Link Budget CalculationMarco Zennaro mzennaro@ictp.itCarlo Fondacfonda@ictp.itLink PlanningHow long can a wireless link be?How much power can we use?What kind of antennas should be use?Do the connection cables effect the link?What is the effect of trees?dB mathdecibels are a relative measurement unit unlike the absolute measurement of milliwattson the logarithmic scale, the reference cannot be zero because the log of zero does not exist!log(0) is undened!dB maththe reference point that relates the logarithmic dB scale to the linear watt scale is:1 mW = 0 dBmthe m in dBm refers to the fact that the reference is 1 mW and therefore a dBm measurement is a measurement of absolute powerdB mathto convert mW to dBm:P(dBm)= 10 log P(mW)to convert dBm to mW:P(mW)= log-1 (P(dBm)/10)the advantage of using dB is that gains and losses are ADDITIVEdB mathgain or loss in an RF system may be referred to by absolute power measurement (10W of power) or by a relative power measurement (half of its power)-3dB = half the power in mW+3dB = double the power in mW-10dB = one tenth the power in mW+10dB = ten times the power in mWdB mathfor example:10 mW + 3 dB = 20 mW100 mW - 3dB = 50 mW10 mW + 10 dB = 100 mW300 mW - 10 dB = 30 mWdB mathChapter 2 RF Fundamentals 33 (10 mW + 10dB ! 100 mW) Conversely, -10 dB will decrease the watt value to one tenth oI that value: (300 mW - 10dB ! 30 mW) These rules will allow a quick calculation oI milliwatt power levels when given power levels, gains, and losses in dBm and dB.Figure 2.14 shows that the reIerence point is always the same, but power levels can move in either direction Irom the reIerence point depending on whether they represent a power gain or loss. FIGURE 2.14Power level chart -40dBm-30dBm-20dBm-10dBm0dBm+10dBm+20dBm+30dBm+40dBm100nW1uW10uW100uW1mW10mW100mW1,000mW10,000mW-12dBm-9dBm-6dBm-3dBm-0dBm+3dBm+6dBm+9dBm+12dBm62.5uW125uW250uW500uW1mW2mW4mW8mW16mW In the top chart oI Figure 2.14, gains and losses oI 10 dB are shown at each increment.Notice that a gain oI 10 dB Irom the reIerence point oI 1 mW moves the power to 10 dBm (10 mW).Conversely, notice that a loss oI -10 dB moves the power to -10 dBm (100 microwatts).On the bottom chart, the same principal applies.These charts both represent the same thing, except that one is incremented in gains and losses oI 3 dB and the other Ior gains and losses oI 10 dB.They have been separated into two charts Ior ease oI viewing.Using these charts, one can easily convert dBm and mW power levels. ExampIes 43 dBm divided into 10's and 3's would equal 10 10 10 10 3.From the reIerence point, the charts show you that you would multiply the milliwatt value (starting at the reIerence point) times a Iactor oI 10 Iour times then times a Iactor oI 2 one time yielding the Iollowing: 1 mW x 10 = 10 mW 10 mW x 10 = 100 mW 100 mW x 10 = 1,000 mW 1,000 mW x 10 = 10,000 mW 10,000 mW x 2 = 20,000 mW = 20 watts So, we now see that 43 dBm equals 20 watts oI power.Another example that takes into dB mathfor example:+43 dBm43dB = 10dB + 10dB + 10dB + 10dB + 3dB1 mW x 10 = 10 mW10 mW x 10 = 100 mW100 mW x 10 = 1000 mW1000 mW x 10 = 10000 mW10000 mW x 2 = 20000 mW = 20 WdB mathexercise:express -26 dBm in Watts-26dB = - 10dB - 10dB - 3dB - 3dB1 mW / 10 = 100 !W100 !W / 10 = 10 !W10 !W / 2 = 5 !W5 !W / 2 = 2.5 !WdB mathwhen quantifying the gain of an antenna, the decibel units are usually represented by dBithe i stands for isotropic, which means that the change in power is referenced against an isotropic radiatoran isotropic radiator is a theoretical ideal transmitter that produces an e.m. eld in all directions with equal intensity at a 100% efciency other decibels units are also used, as for example the dBd which is a measure of the gain of an antenna system relative to a dipole antennaantenna gainan antenna element is a passive devicethe antenna can create the effect of amplication by virtue of its physical shapeby intentional radiator we mean the RF device and all cabling and connectors up to, but not including, the antennaintentional radiator28 Chapter 2 RF Fundamentals Antenna Gain An antenna elementwithout the ampliIiers and Iilters typically associated with itis a passive device.There is no conditioning, ampliIying, or manipulating oI the signal by the antenna element itselI.The antenna can create the eIIect oI ampliIication by virtue oI its physical shape.Antenna ampliIication is the result oI Iocusing the RF radiation into a tighter