Let's suppose I am in the middle of the desert or some kind of jungle, and I have no 2G/GSM Signal. I would like to know if there is any civilian technology (that is or maybe isn't available to the general public) that would allow to have signal in this particular situation? Like some kind of portative cell tower?
What you might be able to use is a kind of portable femtocell but as wikipedia explains this requires an internet connection so this would probably not help in your desert or jungle scenario (it's more targeted at e.g. basement offices). Theoretically I suppose you could hook it up to a satellite based internet connection, but you would also need to power both the femtocell and the satellite setup so you'd need big batteries or a generator, making it less portable. If this would work at all, as I can imagine that the lag on the satellite link might cause issues. And if you're going to use satellite technology anyway then why not simply use a satellite phone in the first place...
BTW this is purely theoretical, I do not have practical experience in this field so am curious to see any comment or other answers.
Edit: slightly off-topic as it's slightly larger in scope but perhaps still an interesting read is this article about cellular service at Burning Man (an event in the Nevada desert) and the technical details.
There are two sides to this technical and legal.
On the technical side the main question is how portable does it need to be? If a system mounted in the back of a SUV or pickup is acceptable then it's certainly doable. Man-portable may be tricker.
You would need a mobile base station and something to power it. If you want to talk to the outside world you would need some kind of uplink (sattelite or possiblly short range microwave). If you want to receive incoming calls on your regular phone number then you would need a roaming agreement with your regular phone provider.
The hardware to run a base station is available off the shelf. See for example http://www.rangenetworks.com/products/ . The software is available as open source.
Then there is the legal side. Mobile phones operate on specific channels within specific bands. To operate legally is likely to require specific authorisation from your country's radio regulator and/or an existing cellular operator.
There is some overlap between the 900 MHz US ham band and the cellular bands used in europe but I don't know if this overlap is sufficient to allow running a functional network. http://n9xlc.blogspot.co.uk/2010/10/ham-radio-cellphone-network.html