RF Footprint…or Usable Cell

I have heard many engineers many a time speak about manipulating the RF footprint, and in particular decreasing the cell sizes when speaking about high-density wireless. This is all well and good, however, semantics really comes in to play here. First, we have to look at the term RF footprint, typically I think of this as the signal emitted by one internal or external antenna connected to an access point (AP), based on the transmit power levels, the antenna gain, and cable loss (if applicable). You will see when you survey one AP location and you have a -85dBm RSSI level at a certain spot, that you are most likely a good distance away from your AP (yes, yes there are other factors involved, but stay with me here). This is your RF footprint or cell for that particular AP/antenna location, and this should be taken in to account when choosing your next AP/antenna location, depending on your survey requirements. Now, even though this may seem like a weak signal for your client to use, there is still a lot going on within that footprint, especially if you have your lower data rates enabled, such as beaconing.

Now, this leads me in to the term “usable cell.”  I like to use this term when working with customers who may not know much about wireless and RF (for those of you that don’t understand RF…am I right NolanWifi 😊).  When I speak about a usable cell, I am referring to the portion of the RF footprint that the client will be using to transmit data, and this is something that is able to be manipulated through data rates.  Cisco has a good graphic on this for reference, see below.

From: Wireless High Client Density Design Guide – Cisco, May 2018
As you can see from the graphic, the higher the minimum data rate is, the closer the client needs to be to the AP, thereby illustrating one way to manipulate the usable cell.  There are physical ways to do this as well, by using antennas with specifically focused patterns such as a high gain patch antenna that has a narrow beamwidth.  This would also manipulate your overall footprint, while allowing the engineer to focus the usable cell even greater. In conclusion, I use these terms almost every day when speaking with customers and teaching them how to understand what their RF footprint is, as opposed to what part of the footprint is actually used by the client.  Sometimes showing them this graphic helps, other times I get to do my best artist renditions on a whiteboard (only if the caps have been properly placed on the markers). Either way, I feel it is my duty to get this point across so my customers learn about their RF environments, but hopefully become better RF neighbors.

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