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 working 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.
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.
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. Apple may have wanted to wait for a fully interoperable standard to be approved before shipping.
But, wait, you ask: are clients also being upgraded to 3x3 arrays with three spatial streams? Clearly, not yet. It's expensive. Which begs the question of why Apple might build a 450 Mbps access point without 450 Mbps receivers. The answer is space-time block coding (STBC), or at least I think it is.
STBC allows an access point to send data to separate device simultaneously over different spatial streams. Which means you can be transmitting at 300 Mbps over 40 MHz to one device, and, say, at 150 Mbps at 20 MHz to another at the same time. This allows the access point to have the greatest flexibility.
STBC was part of the Wi-Fi Alliance's latest updates to certification, and I expect Apple will be dropping a Wi-Fi driver update (which it calls AirPort Software) any time now to handle the client side of this equation--although from what I can tell, it's possible that no client update it needed to take advantage.
I'm not sure I have this all correctly figured out, but all the bits and pieces add up. I'll provide more information or update this post as I find out more.
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