Rapid Advancement In Mobile Computer Science Essay

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Due to the rapid advancement in mobile and wireless communication system the need for efficient antenna design with good radiation capability, multiple simultaneous steerable beams and small size is becoming imperative. Research work to improve the antenna design for obtaining desired goals has been carried out by various researchers in the last few decades. Smart antennas with wide scanning angle capabilities have emerged as a result of continuous effort in making efficient antenna design. Modern day cutting edge applications like Radar and satellite communication require antennas with wide scanning angle capabilities and good performance over broad frequency range. The spectrum is limited and the frequency of operation for a device keeps on increasing. At higher frequencies, only direct waves are useful and the effective range is greatly reduced. Hence as the frequency increases, the signal carrying large bandwidth information becomes more and more directional. At higher frequencies the absorption decreases and maximum use of sky waves can be done to reliably transmit and receive the information from certain direction. A beam-forming device that produces true-time delay, wideband, wide-angle and steerable beam is desirable. These antennas are efficient, and can be made sharply directive, thus greatly increasing the strength of the signal transmitted in a desired direction. The power received is inversely proportional to the square of the distance from the transmitter, assuming there is no attenuation due to absorption or scattering. The narrow beam produces good isolation between adjacent radiation elements using space diversity; hence multiple beams are possible to be simultaneously obtained by reusing the antenna structure.

A phased array is the essential device that utilizes the beam-forming network to radiate

energy into free space. Since decades, it has been widely adopted in many radar and

satellite systems to produce electronically-controlled beam scanning. Earlier, array

systems have been restricted for military applications due to high cost and complexity. In

recent years, low cost high performance array and its supporting devices have been

realized using printed circuit technology. Array-based commercial applications such

as wireless point-to-point communications and auto-collision avoidance radar have

emerged. Due to allocation of new bands for commercial ultra-wideband (UWB) [1] and Extremely High Frequency (EHF) applications [2], low-profile high performance arrays have been under investigation. The low-cost high-performance beam-forming networks would facilitate new application development.

The aim of this research work is to optimize the performance of Rotman lens in terms of minimizing the phase error and improving the scanning capabilities with low loss using GA optimizing techniques. The antenna should be capable of producing multiple beams which can be steered without changing the antenna orientation. The lens feeds a linear antenna array of Microstrip patch antennas which acts as radiating elements. Existing design theory will be improved in terms of minimized phase errors and scanning capabilities. Prototype of the Microwave lens has been implemented. Optimization technique has been proposed to improve the scanning capabilities of the lens. A Rotman lens and Microstrip patch antenna have been designed, fabricated and tested, and they are covered throughout these topics. The simulation and measurement data of the fabricated PCB's are used to support the proposals in this dissertation.

Before we begin, it is important to familiarize with the basic concepts of microwave, electromagnetic wave theory, antenna fundamentals, phased array and beam-forming network because it forms the basis of this dissertation work.

1.1 Basics of Microwave and Electromagnetic Wave theory

Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimeter, with frequencies between 0.3 GHz and 300 GHz.

Depending on the range they are classified as various bands

L-band: 1-2 GHz (1,000-2,000 MHz)

S-band: 2-4 GHz (2,000-4,000 MHz

C-band: 4-8 GHz (4,000-8,000 MHz)

X-band: 8-12.5 GHz (8,000-12,500 MHz)

Lower K-band: 12.5-18 GHz (12,500-18,000 MHz)

Upper K-band: 26.5-40 GHz (26,500-40,000 MHz)

Marine radar systems commonly operate in the S and X bands, while satellite navigation system signals are found in the L-band. The break of the K-band into lower and upper ranges is necessary because the resonant frequency of water vapor occurs in the middle region of this band, and severe absorption of radio waves occurs in this part of the spectrum. Effects of reflection, polarization, scattering, diffraction, and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation. Micro in Microwaves means small compared to waves used in typical radio broadcasting, in that they have shorter wavelengths. Microwave technology is extensively used for point-to-point telecommunications (i.e., non broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrow beams than radio waves; their comparatively higher frequencies allow broad bandwidth and high data flow, and also allowing smaller antenna size because antenna size is inversely proportional to transmitted frequency (the higher the frequency, the smaller the antenna size). Microwaves are the principal means by which data, TV, and telephone communications are transmitted between ground stations and to and from satellites. Microwaves are also employed in radar technology.

Electromagnetic radiation is a form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space. EMR has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation. In a vacuum, electromagnetic radiation propagates at a characteristic speed, the speed of light.

