Digital predistortion techniques for linearizing power amplifiers of wireless mobile terminals

Abstract

Power amplifiers (PAs) are essential device to transmit signals in wireless communication systems. However, since the efficiency of PAs is very low, large portion of energy is consumed by PAs. But usually, increasing the efficiency also increases the nonlinearity of PAs. In modern communication systems, higher spectral efficiency is required and the linearity of the transmitters is also becoming more important issue. Typically nonlinear characteristics of PAs cause two major problems. One is spectral regrowth at PA output that causes violation of spectral regulation of wireless standards. The other is degradation of receiver performance such as bit error rate (BER). To mitigate these effects, input power of PA are backed-off from its saturation and the PA is operated in linear region. Since current modulation techniques such as CDMA (code division multiple access) or OFDM (orthogonal frequency division multiplexing) tend to have high peak-to-average power ratio (PAPR), the back-off technique requires large back-off from the saturation and the efficiency of PAs is reduced dramatically.

To compensate the nonlinearity of PAs and increase the efficiency, many techniques have been proposed. Among them, digital predistortion (DPD) is recognized as the most cost effective technique for linearizing PAs. DPD techniques utilize a predistorter in the baseband domain, whose characteristics are the inverse of the PA, to compensate the nonlinearity of PAs. DPD has been widely used in base stations and relays, and these days, its application is being extended to the mobile stations. As the cost and the size are more important issue in mobile stations, new problems are being raised. Among them, we consider two issues. First, novel system structures are proposed to reduce the implementation cost of DPD, and then, extended DPD schemes are proposed to include additional impairments caused by low-cost/small-size implementation of mobile stations. More specifically, this dissert...