# Traffic Offloading in Unlicensed Spectrum for 5G Cellular Network: A Two-Layer Game Approach
## 4 Coalition Game
### Merge-and-Split Based Coalition Formation Algorithm
In this case, this coalition is overloaded, if there are other lighted-AP in WiFi network, it could join in this coalition to alleviate the traffic and optimize the WiFi throughput. Figure 5: Coalition forming based on merge-and-split rule. Figure 6: Throughput changes after coalition forming (three APs in the coalition). Figure 8 reveals the utility changes of LAA and WiFi with the number of transferred stations as well as the total number of stations in the AP coalition when using \(\theta^{*}\) in Equation (24), respectively. From the observation, once the station number of the AP coalition exceeds the optimal value, this coalition has the motivation to offload some data to the LAA network to optimize its network performance. The more stations transferred to the LAA, the larger utility function may be obtained by the WiFi network. In addition, the compensated slot by the WiFi can provide considerable benefits for the LAA. Furthermore, in the case of transferring the same number of stations, a larger total number of stations in AP coalition, \(N\), could lead to more profits after the auction. Because the coalition is more overloaded, it has stronger demands to transfer users to the LAA to reduce collisions, which means LAA could get more rewards from this heavy-loaded AP and would give priority to help this AP. Besides, more stations are transferred to the LAA means more time slots would be compensated for the LAA; consequently, higher throughput can be obtained by using the centralized-control mechanism and better management technology of LTE. Figure 9 illustrates the network throughput comparison among five schemes. \"Basic\" means the original LBT-based CSAT mechanism, collisions may occur at the moment of starting the \"on\" period and \"Modified CSAT\" indicates the modified mechanism that is discussed in Section 3. \"MM\" denotes the scheme in [19] using a Min-Max Fairness Algorithm to make a traffic transaction using a coalition game (15% of macrocell throughput is offloaded to WiFi APs). \"DO\" indicates the scheme in [16] which offloads parts of data from WiFi to LAA based on the distance and channel condition. In addition, M-CAGT means the CAGT scheme with modified CSAT used in this paper. In terms of the average total throughput, the proposed Coalition-Auction scheme based on modified CSAT can achieve about 41.7%, 31.8%, 13.74% and 5.9% improvement for LAA compared to baseline CSAT, modified CSAT, \"MM\" and \"DO\", respectively. In addition, the scheme in this paper can obtain about 24.2%, 6.85%, 15.51% and 6.4% promotion for WiFi compared to baseline CSAT, modified CSAT, the scheme in [19] and the scheme in [16], respectively. Figure 7: Encourage factor changes after coalition forming (three APs in the coalition). ## 7 Conclusions

In this paper, we investigate the coexistence between LAA and WiFi systems in unlicensed bands. Especially, we propose a modified CSAT mechanism, which could avoid the collision at the start of CSAT \"on\" moment. In addition, a two-layer CATG scheme is proposed which could optimize the performance of both LAA and WiFi system simultaneously. Simulation results show that the proposed scheme improves the whole network's throughput effectively. We expect that the proposed scheme is a promising approach for enhancing the performance of coexisting networks. This work was supported in part by the National Natural Science Foundation of China under Grants 61571038 and 61471030; the Important National Science & Technology Specific Projects of China under Grant 2016 ZX03001011-004; and the Fundamental Research Funds for the Central Universities under Grant No. 2017 YJS024. [PERSON] and [PERSON] commonly finished the manuscript. All authors have read and approved the final manuscript. Figure 8: The utility of players in the auction game. (**a**) The utility of the LAA network; (**b**) the utility of the WiFi network. Figure 9: Throughput comparison.