Digital techniques to mitigate feedback impairment and noise of look-up table digital predistortion

Abstract

In this article, some new effects of feedback impairment and noise on look-up table (LUT) digital predistortion (DPD) are presented. Several digital techniques are proposed to mitigate these effects. A smoothing filter (SMF) for LUTs is used to eliminate fluctuations of the transfer function of the overall DPD amplification system. By combining SMF method with iterative average (IA) method, the LUTs iterative process becomes stable and well converged. A postcompensator is established according to the proposed two-box model for feedback impairment. For the demonstration, both simulation and experiments are carried out based on a Hammerstein-type PA. Simulation results give some preliminary cognition of these new effects and the effectiveness of proposed techniques. Experimental tests are performed on an S-band amplifier excited with single-carrier WCDMA signal. The adjacent channel power ratio (ACPR) at 5-MHz offset is −48 dBc after DPD with SMF method and postcompensator. A total of 8 dB extra improvement of ACPR is obtained compared with that with neither SMF nor postcompensator. The result clearly shows that the proposed digital techniques are qualified for LUT DPD system, especially when it suffers significant feedback impairment and noise. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.