# Traffic Offloading in Unlicensed Spectrum for 5G Cellular Network: A Two-Layer Game Approach
## 1 Introduction  
Global mobile data usage has grown by 70-200 percent per annum [1]. Long Term Evolution (LTE) has been proven to be a great success in satisfying the growing user data rate requirements owing to its high spectral efficiency, seamless coverage, and excellent centralized coordination and management services. However, the current capacity of LTE/LTE-Advanced networks will not be able to support the demand in the future. In recent years, enabling cellular small cells in unlicensed frequency bands has garnered attentions as a promising solution to the scarcity of the licensed spectrum for cellular networks [2; 3]. The main incumbent system in the 5 GHz band is the Wireless Local Area Networks (WLAN) [4] which is designed in the premise of trading off performance for low cost and simple implementation for spectrum sharing [5]. The 3 rd Generation Partnership Project (3 GPP) proposed to extend the use of LTE to the unlicensed spectrum, which is named LTE in Unlicensed (LTE-U), also called License Assisted Access (LAA) [6]. Many articles focus on the fairness and friendly coexistence between LAA and WiFi, however, it is also critical to enhance the spectral efficiency of both two systems and make better use of the valuable spectrum resources. Some coexistence mechanisms for LAA and WiFi have been studied in [7; 8; 9; 10; 11; 12], which show that LTE and WiFi could coexist in a friendly manner, and improve the performance of the whole network. There are, at this stage, three main LTE mechanisms under considerations for ensuring fair coexistence with WiFi, namely, Listen Before Talk (LBT) [7; 8], Carrier Sensing and Adaptive Transmission (CSAT) [9; 10; 11; 12; 13; 14] and traffic offloading [15; 16; 17; 18; 19]. In the LBT mechanism, a transmitter would listen to the medium and determine whether it could transmit [6; 7], which is similar to the incumbent Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism in IEEE 802.11 [5]. In practice, due to the occurrence of the collision, the LBT mechanism cannot clearly reflect the advantages of LTE, especially when too many users access to the channel. Furthermore, this method will reduce the whole networks throughput. To exert the advantages of LTE better, a LBT combined with CSAT (LBT-based CSAT) scheme is proposed in [9] which uses LBT to listen to the channel firstly and make sure the channel is free to enter, and then the CSAT could perform \"on\" for coexistence [10]. In addition, CSAT could be applied without changing the current LTE/LTE-Advanced technology, which adopts \"on\" and \"off\" state transition within a single duty cycle [11; 12]. Moreover, CSAT could achieve the same fair effect as the LBT by adjusting parameters appropriately [13; 14]. However, collision still may occur when the WiFi backoff time is counted down to zero and the LTE is about to send packets simultaneously at the start of \"on\" periods. To deal with the problem mentioned above, a modified LBT-based CSAT scheme, which increases a protection mechanism is proposed in this paper. In addition, data offloading is also a good way to share the spectrum. Offloading parts of data to the WiFi network [15] can effectively alleviate the LAA network congestion and enhance the user experience. The author in [16] proposed a new mechanism that is contrast to the traditional mobile data offloading, which transfers some WiFi users to be served by the LAA system. Meanwhile, some unlicensed spectrum resources may be allocated to the LAA system in compensation for handling more WiFi users. In this way, a win-win situation would be generated since LAA can generally achieve better performance than WiFi due to its capability of centralized co-ordination [4; 20]. The IEEE 802.11 Working Group (which standardizes WiFi) is continuously developing new amendments to improve the WiFi performance in emerging scenarios [5]. However, some Access Points (APs) have overlapping coverage to share the network, which would greatly increase the number of devices competing for access.