beam, just as the bulb oI a Ilashlight can be Iocused into a tighter beam creating a seemingly brighter light source that sends the light Iurther.The Iocusing oI the radiation is measured by way oI beamwidths, which are measured in degrees horizontal and vertical.For example, an omni-directional antenna has a 360-degree horizontal beamwidth.By limiting the 360-degree beamwidth into a more Iocused beam oI, say, 30 degrees, at the same power, the RF waves will be radiated Iurther.This is how patch, panel, and Yagi antennas (all oI which are semi-directional antennas) are designed.Highly directional antennas take this theory a step Iurther by very tightly Iocusing both horizontal and vertical beamwidths to maximize distance oI the propagated wave at low power. IntentionaI Radiator As deIined by the Federal Communication Commission (FCC), an intentional radiator is an RF device that is speciIically designed to generate and radiate RF signals.In terms oI hardware, an intentional radiator will include the RF device and all cabling and connectors up to, but not including, the antenna, as illustrated in Figure 2.12 below. FIGURE 2.12ntentional Radiator cablecableconnectorconnectorconnectorantennaComponentsincluded in thententional RadiatorBridge Any reIerence to "power output oI the Intentional Radiator" reIers to the power output at the end oI the last cable or connector beIore the antenna.For example, consider a 30-milliwatt transmitter that loses 15 milliwatts oI power in the cable and another 5 milliwatts Irom the connector at the antenna.The power at the intentional radiator would be 10 milliwatts.As an administrator, it is your responsibility to understand the FCC intentional radiatorany reference to power output of the Intentional Radiator refers to the power output at the end of the last cable or connector before the antennafor example:if a 30 mW transmitter loses 15 mW of power in the cables and another 5 mW in the connectors, the power of the intentional radiator is 10 mWintentional radiator Chapter 2 RF Fundamentals 35 FIGURE 2.15Sample wireless LAN configuration cablecableconnectorconnectorconnectorantennaAccess pointPoint A Point BPoint C Point D Access Point Point A Point B Point C Point D 100 mW-3 dB -3 dB -3 dB+12 dBi =100 mW!2 !2 !2(x2 x2 x2 x2) =100 mW!2 !2 !2x16 =50 mW !2 !2x16 =25 mW !2x16 =12.5 mWx16 =200 mW Accurate Measurements Although these techniques are helpIul and expedient in some situations, there are times when rounded or even numbers may not be available.In these times, using the Iormula is the best method oI doing RF calculations.Since the decibel is a unit oI relative power measurement, a change in power level is implied.II the power level is given in dBm, then change in dB is simple to calculate: nitial power = 20 dBm Final power = 33 dBm Change in power, "P = 33 20 = +13 dB, the value is positive, indicating an increase in power. II the power levels are given in milliwatts, the process can become more complicated: nitial power = 130 mW Final power = 5.2 W Change in power, EIRPthe EIRP is the Equivalent Isotropically Radiated PowerEIRP is the power actually radiated by the antenna element and is important becauseit is regulated by the FCC or other regulatory agencyit is used in calculating whether or not a wireless link is viableEIRPthe EIRP takes into account the gain of the antennaChapter 2 RF Fundamentals 29 rules relating to Intentional Radiators and their power output.Understanding how power output is measured, how much power is allowed, and how to calculate these values are all covered in this book.FCC regulations concerning output power at the Intentional Radiator and EIRP are Iound in Part 47 CFR, Chapter 1, Section 15.247 dated October 1, 2000. EquivaIent IsotropicaIIy Radiated Power (EIRP) EIRP is the power actually radiated by the antenna element, as shown in Figure 2.13.This concept is important because it is regulated by the FCC and because it is used in calculating whether or not a wireless link is viable.EIRP takes into account the gain oI the antenna. FIGURE 2.13ERP cablecableconnectorconnectorconnectorantennaBridgeERP(output power)RF Beam Suppose a transmitting station uses a 10-dBi antenna (which ampliIies the signal 10-Iold) and is Ied by 100 milliwatts Irom the intentional radiator.The EIRP is 1000 mW, or 1 Watt.The FCC has rules deIining both the power output at the intentional radiator and the antenna element. ! Failure to