microsoft powerpoint - chap5 rf

7/30/2019 Microsoft PowerPoint - Chap5 RF

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SP - EEE ET0246 Wireless Network & Security- 2-

ObjectivesUpon completion of this part, you will be able to:

Define and apply the concepts of Radio FrequencybehaviourIdentify and understand application of basic RF andantennaCalculate the simple RF parameters for building FCCcompliant systemsDefine spread spectrum technologiesCompare and contrast between FHSS and DSSSList the factors that impact signal throughput and range


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What is RF?A solid fundamentals of radio frequency (RF)theory is important to wireless LAN administrators

Wire LAN is the culmination of two unrelatedtechnologies: RF and networking

Radio Frequency

High frequency AC signals that are passed along acopper conductor and then radiated into the air via anantennaAn antenna converts/transforms a wiredsignal to a wireless signal and vice versaThese radio waves propagate (move) awayfrom the source (the antenna) in a straightline in all directions


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RF BehavioursMany factors may cause undesirable resultsIts important to understand RF behavioursGain is the term used to describe an increase inan RF signal's amplitude.

Gain can be positive or a negative


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RF BehavioursLoss

Many things can cause RF signal loss, whatfactors???

Resistance, mismatch of impedance, signalabsorption, attenuation


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RF BehavioursReflection

Occurs when an RF signal bounces off a smooth surface,changing the direction of the signalThe object has very large dimensions when compared tothe wavelength of the propagating waveThis reflecting of the main signal from many objects inthe area of the transmission is referred to as multipathSignal up fading, down fading


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RF BehavioursRefraction

Bending of a radio wave as it passes through a mediumof different densityRefraction can become a problem for long distance RFlinks. As atmospheric conditions change, the RF wavesmay change direction, diverting the signal away from the

intended target


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RF BehavioursDiffraction

A change in the direction and intensity of a group of

waves after passing by an obstacle. Refers to thebending of RF signal as it travels past or around anobject.The wave front strikes an obstacle

The rest of the wave front maintains the same speed of propagation.Diffraction is the effect of waves turning, or bending,around the obstacle.


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ScatteringOccurs when RF signal strikes an uneven surface causingthe signal to be scattered in such a fashion that theresultant signals are insignificant foliage, street signs,and lampposts Occur as a signal wave travels through particles in the

medium such as heavy dust content

RF Behaviours


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RF BehavioursAbsorption

Occurs when the RF signal strikes an object and isabsorbed into the material of the object partiallyAbsorbed signal does not pass through, reflect off, orbend around the object


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VSWRVoltage Standing Wave Ratio (VSWR)

Antenna is located some distance from the transmitter

and require cables and connectorsVSWR is a measure of impedance mismatch between thetransmission line and its load. The higher the VSWR, thegreater the mismatch

Caused by an impedance mismatch between connectorsor devices.Impedance is a measure of the resistance to currentflow in Ohms

VSWR results in return loss


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VSWRHave you seen this???


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VSWRVSWR Measurements

Using SWR meter1:1 indicates a perfect matching with noreturn lossTypical WLAN device has a VSWR valueto 1.5:1Military requirement is 1.1:1 or better


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VSWRVSWR effects:

Decreased amplitude of the main RF signal

Transmitter and/or amplifier failure: burn , low outputSolutions:

Never use 75-Ohm cable with 50-Ohm devicesTight connections between cables and connectors

Warning: Do not turn on transmitter without atermination (load) or an antenna attached !

Open end stands a high impedance

Sustained VSWR at high power could damage thetransmitter!


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AntennasConvert electrical energy into RF waves in thecase of a transmitting, RF waves into electricalenergy in the case of a receivingThe physical dimensions of an antenna, e.g.length, are directly related to the frequency

Essential points:Line of sightFresnel Zone

Antenna gainBeamwidths


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AntennasLine of sight

Visual LOS (LOS) is defined as the apparently straight

line from the object in sight (the transmitter) to theobserver's eye (the receiver)RF line of sight: If there s visual LOS, and there are noobjects intruding on the Fresnel Zone by more than

20% , then RF LOS exists


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Fresnel ZoneThe area around the visual line-of-sight that radio waves

spread out into after they leave the antenna. This areamust be clear or else signal strength will weakenObjects in the Fresnel Zone such as trees, hilltops, andbuildings can change the RF LOS

Effects:Introduce RF signal interference if blockedAbsorb or scatter the main RF signal, causingdegradation or complete signal loss if blocked

Introduced by Fresnel in 1814 to explain diffraction

Antennas


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AntennasThe Fresnel Zone of the RF link should not be blocked more than20%,

otherwise RF LOS will be affected


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AntennasEarth curvature needs to consider for a distance more than 7

miles


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Antenna GainAn antenna element is a passive device, then why Gain ?

