The Darker Side of 802.11n

A reporter recently posed a very interesting question to us: "If I have an 802.11g client running talking to an 802.11n access point, will I see any improvements?"

Interestingly enough the answer was an astounding "maybe."  And she hated it.  "It's too [EXPLETIVE] complex," she complained.  "This is the dark side of 802.11n that I didn't want to know about."   Here's what we told her:

802.11n uses multiple radios to drive multiple power amplifiers for transmitting data.  Typically each of these power amps would be identical to the single power amp found in a standard 802.11g client.  If you can effectively combine the power amplifiers, you can double client's transmit power. 

This can result in 3 dB of additional link budget (read you can go a bit faster or a bit farther), that's the goal at least. Turns out, you can't simply transmit the same signal on two antennas without it interfering with itself in many directions.  If you do, unintentional, uncontrolled beamforming will occur. This is sort of like pointing a high gain directional antenna in some arbitrary direction and results in random coverage gaps.

Actually realizing this 3 dB gain doubling requires a bit of an engineering trick. The trick is a technique specified within the 802.11n spec called "cyclic delay diversity." CDD essentially tries to keep the two copies of the signal from interfering with each other by constantly varying the phase of one relative to the other. Which, skipping more gory details (text us if you want them) avoids the coverage gaps associated with unintentional beamforming.

When an 802.11n client is receiving 802.11g signals, that client will use both radios and their associated antennas to figure out how to combine multiple signals it hears to maximize the signal-to-noise ratio.  Each radio on an 802.11n client can listen for the same signal and put the best of both signals together. It's like having two flawed pictures of the same thing but when you put them together you get a perfect picture.  This is what's happening.  A technique called Maximum Ratio Combining (MRC) is implemented in typical 802.11n chipsets to do all this crap.

So what's the net-net of all this?  802.11n is complex.  You have at least twice of almost everything and lots can go wrong. But when you get it right you'll see better performance and range - even when speaking 802.11g.

Now you know why she was pissed.