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TECHNICAL CHARACTERISTICS OF DIGITAL SYSTEMS
Stanko PERPAR
Chairman ITU-R TG 6/8
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ITU DTTB model
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Standards for Digital Terrestrial Television Broadcasting
• Digital system A: ATSC • Digital system B: DVB -T• Digital system C: ISDB – T
• System A: Single carrier modulation
(8-VSB)• Systems B and C: Multicarrier modulation
(OFDM)
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Coded Orthogonal Frequency Division Multiplex
Number of carriers per symbol:
• 2k - 1705
• 8k – 6817
Modulation:
• QPSK
• 16 QAM (uniform or non-uniform)
• 64 QAM (uniform or non-uniform)
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OFDM Carriers
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Guard interval
Ts = Tu + Tg
Tg = ¼, 1/8, 1/16 or 1/32 Tg
• Ts = symbol duration• Tu = usefull symbol duration• Tg = guard intervalSome carriers are pilots used for synchronisation,
transmission of parameters and signal recovery purposes
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Forward Error Correction
• Outer code: Reed Solomon (204, 188, 16)
• Interleaving
• Inner code: convolutional with Viterbi soft decision decoding.– Coding rates: ½, 2/3, ¾, 5/6, 7/8
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Multipath propagation
Reflected path
Direct path
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Reception modes
• Fixed reception
• Class A portable reception (outdoor)
• Class B portable reception (ground floor indoor)
• Mobile reception (moving with such speed that Doppler effect appears)
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Coverage
• Location (0,5 x 0,5)m covered for 99% of the time
• Small area (100 x 100) m– Good coverage for > 95% of locations
– Acceptable for > 70% of locations
• Coverage area – sum of individual small areas
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Small area field strength distribution
Distance [m]
[m]
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Percentage of locations50%
50%
50%
Minimum field strength for 50% locations
E
distance (m)
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Percentage of locations70%
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Percentage of locations50% to 70%
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Limit value for planning
• Analogue: S/N = 30 dB (unweighted)
• Digital: BER 2x10-4 (Quasi Error Free)
• - 3 dB difference means ½ grade on quality scale in analogue picture, but
• no picture in digital
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Cut-off characteristics
E (dB)
Excellent
analogue
digital
threshold
midopinion
Unusable
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Transmission Channels
• Gaussian Channel – direct sight, no multipath (dB
• Rice Channel (1 < < 3 dB) – stationary reception using directional antenna
• Rayleigh Channel ( > 3 dB) – portable reception using omnidirectional antenna
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Ricean Channel
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Rayleigh Channel
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Required C/N
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Planning parameters
• Minimum field strengths
• Protection ratios– Co-channel – Adjacent channel
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Minimum field strengths Fixed reception - 64 QAM 2/3 Rice channel
BAND III IV V
Analogue 55 65 70
Digital
70% locations 39 44 48
95% locations 45 50 54
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Minimum field strengths Portable outside reception 64 QAM 2/3
Rayleigh channel
BAND III IV V
Analogue 55 65 70
. Digital
Locations 70% 59 65 69
Locations 95% 64 71 75
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Minimum field strengths Portable inside reception 64 QAM 2/3
Rayleigh channel
BAND III IV V
Analogue 55 65 70
Digital
Locations 70% 66 73 77
Locations 95% 73 83 87
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Minimum field strengths Portable inside reception 16 QAM 1/2
Rayleigh channel
BAND III IV V
Analogue 55 65 70
Digital
Locations 70% 59 66 70
Locations 95% 68 76 80
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Mobile reception - Typical urban 16 QAM 1/2 non diversity
BAND III IV V
Analogue 55 65 70
Max.speed 254 102 64
Locations 70% 58 65 69
Locations 95% 64 71 77
Locations 99% 68 75 79
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Mobile reception - Typical urbanantenna diversity
BAND III IV V
Analogue 55 65 70
Max.speed 508 203 127
Locations 70% 52 59 63
Locations 95% 58 65 69
Locations 99% 63 69 73
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CO - CHANNEL INTERFERENCE
ANALOGUEOffset 8/12
Norm.offset500 Hz
Prec. offset1 Hz
Tropospheric 30 22
Continiuous 40 27
DIGITAL Rice Rayleigh
64 QAM 2/3 20 23
16 QAM 1/2 11 13
ATSC 15 (19) 15 (19)
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ADJACENT CHANNEL
ANALOGUE
Adj.channel N - 1 N +1
Tropospheric -9 -12
Continuous 1 -2
DIGITAL
DVB -T -30 -30
ATSC -27 -27
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Direct path
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Multipath
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Multipath analogue
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Advantages of DVB-T
• Flexible approach ( cca 120 possibilities)
• SFN
• Mobile reception
• Possible use of TABO channels
• Robust to multipath effects
• Similarity with DVB-S, DVB-C
• Set-top box
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UPGRADE FROM ANALOGUE TO DIGITAL TV
TRANSMITTER
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Analogue TV Transmittercommom mode amplification
Analogueexciter
Poweramplifier
Lowpassfilter
Video input
Audio input
RF output
DVB-T exciterwith COFDMmodulator
Poweramplifier
Bandpassfilter
ASI input
MPEG-2transport stream
RF output
ANALOGUE TV TRANSMITTER
DIGITAL TV TRANSMITTER
Analogueexciter
Poweramplifier
Lowpassfilter
Video input
Audio input
RF output
DVB-T exciterwith COFDMmodulator
Poweramplifier
Bandpassfilter
ASI input
MPEG-2transport stream
RF output
ANALOGUE TV TRANSMITTER
DIGITAL TV TRANSMITTER
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Analogue TV Transmittersplit amplification
RFoutputVideo input
Audio input
DVB-T exciterwith COFDMmodulator
Poweramplifier
Bandpassfilter
ASI input
MPEG-2transport stream
RF output
Notchfilter
Analogueexciter
Lowpassfilter
Videopower
amplifier
Audiopower
amplifier
Video/audio
combiner
ANALOGUE TV TRANSMITTER
DIGITAL TV TRANSMITTER
RF video
RF audio
RFoutputVideo input
Audio input
DVB-T exciterwith COFDMmodulator
Poweramplifier
Bandpassfilter
ASI input
MPEG-2transport stream
RF output
Notchfilter
Analogueexciter
Lowpassfilter
Videopower
amplifier
Audiopower
amplifier
Video/audio
combiner
ANALOGUE TV TRANSMITTER
DIGITAL TV TRANSMITTER
RF video
RF audio