Limit SF range for auto configuration

[udovic2100]

I really like this example: very well documented and easy to set up.
I would like to change the SFs that the auto configurations uses. I am quite far away from the next gateways and only messages with SF > 9 will reach the next gateway. Is there a way to change the SF range from 7 to 12 to 10 to 12?

[Leonel Lopes Parente]

Thanks for your compliments.

Yes it is possible to set SF manually but its effect depends on several factors.

When using OTAA activation, you can initially set the SF but it will be used for the join only. During the join the SF will be updated by the network server automatically with a SF that appears suitable to the networkserver.

When using ABP activation there is no join and the SF will not be changed. If ADR is enabled then SF may be changed over time (IIRC the first ADR moment is after the first 20 uplinks) by ADR MAC downlink command from the network server. In case of ADR the network server will select a SF that appears most suitable for the node and its signal quality (which e.g. depends on distance to gateways).

It is preferred to use OTAA for multiple reasons.

Use ADR only for stationary nodes (end devices) but not for mobile devices that are on the go.

SF is normally set automatically but if you want to set a value explicitly this is can be done with the LMIC API function LMIC_setDrTxPow(dataRate, txPower). This is a combined function that sets both data rate and transmit power.
For details see the MCCI LMIC library API documentation.

In LMIC-node SF can be specified in the initLmic() call in setup (so you don't have to call LMIC_setDrTxPow() explicitly).
When using ABP it suffices to only specify adrEnabled and dataRate (e.g. DR_SF9 while DR_SF7 is the default).
When using OTAA you will have to specify all 4 parameters when calling initLmic() because it will only use the dataRate and txPower parameters for OTAA if setDrTxForOtaaExplicit is set to true.

initLmic() signature:

void initLmic(bit_t adrEnabled = 1, 
              dr_t dataRate = DR_SF7, 
              s1_t txPower = 14, 
              bool setDrTxPowForOtaaExplicit = false) 

Assuming ADR is used, to use SF9 you would have to use the following initLmic() call in setup():
(To disable ADR the first parameter should be 0.)

For OTAA:

initLmic(1, DR_SF9, 14, true);    // This DR_SF9 will only be used for the initial join

For ABP:

initLmic(1, DR_SF9);

Note: the setDrTxPowForOtaaExplicit parameter will be removed in an upcoming LMIC-node version so it does not need to be specified explicitly. initLmic() will then require the same parameters for both OTAA and ABP . See #12

[udovic2100]

Thanks for the really quick answer. I'm using OTAA and changed the initLmic() as you suggested in the setup. However I get the following terminal output:

LMIC-node

Device-id:     rpi-pico
LMIC library:  MCCI
Activation:    OTAA
LMIC debug:    2
Interval:      60 seconds

RXMODE_RSSI
000000184937:  Event: EV_JOINING
184990: engineUpdate, opmode=0x4

000000185097:  doWork job started
261466: engineUpdate, opmode=0x4
000000261593:  Event: EV_TXSTART
261684: TXMODE, freq=868100000, len=23, SF=7, BW=125, CR=4/5, IH=0
577299: setupRx1 txrxFlags 00 --> 01
start single rx: now-rxtime: 3
578011: RXMODE_SINGLE, freq=868100000, SF=7, BW=125, CR=4/5, IH=0
rxtimeout: entry: 578459 rxtime: 577924 entry-rxtime: 535 now-entry: 4 rxtime-txend: 312375
639799: setupRx2 txrxFlags 0x1 --> 02
start single rx: now-rxtime: 3
640507: RXMODE_SINGLE, freq=869525000, SF=12, BW=125, CR=4/5, IH=0
rxtimeout: entry: 654782 rxtime: 640424 entry-rxtime: 14358 now-entry: 4 rxtime-txend: 374875
655034: processRx2Jacc txrxFlags 0x2 --> 00
000000655156:  Event: EV_JOIN_TXCOMPLETE

Correct me if I'm wrong, but it looks as if my change didn't have any effect on the joining process. I was hoping for SF=10 on the first try.

Thank you for your help in advance.

[Leonel Lopes Parente]

You're right. That shows SF7 and not SF10.

Partially I'm to blame for that because I forgot that LMIC-node (currently) only calls LMIC_setDrTxpow() for OTAA if ADR is disabled.

I just did some tests with a slightly modified version of LMIC-node where the ADR check is removed, but unfortunately that does not make any difference (independent of whether ADR is enabled or not). This behavior is determined by the LMIC library, not by LMIC-node.

This matter is not simple because the initial SF used for a join is LoRaWAN region dependent and so is the (max) txPower value. Also there is no API to set SF and txPower independently from each other and there is no API support to get the default values for these parameters. For additional information see the following related discussion.

So currently it is not possible to set a custom SF for a join.

[udovic2100]

Thank you very much for your explanation. I will follow the discussion.