Focusing the RF radiation into a tighter beam can extendthe transmission distanceE.g. the bulb of a flashlight can be focused into a tighterbeam creating a brighter light source that sends the light

further.Beamwidths , measured in degrees horizontaland vertical

For example, an omni-directional antenna has a 360-degree horizontal beamwidth.Focusing both horizontal and vertical beamwidths canmaximize distance of the propagated wave at low power

Antennas


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AntennasIntentional Radiator

(FCC): A device that is specifically designed togenerate and radiate RF signalsInclude the RF devices ,cables and connectors upto the antenna (but not including the antenna)"power output of the Intentional Radiator" refers tothe power output at the end of the last cable orconnector before the antenna


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Antennas

Equivalent Isotropically Radiated Power(EIRP)

EIRP is the power radiated by an equivalent idealisotropic antennaExample: Suppose a transmitting station uses a 10-dBiantenna and is fed by 100 milliwatts from theintentional radiator. The EIRP is 1000 mW, or 1 Watt


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Antennas


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RF MathematicsFour considerations:

Power at the transmitting deviceLoss and gain of connectivity devices betweenthe transmitting device and the antenna - suchas cables, connectors, amplifiers, attenuators,

and splittersPower at the last connector before the RFsignal enters the antenna (Intentional Radiator)

Power at the antenna element (EIRP)


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Units of Measurements in RFUnits of Measure

Watt a standard unit of power measurement that is

used to measure the rate at which power is dissipatedPower P =V x I (V:voltage, I: current)

Milliwatt (mw) standard unit of power measurement inthe communications industry equating to 1/1000 of a

Watt1 Watt = 1000 mwPower levels on a single wireless LAN segment isbetween 30 100mW

Special case: >100 mW would be used for outdoorconnections between buildings


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Decibels (dB) in the communications industry, thedecibel is used as the logarithmic expression of the ratiobetween two signals output power

To measure gain or loss

A 10 mw signal is input to the amplifier and 100 mwsignal is produced. How much is the gain of theamplifier?

10log(100/10)=10 dB

dB


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dBidBi decibels referenced to an isotropic radiatorWhen quantifying the gain of an antenna

dBi refers only to the gain of an antennaThe i stands for isotropic : against anisotropic radiator

An ideal isotropic radiator sends RF in alldirections with equal intensity, in three-dimensional spaceAn isotropic radiator has a gain of 0 dBi


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dBiExample:

10dBi equals to 10 times,10log( 10 )=10dB1W + 10 dBi = 10W

Means 1 w input to this antenna has the sameradiation efficiency along the transmittingdirection as 10 w applied to an ideal isotropicantenna !


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Equivalent Isotropic Radiated Power

Example 1 : Given the RF circuit, determine the power atall marked points

EIRP

Overall Gain = -3 -3 -3 +12 =3 dB output P=100x2=200mW

)()()( dBi Ant dB LossdBmTX GGP EIRP +=


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EIRPExample 2:

Output from AP 100mwCable loss 3dB

Antenna gain 6dBi

Calculate EIRP of the antenna?

Solution:

Convert mw to dBm:P(dBm)=10log(P(mw))=10log100=20dBm

EIRP=P(dBm)+G(cable)+G(ant)=20-3+6=23 dBm


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EIRP

100mw=20dBm

EIRP=20-0.5x10-3+24 =36 dBm ???mw

Power of IR= EIRP-24=12 dBm ???mw

Example 3:

An AP with 100mw power output is connectedto a 24dBi antenna through a 10 feet lengthcoaxial cable with a loss of 0.5dB/foot.Connector loss is3 dB

a) Calculate EIRP in dBm?

b) Power output of the Intentional Radiator in dBm?


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Narrow Band and Broadband Transmission

Narrow bandA narrowband transmission is a communications technology

that uses only enough of the frequency spectrum fortransmissionAdvantages:

Frequency is kept as narrow as possible to save the resource

Undesirable crosstalk between channels is avoided by carefullycoordinating different users on different frequencies

Drawbacks:End user must obtain an FCC license for each site where it is

employedInterference by in-band noise or jamminghigh peak power


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Narrow Band and Broadband Transmission

Broadband transmission (Spread Spectrum)When the bandwidth is much wider than what is required

to send the information. Signal is in noise formatAdvantages:

License free wide frequency bandSpread the signal out over a much larger frequency

range, so reduce the probability of corruption or jammingLow power spectral densitySecured communication


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Spread Spectrum Technologies

Why low power spectrum density??? By increasing the bandwidth, Signal/Noise ratiomay be decreased without decreased BERperformance. Why???

