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Comment by willidiots | Hacker News Reader

Yes and nothing special, as long as the clients and AP are moving consistent to each other.

If the client is moving >10MPH relative to a fixed AP (i.e. you're driving past the AP in your car), Wi-Fi doesn't handle that well. Your movement creates Doppler shift in your RF signal, which Wi-Fi can't adjust for (vs. e.g. LTE which does).

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Doppler shift in a radio wave at 10 mph? Wouldn't the Doppler shift be around 36 Hz in that case [1]? That's about 0.015 PPM. Seems temperature would have a bigger impact on frequency stability than that.

[1] ∆f = f0 * (∆v/v0) = 2.4 GHz * (4.5 / 3e8) = 36 Hz (using 10 mph = 4.5 m/s)

Edit: I'm certainly not an expert in RF. I checked my intuition against the datasheet of a radio module I've been working with (RFM69HCW), and found in a footnote of an equation (on page 31):

> crystal tolerance is in the range of 50 to 100 ppm

Here's a link to that datasheet: https://cdn.sparkfun.com/datasheets/Wireless/General/RFM69HC...

It seems that the Doppler shift (measured in PPM) would be well within 0.1% of the crystal's tolerance.

Edit #2: Doppler shift is 0.015 PPM; not 0.150 PPM as I originally stated.

Keep in mind, modern WiFi uses OFDM where the signal is transmitted via a number of simultaneous carriers offset by very small frequencies. So while the center of the 20Mhz band might be shifted just a little the small carries might be shifted far enough onto each other that the signal is unintelligible.

If the individual carriers aren't overlapping in the transmitter, they won't be overlapping in the receiver, either. Each one will be shifted proportionally (∆f/f0=∆v/v0) in the same direction. The signal is still intelligible. It's just shifted.

A radio is a physical device subject to manufacturing variance, temperature fluctuations, and aging effects. It has to be manufactured to tolerate frequency deviations caused by these effects. What I was trying to show in my analysis is that the effect due to Doppler shift at 10 mph is orders of magnitude less than at least one of these effects. If the Doppler shift at 10 mph makes the signal unintelligible, then the other effects I mentioned should as well.

For example, a transmitter with 50 PPM accuracy could cause a 2.4 GHz signal to drift by up to 120 KHz (0.00012 GHz). That's about 3000 times more than the 36 Hz that I calculated for Doppler shift at 10 mph.

> If the client is moving >10MPH relative to a fixed AP (i.e. you're driving past the AP in your car), Wi-Fi doesn't handle that well. Your movement creates Doppler shift in your RF signal, which Wi-Fi can't adjust for (vs. e.g. LTE which does).

Perhaps current-generation WiFi has more of an issue with that, but I've helped build systems that used a much older generation of WiFi in an ad-hoc (no AP) mode between nodes with a speed differential of Mach 1 (between a rocket and the ground). It worked fine, and lost almost no packets over the course of the flight.

Goodness gracious. What was the range of your network? Trying that on a consumer AP would instantly lose the signal, I'd imagine

Miles, easily. But we had three major advantages:

1) We had a shoulder-mounted directional antenna pointed at the rocket.

2) The rocket had a cylindrical patch antenna wrapped around it.

3) Wifi channel 1 is in an amateur radio band, which means we can amplify it beyond the normal limits. We ran it at 1W.

Eh, I run 1W EIRP on consumer equipment and it's legal[1]. Assuming you are in US? Or not?

[1]: https://www.air802.com/fcc-rules-and-regulations.html

US, but TTBOMK it wouldn't have been with the combination of amp and antenna. At the time, we'd looked it up and the legal limit without an amateur license was somewhere in the hundreds-of-milliwatts range.

Normal Wifi is extremely low power by regulation -- the medium itself is capable of much more, and I assume this project used different antennas than you find in your router.

I worked a place that did roaming from one AP to another one on a train in Norway in the early 2000 / late 90s, think we was the first in the world to do it.

To don't have a huge network drop we used a VPN connection on the client computer so it had the same IP while jumping from one wifi over to the other one.

Can you not sue modem providers for false advertising? America is a litigious country, I wonder why this hasnt happened?

I filed a FCC complaint on Frontier delivering 10th of their speeds. They gave me an upgraded connection that gave me full 30Mbps within a week.

The only way Corporations are going to learn how to back off is when they have to pay for their mistakes.

Did you reply to the wrong comment? Or maybe the phone app I'm using misorderered...

I think it's the wrong comment, or maybe our wifi connection misordered content.. /s

I wonder if there are also multi-path effects that become relevant with motion.

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