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Author Topic: Working around limited usable radio frequency amount  (Read 2180 times)

Offline McKay

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We use cell type grids: signal amplification and frequency re-use.
We need frequency re-use because there is a limited amount of radio frequencies we can use/ measure (there are infinite amount of frequencies, but our instruments are not infinitely precise) and people cant just communicate directly over few dozen kilometers directly using radio waves. Well, they can, but its limited - have to have a frequency license or something for long range and short range, aside from short range, is still limited to few frequencies and not much privacy. (and I dont understand why our uber advanced smartphones dont have real walkie talkie funtion in it. Not simulate walkie-talkie via internet, but real, short range, walkie talkie. )

How about pinpointing exactly from where the call (signal, whatever) [for your devices ID] is coming from and then listening strictly only to the signal that is coming from the determined location?
I know determining from where the signal is coming from is possible, but how precise can it be made and how small can the system be made?
Would that kind of system be usable (if/ when technology advancements will allow)?

Few problems I foresee:
1)What if there is another caller from the same direction (with whatever precision the device could tell the direction), but just further or closer?
            *)Perhaps additionally measuring the intensity of the signal? And, still, different frequencies can
                be used if really necessary. How much cellphones can be used simultaneously at any one
                cell?- thats how many people could be in a line with similar signal strengths.
            *)Or not only determining the direction, but also distance using triangulation or trilateration or
                something gaining a precise point of origin? (yes, the technology would have to be
               ridiculously miniaturized and improved on precision, but, theoretically, possible, no? )
2)If the caller (or listener) moves:
            *)Well, it could be tracked if an error window is allowed, from which signals will still be
                accepted, around the determined origin point - comparing the original point with the current
                point system can keep track and move the error window accordingly. (like looking trough a
                telescope at something moving.. )
« Last Edit: 04/04/2014 00:52:20 by McKay »


 

Offline CliffordK

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Re: Working around limited usable radio frequency amount
« Reply #1 on: 04/04/2014 02:49:49 »
Most children's walkie talkies have pretty poor quality, although higher end CB radios have a bit better quality, and 40 channels or so.  The squelch function does allow some range adjustment.

NexTel traditionally had a "push to talk" function which was very much like a walkie talkie with unlimited (in-network) range, and was phone to phone so you would only talk to the person you wished to talk to.  In a sense it was handy, but not too much different from just making a phone call.  Good, though, if you only need a one or two word answer.  Now that NexTel is defunct, perhaps other carriers will have the service.  ATT?

Modern digital technology can specify the source and destination without complex distance/direction filters.  One may have a benefit of phone to phone communications rather than phone to tower to phone communications, but that may be limited.  Some phones will allow using phone to WIFI to other net stuff which is an alternative for around home.

 

Offline evan_au

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Re: Working around limited usable radio frequency amount
« Reply #2 on: 04/04/2014 10:27:50 »
A number of cellular mobile phone operators are now deploying the "LTE" technology (an acronym derived from "Long Term Evolution").

It already does a number of the things you suggest, with further improvements planned over the next few years (LTE-Advanced).

Quote
there is a limited amount of radio frequencies we can use
Operators like frequencies around 700MHz because they have a long range, and can penetrate walls of buildings and trees; they aren't badly affected by rain, compared to higher frequencies.
The Shannon limit indicates the minimum amount of bandwidth that you need to transmit a given amount of information. If you have purchased 20MHz of spectrum, and each signal has a bandwidth of (say) 100kHz, then you can have around 200 simultaneously-active transmissions within the range of the cellular tower.

You can have more than 200 users in the cell, because most information today is represented as digital format, and is sent in short bursts. You can send a burst to one user in one millisecond, and to another user in the next millisecond.

You can get longer reach by increasing the transmit power. However, this interferes with more users, and actually reduces the number of people who can use a system. In fact, the trend is to progress to smaller cells, with closer base stations, with the ultimate goal of having a base station inside each home or business. This shorter range allows the use of higher frequencies, where bandwidth is relatively cheap.

Quote
then listening strictly only to the signal that is coming from the determined location?
This is done now. Many older cellular towers have the antennas mounted in a triangle. This breaks up the cell into 3 sectors which are handled independently. They often have 3 antennas on each side of the triangle, which allows the direction of transmission and reception to be tuned by adjusting the phase of signals to each antenna.
Smartphones also have multiple antennas built into the case (ie pretty small), allowing the mobile phone and the base station to steer their signals, and to integrate signals which have bounced of various buildings into a coherent signal.
By using MIMO, they can use these multiple antennas to form multiple communication channels between the user and the base station.

