The physical layer defines how bits are sent between the RFID reader and
tags. Much of it uses methods for sending wireless signals that we have seen previously.
In the U.S., transmissions are sent in the unlicensed 902–928 MHz ISM
band. This band falls in the UHF (Ultra High Frequency) range, so the tags are
referred to as UHF RFID tags. The reader performs frequency hopping at least
every 400 msec to spread its signal across the channel, to limit interference and
satisfy regulatory requirements. The reader and tags use forms of ASK (Amplitude
Shift Keying) modulation to encode bits.
They take turns to send bits, so the link is half duplex.
There are two main differences from other physical layers that we have studied. The first is that the reader is always transmitting a signal, regardless of whether it is the reader or tag that is communicating. Naturally, the reader transmits a signal to send bits to tags. For the tags to send bits to the reader, the reader transmits a fixed carrier signal that carries no bits. The tags harvest this signal to get the power they need to run; otherwise, a tag would not be able to transmit in the first place. To send data, a tag changes whether it is reflecting the signal from the reader, like a radar signal bouncing off a target, or absorbing it.
This method is called backscatter. It differs from all the other wireless situations we have seen so far, in which the sender and receiver never both transmit at the same time. Backscatter is a low-energy way for the tag to create a weak signal of its own that shows up at the reader. For the reader to decode the incoming signal, it must filter out the outgoing signal that it is transmitting. Because the tag signal is weak, tags can only send bits to the reader at a low rate, and tags cannot receive or even sense transmissions from other tags.
The second difference is that 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. To send data to the tags, the reader uses two amplitude levels. Bits are determined to be either a 0 or a 1, depending on how long the reader waits before a low-power period. The tag measures the time between low-power periods and compares this time to a reference measured during a preamble. As shown in figure, 1s are longer than 0s.
Tag responses consist of the tag alternating its backscatter state at fixed intervals to create a series of pulses in the signal. Anywhere from one to eight pulse periods can be used to encode each 0 or 1, depending on the need for reliability. 1s have fewer transitions than 0s, as is shown with an example of two-pulse period coding in figure.
There are two main differences from other physical layers that we have studied. The first is that the reader is always transmitting a signal, regardless of whether it is the reader or tag that is communicating. Naturally, the reader transmits a signal to send bits to tags. For the tags to send bits to the reader, the reader transmits a fixed carrier signal that carries no bits. The tags harvest this signal to get the power they need to run; otherwise, a tag would not be able to transmit in the first place. To send data, a tag changes whether it is reflecting the signal from the reader, like a radar signal bouncing off a target, or absorbing it.
This method is called backscatter. It differs from all the other wireless situations we have seen so far, in which the sender and receiver never both transmit at the same time. Backscatter is a low-energy way for the tag to create a weak signal of its own that shows up at the reader. For the reader to decode the incoming signal, it must filter out the outgoing signal that it is transmitting. Because the tag signal is weak, tags can only send bits to the reader at a low rate, and tags cannot receive or even sense transmissions from other tags.
The second difference is that 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. To send data to the tags, the reader uses two amplitude levels. Bits are determined to be either a 0 or a 1, depending on how long the reader waits before a low-power period. The tag measures the time between low-power periods and compares this time to a reference measured during a preamble. As shown in figure, 1s are longer than 0s.
Tag responses consist of the tag alternating its backscatter state at fixed intervals to create a series of pulses in the signal. Anywhere from one to eight pulse periods can be used to encode each 0 or 1, depending on the need for reliability. 1s have fewer transitions than 0s, as is shown with an example of two-pulse period coding in figure.
Reader and tag backscatter signals.
No comments:
Post a Comment