# Unlicensed Spectrum Allocation for LTE and Wi-Fi Coexistence with HAP
## 2 Related Technology and Works
### Related Works on the Coexistence of LTE and Wi-Fi  
Many studies are devoted to the coexistence of LTE and Wi-Fi in the unlicensed band. The following is a brief survey of existing approaches. In [9], [PERSON] et al. proposed a novel Hopfield neural network-based mechanism as an efficient and fair coexistence mechanism in the unlicensed bands for an LTE-U base station alongside Wi-Fi access points (APs). The coexistence problem was modeled as an optimization problem, in which both the LTE-U data rate and the QoS of the Wi-Fi network are considered for fairness. Another scheme, named mLTE-U, was proposed in [10]. The proposed approach adopts an adaptive LBT scheme. After a variable transmission opportunity (TXOP), mLTE-U has a variable muting period, which the Wi-Fi networks can exploit to gain access to the media. A Q-learning technique is employed to achieve fair coexistence between mLTE-U and Wi-Fi networks. The proposed scheme can decide on an appropriate mixture of TXOP and muting period for fair coexistence. In [11], [PERSON] et al. address issues neglected in previous works, such as the uncertainties in LAA-basedcoexistence systems. It could be that only partial or no information on MAC and physical layer protocol are available in the systems. The lack of such accurate information may inaccurately estimate the key performance indicators. A novel machine learning mechanism that combines a neural network with a logistic regression algorithm is proposed in the paper. It can track and estimate key performance indicators (KPIs) and probability of coexistence (PoC) of LTE-LAA and Wi-Fi networks without the information of MAC and physical layer parameters. A reinforcement learning-based sub-channel selection technique is introduced in [12] for a coexistence scenario with multiple LAA and Wi-Fi competing for channel access in an unlicensed band. The proposed scheme allows access points and eNBs to select the best sub-channel by the MAC protocol considering the physical layer's parameters. In [13], a novel proportional fair allocation scheme was proposed to guarantee fair coexistence between LTE-U and Wi-Fi networks. It allocates the channel access time in a proportional-fair manner to each entity without message-passing between LTE-U and Wi-Fi networks. In [14], a cross-layer proportional-fairness-based framework is proposed to achieve throughput-oriented proportional fairness between the LTE-U and Wi-Fi networks. When LTE-U eNBs operate with the LBT scheme to access the channel, the interactions between the LTE-U and Wi-Fi networks can be modeled by two interactive Markov chains. [PERSON] et al. analyzed the throughput of LTE-U and Wi-Fi and formulated a utility function of throughput, transforming the problem into an optimization problem. The architecture with Hyper-AP (HAP) is proposed in [15]. In [16], the operation of HAP is divided into a contention-free period and a contention period for LTE-U and Wi-Fi users, respectively. To improve system throughput and user fairness, [PERSON] et al. take the resource allocation and user association into consideration to maximize the network utility based on the Nash bargaining solution (NBS). To decide what percentage of a repeated cycle in the time domain should be allotted to the LTE users, the NBS (Nash bargaining solution) algorithm is utilized to solve the allocation of limited resources among many contestants. We would refer to the scheme as NBS. Any competitor in the game would like to obtain a maximum benefit. When one of the participants receives profits, it will cause others to lose some profits. After the bargaining process, the distributions of resources will achieve a balance status among contestants. In [17], [PERSON] and [PERSON] propose a semi-distributed LTE-Unlicensed scheme in which the Wi-Fi-like carrier sense, back-off, and QoS mechanism are equipped in the LTE base station. The proposed scheme also uses the almost blank sub-frame (ABS) to control the interference.