MIMO (Multiple-Input, Multiple-Output) is one of the important technologies to be implemented in the upcoming 802.11n IEEE wireless standard. MIMO promises higher throughput and greater range, by turning what has traditionally been a shortcoming with wireless communication into a benefit. That shortcoming is multipath propogation. Multipath Propogation occurs when the signal from your antenna deflects or reflects off different objects and the signal arrives at its destination in multiple instances at different times. Incidentally, this is what causes ghosting in TV transmissions; you’re actually receiving at least 2 signals and they get superimposed on each other, but one is just out of sync with the other. Already, many vendors are producing what they are calling “Pre-n” wireless routers, but don’t be fooled: these most likely will not be compatible with the upcoming 802.11n standard.
Theory
The first paper on the subject of using multiple antennae was published in 1984, but it wasn’t until 1996 that Greg Raleigh and Gerald J. Foschini devised new approaches to MIMO and wrote a number of new papers on the subject. Greg Raleigh went on to form Airgo Networks and to push for inclusion of his technology in 802.11n. Airgo already has chips in quite a few routers on the market today.
Instad of sending out one signal via one antenna and hoping for the least amount of multipath propogation, MIMO intentionally uses multiple antennae to send out the information stream broken out over those different transmission vectors. The receiver also utilizes multiple antennae to capture those multiple streams and superimpose them on each other to obatin the orignal information stream. Keep in mind that ‘superimposing’ is a gross oversimplification – the multipath propogaion can weaken some signals or disrupt them in parts and the algorithms used to recombine the signals is quite complex.
Interestingly, the more multipath propogation that occurs, the better your chances of obtaining an accurate combined signal due to the fact that you now have more separate signals from which to determine your original signal. The second interesting effect of MIMO is increased range. This happens because a MIMO receiver can take weak signals and recombine them with other weak signals to get a normal signal. Traditional transmission systems that only relied on 1 signal broke down the minute that the one signal suffered degradation due to multipath propogation. The upcoming standard is expected to deliver up to 6x the speed and 8x the range of current 802.11g networks.
Standards and Products
There is currently quite a battle by some of the vendors for determining what gets included in the upcoming 802.11n standard, which is currently in the hands of the IEEE. On either side of the fence are Airgo Networks and Atheros Communications, 2 companies that are using the MIMO term to further their own products. They belong to the WWiSE and TGn Sync consortiums respectively.Both groups include their share of heavyweight vendors such as Intel, Motorola, Qualcomm, Siemens, Sony and many others. Although Motorola is part of the WWiSE group, they’ve also proposed their own recommendation (MITMOT) to the IEEE 802.11 working group. It would certainly be a boon to the vendors if they got some of their intellectual property included in the standard.
The IEEE 802.11 working group has a target date of April 2007 [use IE if page does not display in Mozilla/Firefox], which leaves a lot of time for a great deal of argument and wrangling to occur. Although WWiSE’s and MITMOT’s recommendations were rejected in January 2005, the proposal by TGn Sync did not receive the required 75% vote to advance, so the other groups still stand of chance of being picked. As of September 2005, the 3 groups proposed to develop a merged solution. I’m sure most people will agree that this will end up serving the standard, the technology and the public best.
As mentioned previously, some vendors have already started slapping the “Pre-n” notation on some of their routers, many of which are really 802.11g wireless routers, but which also contain some MIMO technology including the multiple antennae. Of course, in order to take advantage of this, your wireless device also has to have a compatible wireless NIC in it. The Wi-Fi Alliance has threatened to pull the certification of vendors that use the term too loosely, although that hasn’t stopped many vendors from continuing to use the marketing term. As a consumer, you should be careful when choosing your next wireless router in the next year and a half or so. Of course, this does not mean that the products are necessarily bad or poor-performing. As a matter of fact, with a compatible wireless network card, the performance can be incredible: one router delivered in excess of 30Mpbs at 200 ft.