# Asynchronous Flipped Grant-Free SCMA for Satellite-Based Internet of Things Communication Networks
## 1 Introduction
For the uplink grant-free SCMA system, the essential issues are to detect the active users, estimate the channel status and decode the data without the information of active codebooks and pilots. Ref. [18] put forward an uplink contention-based SCMA design, which was a prototype of the grant-free SCMA system, and verified its feasibility by simulations. After that, many blind-detection algorithms for the grant-free SCMA have been proposed [19; 20; 21; 22; 23; 24; 25]. The authors in [19] designed a receiver consisting of an active pilot detector and a joint data and active codebook detection algorithm (JMPA), which were used to detect the potential active users and decode the user data respectively without the knowledge of active codebooks. By introducing an extra zero constellation point in the codebooks, the capability of JMPA for distinguishing the active and inactive users was improved. Based on [19], schemes aiming to promote the blind-detection accuracy of active users were proposed in [20; 21]. A joint user activity and data detection algorithm combined approximate message passing (AMP) with expectation maximization (EM) algorithms was introduced in [25]. The above works mainly focus on designing algorithms that can detect the active users and the transmission data blindly and further improve the performance in terms of BER. Ref. [26] presented an uncoordinated NOMA scheme where each user transmitted the selected pilot and data simultaneously. Based on the proposed interference model, the outage probability of the NOMA scheme was derived, and the throughput achieved, by using both successive joint decoding (SJD) and SIC schemes, was analyzed. In [27], authors compared the network throughput of a grant-free multiple access (GFMA) to that of slotted ALOHA and grant multiple access in LTE system. Please note that the related works on grant-free SCMA mentioned above follow the assumption of time synchronization, i.e., all the received symbols from different users are time aligned, which is inefficiently applied to the straightforward SAT-IoT networks. Since MTDs need to send extra signaling overhead to keep time synchronized, it is unaffordable to the energy-sensitive devices. Moreover, signaling interaction between terminals and satellite will enlarge the access latency, which is intolerant for delay-sensitive traffic. In addition, it is unrealistic to synchronize the whole network, especially for future satellite-terrestrial integration communication networks. Therefore, it is necessary to study asynchronous grant-free SCMA in the SAT-IoT networks. An asynchronous grant-free NOMA protocol was investigated in [28]. The authors considered both time and frequency offsets and exploited packet replicas to trigger the SIC procedure. In [29], a novel asynchronous interference cancellation technique named triangular SIC (T-SIC) for NOMA was introduced. The receiver tried to decode the overlapping signals in a triangular pattern based on the a priori information of all the decoded symbols. However, the T-SIC method requires the time offsets to be compensated within one symbol interval, which is not a strict grant-free scheme. In fact, the above two NOMA schemes are based on power domain, which are not appropriate for the SAT-IoT networks, because the distance between MTDs and satellite is so long that the differences of the received power are not big enough. Moreover, the MTDs are usually energy-sensitive, which cannot afford high transmission power. Furthermore, the performance of power domain NOMA schemes depends on the power allocation policies to a great extent, and the power allocation procedure will bring in extra signaling and delay. Therefore, it is meaningful to design an asynchronous SCMA scheme for the future SAT-IoT networks. However, without time synchronization, the SCMA packets may be partially interfered, which makes the traditional successive joint decoder [26] not being applied straightforward.