Electromagnetic radiation is a particular form of the more general electromagnetic field (EM field), which is produced by moving charges.Charges and currents produce EMR only indirectly-rather, in EMR, both the magnetic and electric fields are associated with changes in the other type of field, not directly by charges and currents. This close

relationship assures that the electric and magnetic fields in EMR exist in a constant ratio of strengths to each other, and also to be found in phase, with maxima and nodes in each found at the same places in space.In classical physics, EMR is considered to be produced when charged particles are accelerated by forces acting on them. Electrons are responsible for emission of most EMR because they have low mass, and therefore are easily accelerated by a variety of mechanisms. Rapidly moving electrons are most sharply accelerated when they encounter a region of force, so they are responsible for producing much of the highest frequency electromagnetic radiation observed in nature.

In the history of electromagnetic theory, significant work specifically in the area of microwaves and their applications was carried out by researchers.Electronic devices operating at high frequencies usually have physical size comparable to the wavelength, thus classical circuit theory hardly applies. It is the Maxwell equations that provide the fundamental theories for many engineers to perform predictive design and pursue solid explanation. In this section, we first review the Maxwell equations.

Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.Conceptually, Maxwell's equations describe how electric charges and electric currents act as sources for the electric and magnetic fields and how they affect each other. (See below for a mathematical description of these laws.) Of the four equations, two, Gauss's law and Gauss's law for magnetism, describe how the fields emanate from charges. (For the magnetic field there is no magnetic charge and therefore magnetic fields lines neither begin nor end anywhere.) The other two equations describe how the fields 'circulate' around their respective sources; the magnetic field 'circulates' around electric currents and time varying electric field in Ampère's law with Maxwell's correction, while the electric field 'circulates' around time varying magnetic fields in Faraday's law.

Equations

1.2 Antenna and its array: An antenna in a telecommunications system is the port through which radio frequency (RF) energy is coupled from the transmitter to the outside world for transmission purposes, and in reverse, to the receiver from the outside world for reception purposes. Antennas had been the most neglected of all the components in personal communications systems. Yet, the manner in which radio frequency energy is distributed into and collected from space has a profound influence upon the efficient use of spectrum, the cost of establishing new personal communications networks and the service quality provided by those networks. Main aim here is to see how the focus shifted from omnidirectional antenna (equal radiation in all directions and no preferable direction) to Directional antenna (one direction)and beyond that from the use of single element to array of elements working as radiators.

1.2.1 Omnidirectional Antennas

Since the early days of wireless communications, there has been the simple dipole antenna, which radiates and receives equally well in all directions.

Figure 1: Omnidirectional Antennas

This was quiet adequate for simple RF environments where there was no knowledge of the users' whereabouts .Due to this unfocused approach, the signal was scattered in all directions trying to reach to the desired users. The energy was wasted by sending the signal in all directions. Given this limitation, omnidirectional strategies attempt to overcome environmental challenges by simply boosting the power level of the signals broadcast. The increase in the power increased the level of interference in the same or adjoining cells. In uplink applications (user to base station), omnidirectional antennas offer no preferential gain for the signals of served users. In other words, users have to shout over competing signal energy. Also, this single-element approach cannot selectively reject signals interfering with those of served users and has no spatial multipath mitigation or equalization capabilities. Therefore,omnidirectional strategies directly and adversely impact spectral efficiency, limiting frequency reuse.

.

1.2.2 Directional Antennas and Sectorized Systems

A single antenna can also be constructed to have certain fixed preferential transmission and reception directions. Sectorized antenna system take a traditional cellular area and subdivide it into sectors that are covered using directional antennas looking out from the same base station location.Operationally, each sector is treated as a different cell in the system, the range of which can be greater than in the omni directional case, since power can be focused to a smaller area. This is commonly referred to as antenna element gain. Additionally, sectorized antenna systems increase the possible reuse of a frequency channel in such cellular systems by reducing potential interference across the

original cell. However, since each sector uses a different frequency to reduce cochannel interference, handoffs (handovers) between sectors are required. Narrower sectors give better performance of the system, but this would result in to many handoffs.While sectorized antenna systems multiply the use of channels, they do not overcome the major disadvantages of standard omnidirectional antennas such as filtering of unwanted interference signals from adjacent cells.

Figure 2: Sectorized antenna system

1.3 Smart Antenna Systems

Smart antennas (also known as adaptive array antennas, multiple antennas and, recently, MIMO) are antenna arrays with smart signal processing algorithms used to identify spatial signal signature such as the direction of arrival (DOA) of the signal, and use it to calculate beamforming vectors, to track and locate the antenna beam on the mobile/target. The antenna could optionally be any sensor.Smart antenna techniques are used notably in acoustic signal processing, track and scan RADAR, radio astronomy and radio telescopes, and mostly in cellular systems like W-CDMA and UMTS.Smart antennas have two main functions: DOA estimation and Beamforming

Direction of arrival (DOA) estimation

The smart antenna system estimates the direction of arrival of the signal, using techniques such as MUSIC (Multiple Signal Classification), estimation of signal parameters via rotational invariance techniques (ESPRIT) algorithms, Matrix Pencil method or one of their derivatives. They involve finding a spatial spectrum of the antenna/sensor array, and calculating the DOA from the peaks of this spectrum. These calculations are computationally intensive.

