Number Of Subscriber Stations Research Articles

A Comparative Study of Power Line Communication Networks With and Without Buffer

Power line communication (PLC) has recently been receiving widespread attention due to its applications in the smart grid. We present a comparative study on the call-level performance of PLC networks with and without buffer through queueing theory. The PLC network model consists of a base station (BS) and a number of subscriber stations that are interconnected with each other and with the BS via power lines. The power line transmission channels are subject to failure during service due to disturbance. A two-dimensional Markov process is used for the modeling. Performance comparison is presented through theoretic analysis and numerical results.

A Performance Study of Downlink Scheduling Algorithms in Wireless Broadband Networks

IEEE 802.16 is also known as WiMAX was developed to produce high performance in Broadband Wireless Access (BWA) systems with a lower deployment cost than wired broadband services. Like other broadband services, IEEE 802.16 is designed to support applications such as Voice over IP (VoIP), video streaming, video conferencing and online gaming. In IEEE 802.16 standard, five types of service classes have been formed to cater the Quality of Service (QoS) needs for different applications. However, the standard does not state any specific scheduling algorithms for either uplink or downlink transmission. Therefore, scheduling algorithm implementations are depending on the vendors, service providers and researchers. In our presented work, an analysis of various available scheduling algorithms in wireless environment has been carried out. Upon the literature study and analysis, Round Robin (RR), Strict Priority (SP), Self-Clock Fair (SC) and Weighted Fair Queuing (WFQ) were tested in downlink scheduling. For each scheduling algorithm, two scenarios were created, 1) 64QAM and 2) the combination of 16QAM & 64QAM. Simulation results indicate that all the schedulers were struggling to perform as the number of Subscriber Station (SS) increases. Furthermore, the impact on having bad quality channel, which is 16QAM, is also one of the reasons that produces poor performance among all the schedulers. Hence, the traditional schedulers are not suitable for the uncertainty condition in wireless environment because they do not satisfy the QoS demand in WiMAX. 

Efficient carrier frequency offset estimation for orthogonal frequency‐division multiple access uplink with an arbitrary number of subscriber stations

An efficient method is proposed to estimate the carrier frequency offsets (CFOs) in the orthogonal frequency-division multiple access (OFDMA) uplink. The conventional alternating projection method is accelerated by utilising the inherited properties of the matrices involved. The multiplication of large sparse projection matrices can be elegantly transformed to a series of products involving small dense matrices, and the inverse operation of these large matrices can be substituted by direct computations. Hence, the computational cost is significantly reduced without compromising the accuracy of the CFO estimation.

Call-Level Performance Analysis of a Power Line Communication Network Under Disturbance

Power line communication (PLC) is a promising technique for information transmission using existing power lines. We analytically model a finite-source PLC network subject to channel noise (disturbance) and evaluate its call-level performance through a queueing theoretic framework. The proposed PLC network model consists of a base station (BS), which is located at a transformer station and connected to the backbone communication networks, and a number of subscriber stations that are interconnected with each other and with the BS via the power line transmission medium. An orthogonal frequency division multiplexing based transmission technique is assumed to be used for providing the transmission channels in a frequency spectrum. The channels are subject to failure during service due to disturbance. We determine the steady-state solution of the proposed model and derive a set of performance metrics of interest. Numerical and simulation results are presented to show the derived metrics with respect to different system parameters. The proposed modeling method can be used for evaluation and design of future PLC networks.

Decentralized Dynamic Spectrum Allocation Based on Adaptive Antenna Array Interference Mitigation Diversity

A new class of decentralized dynamic spectrum allocation (DSA) algorithms that exploit adaptive antenna array interference mitigation (IM) diversity at the receiver, is proposed for interference-limited environments with high level of frequency reuse. The system model consists of base stations (BS) that can optimize uplink frequency allocation to their subscriber stations (SS) to achieve the least impact of IM on the useful signal, assuming no control over band allocation of other BSs sharing the same bands. It is demonstrated that when the number of SSs that share the same frequency approaches the number of antenna elements at a BS, the potential performance gain is most significant for the IM-based DSA compared to the random frequency allocation. A ?good neighbor? decentralized DSA strategy is introduced. The convergence and convergence rate of IM-based DSA are investigated by means of the theory of absorbing Markov chains and statistical simulations. It is shown that the ?good neighbor? strategy leads to much better convergence properties of the entire system compared with the conventional ?selfish? approach. This suggests that the ?good neighbor? IM-based DSA techniques may have a practical perspective even in the scenarios, where global convergence with probability one cannot be established.

