Our Mesh group in Michigan's Upper Peninsula have purchased 15 of the Nanobridges M2.
Does anyone have data on needed vertical separation if mounted on a single tower in almost opposite directions?
Or when mounted with 45 degrees azimuth and connected with cat 5?
Thanks - Vance - KC8RGO
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Nanobridge m2 Multiple mounts
Sat, 09/17/2016 - 18:14
#1
Nanobridge m2 Multiple mounts
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1) If the area wifi noise allows it, consider 5MHz channel width to have 2 channels on -1 and -2. (ch -1 does have some overlap with ch 1 @ 20Mhz signals.)
2) We have seen 2 tower 120 sector panels use these 2 adjacent channels and found there was no interference between channels. The antennas were on different levels of the antenna, like 50' apart with no RF shielding. For a specific situation you'll have to test and hope for the best, but design in to put max distance apart. We could all use some more data points to better understand how close is too close, but probably within 5' it's a given there will be adjacent channel interference of some amount. So it's the lesser of the evil choices A) sharing channel and down 50% thoughput; B) split the channels and degradations due to adjacent channel interference and overlap with ch +1 wifi.
3) if you share the channel with 2 devices on the tower, do not try to shield them. In this case, we want them talking to each other to coordinate the channel. If traffic is hopping through the tower site, the throughput is now 50% or less as the frequency is now shared. In 802.11, in this situation (hidden transmitter) the protocol will use RTS/CTS management packets to mitigate hidden transmitter issues on the channel. We don't want to degrade the SNR with shielding between them to be able to coordinate and do this handshaking. RTS/CTS works across SSIDs, but not channel widths.
4) On the next bulk purchase of equipment, buy the 5Ghz NanoBeams to have a lot more channel clear sailing options above ch 165 where WISP operators do not have FCC licensing to use today. Only need to coordinate with your local and ARRL band plans. The 5Ghz Nanobridges also yield +4dB and +7dB higher gain antennas.
In looking at the AREDN map of devices across the world, 2Ghz seems to be the preferred band. However, 2GHz has very limited channel options and lower gain for same size antennas as compared to 5Ghz. Also, contrary to what we think of desirable for a "mesh" (which is up at the layer 3 tcp/udp level in AREDN), If everyone everywhere in the area is on the same 802.11 channel (not using a mesh protocol at this lower level), the throughput can grind to a halt. If 10 different links can all hear each other on the same channel, and we are passing traffic at the same time on all 10 links, then we suddenly have <10% the throughput compared to unique channels. Best to design "cell sites" that do not hear each other, and antenna coverage that avoids straying out of that coverage area.
Sorry, started tossing in a few more issues, but related, than the topic suggests :) ...
Joe AE6XE
Regarding point 2: There is actually a mathematical formula for this of how much DB of (theoretical) Isolation you get for each foot of separation.
Commscope has a similple caluclator online: http://www.commscope.com/calculators/qvisolation.aspx
More detailed information can be found in this publication: https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-M.2244-2011-PDF-E.pdf
For complete isolation you would want at least 128dbm of isolation (commscope caps their calculator out at 70dbm stating its the max they tend to see) (Recivers are speced at -95dbm average, so taking it to -100dbm to add a bit of room to avoid noise floor, and then the transmitter is at +28 so you end up with 28-(-100)=128dbm isolation
The Commscope calculator says its 66db of isolation at 3feet and they cap it at 70db isolation at 4feet (their calculator is a simple one the true forumula is actually: Iv[dB] = 28 + 40*lg(dv/λ) though the doc above notes that since its very hard to have the nulls together exactly formula PRx/PTx = (GTx * SL(ρ)Tx)(GRx * SL(θ)Rx)(λ/4π dh)^2 may be better
I'll disagree with point 3 to some degree, in that IF you could achieve full shielding between the two devices (neither can hear each other so +128dbm or better) than shielding is an absolute advantage and should be done as if they can't hear each other they can not interfere with each other. I am not sure how possible that would be however since bounce back from a hill, the tower itself, cat5 lines, etc all add up to reduce the isolation.
Even in the case where you cant fully isolate them, if shielding still doesn't fully isolate the transmitters that means CTS signal from the tower top mesh node should 'silence' the other tower top mesh node (assuming a stronger deployed node didn't cover up the reception of the CTS signal in which case unless it was because neither packet was decoded this means contact can continue even with the tower top node transmitting because there is enough isolation)
I think more research is needed before such broad comments about not shielding can be made.
We are a vacation area and much of it has little 2.4Ghz.
Thanks again - Vance - KC8RGO
Sticking with my story :)
I've got 2 nodes on 5Ghz on the same tower. Both have full RF Armor shielding including the Rockets and face in opposite directions at 2 levels and opposite side of the tower about 50' apart. Checking just now, they receive each others signals at -81dBm.
3 weeks ago a WISP brought up a signal directly on our part 97 channel on a tower about 150 yards away. They generally point away from my tower and are behind my RF shielding. Their signal on the same channel still came in at -84dBm (at 20MHz width) and made our primary link unusable (using 10MHz width). It was easy to shut them down--they don't have FCC licensing to be on this part 97 channel. Magically the signal disappeared the same day just by an inquiry to the 3 WISP operators from the site owner. No one fessed up. Note it was 10dB higher about ~74dBM on my 5Ghz pointing directly at the other tower.
While I'm sure more can be done to shield and have isolation on the same channel, I don't have a practical way to do this in these examples. I'll stick with this being the general case, but would be happy if someone can demonstrate otherwise to change my story.
Image is of the occasional bad apple WISP tower.
Joe AE6XE