code: func (b *Backend) SendTXPacket(txPacket gw.DownlinkFrame) error {
  	if txPacket.TxInfo == nil {
  		return errors.New("tx_info must not be nil")
  	}
  
  	gatewayID := helpers.GetGatewayID(txPacket.TxInfo)
  
  	return b.publishCommand(gatewayID, "down", &txPacket)
  }
docs: // SendTXPacket sends the given downlink-frame to the gateway.