Guest Editorial: Special Issue on: WiMAX security and applications

Worldwide Interoperability for Microwave Access (WiMAX) aims to provide high-speed, high-bandwidth, and high-capacity Broadband Wireless Access (BWA) for residential as well as enterprise applications. WiMAX is designed to provide high data rate multimedia services, e.g., voice, data, Internet, email, instant messenger, and games. The security of WiMAX has been under researchers’ scrutiny for the last few years. Both architectures and protocols on WiMAX security have been extensively studied in academics and industries. Several articles, survey reports, and research papers have been published and a number of flaws have been highlighted in the literature. The IEEE Committee has published several revisions of the IEEE 802.16 standard since its initial release in 2001, but many of the vulnerabilities have still been left unattended in the standard. Ensuring the security of these complex and yet resource constraint systems has emerged as one of the most pressing challenges. The aim of this Special Issue is to bring together researchers and practitioners in related areas and to encourage interchange and cooperation under the context of WiMAX security between the research community and the industrial/ consumer community. With regard to such cyber security aspects, there is an increasing demand for measures to guarantee and fully attain the authentication, confidentiality, data integrity, privacy, access control, non-repudiation, and availability of system services. The first paper in this Special Issue by Barghava et al.: discusses security problems, with a focus on collaborative attacks, in the WiMAX scenario. The WiMAX protocol suite, which includes but is not limited to DOCSIS, DES, and AES, consists of a large number of protocols. The authors presented briefly the WiMAX standard and its vulnerabilities. They pinpoint the problems with individual protocols in the WiMAX protocol suite, and discuss collaborative attacks on WiMAX systems. Then several typical WiMAX attack scenarios were presented, including: bringing a large number of attackers to increase their computation power and break WiMAX protocols; assembling a sufficient number of attackers to influence the decision-making of core machines, which includes routing attacks and Sybil attacks, and exploiting implementations that do not conform to the WiMAX specification completely, causing interoperability problems among various protocols, including the ones in typical WiMAX/WiFi/LAN deployment scenarios. Then the authors presented theoretical models and practical solutions to profile, model, and analyze collaborative attacks in WiMAX. Employed attack graphs to vulnerability were analyzed. Experimental results are carried out to verify the models and validate the analysis. Du et al.: study the rekeying issue in IEEE 802.16e WiMAX networks. The existing rekeying scheme— the Multicast and Broadcast Rekeying Algorithm (MBRA) unicasts new keys to each subscriber station (SS). This scheme does not scale well since it incurs large communication overheads when the number of SSs increases. In this work, first a general tree-based rekeying scheme was proposed, which is more efficient than the MBRA. Also formulate an optimization problem to determine the optimal tree structure for given number of SSs. Furthermore, a novel and efficient rekeying scheme for WiMAX networks was presented. The new rekeying scheme utilizes efficient

Cross-layer Based Delay-constraint Adaptive Polling for High Density Subscribers in IEEE 802.16 WiMAX Networks

