以數位信號處理輔助功率放大器反失真線性系統的最佳化設計

DSP-ASSISTED OPTIMIZATION FOR POWER AMPLIFIER PREDISTORTION-BASED LINEARIZATION

林志鴻(Chih-Hung Lin)

指導教授 ： 陳博現

國立清華大學/電機資訊學院/通訊工程研究所/博士(2007年)

摘要

在無線通訊系統，射頻發射機的線性度已經變成一個世界各地熱門的研究主題。隨著高速率資料傳輸的需求和先進的調變技術已經被許多無線通訊標準採用。然而，為了達成這個目標，射頻射頻發射機的線性度變成愈來愈關鍵。取代原先類比功率放大器電路的設計，我們提出一些利用數位信號處理補償的技術來幫忙校正功率放大器非線性的問題，依次來改進整體射頻發射機的線性度的效能。在第二章節，我們探討功率放大器的非線性與現有利用數位信號處理反失真的線性方法。
在基頻利用數位反失真來對功率放大器作線性化是一種有效、低成本的方法，尤其針對無線通訊系統中使用非常數調變信號，可降低鄰近通道的干擾。以查表法為基礎的數位反失真方法，在無線應用上是一種低成本、有效來對功率放大器作線性化。然而，許多現有以查表法為基礎的數位反失真方法，其效能改進成果大都是次最佳的，因為這些方法都採用等間距查表法，不管系統的狀態資訊，例如：功率放大器的特性、輸入信號的統計特性。其他現有以查表法為基礎的數位反失真方法，大都假設或是部份假設已知狀態資訊來對查表法間距做最佳化且固定已最佳化的間距。在第三章節，我們提出不需要事先假設已知狀態資訊，可以自動狀態資訊學習、低複雜度的程序來對以查表法為基礎的數位反失真方法最佳化。所提出的程序可以線上針對各種不同非線性的功率放大器特性，各種不同輸入信號的統計特性，各種不同隨時間改變的無線環境特性來調整查表法的間距。
正交分頻多工系統由於有著極大的峰對均值的比例值，於是對於非線性失真有著高度敏感度。然而，大多現有降低峰對均值的比例值的做法，都沒有彈性且是次最佳的效能。因為這些做法採用無失真或失真的降低峰對均值做法，都沒有考慮到功率放大器的特性。功率放大器造成的非線性失真，有截取失真與壓縮增益失真。在第四章節，我們提出一個分別對降低峰對均值的比例值的做法與以查表法為基礎的數位反失真方法做最佳化的方法，可以緩和截取失真與壓縮增益失
真。所提出的程序可以線上針對各種不同非線性的功率放大器特性，各種不同信號失真與雜訊比的特性，各種不同隨時間改變的無線環境特性來調整查表法的間距。

關鍵字

功率放大器 ； 反失真線性化 ； 查表法 ； 峰對均值比例值

並列摘要

Radio frequency (RF) transmitter linearity has become a topic of intensive research worldwide in the wireless communication. With increasing demands for higher data rate and advanced modem techniques are being adopted by many wireless standards. However, to achieve this goal, the linearity of RF transmitter towards more and more critical. Instead of doing the analog power amplifier (PA) circuitry design, we propose several digital calibration techniques to cooperate with the analog RF PA and these digital compensation techniques help to correct for the PA nonlinerity which in turn improve the overall RF transmitter performance. In chapter 2 of this dissertation, we discuss the PA nonlinearity and exiting digital predistortion linearization schemes.
Digital predistortion at baseband is an efficient and low-cost method for the linearization of a PA in a wireless system employing a non-constant-envelop modulation scheme, so as to reduce the adjacent channel interference. The look-up-table based digital adaptive predistortion (DAPD-LUT) approaches are low-cost and effective for PA linearization in wireless applications. However, most existing DAPD-LUT schemes are sub-optimum because they adopt uniformly-spaced LUTs regardless of
the system state information (SSI), i.e., the PA characteristics and the input signal
statistics. Other existing DAPD-LUT schemes assume either full or partial knowledge
of the SSI to optimize and then to freeze the LUT spacing. Without prior knowledge
of the SSI, in chapter 3 of this dissertation, we propose an SSI-learning low-complexity
procedure to optimize the LUT spacing for a DAPD-LUT scheme. The proposed procedure is capable of online adapting the LUT spacing for PAs with various nonlinear characteristics, for input signals with various statistics, and for wireless environments with various time-varying properties.
Orthogonal frequency division multiplexing system is sensitive to nonlinear distortion due to its large peak-to-average power ratio (PAPR) value. However, most existing PAPR reduction schemes are inflexible and sub-optimum because they adopt distortion or distortionless of PAPR reduction regardless of the PA characteristics. The nonlinear distortion caused by a PA is due to clipping distortion and compressive gain distortion. In chapter4, a separately optimized PAPR reduction and LUT-based predistortion scheme is proposed to efficiently mitigate both clipping distortion and compressive gain distortion. The proposed procedure is capable of online adapting the parameter of PAPR reduction scheme and the LUT spacing for PAs with various nonlinear characteristics, for various signal-to-distortion and noise ratio, and for wireless environments with various time-varying properties.

並列關鍵字

Power Amplifier ； Predistortion Linearization ； Look-Up-Table ； Peak-to-Average Power Ratio

參考文獻

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Amplifier Linearizer,” Proc. IEEE Int. Conf. Universal Personal Commun.,

[2] Jiunn-Tsair Chen, Huan-Shang Tsai, and Young-Kai Chen, “The Optimal RLS

Parameter Tracking Algorithm For A Power Amplifier Feed-Forward Linearizer,”

IEEE Trans. on Circuits and Syst. II:. Analog and Digital Signal Processing, vol.