Split MR86 antenna over 2 warehouse rows

[GIdenJoe]

I know this topic is a bit controversial when it comes to antenna diversity specialists but I do want an outside opinion.

So I have a customer with an existing wireless deployment in a freezer warehouse environment.  There are some phone issues where calls can drop at the end of some aisles where I found the phone to try to roam from one AP to another while both only have about -85 dBm signal strength which is to be expected.

The environment has one tricky issue and that is that the racks are mobile.  So each rack is 2.5 meters wide and in each grouping of racks there is always one space of 4 meters.  So yes I have checked and there always is a 1.5 meter overlap where I could place an AP to cover 2 potential rows.  In the current setup that consists of Zyxel access points they have put 3x3 AP's where one directional antenna is about 3 meters to the left of the AP and another is to the right.  They have not connected the 3rd antenna slot because Zyxel told them their AP's detect that and go into 2x2 mode.  Since not all antennas are lined up to every possible open row configuration the above mentioned problem happens predominantly when an antenna is not lined up to the open row.

So in a potential new deployment I would like to of course take into account all the rows.

However since Cisco and Cisco Meraki is just a teensie weensie bit more expensive than a Zyxel solution I am a bit worried about mainly the cost but also the potentia of having to double the AP count.

I was wondering since the MR86 can be paired with single element or dual element antennas without having issues.  In essence would it be possible to have one antenna connected via a 3 meter low loss cable to the left side of the AP covering the first gap (2 row combinations) and having the other antenna do the same to the right covering the second gap.  I know that antenna diversity will be less than stellar.  It's not that there won't be any coverage from the second antenna that is blocked by the rows but that will be poor while the visible antenna will have great coverage towards the entire row.

The second slight worry I have if that if this is entirely out of the question I will have to double the amount of AP's and thus double the amount of airtime utilization since I only have the option to place the AP's at the open wall in front of the rows and not above the rows themselves.

I will apply for a demo to test this scenario out but I'm curious about my fellow wireless guys with more experience what you would say about this.

Btw the clients are a mixture of 1x1 wifi phones, and 2x2 Zebra barcode scanning devices.

[KarstenI]

You say yourself it's controversial... I think it was already a bad design when we only designed for coverage ...

Does it have to be Meraki? To save on APs, I would run C9120E in dual 5GHz mode with one warehouse antenna connected with RP-TNC and a second one connected with the DART connector. Sadly, nothing from Meraki to build something like this. Neither an AP nor the Antenna (like Acceltex ATS-01088 that has Beamwidth 15 horizontal/120vertical).

[GIdenJoe]

Yes it has to be Meraki.  The other branches of the customer is becoming all Meraki.
I have considered a controller with C9120 as an option but then in these freezer areas they would require heated enclosures which kinda nullifies the cost difference and increases the configuration and maintenance difficulty.  And combining full Meraki with controller based in a single Meraki network is not possible.

I reckon for now that since the MR86 is 4x4 you can use a ANT-25 or 27 since it has at least polarization diversity to have some merit when only receiving one physical antenna at full strength.  Before suggesting this installation type I will however try do to a good enough test with the phones and the scanners to see if that kind of setup is okay or not.  This weird setup has been working fine in the locations where 1 antenna does line up to the open row.

Have you ever had any testing done with situations like this?

[KarstenI]

Luckily, I never had to deal with that. But although you design the "hidden node" problem into this solution, I still would expect it to work. RTS/CTS to the rescue. Well, at least while the airtime consumption is low. Don't forget to write/blog/youtube about your findings. 😉

[GIdenJoe]

That's a promise 🙂

I did a few captures on the other site and yes all the clients use RTS/CTS nicely.

You are correct about the airtime.  Since the current solution is using 2.4 GHz exclusively and even with the amount of AP's they use now they have areas with 20 to 30% usage even when no employees are present at that exact moment.  The current deployment is a bit poor and they actually set the min bitrate all the way down to 1 Mbps to still get it to work.

[PhilipDAth]

Your experience in this area is greater than mine, so I mostly offer moral support.  🙂

>However since Cisco and Cisco Meraki is just a teensie weensie bit more expensive than a Zyxel solution I am a bit worried about mainly the cost but also the potentia of having to double the AP count.

