Energy efficient wireless transmitters: Polar and direct-digital modulation architectures

Power consumption is an increasingly important issue in highly integrated wireless systems. While advances in semiconductor technology have driven continuous integration of features and services into portable devices, power consumption is now a major limiting factor on computational complexity and the ability to communicate over long distances. In portable communication devices, the wireless transmitter is often the dominant source of power consumption, such that in recent years there has been a major effort to improve the power efficiency of transmitter circuits, especially the power amplifier (PA). In addition to power consumption, it is now apparent that energy consumption is an important metric for transmitter circuits. Energy consumption more accurately predicts the battery life, especially when a portable system operates with a wide range of output power.;In this work, polar transmitter architectures are presented as a promising alternative to conventional Cartesian architectures. Traditional polar systems use dynamic modulation of the PA power supply to transmit amplitude information. This helps polar systems achieve higher average (energy) efficiency than Cartesian systems. Polar systems have suffered from drawbacks related to linearity, time-alignment of amplitude and phase signals, and power supply noise. Furthermore, the amplitude and phase paths require bandwidth significantly higher than the Cartesian I-Q basis vectors to represent the wideband polar representation of the wireless signal.;This work focuses on several contributions related to improving the operation of energy efficient polar transmitters. The first contribution is a power supply noise analysis framework for nominally linear power amplifiers. This analysis helps predict upconversion of supply noise to RF frequencies---a scenario that is likely in polar and envelope tracking supply-modulated transmitters. Supply noise upconversion can cause violation of the spectral mask and it is important to understand the underlying circuit mechanisms for this phenomenon.;A second contribution is an optimal operating strategy for hybrid switching-linear voltage regulators. These regulators are attractive for polar systems since they achieve the wideband, high-fidelity performance of a linear regulator with the power efficiency of a switching regulator. We show that past implementations of hybrid regulators could achieve higher efficiency with an optimized control objective. We use time-domain averaging to determine the optimum current for the DC-DC converter in the hybrid regulator. The optimized solution achieves substantially higher efficiency across the output power range than the traditional solution. We develop expressions for optimum efficiency as a function of characteristics of the envelope signal and the supply voltage for the linear regulator.;The final contribution is a digital-polar transmitter based on pulse-density modulation of the RF carrier. Instead of using power supply modulation, the amplitude path is controlled with a digital noise-shaping process. To reduce power consumption, noise shaping is implemented in two stages: a baseband DeltaSigma modulator operating at 100MHz and a programmed pulse-density modulator operating at 2.4GHz. A circuit implemented in 90nm CMOS uses a class D PA to achieve up to 20dBm output power. The system achieves peak efficiency of 38.5% at 2.4GHz including power of the PA drivers and filter insertion loss. EVM is approximately 2.0% for 8DQPSK and pi/4DQPSK constellation trajectories. The spectral mask for Bluetooth 2.1+EDR is satisfied under normal conditions.;Overall this work highlights several techniques that help improve energy efficiency of wireless systems. Hopefully these solutions will paint a roadmap of future work that will help commercial development and lead to many challenging research problems and academic contributions.