MIMO wireless - stability

We use wireless at market research meetings where we use wireless between the viewing room and the interview room through a one way mirror. The stability of the wireless connection is cause for concern - it occasionally drops out. I appreciate that this often happens with 802.11g wireless - it happens at the office and it happens here at home.

802.11n (MIMO) is touted as having increased range, increased speed and increased stability. The first two are obvious to demonstrate but what about increased stability?

I suspect that the problem we get is a combination of sending the signal through a silver mesh (the one way mirror) plus other surrounding wireless networks causing the signal to bounce all over the place. 802.11n is supposed to work with this effect instead of fighting against it so it could be better.


Thanks, Rob.

Reply to
Rob Nicholson
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It may happen anywhere on the planet. It's usually caused by interference from everything from microwave ovens to radio controlled airplanes. See a list of possible culprits at:

Where did you get the idea that you'll get increased range with MIMO? Basically, you can trade range for speed. Double the range, and you have to slow down to 1/4th the speed. Similarly, cut the range in half, and you can go 4 times as fast. For long range, you'll never get faster than 802.11g speeds. MIMO is about improved resistance to reflections and multipath. 802.11g sorta tolerates multipath by being tolerant to frequency selective fading. MIMO uses the multipath to create additional data streams (spacial multiplexing), which will increase the speed, but not the range.

I've never tested a one-way mirror for RF attenuation. My guess is that it's about the same as aluminized reflective window coatings commonly used in office buildings for energy efficiency. These block signals like a brick wall. You're lucky that any signal goes through.

The surrounding wireless networks cause interference. If this is the case, move the access point to a less polluted channel (1, 6, or 11), move the access point away from windows and doors so that it doesn't pickup as much junk, or elect public officials that don't support municipal wireless networks running high power (1 watt) radios.

Pre 802.11n Draft 2 is suppose to be faster. It's also somewhat more resistant to reflections. It slows down to 802.11g modes in the presence of interference and is therefore no better than 802.11g. I guess you can call that more reliable.

Keep it simple. Play with antenna patterns. Change channels or bands (802.11a is less polluted for now).

Also, don't elect officials that promote "free" wireless networks that neither work or pay for themselves.

Reply to
Jeff Liebermann

Err, articles like this:

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Range isn't actually important for this requirement - the receiving equipment is 5m away from the transmitter.

A one-way mirror isn't solid - it's dots of silver with gaps inbetween and only works because the viewing room is kept dark whereas the other room is bright light so the majority of the light reflects back. Turn the lights off in the meeting room and on in the viewing room and the one-way mirror reverses. I always wondered how they worked :-)

So that's probably how the signal is getting through but it's probably causing a lot of degredation. I'll get them to check the signal strength next time.

These meetings are often staffed by non-technical people so we have to keep it as simple as possible. Yes, it's a challenge :-)

Another alternative is a video sender - the PC in the viewing room is currently remote controlling the one in the meeting room. Instead, we're considering a video sender using the S-Video output of the laptop and the input of the data projector. This would have similar problems (same frequencies I think) but I've got one at home and it's rock solid. We've got a concern over the video quality but most of the time it's big PowerPoint slides on the screen.

Cheers, Rob.

Reply to
Rob Nicholson

OK, I'll bite. It says: "We were impressed with MIMO's range capabilities and found it delivered exceptional coverage in a normal home environment."

The big question is what they mean by "coverage". Also whether they were testing in the various MIMO modes, or were falling back to the

802.11b/g modes when testing the coverage. I'll leave it an open question as I fail to see any substantiating numbers. They do indicate that the MIMO router is more "reliable" which they interpret as experiencing fewer dropouts. My guess is that they were testing in a highly reflective indoor environment, where MIMO does do better.

Well, there you have me. The article mentions: "MIMO technology has attracted attention in wireless communications, since it offers significant increases in data throughput and link range without additional bandwidth or transmit power."

However, searching the article, I find no substantiation of this claim and no furthur mention of the buzzwords "range", "coverage", or "distance". In short, it's another recycled claim devoid of any substantiating numbers.

Then, you should be getting "MIMO performance", which is a good thing. The major benifit will be increased speed and a much more reliable connection with fewer disconnects due to reflections and multipath.

Well, it doesn't say how the mirror is constructed but other references mention dots and "smears". If they're electrically connected to each other (they would have to be if the coating was electrically deposited), then it would probably block RF. My uninformed guess is that it wouldn't be as bad as a solid wall, but would still block some RF. I could run a test, but I don't have a one-way mirror handy.

Check the signal strength for an equal distance but without the mirror. Then test with the mirror over roughly the same distance. The signal strength difference is the mirror attenuation. You may need to make a series of measurements as antenna positions and room differences will have a substantial effect.

Explaining microwave propogation and antenna characteristics isn't too horribly difficult. If they are totally clueless, an optical analogy works well enough. Anything light can go through, so can RF. Water filled balloon for antenna pattern demos.

I just bought a cheapo 2.4Ghz wireless TV contraption. It rips on my

802.11g thruput and my access point mangles the video. It's anything but rock solid fading in and out as people walk around between the camera and the receiver. I guess they vary in quality.
Reply to
Jeff Liebermann

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put the antenna on the other side of the mirror.

or use one of the powerline networking boxes with a built in AP to give you a link between the 2 rooms - as long as they are on the same mains distribution you should get a reasonable link.

or just run some Cat5 patching into the local flood wiring system and avoid wireless completely?

if you can afford glass with silver, then a bit of cable shouldnt hurt.

if security is an issue then why use wireless anyway?

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