Wireless data transfer has become commonplace in recent years as the number of electronic devices which communicate with each other has increased.
Wireless communication links can be direct “line-of-sight” communication links in which there is a direct path between the transmitter of one device and the receiver of the other device. In such communication links, the receiver must be located within the path of the signal used to transmit data between the devices. Of course, the path of the signal may not necessarily be a straight line between the transmitter and receiver—the signal may, for example, be reflected off one or more surfaces. An example of a “line-of-sight” communication link is an infra-red communication link in which an electromagnetic wave is generated at the transmitter in the infra-red spectrum of the electromagnetic spectrum (“an infra-red signal”). In such communication links, the transmitting device will generally “know” that it is transmitting to a given device if there are a number of possible receiving devices in its vicinity. This is because the transmitting device will have to be directed towards the given device for communication to be possible.
However, short range communication links may be direct communication links in which there does not need to be a direct “line-of-sight” signal path between the transmitter and receiver (“a non-line-of-sight communication link”). The transmitter does not need to be directed in any particular direction for a signal to be received by the receiver. An example of a non-line-of-sight communication link is a short range radio frequency (RF) communication link in which an electromagnetic wave is generated at the transmitter in the radio frequency range (“an RF signal”). In such communication links, a transmitting device cannot “know” with which receiving device it is communicating unless the receiving device has been previously identified.
Radio frequency identification tags (RFID) use short range RF communication. A memory tag is an example of an RFID tag.
RFID tags come in many forms, but all comprise an integrated circuit on which data can be stored and a coil which enables it to be interrogated by a transceiver that also powers it by means of an RF wireless communications link. Some RFID tags include read-only memory (ROM) and are written to at the time of manufacture, whilst others have read and write capability.
RFID tags incorporate a number of elements. These include an antenna which couples inductively with an antenna in a tag transceiver, an RF decoder for decoding radio frequency signals received via the antenna, a processor for processing the received signals and an area of non-volatile memory. A voltage regulator in the processor operates to provide a constant voltage for powering the RFID tag.
In many situations, there will be no direct “line-of-sight” communication path between the RFID tag and the tag transceiver. For example, the RFID tag could be contained inside an article or piece of clothing. The RFID tag can also be inserted into sheets of paper or card which are then stacked together so that there is no direct “line of sight” communication path between individual tags in the stack.
FIG. 1 shows an existing wireless communication system employing an RFID tag.
A transceiver 10 includes a radio frequency (RF) generator 11 and a data source 12. Digital data from the data source 12 is amplitude modulated in a modulator 13 onto an RF carrier output by the radio frequency generator 11. The resulting RF signal 15 is output from the transceiver 10 via a transmitter antenna 14.
A receiver 16 receives the RF signal 15 via a receiver antenna 17. The transmitted data is extracted from the RF signal 15 in demodulator 18 and passed to processing circuitry 19 in the RFID tag. In addition, the RF signal is passed through a diode 20 and across a capacitor 21 to generate a DC voltage. The DC voltage is passed through a voltage regulator 22 to generate a constant output voltage to act as a power supply for the processing circuitry 19.
As will be appreciated, there may be a number of RFID tags in the vicinity (and within communicating range) of a tag transceiver. It is desirable to be able to identify each tag independently so that data can be written to and received from an identified tag and subsequent communication can be directed to and from an identified tag.
One example of a situation in which it is important to be able to identify RFID tags is when they are stacked on top of each other, for example in memory tag enabled paper. It is important to be able to identify the uppermost tag in a stack of paper and the order of other tags in the stack so that data can be written to and received from the uppermost tag or a tag at a given position within the stack.
One problem with using RF signals to identify the proximity of tag to a transceiver is that dedicated circuitry is required within the tag to generate a received signal strength indicator (RSSI). Minimising the components used in a tag is important to reduce the cost and power consumption of individual tags—there is often no internal power source in memory tags.
Another problem with using RF signals to identify the proximity of tag to a transceiver is that misleading information may often be generated simply by relying on the RSSI because, although a particular device may be physically closer to an interrogator, it may be shielded by an obstacle that reduces the received signal strength which would indicate that it was further away than another device which was not shielded by the obstacle and which was, in fact, located further away.
Existing techniques which are used to distinguish between wireless devices provide a unique identifier, e.g. a Media Access Control (MAC) address or an Internet Protocol (IP) address, for each device. However, these enumeration techniques do not provide the proximity of the wireless device. In fact, such enumeration techniques require large computing overheads and set up time. For example, an IP address (using version 6—IPv6 from The Internet Engineering Task Force) requires 128 bits of memory to store the IP address and substantial computing power to enumerate such an address.
The small size and power constraints of RFID tags means that it is desirable to minimise the computing power required to enumerate the address of a wireless device.
Using a unique identifier means that each wireless device has to be pre-programmed with its unique identifier during manufacture which increases its manufacturing cost.