While I understand that a lot of 802.11a/b/g and now 802.11n networks have been sold as "location ready", it is questioned what does this mean by "location ready"? Does this mean that the WLAN appliance has been purchased or the AP infrastructure has been built out? In any case these are some caveats and realities of WIFI based location accuracy and infrastructure costs.
1. Standard AP spacing for 802.11a/b/g for data is approximately 120 ft. Maybe closer together for voice over IP, but in reality VoIP just needs a reliable -65dBm signal while roaming. Yes if you had the location appliance, and WiFi based tags you could locate some equipment, but in reality actual location based accuracy is around the vincity of/(around 4000 to 5000 square ft), that is based upon best of practices for a 802.11a/b/g or 802.11n WLAN for data,( spacing of 120ft more or less), via www.airmagnet.com site survey. (What I would do).
2. Cisco Systems, Inc. recommendation for location accuracy to actually work, means you need an AP to AP spacing of approximately 70 sq. ft. (So essentially almost doubling the required AP count). (See diagram below.) Even then you will probably only obtain an accuracy (WiFi tags and clients), around 10 meters or 30 ft. You will need extra "exciters" (extra in-room readers), to obtain any granularity of location accuracy. This has a ripple effect of when you double your AP count, you then double your port count, of which could mean additional L2/L3 switches in the IDF to handle the increased port count. Let alone maybe a forklift upgrade if you now need a 48 port switch instead of a 24 port switch.
3. For health care institutions to derive any meaningful "location based information", granularity of location accuracy is a must, but there will be a requirement to have this ability of WiFi, RTLS over every square inch of your facility. This way you obtain meaningful location based information that can be applied to improvements in work-flow and process automation. Any less of coverage will defeat the business model. You might as well just go back a departmental based RTLS solution, that over the years, have been a lot of "science projects."
Healthcare is one of the worst environments for multi-path,(1) due to metal, fluids and a lot of moving around. The second figure shows (a) Power Law Propagation, (b), shadowning, for example by two walls(c) multipath fading.
In summary, power law propagaton and shadowing, if determined through calibration and/or reference tags, help with location accuracy, yet shadowing, if unknown, and multi-path hinder locations accuracy.
(1) Multipath propagation occures when an RF signal take different paths when propagating from a source (from a radio client (WiFi tag to destination point (access point). When the signal is en-route, walls, chairs and other items are in the way and this causes the signal to bounce around. A portion of the signal may go to the AP directly, while the rest of signal bouces off of chairs and other items….and then gets to its destination. As a result, some of the signal will encounter delay and travel longer paths to the receive, (AP). Multipath delay causes the information symblos representated in an 802.11 signal to overlap, which confuses the receiver. This is what is called intersymbol interference (ISI). Antenna diversity (two antennas), does help, but multi-path is a reality in healthcare.
It is questioned what the ROI model is like to actually deploy such a WiFi based RTLS solution?
Attachments include Cisco Systems extensive design and deployment guidelines for WLAN RTLS.
Any comments, just let know if I am on base or off-base?
Download Cas-faq Download LocatnDownload Lbswifig_external
