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

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 power-hungry 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.