Abstract
We propose a spatially superposed 16-quadrature amplitude modulation (QAM) system composed of two offset-quadrature phase-shift keying (OQPSK) signals and investigate its performance over typical nonlinear channels. The system incorporates two OQPSK modulators. Their output signals are fed to high power amplifiers (HPAs), where each OQPSK signal is separately power-amplified in a highly efficient nonlinear region of an HPA. The amplified signals are then combined in a vector-sum manner with two beams formed by spatial power-combining technology to spatially produce a 16-QAM signal. The amplitude change of OQPSK is smaller than the change of 16-QAM, and this enables the system to be operated on power amplifiers with high efficiency as compared to conventional 16-QAM or 16-amplitude and phase-shift keying (APSK) systems. Moreover, we explain here that even after the nonlinear amplification of OQPSK-modulated radio frequency (RF) signals, the frequency side-lobe levels are similar to ones of conventional 16-QAM or 16APSK signals. Thus, we can achieve more efficient RF power amplification of 16-QAM signals without expanding the spectral occupancy. Finally, the power consumption is qualitatively discussed based on specific HPA characteristics, and its performance is compared to conventional 16-QAM and 16-APSK, with a remarkable gain shown in both power and spectral efficiency. We show that a reduction in power consumption of 50% is achievable compared to a conventional system. We found through this study that the proposed system is feasible and will enable broadband transmission with efficient use of power and bandwidth.
Published Version
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