Direct cellphone to cellphone communication via a thin relay

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.
Integrated Cellular and Ad Hoc Relay (iCAR) Systems: Pushing the Performance Limits of Conventional Wireless Networks

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.
UCAN: A Unified Cellular and Ad Hoc Relay Architecture

If all this works, you can have all your old camera phones plugged in different rooms as security cameras, or for medical survelliance, or just to enable the digital home vision in a cheap way. The problem however is that 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.
Recyclying a billion cellphones

I was however thinking that low bandwidth ad hoc communication might still be possible on current dat unmodified cellphones on the cellular GPRS/CDMA interface by using a low cost relay acting 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 (which is unencrypted) for uplink communication.
4. Forward SMSes to the cellphones on the paging channel (which is also unencrypted) for downlink communication.

Thus, the paging and access channels can be used with SMS as a transport layer. SIM cards will not be needed, and no communication with the infrastructure will be required. I agree that high bandwidths will not be possible, but this will suffice for the kind of applications that do a lot of local processing and do not have huge amounts of data to transmit.

With all the work available on SDRs, designing such a relay should not be too difficult. 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. On the device side, 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.

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.

I will hasten to 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.