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Comment by anfractuosity | Hacker News Reader

Has anyone ever seen a software defined radio based phone/client out of interest?

Also I've not properly read the paper yet but it mentions a VCTCXO (Voltage Controlled Temperature Compensated Crystal Oscillator), used with some of the SDRs. A while ago I bought a rubidium clock, which I thought may have been decommissioned from a cell tower. I'm curious how important the clock used is, for a cellular system?

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> I'm curious how important the clock used is, for a cellular system

Very, that's why they bother with things like rubidium holdover clocks. A lot of things depend on precise synchronization, both locally and between multiple towers.

Thanks, that makes sense, curious how they seem to get away with a VCTCXO in this case (or at least I couldn't seem to find mention of rubidium clocks in addition).

Different qualities: Quartz crystals are better than Rb when looking at short periods. Long-term stability is normally provided by GPS. Rubidium beats crystals when it comes to stability over hours - it is added as a fallback for when GPS fails. If you don't have a requirement that your system can work lets say 24 or 48 hours without GPS sync, you can skip the Rb.

I too have some kind of GPS-disciplined high accuracy clock (I don’t recall the exact tech but I think rubidium sounds right). From what I recall, precision timing is quite critical on the infrastructure side of things so that the user device can sync to it and accurately transmit and receive in the correct TDD windows.

One of the niftiest things was using the 10MHz reference clock as the clock source for my SDR, and tuning the receiver to the centre carrier of a nearby cell tower. I got a ~0.25Hz beat frequency out of it, which means this cheap eBay clock is accurate to about 1/3.2e9 Hz (cell tower was around 800MHz). Neat!

Yes, for example there is srsue in https://github.com/srsran/srsRAN_4G (the predecessor of the stack used in TFA)

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