Transcript U NIVERSITY OF TURKU Modeling of DVB-H Link Layer Heidi Joki

UNIVERSITY OF TURKU
Modeling of DVB-H Link Layer
Heidi Joki
Deparment of Information Technology
University of Turku
Supervisor: Professor Jorma Virtamo
Instructor: Jarkko Paavola, M.Sc.
UNIVERSITY OF TURKU
Agenda
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Background: Why was DVB-H developed?
Services
From DVB-T to DVB-H
The DVB-H system
DVB-H standards family
Presentation of the DVB-H Link Layer
Simulation model
Simulation results
New decoding algorithms
Conclusions
Further work
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Background: Why was DVB-H developed?
• There was a wish to bring TV-like
services to mobile phones
• UMTS does not fulfil requirements for
high bandwidth Internet applications,
such as streaming video
• Mobile broadcasting is the best way to
reach many users with reasonable cost
• DVB-T is not suitable for handheld
battery powered devices
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Services
• Real time applications
– TV broadcasting, info linked to events,
games or quizzes
• Data carousel applications
– Like teletext; stocks, weather, sports
• File Download
– Buy newspaper, tourist map of city
• DVB-H in mobile phones => cellular network
as return channel for interactivity, billing and
authentication
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From DVB-T to DVB-H
• DVB-H is amendment of DVB-T for handheld
devices
• Lower power consumtion in the receiver
• More flexibilyty in network planning
• Technical changes:
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Time-slicing (Link layer)
MPE-FEC (Link layer)
4K OFDM mode (Physical layer)
IP datacast (Network layer)
Signaling
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The DVB-H system
MPEG2 TV service
MPEG2 TV service
MPEG2 TV service
MPEG2 TV service
MUX
MPE
MPEFEC
DVB-T Modulator
8k
DVB-H
IP Encapsulator
IP
TS
4k
2k
DVB-H TPS
Transmitter
Time
slicing
RF
New to DVB-H
Channel
Receiver
RF
DVB-T Demodulator
TS
8k
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4k
2k
DVB-H TPS
DVB-H
IP Decapsulator
MPE
Heidi Joki
MPEFEC
IP
Time
slicing
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Presentation of the DVB-H Link Layer
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Link Layer Packets (TX)
Time-Slicing
MPE-FEC
Reed-Solomon(255,191)
MPE- and FEC-sections
Transport Stream
Section parsing and Decapsulation (RX)
Erasure Decoding (RX)
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Link Layer Packets (transmitter)
Network Layer:
IP datagram
IP header (20B)
Payload (0-1480B)
Application data table
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CRC-32 (4B)
MPE-FEC header (12B)
...
TS Header (4B)
Payload (184B)
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Last punctured RS column
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First punctured RS column
Parity bytes in last FEC section
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(Header includes 4B
Real time parameters)
IP datagram
Parity bytes carried in section 2
Parity bytes carried in section 1
Last data padding column
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MPE header (12B)
First data padding column
Padding bytes
Last IP datagram
MAC sublayer:
MPE and MPE-FEC sections
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3rd IP dg
2nd IP dg cont..
2nd IP datagram
1st IP dg cont.
1st IP datagram
1
Nbr of rows
256, 512,
786 or 1024
MPEG-2
Transport Stream
RS data table
1
LLC sublayer:
MPE-FEC frame
Column (max 1024B)
TS Header (4B)
Payload (184B)
CRC-32 (4B)
...
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Time-slicing
• Data sent in bursts, one
burst per MPE-FEC frame
• Enables power saving (≤90%)
• Delta-t, time to start of next burst, is
announced in the section header
• No separate synchronization needed;
Receiver clock has to be stable only until
next burst
• Supports use of receiver for network
monitoring during off-periods
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MPE-FEC in TX (1/2)
RS data table
Application data table
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191 1
1
Last
punctured
RS column
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First punctured RS column
Parity
bytes
in
last
FEC section
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Parity bytes carried in section 2
Parity bytes carried in section 1
Last data padding column
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First data padding column
Last IP datagram
Padding bytes
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Nbr of rows
256, 512,
786 or 1024
2nd IP dg cont..
