Lecture 4 - Spread Spectrum Technologies
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Spread Spectrum
Technologies(1 September, 2006)
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Define spread spectrum technologies and howthey are used
Describe modulation and the different data rates
Explain and compare FHSS, DSSS and OFDM
List the factors that impact signal throughput andrange
ObjectivesUpon completion of this chapter you will be able to:
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Spread Spectrum
Spread spectrum is a communication technique that
spreads a narrowband communication signal over a wide rangeof frequencies for transmission then de-spreads it into theoriginal data bandwidth at the receive.
Spread spectrum is characterized by:
wide bandwidth and
low power
Jamming and interference have less effect on Spreadspectrum because it is:
Resembles noise
Hard to detect
Hard to intercept
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Narrowband vsSpread Spectrum
Frequency
Power
Spread Spectrum(Low Peak Power)
Narrowband(High Peak Power)
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Narrow Band vs Spread Spectrum
Narrow Band
Uses only enough frequency spectrum to carry the signal
High peak power
Easily jammed
Spread Spectrum
The bandwidth is much wider than required to send to thesignal.
Low peak power
Hard to detect
Hard to intercept
Difficult to jam
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Spread Spectrum Use
In the 1980s FCC implemented a set of rules making Spread
Spectrum available to the public.Cordless Telephones
Global Positioning Systems (GPS)
Cell Phones
Personal Communication Systems
Wireless video cameras
Local Area Networks
Wireless Local Area Networks (WLAN)
Wireless Personal Area Network (WPAN)
Wireless Metropolitan Area Network (WMAN)
Wireless Wide Area Network (WWAN)
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FCC Specifications
The Code of Federal Regulations (CFR) Part 15 originallyonly described two spread spectrum techniques to be used in
the licensed free Industrial, Scientific, Medical (ISM) band,2.4 GHz, thus 802.11 and 802.11b.
Frequency Hopping Spread Spectrum (FHSS)and
Direct Sequence spread Spectrum (DSSS)
Orthogonal Frequency Division Multiplexing (OFDM)wasnot covered by the CFR and would have required licensing.
802.11a, employing OFDM, was created to work in the 5GHz
Unlicensed National Information Infrastructure (UNII)
In May, 2001 CFR, Part 15 was modified to allow alternative"digital modulation techniques".
This resulted in 802.11gwhich employs OFDM in the 2.4
GHz range
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Wireless LAN Networks Wireless LANs RF spread spectrum management techniques
Frequency Hopping Spread Spectrum (FHSS).
Operates in the 2.4 Ghz range Rapid frequency switching2.5 hops per second w/ a dwell time of 400ms.
A predetermined pseudorandom pattern
Fast Setting frequency synthesizers.
Direct Sequence Spread Spectrum (DSSS)
Operates in the 2.4 GHz range Digital Data signal is inserted into a higher data ratechipping code.
A Chipping code is a bit sequence consisting of a redundant bit pattern.
Barker, Gold, M-sequence and Kasami codes are employed
Orthogonal Frequency Division Multiplexing (OFDM)
Operates in both the 5 Ghz and 2.4 GHz range with a data rate of between 6and 54 Mbps. 802.11a divides each channel into 52 low-speed sub-channels 48 sub-channels are for data while the other 4 are pilot carriers.
The modulation scheme can be either BPSK, QPSK or QAM dependingupon the speed of transmission.
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FCC Radio Spectrum
VLF 10 kHz - 30 kHz Cable Locating Equipment
LF 30 kHz - 300 kHz Maritime Mobile Service.
MF 300 kHz - 3 MHz Aircraft navigation, ham radio andAvalanche transceivers.
HF 3 MHz - 30 MHz CB radios, CAP, Radio telephone,
and Radio Astronomy.VHF 30 MHz - 328.6 MHZ Cordless phones, Televisions, RCCars, Aircraft, police and business radios.
