## 5 Discussion  
Traditional low-power wireless communications in the industrial domain have been based on the IEEE 802.15.4 standard, using the OQPSK-DSSS modulation and operating in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band. Given the expected communication range (i.e., typically below 50 m) caused by multi-path propagation and external interference effects, networks based on such a standard have typically relied on mesh technologies, which allow extending the range by forwarding neighbor packets towards the gateway in a multi-hop fashion. However, deploying and maintaining mesh networks has proven to be challenging due to the changing nature of the environment. Thanks to the deployment in a real-world industrial setting, in this paper, we have shown that the use of the sub-GHz band combined with the robust IEEE 802.15.4-2015 SUN modulations provides good communication characteristics in an industrial setting, with links up to 273.5 m and effective data rates of 50 kbps. This allows to simplify the network deployment and operation compared to mesh networks based on the traditional IEEE 802.15.4 standard. However, despite the propagation and robustness advantage compared to traditional IEEE 802.15.4 communications, we have observed that communications between the nodes and the gateway are still severely impacted by both multi-path propagation and external interference effects, limiting network performance in terms of reliability and availability. In fact, the average PDR is only 75.9%, which is well below the typical 99.9% requirement for industrial settings. To cope with packet losses caused by such propagation and interference effects, we used blind packet repetitions, a common technique used to increase the reliability of LR-WPAN (e.g., WirelessHART, ISA 100.11a, 6 TiSCH) [19] and LP-WAN (e.g., LoRaWAN, Sigfox, NB-IoT, etc.) [20] networks. However, the results from our deployment show that even three transmissions cannot guarantee a PDR \(>\) 99.9%. In fact, with three transmissions, it is only possible to reach an average of 94.2% with SUN-FSK, 94.1% with SUN-OQPSK, and 86.0% with SUN-OFDM, which is still far from the target PDR requirement. Further increasing delivery success adding more transmissions is possible, but it has a high impact on the battery life and is limited by the duty-cycle regulations, as it causes interference to the remaining nodes, as well as to other networks operating in the same area. A widely used technique to further mitigate the propagation and interference effects is frequency diversity (i.e., channel hopping). However, while frequency diversity is suitable in the 2.4 GHz band given that the available bandwidth is large and the channels are uncorrelated, the benefits in the sub-GHz band (i.e., 868 MHz in Europe and 915 MHz in United States) are mostly limited to mitigating interference effects. In addition, adding frequency diversity requires synchronization between the nodes and the gateway, which adds complexity to the solution and limits certain use cases where there are multiple gateways and/or nodes are mobile. Interestingly, modulation diversity (i.e., changing modulations for every transmitted packet) has the potential of providing similar effects as frequency diversity, since each modulation has its own properties with regard to robustness against both propagation and interference effects. For example, narrow-band modulations (i.e., SUN-FSK) are robust and provide a large number of channels, but are prone to frequency drifts caused by temperature changes and component aging. In contrast, wide-band modulations (i.e., SUN-OQPSK) are inherently robust to narrow-band interference and can cope better with frequency drift, but have a limited number of channels available. Finally, multi-carrier modulations (i.e., SUN-OFDM) can use time and frequency repetition to provide robustness against both multi-path fading and external interference effects, while providing a trade-off in terms of channels available.