3rd IP dg
1st IP dg cont.
2nd IP datagram
1st IP datagram
1
Payload (0-1480B)
IP header (20B)
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MPE-FEC in TX (2/2)
• Max 1500B IP datagrams (as Ethernet)
• IP datagrams encapsulated column-wise into the
Application Data Table (ADT)
• ADT encoded row-wise with RS(255,191)
• Virtual interleaving is achieved!
• Code shortening and puncturing used for achieving
different MPE-FEC code rates
• Different number of rows in MPE-FEC frame give
different burst sizes
• Number of rows and the use of MPE-FEC is signalled
to the receiver
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Reed-Solomon(255,191)
• Hamming distance d = n-k+1 = 65
• Correction capabillity
– tu = 32 errors if pure
error correction used
– te = 64 erasures if pure
erasure correction used
d  2tu  1  te
• Hamming distance depends on the
amount of transmitted RS columns
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MPE- and MPE-FEC sections
• IP datagrams form payload of MPEsections
• RS data columns form payload of MPEFEC sections
• 12B section header added
• CRC-32 calculated and 4 redundancy
bytes placed at the end of the section
• CRC-32 is used for error detection in the
receiver
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MPE section header
Syntax
MPE-FEC section header
bits
Syntax
bits
table_id
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table_id
8
section_syntax_indicator
1
section_syntax_indicator
1
private_indicator
1
private_indicator
1
reserved
2
Reserved
2
section_length
12
section_length
12
MAC_address_6
8
padding_columns
8
MAC_address_5
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reserved_for_future_use
8
reserved
2
Reserved
2
payload_scrambling_control
2
reserved_for_future_use
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address_ scrambling_control
2
LLC_snap_flag
1
current_next_indicator
1
current_next_indicator
1
section_number
8
section_number
8
last_section_number
8
last_section_number
8
Real_time_parameters
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Real_time_parameters
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Real time parameters
msb
msb
lsb
MAC_address_4
delta_t
lsb
table
frme
MAC_address_3
msb
MAC_address_2
address
lsb
MAC_address_1
• Delta-t = time to beginning of next burst
• Table_bounary = ’1’ for last section of ADT or
RS data table
• Frame_boundary = ’1’ for last section of a
MPE-FEC frame
• Address = number of cell in the MPE-FEC
frame for the first byte of the payload carried
in that section
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Transport Stream
MAC sublayer:
MPE and MPE-FEC sections
MPE header (12B)
IP datagram
(Header includes 4B
Real time parameters)
MPEG-2
Transport Stream
CRC-32 (4B)
MPE-FEC header (12B)
...
TS Header (4B)
Payload (184B)
Column (max 1024B)
TS Header (4B)
Payload (184B)
CRC-32 (4B)
...
• TS packet = 4B TS header + 184B payload
• 13 bit PID in the TS header indicates
Elementary Stream and data type
• transport_error_indicator (1 bit) set to ’1’
by physical layer RS(204,188) decoder in
the receiver if error correction failed
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Section parsing and decapsulation
in the Receiver
• RX receives TS with a certain PID
• Find first byte of the section
– table_id = 62 (MPE) or 120 (FEC)
• Find section length
• Do CRC-32 check
– OK -> find address and decapsulate the
section payload into the frame
– Failed -> mark bytes as erasures
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Erasure decoding in DVB-H
• Erasure Info Table (EIT) of same size as
MPE-FEC frame
• ’0’ = reliable byte, ’1’ = erasure
• If a section fails CRC-32 check, the
complete datagram/RS column is
marked as ’erasure’
• RS decoder can correct 64 erasures/row
if all RS columns are transmitted
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Simulation model of Finnish WingTV consortium
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Simulation model: motivation
Parameter
Options
Explanation
Modulation
3
QPSK, 16QAM, 64QAM
FFT-size
3
2K, 4K, 8K
In-depth interleaver
2
On / Off (only for 2K and 4K)
Guard Interval
4
1/4, 1/8, 1/16, 1/32
CC rate
5
1/2, 2/3, 3/4, 5/6, 7/8
MPE-FEC code rate
6
1/2, 2/3, 3/4, 5/6, 7/8, 1
Burst size
4
256, 512, 768, 1024 rows
Burst bit rate
2
Number of
combinations
14400
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• The number of link
layer and physical layer
parameters add up to
14400!
