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The IEEE 802.11 medium-access control (MAC) protocol is usually considered to be a default standard in multihop wireless networks. However, in a multihop network with a large interference range, the request-to-send/clear-to-send (RTS/CTS) handshake and virtual carrier sensing mechanism may not be able to eliminate interference or solve hidden- and exposed-terminal problems. This paper proposes two new MAC protocols, i.e., the power-fixed dual (PFD) and power-aware dual (PAD) busy-tone schemes, both of which are able to effectively prevent collision of data/acknowledgment (ACK) packets and are applicable in various open-space environments with different path-loss characteristics. Analytical models are developed to evaluate their performance in terms of the blocking area, saturation throughput, and capability of mitigating aggregate interference of simultaneous transmissions. Numerical examples are presented to show the effectiveness of the proposed MAC protocols and the interaction between performance metrics and key parameters. Analysis and simulation results indicate that both PFD and PAD schemes can achieve a much higher throughput and a lower packet-collision ratio than IEEE 802.11 distributed coordination function (DCF), conservative CTS reply (CCR), and dual busy-tone multiple-access (DBTMA) schemes. Index Terms--Ad hoc network, IEEE 802.11, large interference area, medium-access control (MAC), multihop wireless network, power control.

In mobile ad hoc networks (MANETs), many applications require group-oriented computing among a large number of nodes in an adversarial environment. To deploy these large-scale cooperative applications, secure multicast service must be provided to efficiently and safely exchange data among nodes. The existing literature has extensively studied security protection for a single multicast group, in which all nodes are assumed to have the same security level. However, such an assumption may not be valid in practice because, for many applications, different users can play different roles and thus naturally be classified into multiple security levels. In this paper, we propose a pyramidal security model to safeguard the multisecurity-level information sharing in one cooperation domain. As a prominent feature, a pyramidal security model contains a set of hierarchical security groups and multicast groups. To find an efficient key management solution that covers all the involved multicast groups, we develop the following three schemes for the proposed security model: 1) separated star key graph; 2) separated tree key graph, and 3) integrated tree key graph. Performance comparison demonstrates that the scheme of integrated tree key graph has advantages over its counterparts. Index Terms--Group-oriented computing, key management, mobile ad hoc network (MANET), multicast, pyramidal security model.

In this paper, we propose a group-based location service protocol named GrLS for Mobile Ad hoc NETworks (MANETs). The novelty of GrLS is in its exploitation of group mobility to improve the efficiency of the location service. GrLS uses different location management strategies for single nodes and for groups of nodes. A single node is responsible for recruiting its own location servers and performing location update. On the other hand, in a group of nodes, only the group leader recruits the location servers and updates its location to a specific home region called group home region. Since the location update cost normally dominates the location service cost for all practical purposes, the overhead of the location service protocol is significantly reduced. Furthermore, when the nodes join or leave groups, GrLS can provide seamless location service handoff. To the best of our knowledge, GrLS is the first location service protocol in MANETs that has explored group mobility and developed group location management for mobile nodes. Both theoretical analysis and simulation results show that GrLS can achieve a higher success ratio of location query and better load balance with much lower overhead than the existing protocols without considering group mobility. Index Terms--Group location management, group mobility, location service, Mobile Ad hoc NETworks (MANETs).

All currently available code-division multiple-access (CDMA) technologies used in second-generation and third-generation mobile cellular systems are interference limited and can be appropriately called first-generation CDMA, whereas next-generation CDMA should provide a nearly interference-free performance. This paper addresses the issues on spreading code generation that is suitable for next-generation CDMA systems. The real environment adaptation linearization (REAL) approach is proposed to generate perfectly orthogonal complementary (POC) codes characterized by multiple access interference (MAI)-free and multipath interference (MI)-free operation. The REAL approach takes into account almost all major impairing factors in real applications, such as multipath propagation, asynchronous transmission, random data signs, and burst traffic, such that a CDMA system using them can offer an interference-resist operation. Two important conclusions are drawn in this paper: First, implementation of an interference-free CDMA will not be possible unless using complementary codes, such as the POC codes. Second, to enable interference-free CDMA, the flock size of the signature codes should preferably be equal to the set size of the codes. A fast algorithm to generate supercomplementary codes (a subset of POC codes) is also presented, and their ideal orthogonality is explicitly proven. Index Terms--Multipath effect, multiple access interference (MAI), next-generation code-division multiple access (CDMA), perfectly orthogonal complementary (POC) code.

This paper proposes a cooperation strategy among rational nodes in a wireless cooperative relaying network as an effort to solve two basic problems, i.e., when to cooperate and how to cooperate. First, a symmetric system model comprising two users and an access point (AP) is presented. In this model, each user plays an equal role and acts as a source as well as a potential relay and has the right to decide the amount of bandwidth it should contribute for cooperation. Second, referring to the cooperative game theory, the above problems are formulated as a two-person bargaining problem. Then, a cooperation bandwidth allocation strategy based on the Nash bargaining solution is proposed, in which if a derived condition is satisfied, users will cooperatively work, and each will share a certain fraction of its bandwidth for relaying; otherwise, they will independently work. Simulation results demonstrate that when cooperation takes place, users benefit from the proposed strategy in terms of utility, and those with longer distance to the AP should spend more bandwidth to cooperate with others. Index Terms--Cooperative diversity, cooperative relay, game theory, Nash bargaining solution (NBS).

