Bài viết này trình bày một predistorter baseband được sử dụng trong các hệ thống OFDM điều hành với một bộ khuếch đại quyền lực phi tuyến cao (HPA). Các tính năng chính của predistorter cư trú trong việc sử dụng của cấu trúc ngược HPA bù biến dạng phi tuyến. | TẠP CHÍ PHÁT TRIỂN KH CN TẬP 11 SÓ 02 - 2008 POWER AMPLIFIER MODELING AND POWER AMPLIFIER PREDISTORTION IN OFDM SYSTEM Tran Duc Tan College of Technology VNU- HN Manuscript Received on August 30th 2006 Manuscript Revised December 05th 2007 ABSTRACT This paper presents a baseband predistorter to be used in OFDM systems operating with a nonlinear high power amplifier HPA . Key features of the predistorter reside in the use of the HPA inverse structure as nonlinear distortion compensator. The performance of the compensated system is analyzed by simulations in an AWGN environment. The receiver also needs furthermore an equalizer in order to combat the distorsion effect. Keywords OFDM DAP HPA Adaptive Equalizer 1. INTRODUCTION Nowadays the OFDM technology is applied widely in the wireless communication because of many advantages such as robustness to severe multipath channels compared to single carrier SC system effective bandwidth to FDM systems and transceiver structures simple based on DFT circuits 1 2 . However in ODFM Systems we can not ignore a distorsion problem introduced by nonlinear High Power Amplifier HPA 3 4 .The main purpose of our paper is to analyze these effects on a high speed OFDM system WLAN2 5 . Then it is focused on designing a Digital Adaptive Pre-distorter DAP to overcome the nonlinear effect of HPA. 2. SYSTEM DESCRIPTION Figure 1 shows the baseband equivalent system of an OFDM system 5 . The input of the system is a serial of binary data mapped onto the M-ary QAM signal constellation to give a stream of complex symbols which are assumed to be statiscally independent. This complex symbol stream is applied to the OFDM modulation block. In the OFDM block the stream is serial-to-parallel converted to produce a sequence c. c is transformed by a inverse fast . ZTT_ . _ .T Fourier transform IFFT unit. A guard interval called cyclic prefix CP with length g is added to this signal yielding a T-spaced discrete-time representation of the transmitted .
