A novel timing synchronizing scheme is proposed for use in inter-vehicle communication (IVC) with an autonomous distributed intelligent transport system (ITS). The scheme determines the timing of packet signal transmission in the IVC network and employs the guard interval (GI) timing in the orthogonal frequency divisional multiplexing (OFDM) signal currently used for terrestrial broadcasts in the Japanese digital television system (ISDB-T). This signal is used because it is expected that the automotive market will demand the capability for cars to receive terrestrial digital TV broadcasts in the near future. The use of broadcasts by automobiles presupposes that the on-board receivers are capable of accurately detecting the GI timing data in an extremely low carrier-to-noise ratio (CNR) condition regardless of a severe multipath environment which will introduce broad scatter in signal arrival times. Therefore, we analyzed actual broadcast signals received in a moving vehicle in a field experiment and showed that the GI timing signal is detected with the desired accuracy even in the case of extremely low-CNR environments. Some considerations were also given about how to use these findings.

IEEE 802.11 MAC is the most commonly used protocol in multi-hop ad-hoc networks which need no infrastructure. In multi-hop ad-hoc networks based on IEEE 802.11, since nodes try transmitting their packets at every possible slot, there is a high packet loss probability. This degrades network performance attributes such as throughput and fairness. In this paper, we focus on achieving the maximum throughput of the given network topology by adding extra backoff time to random backoff time based on IEEE 802.11. Specifically, the optimal load rate is obtained with constraints of Carrier Sensing Multiple Access/Collision Avoidance (CSMA/CA) mechanism.

IEEE 802.11 Media Access Control (MAC) is the most widely used protocol in Wireless Ad-hoc Networks (WANETs). However, it cannot guarantee fair channel allocation among the flows contending for the channel. In this paper, we propose a new media access control algorithm to achieve per-flow fairness while maximizing WANETs throughput.

We propose a medium access control (MAC) protocol for real-time applications in one-hop ad-hoc wireless networks. It is a distributed mechanism that takes account of priority and has a bounded packet delay. Nodes use energy signals to contend for the right to access the channel. Nodes, which have a packet to transmit, send energy signals or listen to the channel based on their binary frame. The node that has sent energy signals and has not heard any energy signals wins the right to access the channel. We use two schemes to determine the binary frame: at the beginning of a session, a node determines it based on its priority level and a random number; after successful transmission, based on a count of successful packet transmissions. With the first scheme, in order to reduce contention losses, the nodes that had won the right to access the channel but failed in transmission have priority over the other nodes. With the second scheme, the node that has the largest count, the one that has been waiting the longest, can send a packet without risking collision. The protocol provides higher probability of successful transmission and a limit on maximum packet delay. An analysis of the protocol provides conditions for the protocol to be stable. We evaluate the performance of the proposed protocol using simulations of a network with a mixed population of data and real-time nodes, whose source is constant bit rate (CBR) and a two state Markov on/off process.

Routing in Ad-hoc networks is unreliable due to the mobility of the nodes. Location-based routing protocols, unlike other protocols which rely on flooding, excel in network scalability. Furthermore, new location-based routing protocols, like, e.g. BLR [1] , IGF [2], & CBF [3] have been proposed, with the feature of not requiring beacons in MAC-layer, which improve more in terms of scalability. Such beaconless routing protocols can work efficiently in dense network areas. However, these protocols' algorithms have no ability to avoid from routing into sparse areas. In this article, historical signal strength has been added as a factor into the BLR algorithm, which avoids routing into sparse area, and consequently improves the global routing efficiency.

Multicast is an efficient transport mechanism for group-based community communications and mobile ad-hoc networks (MANET) is recently regarded as a promising solution for supporting ubiquitous computing as an underlying network technology. However, it is challenging to deploy the multicast mechanism used in a wired network directly into MANET owing to scarce resources in wireless networks and unpredictable changes in network topology. Several multicast mechanisms have been proposed in the literature to overcome these limitations. In MANET, especially, overlay multicasting schemes present several advantages over network-based multicasting schemes. However we have observed a common limitation of previously proposed overlay multicasting schemes. They introduce redundant data transmissions that waste network bandwidth and the battery of relay nodes. The observation motivated us to propose an efficient way to create and maintain a "semi-overlay structure" that utilizes a few nonmember nodes selected as branch nodes. The proposed scheme, called "SOMRP (Semi-overlay multicast routing protocol)," has been evaluated by using extensive network simulation in two different scenarios, comparing the performance of SOMRP with two previously proposed schemes. Simulation results show that SOMRP outperforms the two schemes in terms of the packet delivery ratio, transmission cost and end-to-end delay.

We use the network utility maximization (NUM) framework to create an efficient and fair medium access control (MAC) protocol for wireless networks. By adjusting the parameters in the utility objective functions of NUM problems, we control the tradeoff between efficiency and fairness of radio resource allocation through a rigorous and systematic design. In this paper, we propose a scheduling-based MAC protocol. Since it provides an upper-bound on the achievable performance, it establishes the optimality benchmarks for comparison with other algorithms in related work.

