Can you try changing the MT to use only 5mhz or 10mhz of spectrum just to see what that does? Or even temporarily change to an unused band with the MT (5.9ghz) to see what happens? It may help isolate what's really going on.


Tom DeReggi wrote:

Thanks, Charles and Dustin,

The challenge I'm working on is to determine if the degregation of my test link, is caused by

A) Distortion on the transmitter, at full power? or
B) Overload or lack of acuracy of the receiver. or
C) Or Just plain interference creeping in. (tested at about -80db)
note: multipath unlikely, as LOS link, 10 miles, parabolic antenna, o wall behind antennas, 100ft above other buildings.

In my Trango test case, w/ 2ft antennas, QAM16, at -55 db I got worse signal Quality quality (packet loss) than at -65db. To me that would infer case A or B was happening.

What was interesting, is my Mikrotik test link w/ range5s, actually got peak rssi (full power) of -47db apposed to Altas's peak signal of -55. (note: path analisys calculated -55 db appropriate, so not a negative for the Trango, but a Plus for the Range 5, exceeding expectations).

With the Mikrotik, the higher the rssi radio power, the better the speed results, and lower the packet loss. So Mikrotik did not seem to be plagued with the same delimna. However, at a surprise, the Mikrotik performed at a slower speed, and had more packet loss, in its best link configuration, than Trango had. So the Trango at -65db QAM16, outperformed the Mikrotik at -47db.

I attribute those results partially, to how the radios deal with interference. One side of the link (AP/MU) had significant noise, causing the Mikrotik to lower modulation more frequently. I proved this, by repeating speed tests with Trango using 5.3Ghz, which performed perfect links (no loss). However, the 10-11 miles was pushing the maxrange of 5.3, and I felt 5.3 was to risky, based on that. I actually had to turnup the Power a little over the legal limit to get the perfect link, but still lower rssi than the 5.8G link. But my point was, when noise wasn't there, the links worked much better.

So the decission I am trying to decide on is,
a) increase the gain (dbi) of the antennas and lower the gain (dbm) of the radio, to improve the link.
For example, upgrade from 2 ft dished to 3 or 4 ft dished.  or

b) get a better 2 ft antenna with more isolation.
For example, upgrade Gabriel cheap 2 ft para to the high performance 2 ft Gabriel Drum style antennas?

Either one could have a possitive effect. Its likely that my noise is comming from my colocated antennas at the same site. The Drum style antenna will likely have much better isolation comming from the sides. Better F/B ratio is not jsut about an antenna behind me, but also beside me, and interference is not always cured by lowering the beamwidth, if the interference is comming from the side. So better isolation antenna could be the choice.

However, if the packet loss was from self generated noise, larger antenna would keep my gain up, even after lowering power. However, I actually would still have a gain improvement, because the antenna increases gain in both directions, where as lowering he TX power only does it in one direction. Because most of my interference is at the AP/MU side my paln was possibly to....

Increase the antenna at the RU/Client, to a 3-4 ft dish. If packet loss at -55db was due to transmitting to high power, and loss was at MU/AP then it would be most importantto lower transmit power at the RU/Client side. Increasing dish size at RU would help this.

Then on the MU/AP side, I would add the high performance 2ft antenna, with better isolation, taking that most of teh interference may be colocation interference. Increasing the antenna size may not block interference comming from the side.

But then again, if interference comming from the front (I have another site 20 deg off to the left), its possible the larger dish and narrower beam may in fact also help isolate interference.

Now to make it complicated, what if the cause is not interference at the radio receivers? But instead its all the RF in between and reflections comming out of phase and distorting my signal before it gets to my radios?

Now I could just add 4 ft high performance drum antennas on both sides, and call the problem done, but then that would be $4000 just in antennas :-( But also means upgrading mounting pole and ballast hardware.

Which brings me back to my original post, is it just cheaper to buy better radios, and which have better C/Is and SNR threshholds?

The orthogon has more power, but no matter how good their path anal tool is, and how good the reputation, when a large part of the problem is noise floor, I have a hard time believing that faster speed can be pushed through a narrower spectrum width, while hard setting speed in each direction.

So what I am learning is its not about whats the best radio, its what tool do you need to solve each unique problem. The hard part of this business is conclusively identifying what problem exists, to know the most cost effective way to solve it.

Decissions, decissions, decissions.

Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband

----- Original Message ----- From: "Charles Wu" <[EMAIL PROTECTED]>
To: "'WISPA General List'" <>
Sent: Wednesday, February 08, 2006 9:29 AM
Subject: RE: [WISPA] BPSK QAM16 DSSS interference

As you start to walk up the modulation line you definitely need more C/I,

but you also start to loose the ability to use full power out of the radio.

A small bit of trivial regarding this issue

With higher order modulation schemes, the EVM (Error Vector Magnitude) can
be so high that even on a perfect link (no noise) the receive chip is
incable of decoding the signal properly into the correct 64 "dots" of the
QAM modulation plot.

This QAM constellation "interference" can be represented by a grid of 8x8
dots that are being blurred by the transmitter not handling the signals with
enough linearity (e.g., the radio power amp is turned to high).  When too
much blur occurs, the adjacent dots touch each other and the receiver will
not be able to decipher the signal (it's blurred)


WiNOG Austin, TX
March 13-15, 2006

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