diff --git a/src/LoRaPhy/LoRaTransmitter.cc b/src/LoRaPhy/LoRaTransmitter.cc
index 2d609b2..865e733 100644
--- a/src/LoRaPhy/LoRaTransmitter.cc
+++ b/src/LoRaPhy/LoRaTransmitter.cc
@@ -73,7 +73,9 @@ const ITransmission *LoRaTransmitter::createTransmission(const IRadio *transmitt
     int payloadBytes = 0;
     if(iAmGateway) payloadBytes = 15;
     else payloadBytes = 20;
-    int payloadSymbNb = 8 + math::max(ceil((8*payloadBytes - 4*frame->getLoRaSF() + 28 + 16 - 20*0)/(4*(frame->getLoRaSF()-2*0)))*(frame->getLoRaCR() + 4), 0);
+    short de = 0;
+    if ((frame->getLoRaSF() == 11) || (frame->getLoRaSF() == 12)) de = 1;
+    int payloadSymbNb = 8 + math::max(ceil((8*payloadBytes - 4*frame->getLoRaSF() + 28 + 16 - 20*0)/(4*(frame->getLoRaSF()-2*de)))*(frame->getLoRaCR() + 4), 0);
 
     simtime_t Theader = 0.5 * (8+payloadSymbNb) * Tsym / 1000;
     simtime_t Tpayload = 0.5 * (8+payloadSymbNb) * Tsym / 1000;