11

Modulation Techniques for Mobile RadioModulation Techniques for Mobile RadioModulation Techniques for Mobile RadioModulation Techniques for Mobile Radio

Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission.

It generally involves translating a base band signal (or source) to a band pass signal, centered at a high carrier frequency.

Demodulation is the process of extracting the base band message from the carrier.

Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission.

It generally involves translating a base band signal (or source) to a band pass signal, centered at a high carrier frequency.

Demodulation is the process of extracting the base band message from the carrier.

22

Modulation TechniquesModulation TechniquesModulation TechniquesModulation Techniques

33

Review of Analog Modulation TechniquesReview of Analog Modulation TechniquesReview of Analog Modulation TechniquesReview of Analog Modulation Techniques

Amplitude Modulation (AM)Amplitude Modulation (AM)

• Message Signal -- Message Signal --

• Carrier SignalCarrier Signal -- --

• AM Signal AM Signal ----

Amplitude Modulation (AM)Amplitude Modulation (AM)

• Message Signal -- Message Signal --

• Carrier SignalCarrier Signal -- --

• AM Signal AM Signal ----

c cA cos(2πf t)

AM c cS (t) = A [1+m(t)]cos(2 f t)

m(t)

44

AM SpectrumAM SpectrumAM SpectrumAM Spectrum

AMS (f) c c c c c = 0.5A [ (f - f ) + M(f - f ) + (f + f ) + M(f + f )]

CarrierSidebands

55

AM ParametersAM ParametersAM ParametersAM Parameters• Modulation IndexModulation Index -- --

• BandwidthBandwidth -- --

• Total Power in AM SignalTotal Power in AM Signal

• Power in the carrierPower in the carrier -- --

• Modulation IndexModulation Index -- --

• BandwidthBandwidth -- --

• Total Power in AM SignalTotal Power in AM Signal

• Power in the carrierPower in the carrier -- --

AM m B = 2 f

AMPc

2 2= 0.5A [ 1 + 2 <m(t)> + <m (t)> ]

c

2c P = A / 2

m c k= A / A 1

66

Single Singleband AM SignalSingle Singleband AM SignalSingle Singleband AM SignalSingle Singleband AM SignalLower SidebandLower Sideband

Upper SidebandUpper Sideband

Where the Hilbert transform is defined as:Where the Hilbert transform is defined as:

; ;

Lower SidebandLower Sideband

Upper SidebandUpper Sideband

Where the Hilbert transform is defined as:Where the Hilbert transform is defined as:

; ;

SSBS c c cA [m(t)cos(2 f t) m(t)sin(2 f t)]

m(t) m(t) h(t) h(t) 1/ t

77

SSB GenerationSSB GenerationSSB GenerationSSB GenerationFilter MethodFilter Method

Baseband filter passes upper or lower sidebands Baseband filter passes upper or lower sidebands

Filter MethodFilter Method

Baseband filter passes upper or lower sidebands Baseband filter passes upper or lower sidebands

Baseband FilterBaseband Filter

SSBS (t)c cA cos(2 f t)m(t)

88

Balanced ModulatorBalanced ModulatorBalanced ModulatorBalanced Modulator

SSBS (t)Carrier fc

-90-90o o phase shiftphase shift

9090oo

∑∑

m(t)

99

Properties of SSBProperties of SSBProperties of SSBProperties of SSB

Bandwidth of SSB is very efficient = fBandwidth of SSB is very efficient = fmm .However, Doppler spreading and Rayleigh .However, Doppler spreading and Rayleigh

fading can shift the signal spectrum, causing fading can shift the signal spectrum, causing distortion.distortion.

Frequency of the receiver oscillator must be Frequency of the receiver oscillator must be exactly the same as that of the transmitted exactly the same as that of the transmitted carrier fcarrier fcc. If not, this results in a frequency . If not, this results in a frequency shift fshift fcc f, causing distortion.f, causing distortion.

Bandwidth of SSB is very efficient = fBandwidth of SSB is very efficient = fmm .However, Doppler spreading and Rayleigh .However, Doppler spreading and Rayleigh

fading can shift the signal spectrum, causing fading can shift the signal spectrum, causing distortion.distortion.

Frequency of the receiver oscillator must be Frequency of the receiver oscillator must be exactly the same as that of the transmitted exactly the same as that of the transmitted carrier fcarrier fcc. If not, this results in a frequency . If not, this results in a frequency shift fshift fcc f, causing distortion.f, causing distortion.

1010

Pilot Tone SSB Pilot Tone SSB Pilot Tone SSB Pilot Tone SSB

Transmit a low level pilot tone along with Transmit a low level pilot tone along with the SSB signal.the SSB signal.

The pilot tone has information on the frequency The pilot tone has information on the frequency and amplitude of the carrier.and amplitude of the carrier.

The pilot tone can be tracked using signal The pilot tone can be tracked using signal processing FFSR - Feed Forward Signal processing FFSR - Feed Forward Signal Regeneration.Regeneration.

Transmit a low level pilot tone along with Transmit a low level pilot tone along with the SSB signal.the SSB signal.

