I am a little skeptical of path analysis software that attempts to calculate multipath in an outdoor environment at the frequencies and over the distances that WISPs use. Just some back-of-the-envelope calculations:
Take the example of a 2 mile path across flat ground between antennas each 20 feet AGL, with a reflection off a pond in the middle of the path. Additional distance for multipath reflection = approx. 1 inch Wavelength at 5 GHz = approx. 2 inches So in this example (which ignores earth curvature), the reflected path is right around a half wavelength longer. If the path is longer or the antennas are higher, the difference will be several wavelengths. I think this is why we see multipath problems change rapidly with crop growth, ground fog, thermal inversions, and even changes in air temperature. At lower frequencies like AM/FM/VHF/TV or on shorter paths (like indoors), it would not be as critical. I know that theoretically OFDM is more resistant to this because each subcarrier sends symbols that stretch out over a much longer time period than 1/f. But it seems to me that a software calculation based on topographic and clutter databases cannot be accurate down to an inch or even a foot, plus other factors like air temperature and vegetation growth will introduce more than an inch of variation. The other thing is that I’ve never seen this supposed OFDM immunity to multipath actually work in the field. For example, when I swapped out hundreds of Trango single carrier radios for Cambium 430 and 450, the customers with multipath problems had exactly the same multipath problems afterward. Same when we swapped out FSK for 450. And remember how we were supposed to use 1/8 or 1/4 cyclic prefix on 430 systems to maximize immunity to multipath? I never found it to make a difference, other than reducing throughput. Evidently nobody else did either, because now the only choice is 1/16 CP. This is not a rigorous analysis and may very well be wrong, but unless someone can cite field experience to the contrary, I’m a skeptic. Not talking about indoor bouncing off walls and furniture, that may very well work. From: [email protected] [mailto:[email protected]] On Behalf Of Lewis Bergman Sent: Wednesday, December 21, 2016 8:04 AM To: Cambium Networks User Group <[email protected]> Subject: Re: [Cambium-users] Multipath Protocols like OFDM can use multi-path (time diverse bit reception) and piece those back together to increase signal strength and SNR. Most if not all the currently offered products including ePMP and 450(i)(m) as well as most of the PTP products use MIMO OFDM or Massive MIMO OFDM which are capable of piecing together bits arriving at different times to enable a clearer constellation and a higher bit rate. On Wed, Dec 21, 2016 at 7:25 AM Sam Morris <[email protected] <mailto:[email protected]> > wrote: When creating propagation maps in TowerCoverage there is a feature called "Two Rays". The explanation in the TowerCoverage WIKI says "Two rays are used for multi-path calculations. Most WISP use radios that cannot do multi-path reflections so by default this option is turned off. If your radio system can use multi-path reflections then you may want to turn this value on." In this context, do Cambium radios use multipath reflections? Whenever I've asked about this before, it seems I was told to leave this off unless you're shot goes over a large body of water. _______________________________________________ Cambium-users mailing list [email protected] <mailto:[email protected]> http://lists.wispa.org/mailman/listinfo/cambium-users
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