Worldwide interoperability for microwave access (WiMAX) is a promising technology for last-mile Internet access, particularly in the areas where wired infrastructures are not available. In a WiMAX network, call admission control (CAC) is deployed to effectively control different traffic loads and prevent the network from being overloaded. In this paper, we propose a framework of a 2-D CAC to accommodate various features of WiMAX networks. Specifically, we decompose the 2-D uplink and downlink WiMAX CAC problem into two independent 1-D CAC problems and formulate the 1-D CAC optimization, in which the demands of service providers and subscribers are jointly taken into account. To solve the optimization problem, we develop a utility- and fairness-constrained optimal revenue policy, as well as its corresponding approximation algorithm. Simulation results are presented to demonstrate the effectiveness of the proposed WiMAX CAC approach. Index Terms--Call admission control (CAC), optimization, orthogonal frequency-division multiple access (OFDMA), time-division duplex (TDD), worldwide interoperability for microwave access (WiMAX).

Due to the collaborative nature of sensor nodes, time synchronization is critical for many sensor network operations. For sensor networks deployed in hostile environments, security is critical to the success of time synchronization. Over the past few years, a number of secure time-synchronization schemes have been proposed for sensor networks. However, these schemes are designed for homogeneous sensor networks and may incur large communication/computation overhead or may cause accumulated synchronization errors (due to multihop relays of time reference messages). Several works have shown that better performance and security can be achieved in heterogeneous sensor networks (HSNs). In this paper, we presenta secure and efficient time synchronization scheme for HSNs by utilizing powerful high-end sensors. We implement the time-synchronization scheme in real sensor nodes, and the experiments show that our scheme achieves higher synchronization accuracy than a popular sensor time-synchronization scheme. The analyses demonstrate that our scheme is resilient to various attacks and significantly reduces communication overhead. Index Terms--Heterogeneous sensor networks (HSNs), security, time synchronization.

Wireless-communication technology can be used to improve road safety and to provide Internet access inside vehicles. This paper proposes a cross-layer protocol called coordinated external peer communication (CEPEC) for Internet-access services and peer communications for vehicular networks. We assume that IEEE 802.16 base stations (BS) are installed along highways and that the same air interface is equipped in vehicles. Certain vehicles locating outside of the limited coverage of their nearest BSs can still get access to the Internet via a multihop route to their BSs. For Internet-access services, the objective of CEPEC is to increase the end-to-end throughput while providing a fairness guarantee in bandwidth usage among road segments. To achieve this goal, the road is logically partitioned into segments of equal length. A relaying head is selected in each segment that performs both local-packet collecting and aggregated packets relaying. The simulation results have shown that the proposed CEPEC protocol provides higher throughput with guaranteed fairness in multihop data delivery in vehicular networks when compared with the purely IEEE 802.16-based protocol. Index Terms--External peer communication (EPC), IEEE 802.16, multihop, scheduling algorithm, vehicle-to-roadside communication (VRC), vehicular networks.

This paper presents a multicarrier code-division multiple-access (CDMA) system architecture that is based on complete complementary orthogonal spreading codes. The architecture has several advantages as compared to conventional CDMA systems. Specifically, it offers multiple-access interference-free operation in additive white Gaussian noise channels, reduces cochannel interference significantly, and has the potential of higher capacity and spectral efficiency than conventional CDMA systems. This is accomplished by using an 'offset stacked' spreading modulation technique. To maintain good performance in the presence of fading, the offset stacked modulator is followed by a quadrature-amplitude modulation map, which is designed to optimize performance in a fading environment. This new modulation scheme also simplifies the rate-matching algorithms that are relevant for multimedia services and Internet Protocol-based applications. Index Terms--Complete complementary orthogonal (CCO) codes, fading channels, multicarrier code-division multiple access (MC-CDMA), quadrature-amplitude modulation (QAM).

In mobile ad hoc wireless LANs, it is very difficult to maintain a targeted network utilization due to the time-varying nature of the contention-based medium access control protocol and the lack of a central control. Furthermore, previous research has been mainly focusing on the aspect of optimizing the performance at each station. But doing so may result in a very low overall network utilization. Therefore, self-regulating network utilization is very important to provide quality-of-service (QoS) in mobile ad hoc wireless networks. Through self-disciplining its own behaviors locally, each station will optimize its protocol parameters to meet the targeted overall network utilization, which is very important for QoS provisioning to multimedia services. This paper proposes and evaluates a fully distributed scheme for each station to self-regulate its behaviors through adapting the local protocol parameters to meet the targeted overall network utilization with the changes in the network environment such as the number of stations and channel quality. Index Terms--Mobile wireless ad hoc network, network utilization, self-regulation.

