Link Budget_DataCommunication_engineering.ppt

Editor's Notes

• #1: © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws. Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners. Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039.   GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
• #2: The considerations that the RF Engineer needs to account for in their design involves all the aspects of the system as it relates to the transport of the information. The diagram above is an indication of an individual communication link. However, in wireless mobility systems there are a multitude of radio links which are active at any times and dynamically changing their complexion through the subscriber being mobile. As identified briefly in the module the RF design needs to also factor in how the management of the radio spectrum is done which is often referred to as frequency planning. Overall, the RF design of a radio system is complex and involves many disciplines involved with RF theory.
• #3: The determination of how much power is required for the system to transmit is driven by many factors. The most important issue is the design criteria that is sought. If the design calls for a 100W ERP from the site then the RF designer must determine how to achieve this goal with the physical installation. The components which typically need to be included in the determination of the transmitter power for the site are listed below. It is possible to have more or less components in the transmitter’s path than shown in the figure included in the slide. The following is a brief list of the major components involved. The important issue is that every communication site is slightly different and that all the components which exist in the generation of the sites radiated power need to be included in the calculation. Power Amplifier dBm Combing Loss (dB) Filter Loss (dB) Jumper Loss (dB) Lightening Arrestor (dB) Feedline Loss (dB) Jumper Loss (dB) Antenna gain dBd or dBi
• #5: RF Engineers utilize various models to estimate the performance of the radio wave. There are numerous models that are used and all are a derivative of the free space model where specific alterations are made to the base model accounting for frequency of operation, scattering, reflections and morphology. It is important to remember that most models have some specific set of parameters from which their modification is based. Therefore it is important to identify and understand those parameters which bound the equation or model. Each model has its own requirements that need to be met in order to be utilized correctly. Some of the typical parameters which bound the path loss model are: Morphology (environment) within a particular frequency range Base station antenna heights Mobile antenna heights Distance between the Tx and the Rx For example, the free space model estimates that the path loss is 20dB/decade, while for Hata and COST231 it is about 36 dB/decade. Also there is no range limitations for the free space equation, however for Hata the equation is valid starting 1km proceeding to 20 km from the site while for COST231 it is valid from ** to **km.
• #6: Free space is a reference for the type of path loss, or rather attenuation, a radio signal undergoes when it transverses between a transmit antenna and the receiving antenna. The value used for free space that is referenced is only dependent upon the distance that it is from the source transmitter. There is a frequency of operation component in the free space equation but this is a one time value. Free space has a value of 20 dB/decade that is used. This means that for every decade, factor of 10, a transverse 20 dB of attenuation is added to the signal. In other words, for every decade of distance that the signal goes through the air, free space attenuation means the signal decreases by 100 times.   The equation used for free space is Free Space Path Loss (dB) = 32.4 + 20 log R + 20 log f where R = km and f = MHz   The 20 dB per decade comment comes from the part of the equation which changes the value which is distance sensitive, 20logR.   The free space equation is usually the reference point for all the path loss models employed. Each propagation model points out that it more accurately predicts the attenuation experienced by the signal over that of free space.  
