An Adaptive Opportunistic Transmission for Unlicensed Spectrum Sharing in Heterogeneous Networks Daehyoung Kim, Pervez Khan, Hoon Kim Abstract—Efficient utilization of spectrum resources is a fundamental issue of wireless communications due to its scarcity. To improve the efficiency of spectrum utilization, the spectrum sharing for unlicensed bands is being regarded as one of key technologies in the next generation wireless networks. A number of schemes such as Listen-Before-Talk(LBT) and carrier sensor adaptive transmission (CSAT) have been suggested from this aspect, but more efficient sharing schemes are required for improving spectrum utilization efficiency. This work considers an opportunistic transmission approach and a dynamic Contention Window (CW) adjustment scheme for LTE-U users sharing the unlicensed spectrum with Wi-Fi, in order to enhance the overall system throughput. The decision criteria for the dynamic adjustment of CW are based on the collision evaluation, derived from the collision probability of the system. The overall performance can be improved due to the adaptive adjustment of the CW. Simulation results show that our proposed scheme outperforms the Distributed Coordination Function (DCF) mechanism of IEEE 802.11 MAC. Keywords—Spectrum sharing, adaptive opportunistic transmission, unlicensed bands, heterogeneous networks. I. I NTRODUCTION D UE to the surge in usage of smart devices, there is an ever increasing demand for spectrum. Therefore, efficient utilization of spectrum resources is a fundamental issue of wireless communications due to its scarcity. To improve the efficiency of spectrum utilization, the spectrum sharing for unlicensed bands is being regarded as one of key technologies in the next generation wireless networks. The unlicensed bands, such as the ones used in 2.4GHz ISM (Industrial, Scientific and Medical) and 5GHz U-NII (Unlicensed National Information Infrastructure), used for a number of access technologies such as 802.11 (WiFi), 802.15.1 (Bluetooth) and 802.15.4 (ZigBee), have traditionally been unsuitable for use with access technologies designed primarily to operate in licensed frequencies, like LTE, that focus on maximizing spectral efficiency and optimizing user experience. Some collaborative operations such as data offload in unlicensed band today is primarily carried out using Wi-Fi which is designed in the premise of trading off performance for low cost and simple implementation for spectrum sharing. Meanwhile, an advanced scenario of unlicensed spectrum usage of LTE features such as Carrier Aggregation (CA) has Daehyung Kim, Pervez Khan, and Hoon Kim are with the Department of Electronics Engineering, Incheon National University, Incheon, South Korea (e-mail: DH.KIM@inu.ac.kr, pkhan@inu.ac.kr). Hoon Kim is with the Department of Electronics Engineering, Incheon National University, Incheon, South Korea (Corresponding author, e-mail: hoon@inu.ac.kr). made it possible to operate these technologies in unlicensed bands as well. From the user perspective, this means an enhanced broadband experience, higher data rates, seamless use of both licensed and unlicensed bands, with high reliability and robust mobility through licensed anchor carrier. A critical element of the design for LTE in unlicensed band is to ensure LTE-U (LTE in Unlicensed band) co-exists with current access technologies such as Wi-Fi on fair and friendly bases [1]. A number of schemes such as LBT and carrier sensing adaptive transmission (CSAT) have been suggested from this aspect [2], [3], [4]. Those schemes might be useful for spectrum sharing between LTE and Wi-Fi, but more efficient sharing schemes are required for improving spectrum utilization efficiency. This work considers an opportunistic transmission approach and a dynamic CW adjustment scheme for LTE-U users sharing the unlicensed spectrum with Wi-Fi, in order to enhance the overall system throughput. The decision criteria for the dynamic adjustment of CW is based on the collision evaluation, derived from the collision probability of the system. The overall performance can be improved due to the adaptive adjustment of the CW. Simulation results show that our proposed scheme outperforms the DCF mechanism, especially in dense environments. The remainder of this paper is structured as follows: Section II presents the system model of the opportunistic transmission and dynamic CW adjustment scheme. Section III addresses the numerical results and conclusions appear in Section IV. II. SYSTEM MODEL As shown in Fig. 1, we assume that there is an LTE-U Access Point (AP) in the coverage of a Wi-Fi AP. The Wi-Fi AP can only use the unlicensed band, while the LTE-U can use both the licensed and unlicensed bands to serve n Wi-Fi and n LTE-U users, respectively. The DCF mechanism is also adopted in the LTE-U AP to enable the LTE-U users to compete each other and with the Wi-Fi users for the unlicensed band. A. DCF Mechanism According to the IEEE 802.11 DCF mechanism, all the stations compete for an unlicensed channel based on binary exponential backoff scheme with the minimum CW size (CW min ), and the maximum CW size (CW max ), is adopted in the Wi-Fi AP. Fig. 3 shows the basic DCF mechanism of the IEEE 802.11 MAC protocol. From [5], the stationary World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:10, No:5, 2016 693 International Scholarly and Scientific Research & Innovation 10(5) 2016 ISNI:0000000091950263 Open Science Index, Electronics and Communication Engineering Vol:10, No:5, 2016 publications.waset.org/10004604/pdf