Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Cheap radio modules & addressing

Status
Not open for further replies.

NJ Roadmap

New Member
An obvious problem when using RF transreceivers for data transmission is the possibility of another transmitter transmitting at the same time and hence causing interference.

Obviously simple addressing will make sure the data is only processed by the intended receiver, but how would one prevent transmitter interference if you can't change frequency channels on cheap transmitter modules? This isn't a problem currently, but I'd like to know whether theres a solution other than making sure that the transmitting ranges of two transmitters do not overlap.
 
As crowded as the radio spectrum is, it's about impossible to insure your data transmission doesn't get stepped on.

It might be easier to adopt a transmission protocol, where the receiver transmits back an acknowledege to each data buffer. Something like a checksum result and a bit count would verify a good receipt. A fauly result prompt the transmitter to repeat the last buffer.
 
This is a big subject in digital radio systems, like digital cellular. The jargon that describes the problem is "channel contention" and there are many schemes to solve contention problems, some very complex. Textbooks have been written on this one subject alone. As you realize, the need for a contention control scheme depends on the amount of traffic that will be put on the channel. If there is little traffic, you use "pure aloha" contention, which is a fancy phrase that means no scheme at all. Just go ahead and transmit and hope there are no collisions. Its best to have some way of checking that the info got through and if there is a collision, you try again. This is the value of the "acknowledgment". Just adding this idea of an acknowledgement to each packet transmitted is very powerful. If you don't get an "ack" then you transmit the packet again. Transmit information in little packets and only retransmit the packets that were lost, not all of them.

You can add complexity by adding things like:
- carrier sensing (a bit like ethernet uses, where you sense when there is traffic on the channel and wait your turn)
- busy bit. signalling on the output data stream flags the busy/not busy status of the channel
and so on.

Or, you can start dividing the channel up into time slots, one slot per user, and create a TDMA system. Or you can divide the channel up into sub-channels in frequency and create an FDMA system. Or you can share the channel using CDMA or similar spread spectrum ideas.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top