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Stubborn WI-FI reflection - multipath issues

Level 1
Level 1



Currently I'm working on a issue which is very stubborn.


One of our customers has very large open areas that are being made of aluminium walls , floors and ceiling and. Next to the room itself there is a lot of metal in the area / machinery , desks etc. These are confined area's. 


Clients have connectivity issues. What we've done this far : 

- when enabling lower data rates (12 - 18Mbps) so effectively enabling QPSK connection seems to be more stable.
- When using patch antenne's it seems to be meer stable. (AP's are typically mounted against the ceiling).

- Mounted AP 30CM below ALU ceiling --> no difference.

- (long) Guard interval does not change anything.

- Started using 20Mhz channels to have less CCI.

- Disabling some AP's to not have to many AP's.
- Test with AP with integrated antenna in the area, some issues. You'll see a connection drop in MCS rate when you move away (only a few meters) from the test AP

- Multiple site surveys have been conducted (using Ekahau by multiple parties) to check for interferers , Rogue's , Coverage etc, all fine.

- issue occurs in both bands but most clients typically connect to 5Ghz.


Typically when a client is moving away from an AP you'll see that the connection drops to MCS 1 - 4.  Also the clients are very sticky in this highly reflective area's. Effectively this leads to situations that a client does not roam to a neighboring AP whilst it has a very crappy connection (MCS1).


When we had not enabled 12 - 18 Mbps in many cases clients could not connect.


WLC reports in many cases a very good RSSI / SNR eventhough the client device thinks otherwise.


In similar smaller areas it works as a charm. In warehouse / office all works fine except these environments. We tried with 8.5 and 8.10 code / 3800 series AP's.


As mentioned using patch antenne's it seems to be more stable, we get less complaints but the throughput is still very low because of the MCS shifting. 

Has anyone some other suggestions ?

17 Replies 17

I've proceeded troubleshooting (yes, half a year later .. ) and also when having only BPSK and QPSK enabled the connections is awful. 


As soon as a client connects and starts a download channel util will go to +94% whilst only 1 client is connected and it only achieves 7-15Mbps throughput (no interference has been seen). This has been tested with long guard interval enabled and with multiple AP's on multiple channels.


I think there is just too much of reflection in the current AP setup (mounted against the ceiling). I will place an AP with internal antenna's on the desk and see what happens.

@aflbakker have you ionvestigated o that Channel Utilization? If you have an spectrum analyzer such as Sidekick or Channalizer then you can try to find if this is due to an existing interference or simply high 802.11 traffic.

Additioanlly, have you tried using patch or directional antennas and pointing to the floor where devices are supposed to be, and to avoid unwanted reflections from side walls?

*** Please rate helpful responses ***

we did implement patch antenne's instead of omni's. Both of them have been mounted against the ceiling. I'm aiming to test with antenna's on different location (like the wall instead of ceiling). Also I will place an AP with internal antenna on the desk to see if this improves it somewhat.


Transmit utilization always increases when channel util. goes skyhigh this rules out non-wifi interference imo.


Basic stuff like spectrum analysis have been performed multiple times by multiple parties but this issue is a bit more complex due to the nature of its environment. Ekahau will report the WIFI environment as excellent coverage no interference etc. Only if you perform an active survey you'll notice strange data rate in the report that could point to strange behavior.



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