An analytical model on network blocking probability

We present a new analytical model on the blocking probability of the three-stage Clos (1953) network. Due to the effect of approximations, a common problem with previously proposed analytical models is that they may not be very accurate in some cases. In particular, the blocking probability in these models contradicts the well-known deterministic nonblocking condition for the Clos network. The most notable feature of the newly proposed model is that it can more accurately describe the blocking behavior of the network and is consistent with the deterministic nonblocking condition     

Efficient estimation of call blocking probabilities in cellularmobile telephony networks with customer retrials

A novel approximate technique is proposed for the estimation of call blocking probabilities in cellular mobile telephony networks where call blocking triggers customer retrials. The approximate analysis technique is based on Markovian models with state spaces whose cardinalities are proportional to the maximum number of calls that can be simultaneously in progress within cells. The accuracy of the approximate technique is assessed by comparison against results of detailed simulation experiments, results of a previously proposed Markovian analysis approach, and upper and lower bounds to the call blocking probability. Numerical results show that the proposed approximate technique is very accurate, in spite of the remarkably small state spaces of the Markovian models     

An adaptive channel preemption model for small‐cell embedded large‐cellular networks

In this paper, we present an analytical model of adaptive channel preemption (ACP) for small‐cell embedded large‐cellular (SCELC) networks. An SCELC network consists of a fixed base station (FBS) with large coverage and many embedded base stations (EBS) with relatively small coverage. Channel capacity in an FBS cell may become insufficient when traffic is unexpectedly increased particularly in some special occasion. This paper considers two aspects of dynamically allocating channels for an SCELC network. First, by increasing one or more EBS cells within an FBS cell, the proposed ACP can reduce blocking probability of new calls. Second, to reduce dropping probability of handoff calls, the proposed ACP allows a handoff call to preempt an on‐going call, when the latter is located in an EBS cell or in the overlapping area of two adjacent FBS cells. For the purpose of performance evaluation, we build an analytical model with 4‐tuple Markov chains. Numerical results reveal that embedding one or more EBS cells inside an FBS cell needs to be done carefully since it results in a tradeoff between the reduction of new‐call blocking probability and the increase of handoff‐call dropping probability. Copyright © 2011 John Wiley & Sons, Ltd.     

Novel velocity and call duration support for QoS provision in mobile wireless networks

This article presents a novel mobility-based resource reservation and call admission control scheme that is applicable to any real wireless multimedia network. The scheme exploits three key mobility parameters - the position, direction, and speed of a mobile unit - together with the duration of a particular call to accurately estimate the cell visiting probability in order to identify a shadow cluster of cells the unit is most likely to visit. Each cell in the cluster reserves resources for an estimated time interval, which is adapted depending on the aggregated probability of all active units visiting a particular cell. Simulation results confirm the superiority of the new scheme over the existing predictive mobility support scheme in terms of three QoS parameters: call blocking rate, call dropping rate, and channel utilization.     

Modeling iCAR via Multi-Dimensional Markov Chains

iCAR is a new wireless system architecture based on the integration of cellular and modern ad hoc relaying technologies. It addresses the congestion problem due to limited channel access in a cellular system and provides interoperability for heterogeneous networks. The iCAR system can efficiently balance traffic loads and share channel resource between cells by using ad hoc relaying stations (ARS) to relay traffic from one cell to another dynamically. Analyzing the performance of iCAR is nontrivial as the classic Erlang-B formula no longer applies when relaying is used. In this paper, we build multi-dimensional Markov chains to analyze the performance of the iCAR system in terms of the call blocking probability. In particular, we develop an approximate model as well as an accurate model. While it can be time-consuming and tedious to obtain the solutions of the accurate model, the approximate model yields analytical results that are close to the simulation results we obtained previously. Our results show that with a limited number of ARSs, the call blocking probability in a congested cell as well as the overall system can be reduced.     

