Wireless Communication Technology Is Rapidly Growing Computer Science Essay

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Wireless communication technology is rapidly growing around the world and this trend is likely to continue in future also. The design of Smart Antenna has received a lot of attention for mobile wireless communication. However, their application in mobile communications and wireless computing network terminals has received little attention, due to the complexity of the antenna structure, size restrictions, and the system demands associated with the antenna technology. Recently, a number of smart mobile terminal antennas (SMTAs) have been developed for wireless computing networks such as Schlub's seven element ground skirt monopole antenna array based on electronically steerable passive array radiator(ESPAR) for wireless ad-hoc computer networks [1] and Lu's dielectric embedded ESPAR antenna array for mobile terminals[2]. Gray's switched Yagi patch antenna for mobile satellite communication [3] and Thiel's beam-switched monopole arrays [4] have also been explored for the application in wireless communications and computing network systems. However, a strong need to improve the antenna array as an access point in the ceiling and facing down to computer users for indoors and outdoors mobile wireless computer network. In present work, novel smart antenna is developed to improve antenna's performances. Smart antenna must have the capability to vary their operating conditions according to the wireless standard, although maintaining optimum performances at the same time for successful implementation. This approach has the potential to reduce the number of RF circuits needed to implement the same functions, yet have the capability to be individually tuned for frequency bands and channels [5].

The challenge is that RF circuits cannot be reconfigured with the bulky passive RF component (like high-Q inductors, ceramic filters, SAW filters, varactor diodes and PIN diodes switches), due to the limitation associated with power handling, linearity, insertion loss and isolation. These limitations make it impractical to design low loss reconfigurable circuits based on bulky passive RF component.

The application of microelectromechanical systems (MEMS) technology to radio-frequency (RF)/ microwave systems is on the verge of revolutionizing wireless communications. The MEMS is being recognized as a core technology to give the greatest impact in constructing the next generation of mobile/wireless communication products. Their primary goal is the implementation of electrical functions whose characteristics can be adjusted during operation, thus enabling reconfiguration for tunable circuits and components [5].

MEMS technologies enhanced RF performances as they offer an innovative solution to solving the RF design challenges that currently limit the functionality of today's semiconductor technologies. Within the MEMS domain, RF MEMS relays are switching elements that offer simultaneously higher Q at high frequencies, low insertion loss, higher isolation, outstanding linearity (>70 dBm) and smaller packaging size. Hence, they deliver expanded RF Performances compared to PIN and GaAs switches.

An important benefit of RF MEMS technologies is that it supports the possibility of System-on-chip (SoC) integration by post processing on top of CMOS technologies or integrated passive devices technologies [6] [7],. Therefore, it merges silicon electrical structures with miniaturized high RF performances mechanical structures. The benefits of SOC integration have been well documented. Space saving not only increase the functionality per unit area, but also improves performances via a smaller electrical path, resulting in lower losses and practises with contributes to higher bandwidth. SOC can potentially increase reusability, reliability, and reduce time-to-market [8]. Hence, the integration of MEMS switches into RF subsystems is expected to provide benefits.

In the past, various actuation approaches have been demonstrated utilizing various operation principles such as electrothermal actuation, the electrostatic actuation, the piezoelectric actuation, and the electromagnetic actuation. Among all of these processes, electrostatic and electrothermal actuators are most attractive. The electrostatic actuators offers low power dissipation and high driving frequency, but suffers from less functional robustness, high actuation voltage and small range of controllable displacement. In contrast, the electrothermal actuators have capability in generating relatively large actuation displacement and force, but a relatively high power consumption as compared with the electrostatic actuators. Therefore, the actuation method should be selected according to the application requirements of the switches [3].

The proposed RF MEMS switch is optimized by using MetalMumps process [4], a commercially available Multiproject Project Wafer (MPW) service. Through MetalMumps process the substrate loss (a trench 25-µm deep is carved under the device, resulting in a suspended switch) can be reduced substantially. Also, it can provide a good solution for the integration of MEMS with IC. Hence, bring the concept of System of chip (SOC) into reality.

1.2 Objective of the Research

The purpose of this report is to present unique configurations and techniques to develop novel electrostatically and electrothermally actuated RF MEMS switches to improve antenna's performances. The subject is divided into three tasks:

(i) Design of novel configurations for electrostatically and electrothermally actuated RF MEMS switches: The aim of this section to design novel RF MEMS switches that have excellent RF performances such as low insertion loss, high isolation, low actuation voltage and good return loss. Therefore, design is developed using Finite Element Simulator (FEM) such as Intellisuite, COMSOL and CST, which are based on Finite Element Method.

