Say I built a private GSM network using a Raspberry Pi 4, a Nuand BladeRF x40, a power supply, writable SIM cards, 1 or more phones, and a Wi-Fi connection. Would it be possible to call somebody who's not connected to the GSM network through the internet?
The a Nuand BladeRF x40 is a software defined radio that is not on the Open BTS hardware list. Note use of these may not be legal to use in your country in this manner or for any form of transmission without approval / licensing.
OpenBTS for 3G states:
OpenBTS-UMTS is a more computationally intensive application than OpenBTS, since the UMTS channel bandwidth is roughly 13x larger than a GSM channel. Generally, a multi-core high performance CPU is required, such as Intel Core i3, i5, or i7 running at more than 1.6Ghz. Intel Atom processors are too weak to support the current implementation
So it is possible that the Pi will not manage (esp. as it hits the USB port very hard).
If you are looking to connect GSM to Internet, the simplest way to go is via a VOIP gateway using a soft 'phone on the Pi. Packages to look at are Asterix/Free PBX or 3CX neither of which are simple installs and VOIP/SIP routing can be a pain if your ISP objects or you have a dynamic IP address. In the case of the dynamic IP - you can set up a program internally to update your DNS records (I use DYNU for this and had great support from them but others exist).
3CX have an install guide at their site here and are a commercial company (currently offering some FOC services)
Fortunately, my router handles VOIP for me now - lots less hassle :)
- Look at VOIP / SIP
- Look at 3CX and Asterix (other solutions do exist)
- No the Nuand card is not on the hardware list.