Design and performance analysis of the algorithms for channelallocation in cellular networks

The authors consider the problem of dynamic channel allocation in cellular networks. Each cell can use any channel, subject to the interference constraints. Channel allocation algorithms are executed by the network switch in a centralized way. The authors show how to design and use objective functions aimed at proper channel allocation and improvement of network performance. As a figure of merit of network performance, they consider the blocking probability in the network as a whole, and the maximum blocking probability in any particular cell of the network (“hot-spot” in the network). They designed three specific channel allocation policies, based on three different objective functions. Compared with two other benchmark policies, the approach shows significant improvement. Performance analysis of various channel allocation policies is virtually impossible without simulations, which are prohibitively time-consuming in the case of small blocking probabilities. The authors propose an original approximate method based on a short simulation and an analytic approximation. The method exhibits good accuracy and significant improvement in efficiency     

单脉冲二次雷达应答分析的抗干扰处理

本文提出了一种新的二次雷达应答分析的抗干扰处理技术,通过准确提取应答框架位置和参考信号特征,利用应答脉冲与参考信号的信号特征一致性进行有效信号判定.采用该方法,能在高密度应答中有效地筛选真实应答,去除幻影和FRUIT干扰.通过理论分析和产品使用证明,即使在高密度询问下和复杂的FRUIT干扰,应答信号存在非常严重的重叠情况时,本文的抗干扰处理技术都具有极高的真实应答的检测概率和极低的虚警概率.     

Performance evaluation of a reservation random access scheme for packetized wireless systems with call control and hand-off loading

We consider a packet switched wireless network where each cell's communication channel is shared among packet voice sources. In this paper, we present a method for the design and analysis of wireless cells using a reservation random access (RRA) scheme for packet access control. This scheme is integrated with a call admission control procedure. We model the state process of a single cell as a vector Markov chain. We compute the steady state distribution of the Markov chain. This result is used to calculate the packet dropping probability and the call blocking probability. By setting limits on maximum permissible levels for the call blocking probability and the packet dropping probability, we obtain the Erlang capacity of a single cell, with and without hand-off traffic. For an illustrative RRA scheme, the Erlang capacity of a single cell is shown to be about twice that attained by a comparable fixed assigned TDMA scheme. We show that a cellular network using this RRA scheme and which applies can be no blocking of hand-off calls, exhibits similar call capacity levels.This work is supported by a University of California MICRO and Pacific-Bell Grant No. 94-107.     

Design of multicast ATM switch

The proposed ATM switch has very high throughput under heavy-traffic conditions. The cell blocking scenario described in the comment [see ibid., vol. 35, no 5, 1999] is of no importance. Directly after the switch is reset, the first four cells will not be placed in the same RAM with the help of the cell-in stage. Under heavy-traffic conditions, the number of cells in each RAM is well balanced and no blocking effect would occur for unicast traffic. That scenario would only arise at the end of transmission when no more cells would be entering the switch. However, the probability of that scenario ocurring is extremely low. Therefore, an internal memory speedup would not be needed and the throughput would not be degraded. The proposed dynamic multicast scheme can fully utilise the available bandwidth     

Performance analysis of channelized cellular systems with dynamic channel allocation

We present an analytical model to compute the blocking probability in channelized cellular systems with dynamic channel allocation. We model the channel occupancy in a cell by a two-dimensional (2D) Markov chain, which can be solved to obtain the blocking probability in each cell. We apply our analytical model to linear highway systems with and without lognormal shadowing and then extend it to 2D cellular systems with lognormal shadowing. We show that, for linear highway systems, distributed dynamic channel-allocation schemes perform similarly to the centralized dynamic channel-allocation schemes in terms of blocking probability. However, for 2D cellular systems, the improvement in the performance is significant and the reduction in the blocking probability in systems with distributed dynamic channel allocation is by almost one order of magnitude, when compared to that in systems with centralized dynamic channel allocation. In practice, our analysis of linear highway systems is applicable to Digital European Cordless Telephony (DECT) and that of 2D cellular systems is applicable to global systems for mobile communications (GSM).     

