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This summer I will be attending a large music festival for the second year. I will be going with other people this time, and I'd like to be able to communicate with the other people wirelessly in some way. Cell phones are out, because even if you could get reception your phone dies quickly trying to fight with the other 50K+ cell phones in the square mile that the festival takes up. Walkie talkies are difficult too- because there are so many people, nearly every channel is occupied.

I'm always open to reasonable commercial solutions but when faced with a problem like this my first thought is, what could I build? I've got basic requirements:

  • A bitrate of >0b/s. Yes, that says bits per second. 0.1b/s would satisfy this requirement.

  • A working distance of about 1km / 0.5 miles. I'm willing to compromise here for...

  • Pocket-sized and battery-powered. Obviously not easy but this rules some tech out immediately.

I can use one bit to indicate "meet at the designated spot". I can use four bits if we just sync up our maps with a legend. So, I don't need practically any bandwidth at all, which might drastically simplify some designs. Also, I'd be willing to get any licenses I needed to work on this, but I am not currently knowledgeable about RF communication. I do have a strong basic electronics and programming background, though, so any ideas would be helpful.

I guess my basic question is: is there a wireless tech available that meets the bulleted requirements and would not have the problem of being completely oversaturated by having 100K people in one square mile?

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    \$\begingroup\$ If the airwaves are full of traffic already, how do you expect your simple on/off Morse Code style thing to work properly without super awesome filtering skillz? I guess a very high quality RF transmit/receive pair with extremely good band-pass filtering could make use of an (illegal) unused frequency and it will probably work okay. \$\endgroup\$
    – KyranF
    Commented Jan 16, 2015 at 1:47
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    \$\begingroup\$ Just for you guys to communicate? What about a 433mhz radio at low bit rate? Or a teathered blimp and some modified laser pointers. Kidding on the last one, sort of :) \$\endgroup\$
    – Some Hardware Guy
    Commented Jan 16, 2015 at 1:49
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    \$\begingroup\$ What comes to mind is something like a smart meter communication method for remote areas where a low amplitude signal is sent multiple times at relatively lower frequencies. By the time the signal reaches the receiver, the signal is below the noise floor. The receiver is designed to track the frequency and when it compiles the noise over time, it can extract the information back out. \$\endgroup\$
    – horta
    Commented Jan 16, 2015 at 2:26
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    \$\begingroup\$ What about to use a wireless USB network adapter like this amazon.com/Etekcity-Wireless-Network-Adapter-Antenna/dp/… \$\endgroup\$
    – GR Tech
    Commented Jan 16, 2015 at 4:59
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    \$\begingroup\$ @KyranF Your question is exactly what he IS asking. IF the airwaves are full, how would I ... . There are many potential answers :-). \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 16, 2015 at 12:52

3 Answers 3

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Amateur radio is perfect for that. Hand-held radios can reach several miles. You can use voice or morse if necessary. There is rarely any congestion, there may even be some repeaters. The initial test is pretty easy to pass as long as you get the manual.

Radios: http://www.aesham.com/ham-radios-handheld/?p=catalog&mode=catalog&parent=274&pg=1&CatalogSetSortBy=price

Study Guides: http://www.arrl.org/ham-radio-license-manual

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  • \$\begingroup\$ My general assumption thus far has been that, given ~100K people in ~1 square mile, any common or easily accessible technology will already be used. \$\endgroup\$
    – TheNoonMoose
    Commented Jan 16, 2015 at 15:20
  • \$\begingroup\$ Given the number of channels open to amateur radio station operators in 70cm and 2m and the low density of operators, I think its safe to assume it wouldn't, especially since it requires an FCC license. For instance, I've taken my radio to the Imperial Sand Dunes near Yuma, AZ on holidays (50k people in a square mile) and its empty spectrum. \$\endgroup\$
    – Eric Cope
    Commented Jan 16, 2015 at 22:01
  • \$\begingroup\$ This is a perfectly reasonable answer, if a bit expensive and out of reach for my non-technical buddies. \$\endgroup\$
    – TheNoonMoose
    Commented Jan 17, 2015 at 16:53
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  • Option 1: You could make use of a small FM bug with an RDS coder. Anyone with an off-the-shelf portable FM radio with RDS display would be able to receive your text information. Assembled FM bugs and RDS coders are easy to find, just a matter of putting it together.

Disclaimer: Transmitting on FM broadcasting channels with EIRP levels above -43dBm( 50nW) is illegal in most countries, so you could have problems with the 1km distance requirement if you do not want to break the law.

  • Option 2: Use radio text messaging on a D-Star portable HAM transceiver. Widely available and if you and and your friends manage to get your HAM licence before the festival, that would probably be the most feasible option.

  • Option 3: Acoustic OFDM. This allows simulcast of data and sound at low bit-rate via the speakers of the PA system. If you look at the size of festival speakers I don't think receiving decodable data at a distance of 1km is going to be a problem.

The downside is that you need to walk to the podium and get permission from the organiser to connect your device to the audiomixer or mike. Off-the-shelf devices are also very hard to come by. The good thing is that you own a piece of technology that virtually nobody has ever seen, and that is going to make you cool @ the festival :).

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If you are ok with a simple morse code process try a watch, a decent laser pointer and a convenient building. Everyone agrees to look at the building at the top and bottom of the hour and count the flashes they see on it.

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