摘要
无线自组网具有无需基础设施,组网快速灵活、鲁棒性和抗毁坏性能强等应用优势;超宽带技术具有高带宽、高速率、低功耗和低成本等优点,超宽带与无线自组网结合可形成更加灵活、高速的无线网络,从而极大地推动无线网络的应用和发展。
但无线自组网和超宽带技术都是未成熟和非民用化的技术。多跳信道共享方式、拓扑结构动态变化等网络特征给无线自组网MAC接入控制和路由设计带来了严重的挑战;超宽带通信系统虽然具有内在的抗多径能力,但当通信速率很高时,多径引起的符号间干扰将变得严重;此外,虽然超宽带技术具有较低的功率谱密度,但它捕获时间也相对较长,为重新建立物理层之间的连接,信令必须具有较长的报头和较高的发送功率,所以应尽量避免冲突重传和成本更高的路由重构。物理层抗干扰性能研究、链路层MAC接入控制和网络层路由协议设计是目前超宽带无线自组网研究的热点,也是论文的主要研究对象。论文课题来源为国防某部“十一五”重点攻关项目《XXXX关键技术研究》。
论文的主要贡献及相关研究工作可分为以下三部分:
(1)给出了较全面而简洁的DS-UWB通信系统误码率近似计算公式及在此基础上较为定量化的DS-UWB通信系统抗多径干扰和系统间干扰性能研究结论。
直接序列超宽带DS-UWB系统是目前UWB通信系统的典型物理层实现方案。论文利用等效的离散信道模型,给出了IEEE 802.25.3a所建议的几种UWB室内信道模型的仿真数据,并对意大利学者G Durisi所给出的DS-UWB通信系统误码率的计算公式进行了修正和完善。在此基础上,用仿真分析方法,研究分析了DS-UWB通信系统抗多径干扰和系统间干扰性能,得出如下结论,即当通信速率高于100Mbps以上时,DS-UWB通信系统多径引起的符号间干扰将变得严重,此时提高信噪比对系统误码率影响不大,而采用5个左右叉指数的Rake接收机时,可获得较满意的系统性能;两个异步DS-UWB通信系统同时工作时,系统间产生的干扰主要取决于两个系统的码片速率,码片速率高的系统对另一系统的干扰更严重,可以采用更长的扩频序列来抑制。
(2)提出了进一步改善高性能自组网广播MAC接入协议FPRP竞争性能的有效方法及基于FPRP协议的单播MAC协议TPRP。
FPRP协议为当前具有代表性的基于动态时分复用的无线自组网广播MAC协议,具有高度可靠性、时隙并发性、操作并行性及可扩展性等良好性能特点。论文对FPRP协议进行了进一步的性能改善:非预约节点的状态转换设计解决了隐藏接收终端问题、竞争周期分散机制的采用降低了因竞争周期集中而带来的竞争冲突、考虑优先级的竞争概率和竞争周期安排算法设计,提供了不同等级的接入服务。为提供最基本的单播业务,论文根据广播和单播的不同特点,对广播FPRP协议信道预留步骤进行了合理的精简,设计了具有同样高运行性能的单播MAC协议TPRP。TPRP协议能有效解决暴露发送站问题,结合优先级设计,还可作为具有一定QoS保障的MAC协议。
(3)提出了一种高可靠、优化的无线自组网QoS路由协议RQRP PE。
RQRP_PE协议以带宽为路由首要约束因素,使所选路由满足最重要的QoS资源需求;基于路径生命期进行路由选择保证了所选路由的高可靠性;基于优先级的带宽预留机制则为所选路由提供了具体实施保证。路径带宽计算和生命期状态预测是RQRP_PE协议的设计基础,且带宽计算是一个NP完全问题。论文采用启发式逐跳方式给出了合理的带宽计算方法,而基于距离的生命期预测方法则使路由协议具有了UWB技术特色;在路由协议的具体设计中,论文采用目的节点预约、目的节点缓存备用路由及目的节点参与路由维护等机制,从多个方面提高了路由发现和维护的效率,同时也降低了路由控制消耗。仿真结果表明RQRP_PE协议有着显著于DSR协议的良好运行性能:能有效提高分组投递率,减少因链路断裂而引起的路由重构,降低分组传输延迟。
Wireless Ad Hoc networks are characterized by flexible and fast development, no acquirement for fixed infrastructure, very low cost and robustness, etc. The main advantages of UWB wireless communications are described in such words as high band width, high bit rate, low power consumption and so on. Combining wireless Ad Hoc networks with UWB transmission techniques could get a more flexible and higher speed wireless network to enormously promote the application and development of wireless networks.