Integrated cellular and ad hoc relaying systems: iCAR

Integrated cellular and ad hoc relaying systems (iCAR) is a new wireless system architecture based on the integration of cellular and modern ad hoc relaying technologies. It addresses the congestion problem due to unbalanced traffic in a cellular system and provides interoperability for heterogeneous networks. The iCAR system can efficiently balance traffic loads between cells by using ad hoc relaying stations (ARS) to relay traffic from one cell to another dynamically. This not only increases the system's capacity cost effectively, but also reduces the transmission power for mobile hosts and extends system coverage. We compare the performance of the iCAR system with conventional cellular systems in terms of the call blocking/dropping probability, throughput, and signaling overhead via analysis and simulation. Our results show that with a limited number of ARSs and some increase in the signaling overhead (as well as hardware complexity), the call blocking/dropping probability in a congested cell and the overall system can be reduced     

Performance analysis of cross- and cigar-shaped urban microcellsconsidering user mobility characteristics

User mobility characteristics and cell shapes in urban microcellular environments are different from those in macrocellular environments because pedestrians walk along streets and radio signals are guided by tall buildings lining both sides of the streets. Urban area microcell shapes follow approximately the street pattern. In this paper, we characterize low-tier user mobility in typical urban microcells: cross-shaped cells and cigar-shaped cells, and then evaluate and compare the two types of microcells. The results show that the more suitable cell shape with respect to handoff traffic and its signaling load is determined by the probability of straight movement at an intersection. On the other hand, blocking probability in low-tier microcells mainly depends on the cell size, not the shape. This framework and its numerical results can be utilized in determining the cell shape and size of urban microcellular systems     

Mobility and traffic analyses in three-dimensional PCS environments

We need to solve various mobility/traffic problems in one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) micro or picocell environments to efficiently plan future personal communications services (PCS). However, mobility/traffic problems have thus far primarily been studied in 1-D and 2-D cell structures. We extend the previous mobility modeling from 1-D or 2-D space to 3-D unbounded indoor building environments having staircase regions by analytically modeling the mobility to estimate the number of handoffs. We also characterize the blocking probability of each cell according to the mobility by predicting the equivalent input traffic per cell. Based on the blocking probability model, we obtain the required number of channels per cell under the given blocking probability constraint. For example, 13 channels per cell are required to be assigned in order to meet the requirement that the blocking probability does not exceed 0.02 in the case where the number of radio ports (RPs) (cells) is four and 180 users are moving with a mean velocity of 2 km/h (horizontal motion) and 2/3 km/h (vertical motion) on each floor. The computer simulation results are also close to the analytical ones. These results can be utilized in the network planning of future PCS     

Dynamic Channel Allocation for Real-Time Connections in Highway Macrocellular Networks

The wide deployment of multimedia services in third generation wireless networks will require handoff designs that can simultaneously reduce the blocking probability of handoff requests and decrease the handoff delay. Reducing the handoff blocking probability is needed to prevent frequent call dropping of real-time VBR/VCR connections and decreasing the delay associated with handoff is needed to prevent QoS degradation for multimedia traffic. In this paper, we present a channel assignment/reassignment scheme for highway cellular networks that achieves both requirements. The scheme can be used to deliver real-time data to a large segment of global highways, namely, highways in which the radio channels used in a given cell cannot be simultaneously used in the two neighboring cells to its left and to its right. The scheme possesses the desirable features of real-time algorithms: the execution time per handoff request has a constant time complexity, the number of transmitted messages per request is small, and the space overhead is also O(1). The scheme uses a non-compact initial assignment of nominal channels to neighboring cells and utilizes a set of pointers in each base station to implement an efficient channel assignment and reassignment strategy. The resulting approach greatly simplifies the selection process and avoids the expensive computation and message exchanges typically needed by dynamic channel allocation schemes. The low communication overhead of the scheme can be further reduced via control thresholds. Performance simulation results show that the scheme achieves low blocking probability and is therefore suitable for handling handoffs of real-time connections in highway cellular networks.     

QoS-guaranteed transmission mode selection for efficient resource utilization in multi-hop cellular networks

This paper considers the problem of efficient usage of subcarriers in downlink Orthogonal Frequency-Division Multiple Access (OFDMA) multi-hop cellular networks. Multi-hop transmission can either save or waste subcarriers depending on quality of service (QoS) requirements, interference from other cells, and the location of a given mobile station (MS). Preventing unnecessary usage of multi-hop transmission requires the use of a transmission mode selection (TMS) scheme, and we propose two types of TMS in this paper. The first of these is distance-based TMS (TMS-D), which determines the transmission mode based on the MS?s location, and the second is subcarrier-based TMS (TMS-S), which selects whichever transmission mode uses fewer subcarriers. Numerical results on blocking probability demonstrate that TMS improves overall subcarrier usage efficiency, meaning that more MSs can be supported with low blocking probability within a cell. Furthermore, the performance of TMSS, even given its higher complexity, is shown to be superior to that of TMS-D.     

