Mobility

802.11n, MIMO, and multipath environments


In two previous posts-Multipath environments and how they affect Wi-Fi propagation and How to make the best of 802.11 multipath environments-I examined the somewhat nebulous topic of multipath fading and technologies that minimize the negative effects on Wi-Fi networks. Now I'm going to pull a 180 and talk about how important multipath propagation is.

One researcher and wireless pioneer, Dr. Greg Raleigh, was especially instrumental in determining how to use multipath environments to an advantage. One of the better known developments resulting from the research is Multiple Input/Multiple Output (MIMO) smart antenna technology.

How MIMO works

Qualcomm, a wireless chipset developer that acquired Airgo, the company founded by Dr. Raleigh, has the best definition of how MIMO works:

MIMO systems divide a data stream into multiple unique streams, each of which is modulated and transmitted through a different radio-antenna chain at the same time in the same frequency channel. A revolutionary technique that reverses 100 years of thinking about how radio signals are transmitted, MIMO leverages environmental structures and takes advantage of multipath signal reflections to actually improve radio transmission performance.

Through the use of multipath, each MIMO receive antenna-radio chain is a linear combination of the multiple transmitted data streams. The data streams are separated at the receiver using MIMO algorithms that rely on estimates of all channels between each transmitter and each receiver. Each multipath route can then be treated as a separate channel creating multiple "virtual wires" over which to transmit signals. MIMO employs multiple, spatially separated antennas to take advantage of these "virtual wires" and transfer more data. In addition to multiplying throughput, range is increased because of an antenna diversity advantage, since each receive antenna has a measurement of each transmitted data stream. With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel.

That describes in a nutshell the basic tenants behind MIMO antenna systems.

Two distinct environmental conditions

To avoid confusion when discussing MIMO and multipath propagation, it's important to define the two different yet related environmental conditions encountered by MIMO RF propagation: RF Line of Sight, and RF Non-line of Sight.

RF Line of Sight (LoS): Under this condition, RF signal propagation-regardless if Single Input/Single Output (SISO) or MIMO technology-will not encounter any physical interference along the link path. This eliminates any multipath advantage gained by MIMO technology and any multipath fading disadvantage seen by SISO technology.

Even with a level playing field, MIMO technology still has a distinct advantage, because it uses a process called spatial multiplexing. In explanation, if a SISO system and a MIMO system are being supplied with an identical data stream, the MIMO system's data rate will be X times the data rate of the SISO system-where X is the number of receive/transmit antenna pairings. Even the minimal doubling of the data rate is quite significant when considering today's bandwidth intensive applications.

RF Non-Line of Sight NLoS): This is a condition where the RF signal encounters significant physical interference along the link path and only altered RF signals reach the receiving antenna. These altered RF signals have the tendency to interfere with each other, often destructively which results in multipath fading, the bane of conventional radios using SISO technology.

Wireless pioneers like Dr. Raleigh decided to take advantage of the specific multipath phenomena in which received signals from one transmitting antenna will have different phase, timing, or signal strength characteristics from received signals transmitted by a different antenna. This line of thinking brought about one of those all too seldom "Ah ha!" moments. Using multiple transmit/receive antenna pairings to overcome multipath fading also compliments the concept of higher data rates being derived from having multiple RF streams.

The last piece of the puzzle and where MIMO technology finally came together was the advent of new and, in my opinion, amazing receiver technology. By using advanced digital signal processing hardware and very sophisticated algorithms that deal with space-time coding, it becomes possible to decipher the multipath differentiated RF signals even though they are all on the same frequency.

Conclusion

I'm amazed at how technology development timelines continue to shorten, with the recent developments in 802.11n, especially MIMO technology, being one example. Even more important is the fact that these advances will effectively change how everyone accesses data networks and the Internet.

The 802.11n standard has additional features and technical breakthroughs that are not as well publicized as MIMO. I'll look at those attributes in my next post.

About

Information is my field...Writing is my passion...Coupling the two is my mission.

16 comments
rmuldavin
rmuldavin

Need steerable antennae, the flat phase detectable arrays seem doable, such were part of Lucent Technology's plans for mounting at building corners and connectable to street corners where the wireless info was supplied by more centralized distributors. But, alas, the "Irradtional Exuberance" of the decade ago or less financial crash, seems to have been caused by the "bull shift snorting market", sorry, drugs do alter a humans' immediate perspectives, There is hope, maybe some of these flat panel steerable antenna can be home made. Roof tops gone green, can go narrow casting, steerable or multiple. New math emerges from the blood baths, War no more, Peace is here now, join the forces for consciousness raising. Working on this now, as I have been doing for the last generation or more. Best, the Phantom of the Operational Organizers Revolution (POOR), who were supposed to inherit the wind mills, not the gin mills, from rmuldavin

JCitizen
JCitizen

One question: I am assuming that this tech depends on the circuit, that has the dual or more(n) antenna processing this digital algorithm, operate as one unit? In other words, lets say your local LAN had a router with only one antenna, but you had peripheral APs like a wireless printer, or laptop, all using 802.11n technology. Do these devices share this MIMO information across the network to process each on board or does the router/device have to have two(or more) antenna and do it all on board within the single device? Am I clear as mud?

catseverywhere
catseverywhere

Wow. As an old-timey short wave radio enthusiast, I have rebuilt and improved not a few old tube-type receivers. I throughly understand receiver theory, or so I thought! This Dr. Raleigh is not only thinking outside the box, he appears to be thinking outside the universe... I'm trying to wrap my mind around this one. Great article, I haven't been this challenged by new technology since Scientific American went south around 1999. What's even more incredible is once they worked out the math, building the logic circuit probably costs 12 cents. This guy ought to be in line for a Nobel prize. Space time coding, hmmm....

