Efficient Lookup Table-Based Adaptive Baseband Predistortion Architecture for Memoryless Nonlinearity

Seydou N Ba,G Tong Zhou,Khurram Waheed

doi:10.1155/2010/379249

Abstract

Digital predistortion is an effective means to compensate for the nonlinear effects of a memoryless system. In case of a cellular transmitter, a digital baseband predistorter can mitigate the undesirable nonlinear effects along the signal chain, particularly the nonlinear impairments in the radiofrequency (RF) amplifiers. To be practically feasible, the implementation complexity of the predistorter must be minimized so that it becomes a cost-effective solution for the resource-limited wireless handset. This paper proposes optimizations that facilitate the design of a low-cost high-performance adaptive digital baseband predistorter for memoryless systems. A comparative performance analysis of the amplitude and power lookup table (LUT) indexing schemes is presented. An optimized low-complexity amplitude approximation and its hardware synthesis results are also studied. An efficient LUT predistorter training algorithm that combines the fast convergence speed of the normalized least mean squares (NLMSs) with a small hardware footprint is proposed. Results of fixed-point simulations based on the measured nonlinear characteristics of an RF amplifier are presented.

Highlights

High-efficiency RF amplifiers have nonlinear amplitude and phase transfer characteristics, which distort the transmitted signals, causing undesired out-of-band spectral regrowth and an increase in error vector magnitude (EVM) and bit error rate (BER)
This paper proposes optimizations that facilitate the design of a cost-effective and high-performance adaptive digital baseband predistorter, while minimizing expensive factory calibration requirements
3.5 dB of peak-to-average power ratio (PAPR), advanced WCDMA waveforms exhibit PAPRs in excess of 6 dB and modern 4G (LTE, WiMax) use more complex signal constellations resulting in PAPRs of up to 12 dB [1]

Summary

Introduction

High-efficiency RF amplifiers have nonlinear amplitude and phase transfer characteristics, which distort the transmitted signals, causing undesired out-of-band spectral regrowth and an increase in error vector magnitude (EVM) and bit error rate (BER). 3.5 dB of peak-to-average power ratio (PAPR), advanced WCDMA (or HSPA) waveforms exhibit PAPRs in excess of 6 dB and modern 4G (LTE, WiMax) use more complex signal constellations resulting in PAPRs of up to 12 dB [1] Such a high PAPR mandates higher linearity requirements from the RF physical layer, which is in sharp contrast to the stronger demand for increased power efficiency and maximization of the handset battery life. The resulting, but often unintended effect, is a concentration of the LUT entries around the higher amplitude region [7, 8] This power indexing scheme is suitable for class-A and mild classAB amplifiers since their characteristics are mostly linear until close to saturation.

Performance of Amplitude and Power LUT Indexing

Adaptation of Complex-Gain LUT Predistorters

Conclusions