cofdm - atv-europe.net · allows receiver to lock onto signal ... tp1_1 tp2_1 tp1_2 tp4_1 tp3_1...
TRANSCRIPT
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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A brief history of COFDM - 1
• Popular in the 1980’s and used for digital audio broadcasting(DAB)
– OFDM + QPSK modulation
• OFDM - A special form of MCM– Patent issued in the US in 1970 (number 3,488445) submitted by R.W.
Chang in 1966
• Time domain signals used to ensure subcarrier orthogonality– Major contribution by Shannon in defining waveforms in Euclidean space,
allowing definitions of orthogonality
– No need for steep band pass filters
– Sub-carrier spectra allowed to overlap– Need for real time FFT’s
• OFDM grew out of Multi Carrier Modulation (MCM)– Military HF radio (late 1950’s)– Divides stream into several parallel bit streams
– Bit streams used to modulate several carriers
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A brief history of COFDM - 2
– DTG (UK based Digital Terrestrial Group, set up in 1995 to make a workingbroadcast solution for the UK to meet Government plans)� First commercial broadcasts in late 1998 with simulcast and later
OnDigital services.
• Various associated bodies– 1992 DVB (Digital Video Broadcast - voluntary group of 200 companies)
� DVB-S, DVB-C in 1994 and DVB-T in early 1997
– dTTb (digtial Terrestrial Television broadcast project)
� Demonstrator to show the feasibility of a commercial receiver
– DVBird (Digital Video Broadcast integrated receiver decoder)
� Technical specifications needed and partitioning of electronic functions
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COFDM
• A brief history
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
• COFDM principles
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What is COFDM ?
C - Coded
O - Orthogonal
F - Frequency
D - Division
M - Multiplex
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FDM (Frequency Division Multiplex)Data signals
Carrier modulation
fc1
Carrier modulation
fc2
Carrier modulation
fcn
:
s1(t)
s2(t)
sn(t)
Σ Transimitter
Transmitter architecture
Output signal
G(t)
S1(f)
S2(f)
Sn(f)
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FDM (Frequency Division Multiplex)
NRZ
1 1 0 0 0 1 1Code
ASK
FSK
PSK
- Modulation
Quadrature (QAM)phase shift uses aπ/2 phase shift.π phase shift is shown here
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FDM (Frequency Division Multiplex)
Guard interval
Frequency
Time
Useful data
Continuous frequencytransmissions G(t)
FC1
FC2
FC3
FC4
FC5
6817 or 1705 frequenciesFCn
Tu
Tu α 1/(Fc1 - Fc2)
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FDM (Frequency Division Multiplex)
n data symbols over time period T
time
G(t)
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FDM (Frequency Division Multiplex)
Receiver architecture
BPFc1
BPF = Band pass filter
BPFc2
BPFcn
:
Carrierdemod
Carrierdemod
Carrierdemod
G(t)
s1(t)
sn(t)
s2(t)
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FDM (Frequency Division Multiplex)
≡
≡
IDFT
DFT
s(ti) = 2WN �
fk=1
N
S(fk)ej2�fkti/N
S(fk) = TN �
ti=1
N
s(ti)e−j2�fkti /N
Carrier modulation
fc1
Carrier modulation
fc2
Carrier modulation
fcn
:
s1(t)
s2(t)
sn(t)
Σ Transimitter
Transmitter architecture
Output signal
G(t)
S1(f)
S2(f)
Sn(f)
Receiver architecture
BPFc1
BPF = Band pass filter
BPFc2
BPFcn
:
Carrierdemod
Carrierdemod
Carrierdemod
G(t)
s1(t)
sn(t)
s2(t)
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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DVB-T framing structure• Fixed number of carriers used
� Allows receiver to lock onto signal
• Modulation used� Increases number of bits that can be transmitted
� '2K' system in UK (1705 carriers)
• Carrier types
� Keeps constant power levels
� Eg each carrier transports 4 bits for QAM-16
� '8K' also an option (6817 carriers)
� Data carriers - 2,4 or 6 bits per symbol, per carrier
� TPS carriers - Transmission information
� Pilot carriers -Channel estimation at receiver, Tx at boosted power levels
� Scattered - 524 in '8K' mode, 121 in '2K' pseudo random within symbol
� Continual - 177 in '8K' mode, 45 in '2K' always in same position within symbol
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DVB-T framing structure
f
Single frequency carrier.One of 6817 (8k) or 1705 (2k)discrete modulation carriers.Either: Data (6048 or 1 512)
Continual pilot (177 or 45)Scattered pilot (524 0r 131)TPS carrier (68 or 17)
f
....6817 carriers (8K)1705 carriers (2K)
OFDM symbol(frequency domain)
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DVB-T framing structure
OFDM symbol (time domain)
....