UHF 328.6 MHz - 2.9 GHz police radios, fire radios, businessradios, cellular phones, GPS, paging,
wireless networks and cordless phones.
SHF 2.9 GHz - 30 GHz Doppler weather radar, satellitecommunications.
EHF 30 GHz and above Radio astronomy, military systems,vehicle radar systems, ham radio.
Band Name Range Usage
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ISM Frequency Bands
UHF ISM 902 - 928 Mhz
S-Band 2 - 4 Ghz
S-Band ISM (802.11b) 2.4 - 2.5 Ghz
C-Band 4 - 8 Ghz
C-Band Satellite downlink 3.7 - 4.2Ghz
C-Band Radar (weather) 5.25 - 5.925 Ghz
C-Band ISM (802.11a) 5.725 - 5.875 Ghz
C-Band satellite uplink 5.925-6.425 Ghz
X-Band 8-12 Ghz
X-Band Radar (police/weather) 9.5-10.55 GhzKu-band 12-18 Ghz
Ku-band Radar (Police) 13.5-15 Ghz
15.7-17.7 Ghz
ISM - Industrial, Scientific and Medical
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FHSS
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Frequency Hopping Spread Spectrum
Carrier changes frequency (HOPS)
according to a pseudorandom Sequence. Pseudorandom sequence is a list of frequencies. Thecarrier hops through this lists of frequencies.
The carrier then repeats this pattern. During Dwell Timethe carrier remains at a certainfrequency.
During Hop Timethe carrier hops to the next frequency.
The data is spread over 83 MHz in the 2.4 GHz ISMband.
This signal is resistant but not immune to narrow band
interference.
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Channel 1 Channel 2 Channel 78
Elapsed Time in Milliseconds (ms)
200 400 600 800 1000 1200 1400 1600
2.401
2.479
TransmissionF
requency(GHz
)
Divide
dinto79
1MHz
Channe
ls
Frequency Hopping Spread Spectrum
An Example of a Co-located Frequency Hopping System
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FHSS Contd
The original 802.11 FHSS standard supports 1 and
2 Mbps data rate.
FHSS uses the 2.402 2.480 GHz frequency range in the ISM band.
It splits the band into 79non-overlapping channels with each channel
1 MHzwide. FHSS hops between channels at a minimum rate of 2.5 times persecond. Each hop must cover at least 6 MHz
The hopping channels for the US and Europe are shown below.
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FHSS Contd
Dwell Time
The Dwell time per frequency is around 100 ms(The FCC specifies a dwell time of 400 ms per carrierfrequency in any 30 second time period).
Longer dwell time = greater throughput.
Shorter dwell time = less throughput
Hop Time
Is measured in microseconds (us) and isgenerally around 200-300 us.
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FHSS Contd
Gaussian Frequency Shift Keying
The FHSS Physical sublayer modulates the data stream usingGaussian Frequency Shift Keying (GFSK).
Each symbol, a zero and a one, is represented by a differentfrequency (2 level GFSK)
two symbols can be represented by four frequencies (4 levelGFSK).
A Gaussian filter smoothes the abrupt jumps betweenfrequencies.
fc + fd2fc + fd1fc - fd1fc fd2
10110100
fc
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FHSS Disadvantages
Not as fast as a wired Lan or the newer WLANStandards
Lower throughput due to interference.
FHSS is subject to interference from other frequencies inthe ISM band because it hops across the entire frequencyspectrum.
Adjacent FHSS access points can synchronize
their hopping sequence to increase the number of co-located systems, however, it is prohibitively expensive.
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DSSS
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Direct Sequence Spread Spectrum
Spread spectrum increases the bandwidth of the signal
compared to narrow band by spreading the signal.
There are two major types of spread spectrum techniques:
FHSS and DSSS.
FHSS spreads the signal by hopping from one frequency toanother across a bandwidth of 83 Mhz.
DSSS spreads the signal by adding redundant bits to the
signal prior to transmission which spreads the signal across 22
Mhz.
