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October 20, 2009

Apple Slipstreams 3x3 into Wi-Fi Base Stations

Apple offered a quiet update note to its two main base station models today with a big boost in speed and coverage: The company put in a note on the data page (see "Even faster performance") and mentioned in passing to media who were briefed that its AirPort Extreme and Time Capsule base stations would see a boost of up to 50 percent in data throughput and an increase in range of up to 25 percent over the immediately preceding models.

How? 3x3. Engadget found the FCC documents that supports that statement before the announcement today, although the writer didn't explain what this means.

In the MIMO (multiple in, multiple out) antenna system that's used in 802.11n, designers have lots of choices in how to build in range and resiliency, and those choices have increased as silicon and antennas have become cheaper.

Most consumer 802.11n access points use a 2x2 MIMO array, which is two receiving and two transmitting antennas. Each antenna pair is typically handled by a separate radio chain. Each radio chain can transmit unique data for higher data rates, or the same data as other radio chains to increase redundancy, and thus provide better reception at lower rates.

These radio chains use spatial multiplexing, which allows a kind of "body english" in which varying power fed through antennas steers a beam so that it travels a unique path through space, using reflection of objects as one of the characteristics that forms the beam. Multiple receiving antennas decode these individual chains and reassemble data into what was sent in the first place.

In 802.11n, each spatial stream in the highest-rate mode can act like a separate full-speed connection. Since roughly 75 Mbps is the raw rate for 20 MHz channels and 150 Mbps for "wide" 40 MHz channels, a two-stream device maxes out at 300 Mbps of raw throughput.

Nearly all 802.11n base stations sold to date use 2x2 arrays coupled with two spatial streams; some also offer 2x3 arrays for redundancy with just two streams. However, chipmakers have been planning for some time on getting 3x3 arrays with three spatial streams into the market with a raw 450 Mbps rate. Apple may be the first consumer access point maker to bite, although there are definitely other 450 Mbps APs on the market. (See next paragraph for update.)

[Update! An informed commenter--see below--notes that there's only a single AP that does three streams. So Apple isn't slipping in higher bandwidth here, just better signal diversity and performance.]

The additional transmit and receive antennas improve how far signals can travel to a client, and how sensitively an access point can pick up distant transmissions. This accounts for Apple's statement on improved range. It also provides improved bandwidth further from the base station; the data rate doesn't drop off nearly as fast as with 2x2. The "up to 50 percent" figure relates to a range of distances, not close up to the base station.

The Wi-Fi Alliance just approved a testing regime for devices with three spatial streams, and all the major Wi-Fi chipmakers were involved in that testing. Our informed commenter says it'll be until late 2010 before we see a large number of 3-stream devices; other opinions?

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Wow, nice bit of digging & sleuthing, Glenn. Really interesting stuff!

Actually, most enterprise APs are 3x3, however, NONE of the AP Wi-Fi chipsets support 3 streams, and I don't expect Apple to be an exception. The existing 3x3 chipsets support only 2 streams, or in other words, 300Mbps. The 3x3 only provides better rate over range and better reception. The only chipset to offer 3 streams is Intel 5300, but it's available only for clients - and has been shipping for a while. Some vendors plan on having 3x3 AP chipsets late 2010, but this is just a plan for now.

Thank you so much. I have revised the article to include this. I'll be following up on this by interviewing chipmakers about the 3-stream roadmap, since it's awfully confusing even to this reporter, who has been following 802.11n since before it was called 802.11n.

It would be nice to see open ended streams that are software selectable. Hopefully, one day, wireless routers will each have a range of 1 km. or more and will overlap with a redundancy that will prevent failures like we have seen with the Telco's during 9/11 and Katrina. Add a waterproof case, an internal cell phone battery and you've got people surfing the web from their roof tops after a hurricane!

This is really confusing. I have 19 of the 'newer' style Airport Extreme 802.11n models, ranging from the originals with only 100BT ethernet up to the first simultaneous dual-band models, four of which I just got a month ago (damn!).
I was under the impression that these all had three antennas and did MIMO, but now it seems they only did 2x2. Very confusing. Do I understand that a firmware update will not be possible to upgrade the early '09 AEn models to the new 3x3?
On a related note, I have searched without success for anyone who has mapped the radiation pattern of Airport Extremes. Yes, they're supposed to be omnidirectional, but I am guessing there are certain directions that the signal is stronger. Are there any sites which post these patterns anymore? I recall there were a few in the "old" days of wifi.

2x2 is MIMO. It's two receive and two transmit antennas. You can't update the older units to 3x3, because that's a hardware change.

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