Cognitive radio, which utilizes the radio frequency spectrum efficiently by recognizing radio resource availability, is an attractive technology for overcoming the shortage of radio frequency. From the perspective of networking, cognitive radio technologies are also useful since they allow flexible network construction. This paper proposes base station networks using cognitive radio technologies. In order to achieve efficient utilization of the radio frequency spectrum and flexible network construction, we also propose a topology management and route control method for our proposed base station network. Our method shares the status of the wireless links along with topology information and establishes routes by using this information. Through simulation, we evaluate that our method significantly improves the throughput by efficient utilization of the radio frequency spectrum. Moreover, we demonstrate that our method works well when the size of the network gets larger.

Fault-tolerance is an important design issue in building a reliable mobile computing system. This paper considers checkpointing recovery services for a mobile computing system based on the ad-hoc network environment. Since potential problems of this new environment are insufficient power and limited storage capacity, the proposed scheme tries to reduce disk access frequency for saving recovery information, and also the amount of information saved for recovery. A brief simulation study has been performed and the results show that the proposed scheme takes advantage of the existing checkpointing recovery schemes.

The directional MAC protocols improve spatial reuse, but require the exact location of destination and have the problem of deafness. In this paper, we propose a dual-channel MAC protocol with directional antennas for mobile ad-hoc networks. In the proposed MAC protocol, RTS/CTS are sent omnidirectionally as nodes do not have the exact location of the destination in mobile environments. Omnidirectional transmissions on control channel overcome deafness, but have low spatial reuse. We propose a new blocking algorithm to improve spatial reuse on control channel. We use the negative CTS (NCTS) to solve the exposed terminal problem. We confirm throughput of the proposed MAC protocol by simulations using Qualnet ver. 3.8 simulator.

In ad-hoc networks, mobile nodes are limited by a range of radio coverage and have an irregular source of power due to their battery. In ad-hoc networks, there are a lot of situations that all mobile nodes need to agree on their key not at the same time but in part and then merge themselves subsequently. This is because ad-hoc networks have specific features such as mobility and allow various conditions during configuration. In this thesis, we propose MCP (Merging Clusters Protocol), a simple key agreement scheme that can effectively deal with merging different adjacent clusters in mobile ad-hoc networks. When nodes of each cluster have already agreed on their own group keys and intend to merge themselves for further secure communications, MCP can be used in an efficient and secure way. In addition, it can be utilized for efficient group key agreement in a large ad-hoc network. We analyze the security and efficiency of MCP and discuss the experimental results according to practical implementation scenarios.

In this paper, we propose a novel highly reliable packet transmission protocol for ad-hoc wireless networks without deciding the route from a source node to a destination node in advance. The proposed protocol uses distributed surrounding nodes as transmitters of the retransmission packet when the transmitted packet contains an error. In this protocol, when a packet is not correctly received at the destination node, the source node and the surrounding nodes that have correctly received the packet simultaneously retransmit the same data packet to the destination node. The transmitting timing is triggered by the control packet transmitted from the source node. These operations are repeated until the packet reaches to the destination node like automatic repeat request (ARQ). Moreover, the retransmitted packet is encoded by one branch of the space time block code (STBC) for improving the performance with the network diversity effect. We call this method as STBC Distributed ARQ scheme. The performance of the proposed scheme is evaluated by the computer simulations.

The dynamics of Mobile Ad-hoc NETworks (MANETs), as a consequence of mobility of mobile hosts, pose a problem in finding stable multi-hop routes for communication between a source and a destination. Traditional ad-hoc routing protocols are proposed to find multi-hop routes based on shortest path routing algorithms, which cannot effectively adapt to time-varying radio links and network topologies of MANETs. In this paper, a novel stability-based ad hoc routing protocol, which is named as Ad-hoc On-demand Stability Vector (AOSV) routing protocol, is proposed to properly and effectively discover stable routes with high data throughput and long lifetime by considering the radio propagation effect on signal strength. Here, a stochastic mobile-to-mobile radio propagation model is proposed to predict path loss as well as received signal strength between adjacent nodes, and the estimation of link/route stability is derived from the prediction of signal strength. With awareness of link and route stabilities, a path finding algorithm is designed to explore the stable route with largest route stability for a source and destination pair. The performance of AOSV protocol is compared with the well-known Ad-hoc On-demand Distance Vector (AODV) routing protocol and other related works. Simulation results indicate that the AOSV routing protocol leads to significant throughput increases up to 70% improvement comparing to AODV, and provides better performance than related works.

