| The demand for wireless multimedia services continues to grow,and the smartphones become widespread.The traffic of traditional cellular networks is approaching the limitation of licensed band capacity.Due to the limited bandwidth of the licensed spectrum,the tremendous amount of audio and video streaming services will seriously block off the licensed channel.3GPP proposes to offload the excessive traffic of cellular network to the unlicensed spectrum.Currently,there are mainly solutions,LTE-U and LAA.LTE-U is suitable for the countries and regions such as China and North America.Meanwhile,LAA is a single global framework which meets the strict requirements on unlicensed band,and it applies to all countries and regions of the world.Wi-Fi with IEEE 802.11 serious standards,is the dominated wireless technology currently working in the 5GHz.If LAA system operate on the 5GHz unlicensed band,it will be inevitable to work with Wi-Fi on the same channel.Therefore,to analyze the system performance of LAA and Wi-Fi when they coexisting in the same unlicensed spectrum,this dissertation establishes the models for the LBT channel detection mechanism of LAA.For the coexisting scenario,it also proposes some solutions for the fairness as well as some coordination schemes for inter/within LAA and Wi-Fi systems.This paper mainly analyzes the system capacity of coexistence of cellular,which belong to the 5G mobile communication standard,and Wi-Fi systems in the same channel in unlicensed spectrum,and studies the technology of resource allocation among coexisting systems.The main work and innovations of this dissertation include the following four parts.Firstly,in this work,the transmission problems of the coexisting LAA and Wi-Fi systems is discussed,under the conditions of ideal channel and the saturated traffic model.The two-stage CCA detection methods in 3GPP technical report TR 36.889,LAA LBT category types Cat 3and Cat 4 are established as two-dimensional discrete Markov models,and then the coexistence performance expressions of LAA and Wi-Fi system are derived.We consider the impacts of network size,competitive window size of Cat 3,and contention window size and backoff stage of Cat 4 on the performance of these two coexisting systems,respectively.The simulation results show that the theoretical analysis models can effectively predict the successful channel occupancy ratio of LAA and Wi-Fi,under the conditions that the number of LAA and WiFi devices is various.Simulation and theoretical analysis results show that,by appropriately altering the LBT Cat 3 contention window size,or the LBT Cat 4 competition window size and backoff stage,the LAA system can achieve a fair coexistence with Wi-Fi system,and the channel is maximize utilized.Secondly,considering the practical application scenario,the transmission problem of the coexisting LAA and Wi-Fi systems is further discussed,under Rayleigh fading channel and unsaturated traffic model conditions.A two-dimensional discrete Markov model,with an idle state,is established for the two-stage CCA detection LBT method Cat 4 of LAA.And the impact of traffic loads size on system performance is analyzed.The effects of capture in Rayleigh fading channel and transmission failure caused by channel's error are considered in the analysis.Meanwhile,we assume that the Wi-Fi transmission duration is fixed,the influence of different length of LAA transmission duration on the system performance is analyzed.The simulation experiment proves that the theoretical analysis model is valid under the cases that LAA uses different backoff stage and contention window size.This model is valid for various network size.The simulation and theoretical analysis results show that,under the Rayleigh fading channel condition,if both LAA and Wi-Fi system serve small traffic load,the successful channel occupancy ratios are related to the service traffic load.Otherwise,if the LAA and Wi-Fi have large traffic size,the successful channel occupancy ratios of the two are related to the LBT parameter of the LAA system.Specifically,the small backoff stage and contention window size can make the LAA system obtain a larger successful channel occupancy ratio while weakening Wi-Fi system performance.By adjusting the LBT parameters of the LAA,the coexisting two systems can obtain similar successful channel occupancy ratios,thereby achieving fair coexistence between LAA and Wi-Fi.Thirdly,we propose the performance improved scheme of the LAA system and offer a strategy of fair coexistence with the Wi-Fi system.The ICCA process of LAA is adjusted.We change the fixed ICCA detection duration to a variable method,and then establish the Markov model of the variable-size ICCA of the Cat 3 and Cat 4 access mechanisms,to reduce the conflicts within LAA system.Then the LAA and Wi-Fi coexistence model is further deduced.The transmission expression of the Wi-Fi system is derived through the LAA transmission result,and the adaptive LBT mechanism is proposed based on these derivation results.The adaptive LBT mechanism suggested in this paper is based on the statistical average value of the LAA transmission results which is used to infer the Wi-Fi transmission parameters,and then LAA system will adjust the LBT scheme to achieve the fairness between LAA and Wi-Fi system.No additional access strategy and inter-system information transfer is required in our method.Additionally,no extra coordination mechanism is needed when this LBT scheme is used.The simulation experiments verify that,the proposed adaptive LBT strategy can make the performance of LAA and Wi-Fi system have a linear relationship.The LAA system and the Wi-Fi system can obtain similar successful channel occupancy by a correction value.Further simulation experiments show that the strategy can allocate successful channel occupancy ratio according to the network size of the two coexisting systems,and realize the fair share channel resource of each device.Finally,the paper proposes an coordination mechanism within the LAA system,and an inter-system conflict avoidance mechanism in the LAA and Wi-Fi coexistence environment.According to the characteristics of energy detection,the LAA system channel detection method,we propose the energy pattern to carry information.The channel is reserved by the successively broadcast energy pattern signal before the transmitting and receiving of the information frame,to reduce the loss caused by collisions,and clear the hidden node.Since the energy pattern signal carries information by energy on the different time-frequency positions,the modulation and demodulation time will not be added,and the modulation coding scheme of the system itself and the original codebook of the operator could not be shared.Simulation experiments show that the scheme is effective for reducing the transmission conflicts of different LAA operators.LAA system uses energy detection method and Wi-Fi can not read the LAA information since the different OFDM symbol they used.On the one hand,we propose the cross-carrier transmission with two-stage sensing,the dual channel detection scheme,for LAA system,to reduce the invalid transmission due to the interference at the receiving terminal.On the other hand,we propose the coexistence auxiliary signal,which is a periodic signal transmitted by the LAA receiving terminal,to reduce the probability that the normal transmission failed caused by other transmission interruptions.Simulation experiments show that this scheme can improve the efficiency of LAA system in LAA and Wi-Fi coexistence scenarios. |
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