• Simulation is the
fastest and most
economic way of
evaluating the impact
of different parameters
• Simulation provides an
opportunity to test new
ways of parsing,
decapsulation and
decoding
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Simulation model (link layer)
IP
IP encapsulation
to ADT
Add padding
to ADT
RS(255,191)
encoding
IP datagram or
RS data column
TS
MPE and MPE-FEC sections
header+payload
MPE and FEC
CRC-32 encoding
sections
TS packets header + payload
TS channel model
TS
TS demux /
PID filtering
Outside the scope of the
DVB-H standard, means for
TS erasure decoding and
hierarchical decapsulation
were also implemented (not
included in the figure).
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MPE- and
MPE-FEC
section parsing
MPE and FEC
sections
CRC-32
decoding
Section decapsulation
Measurements
EIT
RS(255,191) decoding
IP
IP parsing and filtering
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TS erasure decoding
• Except the CRC erasure decoding,
means for TS erasure decoding was
implemented
• Symbols in the MPE-FEC frame are
marked as reliable or unreliable
based on the
transport_error_indicator in the TS
header
• IP datagram lengths not considered
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The error pattern
MPEG-2 Test
Signal
Different DVBT modes
Hardware channel simulator and
noise generator:
COST 207 TU channel
Fd
C/N
DVB-T
Modulator
MPEG-2
Source
Channel
Simulator
Provided by Nokia
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Noise
Generator
Only the TS error
statistics were saved
into the file
DVB-T/H
Receiver
Logic
Analyzer
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TS error
Data:
100111…
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Simulation parameters
The effect of the following parameters on the MPE-FEC FER can
be examined:
• Burst size, i.e. number of rows in MPE-FEC frame
• MPE-FEC code rate
• Length of IP datagrams
• FEC decoder type: TS erasure decoding vs. CRC erasure
decoding
• The length of the burst, i.e. the interleaving length
The above mentioned parameters can be simulated with the
following physical channel parameters:
• Modulation
• Doppler frequency
• Convolutional code rate
• Channel model: TU6, indoor, pedestrian, etc.
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Performed simulations
Channel model:
TU6
Modulation:
16 QAM
Doppler frequency:
10 Hz
CC rate:
½
Amount of TS packets:
4 193 000
Amount of TS data:
788 MB
IP datagram length:
1500 Bytes
Amount of IP data:
256 rows: 560 MB
• The simulations were
performed with 256and 1024-row frames
• IP datagram length was
1500 bytes
• Two different
simulations were carried
out
1024 rows: 570 MB
MPE-FEC code rate:
¾
Signal to noise ratio:
17 – 20 dB
Amount of MPE-FEC
frames:
256 rows: 11 686 frames
1024 rows: 2927 frames
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– CRC erasure decoding
– TS erasure decoding
• The aim was to compare
the two different
methods and to study
the amount of
unnecessary erasures
added to the EIT by the
CRC decoding
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CRC erasure decoding vs. TS erasure
decoding
FER with and without erasure info decoding, 1024 rows
-1
-1
10
FER with and without erasure info decoding, 256 rows
10
EIT
real32
real64
TS PER
Frame Error Ratio
Frame Error Ratio
EIT
real32
real64
TS PER
-2
10
-3
10
-2
10
-3
17
18
19
20
SNR [dB]
10
17
18
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EIT64
The RS decoder, using erasure information, is able to correct 64 bytes of CRC-32
erasure data per row in an MPE-FEC frame.
Real 32
The RS decoder is able to correct 32 erroneous bytes per row. The error locations
are unknown. Errors are lost TS packets. The length of the IP datagram is
ignored.
