Performance Analysis And Optimal Design Of Coverage Enhancement Mechanism In Narrow Band Internet Of Things

NB-IoT(Narrowband Internet of Things)is one of the LPWA(Low Power Wide Area)technologies proposed by 3GPP for sensing and data acquisition applications such as smart meters and environmental monitoring,facing technical challenges such as massive connectivity,ultra-low power consumption,wide-area coverage and deep coverage,and mutual triggering of signaling and data.In order to meet the goal of NB-IoT coverage enhancement,3GPP has designed a special link adaptation technology that the user equipment determines its coverage class according to its channel environment and adopts the corresponding coverage enhancement configuration,such as different preamble repetitions and transmitting power.Coverage class is a new concept introduced by 3GPP for NB-IoT.When the channel environment of NB-IoT in a cell is quite different,the same coverage enhancement configuration for all the user equipments will result in waste of resources or deterioration of user equipments performance in poor channel environment.NB-IoT defines a cell with up to 3 coverage classes to meet differentiated coverage enhancement requirements.The main innovation work in paper as follows:(1)A test scheme of NB-IoT indoor and outdoor channel propagation environment is designed.The test scenarios are determined according to the factors such as user equipments density and penetration loss.Statistical characteristics analysis and multi-peak Gauss fitting are carried out for the test data of different scenarios.The rationality of introducing coverage class into NB-IoT by 3GPP is verified.An algorithm to determine the initial coverage class is proposed,and the number of coverage classes and the optimal decision threshold for typical scenarios are given.Simulation results show that the NB-IoT cell in urban area can set two coverage classes,and the optimal decision threshold can be set to-101.8 dBm;the NB-IoT cell in rural area can set three coverage classes,and the optimal decision threshold can be set to-107.7 dBm and-79.63 dBm.(2)The Markov chain of NB-IoT coverage class updating mechanism is established with coverage class as state,which accurately described the dynamic updating process of NB-IoT coverage class.a coverage class updating model with backoff mechanism is proposed to reduce the extra power consumption caused by the initial coverage class determination.A multi-objective optimization model is established with the maximum retransmissions of preamble as variable and the average access failure probability and the average power consumption of the system as objectives.The optimal configuration of the maximum retransmissions of preamble for each coverage class is given.The effects of repetitions of preamble,system load and global maximum retransmissions on the optimal value of maximum retransmissions and the corresponding average access failure probability and average power consumption are analyzed Numerical analysis results show that the maximum transmission number of normal coverage and extended coverage have a great influence on the system performance and their value ranges should be set within [1,5] and [1,7] respectively.However,the maximum transmission number of extreme coverage has little influence on the system performance,its value can be any one in [1,10] but the recommended value is 1.In addition,the average power consumption of the model that introduced coverage classes' rollback mechanism is about 95% lower than the model proposed by 3GPP.(3)Uplink time-frequency resource utilization rate of NB-IoT random access channel and Uplink shared channel multiplexing is studied.A multi-objective optimization model is established with the preamble repetitions as variable,average access success probability and uplink time-frequency resource utilization rate as objectives.The optimal configuration of repetitions for typical multiplexing scenarios without considering the maximum retransmissions of preamble is solved and the influence of the maximum retransmissions of preamble on the optimal configuration of repetitions is analyzed.Simulation results show that the time-frequency resource utilization rate decreases with the increase of repetitions,and the average access success probability increases with the increase of repetition times.The coordination of maximum retransmissions and repetitions of each coverage class can achieve ultra-low power consumption and high uplink time-frequency resource utilization rate with the probability of success of random access being satisfied.