# Spectral Coexistence of QoS-Constrained and IoT Traffic in Satellite Systems
## 1 Introduction  
Massive machine-type communications (MMTC) and the Internet of Things (IoT) are attracting steadily growing research and industry attention, emerging as a fundamental component for next-generation wireless systems. Driven by a blooming number of IoT applications, this novel communications paradigm aims at serving vast populations of often low-power, low-complexity terminals that generate sporadic traffic in the form of short data packets. Examples of practical relevance span a wide set of scenarios, ranging from smart agriculture or industry, where sensors may collect data (e.g., temperature, pressure, and presence of chemical substances) and deliver status information to a common gateway, to environmental monitoring or asset tracking [1, 2, 3]. Support for this multitude of use cases is already provided by terrestrial networks in a number of well-established commercial products [4], e.g., LoRa [5, 6, 7], SigFox [8], Ingenu [9], as well as by standardized approaches, such as NB-IoT and LTE-M [10]. In parallel to this, satellite-based solutions have recently gained traction as a key enabler to provide global coverage to mMTC services [11]. Favored, among other factors, by the significant reduction in launch cost and by the use of off-the-shelves components, such as reprogrammable software defined radios (SDRs), a revived interest toward the deployment of low-Earth orbit (LEO) satellites has characterized the past few years. From this viewpoint, large constellations such as Starlink, OneWeb or the planned Amazon Kuiper are flanked by a growing number of smaller LEO networks focusing on specialized commercial services, e.g., [12, 13, 14]. Satellite IoT connectivity is one of the key scenarios included in the non-terrestrial networks (NTN) standardization efforts within the 3 GPP ecosystem, and is aimed to become part of the standard already from Release 17. The increasing interest toward mMTC has spurred significant research efforts to tackle a number of challenges that span the whole communications protocol stack. In particular, the intermittent transmission of short packets from IoT terminals calls for innovative approaches. At the physical layer, for instance, the design of the channel codes that operate efficiently over blocks in the order of a few tens to hundreds of bits and with possibly limited channel state information is fundamental [15]. Moreover, the grant-based medium access control policies encountered in traditional human-centric communication systems is largely ineffective for the traffic profile encountered in machine-type applications. As a matter of fact, the overhead needed to coordinate resource sharing in the presence of a massive population of sporadically active transmitters is highly inefficient. In this perspective, a flourishing line of research is emerging, with the proposal of a number of novel _modern_ random access protocols [16; 17; 18; 19]. These solutions lean on a joint design of coding and medium access, allowing to achieve spectral efficiencies comparable to those of coordinated schemes under a truly grant-free paradigm, and offer a promising way forward for next-generation mMTC systems. At the same time, random access is already at the core of current IoT communications, as variations of the basic ALOHA strategy [20] are employed by many widely used mMTC solutions [5; 8; 11]. From this standpoint, grant-free and coordinated access policies offer complementary characteristics, and are commonly employed side by side. Indeed, many communications systems host and provide support to a variety of use cases, ranging from sporadic and often lower-priority mMTC traffic to services with more stringent demands in terms of data rate, reliability and quality of service (QoS). This combination is typically achieved by assigning blocks of orthogonal resources (slices)--in time, frequency, or a combination thereof--to applications with distinct requirements.