The Study Of Martingales-Based Grant-Free Access Algorithms

Ultra-reliable and low-latency communications(URLLC)is one of the three core application scenarios of 5G,mainly used to carry new services such as industrial automation,autonomous driving,and telemedicine.URLLC requires to ensure the reliability of at least99.999% within the ambitious latency bound of 1ms.Supporting this kind of service with extremely stringent requirements for delay quality of service(Qo S)and reliability is a huge challenge for the design of machine type communication(MTC)network radio access.As a simple single-phase transmission strategy,grant-free access is belived to be an effective way to implement URLLCs.However,unless a theory-based fine evaluation on dynamic delay,we cannot hope to overcome the natural defects of random access and so effectively utilize the time-frequency resources.In this paper,we propose a novel multi-channel ALOHA-type(M-ALOHA)grant-free random access algorithm for heterogeneous MTC networks with URLLC-type terminals and delay-tolerant massive MTC(m MTC)-type terminals co-existence.The main work and contribution of this paper are concluded as below.(1)In the constructed MTC network with URLLC/m MTC-type termianls co-existence,we propose a multi-channel ALOHA-type grant-free access scheme.We construct the service process model for a terminal with jointly considering the M-ALOHA access scheme,the characteristics of short packet transmissions and the features of frequency-selective fading channels.In our distributed M-ALOHA access mechanism,the distinct delay and reliability Qo S requirements for URLLC-type and m MTC-type terminals are guaranteed by controlling their access probabilities.(2)Considering the sporadic arrivals of MTC traffic and the great advantages of service-martingales theory in random queuing analysis,we present a precise and ingenious delay analysis for each terminal and abtain the martingales-based formulation of delay-bound violation probability.We divide the queue of a terminal's buffer into an empty queue and a non-empty queue logically.For the non-empty queue,relaying on the service-martingales theory,we analyze the queuing process in martingale domain,and calculate the martingale parameters of our formulated service process.For the empty queue,we derive the probability of successful transmission with respect to the specified delay threshold.Finally,the delay-bound violation probability of each terminal is derived through the full probability formula.(3)We formulate our M-ALOHA algorithm as a system throughput maximization problem subject to martingales-based statistical delay-Qo S and the system total bandwidth.The constrained optimization problem(COP)is handled by the bi-objective multi-variable-grey wolf optimizer(BOMV-GWO)algorithm.As a result,we obtain the access probability for each terminal and the optimum value of the key parameters for our system design,such as the number of sub-channels,the bandwith of each sub-channel and the packets transmission rate.Simulation results show that by dividing the total system bandwidth into multiple sub-channels,system performance is improved greatly.Specially,for a given total bandwidth,only multi-channel ALOHA can meet the extremely stringent Qo S requirements of URLLCs.And compared to the single-channel system,the timeslot length for multi-channel system is enlarged,which contributes to both hardware and software.(4)Besides supporting heterogeneous terminals' grant-free access,the proposed M-ALOHA algorithm also provides a theoretical framework for service configuration of our heterogeneous MTC network.It guides the design of a number of important parameters including the number of terminals and their access probabilities for both URLLC-type and m MTC-type terminals.It helps to realize the co-existence of URLLC/m MTC-type terminals reasonably,and so improve the system spectrum resources utilization effectively.