RE: [Discuss-gnuradio] Re: Cellular relay

[Beck, Andrew Thomas - BECAT001]

Hi,
Firstly, I'm totally into re-programming old (GSM) cellphones for fun
and profit.
However, from the little I know & have seen, it is far far tougher than
you are talking about here. J2ME is an Application level interface & though
I have seen test code poking messages into lower layers, this was not
via a publicly documented interface. I see no easy way to control the pysical
layer software which is what I think would be required.
I do think you could reverse engineer a particular phone & get homebrew software
running on it. This would require reverse engineering h/w & s/w interfaces & would
take an incredible effort. I've seen the work of playstation2 homebrew community
& so believe anything is possible, but it's an incredible effort & this would be
for just one model phone from one manufacturer. Although many phones are based on
IP from just a few companies, it is invariably modified and customised in various
ways for each manufacturer.
Also, both CDMA & GSM use different bands for Tx & Rx. Perhaps the radio is flexible
enough to transmit in the rx band so another can receive it, but perhaps not. There are a
few semi-standardised interfaces to mixed signal chips, so it might be that you don't need
to solve the same problem on every phone, but perhaps not.
Anyway, I think writing software for old phones is a very noble goal. However, I'd be
impressed if you get a 'hello world' native code application running on one phone
based on currently available public documentation. That's a good place to start attacking
the radio functions.
Of course, if you could get some manufacturer to turn over all the required documentation,
then that's a whole different story!!

cheers,
andrew

-----Original Message-----
From: address@hidden on behalf of address@hidden
Sent: Thu 22/09/2005 4:22 PM
To: David Young
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] Re: Cellular relay

Quoting David Young <address@hidden>:

> Aaditeshwar,
>
> It is a shame if I have to buy or build a basestation to get any use
> out of old cell phones in my home, even if I can use a USRP and GNU
> Radio to do it. :-) Are any cellular modes more symmetric than CDMA?
> Is direct phone-to-phone communication possible, even without WiFi?
>
> Do you know whether or not it is possible to get instructions for
> re-programming the old phones?
>
> Dave

Dave,

I agree that the idea of having a thin SDR relay for networking old cellphones
might not have been the brightest of all ideas, but it is not altogether
impractical for the following reasons:

- There are companies like Clariton that tie up with cable modem ISPs and
provide a celluar RF to fibre conversion at the modems, and a reconversion to RF
at tap points closer to cellular base stations. This helps alleviate the problem
of poor cellular coverage inside buildings. The SDR relay could simply be
integrated with their solution.

- Hardly any phones discarded today have WiFi on them. Some do have bluetooth
radios but the range is very small. Hardware hacks inside phones will be needed
to increase the range.

- The SDR relay does not have to be a full fledged base station at all. The idea
here is to have some cheap device that just understands 3 channels, can be
plugged anywhere inside homes, and can link together old cellphones that do not
even have a SIM card.

To further answer your questions:

- Symmetric communication in CDMA: It is actually completely asymmetric. I also
couldn't find anything better than to use the paging and access channels given
that the entire CDMA digital processing, MAC, and link layer protocols are built
right into the chipsets which cannot be modified. Therefore, even the CDMA data
channels cannot really be used.

- Reprogramming cellphones: Most cellphones support a J2ME or BREW virtual
machine with downloadable apps, and that is all that is needed on cellphones to
enable this solution. In fact, this was a major reason to use simple
out-of-the-box mechanisms because the J2ME or BREW app on cellphones only needs
to send and receive SMSes now.

I'll also add that putting this SDR relay inside homes may not be the best
application at all. What might instead be feasible is to include relay
functionality in the cellphones itself. This will be a move towards cognitive
radio where cellphones will sense the environment so as not to create any
interference, and yet be able to communicate with peer cellphones directly. What
seemed cool to me was that such relayed communication was indeed possible.

Regards,
Aaditeshwar.


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