Wireless nodes take power from ambient TV broadcast signals
The technology demonstrators communicated over around 50cm at 1kbit/s using a packet protocol.
Communication is bi-directional and includes reception acknowledge as well as a simple anti-collision technique which allows point-to-point communication in an environment where multiple nodes operating.
Like passive RFID, the nodes ‘transmit’ by changing the reflectance of their antenna – in this case by briefly shorting (making it reflect) the two electrodes of a 258mm dipole tuned to a 50MHz band of UHF TV signals around 539MHz.
Changing reflectance effectively amplitude-modulates the field around the dipole, radiating an AM signal.
The dipole is busy as it is not only the transmit antenna, it is also the receive antenna and the source of power for the on-board microcontroller and receiver.
UHF power it extracted straight from the dipole through a four-stage passive diode-capacitor charge pump, the result of a previous University of Washington research project that was done on conjunction with Intel. In the original case, connected to a TV-top log-periodic antenna, the circuit delivered 60µW of dc 4km from the TV transmitter.
The receiver relies on the huge difference in modulation frequency between the ambient TV signal (6MHz) and the bit rates of the nodes (1kbit/s).
It is simply a crystal set – a diode and capacitor connected across the antenna (see diagram) followed by a self-adjusting comparator (a TS881) acting as a detector. The resistors of the comparator act to load the demodulator capacitor, and the various time constants are set to reject any AM from the TV modulation while allowing through AM at the desired data rate.
In practice the circuit can extract a signal that is invisibly buried in the TV signal.
The squared comparator sends its output to a MSP430 microcontroller for preamble correlation, header decode, data decode, and packet cyclic redundancy checking (CRC).
The modulation scheme chosen – at least one transition per bit and two transitions if the bit is a ’1′ – gives a fairly even stream of 1s and 0s during transmission.
The anti-collision scheme involves listening before transmission until the comparator output is largely 1s, or largely 0s, which means no nearby node is transmitting. This is not computationally intensive and needs little power.
Sending the message involves transmitting a 10101010 sequence before the traditional pre-amble to give receivers a good chance of synchronising. Then the 64bit pre-amble, address, and 96bit data are sent.
A range of 45cm was demonstrated indoors and 65cm outdoors, and at up to 10km from the TV transmitter.
Power consumption of the comparator and microcontroller is under 1µW most of the time.
The scheme is described in a paper: “Ambient backscatter: Wireless communication out of thin air“.
The prototype nodes also include three touch pads and an LED (flashing only briefly) for interactivity, and were limited by the microcontrollers need for 1.8V.
In a proposed smartcard application, the team suggest power would be shared: transmit modulator <1%, demodulator 1%, power management 8%, LEDs and touch sensors 26%.
The original idea for the power source is described in the paper: ‘Experimental results with two wireless power transfer systems‘.