A complete complementary code based direct sequence code division multiple access (CC/DS-CDMA) system has been proposed recently as a potential candidate for beyond third generation (B3G) wireless communications. This paper addresses the issues that design of efficient code assignment schemes should be based on a flexible physical layer support, which is extremely important for emerging cross-layer designs in future wireless applications. The study in this paper considers a CC/DS-CDMA system with multiple time slots, three traffic classes and two dynamic code-flock assignment schemes, namely random assignment (RA) and compact assignment (CA). Simulation results show that the CC/DS-CDMA system has variable capacity property (VCP), which is sensitively affected by different code-flock assignment schemes. In general, CA can offer lower blocking probability, whereas RA can offer a larger mean system capacity and higher throughput when offered traffic is heavy. Index Terms--Code division multiple access (CDMA), complete complementary codes, resource allocation.

This paper proposes an analytical method to study multipacket capture effects in a code-division multiple-access (CDMA) communication system considering both signal-to-interference ratio (SIR) levels and packet arrival delay offsets. Single-and double-packet capture probabilities are derived explicitly based on CDMA cochannel interference threshold, which can be generalized to study n-packet capture effect in a CDMA packet data network. Performance of a common coded spread-spectrum (SS) packet network is taken as an example to exemplify the validity of derived analytical model for studying double-packet power-delay capture. It is concluded from the analytical results of this paper that the n-packet-capture capability can significantly enhance performance of an SS packet data network, which is usually underestimated in the literature otherwise. Index Terms--Capture-effect, code division multiple access (CDMA), spread spectrum (SS), wireless packet network.

In this paper, joint power-delay double-capture probabilities of a spread-spectrum packet network are derived. What we mean in terms of 'double-capture' is that a receiver can simultaneously capture the first two incoming packets (from different transmitters) encoded by the same spreading code. The power capture model concerned takes into account multiuser interference, multipath Rayleigh fading, shadowing, and propagation path loss. To derive delay capture probability with an arbitrary arrival delay, a discrete-time approximation is applied. The method of deriving double-capture probability can also be extended to calculating multiple-packet (more than two) capture probabilities. Performance of a common-code slotted ALOHA system is evaluated with the double-capture effect and the results are compared with those of a traditional ALOHA system. It is shown that the channel impairing factors can be greatly mitigated by exploiting joint power-delay double-capture effect. Index Terms--Code division multiple access (CDMA), joint delay-power capture effect, wireless packet network.

This paper introduces asynchronous orthogonal decision-feedback detector (AODFD) for asynchronous CDMA multiuser detection. The AODFD based on entire message-length detection is studied first. A realizable scheme, sliding-window AODFD, is then proposed and its performance is analyzed. In spite of its simple structure, the sliding-window AODFD can perform as desirably as ADDFD, which has a much higher complexity. The reduced complexity of the sliding-window AODFD is due to the use of orthogonal matched-filtering and a short window size. Different from ADDFD that requires computational intensive z-transformed matrix inversion and spectral factorization, AODFD uses agile Gram-Schmidt procedure. It is possible for AODFD to adopt a simple updating algorithm and parameter updating is no longer always necessary when users leave the system. The comparisons are also made with other orthogonal-based detectors and the bit error probability results show that the AODFD is an attractive multiuser detector. Index Terms--CDMA, multiuser detection, decision feedback.

This paper introduces asynchronous orthogonal decision-feedback detector (AODFD) for asynchronous code-division multiple-access multiuser detection. The AODFD based on entire message-length detection is studied first. A realizable scheme, sliding-window AODFD, is then proposed and its performance is analyzed. In spite of its simple structure, the sliding- window AODFD can perform as desirably as ADDFD, which has a much higher complexity. The reduced complexity of the sliding-window AODFD is due to the use of orthogonal matched-filtering and a short window size. Different from ADDFD that requires computational intensive z-transformed matrix inversion and spectral factorization, AODFD uses agile Gram--Schmidt procedure. It is possible for AODFD to adopt a simple updating algorithm and parameter updating is no longer always necessary when users leave the system. The comparisons are also made with other orthogonal-based detectors and the bi-error probability results show that the AODFD is an attractive multiuser detector.

This paper proposes an asynchronous code-division multiple-access multiuser detector, zero-insertion adaptive minimum mean-square error (ZA-MMSE) multiuser detector, in which a data stream is partitioned into blocks by inserted zero bits and detection proceeds block by block to ensure a balance between processing delay and detection efficiency. The bit error rate performance of the ZA-MMSE detector is evaluated under different multipath scenarios with varying severity characterized by average relative strength of the paths. The results are compared with decorrelator, revealing that the ZA-MMSE detector offers promising detection efficiency at a relatively low complexity, which is linear to the product of the number of users and block length. As an-effort to implement it adaptively, the transient behavior of the ZA-MMSE detector with different recursive algorithms is studied. It is concluded that the least mean square algorithm is not suitable due to its power-dependent convergence; whereas the recursive least square algorithm offers a consistently fast convergence regardless of the received power, making it ideal for application in a time-varying channel with near--far effect. Index Terms--Code-division multiple access (CDMA), minimum mean-square error (MMSE), multiuser detection, spread spectrum.