The IEEE 802.16 standard (namely WiMAX) is proposed to support QoS-aware transmission of real-time service in Wireless Metropolitan Area Networks (WMANs). WiMAX also can operate as a wireless backbone, and then cooperates with WiFi to form a heterogeneous wireless network. Thus, hot spots of WiFi belonging to different WiMAX networks can communicate with each other. For achieving that a Base Station (BS) serves a large number of Subscriber Stations (SSs), WiMAX uses a centralized polling access mechanism instead of a random access mechanism. WiMAX thus avoids occurring access collisions but causes long polling delay under a high-density SSs situation. Consequently, IEEE 802.16 does not schedule real-time service (rtPS) while the BS is in multicast polling mode. This problem of long polling delay becomes worse when the BS serves high-density SSs, because the BS only polls some SSs or groups in every frame time. This paper thus proposes an adaptive polling approach with a cross-layer mechanism, which dynamically polls SSs among different polling modes while continuously supporting the rtPS service. The approach consists of two phases. The first phase adopts two hysteresises of number of SSs and residual bandwidth to determine the optimal polling mode for each connection. The second phase presents a novel QoS-aware rtPS service (QrtPS) that provides an Average Delay requirement instead of the Maximum Delay requirement to take over the rtPS service when a SS is polled by the multicast mode. Additionally, a cross-layer mechanism is proposed to achieve that the real-time applications can be sent with a high-quality or low-quality video codec when the SS is in unicast or multicast mode, respectively. Numerical results demonstrate that the proposed approach significantly outperforms IEEE 802.16 in average delay and network utilization. Furthermore, the collision probabilities of using different numbers of contention slots of an uplink subframe and different numbers of groups within a BS are evaluated for the multicast mode. The results meet the experiments.

Bandwidth Allocation with Half-Duplex Stations in IEEE 802.16 Wireless Networks

IEEE 802.16 is a recent IEEE standard for broadband wireless access networks. In IEEE 802.16 networks, the medium access control (MAC) protocol is centralized and explicitly supports quality of service (QoS). That is to say, access to the medium by a number of subscriber stations (SSs) is centrally controlled by one base station (BS), which is responsible for allocating bandwidth to several MAC connections in order to provide them with the negotiated QoS guarantees. However, although the network can be operated in frequency division duplex (FDD) mode (that is, transmissions from the BS (downlink) and SSs (uplink) occur on separate frequency channels), the standard supports SSs with half-duplex capabilities. This means that they are equipped with a single radio transceiver which can be used either to transmit in the uplink direction or to receive in the downlink direction. This may severely hamper the capacity to support QoS. Therefore, in order to allocate bandwidth, an IEEE 802.16 BS has to solve two related issues: (1) how it can schedule bandwidth grants to SSs in order to meet the QoS requirements of their connections and (2) how it can coordinate the uplink and downlink scheduled grants so as to support half-duplex capabilities. In this paper, we derive sufficient conditions for a set of scheduled grants to be allocated so that the transmission of each half-duplex SS does not overlap with its reception. Based on this, we propose a grant allocation algorithm, namely, the half-duplex allocation (HDA) algorithm, which always produces a feasible grant allocation provided that the sufficient conditions are met. HDA has a computation complexity of 0(n), where n is the number of grants to be allocated. Finally, we show that the definition of HDA allows us to address the two issues mentioned above by following a pipeline approach. This is when scheduling and allocation are implemented by separate and independently running algorithms, which are just loosely coupled with each other. We show via extensive simulations that the performance of SSs with half-duplex capabilities, in terms of the delay of real-time and non-real-time interactive traffic, using HDA almost perfectly matches that of full-duplex SSs, whereas an alternative approach, based on the static partitioning of half-duplex SSs into separate groups, which are allocated alternately, is shown to degrade the performance.

Performance Analysis of the Contention Resolution Scheme in IEEE 802.16

At present, the mandatory contention resolution that shall be supported by IEEE 802.16 is based on truncated binary exponential backoff algorithm. In this paper, the differences of contention mechanism between IEEE 802.16 and IEEE 802.11 are analyzed. The calculations of performance metrics such as the utilization of transmission opportunity u, the delay of bandwidth request d and the drop probability of bandwidth request pd are presented. Based on simulation results, the effects of contention parameters such as the initial window W, the number of subscriber station n and the number of transmission opportunity Nto are discussed. Some policies to adjust the parameters are also provided. These policies are useful for BS to schedule its uplink bandwidth resource.