Side Lobe bandwidth capacity

RouterOS Wireless Networking
1

Hey all, I’m hoping some of the wireless experts on here can sort out a friendly disagreement I am having with my manager.

In this hypothetical example, we have a Mikrotik wireless AP (RB922UAGS-5HPacD) PtMP running in MIMO, N only. It’s using 90 degree directional antennas.

In the scenario, there are two clients. One of the client radios is in the sweet spot on the main lobe. The 2nd client is on a side lobe, outside the 90 degree coverage. However, due to distance or some other factor, both clients have identical signal strengths as reported by the AP and the clients.

signal-strength: -57dBm
signal-strength-ch0: -61dBm
signal-strength-ch1: -60dBm
tx-signal-strength: -58dBm
tx-signal-strength-ch0: -60dBm
tx-signal-strength-ch1: -61dBm
noise-floor: -114dBm

My contention is that signal is signal. Despite being on a side lobe, if the signals are the same, both clients have the same functional bandwidth capacity. Signal determines modulation which determines capacity of the wireless link.

His assertion is that there is less bandwidth “available” through that side lobe because the radiation is weaker.

Any thoughts?

2

It is all about signal to noise ratio. The less noise, the better.

Avoid at all costs that clients will be in the sidelobes. They can be very tricky. One day it works fine, and then the next day there is a person who started to download a lot, and the side lobes started to get less power. So these clients gets worse connections.

Use a better antenna to reduce the sidelobes. I know it is not 100% possible to cancel them, but if they are 20-30 dB lower than the main lobe, that is already very good.

3

In your hypothetical example, IMHO, both clients would have similar bandwidth capabilities but the initial conditions should stay stable (freeze).
You maybe should ask yourself why the conditions are so similar:

  • bad sector (huge side lobes, behaving as omni, ..)
  • client B (on side lobe) have better gain antenna or is nearer than A (on main)
  • client B has picked up a very lucky reflection
  • ..

If your scenario is not hypothetical, stress it with bandwidth test and you’ll have the definitive answer :laughing:

4

SnR and CCQ will be worse on the sidelobe.

If you have 5 clients connected and 1 or 2 is on a side lobe there wont be much difference.

Problems will start when you are adding more and more clients onto that sector.

Best advice is, don’t connect on sidelobes.

Rather replace the sector with a 120deg if there is not a lot of interference or add a additional 90 or 45deg

5

Thanks for the responses all. It’s not a hypothetical situation and bandwidth testing does indicate that the client works on both APs equally well. However, for some, seeing is not believing, so I appreciate the input. We do try to avoid side lobes when possible.