As our design complexities increase we need to become more creative in our installations. For years the simple solution has always been to install omni antennas and simply cover an entire area with RF. This was the spray and pray methodology of deploying wireless networks. When we only cared about coverage it didn’t really matter too much where our RF went as long as it was out there. As we move towards higher density networks that are designed for capacity instead of coverage we need to evaluate our design decisions and stop using omni antennas everywhere. As a teacher we relate antenna design considerations to that of a building lighting engineer. As a lighting engineer your goal is to put light where you need it but not where you don’t want it. This means using different types of lights such as floods, spotlights, or sconces. Applying this to wireless that means we need to use omnis, directional, and wall plates.
The Dreaded Hallway
Let’s say for example you are designing for a hotel and the property owners say you can only place them in the hallways. Hopefully you immediately realize this is bad and creates the tunnel effect as shown below.
This is bad because our signal is bouncing down the hallway which can cause problems with ARM. You might come up with a design that requires 6 225’s for example in a row down the hallway, when you do your predictive design or survey you will see your coverage meets your requirements but does capacity and channel usage pass? We know we will have some reuse in 2.4GHz and can easily assume that we will have degraded SNR inside the rooms due to thick walls and heavy loss. This is represented below, looking at RSSI you would assume you have coverage, but ARM would end up reducing power levels changing your predictive design.
So what’s another option to be able to solve this problem but still place APs in the hallways? One possible solution is to use directional antennas alternating down the hallway. You cover a couple of rooms with the back lobe on one side of the hallway and on the other side of the hallway you cover rooms with the front lobe. On the next AP in the hallway you place it on the opposite side of the hallway from the previous and point it in the opposite direction. While you still have signal propagation down the hallway it won’t be as strong as with omnis. You can see the impact of this design below.
I’ve utilized this design quite a few times and it works well when you can do it. I aesthetics are a concern you can always use hidden antennas from 3rd party solutions such as Ventev. The flexibility that external antennas gives you is key in modern designs. Because most hotel rooms also have an existing cable plant you could use wallplate APs such as the 205H.
How have you used external antennas to solve design scenarios?
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