1201 1-4244-1176-9/07/$25.00 ©2007 IEEE. Digital Predistorter Architecture with Small Signal Gain Control for Highly Nonlinear RF Power Amplifiers Oualid Hammi, Slim Boumaiza, and Fadhel M. Ghannouchi iRadio Lab., Dept. of Electrical and Computer Engineering Schulich School of Engineering, University of Calgary Calgary, AB, Canada ohammi@ucalgary.ca Bill Vassilakis Powerwave Technologies, Inc. Santa Ana, CA, USA Abstract—In this paper, a digital predistorter (DPD) architecture is proposed for the linearization of highly nonlinear RF power amplifiers. This digital predistorter architecture uses a complexity reduced and a computationally efficient procedure to synthesis the predistortion function. Unlike conventional digital predistortion architectures that require more than one characterization to get a perfect match between the PA’s nonlinearity and that of the DPD, the proposed architecture uses a single characterization and iteratively optimizes the predistortion function performance by controlling the predistorter’s small signal gain. Experimental validation carried on a highly nonlinear RF power amplifier demonstrates the ability of the predistorter’s small signal gain control to improve the linearity performance. I. INTRODUCTION RF power amplifiers deployed in current wireless communications base stations are required to meet stringent linearity requirements while achieving the highest possible power efficiency. The linearity constraint, that consists in meeting the spectrum emission mask requirements, is due to the nature of the transmitted signals which have non constant envelops. In addition, the efficiency of the power amplifier needs to be maximized since it will dominate the running costs of the base stations in terms of DC power consumption and power dissipation. This calls for linearity versus efficiency trade-off. Such trade-off is very critical in the case of modern communication signals used in third generation (3G), 3G and beyond (3G+) and WiMAX systems. In fact, these communication standards employ advanced code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) techniques that result in time domain signals having high peak to average power ratio (PAPR). Accordingly, the use of brute force power amplifiers requires considerable back-off levels and will lead to poor power efficiency. Currently, linearization techniques are being considered to extend the linear region of continuously driven power amplifiers. For a given linearity level, this reduces the required back-off and increases the achievable power efficiency [1]-[3]. Among the linearization techniques that have been reported in the literature, predistortion, and especially digital predistortion, is the most suitable for base station power amplifiers applications. In fact, feedforward systems are complex in nature and usually lead to low overall power efficiency due to the linear amplifier used in the distortion cancellation loop. In addition, feedback systems are very narrowband and are unable to handle the signal bandwidths used with base station power amplifiers and especially multi-carriers power amplifiers (MCPA). In contrast, predistortion technique presents inherent ease of implementation, low cost and good linearity versus efficiency trade-off in comparison with feedforward and feedback based linearizers. Furthermore, the use of digital predistortion, that has the ability to accurately synthesize the AM/AM and AM/PM compensation functions, makes possible the migration from mildly nonlinear power amplifiers to highly nonlinear power amplifiers. This further boosts the overall power efficiency of the linearized power amplifier. The performance of digital predistortion systems fundamentally relies on the perfect match between the synthesized predistortion functions and the actual nonlinear characteristics of the power amplifier. Since the behavior of the power amplifier depends on the statistics and bandwidth of the input signal (continuous wave (CW), two tones, CDMA, etc,…) [4] as well as its average power [5]-[6], it is important to keep the same signal type and especially the same average power at the input of the power amplifier between the characterization and the linearization steps. The use of the instantaneous input and output waveforms based characterization technique reported in [4] ensures the first condition relative to the nature of the driving signal. For This work was supported by the Alberta Informatics Circle of Research Excellence (iCORE), the Natural Sciences and Engineering Research Council of Canada (NSERC), and Canada Research Chairs (CRC).