摘要
随着地面无线接力、卫星和地面蜂窝等传输技术的突破,无线通信得到了飞速发展。近二十多年来,无线和有线两种通信系统相结合,构成了几乎覆盖全球的现代通信网,形成了信息事业蓬勃发展的大好形势。但是,目前已有的通信系统仍各有弱点。有线通信只适用于固定用户。蜂窝移动通信需要建设大量的基站和天线,才能有足够大的覆盖范围;同步卫星的覆盖范围虽然很大,但它离地面太远,传输损耗很大,延时长;中低轨卫星必须采用几十颗星组成星座才能持续为一个区域服务。正在发展中的高空平台站(HAPS),其高度介于地面和各种通信卫星之间。它利用良好的电波传输特性,通过平台实现地面用户之间、平台之间或平台与卫星之间的通信连接,具有布局灵活、应用广泛、成本低廉、安全可靠等优点。由高空平台构成的信息系统将是新一代的无线通信系统,它融合了陆地和卫星通信系统的优势,又不同程度地避免了两者的缺点,在通信领域的应用得到了广泛认同,是现有通信方式的有效补充。积极开展高空平台与有关信息系统的研究,有助于我国在通信事业上与国际先进技术保持同步,为通信事业开拓一个新的领域。
高空平台通过使用多波束天线对覆盖区域进行分区覆盖,实现了CDMA和SDMA混合多址接入技术,使得宽带通信网系统容量和频谱利用率的大幅度提高成为可能。天线技术,小区划分,覆盖方案,链路特性,干扰特性等都是高空平台通信系统的关键技术,对于HAPS通信系统的设计和应用具有非常重要的意义。在此背景下,本文重点对高空平台通信的小区划分,覆盖,链路特性及小区干扰等从理论给予推导和数值分析,并在此基础上对高空平台准静止状态的呼叫接入控制(CAC),热点区域通信及系统容量估算等关键技术进行深入和系统的研究。
首先,本文给出了HAPS通信的基本结构,对HAPS通信覆盖和链路特性做了大量的研究。通过给出3种小区划分方法并对其性能和实现等方面给予比较。分析了功率控制对小区链路的影响,给出了有无功率控制下对系统性能的分析。为下一步的研究工作做充分的铺垫。
其次,由于高空平台的准静止状态导致平台接收信号的变化,要实现信号动态变化情况下的呼叫接入控制机制,并且保证各种业务的服务质量,需要采取特定的措施来避免平台准静止状态对接入控制机制的影响。针对现阶段对HAPS研究基本都假设平台静止不动或调整波束指向能够补偿由于高空平台移动造成的剩余指向误差。本文重点研究高空平台通信的准静止状态,并在链路特性中反应准静止状态带来的影响。本文通过建模把平台的不稳定特性通过上行链路的非理想功率控制来体现。为解决平台不稳定等造成接收信号随机变化很大导致接入困难的问题,本文提出了基于平均Eb/N0测量值的呼叫接入控制策略,与基于瞬时Eb/N0测量值相比,准确真实的反映平台接收信号的情况,消除平台准静止状态而导致信号变化对呼叫接入控制的影响。同时通过对每种业务的新呼叫和切换呼叫设置不同的Eb/N0接入门限,该CAC机制很好的支持了多媒体业务。
再次,在高空平台通信系统中,当多个高空平台为同一区域服务或覆盖区域有重叠的时候,位于覆盖区域内的用户可以和多个高空平台进行通信。利用高空平台通信结构这种独有的优势,网络能够极大的增加系统服务区内的移动用户容量,解决由于负载不均衡造成的无线信道资源无法满足用户需求的情况,为热点区域通信问题提供很好的解决方案。本文提出了基于资源代理的资源分配算法,实时监测高空平台通信系统的网络性能,处理资源分配过程中出现的各种情况,解决由于负载不均衡造成的无线资源无法满足用户需求的问题。本文对热点区域移动用户群的通信做了扩展研究,通过对移动用户群采用业务量预测与呼叫量变化预测的方法,可以为移动用户群预留合适的资源,避免了移动用户群切换时所需要大量资源而系统无法提供导致移动群通信中断的问题,具有很好的应用前景。
最后,在当前的无线通信体制中,系统容量主要受到来自带宽与功率这两方面物理条件的制约。HAPS作为无线通信的一个分支,系统容量主要也是受到上面两个物理指标的约束。而且在不同的系统和不同的技术体制下均需要建立相似但又不同的估算模型,涉及到大量的参量,实现复杂,工作量巨大。本文基于高空平台通信的特点,在联合带宽和功率下给出了系统在频分,时分,码分和空分等多种混合多址技术体制下的容量估算,避免了以前单一指标、单一技术体制的系统容量估算。本文给出的容量估算方法简单,系统设计者利用该容量估算方法能够很快地对系统容量进行预先估算,对系统性能作出整体评估。因此,本文给出的系统容量估算方法具有很强的工程实用价值。
With such transmission breakthroughs as the radio relay, communications satellite and terrestrial cellular system, the wireless communications has been rapidly developing. In the last two decades, thanks to the combination of the fixed-line and wireless technologies, a modernized communications network has been almost covered the whole globe and has been enabling the information industry to develop in a flourishing way. Nevertheless, each of the existing communications system has shortcomings. A wire-line system is only applicable to fixed users. A terrestrial cellular mobile communications needs construction of numerous base stations and antennae for achieving sufficient coverage. A geostationary satellite has quite vast coverage, however, it is far from the earth, great transmission attenuation and time delay can be caused. With regard to a medium or low orbit satellite system, dozens of satellites are required to form a constellation so a number of problems may appear. The developing of high altitude platform stations (HAPS) network is intended to be located at a position that is at a point between the different kinds of satellites and the earth. By using good radio wave transmission behavior, this new system can realize communications connections between the platform and its terrestrial users, between two platforms as well as between a platform and a communications satellite, therefore, it possesses advantages of flexible arrangement, vast-range, cost-effective, safe and reliable communicating, etc. An information system formed by HAPS will be a new generation-system for the wireless communications and HAPS communication system combines the advantages of both terrestrial and satellite communication systems and avoids, to different extents, their disadvantages. It has been identified as a promising communication style in the field of telecommunication and an effective complement to the existing communication system. To actively carry out the research and development work for the HAPS and its related information system is helpful to China to keep pace with internationally advanced technologies in the communications field and open up a new field for the telecom business.
