# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 1 Introduction  
Wireless communications play a vital role in modern societies by enabling a wide range of applications critical for our daily life. Modern communication systems ensure a reliable, seamless, and ubiquitous data transfer that is in the path to connect everyone and everything. This calls for the development of communication systems infrastructure that can handle an increasingly large number of users and an ever-expanding amount of data. Networks that are 5G and beyond offer a plethora of solutions to accommodate these needs in varying applications such as healthcare, inter-vehicular communication, environmental monitoring, and other Internet of things (IoT) applications [1, 2, 3]. However, these advances can only be achieved through the design of advanced communication hardware that can successfully meet the stringent requirements of modern applications. This work focuses on the radio frequency (RF) front-end of wireless communication systems, and, more specifically, on the power amplification stages where opposing requirements need to be managed carefully. Radio frequency power amplifiers (PAs) represent one of the most critical subsystems in the RF front-end of modern communication systems. This is mainly due to the noticeable impact that the power efficiency and linearity of the power amplifier have on the overall performance (efficiency and linearity) of the entire transmitter. Power efficiency is an important performance criterion since the power amplification stage is typically the subsystem that consumes the most amount of power in the communication infrastructure (basestations). Enhancing the power efficiency of the PA will reduce the DC power consumption for a given output power, and therefore lessen the amount of power being lost through thermal dissipation. Not only is the unused power being lost, but it often requires the use of cooling mechanisms that will also consume additional power. Hence, power efficiency can have a significant impact on the running cost of base stations. Most importantly, high power consumption increases the carbon footprint of the base station and goes against the need for greener and more environmentally friendly communication infrastructure. Linearity, on the other hand, is directly related to the quality of service and the ability to reliably transmit data without loss of information. Nonlinear distortions, mainly caused by the power amplification stage, cause interference with the adjacent channel and are strictly monitored by spectrum emission regulations in each wireless communication standard. Given that a power amplifier cannot operate in a power efficient mode while linearly amplifying the signal, it is essential, in order to reach an acceptable tradeoff between linearity and efficiency, to operate the power amplifier in a nonlinear but power efficient mode and use adequate system level linearization techniques to counteract the effects of the amplifier's nonlinear distortions. Among the various linearization techniques, predistortion stands out as the preferred approach for modern base stations [4]. This is mainly due to its ability to achieve adequate performance with a reasonable overhead by taking advantage of digital implementations of the predistortion function. While digital predistortion (DPD) technique is a mature technology for wireless communication infrastructure, it is facing continuous challenges due to the ever-increasing requirements set by evolving wireless communication standards. For example, compared to 4G systems, 5G systems employ signals of much larger bandwidth that can reach up to 100 MHz in the sub-6 GHz frequency range 1 (FR1), and up to 400 MHz in the millimeter wave frequency range 2 (FR2), as explored in [5; 6]. Such wideband signals set tough requirements on the hardware needed for the digital predistortion function (especially for the digital-to-analog and analog-to-digital converters used in predistortion's forward and feedback paths, respectively).