Shannons information theoryC = W log2 (1+ S/N)

C = Channel capacity in bits

W = Bandwidth in HertzS = Signal Power N = Noise Power


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Spread spectrum is used for:Wireless Local Area Network (WLAN)Wireless Personal Area Networks (WPAN)Wireless Metropolitan Area Networks (WMAN)Wireless Wide Area Networks (WWAN)

FCC regulations describe two spread spectrum technologies:Direct sequence spread spectrum (DSSS)Frequency hopping spread spectrum (FHSS)

IEEE follows FCC regulations on spread spectrum for use of 802.11 networks.

Also specified Orthogonal Frequency Division Multiplexing(OFDM) to achieve higher data rate in 802.11a and 802.11gprotocols.OFDM is not spread spectrum. Uses multiple sub-carriers totransmit data

Spread Spectrum Technologies


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Frequency Hopping Spread Spectrum

(FHSS)Frequency Hopping Spread Spectrum (FHSS)

Technique to send data using a narrowband carrier signal

that hops from frequency to frequency as a function of time over a wide band of frequencies more than 83MHz


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FHSSHow FHSS Works?

the carrier signal changes frequency, or hops , according toa pseudorandom code sequence

The pseudorandom sequence is a list of severalfrequencies to which the carrier will hop beforerepeating the pattern (also called a channel )

The transmitter will remain at a certain frequency for aspecified time ( dwell time ), then take a small amount of time to hop to the next frequency ( hop time )When the list of code sequence has been exhausted, thetransmitter will repeat until the information is transmittedcompletelyThe receiver is synchronized to the transmitter in order toreceive properly (hop in the same way)


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FHSSFHSS systems are resistant but not immune to narrowband interference

An interference signal may occupy several MHzSince FH band is over 83 MHz wide, this interferingsignal will cause little degradation of the spreadspectrum signal


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FHSSFCC regulations on FHSS:

Divide into pre- 8/31/2000 rules and post-8/31/2000 rulesManufacturers can choose either rules to be boundedpre- 8/31/2000 rules:

Use at least 75 of the possible 79 carrier frequencies as thecodeMaximum power 1 Watt in a PTMP system

1MHz per carrier bandwidth (79 carrier frequencies at2.4GHz)Maximum dwell time 400ms per carrier frequencyHop time within 200-300 s

post-8/31/2000 rules:At least 15 hops125 mw power in a PTMP system5 MHz maximum carrier frequency bandwidth

Direct Sequence Spread Spectrum


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DSSS combines a data signal at the sendingstation with a higher data rate bit sequence,

which is referred to as a chipping code or processing gainA high processing gain increases the signal sresistance to interferenceThe number of chips in the code willdetermine how much spreading occurs

Most widely recognized form of spread spectrumFast and inexpensiveEasy implementation

Direct Sequence Spread Spectrum(DSSS)


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DSSSGenerates redundant bit pattern for each bit to betransmitted

This bit pattern is called chip/chipping code (processinggain) Longer the chip damaged bits during transmission can be recovered withno retransmission

View DSSS as a low-power wideband noise & rejected bymost narrowband receivers


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DSSSThe minimum linear processing gain that FCCallows is 10

Most commercial products operate under 20The IEEE 802.11working group has set theirminimum processing gain requirements at 11A chipping code is assigned to represent logic 1and 0 data bits


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DSSS

D = rate of data signalBreak each bit into k

chipsChips are a user-specificfixed pattern

Chip data rate of newchannel = kD


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DSSS


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DSSS Signal Coding

If k = 6 and code is a sequence of 1s and -1sFor a 1 bit, A sends code as chip pattern

For a 0 bit, A sends complement of code

Receiver knows sender s code and performselectronic decode function

< d1, d2, d3, d4, d5, d6> = received chippattern

< c1, c2, c3, c4, c5, c6> = sender s code


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DSSS Signal Coding

User A code = To send a 1 bit =

To send a 0 bit = < 1, 1, 1, 1, 1, 1>User B code =

To send a 1 bit =

Receiver intends to receive A s code(A s code) x (received chip pattern)