OFDM frame
0 67
Tf
t
Useful data
Guard interval
∆ Tu
Ts
t
t
OFDM super frame
4 x OFDM frames
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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COFDM functional blocksOuter coding*(R/S bytes added)
204 204
RS
MPEG-2 Transport streaminput
Randomisation*
PRBS
188 188 188
Outer interleaving*(Forney)
Inner coding*
|||||||........||||||||
Bit and symbol interleaving
....011001010001.... |||||||........||||||||
|||||||........||||||||
Amplitude/phasemapping
R|||||||........||||||||
|||||||........|||||||| I
Pilots and TPS addition
Inverse FFT........................
|||||||........||||||||R
|||||||........||||||||IIFFT
FIR
Time shift and combination
Guard intervalinsertion
|||||......|||||||||||......||||||
GI
Analogueconversion Upconversion
F
DAC
Filtering Transmission
* Same as DVB-S
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DVB transport stream
TELETEXT_1
VIDEO_1
AUDIO_1
TELETEXT_2
VIDEO_2
AUDIO_2
TELETEXT_3
VIDEO_3
AUDIO_3
TELETEXT_4
VIDEO_4
AUDIO_4
TP1_1 TP1_2 TP1_3 TP2_1 TP2_2 TP2_3 TP3_1 TP3_2 TP3_3 TP4_1 TP4_2 TP4_3
ES
TP
PROGRAMS
TP1_1 TP2_1 TP1_2 TP4_1 TP3_1 TP2_2 TP2_3 TP1_3 TP4_2
TRANSPORT MUX
TRANSPORT STREAM
PCR_1 PCR_2 PCR_3 PCR_4
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Data scramblingPseudo Random Binary Sequence (PRBS)
Energy dispersal to ensure adequate binary transitions
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Enable (1)
Data input
Randomized dataoutput
1 0 0 1 0 1 0 1 0 0 0 0 0 0 0Initialisation sequence
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Error correction• Error prone environment hence small packets (188 bytes) with
additional error correction data (16 bytes)– Known as Forward Error Correction (FEC)– Also known as channel coding
– Two main parts:
– Outer coding for burst errors (Reed - Solomon and Forney)
– Inner coding (Convolution coding)
QEF Channel
TransmitterFEC
ReceiverFEC
Data Data
Energydispersal
Outercoding(RS)
ForneyInterleaving
Innercoding(Convolution)
BER < 10-10
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Outer coding
• Reed Solomon– Operates over individual packets– Corrects up to 8 erroneous bytes per packet
– Non correctable flag for > 8 byte errors
– Bandwidth overhead is 8%
• Forney convolution interleaving– Increases efficiency of the RS coding– Spreads errors over a greater area
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Inner coding
• Convolution coding– 2 identical streams produced from outer coded stream– Output stream formed from combination of these new streams
– Not all simultaneous bits taken - hence rate defined
(DVB-T code rates: 1/2, 2/3, 3/4, 5/6, 7/8)
• Puncture rate impact on data rate– Puncture rate of 3/4 means 1 out of 4 bits is removed– Data rate becomes: (1/2)*(4/3) = 2/3 of original (ie code rate is 2/3)
No puncturing data rateis halved since convolutionencoder produces twoidentical streams
Every 4th bit removed
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Bit and Symbol Interleaving
• Bite-wise interleaving– Inner coder has two output streams– Bit wise interleaver produces 2, 4 or 6 streams for QPSK, 16-QAM and
64-QAM respectively
• Symbol interleaving– The 2, 4 or 6 bit words are mapped onto the OFDM carriers– 1512 for 2k mode or 6048 for 8K mode
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Amplitude and Phase Mapping(example)
Q
I
12 phases / 3 amplitudes2 amplitudes appear on 4 phases1 amplitude appears on 8 phases
16-QAM
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Pilots and TPS addition• Pilots
– Continual pilots� Always in the same place within the OFDM symbol� 45 in 2k mode, 177 in 8k mode� Transmitted at increased power levels� Used to estimate the channel characteristics and therefore make corrections
– Scattered pilots� Located as a pre-defined pattern such that there is an equal number per
symbol� 131 in 2K mode, 524 in 8k mode� Transmitted at increased power levels� Used in conjunction with continual pilots to estimate the channel distortion
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Pilots and TPS addition• TPS (Transmission Parameter Signalling)
– Type of modulation used– Hierarchy information
– Guard interval
– Inner code rates
– Transmission mode (ie 2k or 8k)– Frame number within a super frame (ie 0 to 3)
– DPSK (Differential Phase Shift Keying) modulation used due to robustness
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IFFT, time shift and combination
• Complex to real conversion– Q (real) and I (Imaginary) are added, sampled and output
• IDFT at transmitter, DFT at receiver– FFT actually used (computational algorithm) for summing operation– FFT’s must be powers of 2, hence ‘2k’ or ‘8k’ modes
– Much faster that normal DFT
– Eg if 8k point DFT takes 670 ms then the FFT takes .53 ms
N2 log NN2
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Guard Interval Insertion• Replication of end of symbol placed at beginning
∆ ∆ ∆
Useful symbol
t
Main signal
Copy of end of symbol
Also means receiver canidentify start of symbol usinga correlation function
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Final stages - Transmission• D/A conversion• Filtering• Upconversion and transmission
FM sound carrierNICAM
COFDM carrier
530 MHz +/- 1/6 Mhz(Centre)
526 MHz
f
CH27
519.25 MHz518 MHz
CH28
Video luminancecarrier
Video pictureinformation
7.61 MHz
Analoguetransmission
Digitaltransmission
Example: UK transmission in channel 28
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COFDM functional blocksOuter coding*(R/S bytes added)
204 204
RS
MPEG-2 Transport streaminput
Randomisation*
PRBS
188 188 188
Outer interleaving*(Forney)
Inner coding*
|||||||........||||||||
Bit and symbol interleaving
....011001010001.... |||||||........||||||||
|||||||........||||||||
Amplitude/phasemapping
R|||||||........||||||||
|||||||........|||||||| I
Pilots and TPS addition
Inverse FFT........................
