UHF GEN2 RFID channels for Europe and the USA
The UHF RFID transmit and receive frequencies with which timing systems operate are not the same worldwide. In each country or region, the authorities responsible for defining these standards set the frequencies. Therefore, frequencies, number of channel, channel sizes, techniques and powers vary from country to country.
In this article we will focus on Europe and the United States, but first I will explain why the definition of these elements is fundamental to the functioning of tags and RFID readers.
- How does RFID work?
- RFID standars in Europe
- RFID standars in the USA
- Protocol differences between Europe and the USA
- Channels in timingsense timing systems
How does RFID work
Lets go step by step.
- RFID readers placed inside the timing system emit a signal. This signal has enough power to power up a tag at a certain distance, but also commands that the tag will be able to interpret and respond to. The most common command is the one in which the reader asks for all the tags that are turned on in its field of action to be identified.
- The tag’s antenna picks up the energy and the tag transforms it until it is turned on, as long as the energy is powerful enough.
- The tag’s antenna has a very specific size that bounces a small part of the signal it receives. This happens because it is tuned to the frequency of the reader. If it wasn’t, it would bounce nothing at all (or almost nothing). Think of a lock and its key: if they are not the same size, the door will not open. We all remember the problem with the iPhone 4’s antenna when the antenna was short-circuited with the hand, changing its effective dimension.
Therefore, we have a tag that is able to turn on, receive commands and bounce some of the energy because its antenna is an ideal size.
How exactly does the tag give its answer? Very easy.
The tag has a switch, a thicker transistor than the others, that connects the top of the antenna to the bottom. Therefore, it is able to modify the ideal size of the antenna at will and thus bounce or not bounce energy. This means that the antenna splits in two or stays together depending on what the tag wants. The reader interprets these intermittent bounces of energy, receiving the response from the tag. Smart, isn’t it?
Simplifying this whole process a great deal, the result could be understood as something like this:
- When the signal is up, the tag has closed the switch.
- When the signal is down, the switch is open and the tag does not return power.
The signal on the image has a frequency like any digital signal. That digital signal adds up to the signal that the reader has sent and the tag is bouncing. Therefore, we might think that the frequency of the reader’s bounced signal is modified by the cuts made by the tag and its switch.
What is the frequency of the response? More or less at the reader’s frequency plus the size of the channel: in Europe 200Khz and in the USA approximately 500Khz. For example, in Europe, between the first and second maximum power channels, is the response channel of the tags whose readers have asked them on the first channel. Between the second and third channels is the response channel of the tags that have been interrogated from channel 2, and so on.
RFID standards in Europe
In Europe, the authority in charge of regulating RFID standards is the CEPT, although each country has the power to establish what it deems appropriate. For example, the frequency range in the UK is different from the rest.
CEPT is based on ETSI standards. ETSI is a non-profit organisation, with more than 800 members from 66 different countries. Its mission is to provide high quality standards for telecommunications technologies. RFID GEN2 is one of them.
The Gen2 RFID standard is the one used by almost all manufacturers of timing technology: timingsense, MyLaps, RaceResult, Chronotrack, Macsha, Ipico…
In Europe GEN2 consists of 15 channels, but a reader can only broadcast on 4 at maximum power. All the readers we have tested for timing only broadcast on these 4 channels:
- 865.7 Mhz
- 866.3 Mhz
- 866.9 Mhz
- 867.50 Mhz
The channel size is approximately 200Khz, depending on the mode. Talking about this would be enough for another post. To give you an idea, an FM radio signal has approximately this bandwidth.
Maximum power is 2W ERP or 33 dBm ERP. I’ll explain this a little more later.
RFID standards in the USA
In the USA, the authority in charge of regulating the standard is FCC. FCC dedicates 25 channels of approximately 500 kHz bandwidth, in frequencies from 902 to 928 MHz. In addition, the maximum power is 4W EIRP or, in other words, 33.75 dBm ERP.
What is the differnece between the USA and European protocols?
There’s not much difference in power, that’s obvious, but there is in the rest.
First of all, the bandwidth per channel is much higher. I will explain this using a comparison with light and colours. These are the wavelengths of light which, like RFID, is also a wave.
If we had to distinguish two wavelengths, it would be much easier to distinguish two waves of light separated by 200nm than by 50nm. If we set our starting point at a wavelength of 650nm, at 200nm it changes from red to blue, while at 50nm it would hardly show a reddish orange much more difficult to distinguish.
The same thing happens with electromagnetic waves, it is more difficult to filter, although the filters of today’s readers are amazing! In the USA, the higher the frequency, the faster the tag responds and the more time there is for more tags to respond. It’s as simple as that.
Keep in mind that the antenna of the system is not able to send all the power it receives because it is not perfect. The power that is not transmitted by the antenna of the equipment and that bounces back to the reader is much greater than the poor signal that a chip responds to, so the filters of the reader must be very good to distinguish the response of the chip.
The most important part for a timer is how both standards handle channels, as this can affect timing.
In Europe it is very simple. The timer controlling the reader chooses the channel or channels on which to transmit from the 4 maximum power channels with one exception. ETSI only allows transmitting on the same channel for 4 seconds if it detects tags and 1 second if it does not.
After this time, you should stop broadcasting for 100ms. That is, if we are on a fixed channel, waiting for the first athlete one to arrive, the reader is silent for 1/10th of the time until it transmits again.
It works differently in the United States. The channels there change due to FHSS (frequency-hopping spread spectrum). The reader will start on one channel and, after a while, in a pseudo-random way, will switch to another channel releasing the previous one.
The pseudo-random thing is so that all the channels end up using the same time. In this standard, all the user can do is limit the reader’s transmission channels to 16. At timingsense we do not use such a limitation.
How have we solved the emissions limitation in Europe on timingsense systems?
In timingsense we have reduced to 3 channels: left, right and ground.
- The left channel corresponds to the channel at 865.7 Mhz.
- The right channel to the channel at 867.50 Mhz.
- The floor channel to channels 866.3 Mhz and 866.9 Mhz.
Thus, if we are at 866.3 Mhz waiting for the first athlete, if a second passes, the reader will free the channel and switch to channel 866.9 Mhz, without stopping broadcasting at any time. If you want to know more about our TS2 timing system I recommend you to read this post or visit our website. If you have any more specific questions, please contact us.