he ability to communicate with anyone on the planet from anywhere on the planet has been mankind's dream for a long time. Wireless is the only medium that can enable such untethered communication. With the recent advances in VLSI and wireless technologies, it is now possible to build high-speed wireless systems that are cheap as well as easy to install and operate. However, the wireless medium is a broadcast medium, and therefore multiple devices can access the medium at the same time. Multiple simultaneous transmissions can result in garbled data, making communication impossible. A medium access control (MAC) protocol moderates access to the shared medium by defining rules that allow these devices to communicate with each other in an orderly and efficient manner. MAC protocols therefore play a crucial role in enabling this paradigm by ensuring efficient and fair sharing of the scare wireless bandwidth. Wireless MAC protocols have been studied extensively since the 1970s. The initial protocols were developed for data and satellite communications. We are now witnessing a convergence of the telephone, cable and data networks into a single unified network that supports multimedia and real-time applications like voice and video in addition to data. The multimedia applications require delay and jitter guarantees from the network. This demand of the network is known as the Quality of Service (QoS) guarantee. These requirements have led to novel and complex MAC protocols that can support multimedia traffic.This article surveys the various MAC protocols that have been proposed in the literature and compares them based on architecture (MAC co-ordination, duplexing), performance (throughput, delay, stability, contention resolution algorithms and fairness) and multimedia support (scheduling, access priorities). We confine our study to systems that span relatively small areas. The article is organized as follows. First we contrast different wireless network architectures. We then bring out the issues unique to wireless MAC protocols. The performance metrics used to compare different MAC protocols are discussed later. We then present a classification of the protocols. We will present the different classes of proposed MAC protocols and compare the pros and cons of the proposed protocols. GENERAL NETWORK CONCEPTSA wireless network is comprised of devices with wireless adapters communicating with each other using radio waves. These wireless devices are called nodes in this dissertation. The signal transmitted can be received only within a certain distance from the sender, which is called the range of the node. A base station (BS) is a special node in the network that is not mobile and is located in a central location. Wireless networks differ in the duplexing mechanism and the network architecture.AJAY CHANDRA V. GUMMALLA AND JOHN O. LIMB, GEORGIA INSTITUTE OF TECHNOLOGY T ABSTRACTTechnological advances, coupled with the flexibility and mobility of wireless systems, are the driving force behind the Anyone, Anywhere, Anytime par...

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Recent growth in the use of wireless wide-area networks (WWAN), the adoption of broadband wireless local-area networks (WLAN), and consumer demand for seamless global access has pushed the wireless industry to support most broadband wireless standards . . These are supported in different geographical areas by supporting multi-band and multimode operation in cellular handsets, access points, laptops, and client cards. This has created a great challenge for engineers. It has pushed RF and antenna design beyond the capabilities of current technologies, opening the door for creative solutions that are 1) multi band, 2) low profile, 3) small, 4) better performing (including MIMO), 5) accelerate time to market, 6) low cost, and 7) easy to integrate in the devices listed above. Conventional state-of-the-art antenna technologies satisfy a subset of these seven criteria; however, they hardly satisfy all of them. In this paper, we apply composite right-left-hand "CRLH-based" RF design to print penta-band handset antennas directly on the printed circuit board (PCB), and balanced-antennas for Wi-Fi access points.Full active and passive performance is presented, while describing key benefits of metamaterial antennas. We also analyze in detail how these antennas operate, while focusing on the main left-handed (LH) mode that enables antenna size reduction, and the ability to print them directly on the printed circuit board.

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Time-variant multiple-input multiple-output (MIMO) channels are measured in an outdoor campus environment at 2.45 GHz with directional patch arrays and omnidirectional monopole arrays. A number of useful metrics are proposed for quantifying time variation in MIMO channels: eigenvalue level crossing rate, eigenvector angular deviation, and capacity loss for delayed transmit and receive channel state information (CSI). Measurements in four different environments confirm the strong correlation between angular spread of multipath and MIMO channel time variability. The rate of time variation is also strongly influenced by the type of array, indicating that directional elements may be advantageous for highly mobile environments. The proposed metrics indicate that although the physical communication layer may need to update CSI several times per wavelength, the required rate of adaptation in transmit rate, modulation, and power allocation is much less severe.