• #7: The Hata model is an empirical model derived from the technical report made by Okumura so the results could be used in a computational model. The Okumura report is a series of charts that are instrumental in radio communication modeling.  The Hata model is shown below.  LH = 69.55 + 26.16log10fc – 13.82log10hb – a(hm) + (44.9 – 6.55log10hb)log10R  hb is the base station antenna height in meters hm is the mobile antenna height also measured in meters R is the distance from the cell site to the mobile in km fc is the transmit frequency in MHz The range for which the Hata model is valid over is listed below. Fc = 150-1500Mhz h.b= 30 –200m h.m= 1-10m R= 1-20km   Therefore the Hata model should not be employed when trying to predict pathloss less than 1 km from the cell site or if the site is less than 30 meters in height. This is an interesting point to note since cellular sites are being placed at time less than 1 km apart and often below the 30 meter height.   In the Hata model the value hm is used to correct for the mobile antenna height. The interesting point is that if you assume a height of 1.5 meters for the mobile, that value nulls out of the equation.   The Hata model employs three correction factors based on the environmental conditions that pathloss prediction is evaluated over. The three environmental conditions are urban, suburban and open. The environmental correction values are easily calculated but vary for different values of mobile height. For the values listed below a mobile height of 1.5 meters was assumed. Urban: 0 dB Suburban: 9.88 dB Open -28.41 dB Example: What is the Hata loss at a distance of 2 km from the cell site using a frequency of 900 MHz, a base station antenna height of 50 m, and a mobile antenna height of 1.5 meters for both open and urban environments ? Answer: LH = 69.55 + 26.16log10(900) – 13.82log10(50) – 0 + (44.9 – 6.55log10(50))log10(2) = 133.52 dB for urban = 105.11 dB for open (obtained by subtracting the correction factor of 28.41 from urban)
• #8: The Hata model is an empirical model derived from the technical report made by Okumura so the results could be used in a computational model. The Okumura report is a series of charts that are instrumental in radio communication modeling.  The Hata model is shown below.  LH = 69.55 + 26.16log10fc – 13.82log10hb – a(hm) + (44.9 – 6.55log10hb)log10R  hb is the base station antenna height in meters hm is the mobile antenna height also measured in meters R is the distance from the cell site to the mobile in km fc is the transmit frequency in MHz The range for which the Hata model is valid over is listed below. Fc = 150-1500Mhz h.b= 30 –200m h.m= 1-10m R= 1-20km   Therefore the Hata model should not be employed when trying to predict pathloss less than 1 km from the cell site or if the site is less than 30 meters in height. This is an interesting point to note since cellular sites are being placed at time less than 1 km apart and often below the 30 meter height.   In the Hata model the value hm is used to correct for the mobile antenna height. The interesting point is that if you assume a height of 1.5 meters for the mobile, that value nulls out of the equation.   The Hata model employs three correction factors based on the environmental conditions that pathloss prediction is evaluated over. The three environmental conditions are urban, suburban and open. The environmental correction values are easily calculated but vary for different values of mobile height. For the values listed below a mobile height of 1.5 meters was assumed. Urban: 0 dB Suburban: 9.88 dB Open -28.41 dB Example: What is the Hata loss at a distance of 2 km from the cell site using a frequency of 900 MHz, a base station antenna height of 50 m, and a mobile antenna height of 1.5 meters for both open and urban environments ? Answer: LH = 69.55 + 26.16log10(900) – 13.82log10(50) – 0 + (44.9 – 6.55log10(50))log10(2) = 133.52 dB for urban = 105.11 dB for open (obtained by subtracting the correction factor of 28.41 from urban)
• #9: As the radio signal propagates, (travels) from the transmitter to the receiver many impairments can and due occur to the radio signal causing it to lose power. The various impairments assume that there are no physical problems with either the source or receiving equipment or system, which can add to impairments but are not propagation related. The propagation impairments are directly related to the morphology that the radio signal passes over. For instance urban, suburban and rural all are different types of morphology classifications. The occurrence of absorption, reflection, refraction, and diffraction may not always present a problem depending on the mode of communication chosen. For instance, reflection is required for radar systems. Blocking is often used to the advantage of the radio engineer for restricting the coverage of the communication site or for mitigation of interference. Refraction is often used for HF communication over the horizon to mention a few. Only when the relative amount of these effects is too great for the receiver to separate or ignore do these effects result in problems. In fact, sometimes these effects can be advantageous and allow the reception of the signal in situations or at distances that normally would be impractical.