The process of adding redundant information to the
signal is calledProcessing Gain .
The redundant information bits are calledPseudorandom
Numbers (PN).
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Direct Sequence Spread Spectrum
DSSS works by combining informationbits (data signal) with
higher data rate bit sequence (pseudorandom number (PN)).
The PN is also called a Chipping Code (eg., the Barker chipping
code)
The bits resulting from combining the information bits with the
chipping code are calledchips - the result- which is thentransmitted.
The higherprocessing gain (more chips) increases the signal's
resistance to interference by spreading it across a greater number of
frequencies. IEEE has set their minimum processing gain to 11. The number
of chips in the chipping code equates to the signal spreading ratio.
Doubling the chipping speed doubles the signal spread and the
required bandwidth.
Si l S di
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Signal Spreading
The Spreaderemploys an encoding scheme (BarkerorComplementary Code Keying (CCK).
The spread signal is then modulated by a carrier employing eitherDifferential Binary Phase Shift Keying (DBPSK), or DifferentialQuadrature Phase Shift Keying (DQPSK).
The Correlatorreverses this process in order to recover the originaldata.
DSSS Ch l
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Fourteen channels are identified, however, the FCC specifies only 11channelsfor non-licensed (ISM band) use in the US.
Each channels is a contiguous band of frequencies 22 Mhz wide with eachchannel separated by 5 MHz.
Channel 1 = 2.401 2.423 (2.412 plus/minus 11 Mhz).
Channel 2 = 2.406 2.429 (2.417 plus/minus 11 Mhz).
Only Channels 1, 6 and 11 do not overlap
DSSS Channels
S t M k
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Spectrum Mask
A spectrum Mask represents the maximum power output for thechannel at various frequencies.
From the center channel frequency, 11 MHz and 22 MHZ the signalmust be attenuated 30 dB.
From the center channel frequency, outside 22 MHZ, the signal isattenuated 50 dB.
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DSSS Frequency Assignments
Channel 12.412 GHz
Channel 6
2.437 GHz
Channel 112.462 GHz
25 MHz25 MHz
The Center DSSS frequencies of each channel are only 5 Mhz apart buteach channel is 22 Mhz wide therefore adjacent channels will overlap.
DSSS systems with overlapping channels in the same physical spacewould cause interference between systems.
Co-located DSSS systems should have frequencies which are at least5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc.
Channels 1, 6 and 11 are the only theoretically non-overlappingchannels.
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2.401 GHz 2.473 GHz
Channel 1 Channel 6 Channel 11
22 MHz
3 MHz
f
P
DSSS Non-overlapping Channels
Each channel is 22 MHz wide. Inorder for two bands not to overlap
(interfere), there must be fivechannelsbetween them.
A maximum of three channels maybe co-located (as shown) withoutoverlap (interference).
The transmitter spreads the signalsequence across the 22 Mhz widechannel so only a few chips will beimpacted by interference.
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DSSS
Encoding and Modulation
DSSS Encoding and Modulation
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DSSS Encoding and Modulation
DSSS (802.11b) employs two types of encoding schemesand two types of modulation schemes depending upon the
speed of transmission.Encoding Schemes
Barker Chipping Code: Spreads 1 data bit across 11 redundantbits at both 1 Mbps and 2 Mbps
Complementary Code Keying (CCK):
Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps
Maps 8 data bits into a unique redundant 8 bits for 11 Mbps.
Modulation SchemesDifferential Binary Phase Shift Keying (DBPSK):Two phaseshifts with each phase shift representing one transmitted bit.
Differential Quadrature Phase Shift Keying (DQPSK):Four
phase shifts with each phase shift representing two bits.
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DSSS Encoding
B k Chi i C d
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Barker Chipping Code
802.11 adopted an 11 bit Barker chipping code.
Transmission. The Barker sequence, 10110111000, was chosen to spreadeach 1 and 0 signal.
The Barker sequence has six 1s and five 0s.
