Maybe I'm misunderstanding. 3db is the correct adjustment for 2x2. I
asked the vendor if I had to reduce by 6db when using 4x4, and they told
me "no, only 3".
On 8/26/2015 12:10 PM, Ken Hohhof wrote:
3 dB is half power, so that is correct even assuming they are summed.
*From:* Adam Moffett <mailto:[email protected]>
*Sent:* Wednesday, August 26, 2015 11:04 AM
*To:* [email protected] <mailto:[email protected]>
*Subject:* Re: [AFMUG] Spacial Diversity - helps how much?
On the tx power issue, I was also told that I only have to lower tx
power by 3 when using the 4x4. Apparently the time offset between the
two pairs of chains means their power is not summed.
On 8/26/2015 11:51 AM, Ken Hohhof wrote:
FWIW, Purewave sold us a 6x6 panel made by Mars that was essentially
three 2x2 sectors in one panel. They told us the WiMAX super sauce
would work better with three individual 2x2 antennas spaced a few
feet apart. But a lot of what PW told us was BS (and I don’t mean
BaseStation). I never tried the experiment, so can’t vouch for how
much the increased spacing would help.
Also note the thing that always seems to get lost in MIMO discussions
is that you’re supposed to lower the xmt power on each chain to keep
the total EIRP within FCC limits, so the performance gain has to be
more than the per-chain xmt power penalty. No getting around the
fact that signal, noise, and channel width are the main factors
determining channel capacity. Anything is possible if you assume
infinite signal, negligible noise, or infinite channel width, but in
the real world there are limits. Channel coding does work, but
typically there is a penalty and the coding gain has to be greater
than the penalty.
A similar question I often ask is how much all those 802.11ac chains
help on the $300 routers where the antennas are spaced inches apart.
*From:* Cameron Crum <mailto:[email protected]>
*Sent:* Wednesday, August 26, 2015 10:38 AM
*To:* [email protected] <mailto:[email protected]>
*Subject:* Re: [AFMUG] Spacial Diversity - helps how much?
Spatial diversity is primarily used to combat multi-path. If you have
clear los, your chances of bad multi-path are fairly small and you
probably won't see a lot of benefit. If you have a lot of objects
between you and the tower that can cause reflections, then it will
help more. Simple enough?
On Wed, Aug 26, 2015 at 10:19 AM, Adam Moffett <[email protected]>
wrote:
There are a couple of products out there selling 4x4 MIMO (Telrad
is one, but there are others).
In Telrad's case, two of the chains have a time offset from the
other two, so you get two chains on each of two polarities.
Their default antenna is a single sector antenna with 4
N-connectors on it, so there's no significant spacial diversity.
In the past it's been suggested that we use two dual pol sector
antennas and space them 3 feet apart to get spacial diversity.
When I asked why they do the single antenna, a source at Telrad
told me that spacial diversity "only helps a little". The party
selling us the two panels considers it to add 6db when they run
coverage projections. I suspect any gain from spacial diversity
is going to depend on a lot of circumstances and I doubt it could
be as simple as adding 6db.
I'm wondering if anyone here has any opinions on the topic?
Maybe even facts :)
(I'm sort of eyeballing a certain guy in Utah who designs
antennas and isn't trying to sell me anything.)
