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.
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.
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.
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).
This is fascinating. Happy to do the research myself, but do you have any recommended reading/sources to learn more about this?
For the practical 'how-to,' I recommend studying GNU Radio and SDR++; they show how to process IQ data or raw audio streams directly, and for sure there are other libraries. On the 'ancestor' side, look at the AX.25 Packet Radio protocol and AFSK (Audio Frequency Shift Keying). These are the same 'softmodem' principles used in FidoNet nodes decades ago.
GSM Arena can help you find phones with integrated FM receivers. You'll notice that many features are market-dependent, meaning: the receiver is often physically present but simply disabled by software.
> For instance, many chipsets have an integrated FM receiver that is essentially a high-sensitivity VHF radio.
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.
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).
Yeah. Even in free space. For example, attenuation at 1 km for 144 MHz (ham VHF band) is about -76 dB while for 2.4 GHz, it is about -100 dB. That 24 dB drop could mean, the signal is below the noise floor of your receiver unless you increase the RF power output which means more battery drain.
For example, BT audio gets cut just moving to the next room despite the RF power of BT transmitters being ~ 5mW( 7 dBm ) and at 10m, the attenuation is -60 dB(just free space loss which is ideal condition), so 53 dBm (7-60) at the receiver is usually sufficient, yet they struggle.
> For example, attenuation at 1 km for 144 MHz (ham VHF band) is about -76 dB while for 2.4 GHz, it is about -100 dB.
This is a common misunderstanding of the free-space path loss formula, which is expressed in terms of the idealized isotropic radiator, the length of which is frequency-dependent. In other words, this calculation is assuming a proportionally (much) smaller antenna for the 2.4 GHz case.
With the same antenna size, the path loss is exactly the same. After all, where else should the radiated energy go?
What do you mean? The size of the dipole or monopole antenna is dependent on the wavelength, so obviously the 2.4 GHz is just a few centimeters and not the same size as a VHF antenna.
> After all, where else should the radiated energy go?
Well, most of RF energy is wasted. There are software that can plot the radiation pattern, but even without knowing the exact pattern, very little RF energy is received at the target.
Sure, if you want to stay omnidirectional, but you don't have to. You can use one of several antennas based on feedback, beamforming etc.
There are tons of cool things society could enjoy if it wasn't for a small handful of shameless actors.