Stanko PERPAR

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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 interval
• Some 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
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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 gt 95 of locations
• Acceptable for gt 70 of locations
• Coverage area sum of individual small areas

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Small area field strength distribution
m
Distance m
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Percentage of locations50
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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
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Transmission Channels

• Gaussian Channel direct sight, no multipath (s
  lt 1dB)
• Rice Channel (1 lt s lt 3 dB) stationary
  reception using directional antenna
• Rayleigh Channel (s gt 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
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Minimum field strengths Portable outside
reception 64 QAM 2/3 Rayleigh channel
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Minimum field strengths Portable inside
reception 64 QAM 2/3 Rayleigh channel
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Minimum field strengths Portable inside
reception 16 QAM 1/2 Rayleigh channel
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Mobile reception - Typical urban 16 QAM 1/2 non
diversity
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Mobile reception - Typical urbanantenna diversity
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CO - CHANNEL INTERFERENCE
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ADJACENT CHANNEL
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Direct path
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Multipath
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Multipath analogue
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(No Transcript)
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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
Video input
Video input
Lowpass
Lowpass
Analogue
Power
Analogue
Power
RF output
RF output
exciter
amplifier
filter
exciter
amplifier
filter
Audio input
Audio input
ANALOGUE TV TRANSMITTER
ANALOGUE TV TRANSMITTER
ASI input
ASI input
DVB
-
T exciter
DVB
-
T exciter
Bandpass
Bandpass
Power
Power
RF output
RF output
with COFDM
with COFDM
amplifier
filter
amplifier
filter
MPEG
-
2
MPEG
-
2
modulator
modulator
transport stream
transport stream
DIGITAL TV TRANSMITTER
DIGITAL TV TRANSMITTER
37
Analogue TV Transmittersplit amplification