Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. Due to high speed of nodes and their frequent disconnections, it is difficult to design a MAC scheme in VANETs that satisfies the quality-of-service requirements in all networking scenarios. In this thesis, we provide a comprehensive evaluation of the mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay, as well as the impact of mobility factors. The study also presents a relation between the mobility factors and the respective medium access behavior. Moreover, a new unfairness problem according to node relative speed is identified for both broadcast and unicast scenarios. To achieve better performance, we propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. Extensive simulation results show the significant impact of mobility on the IEEE 802.11p MAC performance, an identification of a new unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.

Driven by both the need for ubiquitous wireless services and the stringent strain on radio spectrum faced in today's wireless communications, cognitive radio (CR) have been investigated as a promising solution to deploy Wireless Regional Area Networks (WRANs) for an efficient spectrum utilization. Communication devices with CR capabilities are able to access spectrum bands licensed for other wireless services in an opportunistic and secondary fashion, while preventing harmful interference to incumbent licensed services. However, a lesson learned from early experiences in developing such macro-cellular networks is that it becomes increasingly less economically viable to develop CR macrocellular infrastructures for increasing data rates in both line-of-sight as well as non-line-of-sight situation of WRAN, and the corresponding quality of service (QoS) in macrocellular networks is also noticeably degraded due to path loss, shadowing, and multipath fading due to wall penetration. Moreover, there are several challenges to make the real-world CR enabling dynamic spectrum access a difficult problem to implement without harmful interference. First, the hardware design of cognitive radio on the physical layer involves the tuning over a broad range of spectrum to detect a weak signal in a dynamic environment of fading channels, which in turn makes identification of the spectrum opportunities hard to achieve in an efficient and accurate manner. Second, opportunistic media access based on imperfect spectrum usage information obtain from physical layer brings up undesirable interference issue, as well as reliability issues introduced by mutual interference. Third, the curial issue is to determine which channels to use for data transmissions in presence of the dynamic and opportunistic nature of wireless environments, in the case where pre-defined dedicated control channel is not available in the complex and heterogenous networks. In this dissertation, a novel framework called Cognitive-Empowered Femtocell (CEF), which combines CR techniques with femtocell networking, is introduced to tackle these challenges and achieve better spectrum reuse, lower interference, easy integration, wider network coverage, as well as fast and cost effective early stage WRAN. In this framework, a sensing coordination scheme is proposed to gracefully unshackles the master/slave relationship between central controllers and end users, while maintaining order and coordination such that better sensing precision and efficiency can be achieved. As such, the network intelligence can be expanded from controlling the intelligence paradigm to better understand the satisfy wireless user needs. We also discuss design and deployment aspects such as sensing with reasoning approach, gossip-enabled stochastic media access without a dedicated control channel, all of which are important to the success of the CEF framework. We illustrate that such a framework allows wireless users to intelligently capture spectrum opportunities while mitigating interference to other users, as well as improving the network capacity. Performance analysis and simulations were conducted based on these techniques to provide insight on the future direction of interference suppression for dynamic spectrum access.

The Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and binary slotted exponential backoff scheme is the basis of the IEEE 802.11 Medium Access Control (MAC) protocol. However, the DCF is not suitable for real-time traffic control since the backoff scheme may cause huge frame delay and jitter. We propose a modified DCF which uses a forward backoff scheme to remedy this disadvantage. In addition, a call admission control (CAC) is also proposed. Our protocol can guarantee service qualities such as the network throughput, frame delay, and jitter for real-time traffics. Besides, the modified DCF is still compliant with the IEEE 802.11 standard. Simulation results have shown that our method performs better than other DCF disciplines.

Typically, MAC protocols for mobile ad hoc networks assume omnidirectional antennas, and use of directional antennas offers many advantages, such as range extension, reduced co-channel interference, increased the degree of spatial reuse, improved the throughput of networks, and reduced the transmission power. There are many wireless ad hoc MAC protocol have been proposed. However, these protocols do not provide significant improvement of network performance because they can¡¦t let nodes to support multiple simultaneous transmissions or receptions ability. In this paper, we proposed a newly designed MAC protocols, we used adaptive beam-forming system to reduce the co-channel interference problem, and our proposed MAC protocol will enable nodes with multiple simultaneous transmissions and receptions ability. The handshake mechanism of proposed MAC protocol is that used receiver initialize handshake mechanism such as MACA/BI. We changed the Ready-to-Receive (RTR) control packet, which was used to poll neighbor nodes to be a dual-used control packet, Ready-to-Receive-Transmission (RTRT). Our simulation results will show that our proposed MAC protocol do exploit the advantage of space division multiple access that significantly reduced the problem of poor networks throughput which caused by bottleneck nodes in wireless ad hoc networks.

