Polling from gateway to every nodes

Hi everyone!
I’m trying to build a LoraWan by using polling to every nodes and get the data back.
Make the nodes are sleep in most of time, only wake up when it listen the specific preamble.
The source code I used are from: https://github.com/sandeepmistry/arduino-LoRa/blob/master/src/LoRa.cpp
I only found “preamble_length” variable in sx127x and default value is 8.
But I don’t know how to exactly setting the different preamble information that can be identified.
Is there any example ? Thanks!

I see this function, try this: lora_set_preamble_length