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Note for Wireless Communication - WC By Naresh B

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Introduction to Mobile Radio Propagation and Characterization of Frequency Bands by Siamak Sorooshyari Course: Wireless Communication Technologies 16:332:559 Lecture #1 Instructor: Dr. Narayan Mandayam I. INTRODUCTION The term wireless communication refers to transfer of information via electromagnetic or acoustic waves over atmospheric space rather than along a cable. The apparent wrinkle between such a scheme and conventional wired systems is the presence of the wireless channel as the medium over which the communication must take place. Unfortunately, more often than not, this medium is hostile in regards to attenuating, delaying, and even completely distorting the transmitted signal. Thus when considering a general digital wireless communication system such as that in Fig. 1, the design of each building block will be dependent on the channel between transmitter and receiver. Therefore, before moving on to specific issues such as modulation, source/channel coding, synchronization, equalization, multi-access analysis, and radio resource management; it makes sense to analyze and appreciate one of the main obstacles that such techniques are trying to account for. For the purpose of this course, we will focus our attention on the more specific digital wireless communication system shown in Fig. 2 for terrestrial communication. Thus, for the particular case at hand, we will assume that the remaining blocks in Fig. 1 have either been taken care of or are not being used. II. ATMOSPHERIC EFFECTS ON MOBILE RADIO PROPAGATION The wireless medium introduces difficulties for communication by its inherent nature. The atmospheric medium most relevant to terrestrial radio propagation may be specified as that of Fig. 3. The troposphere is the first layer above the surface of the earth, and contains approximately half of the earth’s atmosphere. This is the layer at which weather takes place. The ionosphere is where ions and electrons exist in sufficient quantities to reflect and/or refract the electromagnetic radio waves. For our specified model, it suffices to consider two types of electromagnetic waves: ground waves and sky waves. The ground wave is the portion of the transmitted signal that propagates along the contour of the earth. Understandably, such waves are directly affected by the earth’s terrain. Ground waves are the dominant mode of propagation for frequencies below 2 MHz. As frequency increases the sky wave separates from the sky wave, enabling long distance communication. More specifically, the sky wave propagates in space and returns to the earth via reflection in either the ionosphere or the troposphere, thereby enabling beyond the horizon communication through successive reflection. It is interesting to note that above 30 MHz the sky wave propagates in a straight line, and actually propagates through the ionosphere. This property is taken advantage of for satellite communication. 1

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Frequency Band Frequency Range (Wavelength) ELF (Extremely Low Frequency) Less than 3 KHz λ 100 km VLF (Very Low Frequency) 3-30 KHz 10 km λ 100 km LF (Low Frequency) 30-300 KHz 1 km λ 10 km MF (Medium Frequency) 300 KHz-3 MHz 100 m λ 1 km HF (High Frequency) 3-30 MHz 10 m λ 100 m VHF 30-300 MHz 1 m λ 10 m (Very High Frequency) 30-60 MHz 5 m λ 10 m UHF (Ultra High Frequency) 300 MHz-3 GHz 10 cm λ 1 m SHF (Super High Frequency) 3 GHz-30 GHz 1 cm λ 10 cm EHF (Extremely High Frequency) 30 GHz-300 GHz 1 mm λ 10 mm                 Propagation Modes  Ground wave  Earth-Ionosphere guided   Ground wave Ground/sky wave for short/long distances. Sky wave, but limited, shortdistance ground wave also. Space wave Space wave        Space wave   Space wave Space wave Table 1: Radio Frequency Allocations. III. CHARACTERIZATION OF FREQUENCY BANDS Due to dissimilar propagation properties of different frequencies traveling over the ionosphere and troposphere, it is logical to assign separate spectrum allocations to different applications. For example, for commercial cellular systems, small antenna size is a premium. This brings about the necessity of using radio waves with small wavelengths and hence high frequencies. Table 1 gives a brief picture of frequency spectrum classifications, below the specifications and applications are discussed:  ELF Extremely Low Frequency : Radio wave propagates between surface of earth and Ionosphere, also penetrating deep into water and ground. Experiences low attenuation, and high atmospheric noise level. The very large wavelength of radio waves requires implementation of large antennas. Applications : Worldwide military and submarine communication. VLF Very Low Frequency : Similar to ELF, slightly less reliable. Applications : Communication underwater and in mines, SONAR. LF Low Frequency : Sky wave can be separated from ground wave for frequencies above 100 KHz. Logically, the ground wave has a larger transmission loss. Absorption in daytime. Applications : Long-range navigation and marine communication, radio beacons. MF Medium Frequency : Sky wave separates from ground wave. Ground wave gives usable signal strength up to    2

