I’m building an AI powered robotic car which needs multiple Pi, one driving, one sensing, one learning etc. The individual boards need to talk to each other, most likely using MQTT, outside of a wireless network. So to put it simply how can I network the boards without a network? Is Bluetooth a possibility?
The easiest way without overthinking it too much, would be to just set up one of the RasPis as an access point that the others connect to. Make your robotic car it's own WLAN.
If you prefer a wired connection, simple communication can work over a basic serial UART. If you have a lot of streamed data, SPI may be a possibility as well. Lastly, you can add a mini LAN switch and use wired ethernet.
If you want to use MQTT though, Bluetooth is out unless you want to use MQTT-SN (MQTT requires TCP).