Green wireless communications: A power amplifier perspective

In this paper, we survey two essential and practical characteristics of radio-frequency power amplifier (PA), namely, linearity and efficiency. Nonlinear amplification yields significant distortion of the transmit signals and strong interference for cochannel users. Imperfect efficiency of the PA causes an overhead of the systems resulting in energy efficiency (EE) degradation. Therefore, the linearity and efficiency of the PA should be precisely characterized in the system design. We first survey the linearity and efficiency models of PA, and then introduce commonly used technologies for improving the EE according to three approaches: i) transmitter architecture, ii) signal processing, and iii) network protocols. We then introduce our recent work on a multiple PA switching (PAS) architecture, in which one or more PAs are switched on at any time to maximize the EE while satisfying the required spectral efficiency (SE). We consider the case where either full or partial channel state information is available at the transmitter (CSIT). Since the transmitter selects the most efficient PA that satisfies a target rate with the least power consumption, EE is improved, and a Pareto-optimal SE-EE tradeoff region can be enlarged as verified in the numerical results with real-life device parameters. For example, we observe around 323% and 50% EE improvements for a single antenna system with a full CSIT and for a transmit antenna selection and maximum ration combining system with a partial CSIT, respectively; as a result, we can surmise that the PAS is one promising technology for green, i.e., energy efficient, wireless communication systems.