Random Access Scheme On The High Timeliness Demand Of Space Internet Of Things

Many Io T applications are increasingly dependent on the exchange of time-sensitive information to enable monitoring and control,such as intrusion detection,emergency event detection,telemedicine,autonomous driving,etc.At this point,the timeliness of communication is critical,as outdated information can lose its value and may even lead to system failure and security risks.Age of Information(Ao I)is a recently proposed performance metric that captures the freshness of information from the perspective of the destinat ion.In this paper,we focus on the study of high-timeliness access strategies in space Io T scenarios.Spatial Io T is a three-dimensional spatial expansion of terrestrial Io T under the integrated communication network between sky and earth,and is a ubiquitous network architecture with more comprehensive coverage.Compared with the traditional terrestrial wireless network,the main difference lies in the electromagnetic wave propagation characteristics: signals with line-of-sight,longer communication distance,i.e.,with non-negligible propagation time delay.Currently,most of the research on Ao I is focused on terrestrial networks and ignores the effect of propagation delay,which is an idealized assumption that is no longer applicable in space Io T.This paper focuses on single-hop distributed access at the MAC layer,which can be summarized as multiple source nodes transmitting their state update packets to a central controller over a shared logical channel.One of the typical random access schemes,CSMA,requires each node to spend time listening to the channel state before transmitting packets,thus requiring fewer transmissions for a successful transmission.But the listening time is positively correlated with the propagation distance.Another random access scheme,Aloha,is that nodes send packets directly when they are available without waiting.But it causes more collisions.In this paper,we establish discrete-time mathematical models for the above two distributed access schemes to analyze their timi ng performance in a symmetric network with non-negligible propagation delay,and derive closed expressions for the age of information for both.The Ao I performance differences between the two mechanisms are compared under the same network parameters,and it is found that there exists a time performance equilibrium point.Once the propagation distance exceeds this point,Aloha will be the transmission strategy with better time performance.The optimal access probabilities of the two mechanisms are solved separately under different network parameters.In addition,this paper establishes a generic system model description when the propagation delay is non-negligible,and gives an ideal lower bound for the Ao I under this network parameter setting,and analyzes the impact of each parameter in the network.Based on this,a dynamic access scheme combining local information(e.g.,information age and channel state)is explored,which brings significant performance improvement by using more local information compared to the traditional fixed access probability approach.Finally,this paper gives a distributed access algorithm flow with low complexity and high operability under asymmetric networks.This distributed scheme has even better performance and more flexibility than the centralized policy to adapt to the asymmetric nature of the network.