A Study of a Millimeter-Wave Transmitter Architecture Realizing QAM Directly in RF Domain

Abstract

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

Reconfigurable Signal Processing and DSP Hardware Generator for 5G and Beyond Transmitters

“…However, generating a narrow pulse at the RF front-end becomes difficult [30]. Finally, RF-QAM uses power amplification of the quadrature phase shift keying (QPSK) signal directly in the RF domain before combining it to facilitate linearity of the PA [31]. The hardware for these emerging architectures can be supported by chisel-based generators like the one proposed, but they are not covered in this work.…”

A Study of Out-of-Band Emission in Digital Transmitters Due to PLL Phase Noise, Circuit Non-Linearity, and Bandwidth Limitation

“…TX circuit blocks, especially those up the TX chain close to the antenna, exhibit nonlinearity that degrades the TX performance in terms of EVM [14]. Additionally, nonlinearity causes spectral regrowth, which elevates the OOBE level.…”

“…The situation is exacerbated when the resulting RF signal passes through nonlinear blocks such as a PA. The signal passing through the PA, due to its non-linearity, experiences AM/AM and AM/PM distortions as well as spectral regrowth, further increasing the OOB power [14], [15]. An ideal band-pass filter (BPF) at the PA output should only pass the inband portion of the signal while eliminating its out-of-band components.…”