Design And Implementation Of Digital Baseband Pre-distortion System Based On DTMB Standard

TDS-OFDM technology is innovatively adopted in multi-carrier mode of DTMB standard developed by our country, which is one of the international DTTB standards. OFDM technology has the advantage of high spectral efficiency, strong ability against multipath effect and frequency selective fading. TDS-OFDM technology uses PN sequence to fill the cyclic prefix, and acts as a pilot signal in the frequency domain, which further improves spectral efficiency, simplifies the receiver synchronization and channel estimation and other operations. However, TDS-OFDM technology has high PAPR, that is, the higher peak to average power ration makes the signal easy to enter nonlinear area of the RF power amplifier in transmitter, causing serious in-band distortion and out-of-band interference, heavy damage to the orthogonality among the various sub-channels, and result in the degration of system performance.In order to solve the contradiction between the high peak to average power ratio of OFDM system and the nonlinear of power amplifier, improve the performance of communication system, starting from the perspective of power amplifier linearization, pre-distortion technology is introduced in DTMB channel processing system in this paper. Measuring the actual characteristic curve of amplifier, using the software algorithm to determine the coefficient of predistortion combined with the technology of pre-distorter based on look-up table, the linearization of power amplifier is achived and the technical criterions of the whole front-end launch system are better. According to the actual work order, firstly manual pre-distorter is designed. The actual test in the dual tone system and actuator system verifies the design ideas and the hardware implemention of pre-distortion. Then considering the insufficient of manual predistortion in practical application, the semi-automatic pre-distortion system is designed, which is consist of the pre-distorter in FPGA and the user control interface of semi-automatic pre-distortion system in PC. With the help of the man-machine interface which is builted in PC, the nonlinear characteristic curve of power amplifier can be adjusted properly according to the change of working environment, and the corresponding pre-distortion coefficient can be produced. Meanwhile, data network communication is completed by Nios Ⅱ microprocessor embedded in FPGA platform. Finally, the Pre-distortion factor will be passed to the hardware side, the look-up table of pre-distorter will be updated in the actuator, and the pre-distortion function can be opened or closed by PC.The design modules studied in the paper above is added to the actuator design, which constitute a complete DTMB exciter system with the function of predistortion. Then the data communications, hardware implementation and actual TV signals are tested. The results show that the semi-automatic pre-distort system can make up for the shortcomings of manual pre-distorter, improve the nonlinear of power amplifier effectively, and correct the image Mosaic phenomenon caused by the nonlinear of power amplifier. This research reachs the expected results and meets the design objectives.