Conception et réalisation d'un modem radio COFDM dans la bande ...
Tutorial COFDM
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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 1980s 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 FFTs
OFDM grew out of Multi Carrier Modulation (MCM) Military HF radio (late 1950s)
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)
9 First commercial broadcasts in late 1998 with simulcast and laterOnDigital services.
Various associated bodies
1992DVB (Digital Video Broadcast - voluntary group of 200 companies)
9 DVB-S, DVB-C in 1994 and DVB-T in early 1997
dTTb (digtial Terrestrial Television broadcast project)
9 Demonstrator to show the feasibility of a commercial receiver
DVBird (Digital Video Broadcast integrated receiver decoder)9 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
Carriermodulation
fc1
Carriermodulation
fc2
Carriermodulation
fcn
:
s1(t)
s2(t)
sn(t)
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 a/2 phase shift. phase shift isshown here
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FDM (Frequency Division Multiplex)
Guard interval
Frequency
Time
Useful data
Continuous frequency
transmissions G(t)
FC1
FC2
FC3
FC4
FC5
6817 or 1705 frequenciesFCn
Tu
Tu 1/(Fc1 - Fc2)
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FDM (Frequency Division Multiplex)
ndata 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
:
Carrier
demod
Carrier
demod
Carrier
demod
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)ej2fkti/N
S(fk) =TN
ti=1
N
s(ti)ej2fkti/N
Carriermodulation
fc1
Carriermodulation
fc2
Carriermodulation
fcn
:
s1(t)
s2(t)
sn(t)
Transimitter
Transmitter architecture
Output signal
G(t)
S1(f)
S2(f)
Sn(f)
Receiver architecture
BPFc1
BPF = Band pass filter
BPFc2
BPFcn
:
Carrier
demod
Carrier
demod
Carrier
demod
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
9 Allows receiver to lock onto signal
Modulation used
9 Increases number of bits that can be transmitted
9 '2K' system in UK (1705 carriers)
Carrier types
9 Keeps constant power levels
9 Eg each carrier transports 4 bits for QAM-16
9 '8K' also an option (6817 carriers)
9 Data carriers - 2,4 or 6 bits per symbol, per carrier
9 TPS carriers - Transmission information9 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/phase
mapping
R|||||||........||||||||
|||||||........|||||||| I
Pilots and TPS addition
Inverse FFT........................
|||||||........||||||||R
|||||||........||||||||IIFFT
FIR
Time shift andcombination
Guard interval
insertion
|||||......|||||||||||......||||||
GI
Analogue
conversion 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
TRANSPORTSTREAM
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 data
output
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
Transmitter
FEC
Receiver
FECData Data
Energy
dispersal
Outer
coding
(RS)
Forney
Interleaving
Inner
coding
(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 amplitudes
2 amplitudes appear on 4 phases
1 amplitude appears on 8 phases
16-QAM
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Pilots and TPS addition Pilots
Continual pilots
9Always in the same place within the OFDM symbol
9 45 in 2k mode, 177 in 8k mode
9 Transmitted at increased power levels
9 Used to estimate the channel characteristics and therefore make corrections
Scattered pilots
9 Located as a pre-defined pattern such that there is an equal number persymbol
9 131 in 2K mode, 524 in 8k mode
9 Transmitted at increased power levels
9Used 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
FFTs 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
N
2 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 carrier
NICAM
COFDM carrier
530 MHz +/- 1/6 Mhz
(Centre)
526 MHz
f
CH27
519.25 MHz518 MHz
CH28
Video luminance
carrier
Video picture
information
7.61 MHz
Analogue
transmission
Digital
transmission
Example: UK transmission in channel 28
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COFDM functional blocksOuter coding*
(R/S bytes added)
204 204
RSMPEG-2 Transport streaminput Randomisation*
PRBS
188 188 188
Outer interleaving*
(Forney)
Inner coding*
|||||||........||||||||
Bit and symbol interleaving
....011001010001.... |||||||........||||||||
|||||||........||||||||
Amplitude/phase
mapping
R|||||||........||||||||
|||||||........|||||||| I
Pilots and TPS addition
Inverse FFT........................
|||||||........||||||||R
|||||||........||||||||IIFFT
FIR
Time shift andcombination
Guard intervalinsertion
|||||......|||||||||||......||||||
GI
Analogue
conversion 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/)
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
C t i i t h d
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Countering against echoes and
reflections
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
Reflected
signal
Set top box
COFDM transmitter 1
Direct signal
COFDM transmitter 2
Distant signal
Single Frequency Networks (SFNs)
C t i i t h d
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Countering against echoes and
reflections
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
Guardinterval
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 (SFNs)
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 and
network 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 + greater
error 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