Wireless Networking COMP-10021 RF Measurements and Mathematics Fall 2011 Polarization * * Electromagnetic Fields RF waves are made up of two energy fields: Electric Magnetic Electric and magnetic fields run perpendicular to each other as they travel away from their point of origin Sum of the two fields is called the electro-magnetic field Energy oscillates back and forth between these fields as the wave moves * E-Field & H-Field Electrical field (called the E-field) runs parallel to the antenna element and is the electric part of the wave Magnetic field (called the H-field) runs perpendicular to the antenna element and is the magnetic part of the wave The E-field is the more important because it establishes the wave’s polarization * E-plane & H-plane E-field is generated parallel to the antenna element and this plane is referred to as the E-plane H-field is generated perpendicular to the antenna element and this plane is referred to as the H-plane Think “H” for horizontal to the ground Most dipole antennas run vertically so, in such cases, the magnetic field (H-field) runs horizontally relative to the earth’s surface E-plane H-plane * Polarization Polarization is the physical orientation of antenna in either a horizontal or vertical position Since the electric field (E-plane) is parallel to the antenna’s radiating element (i.e. the metal part radiating RF waves), polarization is controlled by the antenna position If antenna is vertical, polarization is vertical If antenna is horizontal, polarization is horizontal * Horizontal & Vertical Polarization Vertical Polarization Electric field is perpendicular to the ground Antenna is perpendicular to the ground Horizontal Polarization Electric field is parallel to the ground Antenna is parallel to the ground Typically AP antennas are setup to use vertical polarization * Antenna Polarization Orientation Communicating antennas must have their polarization orientation aligned with each other in order to maximize the strength of the communication link Receiving antenna picks up incoming E-field energy and passes this energy on to the receiving RF system By aligning the receiving and transmitting antennas, the amount of received electrical energy is maximized Polarization orientation alignment means matching the polarization between the sending and receiving antennas E.g. Both set up for vertically polarization * Antenna Polarization Orientation Analogy If a flashlight is pointed at two identical walls with identically aligned slots then more light energy passes through the slot in the second wall than would be the case if the slots are perpendicular to each other If the E-planes of two antennas are aligned, significantly more RF energy will be received Gain / Loss * * How are Radio Frequency Signals Generated? High frequency alternating electrical current (AC) signals are sent from the RF transmitter to the antenna through a metal conductor (e.g. coaxial cable) Antenna converts this high frequency AC energy into radio waves that radiate from the antenna Radio Frequency Transmitter High Frequency Alternating Current (AC) RF Waves AP/Client Adapter’s Radio Component * Key Discoveries In 1820 Hans Christian Oersted discovered that the flow of DC electricity along a wire creates a magnetic field around the wire His original work in the field of electromagnetism served as the foundation for many key elements in today’s RF technology In part, this is the technology upon which RF antenna transmission is based * Key Discoveries In 1831 Michael Faraday discovered when a magnetic bar is moved in and out of a metal loop, an electrical current is induced onto the loop This is called electromagnetic induction and, in part, is the technology upon which RF antenna reception is based * Gain Gain refers to an increase in the RF wave’s amplitude (i.e. increase in signal strength or energy) No change in frequency or wavelength Gain is measured in decibels (dB) * Active Gain Gain can be increased intentionally by amplifying the transmitter’s AC electrical signal which increases the amount of outgoing AC energy that reaches the antenna E.g. Using an RF amplifier to amplify the signal This is called “active gain” Active gain is NOT related to the antenna’s design Radio Frequency Transmitter RF Waves AC Outlet AC Power RF Amplifier * Passive Gain