Each data bit, 1 and 0, is modulo-2 (XOR) added to theeleven bit Barker sequence.
If a one is encoded all the bits change.
If a zero is encoded all bits stay the same.
Reception.
Azero bitcorresponds to an eleven bit sequence ofsix 1s.
Aone bitcorresponds to an eleven bit sequence ofsix 0s.
B k S
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Barker Sequence
One Bit
1 0
1 0 1 1 0 1 1 1 0 0 0 1 0 1 1 0 1 1 1 0 0 0
Chipping Code(Barker Sequence)
Original Data
Spread Data0 1 0 0 1 0 0 0 1 1 1 1 0 1 1 0 1 1 1 0 0 0
Six 0s = 1 Six 1s = 0
One Bit
10110111000
Di S S d S C d
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Direct Sequence Spread Spectrum Contd
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Complementary Code Keying (CCK)
Barker encoding along with DBPSK and DQPSK modulation
schemes allow 802.11b to transmit data at 1 and 2 Mbps
Complementary Code Keying (CCK)allows 802.11b totransmit data at 5.5 and 11 Mbps.
CCK employs an 8 bit chipping code. The 8 chipping bit pattern is generated based upon thedata to be transmitted.
At 5.5 Mbps, 4 bits of incoming data is mapped into a
unique 8 bit chipping pattern.
At 11 Mbps, 8 bits of data is mapped into a unique 8bit chipping pattern.
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Complementary Code Keying (CCK) Contd
To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
The unique 8 chipping bits is determined by the bit pattern of the 4 databits to be transmitted. The data bit pattern is:
b0, b1, b2, b3
b2 and b3determine the unique pattern of the 8 bit CCK chippingcode.
Note: j represents the imaginary number, sqrt(-1), and appears on the imaginaryor quadrature axis of the complex plane.
C C CC C
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Complementary Code Keying (CCK) Contd
To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
The unique 8 chipping bits is determined by the bit pattern of the 4 databits to be transmitted. The data bit pattern is:
b0, b1, b2, b3
b0 and b1 determine the DQPSK phase rotation that is to beapplied to the chip sequence.
Each phase change is relative to the last chip transmitted.
C l C d K i (CCK) C d
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Complementary Code Keying (CCK) Contd
To transmit 11 Mbps 8 data bits is mapped into 8
CCK chipping bits.
The unique 8 chipping bits is determined by thebit pattern of the 8 data bits to be transmitted. The
data bit pattern is:b0, b1, b2, b3, b4, b5, b6 ,b7
b2, b3, b4 ,b5, b6 and b7selects one unique
pattern of the 8 bit CCK chipping code out of 64possible sequences.
b0 and b1 are used to select the phase rotationsequence.
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DSSSModulation
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Differential Binary Phase Shift Keying (DBPSK)
0 PhaseShift
A Zero phase shift from theprevious symbol is interpreted asa 0.
A 180 degree phase shift fromthe previous symbol is interpretedas a 1.
180 degreePhase Shift
180 degree
Phase Shift
Previouscarrier symbol
Differential Quadrature Phase Shift Keying (DQPSK)
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Differential Quadrature Phase Shift Keying (DQPSK)
A Zero phase shift from the previoussymbol is interpreted as a 00.
Previouscarrier symbol
0 PhaseShift
A 90 degree phase shift from the previoussymbol is interpreted as a 01.
A 180 degree phase shift from the previous
symbol is interpreted as a 11.
A 270 degree phase shift from the previoussymbol is interpreted as a 10.
90 PhaseShift
180 PhaseShift
270 PhaseShift
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DSSS Summary
1 Barker Coding 11 chips encoding 1 bit DBPSK
2 Barker Coding 11 chips encoding 1 bit DQPSK
5.5 CCK Coding 8 chips encode 8 bits DQPSK
11 CCK Coding 8 chips encode 4 bits DQPSK
Data Rate Encoding Modulation
FHSS vs DSSS
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FHSS vs DSSS
DSSS is more susceptible to narrow band noise. DSSS channel is 22 Mhzwide whereas
FHSS is 79 Mhzwide.
