Analyzing LINC Systems

In wireless communication systems, the radio-frequency (RF) power amplifier (PA) is one of the most critical components in the design of wireless transmitters. This is true whether one is interested in low-power portable devices transmitting hundreds of milliwatts, high-power cellular base stations transmitting tens of watts, TV transmitter pumping out hundreds of watts or other space-based and terrestrial application operating at various power levels. In all of these applications, the design and manufacturing of the PA requires the engineer to address several concurrent, and often conflicting, requirements on power efficiency, linearity, cost per watt and size. The exact relative weight a designer must attach to each of these requirements, and their relative order of importance, depends largely on the targeted application, the expected volume, and the nature of the signals to be amplified. Consequently, trade-offs are often made between the requirements/specifications, while taking into account these added considerations [1]. Recent trends in efficient and linear PA research have begun focusing more on the use of two-branch amplifier systems and away from the classical single-ended amplifier topology combined with the use of digital predistortion techniques [2]–[4]. Among these dual-branch systems, the most popular are the Doherty amplifier [5]–[7]; the envelope elimination and restoration (EER) technique and variations thereof [8], [9]; the linear amplification with nonlinear components (LINC) technique [10], [11]; and the modified implementation of the LINC concept (MILC) technique [12]. This article focuses on the LINC techniques and its inherent challenges to design engineers.