# Unlicensed Spectrum Allocation for LTE and Wi-Fi Coexistence with HAP
## 5 Performance Evaluation
The overall throughput will be compared under different loading situations. To be more realistic, we change the number of LTE users and Wi-Fi stations in different experiments to observe the performance variation in typical environments. We compare the proposed scheme with other coexistence mechanisms, including the original scheme without a coexistence mechanism (denoted as original), NBS (Nash bargaining solution) [16], LAA (Licensed Assisted Access) [24; 25], and LTE-U (LTE-Unlicensed) [2; 5; 6]. Overall throughput is the primary performance index for the comparative study. Figure 11 shows the throughput of Wi-Fi and LTE using different mechanisms in a lightly loaded environment. Because of the low load environment, the throughput of the original scheme between Wi-Fi and LTE is much closer in the absence of a coexistence mechanism. When the coexistence mechanism is adopted, the throughput of Wi-Fi degrades because some resources of the unlicensed band are allocated to LTE users. On the other hand, LTE can utilize the license-free band to improve throughput. Therefore, the throughput of LTE increases significantly. Our proposed scheme is based on the utilization function to adjust the time ratio for time-division multiplexing between the Wi-Fi and LTE to maximize the overall throughput. However, the LTE performance of NBS is a little better than our proposed scheme since NBS sacrifices Wi-Fi performance. The proposed solution provides a fairer access method for Wi-Fi and LTE. Interestingly, the technology of LAA makes Wi-Fi and LTE have the same priority to access the unlicensed band. Compared with the proposed scheme, it can give more access opportunities to Wi-Fi stations in a lightly loaded situation. As a result, the throughput of Wi-Fi is better than those of other schemes. Due to the LBT mechanism, LTE users with LAA will compete with Wi-Fi stations to access the unlicensed band, which wastes bandwidth resources. Therefore, the throughput of LTE decreases dramatically. On the other end, instead of using the LBT mechanism to prevent LTE from interfering with Wi-Fi stations, NBS and our proposed scheme integrated the HAP to coordinate LTE users and Wi-Fi stations to access the unlicensed band. Doing so can avoid interference between LTE and Wi-Fi. Differently, LTE-U is based on the CSAT scheme, which can calculate a suitable duty cycle according to channel status. But without HAP to coordinate LTE users and Wi-Fi stations, Wi-Fi stations cannot know when LTE users will utilize the unlicensed band. Therefore, it will increase the collision rate in the  
Figure 10: Simulation scenario. unlicensed band, decreasing the throughput of Wi-Fi. We have a similar observation when Wi-Fi is heavily loaded. Table 2 presents the same experiment results from a different perspective. It is evident that the proposed scheme possesses the highest total throughput compared with other approaches, as intended. Notice that NBS has a secondary total throughput close to the proposed approach. However, NBS's fairness is inferior to the proposed scheme, as we shall see in later experiments. To understand the effect of the number of Wi-Fi stations on the overall throughput, the number of STAs gradually increased without changing the number of LTE users. Figure 11(a) shows the change in the overall Wi-Fi throughput. Under the original mechanism without a coexistence mechanism, the throughput dropped rapidly with the increase in STAs. Because LAA utilizes the pure contention-based mechanism named LBT, the Wi-Fi throughput also dropped significantly due to the increment of the number of STAs. LTE-U adopts the CSAT mechanism to coexist with Wi-Fi, which can detect and analyze the number of neighboring base stations, including LTE-U base stations and Wi-Fi APs. Based on the observation, the access time will be divided into several adaptive duty cycles in which LTE users and Wi-Fi stations operate in a time-division manner such that the neighboring base stations can equally share resources.