Modeling of DVB-H Link Layer

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Modeling of DVB-H Link Layer

• Heidi Joki
• Deparment of Information Technology
• University of Turku
• Supervisor Professor Jorma Virtamo
• Instructor Jarkko Paavola, M.Sc.

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Agenda

• 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 gt 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
• 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
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(No Transcript)
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Presentation of the DVB-H Link Layer

• 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)
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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)
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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-k1 65
• Correction capabillity
• tu 32 errors if pure error correction used
• te 64 erasures if pure erasure correction used
• Hamming distance depends on the amount of
  transmitted RS columns

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MPE- and MPE-FEC sections

• IP datagrams form payload of MPE-sections
• RS data columns form payload of MPE-FEC 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 MPE section header MPE-FEC section header MPE-FEC section header
Syntax bits Syntax bits
table_id 8 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 8 reserved_for_future_use 8
reserved 2 Reserved 2
payload_scrambling_control 2 reserved_for_future_use 5
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 32 Real_time_parameters 32
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Real time parameters

• 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

• 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 -gt find address and decapsulate the section
  payload into the frame
• Failed -gt 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

• 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

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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Simulation model (link layer)
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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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
Provided by Nokia
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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

• The simulations were performed with 256- and
  1024-row frames
• IP datagram length was 1500 bytes
• Two different simulations were carried out
• 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

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
Amount of IP data 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
Amount of MPE-FEC frames 1024 rows 2927 frames
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CRC erasure decoding vs. TS erasure decoding
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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Symbol error ratio using CRC erasure decoding

• 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
  MPE-FEC 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

• 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
• Heidi.Joki_at_utu.fi