Continuing in the 802.11ac wave 2 series where I dissect each feature in the “wave 2” grab-bag of technologies, this next post in the series explores the impact of beamforming.
A couple of years ago, I wrote about how beamforming is like an investment. Just like investing in a stock, you start by paying a cost, and hope that the return is higher than the cost.
The steps in beamforming are:
- Measure the radio channel. This process involves exchanging frames between the two devices. Frame exchanges involve using airtime, so the frame exchange is a cost to overall throughput because it is airtime that does not transmit user bits.
- Calculate the “steering matrix,” a mathematical object that describes how to transmit your frame in a given direction. Use the resulting steering matrix to transform your omnidirectional packet into one that has a preferred direction.
- Transmit the beamformed frame. Here’s where the returns come in. If beamforming raises the data rate on the packet successfully, it requires less airtime for transmission. In the desired scenario, the lower airtime saved in the transmission step is greater than the channel measurement overhead required in the measurement step.
In practice, what does this look like? Well, the data rates in 802.11 are based on SNR, not on distance, and there’s a cluster in the middle of the data rates where they are close together. Beamforming doesn’t help when the receiver is close to the transmitter because you’re already at the top rate. When you’re far away, the SNR distance between two data rates is so big that you can’t make the leap. It’s in that middle range.
When I went looking for examples of real-world assessments of beamforming gain, they were similar. I’m partial to this write-up from Tim Higgins last year because he made an effort to test at distinct ranges and report results. (That he opens quoting me didn’t hurt, either). What you see in Tim’s results is that beamforming helps in the mid-ranges, but it’s hardly a game-changer.
So if beamforming isn’t a game-changer, why are we talking about it? Because it’s necessary for MU-MIMO, as we’ll explore as this series continues.
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Other posts in this series on 802.11ac wave 2:
802.11ac wave 2: A brief history of beamforming(By Matthew Gast)
802.11ac wave 2: Regulations around 160 MHz channels (By Matthew Gast)
160 MHz channels in 802.11ac wave 2: facts and fictions (By Matthew Gast)
What does 802.11ac "wave 2" mean and do you need it? (By Matthew Gast)
Wait on 802.11ac Wave 2 until mobile devices catch up (by Metka Dragos)
Deploying 802.11ac won't force a wired network upgrade (by Metka Dragos)