What are semi-active or semi-passive tags?

These are passive tags, but they also have an on-board battery to boast the transmit power thereby extending the read range. They will read at short ranges even if the battery is dead. They are typically less expensive than active tags and more expensive than passive tags. They increase the read range from 10-30 feet to 100-150 feet. For example, if one put one on a semi-trailer of telephone poles, one could read everything on the trailer as it drove through an exit gate.

Practical considerations

• Local or global: If your application will be used in multiple countries, the choice of frequencies is reduced substantially.
• Liquids and metals: To transmit through liquids, or around metals and corners, the choice of frequencies is again very important.
• Cost of tag vs. interrogators: Some applications are better off using a more expensive active RFID tag that have a low cost integrator if many interrogators are used with only a few tags. Others applications may require the least cost passive RFID tag with an expensive interrogator.
• Data security and encryption: This is an issue with some applications where it will demand semi-active or active tags.
• Middleware solutions: Can cost anywhere from $15,000 to $100,000 and are used to interface between the RFID interrogators and some customer specified database management program.

Lower frequencies up to 433MHz are typically used when the RFID signal must pass through liquids or even the human body. Water absorbs much of the energy in frequencies about 433MHz. LF around 125KHz or HF around 13.56MHz have been used for access control at door entrances for years. Recently, quite a few new UHF tags that are in the 860-960MHz range have been specifically design for use on metals or liquids. This does not mean they will transmit through liquids of metals, it just means they will have reasonably read ranges when adhered or fastened to metals or liquid containers. These “metal mount” tags are more expensive than the generic UHF tags.

Active tags at micro-wave frequencies such of 2.4GHz typically read at have the distance of a UHF active tag. LF and HF tags have typical read distances less than 3 feet. The data rate or speed of transmission is greater with higher frequency tags permitting faster readings. UHF tags are the lowest cost passive tags because this technology has been pushed by Wal-Mart and the U.S. Department of Defense (DoD) to track the shipment of goods.

Meeting country standards

If the RFID tags are going to be used by a single country or region of the world, then the frequency selection is easier. However, if you are going to ship product globally, then the frequency selection is critical. 2.4 GHz, and 13.56 MHz are two frequencies that are the most universally accepted and can be use around the globe.

315MHz can only transmit every 10 seconds automatically in a repetitive mode. 433 MHz is similar unless the power level is low yielding a read distance of about 35 feet.

UHF tags specially designed to meet Wal-Mart’s EID (Electronic Identification) or the DoD’s mandate have different frequencies and power level requirements for various parts of the world. For example, the U.S. allows tags around 915MHz, Europe allows around 868MHz, and Japan allows around 950MHz. Some UHF tags read at all those frequencies and only require a country specific reader.

The following are examples of passive RFID applications:

• LF
  • 125 KHz          Access Control
  • 134 KHz          Ear tags used for dairy cows and pets
• HF
  • 13.56 MHz      Access Control
    • Pharmaceutical item tracking
• UHF
  • 868 MHz         Item/case/pallet tracking in Europe
  • 915 MHz         Item/case/pallet tracking in North America
  • 950 MHz         Item/case/pallet tracking in Asia

Wi-Fi

Wi-Fi is the underlying technology of wireless local networks based on the Instrument Society of Electrical Engineers (IEEE) 802.11x specifications. If there is an existing Wi-Fi network used to interconnect computers or Voice-Over-Internet Protocol phones, then the reader infra-structure may be already in place to read the tags that can track people or assets and transmit sensor information such as temperature. Wi-Fi tags are capable of communicating large amounts of data, making them very robust with automatic retransmission in the present of noise. They can also be connected to remote sensors such as motion, temperature, and vibration. However, these features come at a price of higher power consumption. Wi-Fi tags tend to consume significant amounts of power when they transmit or when listening in receive mode. For long battery life, it is imperative to keep the length of transmission and the frequency of transmission as short as possible. For example, if asset movement is the interesting measurement, then low power motion sensors can be incorporated to wake the tag up when is it moved to a new location. This is also accomplished by incorporating a LF 125-134KHz receiver that triggers or wakes up the tag when in range of the LF beacon. The LF beacon is strategically placed at choke points such as doorways. Another example for Wi-Fi tags is for real-time location systems (RTLS). With the use of many access transceivers that read the Wi-Fi tags simultaneously, the received signal strength indication (RSSI) can be used to determine which room you are in. A typical application would be hospitals to track assets and staff.

IEEE 802.15.4 Star, Point-to-Point sensor networks

This standard specifies the protocol for connecting tag data in star network where one coordinator communicates to each tag directly, or a single tag can talk to a second tag directly. This type of network satisfies over half of the industry needs.