Gain can be increased intentionally by using a higher gain antenna which focuses (i.e. narrows) the antenna’s beam which extends the distance This is called “passive gain” Passive gain is achieved through the antenna’s design NOT related to the amount of AC power the antenna receives Gain is achieved by focusing the energy not by increasing the outgoing energy received by the antenna Receiving Station Transmitting Station Antenna Gain If an antenna could radiate power equally in every possible direction the antenna’s gain would be zero Such an antenna is called an isotropic radiator It is not possible to manufacture such an antenna Every man-made antenna generates more energy in some directions than in other directions and the measurement of this focused energy is referred to as antenna gain High gain antennas focus their energy more than low gain antennas * * Unintentional Passive Gain Gain can increase unintentionally After waves leave the antenna, conditions such as reflection may cause the wave front to split off into several different paths If these different sets of reflected waves converge in phase at the receiver’s antenna it would create a wave signal whose amplitude is greater than what would happen if the waves had not converged Creates a condition called multipath Recombined signals can NEVER equal or exceed the original signal strength due to free space path loss * Loss Refers to a reduction in the RF wave’s amplitude (i.e. decrease in signal strength) Loss does not change the wave’s frequency or wavelength Like gain, loss can be any of the following: Active Passive Intentional Unintentional Like gain, loss is measured in decibels (dB) * Active Loss Loss can be achieved intentionally Loss can be increased prior to the signal leaving the antenna while still in AC electrical signal form by using a RF attenuator (i.e. resistor) Radio Frequency Transmitter RF Waves AC Power RF Attenuator * Passive Loss The following will cause unintentional passive loss: Prior to wave transmission Resistance in cables and antenna Mismatched impedance between various components During wave transmission Normal attenuation (i.e. free space path loss) If two sets of waves on different paths converge and if the two waves are out of phase then a single wave with a reduced amplitude can be created Potential Multipath Results Green and blue waves indicate two converging waves Red wave shows the resultant wave after they converge * Upfade Downfade Nulling * Reasons for Intentionally Increasing Gain and Loss Intentional Gain Enables greater distances between transmitter and receiver Allows STAs to transmit at higher rates (i.e. Dynamic Rate Switching) Intentional Loss Ensures transmitter is not exceeding the legal power output limit Intentional Radiator & Equivalent Isotropically Radiated Power * * Transmitter - Converts data bits received from the WLAN node into a modulated, high frequency, AC signal and sends it through the cable system to the antenna Transmitting Antenna – Radiates the AC signal in the form of RF waves Receiving Antenna – Converts RF waves back into a modulated, high frequency, AC signal and passes it through the cable system to the receiver Receiver – Converts the modulated, high frequency, AC signal into data bits and sends this on to the destination node (e.g. another computer system) * 2-Way Link In WLANs, a single wireless component called a transceiver operates as the transmitter when sending and as the receiver when receiving Diagrams often label the WLAN component as either a transmitter or as a receiver but WLANs are always 2-way links that are typically connect to a cable-based network at one end of the link Transceiver can transmit and receive but not both simultaneously * RF Subsystems In terms of power output calculations for RF systems, the hardware is divided into two subsystems: Intentional Radiator Component design and power settings for this subsystem determine the IR (Intentional Radiator) power calculation Antenna The design of this antenna assembly subsystem determines the EIRP (Equivalent Isotropically Radiated Power) calculation * Intentional Radiator (IR) IR is any device designed to generate radio waves on purpose (intentionally) E.g. Cell phone, garage door opener, WLAN card Main components include: Transmitter (i.e. transceiver) Cabling & connectors running from the transmitter to the antenna Other optional devices in the cabling link such as amplifiers, attenuators, lightening arrestors The Intentional Radiator includes all hardware components EXCEPT the antenna * IR calculation - measures the amount of power reaching the antenna Measured at the connector that attaches directly to the antenna EIRP calculation - measures the amount of power radiated (transmitted) by the antenna Measured at the point where the strongest RF wave signal is radiated by the antenna * Why IR & EIRP Are Not Identical Antennas always create some gain (they are not true isotropic radiators) so EIRP power will always be greater than IR power The grey triangles above indicate the antenna’s coverage area As the triangle moves from left to right, the grey becomes lighter indicating the RF wave strength is diminishing Higher gain, higher EIRP Lower gain, lower EIRP Assume both antennas receive the same IR power * Why Power Calculations Are Important! Two major reasons for doing WLAN power calculations are: Ensuring government regulations are met Maximum IR and EIRP power limits are enforced by government agencies Controlling the RF coverage area size: To prevent interference issues between wireless devices For security reasons – limit coverage area to the organization’s property * Major WLAN Power Elements The 4 major power elements for WLANs are: Transmitting device’s power Connectivity component gain or loss Gain/Loss created by devices, cables and connectors between the transmitter and the antenna Intentional Radiator (IR) power Power output at the last connection where power enters the antenna EIRP (Equivalent Isotropically Radiated Power) Antenna power output * RF Band Regulation In order to prevent anarchy in the radio band frequency range government agencies regulate such things as: What each frequency range can be used for Who (if anyone) is licensed to used a particular frequency How much power a licensed user can generate for the allotted frequency (controls transmission range) In the U.S.A., the Federal Communications Commission (FCC) has this responsibility In Canada, it is Industry Canada who is responsible RF Math * * Units of Measurement The primary units of measurement for WLAN power calculations are: Watts Decibels * Watts (W) Watt is a unit of measurement for electrical power Calculated as the rate of current flow (in amperes) multiplied by the voltage of that flow (in volts) 1 ampere * 1 volt = 1 watt IEEE 802.11 Point-to-Multipoint WLAN networks (e.g. AP with multiple wireless clients) typically operate using power levels far below the 1 watt range Typically in the 30 mW to 100 mW range * Water Flow Analogy To better understand electrical current and electrical voltage, consider water flow through a garden hose Water pressure in the hose is similar to voltage in an electrical circuit measured in volts (V) Water volume flowing through the hose is similar to the electrical current measured in amperes (A) Increasing either the water pressure or the water volume (or both) results in a more powerful water flow * Milliwatts (mW) Many WLAN applications require less than one watt of power so measurements are often in milliwatts 1 watt = 1,000 milliwatts The abbreviation is “mW” * Why Such Small Power Levels? Even a 100 mW power level (i.e. one-tenth of a watt) has the potential to radiate an RF signal up to 0.83 kilometers (0.5 miles) WLAN access points use these relatively low power levels (e.g. typically 30 to 100 mW) in order to limit their transmission range due to potential interference and security concerns * Decibels (dB) Some very sensitive receivers can read signals having as little as 0.000000001 Watts of power (0.000001 mW) Differences between signals measured at such tiny numbers are difficult for people to relate to so a more comprehensible decibel measurement system is used The decibel system measures relative differences between power levels E.g. Twice as strong, half as strong, ten times stronger Decibel (dB) is the unit of measurement You have probably seen decibels used in relation to sound however it is also used to measure other types of power levels including RF signals * What are Decibels? Decibels is the standard unit for measuring transmission gain or loss (i.e. changes) as derived