Hacker News Reader

Comment by can16358p | Hacker News Reader

What would be a realistic effect of Wi-Fi still being on 5GHz when a radar scans the area?

Any actual radar would probably be far away from my home 5GHz Wi-Fi anyway, which has relatively low tx power. Is there a realistic scenario where a 5GHz-scanning radar sends a pulse from far away, and my home Wi-Fi's signal gets picked up by it?

View on HN

My Bluetooth headphones stop working around the train station at rush hour. While the coding used is spread spectrum and many devices can use the same frequencies, each transmitter lowers the overall effective signal-to-noise ratio for all other users in the area. At the train station with thousands of radios transmitting on the same frequencies within a stone's throw, there is so much noise my headphones can't hear my smartphone despite being one metre away from it.

Same idea with radar aimed at an urban area. Thousands of WiFi devices in a high rise will add up. Tens of watts, hundreds of watts of noise. A large and dense downtown core will be transmitting kilowatts of radio noise from millions of devices. More noise means less SNR. And less SNR in radar means a blurry/noisy radar view, and lower effective range.

Is this why there’s such shitty service when the local college is having a home game?

That's a slightly different thing with mobile cells with CDMA style techniques reducing in range when they're busy. Called cell breathing.

https://en.wikipedia.org/wiki/Cell_breathing_(telephony)

This is because CDMA is basically a cacaphony of stations screaming for attention and as a cell gets busier the furthest stations (phones) won't make it through the noise.

GSM was a lot better at this with its fixed timeslots, however as a result it had much more flexible capacity limits. From 3G onwards all standards are based on CDMA (not just for technical reasons, also because of Qualcomm's lobby who are the ones that own many CDMA patents).

The radar is looking at reflected analog signal bouncing off rain drops. I'm not too surprised an active transmitter blasting digital data can cause interference. Raw 802.11 packets can go several miles and be successfully picked up without corruption. The useful range is simply limited by the bit error rate exceeding what the forward error correction is capable of correcting. Also, each 802.11 frame preamble is transmitted at a relatively low symbol rate, maybe like 6Mbps for 5Ghz channels. Even if the data itself is at a higher rate that's spread over more spectrum, the preamble is going to maximally stick out; that's the whole point of it, to be maximally received by all radios.

Yes, because the radar is looking for a return signal from a cloud, which can be pretty faint. A powered transmitter can be more powerful than a radar return.

Doppler weather radar shows interference from 5 Ghz networks (https://en.wikipedia.org/wiki/Dynamic_frequency_selection), but is not used in chunks of the US (don’t even exist on the west coast), but is critical in places with lots of rain storms (south, northeast, midwest).

What's your source for areas of the U.S. without doppler? It looks like NEXRAD dopplers are "everywhere" in the U.S.

https://en.m.wikipedia.org/wiki/NEXRAD

It's not for NEXRAD. The frequencies in question are used for Terminal Doppler Weather Radar (TDWR), which is a special radar used for detecting windshear and higher resolution precipitation data at select airports.

See: https://en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar

The radar will likely use Gold codes or another sequence with strong autocorrelation properties to reject any signals other than its own reflected beam. It generally won't be a problem.

This is incorrect. Wifi systems that don't comply with DFS show up brightly on the radar display. Here's an example from a real-world system of what a misbehaving 5 GHz AP looks like:

https://journals.ametsoc.org/view/journals/bams/97/7/full-ba...

The FCC has a list of WISP companies they've gone after for misconfiguring their wireless equipment:

https://www.fcc.gov/general/u-nii-and-tdwr-interference-enfo...

Interesting, I'm genuinely surprised that's necessary. WiFi looks like white noise, and should be more or less trivial for a radar to reject.

If removing “noise” was trivial, we would all have perfect radios.

What makes you think that would be trivial?

Years of engineering experience in RF and DSP applications.

A radar that can be jammed by a WiFi access point amounts to an HR failure at the company that made it.

It's not jammed, it's dealing with a high noise floor and a weak signal.

You can only do so much to identify your own signal when it's smeared over a trillion water droplets.

I think you’ve misunderstood what we are talking about.

This isn’t jamming.

If it was trivial to identify signals from high noise situations all our radios would be perfect.

Reply on news.ycombinator.com