(ii) Design of Smart Antenna: The aim of this section to design smart antennas with improved performances such as design smart patch antenna array with hexagonal elements that can significantly increase antenna gain and wireless security in WLAN systems operating at 2.4 GHz and design nine-element dielectric-embedded electronically switched multiple-beam (DE-ESMB) antenna array, in which physical size of the antenna array can be significantly improve to improve its performances. Design processes and analysis concepts are developed to analyse the problem based on computer modelling and optimization techniques.

(iii) Integration of RF MEMS switches for Smart Antennas: The objective of this section to integrate RF MEMS switches for smart antenna's application to improve performance and reducing the production costs is necessary. Switching will be applied by RF MEMS switches for beam steering purpose and for that reason it will be possible to build a smaller and lighter device, which is reducing the material costs. MEMS switches are replaced with PIN Diode as Diode can seriously degrade antenna performance.

1.3 Structure of the Report

This report is subdivided into five chapters. Chapter 1 present motivation and the research objectives are outlined while Chapter 2 elaborates the RF MEMS switches and presents the state-of-art in this field. The switches are classified based on the contact mechanism or the implementation perspective. The two types of switches namely electrostatically and electrothermally actuated MEMS switches are the most widely reported devices. The respective merits and shortcomings are briefly described with references. It is followed to applications of MEMS technology in the radio frequency field, with emphasis on RF MEMS switching devices. The description outlines the working principle, the shortcomings in state-of-art devices, and the potential applications in the field of communication. This puts the chosen research topic in proper perspective which is described next, outlining the main aspects and the methodology adopted.

Chapter 3 discusses the dependence of electrical parameters on material properties and switch geometry, with examples from the design of switching devices considered in this report, which lead to the final design considerations. The devices are implemented in standard coplanar wave guide with 50 Ω characteristic impedance configurations, the most preferred connection medium for RF MEMS devices. Though CPW configuration was simulated using CST software, a brief introduction on CPW basics is provided in order to present a complete perspective of the RF MEMS device.

2 Literature Review

2.1 RF MEMS Switches

2.1.1 RF MEMS

RF MEMS switches are devices that use mechanical movement to achieve an open or short circuit condition in an RF transmission line or an antenna. The first reported work on MEMS switches is by K.E Peterson (1979), showing the possibilities of the emerging micromachining technology. In 1991, L.E Larson developed RF MEMS switch for few tens of GHz that brought many research institutes and universities on the track of RF MEMS. With the development of advanced technology for micro-fabrication and the appearance of information technology in the 1990s, devices made by means of MEMS technology have found a great variety of potential applications that includes wide band phase-shifters realized with MEMS switches or micro machined tunable capacitors, reconfigurable antennas, voltage controlled oscillators, and impedance tuning circuits just to give a few examples.

In a total, MEMS devices offer higher Q at high frequencies, low insertion loss and high signal linearity for the switches, higher isolation and smaller packaging size, but on the other hand also by their fabrication complexity, reliability issues, the difficulty of integrating them with traditional electronics circuits, by their special packaging requirements, and by device specific problems, such as relatively low power handling ability of MEMS switches.

Due to rapidly increasing wireless technology, it demands low size, low weight, low power consumption, re-configurability, and good signal properties of the devices to fulfil new telecommunication standards. These requirements make RF MEMS devices very suitable for replacing bulky passive off-chip components that currently constrain further miniaturization of wireless equipment since they consume most of the circuit board size and, in contrast to RF MEMS devices, cannot be integrated on chip without sacrificing performances. Table 1 compare the performances of switches based on PIN diode, FET, conventional electromechanical relay (EMR) and MEMS.

However, Wireless applications are a low cost and high volume market where new mechanical devices with uncertain reliability entail high risks. Therefore, it will be very difficult for MEMS switches to compete with semiconductor switches for wireless applications as semiconductor switches are available in high volumes for 0.3-0.6 USD per circuit. Also, the down turn of the telecommunication sector since 2001 with an investment stop in emerging technologies was a barrier in the ascension of RF MEMS. As a result, many companies specialized in RF MEMS development crashed because their cash was not sufficient to support cash outlay operations until volume orders could be delivered and paid. However, RF components such as Microwave acoustic devices are already in high volumes. The success of these microwave acoustics devices anticipates the market potential and the market's need of RF MEMS devices to even further push technological limits and fulfil customer demands.

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