Digital television Modulation techniques
Transcript of 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
2π
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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