## 1 Introduction  
The emergence of LPWANs shall transform Industry 4.0 into a reality in the upcoming years. M2M communication is believed to be the key enabler for this transformation. The concept of M2M has evolved to Internet of things (IoT) after increasing attention from scholars and practitioners [1]. The penetration of IoT into industry, termed as Industrial Internet of Things (IIoT), has revolutionized the paradigm of industrial communication systems (ICSs) by coping with stringent requirements [2]. From a wider perspective, ICSs are supposed to provide seamless access to the network segments or single nodes placed at the lowest level of an industrial automation system. The most important requirements are timeliness, reliability, and flexibility. The timeliness implies that the system must be capable of carrying out communication tasks to deliver a message within the stipulated time [2]. Reliability is another strict requirement for industrial scenarios. Link reliability can be achieved by spatial and temporal redundancies [3]. The flexibility includes scalability, reconfiguration, and reassembly [2, 4]. ICSs can be classified into critical and non-critical systems [5]. The critical systems are specified by high reliability and data rates that enable real-time communication, while moderate data rates are acceptable for non-critical systems. ICSs mainly comprise fixed transmitters and receivers as the industrial assets are least likely to move. However, establishing a reliable communication is highly challenging because of heavily obstructive environment. Robustness and data rate are therefore primary design requirements in critical systems. These design goals greatly influence the design decisions such as modulation techniques, diversity techniques, and spectrum (licensed/unlicensed) to be used. Studies have been conducted to investigate the suitability of various LPWAN technologies to enable industrial communication [4; 5; 6]. [PERSON] et al. [5] and [PERSON] et al. [7] recommend NB-IoT for long range critical systems while LoRa for medium range. [PERSON] et al. [8] consider LoRa, Wi-Fi HaLow and NB-IoT as the most suitable technologies over a range comparable to industrial site. The reliability mechanism of LoRa is orthogonal spreading factors (SF), while Wi-Fi HaLow and NB-IoT use forward error correction (FEC). LoRa data rate is 50 kbps which is the lowest of the recommended technologies. Wi-Fi HaLow outstands with a minimum data rate of 150 kbps while NB-IoT offers a data rate that does not exceed 125 kbps. In addition to the minimum data rate, LoRa compromises the link reliability in a harsh environment with elevated perturbations. Experiments carried out by [PERSON] and [PERSON] [9] in the reverberation and anechoic chambers with SF of 7-9 have shown 100% packet error rate (PER). LoRa shows a better resilience to multipath only with SF from 10-12 but higher spreading means lower data transmission rate. A simulation study on the use of LoRa in underground mines has reported a 2.5 dB to 6 dB performance reduction to achieve a bit error rate (BER) as low as \(10^{-3}\)[10]. NB-IoT and Wi-Fi HaLow offer high reliability, but they have limitations on data rate and range, respectively. As mentioned earlier, the maximum data rate of NB-IoT is 125 kbps while the maximum range covered by Wi-Fi HaLow is 1 km [11]. Therefore, certain trade-off decisions are involved in selecting a suitable LPWAN technology for an industrial scenario. Modulation technique is an important design decision in deployment of the physical layer (PHY) of an ICS [5;6]. Complex modulation schemes can cause an increase in power consumption of transmitter and receiver. Modulation technique also affects latency [4], latency variation (jitter) and communication range [8]. Modulations with fewer constellation points are more reliable but they are slower at the same time. Higher data rates can be achieved by encoding more data into the physical signal.