## 1 Introduction
### State-of-the-Art
Requirements such as simplicity, storage, and energy consumption should be incorporated into the designs of MAC protocols for satellite IoT networks [20]. With a focus on DtS-IoT networks, a taxonomy of MAC protocols is presented in [20], including four groups: (i) Aloha-based; (ii) reservation and adaptive protocols; (iii) interference cancellation based; and (iv) hybrid protocols. A detailed analysis on the trade-offs involving complexity and scalability is provided. The authors finished their review with the conclusion that a better balance among different metrics should drive the design of novel MAC protocols for DtS-IoT networks. In particular, it is apparent that the choice of a MAC protocol for satellite IoT networks should carefully consider the complexity of implementation and energy consumption. Therefore, random access (RA) protocols based on Aloha [21] are good candidates for the MAC layer in LEO satellite IoT networks, both in terms of simplicity of implementation and delay [20]. Indeed, they have been used for satellite communications for a long time and are even used in modern terrestrial networks, such as LoRaWAN [3] and SigFox [4], and are becoming attractive alternatives for some future 6G use cases [22]. However, for a large number of transmitters, the system's performance can be severely affected by collisions. One solutions for this issue is the introduction of diversity in Aloha. Modern RA schemes [23] based on Aloha often apply SIC for interference cancellation, while allowing devices to transmit multiple copies of their messages [24; 25; 26; 27; 28]. For instance, in Centention Resolution Diversity Slotted Aloha (CRDSA), devices transmit fixed numbers of replicas of their messages while successive interference cancellation (SIC) is applied at the receiver for removing all copies once of the messages is successfully decoded, considerably improving performance [26]. In Irregular Repetition Slotted Aloha (IRSA), devices may transmit different numbers of replicas, improving even more the network throughput [27]. An application of IRSA in the context of satellite communications, where the number of replications per user is optimized, is exploited in [28]. However, methods such as CRDSA and IRSA produce time diversity by means of replication, which may lead to considerable increases in complexity and power consumption at the transmitters, while also demanding substantially in terms of memory and computational complexity at the receiver for the SIC operations. Therefore, in the context of LEO satellites with limited computational resources, time diversity techniques such as CRDSA and IRSA may still be prohibitive. Another alternative is the use of spatial diversity at the receiver [29; 30]. Nowadays, even the use of massive Multiple Input Multiple Output (MIMO) antenna systems at the receiver has been considered in the context of machine-type communications using Aloha [31; 32]. Although promising, a small LEO satellite with several receive antennas may still not be a very practical option. Spatial diversity can also be exploited with Aloha by having multiple single antenna receivers [33; 34]. This idea fits well in a scenario where several satellites cover a given region, which should be more and more common with the predicted launch of hundreds of satellites in the next years [35]. For instance, the links between devices and the multiple relays (e.g., LEO satellites) in coverage have been modeled considering on-off fading (i.e.,erasure probabilities at the satellites) in [15; 36], where different analysis and optimizations were carried out. In [15] critical and non-critical devices coexisted in a slotted Aloha based communication system with multiple satellites. Then, the satellites forwarded the received data to a sink over another shared backhaul link. Orthogonal and non-orthogonal strategies for allocating resources among critical and non-critical services were investigated. The multiple relays can be seen as a constellation of LEO satellites, and the common sink can be an Earth station.