KP performance new x4 antenna wrong polarization?


#1

I was researching the C5’s and the A5c. it appears that the new official antenna is H/V polarized but the C5 is Dual 45° slant polarization. Correct me if I am wrong but with this mismatched polarization you will lose 3db?

Is this intended or an oversight? if intended why have it offset and lose the 3db?


#2

Oddly, the A5-360 is circular polarized, which I can understand when matched with the C5. Would love to hear the reasoning on the slant/45 polarization of the KP antennas.


#3

I can only assume that this is because they expect most people will be matching C5c with antenna of their choice to the A5c. I see the logic, but it makes things a little trickier.


#4

I don’t understand the reasoning for it… Would it just be better to get 2 other slant 45 mimo antennas and use them? 3db sometimes doesn’t matter much but on some longer shots it really can make the difference.


#5

David,

Thanks for the question. We are working on a much more detailed white paper that will cover this exact topic and all the permutations of linear, slant and circular polarizations. I’ll be sure to post it here when it is ready.

In our MIMO implementation, one of the fundamental benefits is the ability to use the MIMO matrix to rotate and correct for signals that are not aligned linearly. In the case of A5c (Linear H/V) and the C5 (Slant –45/+45), as long as the polarizations being transmitted are orthogonal (90 degrees apart), then we can decode them back to two discrete data streams. Although the polarization mismatch is perceived as a –3 dB loss, this signal is split between both receiving polarizations.

As an example, I have a radio that is transmitting a –45° and +45° polarized signals towards a V polarized antenna. If I was transmitting V to V, this would have been received at –50 dBm. Because of the 45° degree offset, the -45° signal will be received at –53 dBm. However I am also receiving the +45° signal at –53 dBm. If I add –53 dBm and –53 dBm, I get -50 dBm, meaning I have in fact not lost any power.

Lee


#6

…but if your client device was also +45/-45 instead of V/V, you would be receiving both at -50, thus a combined effective signal of -47, so in MIMO terms, you are throwing away 3db of usable power, in exchange for some flexibility in antenna options.


#7

Tyler,

If communicating between two radios where the antennas are aligned (H/V to H/V), we still only get - 50 dBm, which is exactly the same as the combined signal strength when we communicate in a mismatched case (H/V to +45/-45). Mimosa radios do have the ability to gain an extra 3 dB on top of the -50 dBm in my example, by dropping to a single stream rather than two streams. However this would result in a halving of throughput, so our rate adaptation algorithm will only do this in adverse RF conditions.

Lee


#8

So if you had a perfectly matched H/V set on both sides where each single stream would only be capable of -50, you’re saying the combined signal strength would not be 3db hotter like most MIMO products? And when you go to a mismatched -45/45 to H/V where the ideal signal strength of each stream would normally be -50, but you lose 3db due to the mismatched antennas, you can still get 3db back by combining the signals? This makes no sense.


#9

My understanding is much like Tyler’s…

The slant to H/V causes a 3db difference that the antenna itself allows to pass to the radio.like a square with the edges cut off. I suppose you could interpret the edges but wouldn’t it be better to just send the square?

Now I am no RF engineer, and I don’t know what possible techniques there are for RF magic. I am just wanting to know if this is good practice or if it will be better with a slant antenna. I would also like to understand why it will combine the chains while mismatched and not when additional RF possessing is required.

Thank you Lee for working on a paper for that :relaxed:
Please keep us posted.


#10

Tyler and David,

Let me see if I can come up with a clearer example.

Linear to Linear
H to H = -50 dBm
V to V = -50 dBm

The V and H are orthogonal, so > 20 dB apart. This means that H only sees H and V only sees V.

Linear to Slant
H to -45° = -53 dBm
H to +45° = -53 dBm
V to -45° = -53 dBm
V to +45° = - 53 dBm

In this case, the two signals I receive from the H polarization on the transmitting radio are split between the -45° and +45° polarized antennas. So if looking just at the case of H to -45°, then it is correct that I lose 3 dBm vs linear to linear. However I pick that 3 dB back up from the +45° polarized antenna when the chains are combined.

So the logical question remains, why do slant at all? The answer is that it really is a decision at the time of building the product as to what we think is going to best performance in that individual product. There is no real RF benefit of us doing linear over slant as I’ve shown above. There is also the case of circular polarizations transmitting to linear/slant which I will cover in my white paper.

Thanks for the questions guys, helps me come up with better ways to describe this.

Lee


#11

Thank you for the explanation Lee. I think I now understand what you mean.

Maybe the only benefit of slant at the CPE end might be less interference? considering most household things are H/V


#12

In theory, but given that most in home routers have movable omni antennas that can be oriented in any direction, this was not a reason for choosing slant over linear in the C5.


#13

How does it work with circular?


#14

Circular is -3db to H/V or Slant


#15

Great, I think my theory still stands though. If they were H/H/V/V each individually received at -50 on both sides, it would still come in at -47. Correct? So while we do get that 3db back with MIMO signal combining, we still are losing 3db over what we -COULD- be getting.


#16

Tyler, I think he is not talking about combining the chains but the signal from both H/V on to a single chain before the chains are combined.

In a H/V to H/V setup:
Chain 0 will see -50/-75
Chain 1 will see -75/-50

The second signal is so low you can’t do anything with it.
this will give you an effective -47 when you combine the chains.

with what I understand now…
with an H/V to Slant
Chain 0 will see -53/-53
Chain 1 will see -53/-53

If each chain is able to combine off of each chain then it would combine to a signal of -50

Then in turn if you combine the chains it would still give you an effective -47


#17

Thanks @David5 I was understanding it how tyler was until your explanation. Its the fact that we are dealing with chains that send 2 streams (4 total) that was throwing me off. In a normal 2x2 situation i think our previous understanding makes sense. i could be wrong on that though


#18

Anybody here truely believe that a transmitted signal will arrive at the receiver in the correct polarization???

Get real peoples…


#19

I don’t think we updated this thread to include the detailed whitepaper, but this gives all the details and visuals to explain polarization differences and (non) impacts to links. Hope it helps!

Part 1: Overview of Polarizations
Part 2: Demystifying Polarization Differences


#20

Better look at the benefit in interference supression from other providers/AP’s still working with H/V polorisations. In fact the major issue for most of us WISPS.

(Indoor uses usually 2,4Ghz and that is a whole new ball game full with overpowered multiantenna weak cpu’d AP routers serving many devices in a messy polorised environment with single + dual chain stations with at times even b/g units molesting the network!)