comply with FCC rules regarding power output can subject the administrator or the organization (or both) to legal action and fines. Radio Frequency Mathematics There are Iour important areas oI power calculation in a wireless LAN.These areas are: ! Power at the transmitting device ! Loss and gain oI connectivity devices between the transmitting device and the antenna - such as cables, connectors, ampliIiers, attenuators, and splitters ! Power at the last connector beIore the RF signal enters the antenna (Intentional Radiator) EIRPsuppose a transmitting station uses a 10 dBi antenna (which amplies the signal 10 times) and is fed by 100 mW from the intentional radiatorthe EIRP is 1000 mW or 1 Wthe FCC has rules dening both the power output at the intentional radiator and the antenna elementEIRPthe FCC enforces certain rules regarding the power radiated by the antenna element, depending on whether the implementation is point-to-multipoint (PtMP) or point-to-point (PtP)PtMP links are typically congured in a star topologywhen an omnidirectional antenna is used, the FCC automatically considers the link a PtMP linkPtMPin the setup of a point-to-multipoint link, the FCC limits the EIRP to 4 Wattsthe power limit set for the intentional radiator is 1 Wattis the transmitting LAN devices are adjustable with respect to their output power, then the system can customized to the needs of the userPtMPfor example:a radio transmitting at 1 Watt(+30 dBm) is connected directly to a 12 dBi omnidirectional antennathe total output power at the antenna is about 16 Watts, which is well above the 4 Watts limitthe FCC stipulates that for each 3dBi above the antennas initial 6dBi of gain, the power of the intentional radiator must be reduced by 3dB below the initial +30dBmPtMPfor our example, since the antenna gain is 12dBi, the power of the intentional radiator must be reduced by 6dBthis reduction will result in an intentional radiator power of 24 dBm, or 36 dM of EIRP which is 4 Wattsthe end result is that the power at the intentional radiator must never be more than 1 Watt, and the EIRP must never be above 4 Watts for a PtMP connectionPtMP158 Chapter 6 Wireless LAN Organizations and Standards GHz ISM band and upper 5 GHz UNII band.Furthermore, the power limit set Ior the intentional radiator (the device transmitting the RF signal) in each oI these bands is 1 Watt.II the transmitting wireless LAN devices are adjustable with respect to their output power, then the system can be customized to the needs oI the user. Suppose a radio transmitting at 1 Watt (30 dBm) is connected directly to a 12 dBi omnidirectional antenna.The total output power at the antenna is about 16 Watts, which is well above the 4 Watt limit.The FCC stipulates that for each 3 dBi above the antennas initial 6 dBi of gain, the power at the intentional radiator must be reduced bv 3 dB below the initial 30 dBm.For our example, since the antenna gain is 12 dBi, the power at the intentional radiator must be reduced by 6 dB.This reduction will result in an intentional radiator power oI 24 dBm (30 dBm6 dB), or 250 mW and an EIRP oI 36 dBm (24 dBm 12 dBi), or 4 Watts.Clearly this rule can become conIusing, but the end result must be that the power at the intentional radiator must never be more than 1 Watt (see Figure 6.2), and the EIRP must never be above 4 Watts Ior a PtMP connection. FIGURE 6.2Point-to-Multipoint Power Limit Table Power at Antenna (dBm) Antenna Gain (dBi) EIRP (dBm) EIRP (watts) 306364 279364 2412364 2115364 1818364 1521364 1224364 The specific information contained in Figure 6.2 is not covered on the CWNA exam.The information is provided as a resource for your administrative tasks. When using an omnidirectional antenna, the rules Ior point-to-multipoint links must be Iollowed, regardless oI whether the actual implementation is point-to-point or point-to-multipoint. Point-to-Point (PtP) PtP links include a single directional transmitting antenna and a single directional receiving antenna.These connections will typically include building-to-building or similar links and must abide by special rules.When installing a 2.4 GHz PtP link, the 4 Watt power limit all but disappears in Iavor oI a sliding power limit.Regarding a PtP link, the FCC mandates that for everv 3 dBi above the initial 6 dBi of antenna gain, the power at the intentional radiator must be reduced bv 1 dB from the initial 30 dBm. Consider our previous example, using