from a ratio of signal amplitudes (i.e. power) 1 bel = 10 decibels Decibels are based on a logarithmic relationship to watts A logarithm is the exponent to which the number 10 must be raised to reach a given value For example: Logarithm (log) of 1,000 is 3 since 103 = 1,000 Logarithm (log) of 100 is 2 since 102 = 100 * Logarithm Basics Although you don’t need to be an expert on logarithms to be a WLAN administrator, you should know a few basics A log can be determined for any positive number E.g. Log (1000) = 3 But there is no log for zero or for a negative number Log (-1000) = undefined Log (0) = undefined * Watts versus Decibels Watts are absolute, linear measurement units that start at zero You can have a reading of 0 Watts which means there is no power Other wattage readings are relative to the zero value Decibels are relative, logarithmic measurement units You cannot have a reading of 0 decibels since the logarithm of 0 is undefined * Why Use Decibels Rather Than Watts? Gain and Loss are sometimes referred to in relative terms and sometimes in absolute terms For example: Absolute: To specify a specific amount of power gain or loss, use watts E.g. 50 mW gain means 50 mW more than original power Relative: To specify a relative gain or loss of power, use decibels E.g. 3 dB gain means the original power is doubled * Cumulative Decibel Losses Losing half of the power in an RF system is equivalent to losing 3 decibels Recorded as -3 dB Assume X is the original power level and a series of three -3 dB power losses occur: Resultant Power Original power lost X - 3 dB ½ X ½ original power lost ½ X - 3 dB ¼ X ¾ original power lost ¼ X - 3 dB 1 / 8 X 7/8 original power lost Each 3 dB loss results in half the previous power being lost Three linear losses of 3 dB each don’t give a linear result * Rule of 10s and 3s To simplify RF Gain and Loss calculations, remember these figures: -3 dB = Half the previous power in mW +3 dB = Double the previous power in mW -10 dB = One tenth the previous power in mW +10 dB = Ten times the previous power in mW The numbers 3 and 10 are the key figures when doing quick gain and loss calculations * Counting by 3s and 10s = 10 - 3 -3 -3 = 3 + 3 + 3 + 3 - 10 = 3 = 10 - 3 - 3 = 10 + 10 - 3 - 3 - 3 - 3 - 3 = 3 + 3 = 10 - 3 = 10 + 10 - 3 - 3 - 3 - 3 = 3 + 3 + 3 = 10 = 10 + 10 - 3 - 3 - 3 = 3 + 3 + 3 + 3 = 10 + 3 = 10 + 10 - 3 - 3 = 3 + 3 + 3 + 3 + 3 = 10 + 3 + 3 = 10 + 10 - 3 = 3 + 3 + 3 + 3 + 3 + 3 = 10 + 3 + 3 + 3 = 10 + 10 Knowing these calculations is the key to quickly doing RF math Also know the negative value calculations from -1 to - 20 * Decibel / Watt Calculation Examples +3 dB doubles the watt value E.g. 10 mW (original) + 3 dB (gain) = 20 mW -3 dB halves the watt value E.g. 10 mW (original) – 3 dB (loss) = 5 mW + 10 dB increases the watt value ten times E.g. 10 mW (original) + 10 dB (gain) = 100 mW - 10 dB decreases the watt value to one tenth E.g. 10 mW (original) – 10 dB (loss) = 1 mW * Additive Gain and Loss Gain and Loss refer to relative changes and are additive (i.e. they can be added and subtracted) For example: Assume in a cable/connector section, the cable had a -2 dB loss and the connector produced a -1 dB loss To calculate total loss you would add the -2 dB and the -1 dB so the total loss would be -3 dB This -3 dB loss means that the resultant power is half the original - 2 dB - 1 dB * dB, dBm and dBi When decibel math is being done for RF systems, different dB variations are needed based on the circumstances: dBm dBm is used when specifying an absolute decibel power measurement such as IR and EIRP dBi dBi is a relative measurement used when referring to an antenna’s gain compared to an isotropic radiator (e.g. +5 dBi antenna) dB dB without either the trailing “i ” or “m” indicates a relative power change and is not specifically related to any particular point in a system * dBm When dBm is used (rather than dB) it means that decibels are being used in relation to milliwatts This usage allows decibels (a relative unit of measurement) to have a common point of reference to milliwatts (an absolute unit of measurement) The m in dBm indicates that the decibel (dB) unit is being referenced to milliwatts (mW) The common point of reference (i.e. baseline) when calculating dBm is: 0 dBm = 1 mW (one milliwatt) dBm is the unit of measurement