Bits are sent between the RFID reader and tags defined by the physical layer. Many of the methods of its use for sending wireless signals that we have seen previously. Transmissions are sent in the unlicensed 902-928 MHz ISM band in the U.S. . This unlicensed band falls in the UHF ( Ultra High Frequency) range, so the tags are referred to as UHF RFID tags. At least every 400 msec reader performs frequency hopping.
- To spread its signal across the channel.
- To limit interference
- To satisfy regulatory requirements.
ASK ( Amplitude Shift Keying) modulation is used by the reader and tags to encode bits. The link is half-duplex because they take turns to send bits.
Two main differences from other physical layers are as follows.
- Regardless it is the reader or tag that is communicating, the reader is always transmitting a signal. To send bits to tags the reader transmits the signal. The reader transmits a fixed carrier signal that carries no bits, for the tags to send bits to the reader. A tag would not be able to transmit in the first place if the tags do not harvest this signal to get the power they need to run. To send data, a tag changes, whether it is reflecting the signal from the reader, like a reader, like a radar signal bouncing off a target, or absorbing it.
This method is called backscatter. In this sender and receiver never both transmit at the same time. For the tag to create a weak signal of its own that shows up at the reader, backscatter is a low-energy way. To decode the incoming signal for the reader, it must filter out the outgoing signal that it is transmitting. Tags cannot receive or even sense transmissions from other tags. Tags can only send bits to the reader at a low rate because the tag signal is weak.
- Very simple forms of modulation are used so that they can be implemented on a tag that runs on very little power and costs only a few cents to make.
The reader uses two amplitude levels, to send data to tags. Bits are determined to be either a 0 or 1, depending on how long the reader waits before a low-power period. The time between low-power periods is measured by the tags. Tags also compare this time to a reference measured during a preamble. As shown in fig(1), 1s are longer than 0s.
Example of two-pulse period coding in fig(1).-
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