Research On LTE-M-WLAN Coexistence Technology For CBTC System In Unlicensed Spectrum

Due to the serious wireless interference in unlicensed spectrum,The CBTC system based on WLAN is suffering frequent train emergency braking at present.To solve this problem,the Urban Rail Transit Association proposed to deploy the CBTC system which is based on LTE-M technology in licensed spectrum.However,during the deployment of the LTE-M system,it is found that the bandwidth of the licensed band is limited,and it is difficult to meet the urban rail transit integrated service bearer demand by the limited resource in licensed spectrum.Therefore,it is proposed to use the unlicensed spectrum along with the licensed spectrum to carry the LTE-M system service together.In order to ensure the secure transmission of LTE-M system services in unlicensed spectrum,this paper will design an LTE-M in unlicensed spectrum-WLAN coexistence scheme based on the existing LTE technology in unlicensed spectrum for urban rail transit scenarios.This paper firstly models the coexistence scenarios of LAA and LTE-U technologies respectively,compares the throughput changes of the two technologies with the interference of WLAN,and verifies the theoretical results through NS-3 system level simulation.It is finally determined that LTE-U is the most suitable technology for carrying LTE-M services in unlicensed spectrum.Then this paper analyze the original CSAT resource allocation algorithm of LTE-U through simulation,and points out that it can not guarantee the service transmission performance of LTE-M when the load of channel is too high,then the fixed minimum duty cycle CSAT and queue-sensing-based CSAT are proposed.The fixed minimum duty cycle CSAT scheme sets the minimum duty cycle of CSAT to a fixed value,ensuring that the data transmission time that LTE-U can obtain under high channel load conditions is not less than the set ratio to ensure system transmission performance.The queue-sensing-based CSAT scheme introduces the FLS algorithm,and calculates the minimum amount of data that the LTE-U device needs to transmit in order to meet the LTE-M system service demand in the current period at the beginning of each duty cycle,and compare to the actual data that was transmitted in the same period.According to the result of comparison,the algrithm adjusts the duty cycle of the LTE-U to guarantee the transmission performance of the LTE-M system service.Through simulation,both improved schemes can guarantee the transmission performance of LTE-U under high channel load conditions.Compared with the fixed minimum duty cycle scheme,the queue-sensing-based CSAT scheme achieves a better performance of fairness through a flexible duty cycle adjustment strategy,while ensuring the security of LTE-M system service.