However, Wireless Ad Hoc networks and UWB techniques are all immature and no civilian. Considering the characteristics of wireless Ad Hoc networks, such as its multihop, the dynamic network topological structure, the restricted energy, it is a challenging task to design the MAC protocol and routing protocol. Although UWB communication systems have the intrinsic anti-multipath interference ability, but as the system transmission speed is very high, the inter symbol interference (ISI) caused by multipath will get serious. In addition, although UWB techniques have the lower power spectrum density, but the capturing time is also relatively longer, this means, longer packet header and higher transmission power are needed to reestablish the connection between the physical levels. Therefore it gets very necessary to avoid conflict and re-transmission of packets, as well as the higher cost routing reconstruction. The researchs of anti-interference at the physical level, MAC protocol design at the link level and the routing protocol design at the network level are the present research hotspots, are also the paper's main research objects.
The paper's main contributions and correlative research works can be divided into the following three parts:
(1) Giving the formula of the bit error probability (BEP) taking into account of more practical factors and the more quantitative research conclusions of the tolerance of multipath interference and intra-system interference in DS-UWB system.
The direct sequence ultra-wide band DS-UWB system is the present typical physical level realization technology of UWB communication systems. In terms of discrete time Impulse response, namely discrete channel models, several kind of UWB channel model simulation data of the indoor channel models suggested by IEEE 802.15.3a were proposed in the paper, and the formula of BEP given by G. Durisi was corrected. On this basis, the tolerance of multipath interference and intra-system interference were explored. The following conclusions were presented: When the data rate is higher than 100Mbps, the system performance obviously degrades due to ISI. In this case, it is inapparent to reduce ISI by improving Signal-to-Noise ratio. Rake receiver with five fingers can be considered; when two asynchronous DS-UWB communication systems work together, the high-rate system generally does more interference to the low-rate system. Such intra-system interference can be restrained by increasing the length of pseudo noise (PN).
(2) Proposing valid methods to further improve the competition performance of the broadcast MAC protocol FPRP for the wireless Ad Hoc networks and a unicast MAC protocol TPRP based on FPRP.
The FPRP (Five-Phase Reservation Protocol) protocol is a representative dynamic TDMA protocol for the wireless Ad Hoc networks, which is possessed of such distinct performance as high reliability, operation parallelism, packet concurrency and extensibility. Farther improvement measures of the FPRP performance were proposed in the paper: The "hidden receiver terminal" problem was solved by the state conversion design for non-reserved node; The competition conflict caused by the centralization of competition period schedules was reduced by decentralizing the competition period. The distinguishing access service was provided by the algorithm design of competition probability and competition period arrangement in which the order of priority was considered. For providing the basic unicast service, according to the differences between the broadcast and unicast, the FPRP reservation procedure was revised, and a unicast FPRP-based protocol termed TPRP(Three-Phase Reservation Protocol) was proposed. The TPRP protocol has high performance likewise. Combining with the order of priority mechanism, TPRP may be used as a QoS MAC protocol.
(3) Presenting a kind of high reliable and optimized QoS routing protocol RQRP_PE for wireless Ad Hoc networks.
The RQRP_PE (A Reliable QoS Routing Protocol Based on Path-lifetime Estimation) protocol takes the path-bandwidth as the principal selection restraint, which has the selected route to meet the most important QoS resource request, depends on the path-lifetime to select route, which enhances the reliability of the selected route, and makes use of bandwidth reservation mechanism based on the order of priority to provide implement guarantee. The calculation of path-bandwidth and the prediction of path-lifetime are the foundation for the design of RQRP_PE protocol. Moreover, the calculation of path-bandwidth is a NP-complete problem. The reasonable calculation methods based on heuristics hop-hop were given in the paper. The predication method of the path-lifetime according to the distance between nodes is more suitable for UWB techniques. In the concrete design of the protocol, it improves the efficiency of the route discovering and maintenance and reduces the routing consummation that the destination node reserves route, memory the standby route and participate the maintenance of route. The simulation results indicated, comparing with the classical DSR protocol, that the RQRPPE protocol can effectively increase packets delivery rate, reduce the probability of reconstruction caused by broken-path, and decrease the average transmission delay.
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