It is possible that system capacity is enhanced and utilization of frequency is increased because HAPS covers a wide area by multi-beam antennas and realizes CDMA and SDMA hybrid access technology. Therefore it is no doubt that antenna technology, cell partition technology, covering scheme, link characteristics and interference are the key techlologies for HAPS. It is very important to design and application of HAPS. Therefore such key technologies are researched theoreticly and some important conclusions are given.Based on this background, call access control (CAC) of quasi-stationary platform, hot spot area communications and system capacity estimation are deeply and systematically developed in this dissertation.
Firstly, HAPS communications system structure is depicted in the dissertation and much research work is done on covering and link characteristics. Three cell partition technologies are provided which performace and difficulty of implementation are compared deeply. Also the influence of power control on link characteristics is analyzed and system performance analysises are maken under perfect and imperfect power control. Therefore some important and useful inclusions are drawn about some basic HAPS communication acknowledge and characteristics through first research. This work is sufficient for next research work in the dissertation.
Secondly, access control mechanism is adopted in HAPS networks due to the limited wireless resource. Based on the guarantee of quality of service of users in the service, the function of access control is to judge whether admit a new call or not. The received signals at the high altitude platform are not stable because of the quasi-stationary state of the high altitude platform. By far, all the research work assumed that the high altitude platform is stationary and offset caused by movement of the high altitude platform is compensated. Therefore, traditional power control mechanism can not resolve the problems caused by the movement of high altitude platform and power control error will appear in HAPS networks. Due to the quasi-stationary state of the high altitude platform, The paper emphasis on the resolution of user access difficulties caused by the variety of the received signals. A new CAC mechanism based on average Eb/N0 tested value is proposed for HAPS CDMA in this paper. Comparison to instantaneous Eb/N0 mechanism, it exactly reflects the received signals of the high altitude platform and cancels the influence caused by the power control error. Also this proposed CAC mechanism sets up different access threshold for new user call and handoff user call of different service and multimedia service is satisfied sufficiently.
Thirdly, In HAPS system, when more than one HAP are employed to serve a common coverage area or the coverage areas of HAPS have an overlapping area, the user in this region can communicate with multiple HAPS. Using this unique characteristic of HAPS system, the network can significantly increase the capacity available to mobile users, and deal with the situation that the wireless resource can hardly guarantee the users’requirements of the quality of service on account of non-uniformly distributed traffic, which provides good solution to the communication problems at hot spots. A new resource allocation algorithm based on resource agent (RA) is proposed to resolve the problem caused by load unbalance in the HAPS network. RA monitors the performance of the networks timely and deals with the resource allocation. With respect to the military request of HAPS networks, extension research work is considered to deal with the mobile user group communications in hot spot areas. By predicting of service and call variety, the HAPS networks can reserve appropriate resource for handoff mobile users group. Consequently the HAPS networks avoid group call failure because of resource lack caused by great handoff group request. This method has a bright applicable future. Lastly, system capacity of wireless networks is limited by two physical
parameters, namely transmission bandwidth and transmission power. As a branch of wireless networks, HAPS has the same problem. Before designing a network, capacity must be estimated. Therefore a lot of similar capacity estimation models are established to estimate different networks and different technologies. So the work is very complex and redundant. A new capacity estimation method based on the characteristics of HAPS communications is proposed in this paper which combines transmission bandwidth and transmission power for hybrid multiple access technology. This proposed method is simple and applicable in engineering. A network designer can find the system capacity easily by the proposed method and can evaluate general performance of the system.
引文
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