Digitally Modulated CMOS Polar Transmitters for Highly-Efficient mm-Wave Wireless Communication

Abstract

A polar transmitter (TX) is implemented at 60 GHz, enabling a power amplifier (PA) to operate in saturation where efficiency is highest, even when handling higher order modulations such as QPSK and 16-QAM. The phase path is upconverted by I-Q mixers, while the amplitude path modulates an RF-DAC. Aimed at 802.11ad applications, the 10 GS/s (i.e., 6x-oversampled) polar TX realizes more than 30 dB alias attenuation, and the input bandwidth exceeds 3.1 GHz. The PA saturated output power is 10.8 dBm with 29.8% drain efficiency at the maximum RF-DAC code. Average output power is 8.1 dBm with 22.3% drain efficiency at $-20.7\;\text{dB}$ EVM for QPSK modulation without RF-DAC predistortion. The corresponding 16-QAM values are: 7.2 dBm average output power with 19.8% efficiency at $-16.5\;\text{dB}$ EVM. With predistortion, a QPSK modulated output achieves 5.3 dBm average power with 15.3% efficiency at $-23.6\;\text{dB}$ EVM, while 3.6 dBm average power with 11.6% efficiency at $-18.1\;\text{dB}$ EVM is realized for 16-QAM. For a sampling rate of 10 GS/s, the TX data rates are 3.33 Gb/and 6.67 Gb/s for QPSK and 16-QAM, respectively. Implemented in 40 nm bulk-CMOS, the core circuit occupies $0.18\text{mm}^{2}$ core of the $2.38\text{mm}^{2}$ total die area, and consumes 40.2 mW from a 0.9 V supply.