The pilot tone has information on the frequency The pilot tone has information on the frequency and amplitude of the carrier.and amplitude of the carrier.

The pilot tone can be tracked using signal The pilot tone can be tracked using signal processing FFSR - Feed Forward Signal processing FFSR - Feed Forward Signal Regeneration.Regeneration.

1111

TTIB (Transparent Tone In-Band) SystemTTIB (Transparent Tone In-Band) System TTIB (Transparent Tone In-Band) SystemTTIB (Transparent Tone In-Band) System

~

m(t)

a

a

b

c d

2f1f f~ ~

~ e

1212

a

d

bc

e

frequencyf2 1BW f f

1f

2f

1313

Properties of TTIB systemProperties of TTIB systemProperties of TTIB systemProperties of TTIB system

Base band signal is split into two equal width Base band signal is split into two equal width segments.segments.

Small portion of audio spectrum is removed and Small portion of audio spectrum is removed and a low-level pilot tone is inserted in its place.a low-level pilot tone is inserted in its place.

This procedure maintains the low bandwidth of This procedure maintains the low bandwidth of the SSB signal.the SSB signal.

Provides good adjacent channel protection.Provides good adjacent channel protection.

Base band signal is split into two equal width Base band signal is split into two equal width segments.segments.

Small portion of audio spectrum is removed and Small portion of audio spectrum is removed and a low-level pilot tone is inserted in its place.a low-level pilot tone is inserted in its place.

This procedure maintains the low bandwidth of This procedure maintains the low bandwidth of the SSB signal.the SSB signal.

Provides good adjacent channel protection.Provides good adjacent channel protection.

1414

Demodulation of AM signalsDemodulation of AM signals Demodulation of AM signalsDemodulation of AM signals

• Coherent Modulation

• Non-coherent demodulation

• Envelope Detectors

1515

Demodulation of AM signalsDemodulation of AM signals

Coherent ModulationCoherent Modulation

Demodulation of AM signalsDemodulation of AM signals

Coherent ModulationCoherent Modulation

LPFLPF o c oA cos(2 f t ) o c oA cos(2 f t )

OUT o r oV (t) 0.5A R(t)cos( ) OUT o r oV (t) 0.5A R(t)cos( )

AM c rS R(t)cos(2 f t )

1616

Frequency ModulationFrequency ModulationFrequency ModulationFrequency Modulation• Message Signal –Message Signal –

• FM Signal –FM Signal –

• Power in FM Signal –Power in FM Signal –

• Frequency modulationFrequency modulation index index ––

W = Highest frequency component in message signalW = Highest frequency component in message signalAAM M = Peak value of modulating signal= Peak value of modulating signal

• Message Signal –Message Signal –

• FM Signal –FM Signal –

• Power in FM Signal –Power in FM Signal –

• Frequency modulationFrequency modulation index index ––

W = Highest frequency component in message signalW = Highest frequency component in message signalAAM M = Peak value of modulating signal= Peak value of modulating signal

m(t)

FM C c f

t

S (t) A cos[2 f t 2 k m(t)dt]

f f Mk A / W f / W

FM c

2P A / 2

1717

Phase ModulationPhase ModulationPhase ModulationPhase Modulation

• PM Signal PM Signal

• Phase Modulation Index Phase Modulation Index

• Power in PM signal Power in PM signal

• Bandwidth Bandwidth

• PM Signal PM Signal

• Phase Modulation Index Phase Modulation Index

• Power in PM signal Power in PM signal

• Bandwidth Bandwidth

PM c

2P A / 2

Mk A

TB 2 f

SPM (t) = Ac cos[2fc t +km(t)]SPM (t) = Ac cos[2fc t +km(t)]

1818

FM methodsFM methods FM methodsFM methods

FM Modulation FM Modulation Direct Method – VCODirect Method – VCO Indirect Method – ArmstrongIndirect Method – ArmstrongFM DetectionFM Detection Slope DetectionSlope Detection Zero Crossing DetectionZero Crossing Detection PLL DetectionPLL Detection Quadrature DetectionQuadrature Detection

FM Modulation FM Modulation Direct Method – VCODirect Method – VCO Indirect Method – ArmstrongIndirect Method – ArmstrongFM DetectionFM Detection Slope DetectionSlope Detection Zero Crossing DetectionZero Crossing Detection PLL DetectionPLL Detection Quadrature DetectionQuadrature Detection

1919

Comparison between AM and FM Comparison between AM and FM Comparison between AM and FM Comparison between AM and FM

FMFM AMAM•FM signals are less noisy, FM signals are less noisy, because amplitude of signal because amplitude of signal is constantis constant

•AM signals are more noisy, AM signals are more noisy, amplitude cannot be limitedamplitude cannot be limited

•The modulation index can The modulation index can be varied to obtain greater be varied to obtain greater SNR(6dB for each doubling SNR(6dB for each doubling in bandwidth)in bandwidth)

•Modulation index cannot be Modulation index cannot be changed automatically.changed automatically.

