# Asynchronous Flipped Grant-Free SCMA for Satellite-Based Internet of Things Communication Networks
## 1 Introduction  
Satellite communication networks as an enhanced and complementary part of the terrestrial communication infrastructure is expected to play a significant role in future Internet of Things (IoT) communication networks [1]. Due to the seamless coverage provided by the satellite networks, satellite-based IoT (SAT-IoT) has the potential to be widely used in fields such as logistics, smart grids, environmental monitoring, etc., which has received increasing attention [2]. Massive machine-type communication (mMTC) is a major communication scenario in SAT-IoT networks, where massive devices with limited energy and computational resources transmit small payload packets to the satellite with low duty cycle [3]. However, to implement the IoT traffic via a satellite, there are some issues that need to be tackled. One of the biggest challenges is the uplink multiple-access scheme, which is required to support a huge number of devices reporting their data efficiently. Time division multiple access (TDMA) is a traditional satellite uplink access scheme. Nevertheless, this kind of fixed-assignment multiple-access scheme is inefficient for mMTC scenario, since not all the massive terminals always have data to send. Some reservation-based multiple-access protocols, such asdemand assignment multiple access (DAMA) [4], are widely used in satellite communication networks. However, these protocols usually use a request-grant procedure via the random-access channel for network registration and resource request. On one hand, the request-grant procedure requires extra energy and large amounts of signaling overhead to ask for the resources, which is inefficient to the bursty traffic and unacceptable for the machine-type devices (MTDs). On the other hand, it will enlarge the access latency, especially for the networks made up of geosynchronous orbit (GEO) satellites, which is intolerant for a service with low latency requirement. Therefore, grant-free uplink access protocols are essential for the SAT-IoT networks to provide better service in mMTC scenarios. ALOHA-like random access (RA) is one of the most common grant-free uplink protocols, which can be seen as a candidate multiple-access scheme of future SAT-IoT networks, because it is insensitive to the bursty traffic, as well as easy to be implement in satellite terminals. However, due to packet collisions, the network throughput of pure ALOHA is only about 0.18 packets/slot [5]. In recent years, some variants of ALOHA [6; 7; 8; 9; 10; 11] have been proposed, aiming to improve the performance of the throughput of the satellite communication network. These protocols exploit successive interference cancellation (SIC) and proper forward error correction (FEC) coding schemes to recover the collided packets. Although the network throughput is improved significantly by adopting these novel ALOHA-like RA schemes, the achievable sum rate can still not match the fast increase of the system traffic load [12]. Recently, some potential multiple-access schemes based on non-orthogonal multiple access (NOMA) have been put forward to tackle the emerging challenges in mMTC scenario [13]. NOMA allows multiple users to share the same communication resources such as time, frequency, and space via power domain or code domain multiplexing, which can further boost the network throughput [14]. Sparse code multiple access (SCMA) [15] is a promising code domain NOMA scheme for the future SAT-IoT system, which exploits multidimensional sparse codewords mapping and joint multi-user decoding to realize massive users accessing the network non-orthogonally. Compared with other code domain NOMA schemes, such as multi-user multiple access (MUSA) [16], and pattern-division multiple access (PDMA) [17], SCMA has lower bit error rate (BER) under the same signal-to-noise ratio (SNR) and it is easier to design the codebook [13]. Moreover, SCMA can be applied well in the uplink grant-free scenario [18], thus we highlight some recent works about grant-free SCMA in the following.