How to make RN2483 node use a fixed channel and spreading factor?

End Devices (Nodes)
3

To only use one channel, the following worked for me for ABP with the TTN libraries along with with RN2483 version 1.0.1 (Dec 15 2015 09:38:09), on EU868.

Of course, this is only for testing. To use, e.g., channel 5:

// Initialise ABP and set up the 8 standard channels:
ttn.personalize(devAddr, nwkSKey, appSKey);

// Disable 7 of the 8 default channels. This basically disables the
// mandatory LoRaWAN channel hopping, so FOR TESTING ONLY.
exec("mac set ch status 0 off");
exec("mac set ch status 1 off");
exec("mac set ch status 2 off");
exec("mac set ch status 3 off");
exec("mac set ch status 4 off");
// exec("mac set ch status 5 off");
exec("mac set ch status 6 off");
exec("mac set ch status 7 off");

Beware that when disabling channels, the resulting single-channel node will have a very low maximum duty cycle of only 0.125%. The wiki explains:

In the European band, a transmission on a channel within a frequency band, also influences the other frequencies in that band.

…and:

As a per-channel duty cycle limit is easier to implement, you can also divide the sub-band duty cycle over the number of channels in that sub-band. So for example, in a sub-band with 8 channels and a duty cycle of 1%, each channel has a duty cycle of 1/8% (that’s 0.125%).

This method is also implemented by the RN2483 module, and as a result, instead of seeing the no_free_ch when you send too quickly after the first message you can send multiple messages before all 8 channels are “blocked” and the duty cycle is enforced.

One can validate that indeed the maximum duty cycle for, e.g., channel 5 is 0.125%:

// Print the default duty cycle: 799 means 100/(799+1) = 0.125%, which
// is 1% for the whole sub-band, divided over 8 channels; see explanation
// on https://www.thethingsnetwork.org/wiki/LoRaWAN/Duty-Cycle
exec("mac get ch dcycle 5");

Though the RN2483 allows one to set the maximum duty cycle for that one channel to be 1%, I think one should not do that. LoRaWAN mandates:

The end-device changes channel in a pseudo-random fashion for every transmission. The resulting frequency diversity makes the system more robust to interferences.

So, I’d say that disabling some default channels from a sub-band does not imply one can then (ab)use the full 1% for the one channel that is used. Instead, I feel that disabling default channels implies the node’s maximum duty cycle is decreased. (And it’s a maximum anyway; better stay below the maximum.)


The exec function as used above needs to be defined like below.

/**
 * Executes the given AT command and prints its result, if any. 
 * FOR TESTING ONLY; might cause a memory leak with String?
 */
String exec(const char* cmd) {
  debugSerial.print(cmd);
  debugSerial.print(F(": "));
  loraSerial.println(cmd);
  while(!loraSerial.available()) {
    delay(50);
  }
  String s = "";
  // This includes \r\n newline:
  while(loraSerial.available()) {
    s.concat((char)loraSerial.read());
  }
  debugSerial.write(s.c_str());
}