• #10: Clutter is a term used to describe the morphology for a wireless system or an area within a wireless system. The use of clutter is essential for estimating the radio propagation. Morphology refers to the unique terrain characteristics for a given area. The unique terrain characteristics could be dense buildings or open flat terrain or rolling hills to mention a few. Morphology is used to help define what the expected radio coverage and performance will be for a given area. There are usually several types of morphology utilized, however their actual values that are assigned to each is often open to debate and much manipulation for the better or worse by RF Engineering. The types of morphology are dense urban, urban, suburban, and rural with the possibility of more morphology classes being added. Dense urban morphology refers to the dense business district for a metropolitan area. The buildings for the area generally are 10 to 20 and above stories consisting of skyscrapers and high rise apartments. Urban Morphology refers to an area where building structures that are normally 5 to 10 stories in height.  Suburban Morphology refers to an area within the network where there is a mix of residential and business with the buildings ranging from 1 to 5 stories, but mainly consisting of 1-2 story structures. Rural Morphology refers to an area within the network which consists of generally open areas with structures not exceeding 2 stories and with that being sparsely populated.    
• #11: In most urban environments as well as suburban, the issue of man-made obstructions is a leading factor in the attenuation of the radio signal. Typically if the radio signal is reflected or absorbed, it does not go where it was headed - it is obstructed. There are several types of obstructions and they are either at the cell site or near the cell site. Obstructions impede the radio signal or the installation of the antenna system. For example, a tall building next to the cell site is an example of an obstruction if the building obscures the radio path. Another form of an obstruction could be a mountain which again obscures the radio path. An installation obstruction that normally occurs is either the HVAC system, window washing apparatus or another competitors radio or antenna equipment. Obstructions can be either good or bad, depending on the circumstances: It is good for containing the signal thereby facilitating frequency reuse through interference control. It is bad primarily because it attenuates the radio signal and therefore reduces the over range of the communication site requiring more sites to be build to provide adequate signal level.
• #12: There are numerous types of natural obstructions but most can be classified as being results of the earth contours. The result of the natural obstruction is the diffraction of the radio signal as it tries to reach the receiver. The figure in the slide depicts a typical diffraction but in real life there are most likely several diffractions in addition to the inclusion of foliage and vegetation losses.
• #13: Diffraction is the description of how a radio signal propagates around and over an obstruction. Depending on the severity of the diffraction the radio signal may be so impaired as to prevent good communication from taking place. There are several types of diffraction methods modeled in RF, two of which are smooth and knife edge. As a general rule of thumb, the larger the obstruction the greater the diffraction or rather attenuation will be experienced. Diffraction is a change in the wave pattern caused by interference between waves that have been reflected from a surface or a point. The interference causes regions of wave strengthening and weakening to radiate out from the point of interference, almost like a new source point. Huygen’s principle states that all points on a wavefront can be considered as point sources for the production of secondary wavelets, which can combine to produce a new wavefront in the direction of propagation. The result is an apparent bending of the wave. This explains how sound can travel around corners, how diffraction gratings work, and how radio waves can travel around buildings. Diffraction occurs in many situations, such as when waves: Pass through a narrow slit Pass the edge of a reflector Reflect off two different surfaces that are approximately one wavelength apart. The K-factor is not the only beneficial effect of diffraction of radio waves. The edges of buildings and other such objects diffract radio waves into areas that are in blocked from direct line-of-sight to the transmit antenna. For land-based radio systems at VHF frequencies and higher, diffraction is a commonly encountered characteristic, especially when far away from the transmitter. If the shadow is caused by blockage due to particular objects such as one or more buildings, the weakest signal will be close to the buildings, with the signal strength recovering at greater distances.
