Tech level: more or less comparable to contemporary.

Location: forsaken, habitable planet with population of approximately 10-20 milion.

Wealth level: first world equivalent

Covering capital, main cities and nearby areas with local mobile phone towers equivalent and connecting them with optical fiber covers most of communication needs. There only remains one minor problem - the remaining 99% of the planet surface.

So it would be reasonable if standard mobile phones at this planet had some additional communication system, that works outside standard coverage range. Presumably of lower quality, ability to send some data package or text message is also really useful. Not even necessary to a nearby tower, juggling such package between nearby phones would also have some value.

For question purposes assume that if desperate such population can manage to maintain industrial base. The question only handles with technical aspect, not how to charge people for abusing emergency communication system to send lolcats (Assume that system of satellite communication would be too expensive)

How good long distance communication can be provided by such additional system in mobile phones assuming that weight of mobile phone has to be kept below half kilogram? (What would be approximately range? Would it bring some clearly visible changes to mobile phone shape like huge battery or antena? Would it be reasonable to use for this single purpose a different frequency like short waves?)

(Yes, I thought about just looking about commercially available walkie-talkies, but I was not certain whether some limitations are not technical but legal)

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    There are such systems in actual use. Satellite phones? Terrestrial Trunked Radio (TETRA)? Plain old shortwave radio? – AlexP May 30 '17 at 13:37
  • AlexP already answered, but an interesting question would be if mobile phones with cellular networks would be needed with so few people and if the alternative should be in fact additional and not just the only option – Raditz_35 May 30 '17 at 13:41
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    There's a reason that humans went with satellite communication over alternative solutions. By comparison to them satellite is cheaper and easier. – sphennings May 30 '17 at 13:57
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    Here's a short article about how teams on a certain underpopulated continent communicate with each other and the rest of the world. – Kys May 30 '17 at 15:53
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    Look up "Where there is no telephone". – a CVn Jun 14 '17 at 8:22

Your options are limited by the physics of radio waves. In descending order of range, your options are:

Satellite phones

Satellite phones communicate with satellites in orbit. So they can work basically anywhere on the planet that can see at least one of your satellites orbiting overhead. The link lists a few different services on Earth that provide this technology. It's expensive, since you have to launch satellites (66 for Iridium which claims global reach, 11 for Immarsat which claims global reach except polar regions). But this is probably the most efficient means to reach the entire planet. If you have the tech to reach other planets, you could probably combine this system with GPS and/or weather and imaging services, etc. to get more use from the satellites than just communications.

Shortwave radio

Shortwave broadcasts can be easily transmitted over a distance of several thousands of kilometers, including from one continent to another. Time of day and weather conditions can effect transmission ranges.

Long wave radio

Low frequency ground waves can be received up to 2,000 kilometers (1,200 mi) from the transmitting antenna. They can also diffract over mountains and other obstacles.

Medium wave radio

Medium wave radio includes the AM radio bands in the US. Practical reception typically extends to 200–300 miles. So you would need repeating stations scattered in some sort of mesh. These towers would need to be connected via their own network as well as having 2-way radios for connecting to users, so either satellite or fiber optic interlinks. They wouldn't be able to cover vast oceans. Time of day and weather conditions can affect transmission ranges.

Cell phone radio

Cell phones have a distance that is technically considered line of sight. There are several equations used to derive the signal distance. Basically, it comes down to height of the tower relative to the receiver. This link shows that a tower with a height of 1500 meters has a reach of 160 km.

Mesh or Peer-to-Peer

A mesh or peer-to-peer (P2P) network works by having lots of relays that interconnect. This exists today, especially for wireless data networks. It doesn't easily scale to global levels though. To reach any spot on the globe, you have to have enough short-range devices scattered everywhere. One way this might could be done is via solar-powered autonomous drones. The elevation a drone can reach might allow for the kind of ranges you need. But this would require hundreds thousands of drones to cover even just the land area of a typical earth-like planet. For specific excursions into the underpopulated areas of a planet, you could set up a line of mesh relays, but that wouldn't really give you global coverage.

