Research And Implementation Of Baseband Digital Predistortion Algorithm For RF Power Amplifier

In modern communication systems, due to the need of high-speed data transmission and the limited spectrum resources, non-constant envelope modulation techniques are widely used. Digital television terrestrial broadcasting signals also adopt a non-constant envelope modulation methods, such as 64QAM, TDS-OFDM, COFDM, whose transmit signals have a high PAPR and wide bandwidth, they are very sensitive to non-linear distortion of power amplifier. When these non-constant envelope and high PAPR transmit signals get through the non-linear high-power amplifier, the signal will be distorted and the performance of system will be affected from two aspects, namely, in-band distortion and out-band noise. In-band distortion is mainly showed as a amplitude distortion and phase distortion which will make the constellation produce offset and reduce the anti-error ability of system; out-band noise, mainly as a spread spectrum signal which will produce adjacent-channel interference and reduce the bandwidth efficiency of system. In order to obtain higher power efficiency, power amplifier is usually worked in the vicinity of saturation point, where the non-linear problems are most serious. So we need a linearization technology for power amplifier to improve its efficiency.Linearization technologies of power amplifier include power back-off method, feed-forward method, feed-back method, analog and digital predistortion and other methods. The power back-off method is making the power amplifier work at the zone away from the saturation point so that the power amplifier will work at the line area. It is based on the cost of sacrificing the efficiency of power amplifier and it has some disadvantages such as a high cost, bulky, power consumption and a heat dissipation difficulty. So it can not be used in the digital TV transmitters and re-transmitter devices. Linearization technology of power amplifier with memory effect based on adaptive digital predistortion has practicality, economy, stability and flexibility. It is considered as the most promising of a power amplifier linearization technology and will be a focus of the future research for some time. It adopts digital signal processing technology and uses adaptive tracking to compensate the errors produced by environmental factors. It has an excellent performance and is very suitable for power amplifier in the digital TV transmitter.This paper firstly introduces the digital TV and its signal characteristics, outlines the research situation and development trend of power amplifier linearization domestic and international. Secondly, analyzes nonlinear characteristics of power amplifier and their impacts, describes the power amplifier model and linearization methods in detail and analyzes predistortion method. Then, according to the actual scheme, proposes a digital predistortion algorithm based on polynomial LUT. Then, introduce a technology combined power amplifier and digital predistortion based on the ADS software for software and hardware co-debugging, use the actual power amplifier to achieve their optimal linearization, verify the feasibility of predistortion algorithm. Finally, according to the program design the part of baseband FPGA board including the realization of predistortion algorithm and adaptive algorithm, the main software development platform is Xilinx's ISE software.