Coexistence,Convergence And Optimization Of LTE And WiFi In The Unlicensed Spectrum

The cellular data traffic has been dramatically increased in the past few years due to the explosive growth in mobile applications,putting a lot of pressure on capacity and data rate im-provement of cellular networks.One effective way to cope with this challenge is targeting more available spectrum opportunities,which has aroused much attention from academia and industry with the support of many leading companies recently.A new long term evolution in unlicensed spectrum(LTE-U)technology has been proposed,which allows users to access both licensed and unlicensed bands under a unified LTE network infrastructure.It can provide better link perfor-mance,medium access control(MAC),mobility management,and larger coverage than simple WiFi offloading.Despite the many advantages of LTE-U,it still faces many technical challenges.One primary issue is the coexistence of LTE-U with the incumbent and wildly popular WiFi net-works.In addition to the coexistence problem,heterogeneous convergence and resource sharing optimization are the other two challenges.This thesis aims to address the above issues.First,LTE systems are specifically designed to operate in the licensed spectrum under the centralized control of network units based on non-contention MAC protocols to prevent packet collision among subscribers.However,WiFi net-works rely on carrier sensing multiple access with collision avoidance(CSMA/CA)to reduce packet collision and use a contention based MAC protocol,namely distributed coordination func-tion(DCF),to resolve package collision through a random backoff mechanism.Thus,how to ensure a fair and harmonious coexistence environment for both networks becomes a major chal-lenge to LTE-U.In this issue,we first introduce a novel hyper-access point(HAP)that integrates the functionalities of LTE small cell base station(SBS)and commercial WiFi access point(AP)for deployment by cellular network operators.Based on a novel hyper access point(HAP)we introduced for effectively embedding LTE-U in unlicensed WiFi band,LTE-U can directly take advantage of the WiFi point coordination function(PCF)protocol.To facilitate the coexistence,our HAP dedicates a contention free period(CFP)to LTE-U users and allows a contention period(CP)for traditional WiFi users,which can avoid interference between LTE-U and WiFi transmis-sion,and thus improve LTE-U throughput and unlicensed resource utilization.Then,we consider the heterogeneous convergence problem.Heterogeneous convergence does not mean that one network replaces the other one,but take full advantages of both networks and use traffic offloading and resource sharing methods to improve overall system performance and user experience.In this issue,we analytically compare the cellular throughput of the traffic offloading and resource sharing methods with the guarantee of per-user WiFi throughput.We find that traffic offloading can achieve better performance than resource sharing when existing WiFi user number is below a threshold,and vice versa.Then,we develop a hybrid method to take full advantages of both traffic offloading and resource sharing methods,where cellular base stations(BS)offload traffic to WiFi networks and simultaneously occupy certain number of time slots on unlicensed bands.Moreover,we derive a closed-form expression for the throughput upper bound in each method.Furthermore,we propose transferring some WiFi users to be served by the LTE system,in contrast to the traditional mobile data offloading which effectively offloads LTE traffic to the WiFi network.In addition,we utilize the Nash bargaining solution(NBS)to develop joint user transfer and unlicensed resource allocation strategy to fulfill the win-win situation for both networks.Finally,resource sharing optimization has been discussed based on the LTE-U energy effi-cient(EE)problem.We can find that EE of the LTE system may be degraded since unlicensed bands are generally less energy efficient than licensed bands.Moreover,we develop a criterion to determine whether unlicensed bands can be leveraged to improve the EE of LTE-U systems,and also investigate an algorithm to provide fair EE among different SBSs based on the Nash bargaining solution(NBS).The research results in this thesis can provide a theoretical base for investigating more com-plex LTE-U systems and also some useful theoretical support and reasonable technical solutions for the 5th generation(5G)communication.