Battery - Powered RFID Deva

In this article, we compare the different types of batterypowered RFID tags that are available today and explore several emerging wireless standards that are candidates for active RFID tagging. We also present an active RFID platform we have developed and a supply chain application implemented on that platform. INTRODUCTION Passive (battery-less) RFID technology has been the primary candidate for tagging of cases and items, where the tags are not reused and the cost of tags is critical. However, in situations where the performance of the RFID tag is critical or a long read range is required, battery-powered RFID tags are preferred. Examples include tagging shipping containers, freight packages, valuable assets, or applications requiring sensors, such as cold-chain shipping, and asset tracking [8]. RFID tags with an onboard power source are also necessary for data-logging applications in which tags must operate independently of the reader. The use of battery-powered RFID tags is rapidly increasing due to two important trends: • The emergence of inexpensive CMOS Radio Frequency Integrated Circuits (RFIC) and microcontrollers now enables very low cost tags. • The cost of batteries is dropping. More interestingly, lowcost thin-film batteries are becoming available, which enable battery-powered tags to be manufactured in the form of thin cards or adhesive labels called the Smart Active Labels [12]. In this article, we compare the different types of batterypowered RFID tags that are available today and explore several emerging wireless standards that are candidates for active RFID tagging. We also present an active RFID platform we have developed and a supply chain application implemented on that platform. TYPES OF BATTERY-POWERED RFID TECHNOLOGIES When considering a battery-powered RFID tag, the two basic options are active RFID tags and semi-passive RFID tags. Battery-powered active RFID is a relatively old technology (Mario W. Cardullo claims to have received the first U.S. patent for an active RFID tag in 1973 [1]). Modern-day active RFID tags are essentially miniature radio beacons, and contain both a radio transmitter and a radio receiver circuit. Historically, the military and animal researchers have used such devices to identify and track airplanes, people, or wild animals. Semi-passive, like passive, RFID tags do not contain any radio transmitter circuit, but simply reflect back a small fraction of the power, which is emitted by the RFID reader. Not surprisingly, this technology grew our of radar research and some of work done by the military in the 1950’s and 1960’s on passive antenna repeaters. In the 1970’s the Department of Energy and IBM later developed a miniature version of such backscatter tags for tracking valuable assets, such as radioactive materials. In the 1980’s this technology was later adopted for use in highway toll collection systems which gave rise to popular brands such as “EZ-Pass”, “Sun-Pass” etc. It is important here to consider the various technical factors that distinguish semi-passive and active RFID technologies. The major factors are listed below. Advantages of Semi-Passive RFID over Active RFID • EPC compatibility – The primary advantage of a semipassive RFID tag over active RFID is that it can be used with existing passive RFID infrastructure. Since both passive RFID and semi-passive RFID use a backscatter mechanism to communicate with the reader, a passive RFID reader does not need to distinguish between these two types of tags. There exist long-term plans for EPC Global to develop an EPC standard for active tags as well (Class 4 or Class 5); however, this will probably not happen for some time. • Lower Tag Cost – Since a semi-passive RFID tag does not require a radio transmitter circuit, the cost of the electronic tag chip can be less than that of an active RFID tag. However, single chip solutions are now available for both, and the difference in cost is small relative to the cost of the battery, for example. It should also be noted that the active RFID chips are also being used in other industries such as sensor networks and manufactured by many vendors; thus in the near future, due to the economies of scale, the cost of an active RFID chip may actually be less that the cost of a semi-passive RFID chip. Advantages of Active RFID over Semi-Passive RFID • Lower reader costs -It is important to point out that the cost of an active RFID infrastructure is generally much less (10X) than the cost of a passive RFID system, because the active RFID tags have longer range and the active RFID readers do not require any high-power radio circuitry. • Longer reading range – Because active RFID tags contain a radio transmitter, their signal falls as the square of the distance (1/r). However, in the case of semipassive tags, which rely on the tiny signal reflected back from the tag, the signal drops off much more severely as the fourth power of the distance (1/r). Therefore at the longer reading distance of 50 feet or so, the signal from a semi-passive tag becomes