[Discuss-gnuradio] Re: Cellular relay

[Aaditeshwar Seth]

Hi

This is a follow-up on a mail I had sent out earlier to the list but 
didn't get a reply. I found something interesting that I thought of 
sharing with you guys. And it'll be great if you can tell me about some 
public CDMA SDR implementations, and whether the people on this list are 
thinking of doing anything about it in the future.

Ok, so making this kind of a relay is indeed possible. And the kind of 
applications it opens up are quite interesting. Have a look at 
http://blizzard.cs.uwaterloo.ca/keshav/mediawiki-1.4.7/index.php/Recycling_a_billion_cell_phones 
which is my PhD supervisor's idea on reusing old cellphones. Now, when 
you have this kind of a low-bandwidth cellular relay, you can have all 
your old cellphones plugged in different rooms as security cameras (all 
cellphones have cameras already!), or for medical survelliance, or just 
to do some cool digital-home kind of stuff. So, it is indeed useful to 
have something like this. And to do it, here is a small email exchange I 
had with people working at a leading cellular provider.

Let me know if you know anything more about this, or you are interested 
in doing this, or anything.

Thanks!

Aaditeshwar.
http://www.cs.uwaterloo.ca/~a3seth

--------------

Adi,

I think that is possible between the mobiles within the same cell (accessing
the same base station). Right now neither the access channel nor the paging
channel SMSes are encrypted, as far as I know of. With the proper tools you
can monitor all the SMSes on the paging channel in a cell, which I think is
true for access channel too. However, we don't have the equipment (we need a
base station equivalent) to listen to the access channel. 

Keep in mind that, right now the base station is very dumb, it does not do
any local processing rather than relaying the communication to the BSC or
the MSC. So the SMS communications today involves a message center and the
MSC. However, as you have mentioned, a thin software can be developed for
the base station to perform such a job.

Hui

-----Original Message-----
From: Aaditeshwar Seth [mailto:address@hidden 
Sent: Monday, September 19, 2005 12:01 PM
To: Hui
Subject: Re: [Fwd: Cellular relay]

Hi Hui,

Yes, cellphone-cellphone communication will not be possible, but I was 
thinking of doing it through a thin relay instead, that acts as a fake 
base station. All that this relay will have to do is as follows:

1. Listen to the pilot channel of the network that is giving cellular 
coverage to the area, and infer what channels are free for low power 
communication to nearby cellphones. This is purely to avoid interference 
with the actual cellular network.
2. Start broadcasting paging channel and access channel parameters on 
its own pilot channel at a low power.
3. Receive SMSes from the cellphones on the access channel for uplink 
communication.
4. Forward SMSes to the cellphones on the paging channel for downlink 
communication.

The doubt I have is whether uplink SMSes on the access channel are 
encrypted (which in turn implies that a prior authentication will also 
be needed). If they are not encrypted and no authentication is required, 
then designing such a relay should not be too difficult, especially with 
all the stuff already available for software defined radios.

What do you think?

Thanks,
Aaditeshwar.


Aaditeshwar Seth wrote:

> Hi,
>
> My question is not directly connected with the SDR radio, but I think 
> there might be people on this list who will be able to help me. Or, 
> point me to a right place to put forth my questions.
>
> You must be aware of research efforts made for cellular ad hoc relay 
> systems which suggested a slightly modified (or an extra) radio on 
> cellphones that could enable ad hoc communication directly between 
> cellphones, or from a cellphone to a thin relay. 
> http://csdl2.computer.org/comp//proceedings/hicss/2002/1435/09/14350300.pdf 
>
>
> The reason why a radio modification is required for such systems is 
> that the chipsets on cellphones are hardwired to communicate to the 
> infrastructure only. Dual mode cellphones can however get around this 
> problem by enabling ad hoc communication on the WiFi interface 
> instead. http://www.bell-labs.com/user/ramjee/papers/ucan.pdf
>
> I was however thinking that low bandwidth ad hoc communication might 
> still be possible on the cellular GPRS/CDMA interface by using a low 
> cost relay access point that pretends to be a real access point. For 
> example, the paging channel or the low bandwidth data channel could be 
> used with SMS as a transport layer. I am not sure, but I think this 
> goes unencrypted. The relay will then have to function at the 
> transport layer. I agree that high bandwidths will not be possible, 
> but a large number of applications can be dreamt of that can benefit 
> from this ad hoc communication.
>
> Thanks!
>
> Aaditeshwar.