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    Long wave transmitter power is typically of the order of megawatts. Medium wave trasmitter power is on the order of hundreds of kilowatts. Not really within the performance envelope of a cell phone battery. And let's not talk about long wave and medium wave transmitter antennas. – AlexP May 30 '17 at 17:32
  • Great answer, I would also advice you to add the possibility of having peer-to-peer networks between individual cellphones giving poor service but should be doable. As long as someone in the p-p network is close to a city you can call anyone in the "global" network – Bomaz May 31 '17 at 7:24
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    @AlexP: Long wave transmitters use megawatts so that the receiver can be very simple. This is reversible: with a simple battery-powered transmitter, you need a very sensitive and therefore complex receiver. – MSalters May 31 '17 at 8:07
  • @MSalters: Citation please. – AlexP May 31 '17 at 8:19
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    @Bomaz I've added peer to peer, but I don't really think it's a viable solution for global coverage. – CaM May 31 '17 at 13:09

Long range communication in something the size of a cellphone is hard. The biggest issue is the power required to transmit further distances.

Sorta long distance

Cellphone on their own have a range of up to 45 miles, assuming ideal conditions, which is the upper limit commercial walkie-talkies of similar size. You could extend this range by using directional antennas to point towards whoever you want to communicate to. Distance Can always be increased bu adding more power, bigger antenna. Software ought to be able to let two cellphone communicate directly with one another.

Longerish Distance

Individual cell phones could be programmed to behave like cell towers, with the messages relayed between them. I imagine the data rate would be very low, but as long as the cell phones care within contact of each other messages could be relayed pretty far. Also, this comes with a heavy battery drain.

  • A bigger antenna, by itself, won't help. Cell phones use radio frequencies that are limited by line-of-sight: if you can't see your target, you can't communicate with it. Sure, adding a parabolic reflector will get you several hundred miles of range, but only if you're standing on top of a mountain. – Mark May 30 '17 at 22:57
  • @Mark I agree completely, I had included that for just the case of communicating off the top of mountains. – Joe Kissling May 30 '17 at 23:00
  • Cellphones may have a range of 160 kilometer (100 miles, GSM @450 Mhz), but that's the range to a large base station antenna. For a mesh network, distances between phones would have to be much smaller. – MSalters May 31 '17 at 8:10

Satellites

Satellite systems are only expensive today because of the cost of launch -- LEO sats are in a radiation benign environment relatively speaking (vs GEO/MEO/Molniya birds which need to be rad hardened thoroughly to avoid making the next AO-10), and are relatively undemanding on their launch platform compared to higher-orbiting satellites. This means that you can pretty much use whatever decent rockets you have on hand for the job, vs needing to build a multi-stage rocket custom tailored to get into a high-velocity GTO, and reusable rocket technology is more readily applicable due to the lower deltaV needed.

Using LEO also gives you the ability to use compact terminals (see Iridium for instance) and a variety of topologies for the network (satellite buffered store-and-forward vs real-time space-to-space circuit switching vs ground interlinked satellites). The downside is you need several LEO sats to do the job of one geostationary satellite, but the much higher geostationary launch and vehicle costs can make up for it.

Ground terminals will look much like what you get with today's Iridium system in this case -- something that looks phone-like, but with a noticeable antenna vs. having one integrated into the case.

Shortwave (HF)

Before the development of satellites, just about all long-haul radio communications was done through HF (shortwave) radio technology, bouncing radio signals off of ionized layers of air high in Earth's atmosphere. By using sensitive receivers, frequency agility, and the knowledge and skill of operators, messages could be passed this way year-round without resorting to high-powered transmissions, and still are to this day in some parts of the world. Voice and slow-speed data are both possible, and improvements in technology have made it possible to leverage HF's strengths while mitigating its weaknesses and reducing the amount of operator intervention needed.

One thing that will be noted here is that HF is a direct, point-to-point technology -- there is no switching going on here. Stations will need to follow a protocol (such as Automatic Link Establishment) in order to find and communicate with each other, and also be able to be automatically frequency agile in order to maintain a link through ionospheric changes.

The resulting terminals will more than likely be a bit more "brick" like due to the need to supply at least a few watts of transmit power, and have long antennae, presumably an extensible whip of the type familiar from old-school transistor radios.