Round One from VA
Round One from VA

"That describes in a nutshell the basic tenants behind MIMO antenna systems." It should be "one of the basic tenets" Tenants are people who rent houses. Otherwise, nice informative article.

Michael Kassner
Michael Kassner

802.11n or proprietary MIMO is divided into two different catagories. Antenna diversity and spatial multiplexing. Antenna diversity is such that if one device has multiple antennas and using diversity that is an improvement over no antenna diversity. If both devices are using multiple antennas, that's even better yet. Antenna diversity technology tracks signal conditions and will transmit using the same antenna that was chosen to receive the most recent packet from the remote device. Spatial multiplexing is a bit different. The receiving device needs to have at least the same number of antennas and ancillary receiving equipment as the sending device's number of transmitters and antennas in order to see the data rate improvement from multiple data streams. The receiver can have more antennas (for example 3x2 versus 2x2) that passively receive signals. As long as the receiver has the proper equipment to coalesce the additional signal information with the others into a stronger aggregate signal. Finally 802.11n equipment with multiple antennas is usually intelligent enough to understand that the remote device is not 802.11n and will revert to antenna diversity and not use spatial multiplexing. As in the example of an 802.11n AP linked to an 802.11g print server.

Michael Kassner
Michael Kassner

We are like minded to be sure. I've been a ham for over 45 years and have many fond memories of hours working on radios, that almost seemed alive with the glow and hum emanating from the cooling louvers. You mentioned that you are a short wave enthusiast. In that regard, you have been using multipath events to your advantage all along. I am sure you realize that the only way radio signals can travel longer than line of sight is to use multipath, i.e., skip or bounce. For example, the RF signal from the radio station on the other side of the world that you are listening to is only getting to your antenna by bouncing off of the ionosphere. The special breakthrough by Dr. Raleigh and others is the ability to decipher/distinguish differences between several multipath events and leverage an advantage from the multiple sources of RF energy. I have not really thought about it, but I suspect that this technology could be helpful at all frequency ranges. Not being an expert by any stretch, I can only make some poor guesses. I am sure you have dealt with shortwave stations fading in and out, which is our friend multipath interference i.e., multiple RF signal components and since shortwave signals are now becoming digitally processed one would assume that the same multipath signal enhancement technology would work.

Michael Kassner
Michael Kassner

Thanks Round One, I must of had a brain fart as I actually know the difference, poor excuse but all I have.

JCitizen
JCitizen

And thanks for your highly informative articles and participation with the questions! I'm going to send all my new WIFI customers to TR and tell them to be especially keen on your articles if they really want the straight poop!

catseverywhere
catseverywhere

You might be right! Could this possibly be applicable to any frequency range? If so, I'm sure given the lower quantity of "information" in the longer waves it would be a simpler processing task. I'd only wonder whether the difference between the end products would factor in. Wifi, though carried on a wave, is modulated digitally. SW is obviously AM. Hmmm... I suppose a wave is a wave is a wave... Seems it should apply. What a concept! Alas, SW being the step-headed red child of the communications world, I can't see anyone beating a path to Grundig's door with this idea any time soon. But if MIMO could be used in an analog receiver (though digitally processed of course) I'd sure as heck try it out. Any chance the question could be put to Dr. Raleigh? As for HAM, I never got into the transmitter side of things. I could never afford it. Wanted to. All the SW radios I've had over the years were dead when I got 'em. I absolutely love tube-type equipment. You can't beat it for audio quality. Solid state induces odd-order distortion, which these highly trained musical ears can hear every time. Right now I'm down to a late '30s vintage Ward's Airline, copper chassis and tubes with numbers like "25." I used to be able to walk down to the TV repair shop and buy tubes. Those days are gone. I bought out the tube supply at two stores when they were dumping the business. But I seem to have run out of the good stuff. Most of what I have left are for use in aviation radios. So, what's your handle? Where do you work? I'm guessing 41 meters? Well, back to work...

Michael Kassner
Michael Kassner

I appreciate that very much. As I mentioned earlier, if there is anyway I can help, just let me know.

Michael Kassner
Michael Kassner

My son belongs to a pickup band (he's the drummer) and they swear that they will never use anything but tube amplifiers for that very reason. They also claim that's the general consensus with main stream musicians as well.

JCitizen
JCitizen

After the proverbial nuclear war; tube radios may be the only communications working. Oddly enough they need only minimal shielding(disconnect antenna). I always wondered why my brother's hand built Heathkit tube amplifier always sounded better than his solid state Scott. Step-headed red child.... That was funny! :^0

JCitizen
JCitizen

High tech or not! Don't listen to any criticism about cultural colloquialisms. You didn't hear any of the astronauts speaking perfect English did you! And many of those guys had PHDs!

Michael Kassner
Michael Kassner

My son also chastised me for the use of an upper Midwest USA slang term in a formal document. I appreciate the comments as it forces me to improve.