used to quantify Intentional Radiator (IR) and Equivalent Isotropically Radiated Power (EIRP) power * Intentional Radiator Calculation Example Assume the transmitter (1) generates 100 mW Assume 6 dB loss by the cabling & connectors (2) What is the Intentional Radiator power (3)? Answer: 25 mW Solution: 100 mW –3 dB = 50 mW 50 mW –3 dB = 25 mW * dBi dBi is the unit of measurement used to indicate the amount of gain created by the antenna E.g. 5 dBi antenna adds 5 dB to the power it receives dBm is for IR and EIRP calculations The “i” in dBi stands for isotropic A true (not equivalent) isotropic radiator is an antenna that generates an equal amount of power in all directions of 3 dimensional space Antennas are not true isotropic radiators since they always generate a stronger signal in some directions than in other directions * EIRP EIRP quantifies the power generated by the antenna measured in absolute units such as watts or dBm EIRP is an important measurement since it quantifies the amount of power radiated by (i.e. leaving) the antenna Each antenna is designed to produce a particular amount of gain (not all create the same gain) and dBi is the measurement unit used to indicate this gain E.g. 5 dBi gain antenna, 11 dBi gain antenna Unless they are malfunctioning, antennas don’t create loss so dBi values are always positive * EIRP Calculation Example Assume Intentional Radiator power (3) is 7 mW Assume an antenna gain of 10 dBi EIRP (4) would be: 7 mW + 10 dBi = 7 mW * 10 = 70 mW Just like dBm in the Intentional Radiator, + 3 dBi doubles the power and + 10 dBi increases power 10 times * Antenna: dBi vs dBm Confusion When doing EIRP calculations there is always a dBi number to deal with (i.e. antenna gain) and sometimes a dBm number (i.e. EIRP value) This can cause confusion – here is what to remember: dBi is a relative value indicating the amount of gain that must be factored into the EIRP calculation dBm is one unit of measurement by which the absolute value of EIRP is measured mW is the other EIRP measurement unit Additional RF Calculation Exercises * Watt to dBm Conversion Question What is the dBm equivalent to 1 W (Watt)? In a previous slides you learned these key points: 0 dBm = 1 mW Hint: Every time 10 dBm is added to the current wattage, power level increases by 10 times * Watt to dBm Conversion Answer Answer: 30 dBm is the dBm equivalent to 1 W Solution: Every 10 dBm gain increase the previous power level ten fold Given: 0 dBm = 1 mW We need to get to 1,000 mW (1 W) Log (1,000) = 3 so we need to add 10 dBm three times 0 dBm = 1 mW 1 mW + 10 dBm = 10 mW 10 mW + 10 dBm = 100 mW 100 mW + 10 dBm = 1,000 mW = 1 W ___________ Total: 30 dBm * EIRP Calculation in Milliwatts Question What is the EIRP at the antenna in milliwatts given the following? WLAN Access Point (RF transmitter) generates 100 mW Intentional Radiator cables and connectors have a 3 dB loss Antenna has a 10 dBi gain * EIRP Calculation in Milliwatts Answer Answer: EIRP is 500 mW Solution: Original Power: 100 mW 100 mW – 3 dB = 50 mW 50 mW + 10 dBi = 500 mW -3 dB halves the power then +10 dBi increases the remaining power by 10 times * EIRP Calculation in Decibels Question What is the EIRP at the antenna in decibels (dBm) given the following? WLAN Access Point (RF transmitter) generates 200 mW Intentional Radiator cables and connectors have a 6 dB loss Antenna has a 9 dBi gain * EIRP Calculation in Decibels Answer Answer: EIRP is 26 dBm Solution: 1. Convert 200 mW to dBm: 0 dBm = 1 mW 1 mW + 3 dB = 2 mW 2 mW + 10 dB = 20 mW 20 mW + 10 dB = 200 mW Total: +23 dBm Total the decibel amounts: +23 dBm – 6 dB + 9 dBi = 26 dBm Not 26 dBi because EIRP is an absolute measurement * EIRP Calculation in Milliwatts Question What is the EIRP at the antenna in mW given the following? WLAN Access Point (RF transmitter) generates 100 mW Intentional Radiator cables and connectors have a 2 dB loss Antenna has a 11 dBi gain Note that 3 and 10 are not being given so look for ways to play with the given numbers (i.e. 2 and 11) to obtain a decibel number you can work with * EIRP Calculation in Milliwatts Answer Answer: EIRP is 800 mW Solution: 1. Gains and losses are additive so total them: 11 dBi - 2 dB = + 9 dB change + 9 dB means doubling original 100 mW power 3 times: 100 mW + 3 dB = 200 mW 200 mW + 3 dB = 400 