The wireless medium being inherently broadcast in nature and hence prone to interferences requires highly optimized medium access control (MAC) protocols. This holds particularly true for wireless sensor networks (WSNs) consisting of a large amount of miniaturized battery-powered wireless networked sensors required to operate for years with no human intervention. There has hence been a growing interest on understanding and optimizing WSN MAC protocols in recent years, where the limited and constrained resources have driven research towards primarily reducing energy consumption of MAC functionalities. In this paper, we provide a comprehensive state-of-the-art study in which we thoroughly expose the prime focus of WSN MAC protocols, design guidelines that inspired these protocols, as well as drawbacks and shortcomings of the existing solutions and how existing and emerging technology will influence future solutions. In contrast to previous surveys that focused on classifying MAC protocols according to the technique being used, we provide a thematic taxonomy in which protocols are classified according to the problems dealt with. We also show that a key element in selecting a suitable solution for a particular situation is mainly driven by the statistical properties of the generated traffic.

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MAC Essentials for Wireless Sensor Networks

The past decade has witnessed a phenomenal market penetration of wireless communications and a steady increase in the number of mobile users. Unlike wired networks, where communication links are inherently stable, in wireless networks, the lifetime of a link is a random variable whose probability distribution depends on mobility, transmission range, and various impairments of radio communications. Because of the very dynamic nature of Vehicular Ad hoc NETworks (VANETs) and the short transmission range mandated by the Federal Communications Commission (FCC), individual communication links come into existence and vanish unpredictably, making the task of establishing and maintaining routing paths between fast-moving vehicles very challenging. The main contribution of this work is to investigate the probability distribution of the lifetime of individual links in a VANET under the combined assumptions of a realistic radio transmission model and a realistic probability distribution model of intervehicle headway distance. Our analytical results were validated and confirmed by extensive simulation.

A Probabilistic Analysis of Link Duration in Vehicular Ad Hoc Networks

The beamforming antenna technology is a promising solution to many challenges facing wireless ad hoc networks. Beamforming antennas have the ability to increase the spatial reuse, improve the transmission reliability, extend the transmission range and/or save the power consumption. If they are effectively used, they can significantly improve the network capacity, lifetime, connectivity and security. However, traditional Medium Access Control (MAC) protocols fail to exploit the potential benefits due to the unique characteristics of wireless ad hoc networks with beamforming antennas. To that end, numerous MAC protocols have been designed over the years to harness the offered potential. In this paper, we survey the literature on MAC protocols proposed for wireless ad hoc networks with beamforming antennas during the last decade. We discuss the main beamforming-related challenges facing the medium access control in ad hoc networks. We present taxonomy of the MAC protocols proposed in the literature based on their mode of operation and the mechanisms used to address the challenges. In addition, we provide a qualitative comparison of the protocols highlighting their features, benefits and requirements. Finally, we provide directions for possible future work.

A Survey On MAC Protocols for Wireless Adhoc Networks with Beamforming Antennas

This article presents comprehensive comparisons of interleave division multiple access (IDMA) and direct sequence code division multiple access (DS-CDMA) in terms of performance and complexity assuming iterative multiuser detection. IDMA can be seen as a special case of DS-CDMA with spreading gain of one using very low rate code and user-specific interleavers for user separation. We focus on three suboptimum linear detectors: minimum mean square error (MMSE), rake (or matched filter), and soft-rake detectors from practical concerns. We analytically prove that the three detectors are equivalent for asynchronous users of IDMA on frequency flat channels for complex modulation alphabets. Such equivalence has been shown only for binary phase shift keying (BPSK) in the literature. The equivalence guarantees the MMSE solution for IDMA without computationally expensive matrix inversions or matrix-vector multiplications. This is generally not the case for DS-CDMA since DS-CDMA is sensitive to user asynchronism. We also discuss complexity aspects when the MMSE detector is used where we focus on essential differences in complexity between IDMA and DS-CDMA, instead of discussing particular complexity reduction techniques. Computer simulations are performed in various scenarios and the performance is analyzed by bit error rate simulations as well as by extrinsic information transfer (EXIT) charts. The analysis reveals the advantages of IDMA over DS-CDMA in terms of performance and complexity under practical considerations, particularly in highly user loaded scenarios.

