TECHNICAL CHARACTERISTICS OF DIGITAL SYSTEMS Stanko PERPAR Chairman ITU-R TG 6/8.

TECHNICAL CHARACTERISTICS OF DIGITAL SYSTEMS

Stanko PERPAR

Chairman ITU-R TG 6/8

ITU DTTB model

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)

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)

OFDM Carriers

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

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

Multipath propagation

Reflected path

Direct path

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)

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

Small area field strength distribution

Distance [m]

[m]

Percentage of locations50%

50%

50%

Minimum field strength for 50% locations

E

distance (m)

Percentage of locations70%

Percentage of locations50% to 70%

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

Cut-off characteristics

E (dB)

Excellent

analogue

digital

threshold

midopinion

Unusable

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

Ricean Channel

Rayleigh Channel

Required C/N

Planning parameters

• Minimum field strengths

• Protection ratios– Co-channel – Adjacent channel

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

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


Minimum field strengths Portable inside reception 64 QAM 2/3

Rayleigh channel

BAND III IV V

Analogue 55 65 70

Digital



Minimum field strengths Portable inside reception 16 QAM 1/2

Rayleigh channel

BAND III IV V

Analogue 55 65 70

Digital



Mobile reception - Typical urban 16 QAM 1/2 non diversity

BAND III IV V

Analogue 55 65 70

Max.speed 254 102 64




Mobile reception - Typical urbanantenna diversity

BAND III IV V

Analogue 55 65 70

Max.speed 508 203 127




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)

ADJACENT CHANNEL

ANALOGUE

Adj.channel N - 1 N +1

Tropospheric -9 -12

Continuous 1 -2

DIGITAL

DVB -T -30 -30

ATSC -27 -27

Direct path

Multipath

Multipath analogue

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

UPGRADE FROM ANALOGUE TO DIGITAL TV

TRANSMITTER

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


Poweramplifier

Bandpassfilter

ASI input


RF output



Analogue TV Transmittersplit amplification

RFoutputVideo input

Audio input


Poweramplifier

Bandpassfilter

ASI input


RF output

Notchfilter

Analogueexciter

Lowpassfilter

Videopower

amplifier

Audiopower

amplifier

Video/audio

combiner



RF video

RF audio

RFoutputVideo input

Audio input


Poweramplifier

Bandpassfilter

ASI input


RF output

Notchfilter

Analogueexciter

Lowpassfilter

Videopower

amplifier

Audiopower

amplifier

Video/audio

combiner



RF video

RF audio

TECHNICAL CHARACTERISTICS OF DIGITAL SYSTEMS Stanko PERPAR Chairman ITU-R TG 6/8.

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