•FM signals occupy more FM signals occupy more bandwidth (good for audio)bandwidth (good for audio)

•AM signals occupy lesser AM signals occupy lesser bandwidth (good for video)bandwidth (good for video)

2020

Digital ModulationDigital ModulationDigital ModulationDigital Modulation VLSI and DSP promoted the advent of Digital VLSI and DSP promoted the advent of Digital

ModulationModulation Low noiseLow noise Easier multiplexing of information (voice, data, Easier multiplexing of information (voice, data,

video)video) Can accommodate digital transmission errors, Can accommodate digital transmission errors,

source coding, encryption and equalization.source coding, encryption and equalization. DSP can implement digital modulators, DSP can implement digital modulators,

demodulators completely in software.demodulators completely in software.

VLSI and DSP promoted the advent of Digital VLSI and DSP promoted the advent of Digital ModulationModulation

Low noiseLow noise Easier multiplexing of information (voice, data, Easier multiplexing of information (voice, data,

video)video) Can accommodate digital transmission errors, Can accommodate digital transmission errors,

source coding, encryption and equalization.source coding, encryption and equalization. DSP can implement digital modulators, DSP can implement digital modulators,

demodulators completely in software.demodulators completely in software.

2121

Basics of digital communicationsBasics of digital communications Basics of digital communicationsBasics of digital communications

In digital communication systems, the In digital communication systems, the message) is represented as a time sequence of message) is represented as a time sequence of symbols or pulses. symbols or pulses.

Each symbol has m finite statesEach symbol has m finite states

Number of bits required for m states:Number of bits required for m states:n = logn = log22m bits/symbol m bits/symbol

In digital communication systems, the In digital communication systems, the message) is represented as a time sequence of message) is represented as a time sequence of symbols or pulses. symbols or pulses.

Each symbol has m finite statesEach symbol has m finite states

Number of bits required for m states:Number of bits required for m states:n = logn = log22m bits/symbol m bits/symbol

2222

Shannon’s bandwidth theorem Shannon’s bandwidth theorem Shannon’s bandwidth theorem Shannon’s bandwidth theorem

Bandwidth efficiencyBandwidth efficiency BB = R / B bps/Hz = R / B bps/Hz

R=Data rate in bits/secondR=Data rate in bits/second

B=Bandwidth of modulated RF signalB=Bandwidth of modulated RF signal Shannon's formula:Shannon's formula:

BmaxBmax = C/B = = C/B = channel capacity (bits/s)channel capacity (bits/s) RF bandwidthRF bandwidth

= log= log22(1 + S/N)(1 + S/N)S/N = Signal to Noise ratioS/N = Signal to Noise ratio

Bandwidth efficiencyBandwidth efficiency BB = R / B bps/Hz = R / B bps/Hz

R=Data rate in bits/secondR=Data rate in bits/second

B=Bandwidth of modulated RF signalB=Bandwidth of modulated RF signal Shannon's formula:Shannon's formula:

BmaxBmax = C/B = = C/B = channel capacity (bits/s)channel capacity (bits/s) RF bandwidthRF bandwidth

= log= log22(1 + S/N)(1 + S/N)S/N = Signal to Noise ratioS/N = Signal to Noise ratio

2323

Practical digital systemsPractical digital systems For US digital cellular standard, R = 48.6 kbpsFor US digital cellular standard, R = 48.6 kbps

RF bandwidth = 30 KHzRF bandwidth = 30 KHz

For SNR 20 dB => 100For SNR 20 dB => 100

C = 30000 * logC = 30000 * log22(1 + S/N)(1 + S/N)

= 30000 * log= 30000 * log22(1 + 100) = 199.75 kbps(1 + 100) = 199.75 kbps For GSM standard, R = 270.833 kbpsFor GSM standard, R = 270.833 kbps

C = 1.99 Mbps for S/N = 30 dBC = 1.99 Mbps for S/N = 30 dB

Practical digital systemsPractical digital systems For US digital cellular standard, R = 48.6 kbpsFor US digital cellular standard, R = 48.6 kbps

RF bandwidth = 30 KHzRF bandwidth = 30 KHz

For SNR 20 dB => 100For SNR 20 dB => 100

C = 30000 * logC = 30000 * log22(1 + S/N)(1 + S/N)

= 30000 * log= 30000 * log22(1 + 100) = 199.75 kbps(1 + 100) = 199.75 kbps For GSM standard, R = 270.833 kbpsFor GSM standard, R = 270.833 kbps

C = 1.99 Mbps for S/N = 30 dBC = 1.99 Mbps for S/N = 30 dB

2424

Line CodingLine CodingLine CodingLine Coding Line codes are used to provide particular spectral Line codes are used to provide particular spectral

characteristics of a pulse train.characteristics of a pulse train. Line codes provide the pulses to represent 0s Line codes provide the pulses to represent 0s

and 1s.and 1s. Line codes can be:Line codes can be:

o Return-to-zero (RZ)Return-to-zero (RZ)o Non-return-to-zero (NRZ)Non-return-to-zero (NRZ)

Line codes are Unipolar (0,V) or Bipolar (-V, V )Line codes are Unipolar (0,V) or Bipolar (-V, V )

Line codes are used to provide particular spectral Line codes are used to provide particular spectral characteristics of a pulse train.characteristics of a pulse train.

