Chapter 3• Radio Frequency Components,

Measurement and mathematics

Communications• For successful communications you need:

– Two or more devices that want to communicate

– Medium of communication– Set of rules to use

• This chapter covers the medium– Components of RF – Transmission of RF signal– Role of each device

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RF Components• Transmitter• Antenna• Receiver• Intentional Radiator• Equivalent Isotropically Radiated Power (EIRP)

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Transmitter• Transmitter begins RF communication• Generates the AC signal

– Modifies the signal based on modulation technique– Carrier Signal goes through cable or direct to antenna

• Transmitter also sets the power level, or transmission amplitude– Dictated by regulation and can be adjusted

• Often integrated with receiver-Transciever

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Antenna• Collects AC signal from transmitter and

radiates RF waves

• With receiver, takes RF Waves and directs to receiver– Receiver converts back to bits and bytes

• Antenna signal is compared to a theoretical isotropic radiator– Point source that radiates equally in all

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Antenna• Antenna Power can be modified by

– Adding power-Active Gain– Focusing Energy-Passive Gain

• Like a lens

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Receiver• Takes carrier signal from antenna and

convert back into 1 and 0

• Signal is much decreased from original amplitude– Free Space Path Loss

• Also altered due to interference and multipath

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Intentional Radiator• Device that intentionally generates and emits

radio frequency energy by radiation or induction– On purpose as opposed to by-product

• All components up to the antenna– Transmitter, cable, connectors, equipment

• The antenna can alter by focusing• FCC dictates max power, usually in MilliWatts

(mW) or decibels relative to 1 milliwatt (dBm)

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Equivalent Isotropically Radiated Power (EIRP)

• Highest signal strength transmitted from a particular antenna

• Measure of the actual output of the antenna– Affected by shape of antenna

• FCC will also regulate this power level

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Units of Power and Comparison• Key ideas for wireless are

coverage and performance

• To measure power, we can measure absolute or relative power– Absolute is compared to a

known scale– Relative is to another signal

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Units of Power and Comparison• Comparative units can help:

– compare coverage areas for different signals– Measure gain or loss

• Measure the change in power

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Units of Power and Comparison

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Units of Power (absolute)

Units of Comparison (Relative)

watt (W) decibel (dB)

milliwatt (mW) dBi

dBm dBd

Watt• Basic unit of power

– 1 ampere of current at 1 volt

• Volts x Amps

• Ability to move/push/etc

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MilliWatt (mW)• 1/1000 th of a watt

• Most 802.11 equipment is measured in milliwatts– Usually 1 to 100 mw

• FCC may allow up to 1 W in some cases, but it isn’t usually needed except in point to point.

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Decibel (dB)• Base unit of comparison, not of power• Represents the difference between two

values• Compare the power of two transmitters• Compare the output of a transmitter and

received at the receiver• From the term bel

– Bell Labs– 10 to 1 ratio

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Decibel (dB)• Bels are logarithmic

– Use the log10 to calculate

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101=10 Log10(10)=1

102=100 Log10(100)=2

103=1000 Log10(1000)=3

104=10000 Log10(10000)=4

Decibel (dB)• Decibels are 10 x a bel

• bel=log10(P1/P2)

• decibel= 10Xlog10(P1/P2)

• No log math on the test!• We use decibels instead of watts as it is

easier to write in many cases.

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Decibel (dB)

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dBi• Antennas are compared to isotropic radiators• The difference between the theoretical isotropic

radiator and the actual antenna can be measures in decibels isotropic (dBi)– Relative measurement– Change in power relative to an antenna– Measure of antenna gain

• Measured at focus point– Always a gain, not a loss– No-gain or unity gain (0 dBi)

• Think antenna GainPg 73

dBd• A second relative measure of strength

• Decibel dipole– Decibel gain relative to a dipole antenna

• Can also compare to dBi– Standard dipole is 2.14 dBi– If an antenna is 3 dBd the total is additive

• 2.14+3=5.14 dBi

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dBm• Absolute measurement

– Decibels relative to 1 mw of power

• So 100 mW= +20dBm

• Can also calc from a dBm value

• PmW=log-1(PdBm/10)

• 1 mW is reference and 0 dBm is 1 mW

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dBm• Why use dBm?

