無合成器之編碼正交相位LINC-OFDM系統

Abstract

正交多頻分工(OFDM)是一種極具效能的調變技術。不過，這種系統存在著一種重大的缺陷-高峰均功率比(PAPR)造成功率放大器的效率下降。對此，有研究者提出了一種使用非線性元件的線性放大器(LINC)來對抗這樣的問題。LINC是一種將輸入訊號拆解成兩固定波包(constant envelope)訊號後，將兩拆後訊號分別使用非線性放大器放大並且透過功率合成器(power combiner)重新合成訊號的模組。然而為了避免使用難以設計的功率合成器，有人因此提出了一種無合成器的LINC-OFDM系統。然而，使用此架構卻會造成頻帶外雜訊(out-of-band noise)比起傳統OFDM系統大很多。本論文提出了一種無合成器之編碼正交相位LINC-OFDM系統來解決這樣的問題。在此系統裡輸入訊號會先分離實部與虛部訊號，接者再將兩個子訊號分開做LINC拆解為四個固定波包訊號。在此系統下頻帶外雜訊能夠大幅的降低，此外我們也提出了一種有效的檢測方式，在模擬的結果中可以看出無合成器之編碼正交相位LINC-OFDM系統除了可以有很低的頻帶外雜訊外，效果也幾乎與傳統的編碼OFDM系統相同。

Orthogonal frequency division multiplexing (OFDM) is known to be an efficient modulation technique. However, a major drawback of the system is its high peak-to-average power ration (PAPR) making the power amplifier (PA) low efficiency. Linear-amplification- with-nonlinear-component (LINC) has been proposed to overcome the problem. LINC decomposes the input signal into two constant-envelop sub-signals, amplifies the signals by nonlinear PAs, and then combines the resultant signals by a power combiner. To avoid the use of the power combiner, which is difficult to design, some researcher proposes the combinerless (CL) LINC-OFDM system. However, one problem associated with the CL-LINC-OFDM system is that its out-of-band noise level is significantly greater than that of the conventional OFDM systems. This thesis proposes a coded CL quadrature LINC-OFDM (QLINC-OFDM) system to solve the problem. In CL-QLINC-OFDM systems, the input signal is first decomposed into real and image signals. Then, each signal is further decomposed into two constant-envelop sub-signals. It can be shown that the out-of-band noise level is significantly lower than that of CL-LINC-OFDM. We also propose a method that can effectively conduct the detection. Simulations show that while the proposed coded CL-QLINC-OFDM system has low out-of-band noise, its performance is almost the same as that of the conventional coded OFDM system.