Real 64
The RS decoder, using erasure information, is able to correct 64 erroneous bytes per
row. Errors are lost TS packets. The length of the IP datagram is ignored.
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SNR [dB]
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Symbol error ratio using CRC erasure
decoding
Input vs. Output SER (TS SER vs. MPE-FEC SER)
-1
10
1024 rows
256 rows
-2
Output SER
10
-3
10
-4
10
-3
-2
10
10
Input SER
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• Input SER equals TS
PER. All symbols in
an erroneous TS
packet are
considered
incorrect.
• Output SER is the
SER after CRC
erasure decoding
using RS(255,191)
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Result analysis
• CRC-32 erasure decoding adds far too
many unnecessary erasures.
• When transmitting 1500B IP datagrams in
the smallest frame, the gain of using FEC
is almost lost if using erasures based on
CRC-32
• TS erasure decoding saves gain in all
simulations
• Using a larger MPE-FEC frame gives
improvement in gain, when burst length
is not considered.
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Drawbacks of the DVB-H standard
• CRC adds too much erasures into EIT
• Lack of protection of the section header
• Standard length of IP datagrams or MPE
sections preferable than various length
– Achieving constant TS bit rate (or almost
constant for streaming video)
– Decapsulation possible, though section header
is lost
• Not 100% certainity of ’reliable’ bytes in MPEFEC frame has to be considered
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Suggestions for improvements
(without changing the standard)
• TX: Introducing standard length of IP
datagrams (e.g. 1 or 2 columns)
• RX: Using TS erasure decoding based on
the transport_error_indicator in the TS
header
• RX: Using hierarchical decapsulation and
decoding if needed (also decapsulate erroneous
packets, most of it is probably correct!)
• RX: Using combination of erasure and
error decoding
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The algorithm for hierarchical decapsulation
and hierarchical decoding
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4.
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Perform hierarchical decapsulation of TS packets, using the
transport_error_indicator when filling in the erasure info table
(EIT). Lost data is market with ‘1’, decapsulated but unreliable
data is marked with ‘2’ and correct data with ‘0’ in the EIT.
Consider all unreliable bytes, marked with ‘1’ or ‘2’ in the EIT,
as erasures.
If the amount of unreliable bytes is less than 64, use the
remaining Hamming distance for error decoding. Perform the
erasure (and error) decoding.
If the amount of unreliable bytes exceeds 64, consider the
bytes marked with ‘2’ in the EIT as reliable and repeat step 3.
The pure erasure decoding could also fail if some of the bytes
marked as reliable are erroneous. In this case step 4 is useful,
since it might leave some more Hamming distance for error
correction.
This algorithm can be combined with CRC or TS erasure
decoding. TS erasure decoding is recommended.
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Further work on the simulator
• Means for the user to input the simulation parameters should be
implemented. At least the following parameters should be read:
–
–
–
–
MPE-FEC code rate
The names of the IP data and error pattern files
Burst size and duration
Decoding method to be used; TS erasure or CRC erasure correction
• The TS erasure decoding should be implemented so that IP
datagram lengths are taken into account. Also combinations of
erasure and error correction should be thought of
• Time-slicing should be implemented
• Besides the FER, the output of the simulator should include IP
data along with erasure information, which is used by a potential
RS decoder at the application layer
• The simulator should be able to handle a multiplex of many
elementary streams
• Hierarchical decapsulation and decoding should be implemented
• A symbol based TS error pattern is needed
• Functions should be optimized for shortening the simulation time
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Future work on DVB-H link layer
and physical layer
• The impact of the IP datagram lengths and the MPE-FEC
code rates should be studied carefully
• The decoding process should be improved and different
decoding algorithms should be studied
• Finding the best means of decapsulation and decoding
using all received data is already quite a challenge.
However, the receiver manufacturers would probably
profit from implementing solutions for decoding based
on a combination of TS erasure and error correction.
• Proper channel models for indoor and pedestrian use
cases should be developed
• Based on the channel models, error patterns based on
symbol or bit errors could be developed on TS level
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Thank You!
Questions?
For more information contact
[email protected]
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