3.3.4 Microwave
Microwave tags operate at either 2.45 or 5.8 gigahertz. This frequency range is sometimes
referred to as super-high frequencies (SHF). Microwave RFID technology has come into use
fairly recently and is rapidly developing. Microwave tags used in practice are typically semipassive
or active, but may also come in passive form. Semi-passive microwave tags are often
used in fleet identification and electronic toll applications.
Microwave systems offer higher read rates than UHF and equivalent passive read ranges.
Semi-passive and active read ranges of microwave systems are often greater than UHF
counterparts. Some microwave active tags may be read from ranges of up to 30 meters,
which is less than comparable UHF tags. However, physical implementations of microwave
RFID tags may be much smaller and compact than lower frequency RFID tags.
There are several downsides to microwave tags. One is that they consume comparatively
more energy than their lower-frequency counterparts. Microwave tags are typically more
expensive than UHF tags. Commercially available active tags cost as much as $25 per tag in
2006.
Another problem is that wireless 802.11b/g (WiFi) networks may interfere with microwave
RFID systems. Devices implementing the upcoming ZigBee 802.15 wireless standard could
also potentially conflict with microwave RFID devices as well.
The ISO 18000-4 and the rejected ISO 18000-5 [11] standards offer respective specifications
for 2.45 and 5.8 gigahertz RFID tags.
3.3.5 Ultra-Wideband (UWB)
Ultra-wideband (UWB) technology applied to RFID is fairly recent. Rather than sending a
strong signal on a particular frequency, UWB uses low-power signals on a very broad range
of frequencies. The signal on a particular frequency used by UWB is very weak, but in
aggregate, communication is quite robust. In practice, some implementations of UWB
operate from 3.1 to 10.6 GHz.
The advantages of UWB are that it has a very long line-of-sight read range, perhaps 200
meters in some settings. UWB is also compatible with metal or liquids. Since the signal on a
particular frequency is very weak, UWB does not interfere with sensitive equipment.
Consequently, an early application was asset tracking in a hospital setting.
A disadvantage of current implementations of UWB is that it must be active or at least
semi-passive. However, since UWB tags broadcast very weak signals, they have relatively low
power consumption. As of 2006, it is unclear whether the technology exists to create a
passive UWB tag2.
UWB RFID technology is still in its early phases and there are few commercial products
available. Costs of US$5 per tag in bulk are reasonable in the near future.