Line codes provide the pulses to represent 0s Line codes provide the pulses to represent 0s and 1s.and 1s.

Line codes can be:Line codes can be:o Return-to-zero (RZ)Return-to-zero (RZ)o Non-return-to-zero (NRZ)Non-return-to-zero (NRZ)

Line codes are Unipolar (0,V) or Bipolar (-V, V )Line codes are Unipolar (0,V) or Bipolar (-V, V )

2525

Unipolar NRZUnipolar NRZ 1 0 1 1 0 1 0 1 1 0

Unipolar NRZUnipolar NRZ 1 0 1 1 0 1 0 1 1 0

VV

0 Unipolar RZ0 Unipolar RZ

VV Bipolar NRZ Bipolar NRZ

-V-V

VV

0 Unipolar RZ0 Unipolar RZ

VV Bipolar NRZ Bipolar NRZ

-V-V

2626

Pulse Shaping TechniquesPulse Shaping TechniquesPulse Shaping TechniquesPulse Shaping Techniques

ISI – Inter Symbol Interference ISI – Inter Symbol Interference errors in errors in transmission of symbolstransmission of symbols

Pulse shaping techniques reduce the inter-Pulse shaping techniques reduce the inter-symbol effectssymbol effects

ISI – Inter Symbol Interference ISI – Inter Symbol Interference errors in errors in transmission of symbolstransmission of symbols

Pulse shaping techniques reduce the inter-Pulse shaping techniques reduce the inter-symbol effectssymbol effects

BandlimitedBandlimitedChannelChannel

2727

Pulse shaping filtersPulse shaping filtersPulse shaping filtersPulse shaping filters Raised cosine filterRaised cosine filter

As the value of As the value of (roll-off factor) increases, the (roll-off factor) increases, the bandwidth of the filter also increasesbandwidth of the filter also increases

As the value of a (roll-off factor) increases, the As the value of a (roll-off factor) increases, the time sidelobe levels decrease. time sidelobe levels decrease.

Raised cosine filterRaised cosine filter

As the value of As the value of (roll-off factor) increases, the (roll-off factor) increases, the bandwidth of the filter also increasesbandwidth of the filter also increases

As the value of a (roll-off factor) increases, the As the value of a (roll-off factor) increases, the time sidelobe levels decrease. time sidelobe levels decrease.

RCh (t)

s2

s

cos( t /T )sin( t /T ) s[ ]( t /T ) (1 4 t /2T )s

2828

Implementation of raised-cosine filterImplementation of raised-cosine filter Implementation of raised-cosine filterImplementation of raised-cosine filter

Use identical [HUse identical [HRCRC (f)] (f)]1/21/2 filters at transmitter and filters at transmitter and receiver receiver

Symbol rate possible through raised cosine filter Symbol rate possible through raised cosine filter

where B is the filter bandwidthwhere B is the filter bandwidth

Use identical [HUse identical [HRCRC (f)] (f)]1/21/2 filters at transmitter and filters at transmitter and receiver receiver

Symbol rate possible through raised cosine filter Symbol rate possible through raised cosine filter

where B is the filter bandwidthwhere B is the filter bandwidth

s sR 1/ T 2B /(1 )

2929

Types of Digital ModulationTypes of Digital ModulationTypes of Digital ModulationTypes of Digital ModulationLinearLinear Non-LinearNon-Linear Spread Spread

SpectrumSpectrumAmplitude of Amplitude of transmitted signal transmitted signal varies linearly with varies linearly with message signal m(t) message signal m(t)

Amplitude of Amplitude of carrier is constant carrier is constant

Transmission Transmission bandwidth >> bandwidth >> signal signal bandwidth bandwidth

Low bandwidth- Low bandwidth- allows more usersallows more users

High bandwidth –High bandwidth –Low noiseLow noise

More users-More users-high bandwidthhigh bandwidth

Example systems:Example systems: BPSK, QPSKBPSK, QPSK

FSK, GMSKFSK, GMSK W-CDMA, W-CDMA, cdma 2000cdma 2000

3030

Linear digital modulationLinear digital modulationLinear digital modulationLinear digital modulation

PSK or Phase Shift Keying of carrier:PSK or Phase Shift Keying of carrier:SSPSKPSK = A cos( = A cos(t + t + kk))

kk = 0, = 0, (BPSK) (BPSK) kk = 0, = 0, (QPSK) (QPSK) kk = =

0,0,330PSK)0PSK)

PSK or Phase Shift Keying of carrier:PSK or Phase Shift Keying of carrier:SSPSKPSK = A cos( = A cos(t + t + kk))

kk = 0, = 0, (BPSK) (BPSK) kk = 0, = 0, (QPSK) (QPSK) kk = =

0,0,330PSK)0PSK)

3131

Constellation DiagramConstellation DiagramConstellation DiagramConstellation Diagram

Q(Quadrature)Q(Quadrature)

I (In Phase)I (In Phase)

Q(Quadrature)Q(Quadrature)

I (In Phase)I (In Phase)

3232

Properties of BPSK and QPSKProperties of BPSK and QPSKProperties of BPSK and QPSKProperties of BPSK and QPSK BPSKBPSK