The FCC regulated that DSSS use a maximum of 1 watt
of transmitter power in Pt-to-Multipoint system.
DSSS costs less then FHSS
FHSS can have more systems co-located than
DSSS.
DSSS systems have the advantage in throughput
The Wi-Fi alliance tests for DSSS compatibility
No such testing alliance exists for FHSS.
FHSS vs DSSS contd
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FHSS vs DSSS contd
DSSS generally has a throughput of 5-6 Mbps
while FHSS is generally between 1-2 Mbps. Both FHSS and DHSS are equally insecure.
DSSS has gained much wider acceptance due to
its low cost, high speed and interoperability.
This market acceptance is expected to
accelerate.
FHSS advancement includes HomeRFand 802.15
(WPAN) (Bluetooth), however, it is expected to notadvance into the enterprise.
Co-location Comparison
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Co location Comparison
1 5 10 15 20
10
20
30
40
Number of Co-located Systems
11 Mbps DSSS
3 Mbps FHSS (sync.)
3 Mbps FHSS (no sync.)
54 Mbps OFDM
Da
teRateinMbps
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OFDM
802 11
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802.11a
IEEE 802.11a Standard.
Orthogonal Frequency Division Multiplexing (OFDM).
Operates in the 5.0 GHz band.
It Operates in the Unlicensed National InformationInfrastructure (UNII).
200 channels ( channels 1-199) spaced 5 MHz apart.
Supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54,
MBps. 6, 12, and 24 are mandatory. All others are optional.
75-80 Feet
64 users /Access Point
802 11a Network Channel Assignments
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802.11a Network Channel AssignmentsArea Frequency Band Channel Center Frequency
USA U-NII Lower Band 36 5.180 Ghz
(5.150-5.250 Ghz) 40 5.200 Ghz
44 5.220 Ghz
48 5.240 Ghz
USA U-NII Middle Band 52 5.260 Ghz
(5.2505.350 Ghz) 56 5.260 Ghz
60 5.280 Ghz
64 5.320 Ghz
USA U-NII Upper Band 149 5.745 Gh
(5.725
5.825) 153 5.765 Ghz157 5.785 Ghz
161 5.805 Ghz
NOTE: 1. U-NII : Unlicensed National Information Infrastructure.2. 802.11a is specific to the US.
OFDM
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OFDM
A mathematical process that allows52 channels to overlap without
losing their orthogonality (individuality).
48 sub-channel are used for data
Each sub-channel is used to transmit data
4 sub-channel are used as pilot carriers.
The pilot sub-channels are used to monitor path shift and
shifts in sub-channel frequencies (Inter Carrier Interference
(ICI)).
OFDM
OFDM selects channels that
overlap but do not interferewith one another.
Channels are separated
based upon orthogonality.
802 11a Channels
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802.11a Channels
Lower UNII Band Middle UNII Band
802.11a use the lower and middle UNII 5 GHz bands to create 8 channels.
Each Channel is 20 MHz each.
Each channel is broken into 52 sub-channels with each sub-channel300 KHz each.
48 Sub-channels are used to transmit data
4 sub-channels are used as Pilot carriers to monitor the channel
8Channels
52Sub-Channels
for each 8channels
Each channel is20 MHz wide
Lower andMiddle UNII
frequency band
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OFDM
Modulation
Modulation Background
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Modulation Background
In order to properly understand OFDM modulation we need to doa quick review of various modulation techniques.
James Clark Maxwell, 1864, first developed the idea thatelectromagnetic magnetic waves arose as a combination electriccurrent and magnetic field an electromagnetic wave.