Zigbee

This is also an IEEE standard for connecting sensors much like Wi-Fi interconnects computers. It uses the underlying IEEE 802.15.4 standard and overlays a high level mesh networking protocol. It is meant to transmit short data packets like switch closures and sensor data and can have up to 65,000 nodes, but practically 16,000 is the limit. This technology advertises a low power network for the “end points” that can have security enabled and has redundant pathways for communicating, making it very robust. It is not meant for streaming large amounts data. While the end points can be programmed for very low power, the “routers” or relay transceivers still consume large amounts of power and are normally “mains” powered. Transmitting and receiving still consume significant power that drains batteries quickly.

In some instances, routers can be synchronized to wake up periodically to listen for messages at the expense of increased latency. Latency is the time delay in getting information from the originating node to the destination node. The industry has adopted the 2.4GHz frequency region as the standard for building automation, but frequencies of 868-915MHz can be used for increased distance with slower transmission rates. Remember these lower frequencies typically travel about twice as far as the micro-wave 2.4GHz frequencies but are not globally universal like the 2.4 GHz.

Mesh topology allows the sensor to find the best path to deliver its message to the target node. If a path gets interrupted, it finds a new path by routing around the bad node. This is sometimes refers to a self healing network. There is one coordinator node that acts as the master for the network. Each node has a unique MAC address similar to Internet devices. Up to 240 end points can be connected to a node. 128 bit AES encryption can be enabled for data security. Routers can be placed to relay data back to a coordinator or to a different end point.

Pros

• Up to 240 end points per router node
• AES encryption option for security
• Low power
• IEEE standard
• Multiple vendors

Cons

• Non-end point nodes act as routers or relays and are not low power. Synchronization helps but is still not the total solution for all applications.
• High latency (delay)
• Low bandwidth (small packets of data only)

Zigbee Mesh Diagram

Many vendors are offering a wide variety of systems able to interoperate if they conform to the Zigbee standard. Some systems have a power amplifier that boosts the transmission range at the expense of more battery consumption. For stationary Zigbee nodes, some Zigbee radios are antenna agnostic (dual antennas) to reduce the likelihood of multi-path signal cancellation problems. Zigbee networks have the ability to search for a clear frequency channel and then lock onto it. Alternatively, the channel frequency can be assigned so multiple networks can operate in the same space without interference. Zigbee networks also have a Personal Area Network ID to prevent any stray Zigbee nodes from joining the network.

Real Time Location Systems (RTLS)

RTLS can determine the physical proximity of roaming active tags. This is accomplished by orientating fixed-mount readers at strategic locations where the active tag can be read simultaneously by two or more readers. By doing this, algorithms can determine the location of the active tags by the RSSI level and triangulation. This gives an approximate location within about 10 feet depending on how many reader access points are used. Some companies are using signal phase for “time-of-flight” for increased positional accuracy. For example, several companies use Wi-Fi tags, Zigbee tags, and proprietary 433MHz to track personnel and high value assets in hospitals using this technique. For sub-room accuracy, additional sensor technologies are incorporated into the tag such as ultra-sound or infra-red.

Ultra-Wide Band (UWB) tags for RTLS

UWB tags use higher frequencies in the 3.1-10.6GHz range. These tags send out a very short burst of data greatly conserving battery usage. This allows the use of economical coin cell sized batteries. Again, signal phase for time-of-flight measurement is used to triangulate the position. This works well in large open areas. These tags are only capable of ID and location and cannot transmit sensor data. The software to determine the positioning can be a significant cost in the system. It is suited for precise asset positioning and people movement in high security applications. These high frequencies are absorbed by liquids and are significantly attenuated by walls.

Near Field Communication (NFC)

NFC is a technology that reads tags within close proximity to the reading antenna where secure and discrete data transaction are necessary. Entire countries have implemented this technology for payments using NFC enabled cell phones. Passport systems are another application for this technology. It can also be used to retrieve product data at a trade show by presenting the NFC cell phone close to the reader and requesting the product data. It is fast and efficient.

RuBee

RuBee is an IEEE 1902.1 standard utilizing Magnetic Long Wave signals less than 450 KHz as opposed to the electric field of radio waves. It is a bi-directional, on-demand, peer-to-peer radiating transceiver protocol. This protocol functions in harsh environments with networks of many thousands of tags and has a read range of 10-100 feet. They have the ability to work well in environments with lot of steel and liquids. It reaches around corners and therefore works in environments where other tags fail.

For a specific application, there is an optimal frequency and technology type to use. The RFID field is constantly changing, and it is a challenge to keep up with all the technology improvements. It does not just cover the wireless devices but spans to interfacing to the Ethernet and middleware/software solutions. Many engineering departments simply do not have the focused expertise in the growing area and find it more productive to seek out an expert in the RFID field to help weed through the many choices available.

ABOUT THE AUTHOR

Ronald Pulvermacher (ronp@matrixpd.com) is founder and president of Matrix Product Development. His firm specializes in high tech electronics with a focus on RFID and wireless devices. He is a member of the Institute of Electrical and Electronics Engineers, Inc., Madison Area Business Consultants, and Past President of Product Development, Management Association (PDMA) and holds several patents.