Digital television Modulation techniques

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11.2.2003 Digital televsion techniques – Lecture 6 www.abo.fi/~jbjorkqv/digitv 1 Lecture 6 Digital television Modulation techniques • Electromagnetic waves • Analog modulation • Amplitude modulation • Angle modulation • Frequency modulation • Phase modulation • Digital modulation • On-Off keying • Amplitude shift keying • Phase shift keying • Quadrature amplitude modulation

Transcript of Digital television Modulation techniques

Page 1: Digital television Modulation techniques

11.2.2003 Digital televsion techniques – Lecture 6www.abo.fi/~jbjorkqv/digitv

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Lecture 6

Digital television

Modulation techniques

• Electromagnetic waves• Analog modulation

• Amplitude modulation• Angle modulation

• Frequency modulation• Phase modulation

• Digital modulation• On-Off keying• Amplitude shift keying• Phase shift keying• Quadrature amplitude modulation

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Electromagnetic waves

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Electromagnetic waves

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Modulation

"modulate" is "To adjust or adapt to a certain proportion."

”Linear” modulation Exponential modulation

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Amplitude modulation

E.g..• AM radio stations• Analog television

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Amplitude modulation

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AM: waveforms and bandwidth

ν AM in frequency domain:

ν AM bandwidth is twice the message bandwidth W:

x t A x t t

A t x t tC C m C

C C m C

( ) [ ( )]cos( )

cos( ) ( )cos( )

= +

= +

1 µ ω

ω µ ωCarrier Information carrying part

1 24 34 1 244 344

X f A f f A X f fC C C C m C( ) ( ) / ( ) /= − + −δ µ2 2

Carrier Informationcarrying part1 244 344 1 244 344 f > 0 ( )for brief notations

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Phase modulation (PM)ν Carrier Wave (CW) signal:

ν In exponential modulation the modulation is “in the exponent” or “inthe angle”

ν Note that in exponential modulation superposition does not apply:

ν In phase modulation (PM) carrier phase is linearly proportional tothe modulation amplitude:

ν Angular phasor has theinstantaneous frequency (phasor rate)

x t A t tC C C

tC

( ) cos( ( ))( )

= +ω φθ

1 24 34

x t A t tPM C C

x t

tC

( ) cos( ( ) )( ),

( )

= +≤

ω φφ φ π

θ

∆ ∆

31 244 344

( ) cos( ( )) Re[e ( )xp( )]C C C CCx t A t j tAθ θ= =

2 ( )f tω π=ω

Ct

[ ]{ }[ ]

1 2

1 2

( ) cos ( ) ( )

cos cos ( ) ( )C C f

C f

x t A t k a t a t

A t A k a t a t

ω

ω

= + +

≠ + +

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Instantaneous frequency

ν Angular frequency ω (rate) is the derivative of the phase (thesame way as the velocity v(t) is the derivative of distance s(t))

ν For continuously changing frequency instantaneous frequency isdefined by differential changes:

( )tφ

/t s

Angle modulated carrier

Constant frequency carrier:1

0 01rad/s, (2 ) Hzfω π −= =

1st =

( )( )

d tt

dtφ

ω = ( ) ( )t

t dφ ω α α−∞∫= 2 1

2 1

( ) ( ) ( )( )

ds t s t s tv t

dt t t −

= ≈ − Compare tolinear motion:

( )iv t

( )q

v t

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Frequency modulation (FM)ν In frequency modulation carrier instantaneous frequency is

linearly proportional to modulation frequency:

ν Hence the FM waveform can be written as

ν Note that for FM

and for PM

(2 ( ) ( ) /2 )[ ]

C

C

f t d t df

tx tf

ω π θπ ∆

= == +

x t A t f x d t tC C C t

t

C t

( ) cos( ( ) ),( )

= + ≥zω π λ λθ

20 0∆1 2444 3444

( ) ( )Cf t f f x t∆= +

integrate

( ) ( )t

t dφ ω α α−∞∫=

( ) ( )t x tφ φ∆

=

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AM, FM and PM waveforms

x t A t f x dFM C C

t

( ) cos( ( ) )= + zω π λ λ2∆

x t A t x tPM C C

( ) cos( ( ))= +ω φ∆

Constant frequency followsconstant modulation waveformderivative

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FM Bandwidth

Normally calculated using Carlsons rule

B = 2 (D +1)W

where W is the maximun modulation frequency andD is the devation rato D = f∆ / W

f∆ is the peak frequency deviation

FM Radio: f∆ = 75 kHz, W = 15 kHz

B = 2* (75 / 15 +1 ) * 15 kHz= 180 kHz

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FM demodulation - Example

Zero-crossingbased demodulation

Other: PLL

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Comparison of carrier wavemodulation systems

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Digital modulation

2 analog signals

digital signal (010101...)

digital signal approximatedusing 5 sinus waveforms

On-off keying (Binary Amplitude Key Shifting) bandwidth?

e.g. keeping 5 components -> B = 18 f_m

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Shannon’s theorem

The capacity C of a channel is

where B is the bandwidth and S/N is the signalto noise ratio (given in watts/watts)

PAL analog 6MHz studio eq. S/N = 65 dB -> 129 Mbit/sPAL analog 6MHz broadcast S/N = 21 dB -> 42 Mbit/s

Practical for digital applications 15 dB -> 30 Mbit/s

Normal digital applications: 6 bps / Hz

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Digital modulation

• Modify carriers amplitude and/or phase (and frequency)• Constellation: Vector notation / polar coordinates

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Modulation scheme - considerations

•High spectral efficiency•High power efficiency•Robust to multipath effects•Low cost and ease of implementation•Low carrier-to-cochannel interference ratio•Low out of band radiation•Constant or near constant envelope

Constant: Only phase is modulatedNon-constant: phase and amplitude is modulated

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Binary modulations

•Amplitude shift keying (ASK)Transmission on-off

•Frequency shift keying (FSK)

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Binary modulations

• Binary phase shift keying (BPSK)• Simple to implement, inefficient use of bandwidth• Very robust, used in satellite communications

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Phase key shifting

• Quadrature Phase Shift Keying (QPSK)• Multilevel modulation technique: 2 bits per symbol• More spectral efficiency, more complex receiver

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π / 4 – Shifted QPSK

• Variation of QPSK

• Restricted carrier phase transitions to +/- π/4 and +/- 3π/4• Signalling elements selected in turn from two QPSK constellations

each shifted by π/4• Popular in Second Generation Systems

• North American Digital Cellular (1.62 bps / Hz)• Japanese Digital Cellular System (1.68 bps / Hz)

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π / 4 – Shifted QPSK

• Advantages• Two bits per symbol• Phase transitions avoid center of diagram, remove som design

constraints on receiver• Always a phase change between symbols, leading to self-clocking

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Quadrature Amplitude Modulation

• Quadrature Amplitude Modulation (QAM)•Amplitude modulation on both quadrature carriers• 2^n discrete levels, if n=2 -> same as QPSK

• Extensively used in microwave links• DVB-T uses QAM

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Quadrature Amplitude Modulation

4 bits / symbol

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Quadrature Amplitude Modulation

6 bits / symbol

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