the same values:1 Watt (30 dBm) at the intentional radiator and a 12 dBi antenna (in this case the antenna will be a directional antenna).The total output power is still 16 Watts.In this example, since the antenna PtPpoint-to-point links include a single directional transmitting antenna and a single directional receiving antennathe FCC mandates that for every 3 dBi above the initial 6 dBi of antenna gain, the power at the intentional radiator must be reduced by 1 dB from the initial +30 dBmconsider the previous example: 1W (+30 dBm) at the intentional radiator and a 12 dBi antenna (directional antenna in this case)PtPthe total output power is still 16 Wattssince the antenna gain is 12 dBi, the power at the intentional radiator must be reduced by 2 dBthis reduction will result in an intentional radiator power of 28 dBm (30-2), and an EIRP of 40 dBm or 10 Wattsin the case of PtP links, the power of the intentional radiator is still limited to 1 Watt, but the limit of the EIRP increases with the gain of the antennaPtPChapter 6 Wireless LAN Organizations and Standards 159gain is 12 dBi, the power at the intentional radiator must be reduced by 2 dB, as opposed to a 6 dB reduction in the previous example.This reduction will result in an intentional radiator power oI 28 dBm (30 dBm2 dB), or about 630 mW and an EIRP oI 40 dBm (28 dBm 12 dBi), or 10 Watts.In the case oI PtP links, the power at the intentional radiator is still limited to 1 Watt, but the limit oI the EIRP !"#$%&'%' with the gain oI the antenna (Figure 6.3).It is very important to clearly distinguish between the rules that govern PtP and PtMP wireless links. FIGURE 6.3Point-to-Point Power Limit Table Power at Antenna (dBm) Max Antenna Gain (dBi) EIRP (dBm) EIRP (watts) 306364 299386.3 28124010 27154216 26184425 25214639.8 24244863 232750100 223052158 The specific information contained in Figure 6.3 is not covered on the CWNA exam.The information is provided as a resource for your administrative tasks. The FCC has a diIIerent rule Ior PtP links in the upper UNII band.Fixed point-to-point UNII devices operating in the 5.725 - 5.825 GHz band may employ transmitting antennas with directional gain up to 23 dBi without any corresponding reduction in the transmitter peak output power.For Iixed, point-to-point UNII transmitters that employ a directional antenna gain greater than 23 dBi, a 1 dB reduction in peak transmitter power Ior each 1 dBi oI antenna gain in excess oI 23 dBi is required.Notice that by having an output power maximum oI 30 dBm at the intentional radiator, and having a maximum oI 23 dBi antenna gain beIore any reduction in transmitter output power is required, this allows these 5 GHz UNII systems to have an output oI 200 Watts EIRP. nstitute of Electrical and Electronics Engineers The Institute oI Electrical and Electronics Engineers (()))) is the key standards maker Ior most things related to inIormation technology in the United States.The IEEE creates its standards within the laws created by the FCC.The IEEE speciIies many technology standards such as Public Key Cryptography (IEEE 1363), FireWire (IEEE 1394), Ethernet (IEEE 802.3), and Wireless LANs (IEEE 802.11). The website for the EEE is www.ieee.org Fresnel zonethe Fresnel Zone occupies a series of concentric ellipsoid-shaped areas around the Line-of-Sight pathRF LOS is different that visual LOS visual LOS is present when one can stand next one antenna and use binoculars to view the other antennaRF LOS requires not only a visual sight line between the antennas but it also requires that a football shaped area between the two antennas be free of obstructionsFresnel zonethe fresnel zone is an area that is larger in diameter at the center and smaller in diameter at either end.Also, the greater the distance between the antennas, the larger the diameter of the fresnel zone in the center. the Fresnel Zone is important to the integrity of the RF link because it denes an area around the LOS that can introduce RF signal interference if blockedobjects in the Fresnel Zone as trees, hilltops and buildings can diffract or reect the main signal away from the receiverFresnel zoneChapter 2 RF Fundamentals 27 FIGURE 2.11Fresnel Zone Fresnel Zone The radius oI the Fresnel Zone at its widest point can be calculated by the Iollowing Iormula, !"#43 . 