Can Cell Phones Communicate Directly Without Towers?

[treehouse]

Hypothetically, could the transmitter-receiver systems in cell phones communicate directly among other such systems at a range comparable to the range they can communicate with towers?

With 4G bandwidth?

 

[256bits]

I have wondered the same thing.
My first guess would be NO because the transmit and receive frequencies would be different.

 

[pantaz]

Even if you matched the frequencies, the cell phones' very low power transmitters would make such an arrangement worthless.
It's not even worth discussing all the necessary changes to the phone's firmware.

 

[xts]

Ask such questions at engineering subforum!

Yes, they could communicate directly, just two issues:
1. it would be pretty difficult to modify any commercially available phone to do so, as the low level protocols are hardwired in their chips;
2. in most countries it would be not only illegal, but really prosecuted, as interferring with operating cell-phone system.

 

[granpa]

a walkie-talkie function is built into some cell phones.

I always wanted one but could never find one.
(not that I really looked that hard since I can't really afford one anyway)

I was surprised that they would sacrifice one of their frequencies for this.
the downside is that it isn't private

 

[Darrenmackenz]

a walkie-talkie function is built into some cell phones.

I always wanted one but could never find one.
(not that I really looked that hard since I can't really afford one anyway)

I was surprised that they would sacrifice one of their frequencies for this.
the downside is that it isn't private

Yeah the function was called PTT (push to talk). Worked just like a walkie talkie. Don't it worked on uk mobiles though, the function was disabled. I think the function was on my old nokia n70.

 

[granpa]

I don't understand most of this:

http://en.wikipedia.org/wiki/Push-to-talk

Push to Talk over Cellular (PoC) is a service option for a cellular phone network which permits subscribers to use their phone as a walkie-talkie with unlimited range. A typical Push to Talk connection connects almost instantly. One significant advantage of PoC / PTT is that it allows a single person to reach an active talk group with a single button press; users need not make several calls to coordinate with a group.

Push-to-talk cellular calls similarly provide half-duplex communications — while one person transmits, the other(s) receive. This combines the operational advantages of PTT with the interference resistance and other virtues of mobile phones.

Traditional mobile phone networks and devices utilize full-duplex communications, allowing customers to call other persons on a mobile or land-line network and be able to simultaneously talk and hear the other party. Such communications require a connection to be started by dialing a phone number and the other party answering the call, and the connection remains active until either party ends the call or the connection is dropped due to signal loss or a network outage. Therefore, the telephone communication protocol does not allow for casual and immediate transmissions to be sent to other parties on the network. While telephone calls require the lengthy process of dialing, network switching and routing, call setup, and waiting for the other party to answer, a two-way radio has a much quicker protocol because of the immediacy of push to talk communication.

Later versions of PTT are based on 2.5G or 3G packet-switched networks and use SIP and RTP protocols. These particular versions of PTT are called Push to Talk over Cellular, which is abbreviated PoC. When used with GSM and CDMA networks, the PTT service commonly does not use up the regular airtime minutes that are available for general voice calls.

 

[xts]

Yeah the function was called PTT (push to talk).

Push-to-talk-Over-Cellular PoC (popular in USA and also available in some European countries, e.g. Spain) is not a direct phone-to-phone communication. Each phone communicates with a cellular system transmitting station, which retransmits signal to other handset. You may test it: it doesn't work if any of handsets is out of BS range, you may not use it if both phones are registered at different operators, etc.

Later versions of PTT are based on 2.5G or 3G packet-switched networks and use SIP and RTP protocols. These particular versions of PTT are called Push to Talk over Cellular, which is abbreviated PoC. When used with GSM and CDMA networks, the PTT service commonly does not use up the regular airtime minutes that are available for general voice calls.

True - they are charged as internet transmission. PoC counts to your bill like transmitting of some internet packets (not that much, anyway)
"Really direct" communication between handsets is not supported by any of [ADDED: civilian] mobile standards, except of DECT (short distance system - few hundred meters - used for communication between your fixed line phone and wireless handsets for home and office use). DECT standard allows for direct pairing, and operates in unrestricted radio band, so many DECT handsets (e.g. Siemens Gigaset, I use...) support it. You may take a pair of handsets from your home for a picnic and talk in a walkie-talkie mode, even if handsets are out of range of your base station.

ADDED: I've forgotten: Most of restricted use cellular systems (e.g. TETRA used by police, firemen, medical emergency, etc. in Europe) support direct communication between phones. But, to avoid interference, they switch to direct communication only if base station is not in range.

 

[treehouse]

I have wondered the same thing.
My first guess would be NO because the transmit and receive frequencies would be different.

So individual cell phones transmitters and receivers can transmit and receive only on a much narrower range of frequencies than a tower?

 

[treehouse]

Ask such questions at engineering subforum!
2. in most countries it would be not only illegal, but really prosecuted, as interferring with operating cell-phone system.

What about shortwave?

 

[xts]

So individual cell phones transmitters and receivers can transmit and receive only on a much narrower range of frequencies than a tower?

For GSM and UMTS-FDD they use different frequencies for up- and down-link, but radio electronics may still be easily retuned. For UMTS-TDD (rarely used, but implemented by phones) they use the same frequency. So that is not a problem.
Problem is that the protocols are asymmetric (e.g. most control functions, like assigning timeslots for transmission are performed only by BS). So you could use only the very lowest layers of communication: radio electronics and modulators, but the transmission control, coding, encrypting, transmission power adjustment, error detection/correction, etc - would have to be implemented by you.

What about shortwave?

Sure, you may use shortwave (except of antenna size rather unconvenient for handheld phone) or any other unrestricted band, like for CB. But what this have in common with cellphone? You don't use its radio part nor its cellular protocol part. OK... you still may use its case, battery, speaker, microphone and keyboard... But I would rather buy CB than build it myself basing on cell-phone.

I missed that your question:

at a range comparable to the range they can communicate with towers?

No - for two reasons: base stations use hi-gain directional (usually sectors are 60°) antennas and advanced noise reduction systems, while handsets use simple single omnidirectional antennas. Two: power transmitted by BS is much higher than by handset.
Max distance for handset-handset communication for TETRA in open area is 3-5km, while handset-BS is up to 25-40km (anyway BS grid is usually below 20km even in wilderness, much more dense in urban areas)