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'" <[email protected]>
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)
-Charles
-------------------------------------------
WiNOG Austin, TX
March 13-15, 2006
http://www.winog.com