BW = 2 RBW = 2 RBB = 2 / T = 2 / TBB

PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]

RRBB – Bit rate, T – Bit rate, TBB – Bit period – Bit period

EEBB – Energy/bit, N – Energy/bit, N00 – Noise spectral density – Noise spectral density QPSKQPSK

BW = RBW = RBB = 1 / T = 1 / TBB

PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]

BPSKBPSK

BW = 2 RBW = 2 RBB = 2 / T = 2 / TBB

PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]

RRBB – Bit rate, T – Bit rate, TBB – Bit period – Bit period

EEBB – Energy/bit, N – Energy/bit, N00 – Noise spectral density – Noise spectral density QPSKQPSK

BW = RBW = RBB = 1 / T = 1 / TBB

PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]

3333

Nonlinear or envelope modulationNonlinear or envelope modulationNonlinear or envelope modulationNonlinear or envelope modulation

Frequency shift keying Frequency shift keying

The frequency of a constant amplitude carrier The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0)signal is switched between 2 values ( 1 and 0)

Frequency shift keying Frequency shift keying

The frequency of a constant amplitude carrier The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0)signal is switched between 2 values ( 1 and 0)

1/ 2FSK h b b c bS V (t) (2E / T ) cos[2 f 2 f ]t,0 T T

1/ 2FSK i b b c bS V (t) (2E / T ) cos[2 f 2 f ]t,0 T T

3434

Properties of FSK Properties of FSK Properties of FSK Properties of FSK Transmission BandwidthTransmission Bandwidth

BBTT = 2 = 2f + 2Bf + 2B

B = Bandwidth of digital base-band signalB = Bandwidth of digital base-band signal If a raised cosine pulse-shaping filter is usedIf a raised cosine pulse-shaping filter is used

BBTT = 2 = 2f + (1 + f + (1 + )R)R Probability of error Probability of error

PPe,FSKe,FSK = Q[(E = Q[(EBB / N / N00))1/21/2]]

Transmission BandwidthTransmission Bandwidth

BBTT = 2 = 2f + 2Bf + 2B

B = Bandwidth of digital base-band signalB = Bandwidth of digital base-band signal If a raised cosine pulse-shaping filter is usedIf a raised cosine pulse-shaping filter is used

BBTT = 2 = 2f + (1 + f + (1 + )R)R Probability of error Probability of error

PPe,FSKe,FSK = Q[(E = Q[(EBB / N / N00))1/21/2]]

3535

Spread Spectrum Modulation techniquesSpread Spectrum Modulation techniques Spread Spectrum Modulation techniquesSpread Spectrum Modulation techniques

Spread spectrum techniques employ a Spread spectrum techniques employ a transmission bandwidth >> signal bandwidthtransmission bandwidth >> signal bandwidth

The system is inefficient for a single user, but The system is inefficient for a single user, but is efficient for many usersis efficient for many users

Many users use the same bandwidth without Many users use the same bandwidth without significantly interfering with one anothersignificantly interfering with one another

Spread spectrum techniques employ a Spread spectrum techniques employ a transmission bandwidth >> signal bandwidthtransmission bandwidth >> signal bandwidth

The system is inefficient for a single user, but The system is inefficient for a single user, but is efficient for many usersis efficient for many users

Many users use the same bandwidth without Many users use the same bandwidth without significantly interfering with one anothersignificantly interfering with one another

3636

Principle of spread spectrumPrinciple of spread spectrumPrinciple of spread spectrumPrinciple of spread spectrum

Spread spectrum signals are pseudo random, Spread spectrum signals are pseudo random, and spreading waveform is controlled by a PN and spreading waveform is controlled by a PN (pseudo – noise) sequence or code.(pseudo – noise) sequence or code.

Spread spectrum signals are demodulated at Spread spectrum signals are demodulated at the receiver by cross correlation (matching) the receiver by cross correlation (matching) with the correct PN sequence.with the correct PN sequence.

Spread spectrum signals are pseudo random, Spread spectrum signals are pseudo random, and spreading waveform is controlled by a PN and spreading waveform is controlled by a PN (pseudo – noise) sequence or code.(pseudo – noise) sequence or code.

Spread spectrum signals are demodulated at Spread spectrum signals are demodulated at the receiver by cross correlation (matching) the receiver by cross correlation (matching) with the correct PN sequence.with the correct PN sequence.

3737

Advantages of spread spectrum techniquesAdvantages of spread spectrum techniquesAdvantages of spread spectrum techniquesAdvantages of spread spectrum techniques

PN codes are approximately orthogonal, and PN codes are approximately orthogonal, and the receiver can separate each user based on the receiver can separate each user based on their codes.their codes.

Resistance to multi-path fading, because of Resistance to multi-path fading, because of large bandwidths and narrow time widths.large bandwidths and narrow time widths.

PN codes are approximately orthogonal, and PN codes are approximately orthogonal, and the receiver can separate each user based on the receiver can separate each user based on their codes.their codes.

Resistance to multi-path fading, because of Resistance to multi-path fading, because of large bandwidths and narrow time widths.large bandwidths and narrow time widths.

