Rf Components, Measurements, And Math PDF

Handout 3 RF Components, Measurements, and Math Course Name: Wireless Networks Course Code: CSN 405 Notes appended and modified to those accompanying “CWNA Certified Wireless Network Administrator: Official Study Guide”, D. Coleman & D. Westcott, John Wiley & Sons - Sybex, 6th Ed., 2021, Ch. 4 RF...

Handout 3 RF Components, Measurements, and Math Course Name: Wireless Networks Course Code: CSN 405 Notes appended and modified to those accompanying “CWNA Certified Wireless Network Administrator: Official Study Guide”, D. Coleman & D. Westcott, John Wiley & Sons - Sybex, 6th Ed., 2021, Ch. 4 RF Components, Measurements, and Math • RF components • Units of power and comparison • RF math 2 RF Components The transmitter is the initial component in the creation of the wireless medium. The computer hands off the data to the transmitter, and it is the transmitter’s job to begin the RF communication. Main responsibilities are: 1. Initial component in the creation of a wireless signal 2. Receives data from the system and begins RF communication 3. Encodes the data 4. Modulates the AC signal 5. Sends modulated signal to the antenna 3 RF Components An antenna provides two functions in a communication system: 1. When connected to the transmitter, it collects the AC signal that it receives from the transmitter and directs, or radiates, the RF waves away from the antenna in a pattern specific to the antenna type. 2. When connected to the receiver, the antenna takes the RF waves that it receives through the air and directs the AC signal to the receiver. 4 RF Components The receiver is the final component in the wireless medium. 1. The receiver takes the carrier signal that is received from the antenna and translates the modulated signals into 1s and 0s. 2. It then takes this data and passes it to the computer to be processed. 3. The signal that is received is a much less powerful signal than what was transmitted because of the distance it has traveled and the effects of free space path loss (FSPL). The signal is also often unintentionally altered due to interference from other RF sources and multipath. 5 Intentional Radiator (IR)  The FCC Code of Federal Regulations (CFR) Part 15 defines an intentional radiator (IR) as “a device that intentionally generates and emits radio frequency energy by radiation or induction.”  The IR is specifically designed to generate RF, as opposed to something that generates RF as a by-product of its main function, such as a motor that incidentally generates RF noise. 6 Intentional Radiator (IR)  Regulatory bodies such as the FCC limit the amount of power that is allowed to be generated by an IR.  The IR consists of all the components from the transmitter to the antenna but not including the antenna. IR power calculated here  The power output of the IR is thus the sum of all the components from the transmitter to the antenna (again not including the antenna). 7 Isotropic Radiator  An isotropic radiator is a point source that radiates signal equally in all directions.  The sun is probably one of the best examples of an isotropic radiator. It generates equal amounts of energy in all directions. 8 Equivalent Isotropically Radiated Power (EIRP) EIRP calculated here  Equivalent isotropically radiated power (EIRP) is the highest RF signal strength that is transmitted from a particular antenna.  EIRP is the theoretical amount of radiated power emitted from an antenna element. 9 Units of Power and Comparison  Units of power are used to measure transmission amplitude and received amplitude. In other words, units of transmitted or received power measurements are absolute power measurements.  Units of comparison are often used to measure how much gain or loss occurs because of the introduction of cabling or an antenna. Units of comparison are also used to represent a difference in power from point A to point B. In other words, units of comparison are relative measurements of a change in power. 10 Absolute Power Measurements  A watt (W) is the basic unit of power, named after James Watt, an 18th-century Scottish inventor. One watt is equal to 1 ampere (amp) of current flowing at 1 volt.  A milliwatt (mW) is also a unit of power. To put it simply, a milliwatt is 1/1,000 of a watt. 11 Units of Power (Absolute) 1. watt (W) 2. milliwatt (mW) 3. decibels relative to 1 mW (dBm) Absolute power measurements are used in discussions of both transmit power and received power. 