Spread-spectrum communication techniques

Spread-spectrum modulation can be used in a radio system to reduce the likelihood of intercept, as well as providing some protection against jamming and interference. These antijam, anti-interference and low-probability-of-intercept properties are highly desirable in secured communication systems. In addition, spread-spectrum techniques have been proposed to combat spectral congestion by improving the efficiency of spectrum utilisation. Spread-spectrum techniques are also widely used in ranging systems and in local-area networks (LANs) or other multiple-access systems. In the present article, the three main spread-spectrum techniques are outlined and then applications of frequency-hopping systems and some of the techniques used are described     

Fairness in wireless microcellular networks

In the design of cellular and microcellular telecommunication networks, it is common to consider service to subscribers in specific regions by more than a single base station. Although the service overlap provides certain advantages, such as performance improvement achieved in dynamic channel assignment, it results in a large variance in the quality of service across the cells. In this paper, we introduce channel restriction to achieve fairness in the network quality of service. We develop analytical and simulation models for network performance and examine the advantages gained in increased system throughput and reduced probability of handover failure. The numerical results indicate that restricting the number of available channels to users in the overlap region results in considerable improvement in the carried traffic and mean probability of blocking. Finally, we discuss the possibility of substituting or scaling down the guard channels with channel restriction to achieve the desired effect of reduced forced termination     

Mobility patterns in microcellular wireless networks

This study investigates mobility patterns in microcellular wireless networks, based on measurements from the 802.11 based system that blankets the Carnegie Mellon University campus. We characterize the distribution of dwell time, which is the length of time that a mobile device remains in a cell until the next handoff, and sign-on interarrival time, which is the length of time between successive sign-ons from the same mobile device. Many researchers have assumed that these distributions are exponential, but our results based on empirical analysis show that dwell time and sign-on interarrival time can be accurately described using heavy-tailed arithmetic distributions that have infinite mean and variance. We also show that the number of handoffs per sign-on can be modeled accurately with a heavy-tailed distribution.     

Probabilistic channel reservation scheme for mobilepico/microcellular networks

Channel reservation based on the handoff probability of a call arriving in an adjacent cell is proposed as a more flexible and dynamic means of providing handoff priority in mobile pico/microcellular networks in which frequent handoff events are to be expected. A basic probabilistic channel reservation scheme is described and shown to yield improvements in performance, especially in terms of channel utilization by successfully completed call, over the well-known static trunk reservation policy     

Reuse efficiency in urban microcellular networks

We study how efficiently radio channel resources can be reused in microcellular networks. Our focus is on dense urban networks in which cell-site antennas are located well below the rooftops of surrounding buildings. Compared with conventional cellular networks, this kind of microcellular system introduces a markedly different propagation environment, characterized by a mix of strong line-of-sight (LOS) propagation and much weaker non-LOS propagation. We address a number of important issues associated with site layout and reuse planning in such microcellular networks (rectilinear street environments) and present numerical results showing the influence of frequency, cell size, link quality requirements, shadow-fading statistics, and power control on the reuse efficiency. For 900-MHz systems, orthogonal multiple-access schemes (e.g., time division), and a typical set of system parameters, we estimate that the reuse efficiency in urban microcellular networks could be up to 50% higher than that in conventional cellular networks with three-sector antennas     

Spread-spectrum techniques in optical communication using transformdomain processing

A method of applying spread-spectrum techniques to optical communication is presented. The interference-suppression capability of spread-spectrum systems is shown to be enhanced by optical transform domain processing, and a design for spectral coding at optical frequencies is given. The encoding system is based on optical pulse compression and shaping. Several possible implementations of this system are suggested, and applications to fiber optics, laser radar, free-space optical communications, and other systems are discussed     

Resource allocation scheme for QoS provisioning in microcellular networks carrying multimedia traffic

We propose a new resource allocation scheme based on the concept of resource reservation and resource renegotiation. The new scheme is aimed at improving performance with regard to new call blocking rate, handoff dropping rate, forced call termination rate, and average bandwidth use. We compare our scheme with other schemes. The performance is evaluated by using simulation. Copyright © 2001 John Wiley & Sons, Ltd.     

网络视频编码技术

随着视频信息在网络上的应用需求的日益广泛和迫切,适合网络传输的视频编码技术成为了现在研究的一个热点,文章旨在综述网络视频编码技术的发展历程。详细介绍并对比了目前主要的网络视频编码技术:传统的视频编码技术在传输中的应用,视频的可扩展性编码以及视频的多描述编码,最后进一步阐述了未来网络视频编码技术可能的发展方向,分析了综合运用各种编码技术的可能。     

MPLS网络的生存性技术

目前，网络上的应用越来越多，网络就必须能够在故障发生时保证其连续性和服务质量。因此，网络生存性已经成为互联网中的一个重要问题。另一方面，随着光纤技术的发展，链路传输速率已达数10Gb/s。一条链路的损坏将影响大量的应用,而当前基于IP路由协议的恢复机制至少需要几秒到几分钟的时间，这将导致大量分组的丢失，从而造成严重服务质量问题以及网络性能的下降。网络的生存性是流量工程的重要方面。按照ANSI TIA1.2工作组的定义，网络生存性就是在故障发生的情况下，网络仍能维持服务连续性的能力。以前有关网络生存性的研究主要集…     

Distributed dynamic channel assignment with violation to the reuse pattern for microcellular networks

In this paper, the frequency reuse and the distributed dynamic channel assignment for microcellular networks are studied. We show that it is possible to use carriers with violation to the frequency reuse pattern with an insignificant degradation of the quality of service. As a result, a new family of distributed dynamic channel assignment algorithms is presented: the DDCA with violation to the reuse pattern (DDCA with VRP) strategies. The DDCA with VRP schemes allow the use of carriers with at most one violation to the reuse pattern, under the restriction that both cells using simultaneously the same carrier must be the farthest cells of their interference neighborhoods. The results show that the use of carriers with VRP is an effective strategy to increase the system capacity at the expense of an insignificant degradation of the quality of service. This is due to the fact that the carriers are employed with VRP by short time intervals in the least harmful situations. We propose and evaluate five DDCA with VRP schemes and everyone overcomes the performance of the maximum packing (MP) algorithm, with limited carrier usage information and without the need of centralized coordination neither global carrier rearrangements.     

