University of Canberra Advanced Communications Topics

1
University of Canberra Advanced Communications
Topics

• Television Broadcasting into the Digital Era

Lecture 5 DTTB Transmission Error Correction
by Neil Pickford
2
Spectrum of COFDM DTTB
7 MHz Carrier Spacing 2k Mode 3.91 kHz 8k Mode
0.98 kHz
AlmostRectangularShape
1705 or 6817 Carriers
6.67 MHz in 7 MHz Channel
7.61 MHz in 8 MHz Channel
3
64-QAM - Perfect Failure
4
OFDM - Features

• Multicarrier - many carriers sharing
• Reduced C/N compared to Analogue
• Resistant to echoes, Interference etc
• Low symbol rate per carrier
• 1 kBaud Long Symbol Period, can Extend with
  Guard Interval
• With FEC becomes COFDM
• Uses Fast Fourier Transform FFT
• 2k and 8k versions
• Single Frequency Networks SFN

5
COFDM DTTB Block Diagram
Error Correction
6
Forward Error Correction (FEC)

• Broadcast transmission
• One way process - Tx to Rx
• Not possible to repeat any errored data
• Forward Error Correction is a technique used to
  improve the accuracy of data transmission
• Extra redundant bits are added to the data stream
• Error correction algorithms in the demodulator
  use the extra FEC bits to correct data errors
• C OFDM uses a Convolutional FEC code

7
Convolutional Coder
6 5 4
3 2 1
0
8
Puncturing Codes (FEC)

• The X and Y outputs of the Convolutional coder
  are selected in a Puncturing pattern

9
Inner Coding

• Convolutional coder generates the X Y codes

• Puncturing operation selects X Y in sequence

• Result then scrambled with an interleaver

Convolutional Encoder
Puncturing
Interleaver
10
Viterbi Decoder

• A special type of data decoder designed to work
  with convolutional FEC codes
• Uses the past history of the data to identify
  valid future data values
• Element in the Receiver Only

11
Reed Solomon (RS)

• RS is a Block data correcting Code
• Hamming type cyclic Polynomial sequence
• Code Generator Polynomial g(x)
  (xl0)(xl1)(xl2)...(xl15), l02 Hex
• Field Generator Polynomial p(x) x8 x4 x3
  x2 1
• Has special ability to correct multiple bursts of
  errors in a code block
• DVB-T uses 204 bytes for each 188 byte
  Packet(ATSC uses 207 bytes for each 187 byte
  Packet)
• Can correct 8 bytes in each 204 byte packet

12
Error Protection - Order
188 Bytes
204 Bytes
Outer Code RS (204,188)
306 Bytes
204 Bytes
2448 Bits
6 bits x 1512 Carriers6 bits x 6048 Carriers
64 QAM
13
Guard Interval
Transmitted Symbol
Useful Symbol
Guard
1/4
14
COFDM - Multipath

TRANSMITTER A
REFLECTIONS
1 Microsecond 300 Metres
DIRECT PATH
SYMBOL PERIOD 1 ms
RECEPTION POINT
SIGNAL
Several µseconds disturbance from echoes. OFDM
inherently resistant. 8VSB needs Time Domain
Equaliser, symbol period short at 93ns
15
COFDM - Multipath

TRANSMITTER A
REFLECTIONS
1 Microsecond 300 Metres
DIRECT PATH
GUARD INTERVAL
SYMBOL PERIOD
RECEPTION POINT
SAFE AREA
SIGNAL
16
COFDM - Pre-Echo

TRANSMITTER A
REFLECTIONS
1 Microsecond 300 Metres
SYMBOL PERIOD
RECEPTION POINT
SIGNAL
17
COFDM - SFN
TRANSMITTER B

TRANSMITTER A
REFLECTIONS
1 Microsecond 300 Metres
DIRECT PATH
GUARD INTERVAL Variable
SYMBOL PERIOD
RECEPTION POINT
SAFE AREA
SIGNAL
18
Mobile Services

• Antenna Performance
• Poor Directivity, Low Gain
• Multipath Dominated environment
• Doppler
• High Speeds for Main Roads and Railways
• Low Speeds for Public Transport in Cities
• Needs to be Rugged
• Choose version of DVB-T that is suitable
• Low Bit Rate, Low C/N, Long Guard Interval?