If you know in your heart that you need to do it a particular way to make it work, then do it that way, regardless of cost.  It is simply the cost of doing it correctly.  Don't be afraid to refuse to do poor workmanship.

>The environment has one tricky issue and that is that the racks are mobile.

Could this be an opportunity?  Could you put an omni AP on a mobile rack and use it in MESH mode?  Battery with an environmental enclosure?

If the environment is less than 24x7 (let's pretend it operates 12 hours a day), could you create a battery operated AP in an enclosure that operates in MESH mode, that is removed at the end of each shift (like the phones) for charging, and then taken in again the next morning when work starts again?

>There are some phone issues

Could the core issue be re-examined - the phones?  Would using a different model or type of phone resolve the issue?  Could it be that the wrong problem is being fixed?

Its probably going to be bad - but could you put an AP on the outside of the freezer unit and then measure how far (if at all) the WiFi can penetrate into the freezer?  Perhaps this might open up additional mounting locations?

Good luck.  It's going to be tough.

[GIdenJoe]

I like your creativity when it comes to moving racks however if you know what exactly moves you wouldn't be able to solve it that way.

Below is a part of a measurement walk I already did in one of the freezers.

The blue racks are all racks however I have traced the fixed racks with a black box.
So each segment is split by fixed racks and in between you can have anywhere between 6 and 10 moving racks.  There is always one open aisle per section.  So here on the floorplan it is drawn at the left side.  So when you want to open another aisle in the middle all racks to the left of it move along a rail towards the left.  So attaching an AP to a rack itself will only make it track that aisle particularly.  For your kind of system you would require an AP mounted to a plate on a rail that knows what aisle is open en where to place itself which is a whole other solution in itself.

If you count the potential aisles in this freezer alone you would require 9 AP's in this one freezer alone if you keep the antennas both at the AP itself.  They now have 3 AP's in this freezer.  And there are many freezers like this.

This is a atmosphere photo of that freezer taken from the right side looking left.

If my hidden node introducing compromise design would technically work then I would achieve it with 5 to 6  AP's.

[KarstenI]

After seeing this floorplan, I don't expect a hidden node problem anymore. If only three isles are open at a time (one on the left, one in the middle and one on the right), My first idea would be 6*9176D1 equally distributed on the lower wall.

With only one AP for an APoS you should be able to test if you can cover multiple isles in this setup.

[GIdenJoe]

Except for the fact that with an internal antenna you are once again limited to 2 aisles per AP which brings the total for that freezer to 9.

And the fact that it's a freezer would require indoor AP's to have an heated enclosure which is also quite costly in buy installation and maintenance.

In my initial validation survey I did explain the hidden node issue that could arise if two aisles too close to eachother would be open.  And thanks to your remark you made me think about how the hidden node problem arises and how RTS/CTS can help mitigate it.  So thanks a bunch for that!

In further investigation I've noticed the phones don't do RTS on their own but they do respond to RTS from the AP's.

This could be one of the reasons these phones have a default of 20% or more retry rate going up to 80% in end of aisles.  But that could also be due to too many reflections of the signal.  I'll have to investigate that too.

[KarstenI]

Oh, I should not read the message and reply a couple of hours later … 😉

I forgot the freezer and also didn’t realize that two aisles in one block could open simultaniously.

But with a D1, I think it’s worth a try if the reflection give you enough coverage for three rows.

Is there any “system” how the rows are opening? Perhaps you find something that you can provide connectivity to adjacent rows with two different APs just by clever placement of the antennas. That would at least reduce the hidden node problem.

And when you say it’s a 1*1 2.4GHz phone. Given the investment for this project, will this device be replaced with something newer that has perhaps more configuration options?

[KarstenI]

I am wondering why the phones don't do RTS on their own. Are these phones at least .11g?

The decision to use RTS/CTS is based on many factors and perhaps the phone only uses framesize as an option which leads it to not using RTS/CTS.

And now (no, I am not drunk, I am just thinking about how things work): If you bring a 802.11b device into the network, the AP should set the protection bit in the ERP IE which should force the phone to use RTS/CTS for every transmission. And if the network is already tuned down to get it working, you likely don't cause any additional harm. Well, the last statement should be evaluated ...