43 ! " where ! is the link distance in miles, " is the Irequency in GHz, and the answer, #, is in Ieet.For example, suppose there is a 2.4000 GHz link 5 miles (8.35 km) in length.The resulting Fresnel Zone would have a radius oI 31.25 Ieet (9.52 meters). Fresnel Zone calculations are not part of the CWNA exam.The formula is provided to you for your administrative tasks. Obstructions Considering the importance oI Fresnel Zone clearance, it is also important to quantiIy the degree to which the Fresnel Zone can be blocked.Since an RF signal, when partially blocked, will bend around the obstacle to some degree, some blockage oI the Fresnel Zone can occur without signiIicant link disruption.Typically, 20 - 40 Fresnel Zone blockage introduces little to no interIerence into the link.It is always suggested to err to the conservative side allowing no more than 20 blockage oI the Fresnel Zone.Obviously, iI trees or other growing objects are the source oI the blockage, you might want to consider designing the link based on 0 blockage. II the Fresnel Zone oI a proposed RF link is more than 20 blocked, or iI an active link becomes blocked by new construction or tree growth, raising the height oI the antennas will usually alleviate the problem. A question commonly asked about the Fresnel Zone when using indoor wireless LAN equipment such as PC cards and access points is how blockage oI the Fresnel Zone aIIects indoor installations.In most indoor installations, RF signals pass through, reIlect oII, and reIract around walls, Iurniture, and other obstructions.The Fresnel Zone is not encroached upon unless the signal is partially or Iully blocked.This is sometimes the case, but is rarely noticed due to most wireless users being mobile.In a mobile environment, the Fresnel Zone is constantly changing so the user normally dismisses it thinking that the coverage is simply "bad" where they are located - giving no thought to why the coverage is bad. Fresnel zonethe radius of the Fresnel Zone at its widest point can be calculated asr=43.3 x sqrt(d/4f)where d is the link distance in miles, f is the frequency in GHz and the answer r is in feetr=17.3 x sqrt(d/4f)where d is the link distance in kilometers, f is the frequency in GHz and the answer r is in metersat 2.4 GHz, for a distance of 10 km, the Fresnel Zone has a radius of 17.6 metersFresnel zoneconsidering the importance of the Fresnel Zone, it is important to quantify the degree to which it can be blockedtypically, 20% to 40% Fresnel Zone blockage introduces little to no interference into the linkit is better to err to the conservative side, allowing no more than 20% blockage of the Fresnel Zonehttp://gbppr.dyndns.org:8080/fresnel.main.cgiPropagation Basicssignal power is diminished by geometric spreading of the wavefront, commonly known as Free Space Losssignal power is attenuated as the wave passes through solid objects as trees, walls, windows and oors of buildingsthe signal is scattered and can interfere if there are objects in the beam of the transmit antenna even if these objects are not in the direct path between the transmitter and the receiverFree Space Lossgeometric spreading happens because the wavefront radiated signals energy expands as a function of the distance from the transmitterusing decibels to express the loss and using 2.45 GHz as the signal frequency, the equation for the Free Space Loss isLfsl = 40 + 20*log(r)where Lfsl is expressed in dB and r in metersAttenuationwhen the RF signal passes through solid objects, some of the signal power is absorbedthe most convenient way to express this is by adding an allowed loss to the Free Spaceattenuation can vary greatly depending upon the structure of the object the signal is passing throughmetal in the barrier greatly increases the attenuationAttenuationtrees account to 10 to 20 dB of loss per tree in the direct path. Loss depends upon the size and the type of treewalls account to 10 to 15 dB depending upon the constructionoors of buildings account 12 to 27 dB of loss. Floors with concrete and steel are a the high end and wood oors at the low endmirrored walls have very high loss because the reective coating is conductiveScatteringRF signals can reect off of many things and the direct signal combines with signals that have reected off of objects that are not in the direct paththis effect is usually described as multipath, fading or signal dispersion236 Chapter 9 Troubleshooting Wireless LAN nstallationsJust as traditional wired networks have challenges during implementation, wireless LANs have their own set