Both the Active RFID and Semi-Passive RFID tags belong to Battery-powered RFID technology. Where the performance of the RFID tag is critical or a long read range is required, battery-powered RFID tags are preferred. The use of battery-powered RFID tags is rapidly increasing due to the decreasing battery costs, and the increasing inexpensive CMOS Radio Frequency Integrated Circuits (RFIC) and microcontrollers that enables very low cost tags.

Active RFID Tagging Technology

Active RFID tags refer to RFID tags (transmitters) which have their own power source, so they can receive a weaker signal from the RFID reader, and the power source on the tag boosts the return signal. These types can have ranges of many tens of meters and even hundreds of meters, but cost more because of their size and sophistication. Battery life can also limit the life of the active RFID tag.

Semi-Passive RFID Tagging Technology

Semi-Passive RFID Tags are also called as Battery Assisted Passive RFID Tags or Semi-Active RFID Tags. Semi-Passive RFID Tags operate similarly to passive RFID tags. However, Semi-Passive RFID Tags contain a battery that enables longer reading distance and also enables the tag to operate independently of the reader. Semi-Passive RFID tags are the tags with a low-power source (usually a low cost battery) which can be used for on tag sensing, but not to boost range. It is important to understand that the communication in semi-passive tags is still completely passive; no power is transmitted – the tag simply reflects back some of the power emitted by the RFID reader.

Advantages of Semi-Passive RFID over Active RFID Tags

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.

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.

Advantages of Active RFID over Semi-Passive RFID Tags

Lower costs for RFID Readers

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 of Active RFID Tags

Active RFID tags contain a radio transmitter, whereas semi-passive RFID tags, which rely on the tiny signal reflected back from the tag, the signal drops off much more severely. 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

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.

Cheaper RFID Reader with 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 as a semi-passive RFID reader does.

Longer battery life of Active RFID Tags

Active RFID 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.

Smaller size of RFID Tag

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.

References: MIT whitepaper “Battery-Powered RFID” By Deva Seetharam, and Richard Fletcher