Starting work on car-based BTS
For several years now I operate a sysmoBTS 1002 at my home, along with a server that runs the requisite suite of osmo-* processes. The result is my own self-made GSM signal at my home; I call this network Themyscira Wireless, named after the fictional home of Wonder Woman. This home network of mine is interconnected with real PSTN via SIP, such that my ThemWi phone has a real phone number, can call any outside numbers and can receive calls from the outside world. For the past 3 years or so I've been using this home GSM system as my primary personal phone - while I do have a traditional POTS land line as well and will never give it up willingly either, ThemWi GSM is more convenient most of the time.
But of course it works only when I am at home, effectively an exotic type of cordless home phone. For the past 2 years or so I have got into the routine where every time I leave my home for an errand, I turn off my ThemWi phone (to save battery), boot up my T-Mobile 2G phone, then drive away from home - then do the same in the reverse when I come back home. A royal pita, plus having to manage two different phone numbers for home stay vs being out and about.
But now there is word going around in Discord and Reddit vintage mobile phone communities that the evildoers at TMO are going to fully shut down their unloved GSM service on August 3. But if they think for even one nanosecond that I am going to roll over and accept a "modern" (VoLTE-based) phone into my life, they are very mistaken... I have never used a "modern" smartphone and never will!
So what do I do now? For some years now I have had the idea of operating a BTS in my car, as a kind of cell on wheels that travels with me wherever I go. Have another little sysmoBTS 1002 (see product link above) reside in the back seat of my car, with a little Raspberry Pi next to it, interconnected with an Ethernet cable. The Raspberry Pi is to be equipped with an LTE data modem - initially in USB stick form factor, later I will explore a more permanent RPi "hat" solution. The software running on the RPi will maintain a WireGuard VPN connection (running over LTE mobile Internet) to one of my servers acting as the MSC, while osmo-bsc will run on the RPi itself inside the car. The resulting arrangement can be seen as a mobile (on wheels with a gasoline engine) LTE to GSM converter, using the local LTE network wherever I go and giving me GSM.
A Digi-Key order for the necessary RPi components has been placed a few hours ago; when the RPi 3B arrives, I will start playing with Devuan on it, and turning this idea into reality. Stay tuned!
P.S. If you are going to comment about legalities, don't bother. If someone without a clue were to do what I describe, then yes, they will very likely cause interference with other networks, FCC will throw the book at them, and they will get what they deserve. However, I know what I am doing - I know where unused blank spots are in GSM-usable spectrum, and I know how to operate my BTS (both home-based and vehicle-based within my known travel range) in such way that it never interferes with anyone and therefore there will never be any basis for anyone to complain to FCC - there is literally nothing to complain about when I don't interfere with anyone or anything.
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Update: yesterday I got the first version of this idea actually working. The hardware set laid out on the back seat of my car is exactly like I planned in my original post: a sysmoBTS 1002, a Raspberry Pi model 3B+, an LTE modem in USB stick form factor plugged into the RPi, and an Ethernet cable connecting the RPi to the sysmoBTS. The RPi needs 5V power and takes it in via a micro-USB power input connector; sysmoBTS 1002 needs the same 5V, but has a barrel-type power input connector. I got a cable that goes from USB-A plug to the right barrel connector for the sysmoBTS, plus a regular USB A to micro-B cable to power the RPi - so now I only need two USB-A power ports for the whole assembly. For my initial tests I used an adapter that provides USB-A 5V from the car's "cigarette lighter" power port - in my car this port remains powered even when the engine is off, so I could lock my car and walk away with the setup still powered. This method works, but is quite dangerous in that I could run down the car starter battery and leave myself stranded if I run it for too long.
In terms of software and logical functionality, the RPi connects to the Internet via LTE, establishes a WireGuard tunnel with one of my servers acting as the MSC, and then acts as the BSC between this distant MSC and the sysmoBTS next to it. The Linux distro I run on this RPi is Devuan - the moral principle of not using systemd is important to me! Devuan still comes with NetworkManager and ModemManager though, with
nmcliandnmtuiconfiguration tools, so I used those tools to configure automatic bring-up of LTE mobile Internet connection and a static IP assignment on the local Ethernet that connects to the sysmoBTS and also allows management access for me. My own customrc.localscript then configures WireGuard and startsosmo-bscandosmo-mgwprocesses. Those Osmocom components were built from source: Osmocom provides prebuilt binaries for systemd-based distros, but not for more principled Devuan. OsmoBSC running on the car-resident RPi connects to OsmoSTP on my MSC server, with SCTP connection running inside the WireGuard tunnel over public Internet accessed via LTE. RTP streams for voice calls also run inside the same WireGuard tunnel.Aside from the danger of running down the car starter battery if I leave this setup on for too long with the car engine off, system performance is mostly OK in the initial round of tests: I can walk a little bit away from my parked car and still have usable signal, and voice quality was mostly OK in my limited tests so far. Going forward, I plan to buy a high-capacity USB power bank (60 Ah is the biggest I found) and use it as the power source for this back seat BTS instead of the car starter battery - then I will be able to exercise this system a little more thoroughly without risk of stranding myself. To be continued...
Another update: today I finally received the large USB power bank from Amazon, and I rewired the BTS setup in my car to take 5V power from this unit, instead of the car starter battery via the "cigarette lighter" jack and a power converter. Preliminary result is that it works great! Further tests to see how long this power bank will last between recharges remain to be done.