oI challenges, mainly dealing with the behavior oI RF signals.In this chapter, we will discuss the more common obstacles to successIul implementation oI a wireless LAN, and how to troubleshoot them.There are diIIerent methods oI discovering when these challenges exist, and each oI the challenges discussed has its remedies and workarounds. The challenges to implementing any wireless LAN discussed herein are considered by many to be 'textbook problems that can occur within any wireless LAN installation, and, thereIore, can be avoided by careIul planning and simply being aware that these problems can and will occur. Multipath II you will recall Irom Chapter 2, RF Fundamentals, there are two types oI line oI sight (LOS).First, there is visual LOS, which is what the human eye sees.Visual LOS is your Iirst and most basic LOS test.II you can see the RF receiver Irom the installation point oI the RF transmitter, then you have visual line oI sight.Second, and diIIerent Irom visual LOS, is RF line oI sight.RF LOS is what your RF device can 'see. The general behavior oI an RF signal is to grow wider as it is transmitted Iarther.Because oI this type oI behavior, the RF signal will encounter objects in its path that will reIlect, diIIract, or otherwise interIere with the signal.When an RF wave is reIlected oII an object (water, tin rooI, other metal object, etc.) while moving towards its receiver, multiple wave Ironts are created (one Ior each reIlection point).There are now waves moving in many directions, and many oI these reIlected waves are still headed toward the receiver.This behavior is where we get the term multipath, as shown in Figure 9.1.Multipath is deIined as the composition oI a primary signal plus duplicate or echoed wave Ironts caused by reIlections oI waves oII objects between the transmitter and receiver.The delay between the instant that the main signal arrives and the instant that the last reIlected signal arrives is known as delav spread. FIGURE 9.1Multipath Line of SightReflected PathRiver Scatteringthe distortion degrades the ability of the receiver to recover the signal in a manner much like signal lossmultipath can cause several different conditions, all of which can affect the transmission of the RF signal differentlyDecreased SignalAmplitudeChapter 9 Troubleshooting Wireless LAN nstallations 237 Effects of MuItipath Multipath can cause several diIIerent conditions, all oI which can aIIect the transmission oI the RF signal diIIerently.These conditions include: ! Decreased Signal Amplitude (downIade) ! Corruption ! Nulling ! Increased Signal Amplitude (upIade) Decreased SignaI AmpIitude When an RF wave arrives at the receiver, many reIlected waves may arrive at the same time Irom diIIerent directions.The combination oI these waves' amplitudes is additive to the main RF wave.ReIlected waves, iI out-oI-phase with the main wave, can cause decreased signal amplitude at the receiver, as illustrated in Figure 9.2.This occurrence is commonly reIerred to as !"#$%&!' and should be taken into consideration when conducting a sight survey and selecting appropriate antennas. FIGURE 9.2Downfade Amplitude decreasedue to Path LossOut-of-phase reflectedsignal is added to themain signalTower withan antenna Corruption Corrupted signals (waves) due to multipath can occur as a result oI the same phenomena that cause decreased amplitude, but to a greater degree.When reIlected waves arrive at the receiver out-oI-phase with the main wave, as illustrated in Figure 9.3, they can cause the wave to be greatly reduced in amplitude instead oI only slightly reduced.The amplitude reduction is such that the receiver is sensitive enough to detect most oI the inIormation being carried on the wave, but not all. Corruption238 Chapter 9 Troubleshooting Wireless LAN nstallationsFIGURE 9.3RF Signal Corruption Resultant receivedsignal is corrupted byreflected signalsReflective surfaceReflective surface In such cases, the signal to noise ratio (SNR) is generally very low, where the signal itselI is very close to the noise Iloor.The receiver is unable to clearly decipher between the inIormation signal and noise, causing the data that is received to be only part (iI any) oI the transmitted data.This corruption oI data will require the transmitter to resend the data, increasing overhead and decreasing throughput in the wireless LAN. NuIIing The condition known as nulling occurs when one or more reIlected waves