Research On The Access Control Modes Of The GEO Satellite System Based On TDMA

This thesis investigates the techniques and issues associated with Medium Access Control Modes for geostationary satellite systems.The concept of providing a global radio communications network using satellite was first proposed in 1945 but it wasn't until the 1970's that the first successful commercial satellite communication systems were deployed.Satellite were initially used to providing long distance trunk telephony services due to the greater capacity they could offer compared with the terrestrial cables at the time,and have been used to provide broadcast television services due to their unique coverage and inherent broadcast capabilities.Satellite capacity is a limited commodity that must be used aas efficiently as possible to support a large number of subscribers and generate high returns for the service provider.The achievable channel utilisation efficiency and the corresponding quality of service thatsubscribers receive are governed by the underlying multiple access technique and access control modes,the effectiveness of which is dependent on the nature of the traffic generated from the source applications.The ultimate aim is to make more efficient use of capacity combined with improvement in the quality of service provided to the end users.The increasing demand for higher data rates and the movement in satellite use from a limited number of high capacity broadcast channels to numerous individual subscriber connections intensifies the need for efficient capacity utilisation.The propagation delay associated with the satellite channel and ever changing source traffic characteristics makes the development of access control modes for satellites particularly challenging.Inrecent years,there is increasing interest in the research area of broadband satellite network. In broadband satellite network, Access Control Modes is employ- ed to improve the utilization of satellite channel while guaranteeing the quality of the broadband services. Research of access control modes plays more and more important role in broadband satellite network. The highlatency of Geo-stationary Earth Orbit (GEO) satellite channel makes the research focusing on how to decrease the end-to-end delay experienced by packets while guaranteeing the efficiency of the satellite channel.In the GEO satellite communication system based on TDMA,there are three primary strategies for assigning capacity to multiple users: Fixed Assignment,Random Access and Demand Assignment. In the Fixed Assignment ,each user is provided with some static slots,but due to the static nature this strategy is limited as the assignment is not adaptive to changing traffic requirement; At the low channel loads, Random Access can contented the dynamic demand of the satellite capacity, but at the high channel loads,for the cklash of data, it will waste so much channel capacity. The two strategies can not suit the broadcast satellite network. A demand assignment strategy equates to the allocation of capacity in response to user requests. The time delay is a limited in the GEO broadcast satellite system.So in this literary, we introduce Combined Free/Demand Assignment Multiple Access and Burst Targeted Demand Assignment Multiple Access. In TDMA satellite system, these designed to provide significant improvementsin the delay performance of Geo-stationary Earth Orbit (GEO) satellite channelssupporting a finite number of ground stations with bursty data traffic.There are a number of variants of CFDAMA ,differing in their provision and strategy for making DAMA requests. They are CFDAMA-PB,CFDAMA-PA and CFDAMA-RA. But when carrying ON-OFF traffic, the delay performance of CFDAMA decreas. So it introduce the CFDAMA-RR and CFDAMA-PR. The BTDAMA include BTDAMA-PD and BTDAMA-FD, there are the frame formats and request and scheduling strategies. The OPNET software was employed in the computer simulation.The OPNET models of the two modes were presented in the dissertation.The performances between CFDAMA-PR with Round Robin request ,CFDAMA-RR, BTDAMA-PD and BTDAMA-FD chemes were compared by computer simulation.Smulation resulation results demonstrate that compared to CFDAMA-RR scheme ,CFDAMA-PR with Round Robin rquest can offer improved delay/throughput of the satellite channel. A one time,at low channel loads,the CFDAMA reachesthe better delay/throughput performances.Due to it can effedtive use the leaving slots.Under high load conditions,BTDAMA provides very low mean end_to_end delay due to the separation of the request and scheduling funcyions.Lastly,the performance of BTDAMA-UG which is simulated by computer indicates that the high group of terminals gets low end-to-end delay at cost of low priority group of terminals.