Wake-up radio is a way of cutting the power consumption of a seldom-used receiver – in this case an active car tag for road tolling.
Inside a wake-up radio there are two receivers: a simple low-power one that is just complex enough to recognise the desired signal, and a more power-hungry full featured receiver that demodulates and decodes data from the signal; as well as a transmitter if two-way communication is required.
The simple receiver awaits the signal and, when it thinks it has found it, wakes the comprehensive receiver.
Best power consumption is achieved when the simple receiver is just sophisticated enough not false-trigger the full receiver too often.
To help the system along, the desired signal can be customised for wake-up reception.
In the case presented at ISSCC, the data part of the signal – modulated at 256kbit/s – is preceded by 15-17 cycles of simple 14kHz AM modulation.
The team rejected a SAW front-end for the wake-up receiver as being too wideband (leading to false triggering), and too lossy to be sensitive enough.
Instead, they used a four transistor LC-tuned high-gain RF envelope detector feeding a comparator as a one-bit ADC (see diagram).
Following this, a low-power delay-based band-pass filter decides if the tell-tail 14kHz signal is present.
Made on 0.13µm CMOS, final sensitivity in the wake-up radio is -45dBm (MI=0.85).
The associated full receiver is similar in architecture, but has an LNA before the envelope detector, and an analogue band-pass filter between that and the one-bit ADC. 256kbit/s decoding is on a separate chip.
Sensitivity is -61dBm (MI=0.85) and current consumption is 19mA.
The +5dBm transmitter on the same chip draws 40mA delivering data at 512kbit/s.
The design is a team effort between KAIST and PhyChips.