19
Mobile Digital TV Onboard a Tram in Cologne -
Germany
20
Bus Route 7 Singapore - 1999
21
Doppler Echo - 7.5 us Coax
0
COFDM 8-VSB
-5
-10
Echo Level E/D (dB)
-15
-20
-25
0
-500
200
-200
500
Frequency Offset (Hz)
22
DTTB Systems Doppler Performance Limits
for currentimplementations
300
250
UHF
200
VHF - Band III
DOPPLERSHIFT(?Hz)
COFDM 2K, 3dB degrade
140
COFDM 2K
100
50
0
0
1000
500
100
200
300
400
600
700
800
900
ATSC see separate curves
SPEED (Km/Hr)
AIRCRAFT
Vehicles
Over Cities
COFDM implementations will inherently handle post
and pre-ghosts equally within the selected guard
interval.
23
ATSC 8-VSB Doppler Performance Limits
for current implementations
10
VHF - Band III
UHF
DOPPLERSHIFT(?Hz)
8VSB, Fast Mode, 3dB degrade
5
8VSB
1
0
0
100
30
23
10
6
2
SPEED (Km/Hr)
Vehicles
Aircraft
8VSB implementations of equalisers are likely to
cater for post ghosts up to 30 uSec and
pre-ghosts up to 3 uSec only.
24
TPS Pilots

• Transmission Parameter Signalling is added on
  selected carriers within the OFDM spectrum(17
  for 2k 68 for 8k)
• TPS Carries
• Frame Number in Super Frame 00 / 01 / 10 / 11
• Constellation Type QPSK / 16-QAM / 64-QAM
• OFDM Mode 2k or 8k
• Constellation Mode Normal/Hierarchical a value
• Inner FEC Code rate
• Guard Interval
• System Bandwidth

25
7 MHz COFDM Modulator Spectrum
0
-10
-20
Power Spectrum Density (dB)
-30
-40
2k 1/32 Guard
-50
0
-1
-2
-3
-4
-5
-6
-7
-8
1
2
3
4
5
6
7
8
Frequency Offset (MHz)
26
7 MHz COFDM Modulator Spectrum
0
-10
-20
Power Spectrum Density (dB)
-30
-40
8k 1/32 Guard
-50
0
-1
-2
-3
-4
-5
-6
-7
-8
1
2
3
4
5
6
7
8
Frequency Offset (MHz)
27
7 MHz COFDM Modulator Spectrum
0
-10
-20
Power Spectrum Density (dB)
-30
-40
8k 1/32 Guard 2k 1/32 Guard
-50
0
-1
-2
-3
-4
-5
-6
-7
-8
1
2
3
4
5
6
7
8
Frequency Offset (MHz)
28
C/N - Signal Level Performance
28
24
20
16
C/N Threshold (dB)
12
8
4
0
10
15
20
25
30
35
40
45
50
55
60
Receiver Signal Level (dBuV)
29
8VSB vs COFDM Australia
30
8VSB vs COFDM Latest
31
DVB-T - Bit Rates 2k
D/Tu 1/4
D/Tu 1/8
D/Tu 1/32
7 MHz
64 us
32 us
8 us
Code Rate
32
DVB-T - C/N Values
GAUSSIAN
RICEAN
RAYLEIGH
16 -
16 -
64 -
64 -
16 -
Code
QPSK
QPSK
QPSK
Rate
QAM
QAM
QAM
QAM
QAM
1/2
3.10
8.80
3.60
9.60
14.70
5.40
11.20
16.00
14.4
2/3
4.90
11.1
16.5
5.70
11.60
17.10
8.40
14.20
19.30
3/4
5.90
12.5
18.00
6.80
13.00
18.60
10.70
16.70
21.70
5/6
6.90
13.5
19.30
8.00
14.40
20.00
13.10
19.30
25.30
7/8
7.70
13.9
20.10
8.70
15.00
21.00
16.30
22.80
27.90
Simulated Theoretical Thresholds (bandwidth
independent)