12 Units of Power (Relative) Relative units of power measurement include the following: 1. decibel (dB) 2. decibels relative to an isotropic radiator (dBi) 3. decibels relative to a half-wave dipole antenna (dBd) Relative power measurements are used in discussions of gain and loss. 13 Decibels  A relative measurement represents “change in power” as an RF signal moves from one point in space to another point in space.  A decibel (dB) is a relative measurement that is a unit of comparison as opposed to a unit of power. bels = log10(P1/P2) decibels = 10 × log10(P1/P2) P = power 14 dBi and dBd dBi - Decibels relative to an isotropic radiator Isotropic Radiator HalfWave Dipole  Used to measure passive antenna gain  0 dBi = no directivity / passive gain, (which means no gain or unity gain) dBd - Decibels relative to a half-wave dipole  Lesser used unit to measure antenna gain  Half-wave dipole = 2.14 dBi  0 dBd = 2.14 dBi  Example: if an antenna has a value of 3 dBd, this means that it is 3 dB greater than dipole antenna (so a 3 dBd antenna is equal to 5.14 dBi antenna.) 15 Conversions of Absolute Power  dBm is an absolute measurement  Decibels relative to 1 mW  0 dBm = 1 mW dBm is often used to measure received power. 16 Rule of the 10s and 3s Simple and fast way to get close to RF signal strength values: 1. For every 10 dB of gain you multiply signal strength by 10. 2. If calculating loss, for every 10 dB of loss you divide signal strength by 10. 3. For every 3 dB of gain multiply the signal strength by 2. 4. If calculating loss, for every 3 dB of loss divide the signal strength by 2. 17 Rule of 10s and 3s • Provides an approximate value (fairly accurate) • Four basic rules – – – – For every 3 dB gain, double the power For every 3 dB loss, halve the power For every 10 dB gain, power times 10 For every 10 dB loss, power divided by 10 Certified Wireless Network Administrator: CWNA – PW0-108 18 Rule of 10s and 3s Setup • To make RF math easier, first build this chart • dBm is decibels relative to 1 mW, therefore – Set dBm column to value of 0 – Set mW column to value of 1 Certified Wireless Network Administrator: CWNA – PW0-108 19 Rule of 10s and 3s Setup (continued) • Next, add a reminder of the allowed numbers and mathematical symbols to each column • Left column is the dB column – Only numbers that you can use are 3 and 10 – Only math that you can use is + and • Right column is the mW column – Only numbers that you can use are 2 and 10 – Only math that you can use is X and ÷ Certified Wireless Network Administrator: CWNA – PW0-108 20 Rule of 10s and 3s Setup (continued) • Think of this chart as a balance scale • If you change the left side of the scale, you must do something to the right to keep it balanced + or – 3 on left + or – 10 on left requires requires Certified Wireless Network Administrator: CWNA – PW0-108 X or ÷ 2 on right X or ÷ 10 on right 21 Rule of 10s and 3s: Example 2 • A wireless bridge generates a 100 mW signal • The cable between the bridge and the antenna creates -3 dB of signal loss • The antenna provides 10 dBi of signal gain Certified Wireless Network Administrator: CWNA – PW0-108 22 Rule of 10s and 3s: Example 2 (step 1) • The initial chart shows 1 mW, however the bridge generates 100 mW • Increase the mW column to 100 • Then balance the scale by performing the partner calculations to the dBm column Remember to only use the allowed values and mathematical symbols on each side Certified Wireless Network Administrator: CWNA – PW0-108 23 Rule of 10s and 3s: Example 2 (step 2) • At this point the chart shows that 20 dBm is equal to 100 mW • Now you need to calculate the -3 dB of loss that is introduced by the cable • Decrease the dBm column by 3 • Again, do not forget to balance the scale Certified Wireless Network Administrator: CWNA – PW0-108 24 Rule of 10s and 3s: Example 2 (step 3) • At this point the chart shows that 17 dBm is equal to 50 mW • Now you need to calculate the 10 dBi gain introduced by the antenna • Increase the dBm column by 10 • Again, do not forget to balance the scale Certified Wireless Network Administrator: CWNA – PW0-108 25 Rule of 10s and 3s: Example 2 (step 4) • At this point the chart shows that 27 dBm is equal to 500 mW • The power at the IR is 17 dBm or 50 mW • The EIRP is 27 dBm or 500 mW  Think of dBm and mW as Celsius and Fahrenheit, two different scales that represent the same thing Certified Wireless Network