IEEE 802.11 has become the standard of medium access control (MAC) in wireless ad hoc networks. However, due to the embedded binary exponential backoff algorithm, the packet delay and jitter incurred by access collisions and frame retransmissions may grow drastically. The lack of time-constraint considerations in IEEE 802.11 makes it very difficult to provide QoS (Quality of Service) guarantees for multimedia services. Therefore, a lot of research works focusing on priority-based MAC protocols for wireless ad hoc networks have been proposed. Yet, no standards come out until now. This thesis presents a priority-based MAC scheme in wireless ad hoc networks, which not only provides differentiated services, but also improves the QoS limitations of the previously proposed schemes. The main idea of the proposed JMAC (jamming-based MAC) mechanism is that traffic flows with different priorities can be differentiated by transmitting jamming noises of different lengths to interfere with one another. The one with the longest length of jamming noise can start data transmission. Besides, in our design, priority can be dynamically adjusted to allow each MH to change its contention priority and the length of jamming noise in accordance with network congestions. To implement the proposed JMAC, three modules are developed in this thesis: Collision Avoidance, Starvation Prevention, and Deadlock Prevention. For the purpose of evaluation, we perform simulations on the well-known network simulator, NS-2. Our scheme is compared with the EDCF (enhanced distributed coordination function) of IEEE 802.11e¡]draft¡^and one of the existing works. The simulation results demonstrate the effectiveness and superiority of our scheme.

The amount of packets on-the-fly in a wireless ad-hoc network increases when the size of congestion window and the number of hop count increase. It is possible that packets may have to retransmit because large amount of on-the-fly packets may increase the media contention. Besides, packet delay can grow rapidly when the wireless network becomes congested or the channel interference remains unresolved. This thesis presents a cross-layer TCP congestion window control mechanism for multihop ad-hoc networks to dynamically adjust the size of congestion window according to the MAC-layer contention statistics measured at each hop along the routing path. With the proposed scheme, the congestion window of each traffic flow can be dynamically set to an appropriate size to reduce packet delay and increase flow throughput. For the purpose of evaluation, we perform simulations on NS-2. The simulation results have shown the advantage of our proposed scheme over the two previous works, NewReno and CWL(Congestion Window Limit), especially when the ad-hoc network is loaded with background traffic.

Nowadays, the methods of improving the Reliability of network packets are existent. Some start with the software and others with the hardware. For example, like the Dual System, Redundancy or Fault Tolerant Network, etc. However, it costs a lot to construct the required mechanism, and these methods are not reliable for some special network packets, like the UDP packet. Is there a method to raise the reliability of UDP packet just to increase the software equipments or hardware equipments? The network controller that improves the Reliability of UDP Packets in this thesis uses the same two packets, and it transfers the two packets into different network paths to the same destination workstation. This mechanism can avoid the network accidents causing by the network wires breaking or the network switch machine turn-off, and the destination workstation can¡¦t be hurt from the damage of losing the information carried by UDP packets. Moreover, this method can detect the network accidents as the function of the Fault Tolerant Network by using the dual packets, and send signals to alert the network manager ahead of time. In addition, by using the network controller, network manager or constructor mentioned in this thesis, the RNP topology can be easily built, just by connecting the RNP type wires to the network switches or network bridges.

Systemic lupus erythematosus (SLE) is a chronic multiorgan autoimmune disorder characterized by abundant immune complex (IC) deposition, with nephritis being a major cause of morbidity and mortality. Yet, IC deposition alone is not sufficient for disease development suggesting that additional factors dictate the propensity for developing target organ injury. Genome-wide association studies have identified polymorphisms in the leukocyte integrin Mac-1 (CD11b/CD18, ITGAM) that associate with lupus nephritis. Although Mac-1 promotes inflammation by triggering leukocyte recruitment and cytotoxic functions, there is emerging evidence that it may also serve protective roles under certain conditions. We demonstrate that Mac-1 deficiency in the context of the uniquely human FcgRIIA a receptor that binds IgG-IC, promotes susceptibility to lupus nephritis in two independent animal models. Analysis of renal tissue and intravital microscopy revealed that Mac-1 modulates neutrophil recruitment by FcgRIIA. The SLE-associated variant of Mac-1 rs1143679 (R77H), results in reduced Mac-1 functions, but the underlying mechanism remains undefined. CD18 integrin mediated adhesion is a multistep process that begins with affinity changes for ligand via transmission of allosteric signals. Moreover, mechanical forces (e.g. shear flow) paradoxically increase the lifetime of integrin-ligand bonds, referred to as "catch-bonds". Here, we show that expression of Mac-1 R77H on neutrophils, and blocking antibodies to the extracellular b-propeller domain in which it resides, markedly impairs Mac-1 adhesion to ligand under shear flow. R77H expressing cells exhibit a shift in equilibrium towards a bent conformation, a lower affinity and on- and off- rate for ligand and an inability to form catch-bonds. Additional mutants and activating antibodies reveal that R77H prevents allosteric signal transmission to the aI-domain required for productive ligand binding.