3838

PN Sequences PN Sequences PN Sequences PN Sequences Pseudo Noise or Pseudo random sequence is Pseudo Noise or Pseudo random sequence is

a binary sequence of 1s and -1sa binary sequence of 1s and -1s PN sequences are generated by using PN sequences are generated by using

sequential logic circuitssequential logic circuits Very low cross correlation exists between any Very low cross correlation exists between any

two PN sequencestwo PN sequences High cross correlation exists between High cross correlation exists between

identical PN sequencesidentical PN sequences

Pseudo Noise or Pseudo random sequence is Pseudo Noise or Pseudo random sequence is a binary sequence of 1s and -1sa binary sequence of 1s and -1s

PN sequences are generated by using PN sequences are generated by using sequential logic circuitssequential logic circuits

Very low cross correlation exists between any Very low cross correlation exists between any two PN sequencestwo PN sequences

High cross correlation exists between High cross correlation exists between identical PN sequencesidentical PN sequences

3939

Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)

A frequency hopping signal periodically A frequency hopping signal periodically changes the carrier frequency in a pseudo-changes the carrier frequency in a pseudo-random fashion. The set of possible carrier random fashion. The set of possible carrier frequencies is called a frequencies is called a hopsethopset..

Bandwidth of channel used in hopset Bandwidth of channel used in hopset Instantaneous bandwidth BInstantaneous bandwidth B

Bandwidth of spectrum over which the hopping Bandwidth of spectrum over which the hopping occurs occurs total hopping bandwidth W total hopping bandwidth Wssss

A frequency hopping signal periodically A frequency hopping signal periodically changes the carrier frequency in a pseudo-changes the carrier frequency in a pseudo-random fashion. The set of possible carrier random fashion. The set of possible carrier frequencies is called a frequencies is called a hopsethopset..

Bandwidth of channel used in hopset Bandwidth of channel used in hopset Instantaneous bandwidth BInstantaneous bandwidth B

Bandwidth of spectrum over which the hopping Bandwidth of spectrum over which the hopping occurs occurs total hopping bandwidth W total hopping bandwidth Wssss

4040

Methodology of FHSSMethodology of FHSSMethodology of FHSSMethodology of FHSS Time duration between hops Time duration between hops hopping period hopping period

TTss

Data is sent by hopping the transmitter carrier Data is sent by hopping the transmitter carrier to seemingly random channelsto seemingly random channels

Small bursts of data are sent using Small bursts of data are sent using conventional narrow band modulation before conventional narrow band modulation before T/R hops againT/R hops again

Hit -> Two users using the same frequency Hit -> Two users using the same frequency band at the same timeband at the same time

Time duration between hops Time duration between hops hopping period hopping period TTss

Data is sent by hopping the transmitter carrier Data is sent by hopping the transmitter carrier to seemingly random channelsto seemingly random channels

Small bursts of data are sent using Small bursts of data are sent using conventional narrow band modulation before conventional narrow band modulation before T/R hops againT/R hops again

Hit -> Two users using the same frequency Hit -> Two users using the same frequency band at the same timeband at the same time

4141

Frequency Hopping ModulatorFrequency Hopping ModulatorFrequency Hopping ModulatorFrequency Hopping Modulator

ModulatorModulator

PN CodePN CodeGeneratorGenerator

Frequency Frequency SynchronizerSynchronizer

Code BlockCode Block

DATADATA

Frequency Frequency Hopping Hopping

SignalSignal

OscillatorOscillator

4242

Frequency hopping demodulatorFrequency hopping demodulatorFrequency hopping demodulatorFrequency hopping demodulator

Wideband Wideband FilterFilter

PN Code PN Code GeneratorGenerator

Frequency Frequency SynthesizerSynthesizer

SynchronizationSynchronizationSystemSystem

BP FilterBP Filter DemodulationDemodulationDATADATA

Frequency Frequency Hopping Hopping

SignalSignal

4343

Properties of FHSSProperties of FHSSProperties of FHSSProperties of FHSS Fast frequency hoppingFast frequency hopping

More than one frequency hop during each More than one frequency hop during each transmitted symbol transmitted symbol

-> Hopping rate ≥ symbol rate-> Hopping rate ≥ symbol rate

Slow frequency hoppingSlow frequency hopping

Hopping rate < symbol rateHopping rate < symbol rate

Fast frequency hoppingFast frequency hopping

More than one frequency hop during each More than one frequency hop during each transmitted symbol transmitted symbol

-> Hopping rate ≥ symbol rate-> Hopping rate ≥ symbol rate

Slow frequency hoppingSlow frequency hopping

Hopping rate < symbol rateHopping rate < symbol rate

4444

Parameters of FH-SSParameters of FH-SSParameters of FH-SSParameters of FH-SS Probability of error for BPSK Spread SpectrumProbability of error for BPSK Spread Spectrum

PPee = 0.5 x e = 0.5 x e -Eb/ 2N-Eb/ 2N00 x (1 – p x (1 – ph h ) + 0.5 p) + 0.5 phh

pphh = probability of hit = 1 – (1 – 1/M) = probability of hit = 1 – (1 – 1/M)K-1K-1

