Fundamentals of Power Amplifier Linearization Using Digital Pre-Distortion

Pre-distortion is basically a method by which one first stimulates a non-linear power amplifier (PA) with baseband samples and then observes the result of that stimulus at the PA output. Then , the amplitude-toamplitude modulation (AM/AM) and amplitude-to-phase modulation (AM/PM) effects of the PA are estimated. These estimated distortions are then removed from the PA by pre-distorting the input stimulus with their inverse equivalents. Figure 1 illustrates a block diagram of a pre-distortion algorithm. Here at the start of pre-distortion session, the gain a(n) and phase φ(n) are initialized to a(n) =1 and φ(n) = 0. After that, for each input baseband sample vm(n), the pre-distorter gain and phase a(n)e–jφ(n) are generated using an error signal that is based on the difference between the power amplifier output distorted baseband sample vf(n) and its corresponding undistorted input sample vm(n). The pre-distorter gain and phase are set such that the overall combination response of the PA and the pre-distorter becomes a linear system. This means that the pre-distorter is actually acting as an inverse PA non-linearity pre-equalizer. To illustrate this principle, let us consider the memoryless AM/AM and AM/PM measured results shown in Figure 2. Assume that the input sample at point n was |vm(n)| 2 = 0.45, and assume that its phase at that instant of time is θm = 0. Looking at the PA transfer function (AM/AM) in Figure 2(a), the PA output sample (obtained from the RF to base band conversion, i.e., RF downconverter, analog-to-digital converter, and timing alignment) is given by |vj(n)| 2 = 0.57, and the PA phase shift is θj(n) = 62o. In other words, the PA complex gain is given by