S A =6 means that A has sent a 1 bit

S A = - 6 means that A has sent a 0 bit- 6 < S A < 6 unwanted signal ignored


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DSSS Signal Coding


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DSSS ChannelsMore conventional definition of channelsEach channel occupies a contiguous bandwidth of 22 MHz

Continuous channels spaced by 5 MHzOnly first 11 channels available in USANon-overlapping channels: USA: 1,6,11 ETSI: 1,5,9,13


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Effects of Narrow Band Interference


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To DSSS

DSSS systems are also resistant to narrow bandinterference due to their spread spectrum

characteristicsAmount of interference energy is reduced bythe spreading factor

A DSSS signal is more susceptible to narrow bandinterference than FHSS , why ???

The DSSS band is much smaller (22 MHz wideinstead of the 79 MHz wide band used by FHSS)The information is transmitted along the entireband simultaneously instead of one frequencyat a time


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Comparing FHSS and DSSS

Commons:Transmission bandwidth is greater than the

information data rateSignal spreads the wide bandwidth to reducepower spectrum density

Spread and de-spread codes are the sameResistant to narrow band interferenceSecurity in communications

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Comparing FHSS and DSSS


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SP - EEEET0246 Wireless Network & Security - 59-

Differences Between FHSS and DSSS

Data rate & throughput: DSSS can achievemuch higher data rates than FHSS s 2 MbpsEquipment compatibility: WECA supports802.11bSystem Support: DSSS is widely supported thanFHSS due to high speed and low cost


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Ch l C l ti A l i


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Channel Co-location Analysis

802.11b DSSS channel co-location analysisA DSSS system has a maximum of 3 co-location channels.

Theoretical maximum throughput= 11 x 3 = 33 MbpsBandwidth efficiency is only 50%, the throughput wouldbe about: 33Mbps x 50% = 16.5 Mbps

Channel Co location Anal sis


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802.11 FHSS channel co-location analysis

12 synchronized FHSS channels can be co-locatedYields a theoretical throughput: 12 x 2 =24Mbps

50% rated bandwidth: 24 x 50%=12Mbps

15 non-synchronized FHSS channel can be co-located

Theoretical throughput: 15 x 2 =30 MbpsOf course, rated bandwidth is less than 50%Actual throughput < 30 x 50% =15Mbps

Channel Co location Analysis


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802.11g OFDM co-location analysis:The same channel definition as in 802.11b

A maximum of 3 co-location channels.Theoretical maximum throughput= 54 x 3 =162 Mbps

The actual throughput per channel is maximum36Mbps in a pure 802.11g environment36Mbps x 3 = 108 Mbps

In a mixed environment, the actual throughputper channel is only 12Mbps

Channel Co location Analysis


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802.11a OFDM co-location analysis:802.11a channels are non-overlapping

A maximum of 8 co-location channels.Theoretical maximum throughput= 54 x 8 =432 Mbps

The actual throughput per channel is maximum36Mbps:36Mbps x 8 = 288 Mbps

In the view of co-location, 802.11a is the best!

OFDM


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OFDMOFDM achieves higher data rate by dividing a single channelinto a large number of smaller bandwidth sub-carriers.

Each sub-carrier has a relative low data rateTransmitting data in parallel on all sub-carriers

simultaneously, high data rates can be achieved.OFDM offers highest data rate and maximum resistance tointerference and corruption of all the manipulationtechniques in 802.11 today.Not considered as spread spectrum, but it shares manyqualities with spread spectrum, including using a low TXpower and wider-than-necessary band-width.

OFDM in 802 11a


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OFDM in 802.11a802.11a uses OFDM to transmit in UNII band

Three UNII bands: high(UNII-3), middle(UNII-2) andlow(UNII-1)Consumer products use the middle and low bands,allowing a total of 8 channels .Each channel consists of 52 sub-carriers

Each band 100MHz

OFDM in 802 11g


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OFDM in 802.11g

Dividing into three modes:ERP-OFDM: Uses only OFDM to transmit entire frames inenvironments where only 802.11g stations are present.DSSS-OFDM:

Uses DBPSK or DQPSK modulation for low-level headers,and OFDM for the rest of the frames.( Hybrid frames)

802.11b stations can hear the low-level headers andknow that a OFDM frame is being transmitted and howlong the transmission will last.Avoid the collision in the mixed environment of DSSS

and OFDM .Pure DSSS : acts as pure 802.11b stations.

Chapter 5 Notes


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Chapter 5 Notes

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