What low power RF Transmitter to use for crowded signal area

Question

Hi I'm fairly green when it comes to RF technology in circuits.

Here's my problem: I need a low power RF transmitter module (3 - 5v) for my application. It's a small embedded device and all it does is broadcasting a short ID to a base station.

My circuit is very small, it's a battery powered AVR. The AVR holds the unique ID to be broadcast when a certain logic criteria is met.

And to make things complicated, There will be multiple of these broadcasting their IDs to the base station, probably up to 1000 devices broadcasting once per 5 seconds which is 200 times per second. I need the range about 500ft or more.

I'm guessing something that broadcasts in the ISM range will work but I'm open for any design ideas.

So long story short; What TX/RX chips would be good to use? Also, how what would be the best way of getting a message through in such a crowded network?

Answer 1

I'd recommend looking at 433MHz or 868MHz (ISM band) transmitters.

TI/Chipcon have the CC1101, a low cost Sub-1GHz transceiver. There's also an 8051 system-on-chip variant, the CC1110 which could remove the need for your AVR (see also CC430).

Silicon Labs have the Si403x or the system-on-chip Si4010.

All of these chips support data rates up to 128kbit/s, which is plenty of time to transmit a packet. The hardware CRC generation and checking will help weed out bad packets.

To stop nodes jabbering all over each other, you will need to invent some kind of MAC (Media Access Control). You could use carrier sense to wait for silence, with an exponential backoff (CSMA). Or, you could coordinate the timings of your nodes and assign a slot to each node. You may also benefit from having a master node transmitting a timer beacon to prevent clock drift.

In theory, ZigBee supports up to 2^16 nodes on a single PAN. ZigBee is built on 802.15.4, which provides a robust MAC layer and mechanisms for network management (joining, leaving, etc). However, an off-the-shelf module like the XBee may struggle with 1000 nodes and it certainly won't be cheap. For a volume ZigBee deployment, consider the TI CC2531 or Ember EM250/EM260. ZigBee usually runs at 2.4GHz, which will not provide such good penetration of terrain as 433/868MHz.

Answer 2

There will be multiple of these broadcasting their IDs to the base station, probably up to >1000 devices broadcasting once per 5 seconds which is 200 times per second

That is a classic application for true spread spectrum modulation. Implementation details are complicated, but essentially you transform the ID codes into patterns (within an allowed bandwidth) and differentiate the transmitters by finding their pattern in the noise (including interference from each other) rather than by finding a narrowband carrier frequency in it.