A 30GHz 4-way Series Doherty Digital Polar Transmitter Achieving 18% Drain Efficiency and-27.6dB EVM while Transmitting 300MHz 64-QAM OFDM Signal

Abstract

Fifth-generation (5G) mm-wave communication systems support high-order modulation schemes with large peak-to-average power ratios (PAPR). This demands transmitter (TX) operation in deep power back-off (PBO), thus degrading its average efficiency. Hence, several mm-wave Doherty PAs have been proposed [1], [3] to address this issue. However, the number of their peaking amplifiers (PAP) has been limited to two, mainly due to poor scalability, and high losses in the Doherty power combiner. Therefore, the efficiency enhancement was restricted to the 10dB PBO range. Furthermore, prior-art chiefly employed an analog class-AB amplifier for the main PA (PAM), degrading the system efficiency (SE) for two reasons. First, the conduction angle of an analog amplifier increases with a reduction of the drive signal amplitude, resulting in a Class-A-type efficiency roll-off that severely degrades the achievable PBO efficiency. Second, the output impedance of analog PAs is almost constant while the PAs' load significantly increases in PBO Doherty operation. This results in a load mismatch at PBO, degrading the PAs' gain and efficiency while giving rise to AM-AM and AM-PM distortion.