Beamforming

Beamforming is the method used to create the radiation pattern of the antenna array by adding constructively the phases of the signals in the direction of the targets/mobiles desired, and nulling the pattern of the targets/mobiles that are undesired/interfering targets. This can be done with a simple FIR tapped delay line filter. The weights of the FIR filter may also be changed adaptively, and used to provide optimal beamforming.

Two of the main types of smart antennas include switched beam smart antennas and adaptive array smart antennas. Switched beam systems have several available fixed beam patterns. A decision is made as to which beam to access, at any given point in time, based upon the requirements of the system. Adaptive arrays allow the antenna to steer the beam to any direction of interest while simultaneously nulling interfering signals .Beamdirection can be estimated using the so-called direction-of-arrival (DOA) estimation methods.

A smart antenna is a phased or adaptive array that adjusts to the environment. That is, for the adaptive array, the beam pattern changes as the desired user and the interference move, and for the phased array, the beam is steered or different beams are selected as the desired user moves.

Phased array or multibeam antenna consists of either a number of fixed beams with one beam turned on towards the desired signal or a single beam (formed by phase adjustment only) that is steered towards the desired signal.

Adaptive antenna array is an array of multiple antenna elements with the received signals weighted and combined to maximize the desired signal to interference and noise (SINR) ratio. This means that the main beam is put in the direction of the desired signal while nulls are in the direction of the interference.

A smart antenna system combines multiple antenna elements with a signal processing capability to optimize its radiation and/or reception pattern automatically in response to the signal environment.Smart antenna systems are customarily categorized as either switched beam or adaptive array systems.Switched beam antenna system form multiple fixed beams with heightened sensitivity in particular directions. These antenna systems detect signal strength, choose from one of several predetermined,fixed beams, and switch from one beam to another as demand changes throughout the sector. Instead

of shaping the directional antenna pattern with the metallic properties and physical design of a single element (like a sectorized antenna), switched beam systems combine the outputs of multiple antennas in such a way as to form finely sectorized (directional) beams with more spatial selectivity than it can be achieved with conventional, single element approaches.

Smart Antennas are arrays of antenna elements that change their antenna pattern dynamically to adjust to the noise, interference in the channel and mitigate multipath fading effects on the signal of interest.The difference between a smart (adaptive) antenna and "dumb" (fixed) antenna is the property of having an adaptive and fixed lobe-pattern, respectively. The secret to the smart antennas' ability to transmit and receive signals in an adaptive, spatially sensitive manner is the digital signal processing capability present. An antenna element is not smart by itself; it is a combination of antenna elements to form an array and the signal processing software used that make smart antennas effective. This shows that smart antennas are more than just the "antenna", but rather a complete transceiver concept.

Adaptive Antenna Arrays

Adaptive antenna arrays can be considered the smartest of the lot. An Adaptive Antenna Array is a set of antenna elements that can adapt their antenna pattern to changes in their environment.Each antenna of the array is associated with a weight that is adaptively updated so that its gain in a particular look-direction is maximized, while that in a direction corresponding to interfering signals is minimized. In other words, they change their antenna radiation or reception pattern dynamically to adjust to variations in channel noise and interference, in order to improve the SNR (signal to noise ratio) of a desired signal. This procedure is also known as 'adaptive beamforming' or 'digital beamforming'.Conventional mobile systems usually employ some sort of antenna diversity (e.g. space, polarization or angle diversity). Adaptive antennas can be regarded as an extended diversity scheme, having more than two diversity branches. In this context, phased arrays will have a greater gain potential than switched lobe antennas because all elements can be used for diversity combining.

In antenna theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array is a group of multiple active antennas coupled to a common source or load to produce a directive radiation pattern. Usually, the spatial relationship of the individual antennas also contributes to the directivity of the antenna array. Use of the term "active antennas" is intended to describe elements whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input. One common application of this is with a The relative amplitudes of - and constructive and destructive interference effects among - the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft Company, Culver City, CA, in 1957 (see Joseph Spradley, "A Volumetric Electrically Scanned Two-Dimensional Microwave Antenna Array," IRE National Convention Record, Part I - Antennas and Propagation; Microwaves, New York: The Institute of Radio Engineers, 1958, 204-212). When phased arrays are used in sonar, it is called beamforming.