Mobility of queued call requests of a new call-queueing techniquefor cellular systems

A queueing algorithm for new call requests has been proposed to increase the system capacity of traditional fixed channel allocation (FCA) handoff channel reservation schemes. However, mobility of queued call requests was ignored in previous literature. In this paper, a detailed analysis on the effects of the mobility of queued call requests is included in the analytic model. This is important because we found that the mobiles with queued call requests are very likely to leave the cell before granted a free channel. Computer simulation is performed and the results are compared with the numerical ones. It is found that the mobility of the queued new call requests reduces the average queueing delay, but increases blocking probability generally     

Blocking probability and channel assignment in wireless networks

We consider a multi-hop wireless network with a connection-oriented traffic model and multiple transmission channels that can be spatially re-used. In such a network the blocking probability of a call that makes a channel request depends on (a) the channel assignment scheme and (b) the transmission radius of the nodes which affects the network link structure. In this work, we study these two aspects for simple wireless networks. Specifically, we develop blocking probability analysis for a wireless line and grid network and explore the tradeoff between transmission radius and blocking probability for multi-hop calls. We show that for a line network a larger transmission radius can substantially reduce the blocking probability of calls, while for a grid network with a more dense node topology using a smaller transmission radius is better. We then, investigate various channel assignment schemes and present a novel non-rearranging channel assignment algorithm for multi-hop calls in a general network. Our algorithm efficiently incorporates spatial channel re-use and significantly reduces call blocking probability when compared to other algorithms.     

Analytical evaluation of blocking probability in a mobile radiosystem with directed retry

The blocking probability in a cellular network implementing directed retry (DR) is analyzed as a function of the main characteristics of the scenario (the density of base stations), the propagation model (path loss and shadowing), and the speed of mobiles. The analytical approach allows the evaluation of the blocking probability as a function of the maximum number of cells that can be considered at call setup by the DR algorithm. It is shown that the effectiveness of the DR strategy depends on the values of the variance of shadowing and the path loss model. Moreover, the blocking probability is found to be approximately independent from the mobile speed. The analytical results are compared to those obtained by means of simulation: a good agreement has been found, and the analytical model has been shown to be able to reflect the main effects of propagation and mobility on the advantages of DR in a real cellular network     

Cooperative directional inter-cell handover scheme in high altitude platform communications systems

A novel Cooperative Directional inter-cell Handover Scheme (CDHS) for High Altitude Platform (HAP) communications systems is proposed, in which the handover target cell and the two cells adjacent to this handover target cell work cooperatively to exploit the traffic fluctuation to improve handover performance. Users in the overlap area of the overloaded handover target cell will be forced to handover directionally before their optimal handover boundary in order to free up resources for the handover calls which would otherwise be dropped due to the shortage of resources and queue time out. Simulation results show that the handover call dropping probability is greatly reduced (at least 60%) compared with the general queue handover scheme, with little performance reduction to the call blocking probability, and the Not in the Best Cell (NBC) average time is only increased moderately. Moreover, an optimal cell radius can be achieved for a specific platform speed by minimizing the unified system performance, which is the linear combination of the handover call dropping probability and the NBC average time.     

同构流媒体集群系统优化内容部署

该文研究了在固定节目流行度的情况下,如何进行内容优化部署以最小化流媒体集群系统拒绝率和降低复制存储消耗的问题。首先运用排队理论知识分析得出优化目标和服务器访问概率之间的数值联系,并且通过某些数值方法确定出系统最小拒绝率情况下的最优服务器访问概率。由于内容部署属于NP-Hard问题且完全决定每台服务器的访问概率,该文设计了副本交换和对等副本访问概率调整两种启发式策略来进行内容部署,以满足在优化内容分布下每台服务器访问概率和最优值之间的差异最小,从而实现降低系统拒绝率和存储代价的目标。最后分别采用数值分析和离散事件仿真验证了模型的正确性和算法的有效性。