Meteor burst

Meteors not only generate spectacular light shows, but ionization bursts as well. This can be used for burst-type, point-to-point links, useful for store-and-forward messaging; a system like this will be a store-and-forward switched network, presumably with some degree of peer-to-peer component to provide resiliency, as well as the ability to establish links automatically.

Generally speaking, meteor burst terminals rely on pointable, directional antennae to aid in tracking the fast-moving ionization clouds from the meteors. The resulting network would have high latency and unusual terminal designs (presumably using a phased array antenna to achieve directionality), but could be useful for emergency or command and control communications over relatively long distances.

Satellite Phones are the most likely answer. Satellite phones communicate directly with satellites, as opposed to regular cell phones that communicate with a tower. Assuming correct satellite coverage, satellite phones will work anywhere on the planet.

Secondly, radio transmissions might be the easiest way to communicate. Shortwave radios work over hundreds or even thousands of miles, existing in frequencies not far above those of AM radios.

  • "Correct satellite coverage" requires remarkably few satellites. – Draco18s May 30 '17 at 17:31
  • "as opposed to regular cell phones" Um, no, not really. The only real theoretical difference between an Earth-bound cell phone and a satellite phone is the intended location of the base station. A satellite phone communicates with a base station in orbit, and a cell phone communicates with a base station near ground. This theoretical difference then results in a large number of practical engineering differences, likely including transmission protocol differences, but the basics of how the two work is largely identical, particularly on the terminal ("end user device") side of things. – a CVn Jun 14 '17 at 8:09

You set up a mesh of Wi-Fi routers powered by Wind, Earth and Fire (also Water). The range is few hundreds meters from a "node".

OOooooooooR

You create a mobile phone that can be also radio broadcaster. It all depends on what wavelength on your planet are better to use for transmission.

Of course the bigger the antenna the better reception and signal strength. So maybe a mobile phone with attachable antenna for the outback travels?

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    Your forgetting everything that was mentionned for instance satellite phones. Wi-Fi routers have poor distance to cover 99.9% of the planet – Mederic May 30 '17 at 13:45
  • I think it would be easier and cheaper to set and maintain a net of routers than satellites. Because I think that if you can send a bunch of satellites as sure you can build a ton of cell towers. – SZCZERZO KŁY May 30 '17 at 14:11
  • You could probably deploy a mesh of Raspberry Pi based repeaters using a point-to-point (targeted) medium range packet forwarding system for a few bucks a pop, if I had to solve this problem that's probably how I'd go. They could be solar powered if they were mostly inactive (Just receiving), broadcasting would take some power but it seems like this is a pretty rare operation. If one were to mass-produce these right now I'd say you could do it for less than \$35/station (I'd guess \$100 to make it with off the shelf products right now). – Bill K May 30 '17 at 20:06
  • @BillK Exactly that what I had in mind. On dx.com cheapest router is \$5 and solar panel is \$15. If you set desing and production around one type of device you can even plan a way of deployment into their design (dropping from planes for example) – SZCZERZO KŁY May 31 '17 at 8:30
  • @SZCZERZOKŁY Also needs some kind of medium-range communication mechanism (Pringles can+small transmitter?)--and if you wanted to you could deploy them on solar powered drones that could make a couple 10 minute hops a day with recharging between. You'd still be under \$100 for mass produced units. Drones give you treetop deployment as well--more range and better line of site might let us use a lightweight optical solution for linking although it would have to constantly compensate for the movement of its perch) – Bill K May 31 '17 at 20:43

Radio, like all of the Electromagnetic Spectrum, is line-of-sight. We beat that limitation by reflecting the signal off of something. Shortwave bounces off of the ionosphere. Ham Radio operators have bounced signals off of the moon. While modern satellites don't just passively reflect a signal (they amplify and rebroadcast), the earliest ones did.

To relay the signal, you'll need something to bounce it off of. Google is experimenting with high-altitude balloons to act as in-atmosphere satellites (see Project Loon) It would be cheap enough for the government to produce a large quantity to provide good coverage. A similar idea is to use a blimp for the same purposes. They would be more expensive and difficult to operate.

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