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This paper presents an efficient digital polar transmitter (DPTX) at mm-wave frequencies that exploits a novel N-way series Doherty combiner (SDC) to enhance its drain and system efficiency at deep power back-off (PBO). The proposed N-way SDC is scalable and can be implemented elegantly using N transformers and N-1 shunt capacitors. As a proof of concept, a 4-way Doherty DPTX is realized with the proposed SDC in which four identical but independent digital phase modulators deliver a phase-modulated constant envelope signal to their corresponding digital power amplifiers to perform the required amplitude modulation. Fabricated in a 40-nm CMOS process, the proposed DPTX occupies a core area of 1.1 mm 2 and exhibits 18.7 dBm saturated output power and <-40 dBc LO feedthrough. It demonstrates a drain efficiency of 33%/36%/22% at 0/4.5/11.5 dB PBO at 29.5 GHz carrier frequency. While transmitting a 300 MHz 64-QAM OFDM signal with a peakto-average power ratio of -10.7 dB, the DPTX achieves 18%/8% average drain/system efficiency, -27.6 dB error vector magnitude, and -27.5 dBc adjacent channel leakage ratio. To the best of the authors' knowledge, this work is the first reported mm-wave Doherty transmitter that includes the entire chain all the way from the binary data stream up to the modulated mm-wave output signal.Index Terms-Series Doherty combiner, Digital polar transmitter, Doherty design guide, Digital power amplifier (DPA), Digital phase modulator (DPM), millimeter-wave transmitter, power amplifier (PA).

Fully digital transmitters (DTXs) have the potential of replacing analog-intensive transmitter (TX) line-ups in future massive multiple-input and multiple-output (mMIMO) systems since they hold the promise of higher system integration level and energy efficiency. DTX operation so far has been limited to low RF output powers. This article introduces a concept that enables high-power DTX operation. A DTX demonstrator targeting both high output power and high efficiency is realized as a proof of concept. It is based on a custom V T -shifted laterally-diffused MOS (LDMOS) technology, which is utilized to implement a segmented high-power output stage operated in class-BE. A lowvoltage high-speed 40-nm CMOS controller drives the individual output stage segments at gigahertz rates. Measurements show the promising results for the proposed high-power DTX concept and provide valuable lessons for future DTX implementations.

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