Experimental AMPS mini-basestation

This is really neat. Mark Atherton has put together a very basic, working, replica of a base station for analog cellular phones. I've blogged about something similar for GSM phones in the past. This analog system is pretty fascinating for me.

We really have the parts to put together something basically similar for ham radio. The audio could be exactly the same as it is now. Add in a control channel and people could call you even if you have the volume turned all the way down. There have been in-channel methods for quite a long time of course. Signalling systems such as CTCSS, DCS, DTMF have been constructed in the past, and barely used. As of yet, noone has constructed an out-of-channel signalling method. The capability is there for APRS I believe. Some APRS capable radios can advertise a frequency you are monitoring in your report. I think one or two may even allow you to QSY to an advertised frequency.

It'd be pretty cool if you could send out an APRS call, have the other parties' radio QSY to your freq if they answer it, test the path, maybe QSY to a mutually accessible repeater if necessary, etc. I have no delusions that this would be constructed or used though. Previous in-channel signalling methods have been around for decades and remain fairly unused, except to access a repeater. Other neat technologies have falling by the wayside, like Kenwood's DCS-based ID tech.

You know what they say, a rolling stone gathers no moss.

Ham Radio Cellphone Network

I highly recommend this article to anyone, it really resonated with me. The article makes a good point that anyone can throw a wire into a tree and call CQ. But there was one key paragraph that sent my mind wondering. I quote:

Future developments in the non-amateur world of radio from that point included cellular technology and the transmission of higher speed data over the air. Commercial applications for broadcast radio and television have changed radically and now include the imposition of digital methods. Military applications for secure battlefield communication use satellite and terrestrial means like mesh networking for voice and data transmission. Our homes, restaurants and coffee shops are bathed in RF transmitted data that keep our mobile devices connected to the Internet.

 Sure, there's the High Speed MultiMedia HSMM experiments... although the working group for that has disappeared and the general attitude I see about it, over and over is that you can do more with Part15 than you can with the higher power afforded with part97. It's a shame really. I've also seen a couple rare web pages discussing experiments with DATV. Much to my surprise, the experimenters preferred DVB-S to ATSC.

I'm not saying that Ham Radio is completely irrelevant. There's a lot of focus on it's use in emergencies and getting ready to help out in a disaster. And that's great. We have digital modes that run with a soundcard interface on a computer and software defined radio. There is a fairly basic digital voice mode called D-star. That's the big developments lately. Other than that, operating is fairly much the same as it was 30 plus odd years ago.

Why not a Ham Radio Cellphone network? I did some searching and this is what I've come up with..
Okay, the article discusses the use of this stuff to "hack" people's cellphone connections and listen in to their traffic. It misses a point that is blindingly obvious to me.

• European GSM cellphones have 900mhz as a band
• American Ham operators have 900mhz as a band
• Hardware exists to set up a homebrew cellphone base station
• How cool would it be to set up a legal ham radio cellphone network!

KJ6GCG, Chris Paget, set up his system to demonstrate the vulnerabilities of the GSM system specifically by spoofing the network ID for an active carrier. It should be entirely possible to set up a "fake" carrier that will not interfere with any commercial one and run it on our 900mhz band. Possibly even restrict access to special SIM card programming that could be posted online for any Ham Radio Operator to access. GSM can be run without encryption entirely, it's another point that allowed Mr Paget to demonstrate the call recording. Running in this mode will avoid any trouble with the regulations on the merits of codes and cyphers. The 900mhz band in the phones should be completely unused in America, that option is there to remain compatible with European networks.

Imagine this: Your area sets up a Ham Cellphone node and various operators get a GSM quadband phone of their choice (probably needs to be unlocked). Now they can carry a form of communication around that allows them to contact other hams at any time. It will always work in an emergency. You could potentially allow for a "phone patch" operation. It would be beyond easy to put in an extension number to allow access to any attached repeaters, echolink, etc. Call ex# 270 to access the 147.270 repeater!

I wonder if the data connection works.. Hello hinternet! GSM-APRS? Text messaging? You could set up a truely cell based network with HSMM backhauls between each cell. Put the backhaul in the Ham allocation of 2.4ghz and have fun.

The OpenBTS project is what makes all of this possible. They use a software defined radio called the Universal Software Radio Project (USRP) along with their own Linux-based software to fully act as a Cellphone Base station. There is a blog written by the OpenBTS developers here.

Incidentally, The OpenBTS people have been running a small cellphone network at the Burning Man festival for the past 3 years or so. They get a special temporary license from the FCC and coordinate it with the phone company that covers (or doesn't, in this case) the area. The Wikipedia article references this but I can't find the blog posts that I remember where they talked in detail about it. This is the authorization for 2008 with temporary callsign WD9XKN.

Of course, after writing this whole thing I run across a Wikipedia page with GSM frequencies worldwide. Some of the allocations fall in the 900mhz band but it's not clear to me if there are any channels that fall completely into the 902-928mhz bandwidth that we are allotted. Can uplink and downlink frequencies be set to fall within the allotment? Will that actually work with any phones? I don't know.

Questions? Comments? Flames? Does anyone really read these things?