MaximumPC 2005 11

r&dBREAKING DOWN TECH—PRESENT AND FUTURE

60 MA XIMUMPC NOVEMBER 2005

be separated, compared, and recombined at the terminus. That process takes time. While MIMO has provisions for any number of antennas on either side of the transmission, the more signals that are sent, the more signals that must be processed (on both sides), and that means longer processing time. To be fair, we’re taking about intervals shorter than the blink of an eye. However, we’re also dealing with transmission rates in millions of bits per second. Tests in optimal conditions indicate speeds nearly twice as fast as 802.11g but the IEEE standards body is still hammering out the final details. As far as it’s related to wireless networking, MIMO remains a work in progress.

MIMO IN THE MARKETPLACE No one doubts that MIMO will be part of the 802.11n specification when that standard finally arrives, but the oft-used “Pre- N” label that some vendors have slapped on their routers has absolutely no real sig- nificance (except to differentiate a product from its predecessors). What’s more, the MIMO research and development effort is proceeding in three directions: There’s Airgo Networks’ True MIMO, Atheros Communications’ Super G and Super AG, and Video54’s BeamFlex Smart MIMO. Airgo CEO Greg Raleigh wrote the first academic paper on MIMO at Stanford in 1996 and has 26 patents in the field. Airgo is pushing a protocol called “spatial multiplexing,” which uses multiple transceivers on each side. Don’t get caught up in the terminology. It’s just technese: Spatial because the multiple antennas are at different positions; multiplexing because more than one signal is being sent at the same time. Airgo’s True MIMO technology can handle multiple data streams at the same time and over the same frequency band. Theoretically, this should increase the

throughput rate by whatever factor of radios there might be. Practically, it’s never quite that neat. Atheros uses a similar setup; its design, however, transmits the same data stream simultaneously, which neglects the spatial multiplexing factor. Video54 has taken a tack somewhere between the two: Its seven-antenna sys- tem can transmit the same data stream over multiple paths simultaneously, but it uses only one radio—again, without spatial multiplexing. Needless to say, both Atheros and Video54 would like to eliminate spatial multiplexing as a requirement in the MIMO specification, maintaining that multiple antennas at both ends of the path are suf- ficient—while how they are used should be optional. The obvious advantage with fewer transceivers per device is a less complex

and costly final package, and any loss of additional throughput, they claim, is mar- ginal over time. There are also some concerns about attempting to mix the various 802.11 schema and technologies. Although you’ll be happily notified that you have 11 possible channels available (in the U.S.) when you scan for a Wi-Fi network connection, there are really only three channels—1, 6, and 11; all the other frequencies overlap with one or more of these three. The one thing that’s certain about current MIMO implementations is that no matter which technology they’re based on, they’re likely to bear little resem- blance to the official 802.11n specification when it’s finally released. If you want MIMO now, therefore, the operational rule of thumb is “you pays your money and you takes your chance.”

MIMO RECEPTION Creating coherence from chaos

=

1110101011100000101011010110101101011010110111101010111000001010111010010100101111111000 1001011001010110110101001010101101011101010111000001010111010111110011100110010100101111 0010100101001100101011011010100100110010101101110110101011011110101101010100101001100100

ÑSignal scattering caused by environmental obstacles leaves each transmission stream with a slightly different profile. Signal-processing firmware on the receiving side combines all the streams into a single coherent signal.

Status of 802.11n Ratification

If you think the government is slow to act, you haven’t monitored the glacial pace of the IEEE standards committees. The organization’s 802.11n Task Group was formed in March 2003, but the 802.11n standard isn’t expected to be ratified until the first quarter of 2007. For a time, three proposals jockeyed for final certification: TGn Sync, supported by Atheros; WWiSE, advocated by Airgo; and MITMOT, backed by Motorola. After 14 hours of presentations and discussion during the 802.11n Task Group’s January 2005 meeting, MITMOT was eliminated from consid-

eration. In March 2005, the group eliminated WWiSE, leaving TGn Sync as the only remaining candidate. Unfortunately, TGn Sync failed to clear the next hurdle: approval from 75 percent of the group’s membership. At the Task Group’s next meeting, in May 2005, TGn Sync’s advocates addressed the concerns of members who had previously voted against their proposal; but after a second vote, TGn Sync still couldn’t manage to obtain the required 75 percent approval. As dictated by the group’s rules, this opened the door for both WWiSE and MITMOT to be reinstated for consideration. When the group reconvened in July 2005, advocates of all three proposals reached a compromise and announced their intention to merge TGn Sync, WWiSE, and MITMOT into a fourth proposal, which will be presented to the 802.11n Task Group in September.

MaximumPC 2005 11

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