– Easier to grasp -100dBm than .0000000001 mW

• Also, the 6dB rule– If you double the distance between a received and

transmitter, the received signal will decrease by 6 dB.– Also, every 6dBi of gain will double the usable

distance of the RF signal

• Also helps when adding units– If transmitter is +20dBm and the antenna is 5 dBi, the

EIRP is 25 dBm

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Inverse Square Law• The 6 dB rule is based on Isaac Newton’s inverse

square law.– Change in power is equal to the

square of the change in distance– If you double the distance, the power will change by (2xD)2

• FSPL = 36.6 + (20log10(f)) + (20log10(D))– FSPL = path loss in dB– f = frequency in MHz– D = distance in miles between antennas

• FSPL = 32.4 + (20log10(f)) + (20log10(D))– FSPL = path loss in dB– f = frequency in MHz– D = distance in kilometers between antennas

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RF Math• No need for LOG on test.• Rules of 10s and 3s

– Provide for approximate values• For every 3 dB of gain (relative), double the

absolute power (mW).• For every 3 dB of loss (relative), halve the

absolute power (mW).• For every 10 dB of gain (relative), multiply the

absolute power (mW) by a factor of 10.• For every 10 dB of loss (relative), divide the

absolute power (mW) by a factor of 10.

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RF Math• For every 3 dB of gain (relative), double

the absolute power (mW).– For example, if your access point is

configured to transmit at 100 mW and the antenna is rated for 3 dBi of passive gain, the amount of power that will radiate out of the antenna (EIRP) will be 200 mW

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RF Math• For every 3 dB of loss (relative), halve the

absolute power (mW).– Conversely, if your access point is configured

to transmit at 100 mW and is attached to a cable that introduces 3 dB of loss, the amount of absolute amplitude at the end of the cable will be 50 mW

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RF Math• For every 10 dB of gain (relative), multiply

the absolute power (mW) by a factor of 10.– In another example, if your access point is

configured to transmit at 40 mW and the antenna is rated for 10 dBi of passive gain, the amount of power that radiates out of the antenna (EIRP) will be 400 mW

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RF Math• For every 10 dB of loss (relative), divide

the absolute power (mW) by a factor of 10.– Conversely, if your access point is configured

to transmit at 40 mW and is attached to a cable that introduces 10 dB of loss, the amount of absolute amplitude at the end of the cable will be 4 mW.

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RF Math• dBm is a measure of power

• dB is a unit of change

• dB can be applied to dBm

• So, if you have +10dBm and increase by 3 dB, you have +13 dBm

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Step by Step Exercise

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3 + X 2

10 - / 10

dBm

mW

0 1

RF Math Summary• Log Functions

– dBm =10 × log10(mW)

– mW = log–1 (dBm ÷ 10) = 10(dBm ÷ 10)

• Rules of 10 and 3– 3 dB gain = mW × 2– 3 dB loss = mW ÷ 2– 10 dB gain = mW × 10– 10 dB loss = mW ÷ 10

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RF Math Summary

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Received Signal Strength Indicator(RSSI)

• Receive sensitivity is the power level of an RF signal required to be successfully received by the receiver

• The lower this level, the more sensitive the receiver.

• For 802.11 receive sensitivity is often defined as a function of network speed– In order to use a certain speed, you must

have a certain level of loss• More loss, less speed.

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Sensitivity Thresholds

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Received Signal Strength Indicator(RSSI)

• 802.11-2007 standard defines received signal strength indicator as a relative metric used to measure amplitude.– 0 to 255

• Usually mapped to receive sensitivity thresholds in dBm

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Received Signal Strength Indicator(RSSI)

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Received Signal Strength Indicator(RSSI)

• 802.11-2007 also defines Signal Quality (SQ)– Singal quality as it affects coding techniques

like barker or complementary code keying– Anything that affects bit error rate (BER) will

trigger SQ metrics

• RSSI and SQ are often refered to together as RSSI metrics

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Received Signal Strength Indicator(RSSI)

• Signal to Noise Ration (SNR) is not signal quality– It is a measure of the difference between the

received signal and background noise (noise floor)

– Noise is -100 dBm and radio receives -85dBm the SNR is 15 dB

• 25dB or greater is considered good

• Vendors get to choose how to map RSSI– proprietary

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Received Signal Strength Indicator(RSSI)

• Most vendors use RSSI for decisions on roaming and dynamic rate switching

• Roaming is when a client switches from one AP to another

• Dynamic Rate Switching is when 802.11 radios switch between data rates– Often due to reduced signal quality or loss

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Link Budget• Sum of all gains and losses from

transmitting radio, through the RF medium to the receiver radio– Calculated to make sure the final received

signal is about the sensitivity threshold

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Link Budget• Link budget calculations include

– original transmit gain– passive antenna gain– 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.

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Link Budget• Loss in many places

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Link Budget

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Link Budget• Simpler example

– +20 dBm + 5 dBi – 73.98 dB + 2.14 dBi = –46.84 dBm

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Fade Margin/System Operating Margin

• Level of desired signal above what is required.– The buffer, or comfort zone– Plan for 10 to 25 dB above the receive

sensitivity

• 10 dB is minimum– Higher if more MILEs away

• Fade Margin buffer is also known as the System Operating Margin (SOM)

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