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What if there is another caller from the same direction
Mobile handsets and the base station need to know their distance quite accurately, which they can derive by timing. They need to transmit their 1ms burst of signal so that it arrives just after another user's 1 ms burst, and stops before the next person's 1ms burst.
There are many ways to handle two people in the same direction, by (a) sending their bursts on different frequencies within the 20MHz band or (b) sending their bursts at different times.

Quote
If the caller (or listener) moves
This is the basis of using a mobile phone in a car or a train.
The performance of a sitting or walking user can be optimised by carefully measuring the reflections and degradations at different frequencies. However, for moving users, this information is rapidly out of date, as a fast-moving vehicle has its signal bounced off different buildings or blocked by walls. This results in the mobile system having to send the information multiple times, or to send extra information that allows the receiver to reconstruct the lost information.
 

Offline McKay

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Re: Working around limited usable radio frequency amount
« Reply #3 on: 05/04/2014 20:24:43 »
Hmm, interesting. Thanks for the reply's.
But what I am really interested is direct device to device communication without a base tower. True peer to peer. Just over longer distances. I know we can make mesh networks out of wifi, but wifi only has the range of 50-100 meters or so (?). I can shout that far.. What a bout a mesh network using lower frequencies - gsm, 2g, 3g.. ?
I can get a small program that simulates wifi router in my pc enabling me to wirelessly (without bluetooth - just wifi) connect two laptops as if they ware connected via wifi router  - can I, in principle, get/ make a program that simulates a base station in my smartphone and call, basically, peer to peer?
It would mess up the existing infrastructure, yes, and would still be limited to whatever range gsm, powered by a smartphone, has, but still - would be pretty handy in case the system goes down for some reason (or I find myself in a dessert and I would like to keep in touch with my friend who went in the opposite direction to find water or something.. )
 

Offline evan_au

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Re: Working around limited usable radio frequency amount
« Reply #4 on: 06/04/2014 07:47:37 »
Mesh networks are an active area of research for:
  • Military: There may be no fixed infrastructure; any fixed infrastructure could be attacked and destroyed at any moment.
  • Sensors: Low powered devices that must operate for a long time to measure the performance of bridges, roads, etc
  • Vehicles: Cars that communicate with each other to avoid hazards. The members of the mesh network are continually changing.
  • Emergencies: Fixed infrastructure may have been destroyed by earthquake, fire, tsunami, hurricane, etc
  • Remote Areas: Some organisations are experimenting with balloons or drones that would carry a transponder for people out of range of conventional infrastructure. However, this is similar to a base-station (or a low-flying satellite).

Mesh networks need careful power control - you need enough power to reach the next participant in the network, but not too much power that causes widespread interference, preventing other participants from communicating with each other. As you mention, directional antennas are helpful here.

The delays in a mesh network increase with the number of "hops" in the path, and there are cases where transmitters could interfere with each other because they both can "see" a next hop destination, but they can't "see" each other (because there is a hill in between, for example). It may be best to aim for SMS-type communication on a network like this, as voice may be rather unreliable.

A mobile handset has a range of over 1km - if it is communicating to an antenna on a 30 meter high tower. It could go even further if there is a clear line of sight, such as across an open valley. You can expect a reduced range if communicating to another hand-held device.

However, the communication protocols deployed in these mobile phones is based on timing signals received from fixed infrastructure, and will only transmit when commanded by fixed infrastructure deployed by the correct telephone company. Telephone companies will only invest the billions of dollars necessary to build such infrastructure if they can be guaranteed that users will be forced to spend billions of dollars with them to communicate with each other. So there is little commercial interest in promoting "free" communications - especially when it will cause interference with operators who paid good money for the spectrum!

About the only way I imagine this could happen is if the mobile standards body mandates a special "Disaster" communications mode - if the mobile phone is out of range of any cell for an extended time, it switches to Mesh mode. However, this runs the risk that a malfunction in the receiver section of a mobile phone will cause the mobile phone to switch to Mesh mode, and start transmitting at high power, causing interference to paying users over a wide area.

If you need to set up communication between fixed points, a laser link or "cantenna" might do the trick.
« Last Edit: 06/04/2014 07:51:42 by evan_au »
 

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Re: Working around limited usable radio frequency amount
« Reply #4 on: 06/04/2014 07:47:37 »

 

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