Cooperative communication is a technique that can be employed to meet the increased throughput needs of next-generation WiMAX systems. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Although distributed space-time coding (DSTC) is being considered by the IEEE 802.16j/16 m standards for spatial diversity gain, it has several inherent drawbacks. These are addressed in the recently invented randomized distributed space-time coding, called R-DSTC. In this paper, we present the framework for the R-DSTC technique in the emerging relay-assisted WiMAX network, and develop a cooperative medium access control (MAC) layer protocol, called CoopMAX, for R-DSTC deployment in an IEEE 802.16 system. Our scheme couples the MAC layer with the physical (PHY) layer for performance optimization. The PHY layer yields significant diversity gain, while the MAC layer achieves a substantial end-to-end throughput gain. Through extensive simulations, we evaluate the performance of CoopMAX and show that it can generate capacity gains of up to about 77% for an IEEE 802.16 network. View full abstract»

The first generation of fully conformant IEEE 802.16-based networks is being deployed throughout the world. Most of these networks do not support full mobility, due to radio access layer limitations. Newer solutions, based on IEEE 802.16-2005 standard, offer mobility support for subscriber stations. Unfortunately, after quickly changing the point of attachment on the WiMAX layer, very slow and inefficient IPv6 reconfiguration takes place. Delays introduced by DHCPv6 stateless automatic configuration and mobile IPv6 can easily diminish or even render useless all benefits gained using the efficient WiMAX - data link layer. IPv6 automatic configuration process was not designed with fast reconfiguration in mind. As handover speed is a crucial requirement in mobile cellular environments, reasons behind delays introduced by IPv6 layer mechanisms have to be analyzed and appropriate countermeasures applied. Proposals include novel use of DHCPv6 relays for remote configuration, solving DAD delays, limiting Binding Update procedure in Mobile IPv6, configuring routing through DHCPv6 communication and some other. This paper describes all stages of full IPv6 handover in IEEE 802.16 environment, focusing on major reasons of reconfiguration delays. A new metric for assessing impact of every stage on handover efficiency is defined. Several proposed improvements to the IPv6 handover process are evaluated and simulation results are presented. A discussion regarding possible generalization of best improvement proposals and further research areas concludes this paper. View full abstract»

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. View full abstract»

The IEEE 802.16 standard specifies the air interface of broadband wireless access (BWA) systems supporting multimedia services. The applications are varied in their nature and demand different performance levels in order to maintain the appropriate quality of service (QoS). The network should guarantee the pre-defined QoS requirements through an adequate resource allocation and admission control schemes. In this paper we propose a connection admission control method that takes into consideration specific QoS parameters of the ongoing connections and the new requests. The method is based on an optimization approach where the maximum allowed mean delay and the mean data queue lengths corresponding to the connections are used as decision criteria for accepting the new requests for service. The solution of the defined optimization problem also gives the optimal resource allocation. Since solving the optimization problem for a large number of connections can be time consuming, we propose a simple and practical algorithm for obtaining the optimal solution. View full abstract»

In this paper, we analyze the IEEE 802.16 OFDMA system with adaptive allocation of the frequency subchannels to the users. We perform adaptive coding and modulation, allocation of the time-frequency resource, the transmit power, and the frequency subchannels to the downlink and uplink service flows, which are scheduled for transmission in the current frame. Moreover, we consider adaptation of the frame boundary between the downlink and uplink subframes. We minimize the sum time-frequency resource, allocated in both downlink and uplink subframes, under the constraints on the maximum transmit power and the time-frequency resource available in the frame, while satisfying the QoS requirements for all downlink and uplink service flows scheduled for transmission. We propose the cross-layer optimization algorithms, which take into account the MAC and PHY layers processing during the optimization. We also present the results of the proposed algorithms evaluation by system level simulation. View full abstract»

This paper proposes an functional architecture supporting wireless mesh networks (WMNs) for IEEE 802.16 standard. Three essential techniques: collision avoiding, packet scheduling and wireless routing are intensively investigated. Based on the mesh extension of IEEE 802.16 medium access control (MAC) layer protocol and the relay-based characteristic of WMNs, the algorithms concerning with those three techniques are briefly reviewed. The suitable algorithms for collision avoiding and packet scheduling mechanisms are analyzed. Meanwhile, the wireless routing algorithm for the proposal architecture is investigated. Especially, the tree level based centralized scheduling for the multi-radio multi-channel with spatial reuse is proposed. The simulation results show that the proposal is efficiency. The future research work is presented and the research problem is focused finally. View full abstract»

