A Spline-Based Hammerstein Predistortion for 3G Power Amplifiers with Hard Nonlinearities

Abstract—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.