arrive at the receiver out-oI-phase with the main wave with such amplitude that the main wave's amplitude is cancelled.As illustrated in Figure 9.4, when reIlected waves arrive out-oI-phase with the main wave at the receiver, the condition can cancel or 'null the entire set oI RF waves, including the main wave. NullingChapter 9 Troubleshooting Wireless LAN nstallations 239 FIGURE 9.4RF Signal Nulling Reflected signals added to themain signal cancel all signalamplitude resulting in nosignal at all.Reflective surfaceReflective surface When nulling occurs, retransmission oI the data will not solve the problem.The transmitter, receiver, or reIlective objects must be moved.Sometimes more than one oI these must be relocated to compensate Ior the nulling eIIects on the RF wave. Increased SignaI AmpIitude Multipath conditions can also cause a signal`s amplitude to be increased Irom what it would have been without reIlected waves present.Upfade is the term used to describe when multipath causes an RF signal to gain strength.UpIade, as illustrated in Figure 9.5, occurs due to reIlected signals arriving at the receiver in-phase with the main signal.Similar to a decreased signal, all oI these waves are additive to the main signal.Under no circumstance can multipath cause the signal that reaches the receiver to be stronger than the transmitted signal when the signal left the transmitting device.II multipath occurs in such a way as to be additive to the main signal, the total signal that reaches the receiver will be stronger than the signal would have otherwise been without multipath present. Increased Signal Amplitude240 Chapter 9 Troubleshooting Wireless LAN nstallationsFIGURE 9.5Upfade Amplitude decreasedue to Path Lossn-phase reflectedsignal is added tothe main signalTower withan antenna It is important to understand that a received RF signal can never be as large as the signal that was transmitted due to the signiIicance oI Iree space path loss (usually called !"#$ &'(().Path loss is the eIIect oI a signal losing amplitude due to expansion as the signal travels through open space. Think oI path loss as someone blowing a bubble with bubble gum.As the gum expands, the gum at any point becomes thinner.II someone were to reach out and grab a 1-inch square piece oI this bubble, the amount oI gum they would actually get would be less and less as the bubble expanded.II a person grabbed a piece oI the bubble while it was still small (close to the person's mouth, which is the transmitter) the person would get a signiIicant amount oI gum.II the person waited to get that same size piece until the bubble were large (Iurther Irom the transmitter), the piece would be only a very small amount oI gum.This illustration shows that path loss is aIIected by two Iactors:Iirst, the distance between transmitter and receiver, and second, the size oI the receiving aperture (the size oI the piece oI gum that was grabbed). TroubIeshooting MuItipath An in-phase or out-oI-phase RF wave cannot be seen, so we must look Ior the eIIects oI multipath in order to detect its occurrence.When doing a link budget calculation, in order to Iind out just how much power output you will need to have a successIul link between sites, you might calculate an output power level that should work, but doesn't.Such an occurrence is one way to determine that multipath is occurring. Another common method oI Iinding multipath is to look Ior RF coverage holes in a site survey (discussed in Chapter 11).These holes are created both by lack oI coverage and by multipath reIlections that cancel the main signal.Understanding the sources oI multipath is crucial to eliminating its eIIects. Multipath is caused by reIlected RF waves, so obstacles that more easily reIlect RF waves, such as metal blinds, bodies oI water, and metal rooIs, should be removed Irom or avoided in the signal path iI possible.This procedure may include moving the transmitting and receiving antennas.Multipath is likely the most common "textbook" wireless LAN problem.Administrators and installers deal with multipath daily.Even Scatteringantenna diversity was devised for the purpose of compensating the multipathantenna diversity means using multiple antennas in order to compensate the conditions that cause multipathantenna switching diversity: multiple antennas on multiple inputs, with a single receiver. The signal is received through only one antenna at a timetransmission