Comment by digiown
24 days ago
Will the walkie talkies work if there are hundreds in a small area all transmitting data with each other? Besides, there's just not that much bandwidth there.
24 days ago
Will the walkie talkies work if there are hundreds in a small area all transmitting data with each other? Besides, there's just not that much bandwidth there.
The smartphone is just an advanced walkie-talkie, currently limited only by the mobile operator, the law, the radio chipset, and the OS.
In a true emergency, who can stop you from modifying that architecture? Once you treat the device as an independent radio node (using its DSP power to run custom modems) you can establish a mesh network with a range of several kilometers.
We have a '4x4 car in our pockets; we’ve just been conditioned to treat it like a toy.
Not disagreeing with you, but you’re papering over a lot of complexity.
Note that cellular radios are highly specialized and the filtering circuits are tuned to specific bands. It’s not exactly like having a software defined radio in your pocket.
Next, at the modem level, you’ll need to implement and then sideload custom firmware. Finally, you’ll need to expose the right APDUs to the kernel to manage the whole thing.
TBH it sounds like a fun side project, but my point is you need to repurpose a lot of different parts of the stack to accomplish what you want.
I was pushing on the walkie-talkie case to gain the maximum results from existing phones, that's a true emergency case.
You’re absolutely right that the 5G/LTE baseband is a black-box nightmare to repurpose. But I’m not looking to hack the cellular modem; I’m looking for the dormant '4x4 car' already available.
For instance, many chipsets have an integrated FM receiver that is essentially a high-sensitivity VHF radio. By taking the raw audio output and applying a Software Modem (AFSK/FSK) in the user-space, you bypass the kernel/firmware complexity entirely. You don’t need to sideload a modem driver if you treat the audio jack or the internal FM bus as your physical layer.
The 'complexity' is real if you try to fight the manufacturer's fences, but it vanishes if you understand the full stack. A pair of wired headphones becomes your dipole antenna, and the phone's CPU becomes your DSP engine. It’s not about rebuilding the Ferrari; it’s about realizing there’s a VHF engine hidden in the chassis that doesn't need 'permission' to receive bits. You just need a software demodulator the catch them, but for sending you'll need an external transmitter (an USB SDR or jack-to-FM).
2 replies →
The smartphone is talking to a highly sensitive receiver fed by a large sensitive antenna listening carefully in the direction of the smartphone. The base station is transmitting back a carefully directed beam with orders of magnitude more power than a smartphone. The system is highly asymmetrical. Ohh and maybe there is not one but many base stations talking concurrently to the smartphone so that if one looses some data the flow is maintained.
Since I’m not able to edit my original comment: rm30 is actually referring to something much more interesting than jailbreaking the LTE/NR stack.
> For instance, many chipsets have an integrated FM receiver that is essentially a high-sensitivity VHF radio.
Walkie talkies as licensed today wouldn't because they are required by law to use exclusively stone-age radio technology. But modern unlicensed radio technology is incredibly good at sharing scarce 2.4 Ghz spectrum. Sometimes devices do interfere with each other, but they remain useful and they are far better at sharing than any expert would have predicted years ago. Let the radio engineers try.
It is not as easy as you think.
RF attenuation is proportional to frequency and at 2.4 GHz, it is very high. Also, the distance over which one could communicate depends on antenna height, so if both parties are at ground level, it is not feasible over a few hundred meters unless both are in wide open space.
Source: ham operator who has played with long distance device to device communication without using a repeater.
> RF attenuation is proportional to frequency and at 2.4 GHz, it is very high.
Through building materials, foliage etc, but not in free space/line-of-sight.
> Also, the distance over which one could communicate depends on antenna height, so if both parties are at ground level, it is not feasible over a few hundred meters unless both are in wide open space.
Isn't it just the opposite? Antenna height is only the limiting factor with line-of-sight, otherwise NLOS considerations like attenuation by building materials, multipath propagation etc. start to matter much more. Modern radio standards are extremely good at that.
Of course line-of-sight usually remains the ceiling, since there usually isn't much in the sky to helpfully reflect signals back down, at least with mobile transmitter compatible transmission levels (i.e. excluding shortwave).
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This is great on paper until some jackass wants to access their home NAS over the public frequency range so they can watch anime all day at their desk, which only works when they use multiple channels at once.
There are tons of cool things society could enjoy if it wasn't for a small handful of shameless actors.