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This question already has an answer here:

According to this website,

a typical cellphone has enough power to reach a cell tower up to 45 miles away

So, can a smartphone send directly a message to another smartphone 20 miles away?

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marked as duplicate by tcrosley, Sparky256, Anindo Ghosh, Blup1980, Peter Smith Jul 9 '16 at 13:40

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ 45 miles? I doubt it. And smartphones are not capable of peer-to-peer communication. \$\endgroup\$ – Eugene Sh. Jul 8 '16 at 18:46
  • \$\begingroup\$ Why did you ask essentially the same question twice in a row? \$\endgroup\$ – tcrosley Jul 8 '16 at 20:12
  • \$\begingroup\$ @tcrosley They're 2 different questions. \$\endgroup\$ – Amn Jul 8 '16 at 20:15
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    \$\begingroup\$ I disagree, in both cases you want to know if a smartlphone can communicate to another directly without going through a tower. How are they different? \$\endgroup\$ – tcrosley Jul 8 '16 at 20:16
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    \$\begingroup\$ Why did you ask the same question simultaneously on two different SE sites? (see Can a smartphone send directly a message to another smartphone 20 miles away?) \$\endgroup\$ – The Photon Jul 8 '16 at 20:57
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No, it is not possible for one cell phone to communicate directly with another. Not for any physical reason of signal strength, but because the protocol design relies on cell towers playing an active role.

The cell towers orchestrate the communication between themselves and the individual phones. The detail varies between the different protocols, but the towers broadcast timing information and tell the handsets when they are allowed to respond to the tower.

The tower doesn't need to be a big tower, small micro-cells are possible but the network is designed to work for connectivity through the strucure of the network, rather than peer-to-peer.

One key point is that at the handset, transmit and receive are at different frequencies, so a handset and only transmit to a base station, and not to another handset. This allows the base station to transmit and receive at the same time (if very selective filters are used).

Finally, 20 miles range for handset to handset would be surprising. Handset to directional (sectored) antenna in an open environment is plausible but not something to rely on.

See Timing Advance for a tiny fragment of the detail of one specific cell-phone protocol.

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  • \$\begingroup\$ What do you mean by "protocol design"? \$\endgroup\$ – Amn Jul 8 '16 at 20:05
  • \$\begingroup\$ @Amn I mean the design of the GSM protocol stack, physical layer and link layer specifically. These define the RF frequencies and modulation schemes, and the details like round-trip delay which set hard limits on physical separation and endpoint velocity. This is effectively how many different handsets work on different networks in different countries. \$\endgroup\$ – Sean Houlihane Jul 8 '16 at 20:09
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Well, technically, even my WiFi signal reaches you wherever you are in the world, just that it is too weak for your household devices to decipher. In Physics, a radio signal never dies, just that it becomes weaker with distance.

Smartphones communicate via Base Transmission Stations which are large transceiver antennas mounted on huge pillars. These stations are located only 2-3 km apart in rural areas, and may be as close as 400 meters in urban areas (where buildings and structures hinder the electromagnetic field generated by the antenna (and the phones).

A signal never dies down, and yes, it will reach a cell tower 45 miles away, but it would be too weak (Signal to Noise Ratio will be very low) for the tower to decipher.

The deep space probes send radio signals from as far as the extent of the Solar System (might be more now), and to decipher those signals, you need HUGE antennas (big ears to listen to weak signals). Google up the DSN (Deep Space Network) antennas which are located at three points on the Earth surface!

Another factor is collisions and bandwidth. A GSM signal might be around 900 MHz (depending on the country) and is allocated channels near this frequency. Frequencies close by interfere with each other and neighboring channels are affected by each other. Since there is a large number of devices using the GSM band, the signal will face collisions from a large number of devices, hence making the maximum communicatable distance very small.

Now addressing your question.

In networking in general, you make up for collisions, packet errors and flipped bits by re transmissions. Technically, every packet transmitted is prone to flipped bits and errors. Once the receiver detects that the received packet is flawed (there are error detection bits and checksums for that purpose), it requests a retransmission. Yes, it is possible that a cell phone sends data and the tower receives it correctly, depending on the terrain. If not, it will request a re transmission. GSM protocol must be having a number of re transmissions which can be requested before the message sending can be assumed a failure.

A cell phone can send a message to a tower 45 miles away, but unfortunately, GSM is not designed to communicate directly to another phone (an Ad Hoc network).

The central infrastructure (the Base Station) acts not just as a relay, but is responsible for assigning the frequency channel to each SMS or call. There has to be such an entity for assigning channels or the devices would choose randomly and choose overlapping channels, hence causing cross-talk. So the answer is no, two cell phones cannot communicate directly, regardless of the distance.

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  • \$\begingroup\$ That's very informative. But unfortunately not really addressing the question.. \$\endgroup\$ – Eugene Sh. Jul 8 '16 at 19:00
  • \$\begingroup\$ @EugeneSh. I agree, but thanks for your answer! \$\endgroup\$ – Amn Jul 8 '16 at 19:03
  • \$\begingroup\$ Check again. Edited. \$\endgroup\$ – U. Muneeb Jul 8 '16 at 19:04
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    \$\begingroup\$ It's still not addressing the core issue: smartphones do not have the capability to act as cell towers. Adding wifi to the mix just confuses the issue. \$\endgroup\$ – alzee Jul 8 '16 at 19:06
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    \$\begingroup\$ Since the OP asked about "smartphone" and not GSM telephone, so 5G relaying might completely change the answer. I do agree with this answer in the sense of current smartphones. \$\endgroup\$ – AndrejaKo Jul 8 '16 at 20:17

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