A Reduced-Complexity Load-Modulated MIMO Transmitter Readily Scalable in 5G Massive MIMO Transmitters

Abstract

This paper presents the design of a four-branch reduced-complexity load-modulated MIMO transmitter. The transmitter is designed to operate at 3 GHz. The transmitter can generate 64-QAM constellations at the input of the radiating elements. The waveforms of the 64-QAM constellations are generated by changing the impedance parameters of the load modulator circuits connected to antennas. At the same time, the output oscillator is kept constant. Thus, a single RF chain can drive the entire transmitter where the power amplifier amplifies a constant signal and does not need to operate in the back-off region. A four-way Wilkinson power divider is used to split the power to the four branches of the transmitter. Microstrip patch antennas have been connected to the load modulators to transmit the desired outputs. RF isolators absorb reflections from each load modulator, allowing branches to generate desired constellations independently and maintaining the system matched at the input all the time. Advanced Design System (ADS) has been used to design, simulate, and fully characterize the transmitter. The proposed transmitter does not require mixers and digital-to-analog converters (DACs). The transmitter has been fabricated, and over-the-air transmission of 64-QAM signals has been tested. The architecture can be scaled to larger array sizes, making it suitable for adoption in 5G massive MIMO systems. High cost, energy, and RF-complexity savings can be achieved if the transmitter is scaled to a larger number of branches.