IEEE 802.16 in its mesh mode relies on the Network Entry Process to authenticate and register new node joining the network. This process is fundamental to form the mesh network topology. However, few research activities have addressed the security of this process. In this paper, we show that the network entry process can fail under many attacks (Topological attacks, Selfish behavior and denial of service attacks), and then we give solutions in order to avoid these threats. View full abstract»

The power-saving class of type I (PSC I), one of the sleep mode operations in the IEEE 802.16e standard, is designed to reduce power consumption for non-real-time traffic. However, the inefficiency of PSC I comes from its configuration and operation. Based on the notions of IEEE 802.16m sleep mode operation, a statistical sleep window control (SSWC) approach is proposed to improve energy efficiency for mobile stations in this paper. The SSWC approach exploits a partially observable Markov decision process (POMDP) to conjecture the present traffic state. Based on the properties of POMDP, the optimal policy for sleep window selection is acquired in the SSWC approach. The performance evaluation is conducted and compared via the simulations. Simulation results show that the proposed SSWC approach outperforms the IEEE 802.16e PSC I scheme and the inferred IEEE 802.16m power-saving mechanism. View full abstract»

IEEE 802.16 standard suite defines two main polling modes: unicast polling and contention-based polling. These polling modes are adaptively adopted by different service classes in order to guarantee different performance requirements from different applications, in which the nrtPS (non real-time Polling Service) service class is defined to allow to adopt both the two polling modes. However, the standard does not specify exactly how these polling modes should be used for nrtPS. In this paper, by simulation, we evaluate the performance of the different polling mechanisms. We present a fact that an optimal unicast and contention-based mixed polling mode for nrtPS could allocate the bandwidth more efficiently and achieve better performance. View full abstract»

With the increase of network service types and userspsila broadband demand, broadband wireless access (BWA) has become the focus of peoplepsilas attention. The workgroup of IEEE 802 proposes the IEEE 802.16 standard to provide BWA. In IEEE802.16 mesh mode, all nodes are organized in an ad hoc fashion. Nodes use a pseudo-random function to compete for their transmission opportunities based on the scheduling information in the two-hop neighborhood. Meanwhile, they use ldquothree-way handshakingrdquo to achieve the allocation of data slot. In this paper, we present a novel congestion control method by adjusting the resources of MAC layer in a hop-by-hop way. A node, e.g. Base station (BS), which wants to send data to its neighbor, must detect congestion first. It will adjust the exponent of transmission holdoff time up to congestion state and send MSH-DSCH message with congestion information to its upstream node and downstream node, which also adjust the exponent or allocate minislot according to the information in the mesh message. Moreover, simulation results and analysis are given in this paper. With this method congestion has been controlled, packet loss rate and end-to-end delay have been decreased and throughput has been increased. View full abstract»

IEEE 802.16 WirelessMAN (WiMax) has emerged as broadband wireless access (BWA) solution for fixed and mobile communication requirements in the metro area. While the 802.16 standard throws adequate light on aspects of bandwidth-request and subsequent grants as well as supported service types, it however does not discuss service scheduling. In this paper, we propose a new scheduling algorithm, referred as service criticality (SC) based scheduling scheme. The SC scheme offers fairness, efficiency and low-latency scheduling and facilitates QoS guarantees. SC is based on a dual of buffer occupancies at nodes and allowable latencies for a particular service. We compare SC scheme with round-robin and max-min fair scheduling schemes under different system loads. We observe that while round robin and max-min scheme does not guarantee QoS support at high loads, the SC scheme facilitates efficient scheduling even at heavy loads. Simulation based study is reported that considers a multitude of services using realistic physical and MAC layer models. Delay and blocking results are investigated. The SC concept shows betterment over conventional max-min scheme as well as round robin. View full abstract»

Efficient transmission of variable-bit rate (VBR) video traffic in Broadband Wireless Access (BWA) networks is currently an active research topic. Due to the dynamic changes in bandwidth requirements of VBR video and the limited network bandwidth, dynamic bandwidth allocation (DBA) is required. Traffic prediction is a promising approach to improve the effectiveness of DBA in BWA networks. In this paper, we propose DEEP (Density Estimation basEd Predictor), a novel prediction scheme for MPEG traffic. In DEEP, the density of probability of MPEG frames is estimated through kernel density estimation method. This density is then used in forecasting the future bit rate of I, P, and B video frames. Simulation results show that DEEP is able to predict the bit rate of MPEG traffic more accurately than the conventional Least Mean Squares (LMS) algorithm, and is less sensitive to traffic variation. We provide application guidelines of this predictor in the IEEE 802.16 standard. View full abstract»

