My biggest hurdles are protruding terrain features and trees. One of my
colleagues asserted that having the two spatial paths might provide two
chances to find a path through the woods. It sounds simultaneously
plausible and crazy, and I don't have enough background knowledge to say
one way or the other.
We could surely get reflections off the ground, but I've always thought
of trees as a source of attenuation rather than reflection --maybe
that's too simplistic, but most of the time I'd bet it's close enough to
the truth for practical purposes.
If spatial diversity is mostly about fighting multipath interference,
then adding 6db to the link budget is not appropriate, and it sounds
like it would be more fair to say that specific circumstances that might
weaken your signal won't weaken your signal. Which means Telrad's "only
helps a little" is the more accurate response.
Is there any downside? Any circumstance where spatial diversity hurts
you? I can tell you cost is not a problem. The material cost is
actually lower to buy two dual pol sectors compared to the 4x4, but you
have a little more labor in assembly.....cost wise it's a wash.
On 8/26/2015 11:47 AM, [email protected] wrote:
Historically, spatial diversity was used on long paths over non
varying terrain, like deserts and lakes. Things that give off what is
called specular reflections and weather refraction effects. Shooting
from mountain to mountain over a bowl shaped valley is pretty bad for
multipath.
For short distance WiFi, I would think it may be helpful with moving
reflectors like people and cars.
*From:* Cameron Crum <mailto:[email protected]>
*Sent:* Wednesday, August 26, 2015 9:38 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]
<mailto:[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.)
