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OM1P: diversity, antenna selection and range

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PostPosted: Fri Nov 19, 2010 10:19 pm    Post subject: OM1P: diversity, antenna selection and range Reply with quote

In poking at some OM1Ps here I've found:
  1. Turning off diversity improves node-to-node ICMP response rate.
  2. Setting rxantenna to 2 improves rssi of associated client stations. (default is 0)
  3. Twiddling txantenna seems to have no effect. [?] (default is 1)
  4. Making the changes stick through a node's reboot cycle is needlessly complex.

On the last point, /proc/sys/dev/wifi0/(attribute) isn't supposed to stick,
/etc/sysctl.conf doesn't stick,
uci set (blah-di-blah) followed by a commit doesn't stick,
editing /etc/config/(blah-di-blah) is the same as running uci and doesn't stick,
edits to /lib/robin/ don't stick.

The magic spot, at least on r2693, is

Antonio documented that here:
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(that's /viewtopic.php?t=3159&mforum=robin for those reading an archived copy without links)

From the madwifi documentation the choices for rxantenna and txantenna are 0, 1 and 2 where 0 means use diversity, 1 and 2 mean use just this one of the physical ports. That makes the flag flipped by the 'diversity' file in /proc/sys/dev look redundant, but who knows.

On the txantenna I was watching the node's rssi from another's shell while flipping the bit back and forth on the first. I tested it between two nodes about 15 feet apart at about the same elevation and at roughly the same orientation. I wound up telling it to use 2 for both of them just to give it fewer I/O pins to twiddle during normal use. Flipping txantenna on its own seemed to have no effect.

The improvement for choosing rxantenna=2 over rxantenna=1 held across every OM1P I tried. I don't think it's the flakey pigtail issue someone brought up earlier. The difference was 10 to 20 rssi units. Each was tested with just the soldered chip antenna and the stock 2dBi stick.

The feedback from athstats with receive diversity enabled suggested the opposite conclusion. Testing a node with a 19dBi dish on the antenna port and diversity enabled I found most of the traffic showing up as having been received on antenna 1 and that associating a client laptop physically close to the router led to occasional use of antenna 2 and a significant drop in performance to a distant node. Maybe there's a bug in madwifi that swaps the ports with the diversity bit. Again, who knows. Turning off diversity made it work better.

For diversity, rxantenna and txantenna on the OM1P my selections are 0, 2 and 2.

r2693: /etc/profiles/default/lib/robin/
                        # documented as 0,1,2 for auto and two connectors
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PostPosted: Sat Nov 20, 2010 7:12 am    Post subject: Reply with quote

This is odd. I am seeing different results, albeit with a different fw version.

I have a custom OpenWRT 8.09 build with madwifi:
# opkg list_installed | grep madwifi
kmod-madwifi -

# sysctl -a | grep antenna
dev.wifi0.txantenna = 1
dev.wifi0.rxantenna = 1

I have 2 OM1P's setup about 15ft apart, same elevation, stock omni pointed vertically, bent at the hinge. I get an rssi of ~55 on both ends.

I have played with this and changed the setting, but I get the largest rssi on both ends if I have:

OM1P with 4 feet on the table. If I turn it on its side, so the internal patch is veritcal, I lose about 7dB of signal.

Antenna 1 in my case is definitely the rp-sma omni as removing it shows an immediate 20dB signal loss.

Changing the rxantenna to 2 on either end causes the signal to drop by 12-14dB on both devices, implying I am not changing rx antenna, but both rx and tx.

Another oddity is:

# sysctl -w dev.wifi0.diversity=1
sysctl: short write

So I have more questions than answers at this point!
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