diversity: transmits out of the antenna last used for reception. Can alternate antennas for retries Scatteringa simple way of applying the effects of scattering is to change the exponent on the distance factor of the Free Space Loss formula like this:L(dB) = 40 + 10 * n * log(r)the exponent tends to increase with the range in an environment with a lot of scatteringcalculating a range can often require some iteration of the exponent to be used Total Losswhen Free Space Loss, Attenuator and Scattering are combined, the loss isL(dB) = 40 +10*n*log(r) + LallowedLink Marginthe performance of any communication link depends on the quality of the equipment being usedthe link margin is a way of quantifying equipment performancean 802.11 link has an available link margin that is determined by four factorsLink MarginTransmit PowerTransmit Antenna GainReceive Antenna GainCable Losses, Connector Losses and Lightning Arrestor LossesMinimum Received Signal Strength or Levelthe link margin is thenTX(power)+TX(ant gain)+RX(ant gain)-RSLLink Marginthe link factors are usually listed in the manufacturers data sheets for the equipment being used note that the minimum RSL is dependent upon rate and the 1 Mbps rate is used for maximum rangeTX power can also be rate dependent but manufacturers rarely indicate thisMaximum Rangethe maximum range is achieved when the signal loss is less than the link margin we must know the equipment parameters and must estimate the allowed loss and the scattering exponent to complete the calculationthe equipment parameters can be found on the data sheetshttp://www.qsl.net/n9zia/Maximum RangeRF Propagation Basics! 2004 Sputnik, Inc. All rights reserved. 3Link Margin. In addition to environmental factors described above, the performanceof any communication link depends on the quality of the equipment being used. Linkmargin is a way of quantifying equipment performance. An 802.11 communicationlink has an available link margin that is determined by four factors:!Transmit power!Transmit antenna gain!Receive antenna gain!Minimum received signal strength or level.The link margin is:Lmargin = TXpower + TXant gain + RXant gain RSL Eq 5The link factors are usually listed in the manufactures data sheets for the equipmentbeing used. For instance, if Sputniks AP160 is used as an access point with an external8.5 dBi antenna to communicate with a laptop computer having a D-Link DWL-G650station card, the factors to be used are:!TX power = 13 dBm!TX antenna gain = 8.5 dBi!RX antenna gain = 0 dBi!Min RSL = -89 dBm!Link margin = 110.5 dBNote that the Min RSL is dependent upon rate and the 1 Mbps rate is used formaximum range. TX power can also be rate dependent but manufacturers rarelyindicate this.Maximum range. The maximum range is achieved when the signal loss expressed inEquation 4 is less than the link margin expressed in Equation 5. The system operatorneeds to know the equipment parameters and must estimate the allowed loss and thescattering exponent to complete the calculation. Example parameters are shown inTables 1 and 2.Table 1. Application Dependent Environment ParametersApplicationAllowed Loss(dB)ScatteringExponent ExampleOutdoor free space 0 2Outdoor, no barriers 02.5 at 200m3 at 400m3.5 > 500m MarinaOutdoor with trees 10 to 20 3 to 4 ParkOutdoor buildings 0 4 Urban cafIndoor - no barriers 0 2.5 Conference roomIndoor partitions 0 3.5 Office cubiclesIndoor walls & floors12 to 27 (floors)10 to 15 (walls) 4 to 5Condo,apartmentLink Margin2/13/04 Pietrosemoli 47!"#$% "&$% '()*+,-$Gt GrTx RxAtArFree Space LossPtPrdBmkmExamplesuppose a university wants to setup a point-to-multipoint linkExampleAccess Point:TX Power: 13 dBmTX Antenna Gain: 8.5 dBiPC Cards:RX Antenna Gain: 0 dBiRX Sensitivity: -89 dBmExamplethe link margin isTX(power)+TX(ant gain)+RX(ant gain)-RSL13+8.5+0-(-89)= 110.5 dBExamplewe can estimate the allowed loss to be 10 dB if the park had a modest number of trees mixed with open spaces and use a scattering exponent of 3to get the maximum range we solve110.5=40+10*3*log(rmax)+10and we ndrmax=102 metersExampleusing the same equation to calculate the range assuming free space loss only(exponent=2, and allowed loss=0)110.5=40+10*2*log(rmax)+0rmax=3264 metersthis example shows that environmental factors can play a signicant role in diminishing the signal strengthThank you for your attention!Contact info:Marco Zennaromzennaro@ictp.ithttp://wireless.ictp.itLink Budget Calculation