|||||||........||||||||R
|||||||........||||||||IIFFT
FIR
Time shift and combination
Guard intervalinsertion
|||||......|||||||||||......||||||
GI
Analogueconversion Upconversion
F
DAC
Filtering Transmission
* Same as DVB-S
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What is COFDM ?
C - Coded
O - Orthogonal
F - Frequency
D - Division
M - Multiplex
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Orthogonality
• Spacing between carriers is minimised– Results close to theoretical maximum are achieved (∆f ∝1/Τ )
1/T
fk fk+1 fk+2 fk+3 fk+4
– Expensive in analogue FDM due to costly band pass filters
• Definition possible due to signals being described as vectors
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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Countering against echoes andreflections
• Repetition of signal to counter echoes– Echoes caused by
– Moving receiver
– Moving transmitter
– Reflection from moving or static objects
Set top boxBuilding
COFDM transmitter
Direct signal
Reflectedsignal
Set top box
COFDM transmitter 1
Direct signal
COFDM transmitter 2
Distant signal
– Single Frequency Networks (SFN’s)
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Countering against echoes andreflections
• Echo length is easily calculated– Assuming 2k Mode with Guard interval 1/32
– 1/32 of the symbol transmits in 7 us– Maximum delay = 7 us
– Distance = 3 x 108 m/s x 7 us– Distance = 2.1 km
Useful data
Guard interval
∆
t
Main signal
Useful data
Guard interval
∆
t
Delayed signal
Max. delay
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COFDM
• A brief history
• COFDM principles
• COFDM transmission sequence
• Countering against echoes and reflections
• DVB-T framing structure
• DVB-T variable parameters
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DVB-T variable parameters
• Carrier mode: 2k or 8k
• Type of modulation: QPSK, 16-QAM, 64-QAM
• Guard Interval: 1/4, 1/8, 1/16, 1/64
• Inner code rate: 1/2, 2/3, 3/4, 5/6, 7/8
• Hierarchical modes
• Selection of transmission bandwidth (6/7/8 MHz)
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DVB-T variable parameters
Modulation
QPSK
16-QAM
64-QAM
Code rateGuard interval
1/2
2/3
3/4
5/6
7/8
1/2
2/3
3/4
5/6
7/8
1/2
2/3
3/4
5/6
7/8
1/4 1/8 1/16 1/32
4.98
6.64
7.46
8.29
8.71
9.95
13.27
14.93
16.59
17.42
14.93
19.91
22.39
24.88
26.13
5.53
7.37
8.29
9.22
9.68
11.06
14.75
16.59
18.43
19.35
16.59
22.12
24.88
27.65
29.03
5.85
7.81
8.78
9.76
10.25
11.71
15.61
17.56
19.52
20.49
17.56
23.42
26.35
29.27
30.74
6.03
8.04
9.05
10.05
10.56
12.06
16.09
18.10
20.11
21.11
18.10
24.13
27.14
30.16
31.67
Useful data rate (M bits / sec)
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DVB-T variable parameters
• Significance of mode and guard interval
– ‘8k’ system allows good reception with long multi-path echoes
– ‘8k’ system is therefore suitable for single frequency networks (SFN’s)
– ‘2k’ system more suited to multi frequency or single transmitter networks
– A larger guard interval implies a lower bit-rate efficiency
– The guard interval value is therefore a trade-off between bit-rate andnetwork tolerance to echoes and reflections
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DVB-T hierarchy codingI
Q
64 QAM constellation
Low priority carriers
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DVB-T hierarchy codingI
Q
In poor S/N ratio conditions16 64-QAM constellation pointscan be demodulated as one QPSKconstellation point
High priority carriers
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DVB-T hierarchy coding
• Transmission of the same or different data for:
– Same or different program can be transmitted in HD and SD + greatererror recovery
– Poor reception areas can view SD if HD not possible
• Transmission of different resolutions / characteristics:
– Reception by different cost receivers (high end, low end, mobile,portable)
• Other data can be transmitted related to the program