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Realization of high-order modulation schemes directly in the RF domain enables the generation of spectrally efficient 4 M quadrature-amplitude-modulated (4 M QAM) symbols using the vectorial summation of M quadrature phase-shift keying (QPSK) signals whose amplitudes are progressively scaled by a constant factor of two. Called RF-QAM, this approach leads to numerous advantages including the elimination of powerhungry digital-to-analog converter (DAC) and the mitigation of stringent linearity requirement of the front-end power amplifier (PA). This paper presents a comprehensive comparative study of RF-QAM and conventional transmitters. The design issues associated with the front end and the mixed-signal blocks for both architectures are investigated, and the performance of these two designs is compared. Various circuit-and system-level simulations verify the superior performance of the RF-QAM transmitter compared to the conventional counterpart.Index Terms-Digital-to-analog converter (DAC), power amplifier (PA), quadrature amplitude modulation (QAM), quadrature phase-shift keying (QPSK), radio frequency (RF), terahertz (THz), transmitter (TX), 6G. I. INTRODUCTIONT HE advent of emerging content-intensive applications has led to the ever-increasing demand for high-speed data transmission, and thus, the emergence of 6G and beyond where the operating frequency is designated to be in the terahertz (THz) range (loosely defined to cover frequencies from 100-to 1000-GHz) [1], [2]. Several transceiver front-ends operating at the low side of the THz band achieving impressive data rates have been reported in the literature [3], [4], [5], [6]. Recently, a number of end-to-end integrated transmitters and receivers operating above 100 GHz have been presentedAchieving high data rate by increasing the center frequency to obtain larger bandwidth (BW) comes with several essential concerns: (1) Operating above f max /2 results Manuscript

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