limited bandwidth for effective use that bandwidth resources. Next generation wireless communication standard commonly use the Orthogonal Frequency Division Multiplexing (OFDM) modulation technology. LTE (Long Term Evolution) system was developed by 3GPP (3rd Generation Partnership Project) proposed by the fourth generation of mobile communication systems. LTE downlink uses OFDMA (Orthogonal Frequency Division Multiple Access) system, which is based on OFDM system evolved, uplink uses single carrier frequency division multiplexing access (SC-FDMA) systems. SC-FDMA is an evolution of the OFDMA, also known as linear pre-coding OFDMA technology. LTE uplink(UL) and downlink(DL) technology is very similar, but SC-FDMA is characterized by the output of a single carrier frequency transmission signal. SC-FDMA is the modulation symbols for each subcarrier at the same time to do linear combination, so each sub-carrier signal of measure vector is respective. Make SC-FDMA system better than OFDMA system with lower peak/average power ratio (Peak-to-Average Power Ratio, PAPR). In this way, it is possible to improve the power transmission efficiency of the mobile terminal and save battery consumption and reduce the cost of the terminal. However, for the downlink, Pre-Distortion show the importance for OFDM system.
In addition, the impact of I / Q Imbalance is allow to affect hardware to interfere with the quality of the signal, so I / Q compensation for the hardware is very important too.
