Outlooks on Radio Transmitter Energy Efficiency and Ultra-Low Power Radio Transmitters

The number of wireless electronic devices in the world continues to grow at a rapid pace. The wireless devices are powered by limited energy sources and require energy-efficient electronic circuitry to maximize the operating time. Radios can consume a significant share of the power budget of a wireless device. Researchers and designers have, therefore, innovated numerous ultralow power (ULP) radio transmitters and receivers. Such ULP transmitters have conventionally used mostly on-off keying (OOK), binary phase-shift keying (BPSK) or binary frequency-shift keying (BFSK). These modulation schemes enable low power consumption – even below 100 μW – as they only require low-complexity transmitter circuitry and a relatively low signal-to-noise ratio (SNR) for a given error probability. This thesis investigates the option of using M-ary pulse-position modulation (PPM) and differential PPM (DPPM) in ULP narrowband transmitters in lieu of the conventionally used modulation schemes. The goal is to evaluate whether or not they could achieve better energy efficiency. The benefits and disadvantages of PPM and DPPM are reviewed and their energy efficiency is compared with OOK, BPSK and BFSK in a new way. In ULP transmitters, the power amplifier and carrier synthesizer generally consume the most power. The new comparison method considers how the choice of modulation impacts the combined energy consumed by these blocks per bit. The results suggest that use of OOK, BPSK and BFSK can consume tens to hundreds of percents more energy per bit compared to PPM and DPPM. The M-ary PPM schemes are predicted to be particularly energy-efficient in low-output-power transmitters. To evaluate the energy efficiencies of transmitter implementations, a new figure of merit (FOM) is derived. In prior ULP transmitter publications, the energy efficiency FOMs have neglected the effect of noise bandwidth and the choice of modulation. However, it is practically the output power and noise bandwidth that together determine the SNR of the generated signal. Moreover, this SNR and the SNR required by the modulation scheme significantly impact the achievable uplink range. By accounting for these metrics in addition to the energy consumption per bit (EPB), an FOM is obtained that provides a more comprehensive view of transmitter energy efficiency than the comparison metrics that have been used before. Two ULP DPPM radio transmitter implementations are presented with measured results. The first one achieves one of the lowest EPBs while still enabling an uplink range of 30 meters. The second one consumes more energy per bit but enables an uplink range of up to 1 km. An FOM comparison with prior transmitters, including recent sub-mW Bluetooth Low Energy transmitters, suggests that the latter transmitter is state of the art in terms of energy efficiency. In addition to the transmitters, an experimental ULP capacitive gesture sensor interface is presented with measured results. It enables hand-sweep and push-gesture detection over a short range.