• #15: The basic building blocks for a receive system are virtually the same for all the types of modulation formats chosen. The chief differentiator for all the types of receive systems lies in the demodulation portion of the receiver itself, i.e. is the receiver designed to demodulate AM,FM, PM or some variant of the three. In addition, the receiver system must be such that it will be able to detect and understand the information that it is trying to extract. It must also avoid picking up and attempting to demodulate other signals that exist in the spectrum. There are numerous factors involved with the receiver system but the main issues are: Antenna System Feedline length Frequency of Operation Sensitivity Intermodulation There are of course others but the above list covers most of the important issues. Antenna The antenna system is the first stage in the receive path for the receiver. The antennas purpose is to de-couple the electromagnetic energy from the atmosphere and transfer this to the feedline for the communication site. Feedline The feedline is the physical device which connects the antenna to the rest of the receive system to the propagation medium that is used to transport the information. The feedline is comprised of cable and associated jumpers that normally connect the antenna to the receive filters. The feedline is an important element in the receive system and has a direct role in determining how well a receive system will operate Filter The filter or filters utilized for a communication system are a key component of a communication system. There are normally several filters employed in a receive path for a communication system. Specifically the filter should pass only the frequencies of interest, reject all the other frequencies and do so with no loss in amplitude for the desired signals. This is not practical to implement when space constraints and cost enter into the decision matrix. However, the filter that is used in the communication receive path has a large role in determining just how well the receiver will ultimately perform. Preamplifier The preamplifier is usually the first active component in a communication systems receive path. The basic function of any RF pre-amplifier is to increase the signal to noise ratio of the received signal. The preamplifier receivers the desired signal at the lowest level in any of the receive stages for the communication site. Since the RF pre-amp receives the desired signal at the lowest level of any receive stage in the cell sites receive path, any noise or other disturbances introduced in this stage has a proportionally greater effect.   The performance of the cell sites receiver with respect to weak signals depends on the performance of the preamplifier or rather the signal to noise ratio of its output. The key issue is that Amplifiers do not discriminate between what is the signal and what is noise or interference within the amplifiers pass band. In fact, the preamplifier will amplify the desired signal plus any noise equally. Demodulation Demodulation is where the radio receiver demodulates the radio signal and extracts the information whether it is analog or digital in nature. The information stage is simply the output of the radio receiver system where the desired information is delivered to the intended target.
• #16: Sensitivity is the ability for a receiver to detect a weak signal and the sensitivity value in dBm or dBv is determined by the exact signal strength level that the receiver can detect the signal. Specifically there is a relationship between thermal noise, the receivers noise figure and the bandwidth of the signal that the receiver is trying to detect. The relationship for receiver sensitivity is defined below.   Sensitivity = 10log(kT) + 10log(bandwidth(Hz))+NF (dB) = -174 dBm/Hz + 10 Log(bandwidth(Hz) + NF (dB) k= Boltzman constant = 1.38X10-23 J/K T = temperature in K B = Bandwidth (Hz)  
• #17: Sensitivity is the ability for a receiver to detect a weak signal and the sensitivity value in dBm or dBv is determined by the exact signal strength level that the receiver can detect the signal. Specifically there is a relationship between thermal noise, the receivers noise figure and the bandwidth of the signal that the receiver is trying to detect. The relationship for receiver sensitivity is defined below.   Sensitivity = 10log(kT) + 10log(bandwidth(Hz))+NF (dB) = -174 dBm/Hz + 10 Log(bandwidth(Hz) + NF (dB) k= Boltzman constant = 1.38X10-23 J/K T = temperature in K B = Bandwidth (Hz)  
• #18: Sensitivity is the ability for a receiver to detect a weak signal and the sensitivity value in dBm or dBv is determined by the exact signal strength level that the receiver can detect the signal. Specifically there is a relationship between thermal noise, the receivers noise figure and the bandwidth of the signal that the receiver is trying to detect. The relationship for receiver sensitivity is defined below.   Sensitivity = 10log(kT) + 10log(bandwidth(Hz))+NF (dB) = -174 dBm/Hz + 10 Log(bandwidth(Hz) + NF (dB) k= Boltzman constant = 1.38X10-23 J/K T = temperature in K B = Bandwidth (Hz)