Research On Random Access Congestion Control Base On Spread Spectrum ALOHA In Satellite Network

With the rapid development of information society,terrestrial networks cannot completely meet the increasing demands for wireless communications.As an important supplementary part for terrestrial networks,the satellite communication systems are of great importance for global seamless communications.Satellite communication systems play a major role in disaster situations since their rapid building speed,high stability and security.Because of the scarcity of satellite resources,people often hope to make the best use of satellite resources.Unfortunately,it is possible to cause congestion when a large number of users access to the same satellite.Thus,it is urgent to control the signaling congestion caused by the random access.In Geosynchronous Earth Orbit(GEO)satellite systems,the signal transmission suffers from long delay.In this situation,if congestion control schemes that are designed to avoid conflict after it occurs,a very large delay will be introduced.Besides,since the limit number of the satellite beams,the ability of wide coverage is achieved by beam hopping.In this case,the satellite is unable to acquire the accurate number of users who are requiring access,which brings challenges to the access congestion control problem.The thesis studies the access congestion control of GEO satellite systems based on the spread spectrum ALOHA.Considering characteristics of the GEO beam hopping,the historical access records are utilized to predict the number of users who will request access in the next beam hop.Hence based on the estimated extent of congestion,further control measures are employed to avoid user congestions and improve the access rate.The main contributions of this thesis are as follows:(1)Research on congestion control based on the heat value of cover regions.Considering GEO systems with some users require regular access behaviors every day,the thesis proposes to predict the heat value of each beam cover region by the least square method.Based on the heat value,once the congestion may occur,the access request probability can be dynamically controlled accordingly.Specifically,some users are disabled for requesting access according to a dynamic probability to decrease the average access delay.(2)Multilevel access congestion control based on user priority over spread spectrum ALOHA in satellite systems.The thesis applies the time diversity strategy to decrease the delay of users with higher priority.Users with lower priority directly transmit the access request signaling and then keep waiting for responses.Once retransmission will be triggered after access request failure.For users with higher priority,a proactive retransmission protocol is further introduced.After the direct transmission,requests are repeatedly retransmitted Z(Z>1)times without requiring the response of the direct transmission.The influence of the length of spread spectrum code is further investigated.When long code length is employed,the access number of users with higher priority is increased by this multilevel access congestion control scheme.Further simulations show that the access delay is decreased when the proportion of users with higher priority is low.