摘要
卫星信道资源是有限的,卫星通信系统可达到的信道利用率和通信服务质量受潜在的多址技术和接入控制方式的限制。宽带卫星通信是当前通信技术最为重要的一个发展方向。宽带卫星网络的接入控制方式在保障各类业务服务的同时,对提高卫星信道的利用率起着重要作用,对宽带卫星网络接入控制方式的研究是宽带卫星网络技术的重要研究课题。对于宽带静止轨道(GEO)卫星网络,较大的传输时延使得在保障卫星信道利用率的同时,减小业务的端一端时延成为宽带GEO卫星网络接入控制方式研究的重点。
本文主要研究了宽带静止轨道卫星通信系统的接入控制方式的技术和特点。在采用TDMA的GEO卫星系统中,传统的接入控制方案有固定分配,随机分配和按需分配。文中研究了其各自的优缺点,得出这三种传统方式均不适应宽带卫星的信息传输的结论。因此又详细的研究了基于按需分配方式下的混合自由/按需分配(CFDAMA)方式和突发目标按需分配(BTDAMA)方式。尤其是后者其独特的请求与调度相分离的方式,在宽带卫星通信中其时延特性更好。并在此基础上提出了用户分组的BTDAMA-UG方式。
最后利用OPNET软件对CFDAMA方式和BTDAMA方式进行仿真评估,其中对CFDAMA-RR,预测预约CFDAMA-PR,BTDAMA-PD和BTDAMA-FD方式进行了比较,仿真结果说明预测预约的CFDAMA-PR方式与CFDAMA-RR相比较能实现更好的时延/吞吐量性能。同时,在低信道负荷时,CFDAMA方式达到了很好的端-端时延特性;在高信道负荷时,BTDAMA方式由于请求与调度分离,所以实现了较好的端-端时延性能。同时比较了终端数分别为100和200时,系统在CFDAMA-PR方式下的时延/吞吐量特性,结果表明终端数增加时,系统的时延将增加。对本文提出的分组的BTDMAM-UG方式也进行了时延/吞吐量性能的仿真,结果表明不同优先级的不同用户得到了不同的端-端延迟。
This thesis investigates the techniques and issues associated with Medium Access Control Modes for geostationary satellite systems. 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.
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 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. 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.
引文
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