Research On Key Technology Of Grant Free For Ultra Reliable And Low Latency

In recent years,with the extensive application of intelligent device,mobile data traffic increases rapidly,which has caused huge signaling overhead for wireless communication.Simultaneously,in the application of the Internet of Things,the traditional mobile communication access scheme will increase the system complexity and delay due to the characteristics of short packet transmission and abundant traffic.Therefore,3GPP has proposed grant-free access technology to support the future network communication system in which people and things co-exist.The GF technology has the following advantages,such as reducing the control overhead,adapting to the sporadic and sparsity of traffic arrival and supporting low latency business in delay-sensitive networks,etc,which have attracted more and more attention from scholars.Meanwhile,retransmission scheme will inevitably be applied as a solution to achieve high reliability and reduce the collisional probability.Therefore,based on the GF access model,this paper will combine two different HARQ protocols to deduce the system outage probability under short packet transmission.Furthermore,considering the real-time information transmission scenario,this paper will analyze the timeliness of information under GF access scheme based on short packet transmission and provide performance indicator to measure the freshness of information for the equipments that need to collect real-time information.Firstly,this paper proposes the method of evaluating the outage probability of GF-HARQ access scheme and derives the closed solution,which can analyze the short packet transmission reliability in URLLC more accurately.Considering a single-layer cellular network,UE will initiate GF access once short packets arrive,connect the typical BS through contending time-frequency resources and start the HARQ mechanism when the transmission fails.Then,the expressions of one HARQ roundtrip transmission delay and SINR are derived respectively.Meanwhile,the outage probability closed solution is calculated by combining SINR threshold and maximum delay constraint.Finally,the outage probability expression of the HARQ-based access scheme is verified by combining Monte Carlo method and MATLAB simulation platform.The simulation results show that the system mechanism has high reliability under the low SINR threshold.Subsequently,this paper deduces the outage probability of the repetition-based GF under small packet transmission.The access mechanism improves reliability and reduces the extra delay due to retransmission.In the repetition-based transmission,the UE can send the same small packets to the BS at continuous TTIs.After several TTIs,the BS will combine and decode the received packets and feed back to the UE.Accordingly,the transmission delay after multiple retransmissions,SINR and outage probability are analyzed respectively.Then,based on probability theory and binomial distribution,the closed expression of outage probability is derived.Finally,the paper verifies the accuracy of the outage probability expression through simulation and compares its performance with that of the HARQ-based scheme in Chapter 3.The simulation results show that the repetition-based system has better reliability and flexibility.Finally,this paper introduces AoI to measure the timeliness of Information in the uplink GF access system under short packet transmission and proposes system AoI minimizing problem,which is a joint optimization problem of short packet length and access probability.Considering a multi-user node network communication scenario,the state update probability of receiver is analyzed without neglecting the error probability and its expression is obtained when the channel quality is good.After that,the system AoI is obtained by deducing the AoI value of each node.Next,the problem model of the minimization system AoI is established and a multi-variable convex optimization algorithm with short packet length and access probability is proposed.Finally,CVX tool is applied to verify that the joint optimization algorithm has lower system AoI and higher timeliness of information under short packet transmission.