Heinrich Hertz , in 1880s, developed the first Radio
Frequency device that sent and received electromagnetic wavesover the air
The name Hertz (Hz)was given to the unit of frequencymeasurement representing one complete oscillation of an
electromagnetic wave. This is also called cycle per second.Kilohertz = thousands of cycles per second
Megahertz = millions of cycles per second
Gigahertz = billions cycles per second
Modulation Background Contd
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g
The oscillating electromagnetic wave, also called a sine wave, is shown below.
This wave can be used as a carrier signalto carry information.
The information can be imposed upon the carrier through a process calledmodulationwhich is accomplished by modifying one of three physical wavecharacteristic. These physical characteristics are:
Amplitude The height of the wave
Frequency the number of oscillation (cycles) per second.
Phase
the starting point of the wave (when compared to the starting point ofthe previous wave.
The are two major types of modulation schemes: Analog and Digital
Amplitude
Frequency
Phase
Sine Wave
Analog Modulation
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Analog Modulation
Amplitude Modulationvaries theheight of the carrier wave.
Frequency Modulationvaries thenumber of oscillation (waves) per
second
Phase Modulationchanges thestarting point of the wave.
Change inPhase
Change inFrequency
Change inAmplitude
1 = 1800 Phase Change0 = No Phase Change
Digital Modulation
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Digital Modulation
1 = 1800 Phase Change0 = No Phase Change
Amplitude Shift Keying (ASK)changes the amplitude of the carrierwave to represent a 0 or 1.
Frequency Shift Keying (FSK)changes the frequency of the carrier
wave to represent a 0 or 1.
Phase Shift Keying (PSK)changes
the phase of the carrier wave torepresent a 0 or 1.
180 degreephase change
Phase Modulation Extended
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Phase Modulation Extended
Phase Modulationchangesthe starting point of the wave.
Change inPhase
1 = 1800 Phase Change0 = No Phase Change
900
2700
180o 0
o
1 0
Phase shiftcan also be represented on an x/y axisconstellation such that:
In this instance we can transmit 1 bit for every phaseshift.
This is called Binary Phase Shift Keying (BPSK)in802.11a
radians)1 = 1800 Phase Change (
0 = No Phase Changeradians)1 = 1800 Phase Change (
0 = No Phase Change
BPSK
QUADRATURE AMPLITUDE MODULATION (QAM)
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900
2700
00
135o
01
11 10
35o
315o225
o
180o 0
o
2 bits/phase
Quadrature Phase Shift Keying (QPSK)
extends this technique to transmit two bits forevery phase shift.
0000
0001
0011
00100110
0111
01010100
1100
1101
1111
1110 10101011
1001 1000
900
2700
180o 0
o
4 bits/phase
Quadrature Amplitude Modulation(QAM)generalizes these techniques toencode information in both phase (byemploying PSK techniques such as BPSKand QPSK) with amplitude.
For example, in the diagram a right, eachquadrature contains 4 amplitudes (16 levels)and can therefore transmit 4 bits per phase.
00 = 350 Phase Change
01 = 1350 Phase Change
11 = 2250 Phase Change
10 = 3150 Phase Change
QPSK
QAM
QAM Extended
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QAM Extended
In the diagram at right, eachquadrature contains 8 amplitudes (64levels) and can therefore transmit 6 bitsper phase.
900
2700
180o 0
o
Summary of OFDM Encoding/Modulation
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64 Phase shifts can encode 6 bits /phase shift resulting is a transmission rate ofeither 48 or 54 Mbps depending upon the number of sub-channels (R) used for errorcorrection.
Coding Rate (R) is the ratio of sub-channels carrying data to sub-channels
carrying error correction code. E.G., 1/2 would indicate that 24 sub-channels (1/2 X48 = 24) are being used for error correction while the remaining 24 sub-channels areused for data transmission.
The Length of the each Symbol is equal to number of sub-carriers times the bits/transition. e.g., 48 X 6 = 288.
Summary of OFDM Encoding/Modulation
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Summary of OFDM Encoding/Modulation
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End of Lecture