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approximately 100 km from the transmitter. Applications : Maritime radio, direction finding, and AM radio broadcast (550-1600 KHz). HF High Frequency : Sky wave is the main mode of propagation. Ground wave used for communication over shorter distances. Applications : Amature radio (HAM), international broadcasting, long distance aircraft and ship communication, citizen band (CB) radios. VHF Very High Frequency : Diffraction and reflection give rise to propagation beyond the horizon. Propagation at large distance, propagates well within buildings. Applications : FM Radio (88-108 MHz), Broadcast TV, radio beacons for air traffic, AM aircraft communication. UHF Ultra High Frequency : Reflection atmospheric layers, losses due to obstacles larger than those encountered in VHF band. Effect of rain and moisture negligible. Applications : GPS, microwave links, wireless personal communication systems: Cellular, PCS, 3G, unlicensed band communication: Bluetooth, 802.11b, LMDS (500 Mbps). SHF Super High Frequency : Propagation distances become limited due to absorption by atmosphere (i.e. rain, clouds). Applications : Satellite services for telephony and TV, LEO and GEO satellite systems, possible future mobile communication services, MMDS (1 Gbps), UNII band (300 MHz @ 5 GHz) communication: 802.11a, Home RF. EHF Extremely High Frequency : Basically all particles become obstacles due to very small wavelengths. Absorption effects greatly limit range/distance. High losses due to water, vapor, oxygen in atmosphere. Applications : Short-distance communication (LOS required), currently being proposed for HDTV, satellite communication.      3

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Large-Scale Mobile Radio Propagation and Path Loss Models for Macrocells by Siamak Sorooshyari Course: Wireless Communication Technologies 16:332:559 Lecture #2 Instructor: Dr. Narayan Mandayam I. INTRODUCTION TO LARGE-SCALE PROPAGATION The general term f ading is used to describe fluctuations in the envelope of a transmitted radio signal. However, when speaking of such fluctuations, one must consider whether a short observation interval (or small distance) has been taken, or whether a long observation interval (or large distance) has been taken. For a wireless channel, the former case will show rapid fluctuations in the signal’s envelope, while the latter will give more of an slowly varying, averaged view. For this reason the first scenario is formally called small-scale f ading (or multipath), while the second scenario is referred to as large-scale path loss. In this presentation we will only focus on the large-scale effect. Received power or its reciprocal, path loss, is generally the most important parameter predicted by largescale propagation models. It is valuable to examine the three main propagation mechanisms that determine and describe path loss: Re f lection occurs when a radio wave collides with an object which has very large dimensions compared to the wavelength of the propagating wave. Reflections are very commonly caused by the surface of the earth and from buildings, walls, and other such obstructions. Di f f raction occurs when the radio path between the transmitter/receiver pair is obstructed by a surface with sharp edges. This causes secondary waves to arise (in any conceivable direction) from the obstructing surface. There is a possibility that the secondary waves can bend around the obstacle and provide an almost artificial LOS between transmitter and receiver. Like reflection, this phenomenon is dependent on: frequency, amplitude, phase, and the angle of arrival of the incident wave. Scattering occurs when the radio wave travels through a medium consisting of objects with dimensions that are small compared to the wave’s wavelength. In such a case the number of such particles per unit volume are usually very large. Typically scattered waves arise when the radio wave meets rough surfaces or small objects in the channel. II. FREE SPACE PATH LOSS MODEL To obtain a more quantitative view of the effects of path loss, it is useful to consider a few examples. The simplest case of which is the path loss model for free space due to the fact that the influence of all obstacles is ignored. Further easing the analysis, consider the model to be isotropic, where the transmitting antenna signaling with power Pt , has its power radiate uniformly in all directions. Examining the path loss will tell us the amount of power available at the receive antenna a distance r meters away. This situation can be modeled as in Fig. 4 where the transmit antenna can be considered to be at the center of a sphere with radius r. The total power density on the sphere (also referred to as f lux density) may be expressed as: Pd  EIRP 4πr2  Pt 4πr2  Watts m2 (1) where EIRP is the effective radiated power from an isotropic source and 4πr2 is the surface area of the sphere. In this model, the power at the receive antenna will be only a function of the transmitted power and the characteristics of the receive antenna: 4

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