mW 400 mW + 3 dB = 800 mW Total: + 9 dB * EIRP Calculation in Decibels Question What is the EIRP at the antenna in decibels (dBm) given the following? WLAN Access Point (RF transmitter) generates 20 dBm Intentional Radiator cables have a 6 dB loss Intentional Radiator amplifier has a 10 dB gain Intentional Radiator connectors have a 3 dB loss Antenna has a 6 dBi gain * EIRP Calculation in Decibels Answer Answer: EIRP is 27 dBm Solution: Gains and losses are additive so total them: + 20 dBm – 6 dB + 10 dB – 3 dB + 6 dBi = 27 dBm * Net dB Gain/Loss Question What is the net gain or loss (i.e. net change) in decibels given the following? Intentional Radiator cable # 1 has a 3 dB loss Intentional Radiator cable # 2 has a 3 dB loss Intentional Radiator amplifier has a 12 dB gain Antenna has a 9 dBi gain Antenna attenuator generates a 5 dB loss * Net dB Gain/Loss Answer Answer: Gain of 10 dB Solution: Gains and losses are additive so total the changes: - 3 dB -3 dB + 12 dB + 9 dBi – 5dB = 10 dB Free Space Path Loss Calculations 6 dB Rule * * Free Space Path Loss Free Space Path Loss is a form of attenuation (i.e. loss of amplitude) caused by the natural broadening of a wave as it moves away from its origin As the length of the wavefront increases the wave’s amplitude (height) is reduced Like stretching a piece of chewing gum – its diameter shrinks the more you stretch it Calculating Free Space Path Loss RF math calculations in the previous slides focused on determining the amount of power reaching the antenna (IR) and radiating from the antenna (EIRP) The next few slides discuss how something called the 6 dB Rule can be used to calculate the amount of power that will reach the receiving STA’s antenna As soon as the RF signal leaves the antenna it starts to lose power due to an effect referred to as Free Space Path Loss * * 6 dB Rule If the distance between the transmitting antenna and the receiving antenna doubles then the received signal strength drops by 6 dB (1/4 of the original strength) Conversely, in order to double the transmission distance and still maintain the original received signal strength, the transmitted antenna power (EIRP) must be increased by 6 dB (i.e. 4 times stronger) This is a useful piece of information when estimating an RF cell size * 6 dB Rule Example * Free Space Path Loss Question Assume EIRP is 30 mW and this is just strong enough to maintain a viable link between two STAs whose antennas that are 50 meters apart What must the transmitting STA’s EIRP be in mW to just maintain a viable link if one of the STAs move so the antennas are now 200 meters apart? * Free Space Path Loss Answer Answer: Gain of 480 mW Solution: Starting distance is 50 meters and starting EIRP is 30 mW: Doubling distance to 100 meters means a 6 dB increase Doubling 100 meters to 200 meters means another 6 dB increase 30 mW + 6 dB + 6 dB = 30 mW + 3 dB + 3dB + 3dB + 3dB 30 mW + 3 dB + 3dB + 3dB + 3dB = 30 mW * 2 * 2 * 2 * 2 30 mW * 2 * 2 * 2 * 2 = 480 mW 6 dB Rule: Outdoor Usage Only The Free Space Path Loss 6 dB rule calculation can really only be trusted in outdoor applications where there is very little chance of multipath Indoors, RF waves can reach the receiving antenna through a variety of paths so the 6 dB rule calculation cannot be applied with any degree of accuracy * Additional Calculations & Measurements * * Received Signal Strength Indicator (RSSI) RSSI is an optional 802.11 parameter that hardware manufacturers use as a relative measurement of the RF power that is received by their wireless devices RSSI is not part of the 802.11 standard but most wireless manufacturer incorporate it into their systems RSSI measurements return a value in the potential range of 0 to 255 although some manufacturers use a smaller range (e.g. 0 to 100) The higher the measured RSSI value, the stronger the signal strength the device is receiving A stronger signal translates to better performance i.e. Higher transmission rate (DRS) * RSSI: Comparing Apples & Oranges If manufacturer A’s wireless card returned an RSSI value of 65 at a given distance from a transmitter while manufacturer B’s wireless card returned an RSSI value of 60 you might assume that manufacturer A’s card was superior Actually card B might be the superior card – here is why! RSSI calculations vary by manufacturer (no set standard) so you can’t be sure how the two results equate 0 to 255 range not used by all manufacturers Manufacturer A may be using a range from 0 to 100 Manufacturer B may be using a range from 0 to 70 60 out of 70 is better than 65 out of 100 so card B would be superior in this case even with a lower RSSI * Why Use RSSI? If RSSI is not a reliable measure as to how much signal strength is being received then why is it used? RSSI readings are used by WLAN clients to determine: Determine if another wireless device is transmitting Whether or not to roam to another AP based on the RSSI values being received from the APs For dynamic rate switching to determine the best transmission rate Do not judge a wireless card’s receiving capability based upon it’s RSSI values, especially if you are looking at cards from different manufacturers – this is not the intended use for RSSI * System Operating Margin / Link Budget System Operating Margin (SOM) is a calculation performed at the receiving end and is determined as follows: Received signal strength – Receive sensitivity If the calculation result is a positive value (i.e. the incoming signal was strong enough to be read) then the communications link is viable – if the SOM result is negative then no communications take place SOM is also referred to as the Link Budget Like a financial a budget – if more money comes in than you need to cover expenses you are okay but if the money coming in doesn’t cover the bills you are in trouble * Receive Sensitivity Receive sensitivity is shown as a negative dBm number As a signal moves away from its point of origin, the signal always becomes weaker (loss occurs) Receive sensitivity measures the weakest point at which the receiving transceiver can read the signal at a given operating speed Remember that -15 has a higher value than -25 Would you rather owe someone $15 dollars or $25 dollars! In the wireless world, a device with a receive sensitivity of -93 dBm is actually more powerful with regard to its ability to read signals than a device with a -85 receive sensitivity (i.e. it can read a weaker signal) * SOM / Link Budget Calculation Example Positive SOM so the link is viable * Why The System Operating Margin Is Not Enough! SOM calculations allows you to determine if there is enough signal strength for a viable link assuming conditions are static but often fluctuations will occur E.g. A change in air temperature or humidity could result in an increase of loss between the antennas In the previous slide’s example there was only a 7 dBm System Operating Margin (Link Budget) Fluctuations could cause the received signal to drop below - 85 dBm (below the receive sensitivity of the receiver) causing the link to be dropped * Fade Margin When calculations are being done to ensure a viable link, a safety cushion of 10 dB to 20 dB would typically be added to the calculation in order to reduce the chance of an unstable communications link due to normal fluctuations For example, Assume the SOM calculation indicated that a transceiver receive sensitivity of -90 dB was sufficient to maintain a viable link under normal operating conditions Adding 20 dB to the calculation would mean that the transceiver must receive a signal of -70 dBm or higher (i.e. closer to 0) in order to be viable This additional dB value included in the calculation is called the fade margin * * Fade Margin – Not Part of SOM! System operating margin (SOM) is the difference between the signal received and the signal required by the transceiver for a viable link under stable conditions The fade margin value is not factored into the SOM calculation Fade margin is a value factored into the overall wireless link power requirements to build in a cushion so the link will be stable if conditions fluctuate somewhat Think of SOM as determining minimum power requirements while the additional fade margin provides some breathing room beyond minimum requirements * SOM Calculation Assume the following: Signal received: -65 dBm Fade margin: 15 dBm Receive sensitivity: -85 dBm What is the SOM? Answer: 20 dBm -65 dBm – (-85 dBm) Fade margin is NOT factored into the SOM calculation However the person designing this network with a 15 dBm fade margin would want to ensure that the received signal is no less than -70 dBm to ensure a viable link End