Nano- gateway rssi

[MJ]

Hi there, i am not the expert. I am using 2 node and 1 gateway where using rssi value to calculate the distance. Distance calculated using the formula :
Distance = 10 ^ ((Tx power - RSSI) / (10*n))

The tx power i obtained in the lora.stats() show me 20dBm in the range of 0 - 2m. tx power keep on 20 dBm with the change of distance. Is it i can use the tx power as well?

[seb]

@mj

I'm afraid I don't quite follow your question. The TX power given by lora stats is the TX power of the module which unless you change will always remain the same regardless of distance. RSSI is the value that changes with distance.

[MJ]

@seb Now i get your point. But then how can i get the power measured in order to fulfill the formula of counting distance?

[seb]

@mj

TX power should be the power of the device that is transmitting, the gateway in your case. Are you using a pycom module as a nano-gateway? The RSSI should be taken from lora.stats on the receiving nodes.

[MJ]

@seb
yes, i am using the pycom module as a nano gateway where the node will send the information to the gateway which using the lora.stats() . But the tx power that i obtain is 20dBm regardless how long or how short the distance is. Is it still applicable to use the tx power?

[seb]

@mj

Yes the tx should not change with the distance. The gateway will always transmit at the same power because it does not know how far away the nodes are.

[jcaron]

In your formula, you should use the TX power of the sender (usually fixed, known in advance), and the RSSI measured on the receiver (this one will vary).

However be aware that this will give you very approximate results. Even with a fixed setup, RSSI can vary quite a bit, and of course there are lots of parameters that will have an effect on the result, including antenna gains, cable losses, obstacles, multi path, etc.

[bmarkus]

Are you talking about ERP? No antenna gain in the formula.

[jcaron]

The basic principle is that you reverse a link budget equation to find the free space path loss component, and then find the distance for that component.

RSSI = TX Power + TX gain - TX loss - Path loss + RX gain - RX loss

So

Path loss = TX Power + TX Gain - TX loss + RX gain - RX loss - RSSI

If you change antenna, the same TX Power and path loss give a different RSSI (and vice versa). This affects the computation (the value of n in the OP's formula).

[MJ]

@jcaron
I am using the RSSI in order to get the distance. But which formula should i refer to? Because this is a backup for my project which in the jungle their will be no signal so using the rssi value i can get the distance.

[bmarkus]

@mj said in Nano- gateway rssi:

@jcaron
I am using the RSSI in order to get the distance. But which formula should i refer to? Because this is a backup for my project which in the jungle their will be no signal so using the rssi value i can get the distance.

You can't use RSSI to calculate distance on a reliable way. Environment has an ifluence, like rain, vegetation plus if device is moving changing antenna position.

[MJ]

@bmarkus
Using rssi to calculate the distance is just a test. So is it still can make it to get the distance?

[bmarkus]

@mj said in Nano- gateway rssi:

@bmarkus
Using rssi to calculate the distance is just a test. So is it still can make it to get the distance?

Sure, the only question is the accuracy and what do you want to achive :)

[MJ]

@seb
As u mentioned the TX power will remained the same unless we change it. But how can we change the TX power in the module?

[seb]

@mj
https://docs.pycom.io/chapter/firmwareapi/pycom/network/lora.html#class-networkloraid0-

Please see the tx_power parameter of the lora constructor.

[MJ]

@seb
Is there any chances to obtain the RX power?

[seb]

@mj

The RSSI value from lora.stats() is "the received signal strength in dBm" as per: https://docs.pycom.io/chapter/firmwareapi/pycom/network/lora.html#lorastats

[MJ]

@seb
But from lora.stat() it does not have the RX power. how can i obtain? Is there a way to obtain?

[seb]

@mj

What do you mean by RX power? The RSSI is the power of the signal that was received measured in dBm.

[Burgeh]

@seb When I have two LoPys close together, and connect through raw LoRa my RSSI is still around -30dBm that seems bad for such a close range. Putting them even further away (10m) puts them around -60dbM. Is this about right?