Administrator: CWNA – PW0-108 26 Rule of 10s and 3s: Other Examples • CWNA book contains additional examples • Animated explanation of the rule of 10s and 3s as well as explanations of the examples in the CWNA book has been created using Microsoft PowerPoint and can be found on Bb – Course Material. Certified Wireless Network Administrator: CWNA – PW0-108 27 dB Loss and Gain (-10 through +10) • Any dB loss or gain can be calculated using 3 and 10 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -10 -3 -3 -3 -10 -10 +3 +3 +3 +3 -10 +3 -3 -3 -10 -10 +3 +3 +3 +3 +3 -10 +3 +3 -3 -3 -3 -3 -3 +10 +10 -3 -3 -3 Certified Wireless Network Administrator: CWNA – PW0-108 1 2 3 4 5 6 7 8 9 10 +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 28 dBm and milliwatt Conversions dBm + 36 dBm + 30 dBm + 20 dBm + 10 dBm 0 dBm -10 dBm -20 dBm -30 dBm -40 dBm -50 dBm -60 dBm -70 dBm -80 dBm -90 dBm milliwatts 4,000 mW 1,000 mW 100 mW 10 mW 1 mW 0.1 mW 0.01 mW 0.001 mW 0.0001 mW 0.00001 mW 0.000001 mW 0.0000001 mW 0.00000001 mW 0.000000001 mW Certified Wireless Network Administrator: CWNA – PW0-108 Power Level 4 watts 1 watt 1/10th watt 1/100th watt 1/1,000th watt 1/10th milliwatt 1/100th milliwatt 1/1,000th milliwatt 1/10,000th milliwatt 1/100,000th milliwatt 1 millionth of 1 milliwatt 1 ten-millionth of 1 milliwatt 1 hundred-millionth of 1 milliwatt 1 billionth of 1 milliwatt 29 Received Power Recommendations  -70 dBm: high data rate connectivity  -65 dBm: Voice over Wi-Fi Discussed in detail in module 13 (WLAN Design Concepts) 30 Noise Floor  The noise floor is the ambient or background level of radio energy on a specific channel.  This background energy can include modulated or encoded bits coming from nearby 802.11 transmitting radios or unmodulated energy coming from non-802.11 devices, such as microwave ovens, portable telephones, and so on.  Anything electromagnetic has the potential of raising the amplitude of the noise floor on a specific channel. 31 Signal-to-Noise Ratio (SNR)  Another reason for planning for –70 dBm coverage is because the received signal of –70 dBm is usually well above the noise floor.  Data transmissions can become corrupted with a very low SNR.  If the amplitude of the noise floor is too close to the amplitude of the received signal, data corruption will occur and result in layer 2 retransmissions. 32 Signal-to-Noise Ratio (SNR) Received signal = -70 dBm Received signal = -88 dBm SNR = 25 dB SNR = 7 dB Ambient noise floor = - 95 dBm 33 Modulation and SNR 34 SINR  For years SINR has been a standard measurement for Wi-Fi networks.  Over the past few years, the term signal-to-interference-plusnoise ratio (SINR) has appeared and is being used by vendors.  SINR is the difference between the power of the primary RF signal and the sum of the power of the RF interference and the background noise. This difference is measured in decibels. 35 RSSI Received Signal Strength Indicator (RSSI)  802.11 defines method of radios measuring signal strength with a value of 0 – 255  Varies between manufacturers Examples: ■ Atheros: 0-127 ■ Cisco: 0-100 36 Receive Sensitivity  Receive sensitivity refers to the power level of an RF signal required to be successfully received by the receiver radio.  Minimum signal strength at which a data rate can be correctly received  Varies for different devices  Complex modulation and coding require better signals 37 Receive Sensitivity 54 Mbps 36 Mbps 18 Mbps 6 Mbps Data Rate (2.4 GHz) Receive Sensitivity 1 Mbps -101 dBm 6 Mbps -91 dBm MCS 0 -90 dBm 11 Mbps -89 dBm 24 Mbps -87 dBm 54 Mbps -79 dBm MCS 7 -77 dBm MCS 15 -75 dBm MCS 23 -74 dBm  Please understand that not all client devices are created equal.  Depending on the chipset vendor, the radios of various WiFi clients have different receive sensitivity thresholds, which are mapped to different data rates.  This means that two client radios receiving an RF signal with the same strength may use a different data rate for modulation and demodulation. 38 Receive Sensitivity 54 Mbps 36 Mbps 18 Mbps 6 Mbps Data Rate (2.4 GHz) Receive Sensitivity 1 Mbps -101 dBm 6 Mbps -91 dBm MCS 0 -90 dBm 11 Mbps -89 dBm 24 Mbps -87 dBm 54 Mbps -79 dBm MCS 7 -77 dBm MCS 15 -75 dBm MCS 23 -74 dBm  Despite the variances between devices and sensitivity, there is still a common denominator.  