A spline-based Hammerstein predistortion for 3G power amplifiers with hard nonlinearities

In this paper, we focus on the digital predistortion for the linearization of power amplifiers (PAs) with intrinsic hard nonlinearities, such as Doherty PAs, envelope tracking (ET) PAs widely explored in the third generation (3G) communication systems. Conventional Volterra-series-based predistortion with high degree nonlinearities suffers from the difficulty of real-time implement and the problem of numerical instability. Accordingly, a block-oriented Hammerstein predistortion with cubic-spline static nonlinearity is proposed, and the separable nonlinear least squares (SNLS) method is used to identify the Hammerstein coefficients, which significantly reduces the dimension of search space. A normalization convergence scheme is also presented to ensure the global optimum. The experiments are carried out on uplink single-carrier WCDMA signals, the results show that both the static nonlinear distortion and the memory effect of the PA under test are compensated significantly, and fast convergence is achieved by the normalization scheme.