IDMA vs. CDMA: Analysis and Comparison of Two Multiple Access Schemes

SUMMARY
Compared with omni-directional antennas, directional antennas have many merits, such as lower interference, better spatial reuse, longer transmission range, and improved network capacity. Directional antennas enable numerous emerging outdoor and indoor applications, which have been addressed in many recent studies. Despite the advances in wireless networks with directional antennas (DAWNs), there are many research challenges in all layers of DAWNs. This paper presents a detailed study on recent advances and open research issues on DAWNs. Firstly, we briefly introduce the classification of directional antennas, antenna radiation patterns, antenna modes, and the challenges in the physical layer of DAWNs. We then present research issues on the medium access control (MAC) layer, followed by the current solutions as well as open research problems on the MAC layer of DAWNs. In addition, we also discuss the research issues on the routing layer and the transport layer. Moreover, other research challenges on the performance evaluation of DAWNs and a brief introduction of indoor DAWNs are given in this paper as well. In conclusion, we summarize the current research issues on DAWNs as well as prospects in the future. Copyright © 2011 John Wiley & Sons, Ltd.

An overview of using directional antennas in wireless networks

The application of directional antennas in the mobile devices of wireless ad hoc networks has the potential to increase the network connectivity and capacity. However, new solutions for medium access control (MAC) are needed. This pa-per provides a survey and overview of recently proposed MAC protocols for this scenario. We summarize problems specific to this setup and categorize proposed protocols.

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A Survey on MAC Protocols for Ad Hoc Networks with Directional Antennas

A receiver includes a receiving unit (101) for scanning a dedicated signaling channel for the presence of an announcement signal. The announcement signal indicates a transmitter willing to access a transmission medium. The receiving unit is further operative to receive a data signal from a further transmitter on a dedicated traffic channel. Furthermore, the receiver includes an interference estimator (105) for estimating, whether an interference caused by a new transmitter is allowable or not. A processor (107) is provided for applying an interference counter-measure, when the interference is not allowable. A transmitter can, without any limitation, transmit the announcement signal. The transmitter will start transmitting, when it receives a ready-to-receive signal from the addressed receiver and when it does not receive an objection tone from another receiver in the network.

Receiver and transmitter for a network having a non-centralized medium access control

We show that randomized beamforming is a simple yet efficient communication strategy in wireless multihop networks if no neighbor location information is at hand. Already small antenna arrays reduce the hop distance between nodes and speed up the flooding of messages. This result is obtained in different scenarios, using accurate models for circular antenna arrays and a line-of-sight link model between randomly placed nodes.

Hop distances and flooding in wireless multihop networks with randomized beamforming

Conventional medium access control protocols are designed to avoid simultaneous transmissions, based on a simple collision model in the underlying physical layer. Recently, strong physical layer capabilities, enabled by multiuser detection techniques, have been studied in connection with simple medium access control protocols, for example, slotted ALOHA. We think that neither of these extreme approaches is optimum, particularly in general scenarios where network nodes with different signal processing capabilities coexist. Instead of dealing with interferences in either of the two layers alone, both medium access control and physical layer functionalities should be designed to cooperate and complement each other. We discuss several key aspects for designing such a protocol, especially with an emphasis on iterative multiuser detection, which can provide a good tradeoff between performance and complexity. We propose a new protocol called MUD-MAC which satisfies these key aspects. We analyze its throughput bound and also perform numerical simulations. The simulation results show excellent throughput improvements. It is also demonstrated that the MUD-MAC protocol provides certain fairness among network nodes with different signal processing capabilities.

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Medium Access in Spread-Spectrum Ad Hoc Networks with Multiuser Detection

Multihop wireless networks will play an important role in future communication networks. Beamforming antennas have shown great potential to improve the connectivity in these networks; however only heuristic approaches exist to assess the benefits of this technology. Using the popular "keyhole" antenna model, we formulate a Mixed Integer Program (MIP) capable of acquiring optimal antenna configurations for path probability with various auxiliary constraints, node degree, and k-connectivity. We employ a problem-specific large-scale optimization approach named "column generation" to be able to cope with realistic scenario sizes without sacrificing optimality. In a case study, we show the feasibility of our optimization approach and demonstrate that "smarter" beamforming heuristics would still have plenty of room to improve the connectivity of such a network.

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Robert Vilzmann

Papers

A Survey on MAC Protocols for Ad Hoc Networks with Directional Antennas

Receiver and transmitter for a network having a non-centralized medium access control

Hop distances and flooding in wireless multihop networks with randomized beamforming

Medium Access in Spread-Spectrum Ad Hoc Networks with Multiuser Detection

On connectivity limits in ad hoc networks with beamforming antennas