## 1 Introduction  
The IEEE 802.15.4 standard [1] was first released in May 2003 and defined a physical layer (PHY) and a MAC (Medium Access Control) layer for WPAN (Wireless Personal Area Networks) operating in the sub-GHz (868 MHz in Europe, 915 MHz in America) and the 2.4 GHz worldwide ISM (Industrial, Scientific and Medical) frequency bands [2]. The PHY layer was built upon the DSSS-OQPSK (Direct Sequence Spread Spectrum--Offset Quadrature Phase Shift Keying) modulation and provided data rates of 20 kbps and 40 kbps in the sub-GHz bands, respectively, and of 250 kbps in the 2.4 GHz band. At the MAC layer, the standard defined slotted/synchronized and unslotted/unsynchronized operation based on the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) channel access mechanism to trade-off bandwidth, latency and energy consumption of the devices. Thanks to its simplicity and low-cost, over the years, the IEEE 802.15.4 standard has become the basis for multiple low-power wireless communications technologies including ZigBee [3], ISA100.11a [4], WirelessHART [5] and 6 TiSCH [6], among others. In fact, the adoption of the IEEE 802.15.4 standard by different standardization bodies and technologies has promoted the revision of the standard (i.e., three times: in 2006, 2011 and 2015) in order to clarify the operation and to add new features to both the PHY and MAC layers. For example, the 2015 standard revision adopted the MAC layer protocols defined in the IEEE 802.15.4e-2012 [7] amendment. Among others, this amendment defined the TSCH (Time Slotted Channel Hopping), a channel access mechanism that combines TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) to support industrial requirements, including reliable packet delivery (i.e., 99.999%) in adverse conditions such as multi-path propagation and external interference. Similarly, the IEEE 802.15.4-2015 standard revision included three new physical layers targeted to SUN (Smart Utility Network) applications, SUN-FSK, SUN-OQPSK and SUN-OFDM, as defined in the IEEE 802.15.4g-2012 [8] amendment. The SUN-FSK and SUN-OQPSK modulations focus on maintaining backwards compatibility with previous standards and commercially available transceivers, whereas the SUN-OFDM focuses on adding robustness and improving spectrum efficiency at the physical layer. The benefits brought by SUN-OFDM are mainly due to the use of parallel transmissions with orthogonal sub-carriers, each transporting a small portion of the information to be transmitted using a narrow-band modulation. Such approach provides better robustness against multi-path propagation (as sub-channels can be considered flat fading, which are inherently robust to inter-symbol and inter-frame interference) and external interference (as narrow-band interference only affects a portion of the sub-channels and frequency repetition can be used to overcome its effects), as well as improving spectrum efficiency (sub-channels are optimally spaced, allowing to trade-off robustness or data-rate depending on the application requirements). The consolidation of the IEEE 802.15.4g-2012 amendment into the IEEE 802.15.4-2015 standard, as well as the appearance of the first commercially available radio transceivers supporting it, entails a shift in the design, development and deployment of low-power wireless networks, with special interest in the industrial domain, where the propagation and interference effects have largely slowed or limited the adoption of such technologies. Taking that into account, in this paper, we focus on empirically evaluating the interference robustness of the SUN-OFDM and compare it against OQPSK-DSSS. Our goal is to provide evidence of the robustness of the different modulations, including their possible configurations and under different operating conditions, to understand the benefits and limitations when deploying such technologies.