# Towards Harmonious Coexistence in the Unlicensed Spectrum: Rational Cooperation of Operators
## 1 Introduction
### Related Work and Motivation of LTE Operators' Behavior in the Unlicensed Spectrum  
Different LBT deployments were used in [14] to analyze the average user throughput of LTE small cell networks and to verify how WiFi systems are affected by LTE in the unlicensed spectrum. The authors of [15] evaluated throughput performance for both LTE with LBT and WiFi systems under indoor and outdoor scenarios based on stochastic geometry. In [16], extensive simulations were performed to investigate the effectiveness of adjusting transmission time for LTE-LAA with coexisting WiFi networks based on the indirect estimation of the data rate and traffic load of WiFi networks. The work of [17] has evaluated various LBT schemes such as 3 GPP category 4 LBT and extended clear channel assessment (ECCA) to quantify the performance tradeoff of WiFi networks. According to [18], indoor-based interference simulations have addressed channel-sharing issues between LTE-LAA and a WiFi network, and different bandwidths of LTE-LAA (1.4/3/5/10 MHz) have shown different throughput degradation of WiFi in experiments. The authors of [19] verified the optimal transmission time of LTE-LAA by considering a MAC protocol design to protect WiFi in the unlicensed spectrum. In [20], the performance of two coexisting WiFi networks were investigated first, and then one of the WiFi networks was replaced with an LTE operator to quantify the existing WiFi network's user-perceived throughput (UPT) which is the amount of traffic over the actual transmission time excluding the idle time. In [21], the authors extensively presented a classification of different techniques that can be applied on co-located LTE and Wi-Fi networks. The work of [22] also investigated the coexistence of LTE and WiFi where the LTE base station uses WiFi-like carrier sense, back-off, and Quality of Service facility techniques. In [23], the impact of coexistence on WiFi performance and LTE performance was analyzed. According to [13], studies on the operation of LTE in the unlicensed spectrum concluded that LTE-LAA and WiFi systems can harmoniously coexist because LTE-LAA using LBT causes less adjacent channel interference to a WiFi network compared to another WiFi network.  
Despite research on coexistence between LTE and WiFi systems, scenarios involving multiple LTE operators in the unlicensed spectrum have not yet been well studied from theoretical and practical perspectives. Unlicensed spectrum debates in Korea highlighted that an opportunity for the 5 GHz band is likely to induce a new paradigm of LTE operators' policies on the competitive environment [24]. To this end, this article first verifies the interaction between multi-RAT systems the in unlicensed spectrum and then determines the best strategy to maximize their utility. Specifically, we focus on the behavior of two operators with user-deployed WiFi networks based on the framework of game theory. Game theory is a useful tool for modeling independent decision makers whose actions affect others' decisions [25]. Many issues related to wireless communications have been extensively examined by game theory to establish intuition about the strategic operation of systems from both short- and long-term points of view [26]. To this end, we introduce the operator's dilemma and discuss how to improve the efficiency of spectrum utilization to achieve harmonious coexistence between multi-RAT systems in the unlicensed spectrum.  
The rest of our paper is organized as follows. In Section 2, basic operation mechanisms for LTE operators and WiFi networks are introduced. The LTE operators' dilemma in the unlicensed spectrum is addressed with the game theory framework, and performance analysis is represented in Section 3. Finally, we draw our conclusions with discussion and future topics in Section 4.