M = number of hopping channels M = number of hopping channels K = Total number of usersK = Total number of users

Processing gain = WProcessing gain = Wssss / B / B

Probability of error for BPSK Spread SpectrumProbability of error for BPSK Spread Spectrum

PPee = 0.5 x e = 0.5 x e -Eb/ 2N-Eb/ 2N00 x (1 – p x (1 – ph h ) + 0.5 p) + 0.5 phh

pphh = probability of hit = 1 – (1 – 1/M) = probability of hit = 1 – (1 – 1/M)K-1K-1

M = number of hopping channels M = number of hopping channels K = Total number of usersK = Total number of users

Processing gain = WProcessing gain = Wssss / B / B

4545

Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)

codecode

timetime

frequencyfrequency

CC11

CC22

CCNN

4646

Properties of Message Signal Properties of Message Signal Properties of Message Signal Properties of Message Signal m(t) is a time sequence of non-overlapping m(t) is a time sequence of non-overlapping

pulses of duration T, each of which has an pulses of duration T, each of which has an amplitude (+/-) 1.amplitude (+/-) 1.

The PN waveform consists of N pulses or chips The PN waveform consists of N pulses or chips for message symbol period T.for message symbol period T.

NTNTC C = T= T

where Twhere TCC is the chip period. is the chip period.

m(t) is a time sequence of non-overlapping m(t) is a time sequence of non-overlapping pulses of duration T, each of which has an pulses of duration T, each of which has an amplitude (+/-) 1.amplitude (+/-) 1.

The PN waveform consists of N pulses or chips The PN waveform consists of N pulses or chips for message symbol period T.for message symbol period T.

NTNTC C = T= T

where Twhere TCC is the chip period. is the chip period.

4747

ExampleExample:: ExampleExample:: N=4N=4

PN Wave for N =4

11

-1-1

-1-1

11

4848

DSSS TransmitterDSSS TransmitterDSSS TransmitterDSSS Transmitter

1m (t)

1S (t)

11

kk

1PN (t)

KPN (t)

c 1cos(2 f t )

r(t)

c kcos(2 f t )

mk(t)

4949

Principles of transmitter operationPrinciples of transmitter operationPrinciples of transmitter operationPrinciples of transmitter operation The narrowband message signal mThe narrowband message signal m ii(t) is (t) is

multiplied by a pseudo noise code multiplied by a pseudo noise code sequence that has a chip rate >> data rate sequence that has a chip rate >> data rate of message.of message.

All users use the same carrier frequency All users use the same carrier frequency and may transmit simultaneously. The kth and may transmit simultaneously. The kth transmitted signal is given by:transmitted signal is given by:

The narrowband message signal mThe narrowband message signal m ii(t) is (t) is multiplied by a pseudo noise code multiplied by a pseudo noise code sequence that has a chip rate >> data rate sequence that has a chip rate >> data rate of message.of message.

All users use the same carrier frequency All users use the same carrier frequency and may transmit simultaneously. The kth and may transmit simultaneously. The kth transmitted signal is given by:transmitted signal is given by:

k s s k k c kS (t) (2E / T )1/ 2m (t)p (t)cos(2 f t )

5050

CDMA ReceiverCDMA ReceiverCDMA ReceiverCDMA Receiver

c kcos(2 f t )

r(t)km (t)

(.)dt(.)dt

kiZ (t)

KPN (t)

5151

Principles of receiver operationPrinciples of receiver operationPrinciples of receiver operationPrinciples of receiver operation At the receiver, the received signal is At the receiver, the received signal is

correlated with the appropriate PN sequence correlated with the appropriate PN sequence to produce desired variable.to produce desired variable.

At the receiver, the received signal is At the receiver, the received signal is correlated with the appropriate PN sequence correlated with the appropriate PN sequence to produce desired variable.to produce desired variable.

1

1

iT1i 1 1 c 1 1

(i 1)T

Z (t) r(t)p (t )cos[2 f (t ) ]dt

5252

Correlator output for ith user Correlator output for ith user Correlator output for ith user Correlator output for ith user

•The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and will yield the dispersed signal correct signal and will yield the dispersed signal and can be demodulated to yield the message and can be demodulated to yield the message signal msignal mii(t).(t).

•The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and will yield the dispersed signal correct signal and will yield the dispersed signal and can be demodulated to yield the message and can be demodulated to yield the message signal msignal mii(t).(t).

1

1

iT1i 1 1 c 1 1

(i 1)T

Z (t) r(t)p (t )cos[2 f (t ) ]dt

1/ 21 s s 1 1 c 1S (t) (2E / T ) m (t)p (t)cos(2 f t )

5353

Parameters of DSSSParameters of DSSSParameters of DSSSParameters of DSSS Probability of bit error Probability of bit error

PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}

KK = Number of users= Number of users

NN = Number of chips/ symbol= Number of chips/ symbol

When EWhen Ebb/N/Noo

PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }

Probability of bit error Probability of bit error

PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}

KK = Number of users= Number of users

NN = Number of chips/ symbol= Number of chips/ symbol

When EWhen Ebb/N/Noo

PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }

5454

Important Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMA

Many users of CDMA use the same frequency. Many users of CDMA use the same frequency. Either TDD or FDD may be used.Either TDD or FDD may be used.

