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).

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