RTLS over Wi-Fi

…guest post from Ahmad Nassiri

RTLS, RFID, location services/tracking, location analytics, or simply location are terms that have been talked about quite often in the last few years.

It is notable that this trend did start with increase in deployment of Wi-Fi networks even though there were other technologies available like Radar, Zigbee, Bluetooth, Infrared and Ultrasound.

It won’t be wrong to say Wi-Fi technology has played a great role in deployment of RTLS solutions mainly in healthcare, retail, education, transport and manufacturing sectors.

Wi-Fi is almost everywhere these days and it is this existing infrastructure the 802.11 based RTLS solution leverages to provide location and asset tracking services. This helps businesses in saving costs associated with deploying and managing a fully dedicated redundant RTLS system.

Implementation of RTLS systems has helped businesses to increase efficiency, better management and monitoring of their assets through inventory control, staff productivity and security of personnel and clients, thus resulting in an efficient and enhanced workflow. Location data captured from tracked devices can provide insight and other useful information that can be leveraged for analytics. For example in retail industry targeted marketing material like promotions and discounts are pushed to client devices, in hospitals staff not only track their equipment but also check medical supply levels and expiry dates to arrange replenishment and restocking.

RTLS and RFID are not the same thing! The terminologies used within the RTLS space are mostly confused, so let’s get the terminology right first.

While RTLS (Real Time Location System) is the actual solution that provides location details of an asset, RFID (Radio Frequency Identification) on the other hand is what RTLS solutions use to track the location of an asset. RFIDs are fairly small electronic devices with antennas in the form of a tag, chip, label and sticker that is attached to an asset for tracking purposes.

There are two types of RFID tags, passive and active.

Passive tags comprise of an antenna, transceiver and transponder. The passive tags have no internal power and are activated when they are near an RFID reader. The antenna is a very important component, the bigger the antenna size the better the range.

Active tags are the same as passive but they are powered by internal batteries and or solar and can continuously broadcasts beacons and communicates with the RFID reader providing their location details.

Whether passive or active, tags transmit information over radio waves that are read by a reader or interrogator. The reader communicates the location information to a location/positioning engine or database which in turn is fed to an application management software that displays the location information of the tag on a floor map. Other information generated includes alerts, analytics and reports regarding the movement of the tag. Some vendors provide software agents that are installed on client devices which works like a tag and enables location tracking. Vendors also provide open Application Programming Interface (API) for integrating with third party systems/applications.

Since our focus is on RTLS services provided over Wi-Fi technology let’s look at some of its artifacts.

There are a number of design requirements on the wireless network that needs to be addressed in order to support RTLS services.

First of all the wireless network supporting the RTLS solution must be stable and highly available. In mission critical organizations such as hospitals this becomes a very important factor, locating medical equipment and personnel in an emergency situation.

Placement and density of wireless APs (Access Points) is very important and must be designed carefully. APs are to be installed within and at the perimeters of the coverage area to form a triangulation pattern and to be able to calculate and locate assets. Assets must be detected by at least three APs for determining its location.

The signal quality required on the floor by RFID tags is different among vendors and must be confirmed, though the range of -67dbm to -75dbm is very common.

Wi-Fi based RTLS solutions uses received signal strength indicator (RSSI) and time difference of arrival (TDoA) mechanisms in determining location of assets.

In most cases a wireless network designed for Voice services meets the requirements for the RTLS with some additional APs installed on the perimeters. That said, there are instances where the design of an RTLS grade wireless network alongside Voice becomes challenging due to additional number of APs, network overhead and interference. In such cases, some AP’s operational mode can be changed to ‘listen only or scanning’ to avoid Co-Channel Interference (CCI). This also depends on each site’s Radio Frequency (RF) characteristics and must be examined prior to full deployment. On the other hand ‘Listen or scanning’ mode APs may cause coverage hole issues, therefore if APs are operating in both frequency bands (2.4GHz and 5.0GHz) can have their 2.4GHz radios on ‘listen only’ mode and allowing the 5.0GHz radios to serve clients.

RTLS solutions benefit most from high density wireless infrastructure due to additional APs and high availability factors.

In multi floor deployments care must be taken not to stack APs in the same spot on all floors as this results in CCI and causing the assets location reported on the wrong floor. If there are limitation in placement of APs, attenuation factors between each floor must be studied and tested to recommend optimal AP transmit power.

Calibration of the wireless network provides a better level of location precision and is highly recommended to be performed for RTLS services.

The following are some general guidelines to consider:

  • Define business objectives and value in investing for RTLS services
  • Level of location accuracy or proximity must be addressed and tested prior to full deployment (Do not believe vendor’s marketing jargons, test and prove it for yourself. As often products perform different in a Lab environment than in real life situation.)
  • Engage vendors in a proof of concept (PoCs) before full deployment to confirm functionality, integration and interoperability of all network components, existing, new and third party. This will minimise the blame game among stakeholders
  • Overall cost and specially the cost of tags, additional APs, application, installation and user training must be addressed
    (Cost and quality of tags depends on the value of the asset tracked, do not go cheap and assume it will work fine)
  • Select the right type of RFID tags for the right equipment and environment, active or passive tags, duress system pendants, labels etc.
  • Study the vendor blue prints and incorporate requirements in the design, for example RSSI range and elevation/height of the ceiling. Tags may not be able to communicate if AP are installed high up, typical height is 5 – 6 meters
  • Battery life of active tags and timely procedure for replacement
  • Upgrade firmware versions and ensure compatibility
  • Placement or mounting of tags on the assets must be done carefully. I have come across instances where incorrect adhesive used to affix the tags causing it to fall, environmental factors like wet and humid conditions affects the adhesive, (quality does matter)
  • Additional licensing for the existing wireless infrastructure to enable RTLS services
  • Allow for failover, redundancy and future proofing in all levels
  • Regularly monitor and perform surveys to ensure compliance and optimal performance.