Question and feature request for Mimosa


#1

Question to Mimosa regarding B5 levels.

I have noticed that excessive signal to noise ratio is actually degrading the performance of the link. It looks like the optimum SNR in MY case is 28-35dB.
I reach 30dB SNR with an RX level of -50dBm and start to see lower performance when I reach levels of -45dBm (35dB SNR). RX level -50dBm is reached with a TX level of 14dBm.

Have a hard time accepting that this is related to RX overload at these levels.
Is this related to TX compression causing distortion? If that is the case, then where does the TX start to compress to the point where it impacts performance?

My problem is that I have 10dB noise floor difference between the two B5’s, so if I adjust the TX power to reach about 30dB SNR in the higher noise end, then I end up with close to 40dB SNR is the lower noise end.

My link distance is 2.4KM with free LOS using 2X80 Mhz.

Low noise end data:

TX power 17dBm
RX power -47dBm
SNR 38dB

High noise end data:
TX power 17dBm
RX power -48dBm
SNR 28dB

It would be very helpful if I could adjust the TX power on each unit individually to optimize the link. Now I have excessive TX power in one end to get good SNR in the higher noise end.
This would help performance and also keep the spectrum cleaner for other users.

It would be even better to have an “AUTO TX-Power” option with a target SNR for users that has varying noise floor.
Could start out with same power both ends and then gradually back off power if SNR too high.

I am very happy with the performance. Thank you for a great performing link.


Frame loss on modulation change
#2

Tx compression starts above 27 dBm, so receiver saturation is more likely. In a B5/B5c, there are two stages of LNAs. At relatively short link distances the LNAs are bypassed completely to avoid receiver saturation.

Although maximum Tx power is set on the AP and sent to the STA (client) via beacons, TPC should automatically reduce Tx power to improve (lower) EVM on each side of the link independently. TPC is limited to 5 dB of downward adjustment from the Maximum Tx power set on the AP, so that may be a factor.

There are many complications with allowing separate Tx power settings on each side of a backhaul link (e.g. maintaining EIRP, DFS sensitivity, Auto Everything calculations, etc.), so additional controls are not currently on the roadmap.


#3

Thanks for the info Chris.

So there is in fact a TX power auto function already. Good to know.
Can I see anywhere if the TPC is reducing the TX power, or is it happening in the background?


#4

The Tx power changes are not shown, but you will see Rx Power and EVM changes over time in the Dashboard > MIMO Status tables.


#5

Chris,

If I change TX power or frequency then I have noticed that the RX power levels out to very similar value on all chains after a few minutes.
Elegant way of balancing stuff.

Thanks for all info and a nice product.


#6

I can understand the complications for fine grained tx power control, hope it does make the roadmap at some point … how about at least till then report on behind the scene adjustments the radios make to the UI or log.


#7

Hi Brian,

I’ll put in a feature request for you.