Due to the high cost of construction and maintenance of infrastructure, engineers and researchers have focused their investigations on wireless mesh/relay networks with lower cost and high scalability. For current wireless mesh/relay networks, the end-to-end QoS guarantee is an important functionality because the demand for real-time multimedia communications has recently been increasing. Therefore, We propose an adaptive end-to-end delay assurance algorithm with the differentiated services architecture of hybrid mesh/relay networks with IEEE 802.16 and IEEE 802.11e to support a QoS guarantee for the mesh/relay networks. By simulating the mesh/relay networks using OPNET Modeler and analyzing the traffic of applications with different QoS requirements, we show that the proposed algorithm with the differentiated services architecture provides excellent end-to-end QoS support. View full abstract»

In this paper, we propose an accurate analytical model to analyze the performance of initial ranging requests in IEEE 802.16 networks. Two metrics, connection probability and average connection delay, are investigated to evaluate the network performance. Performance observation demonstrate that the connection probability is not heavily influenced by the contention window size and reconnection retry limitation but the average connection delay is sensitive to the above two parameters. Moreover, we find that improving the service capacity and buffer size of base station can optimize the connection probability and average connection delay View full abstract»

Due to the high scarcity and high costs of radio spectrum, more and more radio services are occupying unlicensed bands for their operation. Due to this, there is a high risk of destructive interference which degrades the performance and fails to support Quality of Service (QoS) for systems operating in these bands. IEEE 802.11 based wireless networks are already operating in unlicensed band. A new competitor for unlicensed bands is the IEEE 802.16 based wireless metropolitan area network. Therefore, spectrum sharing between coexisting competing wireless systems like 802.11 and 802.16 is an upcoming challenge. To understand the characteristics of interference in such a heterogeneous scenario, an analysis of possible interference is presented and the performance of the legacy systems is evaluated. Then a spectrum sharing concept is proposed which can generally be applied to both systems. In this paper, the proposed concept is adapted for coexisting 802.16 and 802.11e based systems, which is an extention of 802.11. In this case, the 802.11e Hybrid Coordination Function (HCF) Controlled Channel Access (HCCA) is extended to provide a protocol for airtime sharing. Simulation results are presented showing that the proposed algorithm provides excellent improvement of system performance in the context of capacity and channel utilization compared to the case without applying any spectrum sharing method. View full abstract»

In an IEEE 802.16(d) (WiMAX) mesh network, network bandwidth can be managed by either the centralized scheduling (CS) mode or the distributed scheduling (DS) mode. Compared with the CS mode, the DS mode provides a larger bandwidth capacity and is more scalable. However, because the DS mode uses an on-demand three-way handshake procedure to establish data schedules, the network quality experienced by application programs may fluctuate drastically. To address this problem, in this paper we propose three schemes to improve the data scheduling efficiency of the DS mode of the IEEE 802.16(d) mesh network. Our simulation results show that network performances and qualities experienced by application programs are significantly improved under our schemes. View full abstract»

Multicast and Broadcast Service (MBS) is intended to efficiently use system resources by transmitting same data to multi users simultaneously. MBS is composed of either single BS or multiple BS. In single BS access, each BS transmits MBS data to user which belong to its own BS. In multiple BS access, BSs in the same geographical area composes the MBS zone to transmit the same data to multi users simultaneously. MBS users combine MBS signals which belong to same MBS zone. To enhance the spectral efficiency, link adaption has been employed in wireless communication networks. It is difficult to enable link adaption algorithm in MBS, because channel information reporting for multi users results in a large amount of uplink overhead. Therefore the conventional methods are developed in order to reduce this overhead. However the conventional methods consider single BS access. This paper presents a dynamic Modulation and Coding Scheme (MCS) for MBS zone with limited channel information. The performance of the proposed scheme is analysed in terms of throughput. The effectiveness of the proposed scheme is verified by computer simulation. View full abstract»

This paper is to compare the performance of uplink packet scheduling schemes by the variation of latency value in IEEE 802.16 systems. In order to compare the performance, we chosen 3 types of uplink packet scheduling schemes(UGS[IEEE 802.16 standard], S-l[reference 10], S- 2[reference 12]). From these results, we know the fact that S- 2 scheme increases the system capacity by about 100% more than in the UGS scheme and about 5-20% more than in the S- 1 scheme. Also, S-2 scheme has effect to decrease average delay time, because an average scheduling interval is reduced in case of small ML value. This study is to execute the performance evaluation for the moving subscribers on service. In practical environment, however, both moving and stationary mobile station coexist. Therefore, the practical capacity should be optimized because the maximum throughput is decided by the proportion of moving and stationary device. This result can be used for calculating the system capacity in the base station in case of WMAX and for traffic engineering techniques in order to accommodate many subscribers per the base station. View full abstract»

