  In this paper, we present techniques and examples to reduce power consumption
and increase energy efficiency of autonomous Wireless Sensor Nodes (WSNs) for
the Internet of Things. We focus on the RF Energy Harvester (RFEH), the data
receiver and the transmitter, all of which have a large impact on the device
cost, lifetime and functionality. Co-design of the antenna and the electronics
is explored to boost the power conversion efficiency of the RF-DC converter. As
a proof of principle, a charge pump rectifier is designed, and its measurement
results are presented. To boost the rectifier output voltage, a DC-DC converter
that employs maximum power point tracking has been designed. A prototype
circuit is also presented that can accommodate an input power level range of 1
{\mu}W to 1 mW and offers peak efficiencies of 76.3% and 82% at 1 {\mu}W and 1
mW, respectively. The co-design principle is also used at the receiver side
where the antenna-electronics interface is optimized. It is shown how this
technique allows improving the noise figure of the Low Noise Amplifier (LNA)
without sacrificing power consumption. As a low power alternative to
narrow-band wireless transmission, sub-GHz ultra-wideband is proposed. As a
proof of principle, the design of a novel low-power sub-GHz
Ultra-Wide-Bandwidth (UWB) transmitter which consumes only 0.28 mW is
presented. Its working principle is verified by means of circuit simulations
and measurements. The low power nature of the transmitter and receiver
principles, combined with the power efficient RF-DC converter paves the way
towards the continuous operation of a WSN.
