A Study On Channel Access Mechanisms And Performance Modeling For LTE-U And WiFi System Coexistence In Unlicensed Spectrum

The implementation of LTE-U and WiFi coexistence in unlicensed spectrum bands has been considered as an important mean to increase the capacity of cellular systems and address the increasingly scarce radio spectrum resources issue.Converged network architecture,interference management,and channel access are the major technical issues that must be resovled in implementing LTE-U and WiFi system conexistence in unlicensed spectrum bands.This thesis studies the theory and methodology for LTE-U and WiFi system conexistence in unlicensed spectrum bands,which are very important to resolving these issues and implementing the reasonable coexistence of an LTE-U system and a WiFi system in unlicensed spectrum bands.The major work and original contributions of this thesis include the following five aspects:(1)A virtualized converged network architecture for LTE-U and WiFi system coexistenceThis work considers an LTE-U and WiFi system coexistence scenario in unlicensed spectrum bands,and proposes a converged network architecuture for supporting LTE-U and WiFi system coexistence.By using the network function virtualization(NFV)technology and the software defined network(SDN)technology,this converged network architecture can implement the protocol-layer network virtualization of LTE-U and WiFi systems using different access standards on a C-RAN platform.Moreover,it can implement unified network resource allocation and management by providing the virtualized network entities and access users of LTE-U and WiFi systems with open north-south API interfaces via a unified network controller.Based on this virtualized network architecture,a mechanism is proposed for interexchanging information between an LTE-U system and a WiFi system in order to implement the protocol-layer convergence of LTE-U and WiFi systems.By interexchanging information between an LTE-U system and a WiFi system,this virtualization technology based converged network architecture for an LTE-U and WiFi coexistence system can dynamically adjust the channel access mechanism or channel allocation mechanism based on the current network status and thus make it possible to realize different applications.(2)A hybrid adaptive channel access mechanism for LTE-U and WiFi system coexistence in unlicensed spectrum bandsThis work considers the LTE-U and WiFi system coexistence problem in unlicensed spectrum bands,and proposes a hybrid adaptive channel access(Hy ACA)mechanism based on the virtualized converged network architecture for supporting LTE-U and WiFi system coexistence.A new channel time-frequency structure is first introduced for LTE-U users and WiFi users to access a channel.Moreover,a modified listen before talk(MLBT)function is introduced for LTE-U users to contend for channel access.Based on the time-frequency structure and the MLBT function,a dynamic channel switch(DCS)component mechanism and an adaptive almost blank subframe(AABS)component mechanism are proposed for channel access under different system traffic loads.When the system traffic load is low,the DCS mechanism is employed.When the system traffic load is high,the AABS mechanism is employed and the number of ABSs is adaptively adjusted based on the current system traffic load status.To judge on the current traffic load status in the coexistence system,the average channel access time per LTE-U base station and the average channel access time per WiFi access point are respectively modeled and analyzed.Based on the modeling and analysis,a calculation model is provided for determining the number of ABSs that needs to be adjusted according to the current system traffic load.Simulation results show that,compared with the ABS mechanism and the load-based LAA LBT mechanism in 3GPP Release 13,the proposed Hy ACA mechanism can achieve better fairness in channel access between the two systems and can effectively improve the average throughput of LTE-U users in the coexistence system.(3)A delay balanced adaptive channel allocation mechanism for LTE-U and WiFi system coexistence in unlicensed spectrum bandsThis work proposes a delay-balanced adaptive channel allocation(DB-ACA)mechanism for improving the channel access performance of an LTE-U and WiFi coexistence system based on the virtualized converged network architecture for supporting LTE-U and WiFi system coexistence.In the considered coexistence system,LTE-U users are allowed to use both licensed spectrum bands and unlicensed spectrum bands,while WiFi users are allowed to use only unlicensed spectrum bands.By using the virtualized converged network architecture,the proposed DB-ACA mechanism enables the coexistence system to consider the usage of both licensed bands and unlicensed bands by the LTE-U system,and adaptively adjust the number of channels on unlicensed bands allocated to the LTE-U system and the WiFi system according to the current traffic load status in the LTE-U system and the WiFi system on unlicensed bands.To ensure the access fairness of the coexistence system,the DB-ACA mechanism introduces a fairness criterion,which requires that the channel access delay be balanced on the unlicensed band between the LTE-U and WiFi systems.For the LTE-U system,a 2-D Markov model is built to describe the state transition of the LTE-U system.In particular,the LTE-U user arrivals on the unlicensed band actually follow an IPP,which is approximated into a 1-D Markov arrival process.For the WiFi system,a classic Erlang model is used to describe the channel access model.Based on the Markov models,the performance models are derived to analyze the performance of the LTE-U system and WiFi system in terms of the blocking probability and access delay.Moreover,according to the fairness criterion,a theoretical model is derived to determine the number of unlicensed channels that should be adjusted dynamically.The effectiveness of the theoretical models is validated through simulation results.Simulation results show that,the proposed DB-ACA mechanism can effectively ensure access fairness of the coexistence system,and significantly reduce the blocking probability and access delay of the coexistence system.(4)Performance modeling and analysis of LAA Cat.4 LBT(Listen Before Talk)channel access procedureThis work studies the modeling and analysis of 3GPP LAA Cat.4 LBT procedure for e Node Bs to access a channel on an unlicensed spectrum band.A 3-D Markov model is first used to describe the channel access process for transmitting a data packet with priority,and establish the relationship between the transmission probability and collision probability of a data packet with priority.Unlike existing LAA LBT procedure models,the 3-D Markov model takes into account the GAP period and transmission priority,which is the main original contribution of this work.Based on the established 3-D Markov model,performance models are further derived for analyzing LAA e Node B's steady-state transmission probability,normalized throughput,and average channel access delay.Finally,the derived performance models are justified through simulation results and the impacts of the GAP period and transmission priority on the system performances are investigated through numerical results.It is found that the GAP period has a big impact on the LAA system performance.An LAA Cat.4 LBT procedure with the GAP period filled using a reservation signal can achieve better performance than one with the GAP period not filled using a reservation signal.This work can provide a basis for further studying the performance of an LAA and WiFi coexistence system.(5)Performance modeling and analysis of LAA and WiFi coexistence system with LAA Cat.4 LBT procedure and 802.11 e EDCA mechanismThis work considers an LAA and WiFi system coexistence scenario,and studies the performance modeling and analysis of an LAA and WiFi coexistence system in the scenario.The LAA system uses LAA Cat.4 LBT procedure while the WiFi system uses 802.11 e EDCA mechanism for channel access.In performance modeling,two 3-D Markov models are first used to describe the LAA Cat.4 LBT procedure and 802.11 e EDCA mechanism,and take into account the impacts of the non-saturation state,GAP period,and transmission priority on the channel access process,which are the main original contributions of this work.Based on the two 3-D Markov models,performance models are further derived for analyzing LAA e Node B and WiFi AP's steady-state transmission probability,normalized throughput,and average channel access delay for transmitting data with different priorities.Finally,the derived performance models are justified through simulation results and the impacts of the GAP period and transmission priority on the performances of the coexistence system are investigated through numerical results.It is found that the GAP period has a big impact on the LAA system performance.In an LAA and WiFi coexistence system,it is necessary to fill the GAP period with a reservation signal in order to make the LAA system fairly coexist with the WiFi system.