Digital-intensive RFIC design techniques for transmitters in ISSCC 2023

Sub-6 GHz multi-standard wireless communication and millimeter-wave (mm-wave) wireless transmission provide users with multi-functionality and unprecedented wireless connectivity. With CMOS process scaling down, system-on-chip (SoC) implementation of wireless systems along with RFIC functionality is highly desirable for low cost and small form-factor. The digital transceiver architecture aligns with Moore’s law to provide compact die area, better interface to digital backend and higher efficiency due to the faster switching nature of core devices[1−28]. The digital PA (DPA) plays a dominant role in digital transmitters (DTXs), which performs digital-to-analog conversion, frequency up-conversion and power amplification all-in-one. Since modern wireless standards widely adopt orthogonal frequency division multiplexing (OFDM) and high-order quadrature amplitude modulation (QAM) to increase throughput and spectral utilization, it requires the DPA to achieve high output power (e.g., >30 dBm), high linearity and high efficiency especially at back-off power levels. Relevant intensive researches at sub-6 GHz have been directed toward class-D based switched-capacitor PAs, which are more amenable to CMOS scaling and have good linearity and efficiency due to fast low-loss switches and precise capacitor matching. In ISSCC 2023, two works on sub-6 GHz DTX/DPA came from Fudan University[5, 7]. In Ref. [5], a singlechannel quadrature DTX supporting multi-mode multi-band NB-IoT/BLE applications is proposed. It introduces a sliding digital-IF quadrature architecture to replace the fractional upsampling module and achieve pure output spectrum. Besides, a compact wideband Doherty DPA with IQ cell-sharing is implemented to enhance output power and back-off efficiency. This multi-mode NB-IoT/BLE DTX chip achieves Watt-level peak power with >40% system efficiency while occupying only 0.79 mm2 core area, which is well-fitting lowcost IoT applications. In Ref. [7], a 4.1 W quadrature DPA with 33.6% peak PAE in 28 nm bulk CMOS is presented. It introduces the cascode and 8-way differential power-combining techniques to enhance output power by 16 times. Besides, the IQ cell-sharing and transformer-based Doherty techniques are introduced to further enhance output power by 2 times and achieve 12 efficiency peaks in the complex domain. Powered by 1.1 V/2.2 V supply voltages and packaged in an QFN format, this DPA chip achieves 4.1 W peak power and close-to-Watt-level average power with competitive efficiency performance even compared with polar DPAs, which is very attractive for compact and fast system integration in 5G applications. mm-wave wireless transmission with multi-Gb/s data rate also demands wideband signal processing, high system efficiency and high output power for large coverage. In ISSCC 2023, a 71–89 GHz DPA came from University of Electronic Science and Technology of China[26]. In Ref. [26], the doubleedge-triggered technique is proposed to double the signal bandwidth with limited sampling clock. Besides, the LO leakage suppression and balance-compensated power-combining techniques are introduced to enhance linearity and efficiency. Implemented in 40 nm CMOS, this mm-wave DPA obtains 12 Gb/s high-speed data rate, 20.5 dBm peak output power and 20.4% system efficiency, which is quite competitive for E-band wireless applications. Moreover, two DTX works are implemented for FMCW chirps from Infineon Technologies[27] and low-power cryogenic controller IC design from Tsinghua University[28]. In Ref. [27], accurate frequency modulation of a direct digital frequency synthesizer is combined with the RFDAC to generate precise wide-bandwidth frequency ramps, which achieves 4 GHz modulation bandwidth with <3 dB power variation. In Ref. [28], a polar architecture with DPA-based amplitude modulation and injection-locking LO based phase modulation is proposed. Its power consumption is 13.7 mW per qubit under active control and the average chip area per channel is only 0.9 mm2. In Table 1, various DTX/DPAs operating from sub-6 GHz to mm-wave bands with multi-mode multi-band, output power, good efficiency, linearity and bandwidth have been summarized. With these advanced metrics, the digital RF front-end will be a good candidate for modern and further wireless applications.