High-gain antennas


#1

I’m using high-gain antennas for my outdoor gateways (such as https://www.taoglas.com/product/barracuda-omb-868-b12f21/ and https://www.taoglas.com/product/barracuda-omb-915-b08f21/).

The ETSI regulation limits the transmission power up to 14 dBm (including the antenna gain and the cable loss) for the eu868 sub-bands g and g1 (and 27 dBm for g3).

Is it possible to setup the antenna gain of a gateway throught the lora-app-server ?

Does the loraserver take into account the antenna gain to limit the TxPower of the downlinks ?

Does the loraserver take into account the antenna gain of the receiving gateways for choosing the “best” gateway when a downlink is required ?

Kind regards.
DD


#2

I believe you should configure the gain in the packet-forwarder configuration so that it corrects the downlink TX power.

LoRa Server uses the RSSI and SNR to find the gateway “closest” to your device.


#3

I don’t know how the forwarder is using the antenna_gain property. I will check if it is used for enforce the Tx power limit of the local regulation.

Yes, I know but with high gain antenna, the gateways can receive frames from devices that are very far and the downlink may be not received by the device due to the Tx power limitation. I guess the choice of the “closest” gateway should take the antenna into account especially when the network is a mix of indoor.gateways with cheap antennas and outdoor gateways with high gain antennas.


#4

You are right. Thanks.
line 2344 in https://github.com/Lora-net/packet_forwarder/blob/d0226eae6e7b6bbaec6117d0d2372bf17819c438/lora_pkt_fwd/src/lora_pkt_fwd.c
txpkt.rf_power = (int8_t)json_value_get_number(val) - antenna_gain;


#5

In case this is needed, you you have a suggestion how LoRa Server should implement this calculation?


#6

My intuition is the antennas’ gains should be substracted from the RSSI ou SNR before comparing all the rxInfos of a received frame. What do you think about this ?


#7

I’m not sure if this is correct. First of all, I’m not an antenna expert, hopefully somebody else can fill in the gaps here. What I understand is that a higher antenna gain means that when transmitting, it will concentrate the signal into a certain direction (e.g. when you know your devices are around the antenna, there is no need to transmit in the upwards and downwards direction). In my understanding this doesn’t mean that the signal will (always) be x dBi stronger when the antenna is receiving from the devices around it.

Hopefully somebody else can jump in here :slight_smile:


#8

RSSI is a function of the gains of both antennas and the path between them. RSSI between two similar antennas outdoors placed 200m apart may be better than between the same two antennas placed indoors, 15m apart but with obstacles between them.

In view of the above, I believe you should be looking at selecting the most effective antenna rather than the closest one. In order to do that effectively, you will need to look at the RSSI on both the receiving and sending devices especially as you have high gain antennas on the gateway. If the Loraserver should compute that, it needs to know the rssi on the nodes and the nodes need to be able to differentiate packets received from different gateways in order to report this correctly.

@brocaar does this information get back to the Loraserver?


#9

In case of LoRa Server “closest” means with the best SNR ratio, after a certain SNR treshold, LoRa Server continues the sorting by RSSI.

it needs to know the rssi on the nodes and the nodes need to be able to differentiate packets received from different gateways in order to report this correctly.

This is not possible. When a device receives a downlink, it doesn’t know from which gateway it came as a gateway only transmits the PHYPayload. It does not send any gateway identifier with it.


#10

In the Downstream JSON data from the lora Server to the Gateway, there is field “powe” - TX output power in dBm (unsigned integer, dBm precision).
I think, this can be added as configuration parameter for every gateway, assigned to the Lora Server… And when the server decides to send the downlink to gateway with high gain antenna, then the lower value for TX power will be used.
I don’t know if there is a global parameter for that.


#11

I believe that is exactly why you need to configure the antenna gain in the packet-forwarder configuration so that the gateway will make this “correction”, see @donsez his response a few messages up:

You are right. Thanks.
line 2344 in https://github.com/Lora-net/packet_forwarder/blob/d0226eae6e7b6bbaec6117d0d2372bf17819c438/lora_pkt_fwd/src/lora_pkt_fwd.c
txpkt.rf_power = (int8_t)json_value_get_number(val) - antenna_gain;


#12

But my idea will be a cool feature for the server, to configure the max TX power for every gateway :slight_smile:


#13

But shouldn’t that configuration come from the selected band configuration? It might also be dependent on the downlink frequency. I don’t believe this has been implemented for any band, but please see: https://github.com/brocaar/lorawan/blob/master/band/band.go#L204.

For EU868 this is currently implemented as:

But there could be a switch for the given frequency.


#14

Just for information, Antenna Gain applies equally to the Antenna’s receive and transmit performance. The gain or extra power has come from moving some energy from one direction to another. Normally this comes from flattening the radiation pattern and reducing the power that radiates at an angle both up and down and moving this to the horizontal direction. In this case the antenna still radiates equally in all horizontal directions, just less towards the sky and less directly underneath the antenna.

A “directional” antenna will also concentrate the power in one direction. No longer does the antenna transmit or receive equally in all horizontal directions.

The key take-away is Antenna Gain is the same in both transmit and receive. A Higher Gain antenna will improve receiver performance as well as increase apparent transmit power toward the horizon.

Typically nodes transmit with less power than a Gateway so the weak link is the Uplink direction and a Gain Antenna can assist a Gateway receive a weak signal. In this case turning down the Gateway transmit power to the legal limit should not reduce the system’s performance.