## 2 Background--Related State of the Art  
The objective of this section is to present the characteristics of key IoT standards operating in unlicensed and licensed bands, namely LoRaWAN and NB-IoT, respectively, in order to identify their differences and their comparative strengths and weaknesses. LoRaWANnetworks are based on star topologies in order to interconnect sensing devices with a gateway. In the physical layer, the modulation is based on LoRa Chirp Spread Spectrum for ensuring efficiency in managing the transceiver power and good operation characteristics in an extended coverage range. It uses proprietary spread spectrum techniques and Gaussian Frequency Shift Keying (GFSK). It also employs wideband linear frequency-modulated pulses, with a varying frequency depending on the encoded information. It requires reduced receiver design complexity also due to the equivalence between transmission and reception timing and frequency offsets [1]. The type of the waveform and the high value of the bandwidth-delay product ensure transmission robustness for addressing issues caused by interference and small-scale fading. This affects the achieved link budget, enabling high-coverage areas for each LoRaWAN gateway that can cover thousands of end devices, for sending intermittently their measurements towards the gateway [2]. A downside of this is related to the relatively low allowable duty cycle for the operation of all these end devices, impacting their individual throughputs and packet rates. In the EU region and in the 863-870 MHz ISM band, there are three obligatory uplink LoRaWAN channels at 868.10, 868.30, and 868.50 that have a maximum duty cycle of 1% [3]. Additionally, LoRaWAN MAC includes three different operation classes: Class A for bidirectional communications consisting of one uplink and two subsequent downlink windows, Class B that includes more scheduled downlink (receive) slots, and Class C with continuously open downlink (receive) windows that are only restricted during uplink transmissions. Moving from Class A to C, latency for downlink transmissions can be reduced at the cost of more power required to maintain the downlink window continuously open [4]. 3 GPP has also developed standards for addressing the requirements of IoT use case scenarios, with a focus on the reduction of energy consumption, especially on nodes close to the edge of coverage and to simplify hardware architectures and chipsets that could be connected to the existing cellular infrastructure and use parts of the licensed spectrum, such as the Extended Coverage GSM (EC-GSM) [5], the LTE Cat-M [6; 7], and NB-IoT [8]. LTE IoT devices have more modest specifications in terms of throughput and power consumption, but they support coverage areas corresponding to power budgets ranging between 155 and 164 dB [7; 8]. NB-IoT can support simpler IoT applications involving static sensor topologies and lower throughputs, short-message transmissions, and having moderate delay requirements, with only the requirement to be accommodated within the legacy LTE or GSM spectrum bands (in the latter case following suitable refarming procedures). It should also be noted that EC-GSM may be directly implemented in areas where there is still available 2G network infrastructure providing acceptable performance figures for IoT applications in terms of latency and throughput [9]. In addition, the MulteFire standard [10] has been developed for interworking with LTE networks, and it includes IoT-targeted enhancements such as coverage extension and support of narrowband operation in the ISM bands in 900 MHz and 2.4 GHz [11]. In parallel to the aforementioned IoT access network standardization activities in both licensed and unlicensed bands, there has been increased interest in assessing the various developed standards and modes, in terms of their performance for various use case requirements and scenarios.