Multipath fading may be substantially reduced Multipath fading may be substantially reduced because of large signal bandwidth.because of large signal bandwidth.

There is no absolute limit on the number of There is no absolute limit on the number of users in CDMA. Rather the system users in CDMA. Rather the system performance gradually degrades for all users as performance gradually degrades for all users as the number of users is increased.the number of users is increased.

Many users of CDMA use the same frequency. Many users of CDMA use the same frequency. Either TDD or FDD may be used.Either TDD or FDD may be used.

Multipath fading may be substantially reduced Multipath fading may be substantially reduced because of large signal bandwidth.because of large signal bandwidth.

There is no absolute limit on the number of There is no absolute limit on the number of users in CDMA. Rather the system users in CDMA. Rather the system performance gradually degrades for all users as performance gradually degrades for all users as the number of users is increased.the number of users is increased.

5555

Drawbacks of CDMADrawbacks of CDMADrawbacks of CDMADrawbacks of CDMA

Self-jamming is a problem in a CDMA system. Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN Self-jamming occurs because the PN sequences are not exactly orthogonal.sequences are not exactly orthogonal.

The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high detected receiver if an undesired user has high detected power as compared to the desired user.power as compared to the desired user.

Self-jamming is a problem in a CDMA system. Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN Self-jamming occurs because the PN sequences are not exactly orthogonal.sequences are not exactly orthogonal.

The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high detected receiver if an undesired user has high detected power as compared to the desired user.power as compared to the desired user.

5656

Modulation performance in fading channelsModulation performance in fading channelsModulation performance in fading channelsModulation performance in fading channels

s(t) r(t) s(t) r(t) r(t) = r(t) = (t) e(t) e-j-j(t)(t) s(t) + n(t) s(t) + n(t)

(t) = gain of the channel(t) = gain of the channel

(t) = phase shift of the channel(t) = phase shift of the channel

n(t) = additive gaussian noisen(t) = additive gaussian noise

Average signal to noise ratio at receiverAverage signal to noise ratio at receiver

= (E= (EBB / N / N00) ) 22

s(t) r(t) s(t) r(t) r(t) = r(t) = (t) e(t) e-j-j(t)(t) s(t) + n(t) s(t) + n(t)

(t) = gain of the channel(t) = gain of the channel

(t) = phase shift of the channel(t) = phase shift of the channel

n(t) = additive gaussian noisen(t) = additive gaussian noise

Average signal to noise ratio at receiverAverage signal to noise ratio at receiver

= (E= (EBB / N / N00) ) 22

Fading ChannelFading Channel

5757

Probability of error in fading channelsProbability of error in fading channelsProbability of error in fading channelsProbability of error in fading channels

Probability of errorProbability of error

= Probability of error for parent = Probability of error for parent modulation, such as BPSK, FSKmodulation, such as BPSK, FSK

Probability of errorProbability of error

= Probability of error for parent = Probability of error for parent modulation, such as BPSK, FSKmodulation, such as BPSK, FSK

e e0

P P (X)p(X)dX

eP (X)eP (X)

p(X) = pdf of x due to fading channel p(X) = pdf of x due to fading channel

= = (1/ )exp( x / ),x 0

5858

PPee (X) and P (X) and Pe e for different systems for different systemsPPee (X) and P (X) and Pe e for different systems for different systems

Coherent binary PSKCoherent binary PSK

Coherent binary FSKCoherent binary FSK

Coherent binary PSKCoherent binary PSK

Coherent binary FSKCoherent binary FSK

1/ 2e B 0P (X) Q[(2E /N ) ]

eP 0.5[1 /(1 )]

1/ 2e B 0P (X) Q[(E /N ) ]

eP 0.5[1 /(2 )]

5959

Differential Binary PSKDifferential Binary PSK

Non-coherent orthogonal binary FSKNon-coherent orthogonal binary FSK

Differential Binary PSKDifferential Binary PSK

Non-coherent orthogonal binary FSKNon-coherent orthogonal binary FSK

e B 0P (X) 0.5exp[( E /N )]

eP [0.5 /(1 )]

e B 0P (X) 0.5exp[( E / 2N )]

eP [0.5 /(2 )]

6060

Coherent GMSK Coherent GMSK

=0.68, BT = 0.25, =0.68, BT = 0.25, = 0.68 = 0.68

=0.85, BT = =0.85, BT = , , = 0.85 = 0.85

BT = Bandwidth – bit duration product for GMSKBT = Bandwidth – bit duration product for GMSK

Coherent GMSK Coherent GMSK

=0.68, BT = 0.25, =0.68, BT = 0.25, = 0.68 = 0.68

=0.85, BT = =0.85, BT = , , = 0.85 = 0.85

BT = Bandwidth – bit duration product for GMSKBT = Bandwidth – bit duration product for GMSK

e B

1/ 2e

P (X) Q[(2 E )]

P 0.5{1 [ /( 1)] 1/ 4

e B

1/ 2e

P (X) Q[(2 E )]

P 0.5{1 [ /( 1)] 1/ 4