The advent of Internet-of-Things (IoT), smart factories, and mobile devices has caused a surge of connected devices and proliferation of wireless sensor nodes. Modern communication systems, which need to be connected everywhere and at all times, require each device to be a compact, and impose stringent standards for the RF systems. Transmitters, in face of the explosive increase in data rates require high speed complex modulation schemes with high spectral efficiency and large peak-to-average power ratios (PAPRs), such as high-order quadrature amplitude modulations (QAMs) and orthogonal frequency-division multiplexing (OFDM) are needed. In addition, due to the nature of modern society that must always be connected, battery life is also becoming increasingly important. As a result, due to the complex modulation that requires high PAPR, efficiency in the power Back off (PBO) state as well as maximum power is becoming more critical. From a receiver perspective, massive RF applications lead to a crowed spectrum, making receivers susceptible to mutual interference. Hence, along with cost and power consumption, interference robustness is becoming a major concern for the radios targeting these applications. As the demands of these modern communication systems become more diverse and complex, it is increasingly necessary to approach them in new ways. Analog RF circuits have been studied intensely, but suffer from many circuit level and system level limitations. For example, analog PAs frequently operates at the output 1dB compression point (P1dB) to ensure its amplitude and linearity, which reduces PA efficiency. In addition, most analog RF circuits have a separate function with matching/filtering networks between each stage to ensure performance, which makes further area-reduction difficult. Digital RF circuits are able to overcome many of these inherent limitations of analog approaches and their achievable performance has been highlighted in proportion to the recent technology of complementary metal-oxide-semiconductor (CMOS).  Therefore, a lot of research has been conducted on digital RF circuits. This thesis work aims at expanding upon this. In particular, this dissertation presents several AM-PM behavior analyses and compensation techniques in digital Power Amplifiers (PAs) and circuit innovation to support completely integrated digital front-end circuitry, which can be one of solutions for modern communication systems.