Previous routing algorithms for mobile ad-hoc networks (MANETs) have focused on finding short-distance path(s) between communicating nodes. However, due to the dynamic and unreliable communication nature of MANETs, previously determined paths can easily become disconnected. Although dynamic routing can be used to circumvent this problem, determining a new route each time a packet needs to be sent involves a lot of overhead. An alternative form of dynamic routing involves maintaining valid routes in routing tables, which can be dynamically updated whenever network changes are detected. This paper proposes a new routing algorithm, referred to as pseudo-distance routing (PDR), that supports efficient routing table maintenance and dynamic routing based on such routing tables.

In this paper, we propose simple but effective modifications of 802.11 MAC (media access control) to resolve efficiency and fairness issues, in wireless ad-hoc networks [2]. Our proposal, based on the flow concept, incorporated faster end-to-end forwarding by assigning higher priority to a node that has packets to relay than others. After completion of end-to-end transmission, the node will be assigned a lower priority level to yield to other nodes. Simulation results show that the proposed scheme (F-MAC) significantly enhances the throughput of ad-hoc networks, while keeping fair sharing of bandwidth among mobile nodes.

Bluetooth is reputed as a wireless networking technology capable of forming ad-hoc networks between digital devices. In particular, the Bluetooth scatternet will be an essential part of the fully distributed ad-hoc networks. However, scatternet is not fully described in the Bluetooth specification. This has been the topic of discussion among researches in relation to the formation algorithm, scheduling scheme, etc. Most of the proposed algorithms reported in past researches on scatternet formation are too large and complex to be implemented in a real commercial Bluetooth hardware. Therefore, the verifications of the proposed algorithms reported in past researches were done through only simulations. In addition, the formation process takes too long and these past researches had been conducted only in static environment where no node enters or leaves the scatternet. In this paper, therefore, we propose a new scatternet formation algorithm called Node Ring Scatternet (NRS), emphasizing on two aspects, i.e. implementation and dynamic property of the algorithm. The algorithm is very simple and compact and is verified to be easily implementable in a real commercial Bluetooth device. For the dynamic properties, the NRS entails relatively short formation delay and a reformation algorithm in a dynamic environment was designed. Therefore, the network of the NRS can be scalable and flexible. In addition, a new protocol called SFMP (Scatternet Formation & Management Protocol) was designed and is presented herein. Using this protocol, the NRS algorithm was implemented in a real Bluetooth device, and the performance was verified through hardware experiments. Based on the experimental results, it was found that the NRS composed of up to 20 nodes is formed and the proposed algorithm has shown improvement in terms of formation delay, formation probability and reformation.

This paper discusses effective configuration methods for peer-to-peer (P2P) network topologies within a mobile ad-hoc network. With recent progress in mobile ad-hoc network technology promoting the creation of new and attractive services, we are examining and developing P2P network systems for operation within ad-hoc networks. Our focus is on identifying methods of network-topology control that provide the best balance between performance and availability. We evaluate three methods through computer simulation and field trials from the viewpoints of resource consumption and network integrity, and clarify their domains of applicability. The results are expected to contribute to the design of future P2P networks for operation in mobile ad-hoc networks.

Since mobile ad-hoc networks have certain constraints, including power shortages, an unstable wireless environment, and node mobility, more power-efficient and reliable routing protocols are needed. Accordingly, this paper propose a new routing protocol, PERRA (Power Efficient Reliable Routing protocol for mobile Ad-hoc networks), that includes the advantages of on-demand protocols, while also providing power- efficient and reliable packet transmission. PERRA uses a new cost function to select the optimum path based on considering the minimum residual energy of the nodes on a path, the total energy consumed by a path to transmit and process a packet, and the path's stability in accordance with the node mobility. As a result, the proposed method increases the power efficiency, decreases the route-reconstructions due to residual power shortages and node mobility, and provides effective route maintenance mechanisms. The performance of PERRA is evaluated by simulations under various conditions.

Flooding is usually utilized to find a multi-hop route toward a node which is not within transmission range. However, existing flooding schemes deteriorate the performance of network because of periodic message exchanges, frequent occurrence of collisions, and redundant packet transmission. To resolve the problem, a lightweight and novel flooding scheme is proposed in this paper. The scheme employs ongoing packets for constructing a cluster architecture as the existing on-demand clustering scheme. Unlike to the existing schemes, it makes use of unicast packet transmission to reduce the number of collisions and to find the flooding candidates easily. As a result, the proposed scheme yields fewer flooding nodes than other schemes. Simulation results prove that it causes fewer packet transmissions and fewer collisions than those of two other schemes.

This paper proposes a user revocation scheme for decentralized networks. User revocation is a method to distribute a group decryption key that is shared by n users in a group so that all but d revoked users can obtain the key. In decentralized networks such as ad-hoc networks, mesh networks, and Peer to Peer (P2P) networks, a sender should revoke the access of a dishonest user or an unauthorized user as soon as possible to protect the security of group communication. However, if the sender distributes the group key to all users aside from the revoked user, it would take a long time to revoke a user in a large group. In addition, users must set shared group keys for each user without a privileged center. We propose a scheme in which the amount of transmission and the key storage of each user are small.