60GHz Fresnel Zone and Multi-path

Fresnel zones are concentric ellipsoidal regions surrounding the LOS path. Objects inside Fresnel Zones will cause a reflected wave, phase shifted by some amount, to constructively or destructively interfere with the desired Line-Of-Sight (LOS) signal at the receiver.

The size of the 60GHz Fresnel zone is very small compared to lower frequencies such as 5GHz. The Fresnel zone size is a function of distance (gets bigger as you go farther) and smaller as you increase frequency.

But even though the Fresnel zone is small, you should still very much also try to avoid another potential problem and that is multi-path.

What is multi-path?

  • Multi-path occurs when the transmitted radio signals reach the receiving antenna by two or more paths.

  • The multi-path component(s) are often phase shifted and combine with the LOS signal in undesirable ways.
  • Metal objects near the antenna or metal roofs may cause multi-path issues for PTMP deployments.
  • Moving antennas up to one meter vertically or horizontally can reduce the effects of multi-path.

Some signs you may have a multi-path issue are:

  • RSSI lower than expected compared to the link budget calculation.
  • RSSI not stable.
  • RSSI changes considerably with the surfaces nearby being wet.
  • Performance metrics such as throughput, MCS rates, retry rates, packet loss etc. much lower than expected.
  • Changing the channel actually improves or worsens the performance (assuming there is not an co-channel interference issue).
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