ESA:Functioning of a Radar system is generally by connecting an antenna to a powerful radio transmitter to emit a short pulse of signal. The transmitter is then disconnected and the antenna is connected to a sensitive receiver which amplifies any echos from target objects. By measuring the time it takes for the signal to return, the radar receiver can determine the distance to the object.To scan a portion of the sky, the radar antenna must be physically moved to point in different directions. An active electronically scanned array (AESA), also known as active phased array radar is a type of phased array radar whose transmitter and receiver functions are composed of numerous small solid-state transmit or receive modules .AESA radars aim their "beam" by emitting separate radio waves from each module that interfere constructively at certain angles in front of the antenna. They improve on the older passive electronically scanned radars by spreading their signal emissions out across a band of frequencies, which makes it very difficult to detect over background noise. AESAs allow ships and aircraft to broadcast powerful radar signals while still remaining stealthy.Modern antenna applications such as MIMO Systems,Smart Antennas, Phased Antenna Arrays, etc, require the capability to handle several beams independently.

Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. The improvement compared with omnidirectional reception/transmission is known as the receive/transmit gain (or loss).

Beamforming can be used for radio or sound waves. It has found numerous applications in radar, sonar, seismology, wireless communications, radio astronomy, acoustics, and biomedicine. Adaptive beamforming is used to detect and estimate the signal-of-interest at the output of a sensor array by means of optimal (e.g., least-squares) spatial filtering and interference rejection. To change the directionality of the array. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter, in order to create a pattern of constructive and destructive interference in the wavefront. When receiving, information from different sensors is combined in a way where the expected pattern of radiation is preferentially observed. Conventional beamformers use a fixed set of weightings and time-delays (or phasings) to combine the signals from the sensors in the array, primarily using only information about the location of the sensors in space and the wave directions of interest. In contrast, adaptive beamforming techniques generally combine this information with properties of the signals actually received by the array, typically to improve rejection of unwanted signals from other directions. This process may be carried out in either the time or the frequency domain.

Applications:

Beamformers have much higher Gain than omnidirectional antennas: Increase coverage and reduce number of antennas!

Beamformers can reject interference while omnidirectional antennas cat improve SNR and system capacity.

Beamformers provide N-fold diversity Gain of omnidirectional antennas: increase

system capacity(SDMA)

4) Beamformers suppress delay spread:improve signal quality

Beamforming can be used for radio or sound waves. It has found numerous applications in radar, sonar, seismology, wireless communications, radio astronomy, acoustics, and biomedicine. Adaptive beamforming is used to detect and estimate the signal-of-interest at the output of a sensor array by means of optimal (e.g., least-squares) spatial filtering and interference rejection.

1.5 Scope

In this dissertation, optimized design of Rotman lens with linear array of microstrip patch antenna as radiating elements is presented. The important electrical parameters deduced are phase error, maximum scanning angle, array factor, side lobe level, spill over losses, return loss, bandwidth, antenna efficiency.Research work on Rotman lens antenna started way back in 1963 when W.Rotman and R.F.Turner published their research work.In this work basic design equations of Rotman lens were derived for improving scanning capability of the lens along with the reduction in beam to array port phase error. This work still remains the bench mark for researchers in this area. In this dissertation, new design equations are explored by applying Genetic algorithm so as to reduce the phase error and improve the scanning angle. By improving these parameters effort has been made to reduce the insertion loss,side lobe level, grating lobes and spill over losses for the designed Rotman lens. Various design parameters of radiating elements are also kept in view like improvement in return loss,VSWR,antenna efficiency,gain and bandwidth. Ultimate aim is integration of Microwave lens and patch radiating elements to generate a beam forming network and achive the desired goal. The classical lens design theories are all based on focal lens schemes, which presumably achieve zero phase errors for limited number of given focal beam ports, thus the non-focal ports have relatively high phase errors. The exploration of microstrip lenses and non-focal lenses leads to a method for improving scanning angle of microwave lenses. Because of the constraints on focal equations, the existing lens theory can only design an asymmetric contour lens, which results of a maximum scanning angle of 60 or 90 degrees. The concept of non focal lens design is chosen in this dissertation allowing using interleaving beam and receiving ports to reoccupy a symmetric lens contour, scanning an azimuth region of 360 degree. It possesses most of the classical lens. Both simulation and measurement of the prototype lens have demonstrated very good results.

The following chapters of the dissertation enumerate the design , analysis and optimization of microwave lens and linear array of microstrip patch antenna which acts as the radiating elements. Chapter 2 gives the details of soft computing techniques used,.Chapter 3 reviews the history, applications and design of microstrip patch antenna.Chapter 4 covers the detailed analysis of Rotman lens antenna and the improved

non-focal lens design scheme. The comparison with existing design methods are

Investigated by numerical simulations. Both simulation and measurement data are used in the analysis of Chapter 4. Chapter 5 describes the integration of Rotman lens and MPA. Chapter 6 describes the prototype designed, fabricated, and tested, and both the simulation and the measurement used to prove the concepts. Finally, the dissertation is closed by conclusions and future perspectives in Chapter 7

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