Efficient utilization of the limited wireless spectrum while satisfying applications’ quality of service requirements is an essential design goal of forthcoming wireless networks and a key to their successful deployment. The need for spectrally efficient systems has motivated the development of adaptive transmission techniques. Enabling this adaptation requires protocols for information exchange as well as mathematical tools to optimize the controllable parameters. In this dissertation, we provide insights into such protocols and mathematical tools that target efficient utilization of the wireless spectrum. First, we propose a distributed CDMA-based medium access protocol for mobile ad hoc networks (MANETs). Our approach accounts for multiple access interference at the protocol level, thereby addressing the notorious near-far problem that undermines the throughput performance in MANETs. Second, we present a novel power-controlled MAC protocol, called POWMAC, which enjoys the same single-channel, single-transceiver design of the IEEE 802.11 Ad Hoc MAC protocol, but which achieves a significant throughput improvement over the 802.11 protocol. Third, we consider joint power/rate optimization in the context of orthogonal modulation (OM) and investigate the performance gains achieved through adaptation of the OM order using recently developed optimization techniques. We show that such adaptation can significantly increase network throughput while simultaneously reducing the per-bit energy consumption relative to fixed-order modulation systems. Finally, we determine the maximum achievable “performance” of a wireless CDMA network that employs a conventional matched filter receiver and that operates under optimal link-layer adaptation where each user individually achieves the Shannon capacity. The derived bounds serve as benchmarks against which adaptive CDMA systems can be compared.

Driven by both the need for ubiquitous wireless services and the stringent strain on radio spectrum faced in today's wireless communications, cognitive radio (CR) have been investigated as a promising solution to deploy Wireless Regional Area Networks (WRANs) for an efficient spectrum utilization. Communication devices with CR capabilities are able to access spectrum bands licensed for other wireless services in an opportunistic and secondary fashion, while preventing harmful interference to incumbent licensed services. However, a lesson learned from early experiences in developing such macro-cellular networks is that it becomes increasingly less economically viable to develop CR macrocellular infrastructures for increasing data rates in both line-of-sight as well as non-line-of-sight situation of WRAN, and the corresponding quality of service (QoS) in macrocellular networks is also noticeably degraded due to path loss, shadowing, and multipath fading due to wall penetration. Moreover, there are several challenges to make the real-world CR enabling dynamic spectrum access a difficult problem to implement without harmful interference. First, the hardware design of cognitive radio on the physical layer involves the tuning over a broad range of spectrum to detect a weak signal in a dynamic environment of fading channels, which in turn makes identification of the spectrum opportunities hard to achieve in an efficient and accurate manner. Second, opportunistic media access based on imperfect spectrum usage information obtain from physical layer brings up undesirable interference issue, as well as reliability issues introduced by mutual interference. Third, the curial issue is to determine which channels to use for data transmissions in presence of the dynamic and opportunistic nature of wireless environments, in the case where pre-defined dedicated control channel is not available in the complex and heterogenous networks. In this dissertation, a novel framework called Cognitive-Empowered Femtocell (CEF), which combines CR techniques with femtocell networking, is introduced to tackle these challenges and achieve better spectrum reuse, lower interference, easy integration, wider network coverage, as well as fast and cost effective early stage WRAN. In this framework, a sensing coordination scheme is proposed to gracefully unshackles the master/slave relationship between central controllers and end users, while maintaining order and coordination such that better sensing precision and efficiency can be achieved. As such, the network intelligence can be expanded from controlling the intelligence paradigm to better understand the satisfy wireless user needs. We also discuss design and deployment aspects such as sensing with reasoning approach, gossip-enabled stochastic media access without a dedicated control channel, all of which are important to the success of the CEF framework. We illustrate that such a framework allows wireless users to intelligently capture spectrum opportunities while mitigating interference to other users, as well as improving the network capacity. Performance analysis and simulations were conducted based on these techniques to provide insight on the future direction of interference suppression for dynamic spectrum access.