A received signal of –70 dBm or higher usually guarantees that a client radio will use one of the highest data rates that the client is capable of. 39 Dynamic Rate Switching, DRS  Mobility can cause shifts in data rates  Weaker signal and lower SNR results in lower data rates  APs and client radios upshift and downshift data rates based on receive sensitivity thresholds 40 Free Space Path Loss (FSPL) FSPL = 36.6 + (20log10(f)) + (20log10(D)) FSPL = path loss in dB f = frequency in MHz D = distance in miles between antennas FSPL = 32.44+ (20log10(f)) + (20log10(D)) FSPL = path loss in dB f = frequency in MHz D = distance in kilometers between antennas 41 6 dB Rule  By doubling the distance from the RF source, the signal decreased by about 6 dB. If you double the distance between the transmitter and the receiver, the received signal will decrease by 6 dB.  No matter what numbers are chosen, if the distance is doubled, the decibel loss will be 6 dB.  This rule also implies that if you increase the amplitude by 6 dB, the usable distance will double.  This 6 dB rule is very useful for comparing cell sizes or estimating the coverage of a transmitter.  The 6 dB rule is also useful for understanding antenna gain, because every 6 dB of extra antenna gain will double the usable distance of an RF signal. 42 Link Budget Free Space Path Loss (Distance and Frequency) Cables & Connectors Antenna Gain (Rx) Antenna Gain (Tx) Tx Output Power Cables & Connectors Factors Amount Receive Sensitivity (minimum goal) -74 at 54 Mbps Fade Margin (additional signal desired) 15 dB Transmit Power 26 dBm (400 mW) Tx Cable and Connector Loss -3 dB Antenna Gain 15 dBi Path Loss (FSPL) -104 dB Rx Antenna Gain 12 dBi Rx Cable and Connector Loss -3 dB Received Power Receive Sensitivity Result -57 dBm  When radio communications are deployed, a link budget is the sum of all the planned and expected gains and losses from the transmitting radio, through the RF medium, to the receiver radio.  The purpose of link budget calculations is to guarantee that the final received signal amplitude is above the receiver sensitivity threshold of the receiver radio. 43 Link Budget Components  Link budget calculations include original transmit gain, passive antenna gain, and active gain from RF amplifiers.  All gain must be accounted for—including RF amplifiers and antennas—and all losses must be accounted for—including attenuators, FSPL, and insertion loss.  Any hardware device installed in a radio system adds a certain amount of signal attenuation, called insertion loss.  Cabling is rated for dB loss per 100 feet, and connectors typically add about 0.5 dB of insertion loss. 44 Link Budget Loss • The sum of all gains and losses from the transmitting radio to the receiver radio • Used to guarantee that the received signal is above the receiver sensitivity threshold Certified Wireless Network Administrator: CWNA – PW0-108 45 Link Budget Example 46 Fade Margin Whenever an outdoor WLAN bridge link is designed, link budget and fade margin calculations will be an absolute requirement.  For example, an RF engineer may perform link budget calculations for a 2mile point-to-point bridge link and determine that the final received signal is 5 dB above the receive sensitivity threshold of a radio at one end of a bridge link.  It would seem that RF communications will be just fine; however, because of downfade caused by multipath and weather conditions, a fade margin buffer is needed.  A torrential downpour can attenuate a signal as much as 0.08 dB per mile (0.05 dB per kilometer) in both the 2.4 GHz and 5 GHz frequency ranges. Over long-distance bridge links, a fade margin of 25 dB is usually recommended to compensate for attenuation due to changes in RF behaviors, such as multipath, and due to changes in weather conditions, such as rain, fog, or snow. 47 Fade Margin • Level of desired signal above what is required • Comfort zone • Received signal fluctuates due to outside influences and interference • Protects reception of signal due to fluctuation of the received signal • 10 dB to 25 dB buffer is common practice • System operating margin (SOM) is the difference between the actual received signal and the signal required for communications Certified Wireless Network Administrator: CWNA – PW0-108 48 Online RF Calculators https://www.everythingrf.com/rf-calculators https://www.pasternack.com/t-rf-microwave-calculators-and-conversions.aspx 49 Questions Home Work 1. Open your book and go through all the review questions at the end of the chapter. 2. Review the answers by using Appendix A. 50