Characterization of Wireless Transmitter Distortions
Wireless transmitters designed for modern communication systems are expected to handle wideband amplitude and phase modulated signals with three major performance metrics: linearity, bandwidth, and power efficiency. First, linearity requires the minimization of distortions mainly caused by the transmitter’s radio frequency (RF) analog circuitry in order to preserve the quality of the transmitted signal and avoid any loss of information during the transmission process. Second, bandwidth is critical for multi-carrier and multi-band communication systems. Moreover, wider bandwidths are needed to accommodate higher data rates. Third, power efficiency is an important consideration that affects the deployment and operating costs of communication infrastructure as well as environmental impact.
In general, distortions refer to the alteration of the signal due to the imperfections of the transmitter’s hardware. Distortions observed in wireless transmitters have various origins such as frequency response distortions, harmonic distortions, amplitude and phase distortions, and group delay distortions, in addition to modulator impairments (including direct current (DC) offset, gain, and phase imbalance), and so on. Among these distortions, the predominant ones are those due to the nonlinearity present in the transmitter’s RF front end and mainly the RF power amplifier (PA). Indeed, wireless transmitters are made of a cascade of several stages including digital-to-analog conversion, modulation, frequency up-conversion, filtering, and amplification as illustrated in Figure 1.1. Among these subsystems, the PA is identified as the major source of nonlinear distortions. Thus, modeling and compensating for the transmitter nonlinear distortions is often trimmed down to the modeling and compensation of the PA’s nonlinearity.
Behavioral Modeling and Predistortion of Wideband Wireless Transmitters, First Edition. Fadhel M. Ghannouchi, Oualid Hammi and Mohamed Helaoui.
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.
Figure 1.1 Simplified block diagram of a typical wireless transmitter
In the remainder of this chapter, the nonlinearity of RF PAs will be described and major metrics used to quantify nonlinear distortions will be presented.