31.7 A 0.7-to-2.5GHz Sliding Digital-IF Quadrature Digital Transmitter Achieving >40% System Efficiency for Multi-Mode NB-IoT/BLE Applications

Cellular narrowband Internet-of-Things (NB-loT) is an important branch of low-power wide-area loT applications, which specifies multiple operation bands over 663 to 915MHz (LB) and 1710 to 2010MHz (MB), up to 23dBm output power for wide-range coverage, and low power consumption for longer battery life. Recently, Bluetooth Low Energy (BLE) at 2.4GHz band (HB) is integrated in NB-IoT modules to provide effective near-field maintenance for NB-IoT terminals, such as intelligent meters, smoke sensors etc. To simultaneously support multiple operation bands for user flexibility, separately optimized LB/MB NB-IoT power amplifiers (PAs) are implemented in [1], which increases design complexity and cost. With advantages of high efficiency, compact die area and direct interface to digital backend, scaling-friendly digital transmitters (DTXs) have become the research hotspot. The polar digital power amplifiers (DPAs) [2], [3] and DTXs [4] with high output power and high efficiency have been implemented to support NB-loT or BLE applications, but they require CORDIC, phase modulator, AM-PM synchronization and calibration circuits. The quadrature DTX has a simple and symmetrical architecture with good linearity, but it has lower output power and efficiency due to I/Q orthogonal combination. In addition, the up-sampling clock of digital baseband signal processing is usually integer-divided from the LO frequency to avoid fractional spurs. With the LO frequency varying over a wide frequency range, a complex fractional up-sampling module with variable ratios is required to match with input data-rate. In this work, a sliding digital-IF wideband quadrature DTX is proposed, which introduces a continuously sliding digital-IF digital upconverter (DUC) to replace the fractional up-sampling module, and realizes a compact wideband quadrature Doherty DPA with IQ-sharing technique to enhance output power and efficiency for multi-mode multi-band NB-loT/BLE applications.