[retrac]

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.

[bozhark]

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

[wkat4242]

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).