Vulnerability analysis is indispensably the first step towards securing a network protocol, but currently remains mostly a best effort manual process with no completeness guarantee. Formal methods are proposed for vulnerability analysis and most existing work focus on security properties such as perfect forwarding secrecy and correctness of authentication. However, it remains unclear how to apply these methods to analyze more subtle vulnerabilities such as denial-of-service (DoS) attacks. To address this challenge, in this paper, we propose use of TLA+ to automatically check DoS vulnerability of network protocols with completeness guarantee. In particular, we develop new schemes to avoid state space explosion in property checking and to model attackers' capabilities for finding realistic attacks. As a case study, we successfully identify threats to IEEE 802.16 air interface protocols. View full abstract»

This paper introduces a cross-layer design framework for robust and efficient video multicasting over IEEE 802.16 (also known as WiMAX) networks in metropolitan areas. In the framework, multiple description coding (MDC) on scalable video bitstreams at the source for achieving multiresolution robustness is jointly designed with superposition coding (SCM) on multicast signals at the channel to overcome multiuser channel diversity in wireless multicast. The coded multicast signals under the proposed framework can cope with multiuser channel diversity and mitigate the impact due to short-term channel fluctuations, which are the two most challenging issues in achieving robust and efficient video multicasting in metropolitan areas. We formulate the proposed framework and analyze its video quality performance in terms of the total receivable/ recoverable bitstreams by a receiver. A heuristic methodology is developed for system parameter selection and performance optimization that can be applied to practical scenarios of video multicasting for IPTV services in WiMAX. Simulation is conducted based on actual standard video sequences to verify the proposed methodology on parameter selection and performance optimization. Performance gains of the proposed cross-layer design framework in the presence of fading channel diversity are demonstrated. View full abstract»

This paper presents a study of generalized chirp-like (GCL) sequence for frequency synchronization in IEEE 802.16- 2004 standard application. GCL sequence, a special class of polyphase sequence that it is, has a lower PAPR preamble for synchronization in OEDM system than PN sequence based preamble. The method of synchronization is based on Schmidl and Cox. The result shows that, GCL sequence based method also has a better estimation result of integer frequency offset than PN sequence due to its good cross-correlation property. View full abstract»

Broadband wireless access technology is developed very fast in recent years. The IEEE 802.16 and 802.11a are two broadband wireless access system standards for metropolitan and local areas respectively. The IEEE 802.11a works in the license-exempt (LE) band, and the IEEE 802.16 is designed such that it can operate in the same LE spectrum too. Although the two systems work in different environments, interference can exist under some scenario. In this paper, the coexistence issues of the two types of networks are investigated and a simple and accurate way for bit-error-ratio (BER) calculation is given. The analysis is also verified by Monte Carlo simulations. Furthermore, in our work, we also show how our research production is used to configure the coexisting IEEE 802.16 and 802.11a systems to mitigate the interference. View full abstract»

This paper analyzes a two-hop extension to the coverage of an IEEE 802.16 cell. There is natural degradation in cell capacity due to multihop communications which can be mitigated by spatial reuse, adaptive modulation and coding. We estimate the available capacity by analyzing the random geometry related to the locations of the base station, the sponsor nodes and the mesh subscriber stations situated two hops away from the base station. The results show the trade-offs of extending the coverage area and the decrease of the capacity. View full abstract»

The IEEE 802.16 standard for wireless metropolitan area networks (WMANs) is defined to meet the need for wide-range broadband wireless access at low cost. The objective of this paper is to study how to exploit spectral reuse in resource allocation in an IEEE 802.16 mesh network, which includes routing tree construction (RTC), bandwidth allocation, time-slot assignment, and bandwidth guarantee of real-time flows. The proposed spectral reuse framework covers bandwidth allocation at the application layer, RTC and resource sharing at the medium access control (MAC) layer, and channel reuse at the physical layer. To the best of our knowledge, this is the first paper that formally quantifies spectral reuse in IEEE 802.16 mesh networks and exploits spectral efficiency under an integrated framework. Simulation results show that the proposed schemes significantly improve the throughput of IEEE 802.16 mesh networks. View full abstract»

Multihop Relay (MR) is defined in IEEE 802.16J for coverage extension and capacity enhancement of the legacy IEEE 802.16e point-to-multi-point (PMP) networks. In MR networks, relay stations (RSs) deployment is crucial in achieving good network performance. In this paper, we explore the optimal RS selection problem. By incorporating highway mobility pattern and the Erceg terrain model, the RS selection can be formulated into a non-linear optimization problem, aiming at finding the optimal RS for achieving the maximal end-to-end capacity for subscriber stations (SSs). Numerical results show that the expected end-to- end capacity for individual SS can be increased by 49.86% by using the optimal RS selection approach. View full abstract»