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How the New 802.11n WiFi Standard Will Impact RTLS

IEEE 802.11n is a forthcoming high-speed wireless networking standard being developed as the next generation of Wi-Fi technology. 802.11n is supposed to be backward compatible with legacy 802.11a/b/g products, but features new in 802.11n access points could impact how legacy WiFi-based RTLS systems perform.
Tags: Standards
Oct 30, 2007This article was originally published by RFID Update.

October 30, 2007—In algebra, "n" is commonly used to express the unknown. It's also a bit of an unknown in the real-time location systems (RTLS) community. Specifically, how the forthcoming IEEE 802.11n wireless networking standard will impact WiFi-based RTLS networks.

Problems aren't widely expected, but changes in the way 802.11n access points process RF signals potentially could impact RTLS performance.

802.11n is a new high-speed wireless networking standard that is being developed by the same Institute of Electrical and Electronic Engineers (IEEE) committee that developed the "WiFi" series wireless networking standards, notably 802.11a, 802.11b, and 802.11g. Final IEEE ratification is not expected until the second half of 2008, but close to 100 pre-standard 802.11n products have already been certified as interoperable by the Wi-Fi Alliance.

The 802.11n standard is based on multiple-input, multiple-output (MIMO) technology, which is a major departure from legacy 802.11 wireless standards. 802.11n access points contain multiple RF antennas, which are used to split data streams for faster, more efficient transmission. The 802.11n standard pledges data rates of 300 Mbps, which is about six times higher than current 802.11a and g technology, and about 30 times higher than 802.11b.

MIMO is intended to eliminate transmission problems resulting from the "multipath" effect, in which signals reach the antenna from multiple paths caused by interference.

"The 802.11n specification was made to use the bounces of multipath waves," Tim Harrington, vice president of technology strategy at RTLS provider Zebra Technologies-owned WhereNet, told RFID Update. Prior to joining WhereNet, Harrington worked at Symbol Technologies (now part of Motorola), where he helped design, commercialize, and standardize some of the first enterprise wireless LAN systems. "Typically, multipath is your enemy for wireless transmissions. With MIMO, multipath is your friend. However that's not necessarily what you want for RTLS," said Harrington, explaining that MIMO technology affects the path of the radio signal, and RTLS determines an asset's location by analyzing the communication between its tag and the wireless access point.

There are two general RTLS techniques: received signal strength indication (RSSI) and time difference of arrival (TDOA). RSSI uses algorithms to determine a tag's location based on the strength of its signals received by wireless access points at known locations. TDOA works by measuring the time it takes for signals to be received. Most WiFi RTLS systems are based on RSSI. WhereNet offers both WiFi and non-WiFi standardized RTLS solutions, but favors non-WiFi, ISO 24730-standard RTLS, which uses TDOA.

WiFi infrastructure and RTLS providers say legacy RTLS tags should perform as well or better on 802.11n networks.

"We certainly wouldn't expect 802.11n to have an adverse effect on location accuracy," Peter Thornycroft, technology advocate at WLAN technology provider Aruba Networks, told RFID Update. WiFi-based RTLS providers AeroScout and Ekahau each told RFID Update they are testing 802.11n access points with their legacy RTLS technology now and expect no adverse performance.

"Today, we're seeing there is really no measurable difference," said Tuomo Rutanen of Ekahau. "There should be no issues [with 802.11n]."

Thornycroft hypothesized 802.11n could actually improve RTLS location accuracy because the increased range might cause more access points to locate each tag, providing more data to calculate the position.

Wireless LAN leader Cisco Systems did not provide comment or information for this article in response to RFID Update's requests. The current 802.11n draft specification is based on a protocol submitted by Cisco, which has released pre-standard 802.11n products and has existing partnerships with WiFi RTLS providers.

The WiFi RTLS market is booming (see WiFi Tag Market to Grow 100% Per Year Through 2010), with current sales made up entirely of 802.11b and g tags. All vendors contacted said no customers or prospects have asked about 802.11n RTLS or expressed concern about future compatibility problems. Vendors contacted said they have no current plans to make 802.11n-based RTLS tags, in large part because current 802.11n radios consume much more power than 802.11b/g units, which would reduce the battery life on RTLS tags. Tag battery life is a key performance characteristic.

802.11n will attract ever-increasing attention as more pre-standard products are released and ratification draws nearer. Market analysts think it may initially find a niche in home networking, with healthcare holding potential as an early adopter in the enterprise market because many hospitals are wireless and there is desire to communicate large medical images. Healthcare is a leading market for RTLS, so early mixed 802.11b/g/n systems could first appear there.

"Are customers asking us about 802.11n RTLS? No, but perhaps the better question is: 'What should I be doing to prepare for 802.11n? How should I design my n network, and how will it integrate with my a/b/g networks?'" said Thornycroft. "The RFID tag will certainly be part of the discussion."
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