Radio Frequency Identification


Why WiFi RTLS systems are expensive and imprecise

In recent years, RFID systems have developed rapidly. Advances in digital electronics and RF led to making the systems smaller and less expensive because of the increased integration capacity of chips. Performance and ability to overcome the wired architectures have been the main driving forces behind this rapid growth. A typical example of this is the WiFi technology now widely used.

Meanwhile, another technology, radio frequency identification (RFID), an old technology first used during World War II underwent a technological upgrade and became used to locate equipment in factories, monitoring equipment, identify products in supply chains. Logistics, security, privacy staff are areas receiving multiple possible uses of RFID and RTLS.

There are two technology trends when it comes to RTLS: the systems developed around the 802.11 WiFi and those that use proprietary technologies specifically designed to identify and locate people and equipment, such as Purelink DILS technology.

Some commercially available WiFi systems are attractive due to seducing commercial speech that make you falsely believe that your few access points WiFi are sufficient to implement an RTLS application.

Any RTLS system requires at least three receivers to receive the signal from the tag so that calculation software can estimate its position. Wireless systems are subject to the same limitations. In addition, because of the heavy protocol required on WiFi, WiFi systems are being confined to a few tens of deployments and tags are becoming very expensive facilities.

WiFi RTLS System is very greedy in Wireless Access Points

WiFi and Purelink's Infrastructure for RTLS

WiFi Infrastructure used for RTLS application vs. Purelink dedicated RTLS.

The main advantage claimed by the suppliers of wireless tags is that you can use your existing WiFi infrastructure for tracking and minimizing deployment costs. However, wireless networks are typically designed to carry voice or data using an access point. To locate a device by triangulation, it must remain in contact with at least three access points at all times. To convert an existing wireless infrastructure so that it can be used as a RTLS network, your wireless network needs to be fundamentally changed and a large number of wired access points should be added, multiplying infrastructure cost by 3 or 4 times.

Adding 4 to 5 times more WiFi access points lead to severe network problems. Indeed, the optimization settings for voice communications and data are not compatible with those of an RTLS application. In addition, issues of channel management for WiFi coverage overlapping led to heavy homelessness and channel management. The high density of access points in a small environment can quickly overload the network and generate cascades of failures. In some cases, it is not uncommon for RTLS system failures, which eventually degrade the transmission of telemetry data and voice.

We must also consider that the WiFi access points transmit relatively high powers. An increase in the number of access point in the environment inevitably increases the density of high power transmission in the environment.

With Purelink RTLS system, Purelink location receivers connect to your existing Ethernet and WiFi infrastructure. All tag/badge receiver communications occur at a separate network infrastructure layer. So your WiFi network maintains its integrity and stability. No need to invest to quadruple the number of existing access points. As PDAs, WiFi phones, PCs and other telemetry devices in your infrastructure, few Purelink location receivers are seen as mere network equipment thus preserving the capacity of your current network for your applications.

system architecture schema: Wi-Fi and Purelink Tracking

RTLS comparison. WiFi systems allow only a very limited number of tags while any Purelink receiver can process up to 10 000 tags, badges, every second.


Location and Tracking Systems are based on three fundamental physical parameters: time, strength or amplitude of the received signal and / or angle of arrival of signals to implement the various techniques and location algorithms such as TOA, TDOA, AOA or RSS.

In indoor environments, due to such phenomena as the reflection of RF signals, the attenuation of RF waves, the techniques of power measurement and angle of arrival result in errors and poor precision in estimating the positions with algorithms. Therefore, the only way to get a precise and accurate position, is to have a precise time measurement. An error of 20 nanoseconds in the time measurement leads to an average error of 15 meters in position, tracking systems must be capable of measuring time with an accuracy of about one nanosecond.

A typical example is the GPS system that requires atomic clocks on board satellites in order to achieve an accuracy sufficient time. WiFi technology was designed to optimize the flow of data between office equipment, computers and peripherals, not for RTLS technologies or for active RFID tags and badges.

To resolve this problem, WiFi system providers will integrate infrared sensor into their tags and ask customers to install in every room and corridor, dozens of infrared transmitters so that WiFi tags can recognize where they are and provide this information when activated. In addition, staff must take the habit of always placing the tags in view, without obstruction, so that they can detect at all times the signals from infrared transmitters placed throughout the building.