Full- and half-square cell plans in urban CDMA microcellular networks

The Erlang capacity per cell and per unit area in urban code-division multiple-access (CDMA) microcellular environments is evaluated. Special emphasis is given to the effect of breakpoint distance and cell size on the system Erlang capacity. Two different cell plans are considered in which the communication between the base station (BS) and the mobile unit always occurs in a line-of-sight (LOS) condition: the full- and half-square cell plans. Our main result has been to obtain analytical expressions for the Erlang capacity, which are evaluated through a cell radius-normalization procedure, as a function of the breakpoint to the cell radius ratio (R/sub b//R/sub c/). This has allowed us to notice some important facts not previously reported in the literature. In particular, we observe that the Erlang capacity is a monotonically decreasing function of R/sub b//R/sub c/ and that close to the highest reuse efficiency is achieved for R/sub b/<0.7R/sub c/. Thus, given a breakpoint distance, higher Erlang capacity per cell can be achieved with greater cell radius. Or, equivalently, given a cell radius, higher system capacity can be achieved with smaller breakpoint distances; that is, with smaller base-station antenna heights and/or smaller frequencies. Also, we show that the capacity increase due to the BSs doubling or cell radius-reduction depends on the antenna heights, frequency of operation, and distance between streets. It ranges from 64 to 100%.     

Modeling techniques for large-scale PCS networks

There has been rapid growth in the demand for mobile communications that has led to intensive research and development of complex PCS (personal communication services) networks. Capacity planning and performance modeling are necessary to maintain a high quality of service to the PCS subscriber while minimizing costs. Effective and practical performance models for large-scale PCS networks are available. Two new performance models are presented in this article which can be solved using analytical techniques. The first is the so-called portable population model, based on the flow equivalent assumption (the rate of portables into a cell equals the rate of portables out of the cell). The model provides the steady-state portable population distribution in a cell that is independent of the portable residual time distribution, which can be used by simulations to reduce the necessary execution time by reaching the steady state more rapidly. Additionally, this model can be used to study the blocking probability of a low (portable) mobility PCS network and the performance of portable deregistration strategies. The second model is the so-called portable movement model which can be used to study location tracking and handoff algorithms. The model assumes that the arriving calls to a portable form a Poisson process, and portable residual times have a general distribution. This model can be used to study location-tracking algorithms and handoff algorithms. It is shown that under some assumptions, the analytic techniques are consistent with the simulation model     

Parallel simulation techniques for large-scale networks

Simulation has always been an indispensable tool in the design and analysis of telecommunication networks. Due to performance limitations of the majority of simulators, usually network simulations have been done for rather small network models and for short timescales. In contrast, many difficult design problems facing today's network engineers concern the behavior of very large hierarchical multihop networks carrying millions of multiprotocol flows over long timescales. Examples include scalability and stability of routing protocols, packet losses in core routers, of long-lasting transient behavior due to observed self-similarity of traffic patterns. Simulation of such systems would greatly benefit from application of parallel computing technologies, especially now that multiprocessor workstations and servers have become commonly available. However, parallel simulation has not yet been widely embraced by the telecommunications community due to a number of difficulties. Based on our accumulated experience in parallel network simulation projects, we believe that parallel simulation technology has matured to the point that it is ready to be used in industrial practice of network simulation. This article highlights work in parallel simulations of networks and their promise     

Advanced techniques for managing telecommunications networks

The advanced techniques presently used by AT&T to manage traffic flow in its Worldwide Intelligent Network are described. The role of traffic network management strategies is reviewed, and the scope and routing plan of the AT&T Worldwide Intelligent Network are characterized. Three examples of modern protective controls are considered: selective trunk reservation, hard-to-reach process, and selective dynamic overload control. Advanced network management expansive controls and AT&T's traffic network management support systems are examined. Dynamic graphical analysis of network data, interactive training of network managers, and future trends impacting traffic network management are discussed     

Satellite handover techniques for LEO networks

Low earth orbit satellite constellations could play an important role in future mobile communication networks due to their characteristics, such as global coverage and low propagation delays. However, because of the non‐stationarity of the satellites, a call may be subjected to handovers, which can be cell or satellite handovers. Quite many techniques have been proposed in the literature dealing with the cell handover issue. In this paper, a satellite handover procedure is proposed, that investigates and exploits the partial satellite diversity (namely, the existing common coverage area between contiguous satellites) in order to provide an efficient handover strategy, based always on a tradeoff of blocking and forced termination probabilities for a fair treatment of new and handover calls. Three different criteria were examined for the selection of a satellite. Each one of them could be applied either to new or handover calls, therefore we investigated nine different service schemes. A simulation tool was implemented in order to compare the different service schemes and simulation results are presented at the end of the paper. Copyright © 2004 John Wiley & Sons, Ltd.     

Handover signalling for microcellular environments

A new strategy for routing handover signalling in microcellular environments is proposed. Simulation results show that time delay for handover packets travelling across the fixed network could be reduced by diverting the signalling through the air interface between the serving and the candidate base station (BS)