| Satellite-based Internet of things(S-IoT)can realize global coverage and provide anytime and anywhere multiple access,which has become one of the important development directions of the next generation mobile communication.However,although S-Io T can provide wide area coverage,how to ensure the efficient access of massive Io T user equipment is still an open challenge.Meanwhile,the long distance between satellite and terrestrial Io T user equipment,which is more than hundreds of kilometers,will produce large propagation delay,which makes the age-optimal massive access of S-Io T for massive machine type communication(m MTC)face great challenges.On this basic situation,a timeliness performance indicator named age of information(Ao I),is introduced,and a grant free age-optimal(GFAO)random access protocol is proposed to satisfy age-optimal massive access requirements for the future wide area coverage of S-Io T.By reasonably increasing the number of access time slots,the access failure probability(AFP)of the system is reduced,which reduces the system average Ao I(AAo I)and improves the system throughput.Further,considering the diversified timeliness requirements in the wide area coverage of S-Io T,unequal timeliness(UT)is designed for GFAO random access protocol to realize the UTGFAO random access scheme,by reasonably allocating pilot and time slot resources to different timeliness priority Io T user equipment groups.The specific research contents are as follows:Facing the short packet burst access requirements of m MTC equipment and the long propagation delay characteristic of satellite-terrestrial link,the number of handshakes is reduced through the grant free mechanism,and the GFAO random access protocol is proposed,where the activated Io T user equipment randomly select the pilot to access,and the satellite can accurately identify the pilot collision by utilizing the received signal power and statistical channel information.Then,based on the instantaneous Ao I evolution of a single Io T user equipment,the expression of system AAo I is derived,and the relationship between AAo I and system throughput is also deduced.The simulation results show that under a given system load segment,there is an optimal number of access slots,which can achieve a minimum AAo I and a maximum system throughput.Aiming at the diversified timeliness requirements of services in the wide area coverage of S-Io T,the UT-GFAO random access protocol for m MTC equipment with different timeliness priority is further studied.By reasonably allocating pilot resources,Io T user equipment with different timeliness requirements can send status update packets efficiently in different subframes.UT-GFAO random access protocol includes two schemes: independent unequal timeliness(IUT)access scheme and extended unequal timeliness(EUT)access scheme,where IUT-GFAO can realize ageoptimal access of different timeliness priority Io T user equipment group on their respective access subframes,based on meeting the AAo I requirements.At the same time,based on the conclusion that under a given number of access time slots,there is a system load can achieve the lowest AAo I derived above,EUT-GFAO allows rational use of the remaining pilot and time slot resources of the high timeliness priority Io T user equipment group to provide user uploading for the low timeliness priority Io T user equipment group,to alleviate the pilot collision and reduce the AAo I of the low timeliness priority Io T user equipment group.The instantaneous Ao I evolution of Io T user equipment with different timeliness priority is analyzed,and the AAo I of different timeliness priority Io T user equipment groups for the two UT schemes are derived.Simulation results show that UT-GFAO random access protocol can ensure efficient access of services with diversified timeliness requirement. |
