Those SDR boards can't have that much power, can they? I mean, a usb powered radio rx/tx board running in your apartment can in no way seriously disrupt other peoples' communications.

Really depends on allot of factors, a 10dbm (which is what the board spec claims to be the limit as 10mw = 10dbm (only true for 10mw since it's a 10 x base 10 logarithmic function), but it could potentially be set to higher TX if it's only limited by software and 20-25db transmitter has a range of of 1000's of sq/m, with 20+ DBM transmit power/gain you can get into 1000M+ transmit range alone) "GSM Booster"/Picocell can easily cover an area of ~200M/sq, that's a radius of 30 ft for 1800-1900 3/4G, and more (can't recall by how much off the top of my head and I'm not doing the math) for 800-900mhz cellular signals.

But you don't need to be able to provide a stable connection to interfere with someone else's phone call a lucky alignment of the heavenly bodies at just the wrong moment can cause a call to drop or your phone to start roaming, deregister etc. at quite likely longer ranges than 30ft (how longer that's another question) and that can be enough you don't need to effectively build a jammer/unrouted rogue station that would constantly blanket everyone's reception in order to cause issues.

But one of the reasons I gave fairly localized examples is that while 30ft may not seem that much and it might not be, in most residential environments it's enough to potentially cause issues for some one else on the street, driving by, next door etc, and you never know what can happen.

If you want to play with GSM or any other potentially disruptive radio spectrum use it at very low power and preferably either with direct and shielded coax connections or within a faraday cage, they aren't hard to build and you can easily put both your phone and your SDR into one and work on them remotely.

P.S. Here's a good basic reference guide for RF Power/Range from digikey, it's very very basic but it's more than good enough for back of the envelope type of stuff http://www.digikey.co.uk/Web%20Export/Supplier%20Content/Lai...

USB3 can pump up to 900mA @ 5V = 4.5W, which is pretty good. But each device has its own specs - e.g. LimeSDR has only 10dBm output power, because it doesn't have a built-in amplifier. So if you (1) don't attach a large antenna to it, (2) you operate in your basement, (3) you choose an unused ARFCn and (4) you operate in closed mode, you should be fine form technical perspective. That said, legality of this is up to you - check your local spectrum rules.

It also seems to have an option for an external power supply so you could potentially add more power to it.

I don't know what the specs for it are, but 1000mW is 20dBM which gives you a very nice coverage range on it's own and it's definitely possible to power that of USB alone.

you can use coaxial cable if you are using it for testing or education. so you don't "pollute" the legit GSM communications.

You can and I've recommended it but it's not that trivial either the path loss in coax signals as well as some other factors might not work with all SDR's and RF equipment and it takes a bit more legwork to set up.

Dumb question ... with coax, do you need an SDR board anymore? I assume not since you can do everything in CPU? Or is there some specialized acceleration you get with Silicon. Complete newb here.

The Coax is just a media for the radio wave to propagate through for all intents and purposes for this experiment it's no different than vacuum it's just a piece of cable you still need a transmitter and receiver to actually generate and receive the radio signal as well as a DAC of some sort to connect it to the PC :)

Basically if you do it via direct coax you basically connect a coax cable to the micro bnc port on your SDR (to which the regular antenna coax cable would connect) and the other end has to be connected to the BNC connector on the device that is talking to you radio, some phones have BNC connectors for testing (older phones used to have it to connect to the car phone fixture for better signal) but for some you'll have to solder a connector manually to the phone board or have some setup to connect to whatever custom connector the phone uses to connect to the antenna.

I know that the iPhone 6/s plus for example has a micro bnc connector for the Wifi antennas but I can't recall what they use for the GSM one but it's either going to be bnc or some custom connector if they are using some fractal antenna printed on flex cables or one that is embedded into the case which is quite common these days.

EDIT: Correction the connectors on the LimeSDR and on most phones are U.FL not BNC, I just have a habit of calling all Coax/RF connectors BNC so if this incorrect in some other post apologies :P

ARGGGG EDIT #2: Apparently the iphone (as well as some other phones) use some "custom" connectors that mechanically look like U.FL but are smaller, so FML and good luck finding a cable to connect to it.

That's a really good idea, btw