extremely weak, requiring a very sensitive reader to detect it. It should be noted that the use of the battery on a semi-passive tag helps to extend the range at which the tag will power on; however the battery does not help increase the range of the backscatter signal from the tag, which is primarily dominated by the radar cross section and geometry of the tag antenna. • More robust performance in real environments – Although in the laboratory it is possible to achieve a reading range of 100 feet or more from a semi-passive tag, this is much more difficult in a real environment. Because the signal from the semi-passive tag is so small and fragile, line of sight is often required to achieve the longer reading distances. In the case of an active RFID tag, line of sight is not required, since the tag transmits its own radio signal. • Dense reader mode The case of multiple RFID readers presents a special challenge for semi-passive tags, and special care must be taken to ensure that a semi-passive tag will not be inadvertently disabled by seeing multiple readers simultaneously. Active RFID tags do not have this problem, since it is easy to implement the medium access control protocols (such as Carrier Sense Multiple Access Collision Avoidance (CSMA-CA)) as the communications between readers and tags are symmetric. • Lower radiated power – Because the active RFID tags actually transmit their own radio signal, it is not necessary for the RFID reader to transmit a large amount of power (4 Watts in the US) as a semi-passive RFID reader does. In the case of an active RFID system, the RFID reader may transmit just a few milliwatts of power, which is sufficient to communicate with a tag that is over 100 feet away. Lower radiated power means less electromagnetic interference and this makes it easy to use multiple readers in dense configurations. Moreover, very low-power (and small) handheld readers can also be made for active RFID tags. • Longer battery life – It is often surprising that in most applications, active tags actually have a longer battery life than a semi-passive tag. Although an active tag does require power to transmit, the amount of time that the tag transmits a radio signal is very short. Most of the time (99% or more), the tag is sleeping or taking sensor readings, and not transmitting at all; it is this steady state mode, which actually dominates the battery life of a tag. When the tag does transmit its information, the active tag simply sends its data packet asynchronously and then goes back to sleep; however the semi-passive tag must communicate back and forth with the reader to exchange commands synchronously, which may require the tag to be on for a longer time, even though less power is needed for this function. In addition, an active tag can be programmed to sleep or wake up upon specific events and can asynchronously transmit its information to the reader; as a result, the active tags do not need to be constantly listening for a signal from the reader, which also preserves battery life. Lastly, the modern-day active RFID tag ICs support variable transmission powers, and a tag can lower its transmit power according to the commands from the reader, which further adds to battery life. • Smaller tag size – The size of a battery-powered tag is dominated by its antenna. The emerging standards for active RFID tags at 2.4 GHz enable the antenna to be less than half the size of a semi-passive tag operating at UHF frequencies. • Compatibility with worldwide standards – The most recent active RFID standards (IEEE 802.15.4) have adopted primarily 2.4 GHz as their operating frequency. This frequency is available worldwide. In contrast, semipassive tags operate at UHF frequency bands whose applicability varies from country to country. CHOICE OF TECHNOLOGIES FOR ACTIVE RFID Although the standards for semi-passive RFID tags is becoming well established through the EPC Class 1 Gen 2 protocol and emerging standards for Class 2 and Class 3, the standards for active RFID have only recently been adopted. Until recently, the majority of the active RFID protocols have been proprietary with no interoperability between manufacturers. Fortunately, this is rapidly changing with the adoption of worldwide radio communications standards in a variety of industries outside RFID, such as consumer electronics and industrial automation. Since EPC Global has been slow to adopt a standard for active RFID, many companies have been looking to adopt other existing worldwide standards for use in active RFID. For this reason, we devote the remainder of this article to discussing the various commercial options for active RFID tags. Due to the recent advances in CMOS fabrication technologies, it has become possible to manufacture advanced microcontrollers and RF communication ICs at very low cost. More significantly, the availability of lowcost devices has led to the development of several shortrange (< 100m) wireless communication standards. In particular, the following international standards are now available for building active RFID systems: