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Multiple Transmitters To One Reciever

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cyprio7

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Background information

I am trying to wirelessly transmit a maximum packet size of about 204 bits from multiple trancievers to a central "base station" which consists of a transmitter and reciever. Lets call them BaseTx and BaseRx.

I have based the manchester transmit and recieve routines on nigels wireless tutorials.

All transmitters, recievers etc use the same frequency of 433.92Mhz. I know how to send data from the base station, to a specific tranciever, or multiple trancievers, sending data from a tranciever to the base station is equally easy.


My question

What happens if two trancievers, try to send at the same time to the one basestation using the same frequency? I could use different frequencies for each tranciever but this is not practical as i may have up to 30 trancievers, although it will probably be about 4 or 5 to begin with.

One idea I thought of was making the base station send out a continuous signal, sychronizing all trancievers, so that, they all have a specific "time" in which to send their data, AKA, time division multiplexing (TDM), however, this means that I will need to use at least 2 different frequencies, one for sending data, one sent out by basestation for sychronizing.


Another idea is, if two trancievers try to transmit simultaneously, after transmission, they wait for a "achnowledgment" from basestation to say data was recieved successfully. If the base station recieved both signals simultaneously on the same frequency, the data recieved is therefore corrupted as it cant differentiate between the two. The basestation therefore doesnt reply, or sends an "error" signal back.

After the trancievers which transmitted simultaneously have been waiting for a certain time with no reply from the base station, they assume their data wasnt recieved properly, therefore, tranciever A will wait for 5 seconds, transmit its data, and tranciever B will wait 10 seconds then transmit its data, which, will hopefully ensure data is transmitted one at a time the nex time around.


What do you guys think about the last method? Would it work? Does anybody know any good way of dealing with multiple transmitters trying to send data one reciever when they accidently send simultaneously?

thank you
 
If it was me, i would poll each transmitter with the base station. Each transmitter have their own "address" and hold the data 'till they're ask to send it.
 
If the modulation is AM then the data from simultaneous transmitters will add and make a mess.
If the modulation is FM then only the strongest simultaneous signal will be demodulated due to the "capture ratio" of an FM receiver. That is why aircraft radios are AM so that a weak signal is not blocked by a strong signal.
 
This is really a network problem, it occurs on ethernet etc. as well.

As mister_e suggested, I would get the base station to poll the remotes in turn, with the remotes never allowed to initiate transmission on their own - this cures and collision problems. But does depend on exactly what the system is doing.
 
I am sending data wirelessly to a single or multiple remote stations in packets of no more than 204 bits from a base station to get them to perform certain functions.

The base station is connected to a PC and acts as an interface between the remote stations and the pc, which is used to process and interpret the data. The remote stations can also send data back to the base station interface however they will never send anywhere near 204 bits, it will be a lot less, maybe 38 or so bits.

Given that the interface basestation is a link between the PC and remote stations via RF, the interface must recieve and transmit manchester data to RF modules, it must also transmit/recieve data with the PC using hardware USART.
 
As long as the remote stations only send data when asked, there's no problem at all, they will never attempt to transmit together.

For the USART PC transmitter, use a PIC to interface between them, or (as you could with all the transceivers) use transceivers with internal Manchester encoders/decoders.
 
nigel, the rtfq transmitter you used, i know it says FM transmitter, but what type of FM? When you are transmitting binary data is it using frequency shift keying or phase shift keying, if you know?
 
cyprio7 said:
nigel, the rtfq transmitter you used, i know it says FM transmitter, but what type of FM? When you are transmitting binary data is it using frequency shift keying or phase shift keying, if you know?

No idea, but it makes no difference, they are essentially identical - at a 'guess' I would imagine it's frequency shifted though?.
 
i personally think its fsk but i cant be certain. I was thinking of buying the AM transmitters such as RTF-4 but i didnt because apparently FM based modules are more immune to interference.

obviously static in the air can change the amplitude of an ASK signal, but if im using the modules in a building, i mean, how much static is there going to be which would be significant enough to effect this?

AM modules are cheaper and they use less power than FM ones because they encode data onto the RF sinewave by reducing and increasing its amplitude, so why is fm so much more superior? are there any other advantages or disadvantages which i am not considering?
 
Multiple transmitters

Hello Cyprio,

in aviation every aircraft has to be equipped with an IFF-transponder (originally used in military aviation as Identification Friend/Foe).

The transponder is interrogated by the ground station and transmits a four digit numeric code (e.g. 0276) as advised by the air traffic controller via voice radio. The transmitted code is displayed on the radar next to the "echo" of the transmitting aircraft to easily identify it. An IFF-transceiver can handle up to 4096 transmitted codes simultaneously.

There is no secret to the IFF functions and technical references should be obtainable via the internet.

Boncuk
 
Hi Cyprio,

Look up information on something called "Motes"

Motes are very similar to what you describe, lots of small individual units that can transmit and receive data while performing tasks (like sensing). They can form "ad hoc" networks or like in your case a centralized network.

A very powerful ability in some designs is being able to "store and forward" data and allow one mote to transfer its data to another mote and so on until it reaches the base station. The originating mote could then be technically outside the base station range but still transfer data to it by going through closer motes. This also helps with interference issues as more motes with less space between them makes the data more immune to interference.

There are readily available off-the-shelf mote units that you can purchase and modify to your needs. They deal with the network communication layer all themselves so all you have to do is learn how to interface your own hardware with the mote.

I know this takes away the fun of doing it yourself but if you're more interested in the product and than the process then this might be more straightforward to build and more reliable since the RF network stuff is already tested and handled for you.
 
If your transmitter uses infrared pulses, interference won't be a problem unless it is on a divisible frequency. An ultrasonic transmitter won't have the capacity to bypass any interference, including dogs. The receiver may ignore all signals and be stubborn.
 
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