Access point performance schema

Typical performances of IEEE 802.11g at 54 Mbps

Purelink location systems require only a few receivers to cover the floors of a building and locate thousands of badges or tags per second, with an average error of ± 2 meters. Without multiplication of WiFi access points, without Exciter, without installing infrared transmitter in each room!

WiFi standard protocols, designed for the transport of Internet data packets are complex and limited by the number of simultaneous connections to an access point. Physical and middle layers of access points defined by the 802.11 standard require sophisticated mechanisms to prevent data loss, data collisions and to effectively manage the flow between users. The management of collisions and interframe spacing (IFS) significantly reduce the bandwidth due to the high number of devices that connect to the network. As with Ethernet hubs, wireless network performance is reduced when the number of users exceeds a certain number.

To ensure a minimum traffic flow, most manufacturers recommend having less than 100 devices on a single WiFi access point. Purelink real-time location system is designed to handle a large number of badges and tags. Each receiver can handle up to 10 000 tags / badges per second. And because Purelink products are specifically designed to locate people and equipment you do not have to invest hundreds of thousands of dollars to install dozens of access points to make your wireless infrastructure capable of supporting your application location of equipment. Do not invest $ 500 000 in infrastructure to locate 200 intravenous pumps! Visit the Build Your Application page and compare!

WiFi and Purelink precision shema For RTLS

Path and granularity of WiFi system compared with Purelink system.

Location precision and frequency

Purelink system provides real-time location and allows a better positions update frequency. Assuming that a person is traveling at a speed of 5 km / h (average walking speed of a person), a WiFi tag that transmits at a rate of once per minute, give a position every 85 meters! In comparison, with Purelink location system, a badge transmitting every second and associated with a person walking at a speed of 5 km / h, the position of the person is updated every 2 meters. Compare features, price and quality of Purelink products with other products on the market.

Battery life

WiFi tags have high energy requirements that limit their autonomy. When these tags pass once per second, their battery can only last four hours. Therefore, the building has to be browsed regularly to look for tags that the batteries are discharged and recharged them!

To resolve this problem, WiFi tags must transmit less frequently. WiFi tags are forced to transmit only once per 10 minutes or only when in motion, making them ineffective for dynamic applications.

Purelink tags and badges, when programmed to transmit every second, function, irrespective of movement, for 5 consecutive years without changing batteries.

Small size, low price but high performance

Active RFID tag size Comparison

Size comparison between a cell phone and PUrelink's location badge

Because of their complicated electronic and high energy requirements, WiFi tags are the size of a cell phone, with a thickness of 2 cm (3 / 4 inches). Most of the thickness for the battery that has to be recharged periodically.

Purelink tags and badges are the size of a credit card with a thickness of 4 mm (5 / 32 in.) thick, is 5 times thinner than WiFi tags. This allows to use them in many applications. Their efficient design also allows for more attractive prices, five times less than the WiFi tags. Visit the Products section to select the tag or badge for your application.

Wi-Fi RTLS systems are subject to numerous limitations. Constraints of communication protocols required by WiFi have consequences for high energy consumption and low positional accuracy of WiFi tags. The number of tags, the battery life, size and price reduce their use in applications that require precision, a rapid position update, with a limited budget. WiFi RFID systems can reach a certain level of efficiency in a small-scale deployment, with a limited number of tags and sensing applications by area.

You can choose the simplicity by adopting the performance and affordability of Purelink products. Visit the Build your application page or the Products section for more information.

Comparative table

Characteristics Purelink WiFi
International regulation ISM-UNII compliant ISM compliant
Temporal resolution 3 ns 40 ns
Maximum position accuracy on 2D map +/- 1.5 m +/- 15 m
Algorithms TDoA, RSS, Fingerprinting RSS
Tag-receiver communication 5.8 GHz 2.4 GHz, 802.11 b/g
Signal bandwidth 25 MHz, channel 161 of 802.11a 40 MHz
System capacity 10 000 tags/seconde 300 tags/seconde
Update position frequency 1/seconde per tag 1/15 min per tag
Battery life 5 years at 1 transmission/second 4 hours at 1 transmission/second
Infrastructure required to cover 10 000 m2 4 location receivers 44 WiFi access points, 200 Exciters, 250 infrared sensors
Security High Low
Badges and tags price 25.95 $/unit 95 $/unit
RTLS Software Software suite with database, multiple functions for security, logistics and safety applications Slow interfaces and tables in webpages. Require consulting to deploy expensive web servers and program applications