Demand for Efficient Amplification
With the growth of fourth-generation (4G) services, progressively more complex signal modulation schemes have been introduced to fulfill the sharp surge of data and video capabilities. The high-level system requirements are mainly determined by the physical limits. The RF PA of each standard generation indirectly defines the available transmission rate limits. In Figure 1(a), the bandwidth and PAPR are the main impact factors for PA designs, resulting in challenges for 4G and beyond. In Figure 1(b), the RF PA evolution from class-AB/B to envelope tracking (ET) is mainly driven by the greater complexity of the digital modulation signal. Statistically speaking, the probability distribution function (PDF) of the evolved signal moves toward a low-power region in the histogram plot. Various high-efficiency PAs emerge as the times require as shown in Figure 2(a) [1], [2], where a comprehensive knowledge of ET and high-efficiency load modulation PA techniques has been introduced.
(a) The category of high-efficiency PA architectures and (b) the typical efficiency and gain behavior of PAs over output power compared with PDFs of typical quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) signal. (From [2]. ©2015 Artech House. Reprinted with permission.)