# Asynchronous Flipped Grant-Free SCMA for Satellite-Based Internet of Things Communication Networks
## 5 Simulation Results
### AF-SCMA MAC Performance Results
For a target \(PLR=10^{-3}\), the maximum throughput of AF-SCMA is 27% better than that of S-SCMA and is 90% better than that of A-SCMA for the case of \(\sigma=0\) dB. When the power unbalance exists (\(\sigma=3\) dB), the maximum throughput of AF-SCMA is 20% better than that of S-SCMA and is 72% better than that of A-SCMA, for the same PLR target. This is because for the heavy traffic load, packet collisions become severe, which leads to codebook collisions occur frequently. Without the help of flipped replica, codebook collisions will cause significant negative impacts on SCMA decoding. Since the probability of packet collisions of the asynchronous system is higher than that of the synchronous system, S-SCMA performs better than A-SCMA. We notice that for the case of \(\sigma=0\) dB, the throughput of S-SCMA is higher than that of AF-SCMA in the heavy traffic scene, because codebook collisions are so serious that few packets can be decoded successfully, which is hard to trigger the IC process. Although S-SCMA is superior to the proposed AF-SCMA in the heavy load scenario, the PLR of S-SCMA has been beyond the practical target, which is intolerable in the SAT-IoT networks. Actually, sending extra flipped replicas could reduce the throughput in some cases. Transmitting more packets (original packet and its flipped replica) simultaneously will degrade the received power of each packet, which causes the power of each packet may be not high enough to resist the interference. Moreover, although the probability of codebook collisions will be reduced by introducing flipped replicas, the interference to a specific packet also increases due to the extra flipped replicas. AF-SCMA decoding performance is related to SINR directly, which requires higher \(\left[E_{b}/N_{0}\right]_{D}\). To illustrate the influence of received power on the performance of AF-SCMA better, we also investigate the performance of AF-SCMA with \(\left[E_{b}/N_{0}\right]_{D}=10\) dB. In Figure 4, the throughput of AF-SCMA with \(\left[E_{b}/N_{0}\right]_{D}=10\) dB is lower than that of AF-SCMA with \(\left[E_{b}/N_{0}\right]_{D}=13\) dB. For a target \(PLR=10^{-3}\), the maximum throughput of AF-SCMA with \(\left[E_{b}/N_{0}\right]_{D}=10\) dB is about 50% and 30% worse than that of AF-SCMA with higher \(\left[E_{b}/N_{0}\right]_{D}\), in the scenarios of \(\sigma=0\) dB and \(\sigma=3\) dB respectively. Therefore, the received power of each packet determines the performance of AF-SCMA, and the proposed scheme can provide a remarkable throughput for the SAT-IoT networks with acceptable \(\left[E_{b}/N_{0}\right]_{D}\). Furthermore, we also investigate the AF-SCMA performance for different codebook cases, and the simulation results are shown in Figure 5. We assume all the codebooks are with fixed size \(M=4\) and the number of non-zero entities \(N=2\). Three cases of codebooks are taken into consideration in the simulation, which are \(K=4\) and \(N_{CB}=6\) (AF-SCMA (4, 6)), \(K=6\) and \(N_{CB}=12\) (AF-SCMA (6, 12)), \(K=6\) and \(N_{CB}=15\) (AF-SCMA (6, 15)) respectively. Typically, the number of SCMA codebooks is determined by \(K\) and \(N\) which is \(K_{CB}\leq\binom{K}{N}\). In Figure 5 we can see that the more codebooks are used, the better performance can achieve. Specifically, for a target \(PLR=10^{-3}\), the maximum throughputs of AF-SCMA (4, 6), AF-SCMA (6, 12) and AF-SCMA (6, 15) are 1.9, 2.4 and 2.6 bits/symbol/carrier respectively without power unbalance. While for the same PLR target, the maximum throughputs of these three cases are 2.4, 2.8 and 3.2 bits/symbol/carrier respectively for the case of \(\sigma=3\) dB. This is because with more codebooks, different MTDs have more opportunities to choose different codebooks, which decreases the probability of codebook collisions. Actually, the theoretical maximum throughput of AF-SCMA (6, 12) and AF-SCMA (6, 15) are 4 and 5 bits/symbol/carrier, respectively. However,the simulation results indicate that AF-SCMA can only achieve about 60%-70% of its best performance for these two cases.