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Modeling of DVB-H Link Layer

Modeling of DVB-H Link Layer. Heidi Joki Deparment of Information Technology University of Turku Supervisor: Professor Jorma Virtamo Instructor: Jarkko Paavola, M.Sc. Agenda. Background: Why was DVB-H developed? Services From DVB-T to DVB-H The DVB-H system DVB-H standards family

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Modeling of DVB-H Link Layer

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  1. Modeling of DVB-H Link Layer Heidi Joki Deparment of Information Technology University of Turku Supervisor: Professor Jorma Virtamo Instructor: Jarkko Paavola, M.Sc.

  2. 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 Heidi Joki

  3. 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 Heidi Joki

  4. 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 Heidi Joki

  5. 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 Heidi Joki

  6. The DVB-H system Heidi Joki

  7. Heidi Joki

  8. 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) Heidi Joki

  9. Link Layer Packets (transmitter) Heidi Joki

  10. 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 Heidi Joki

  11. Last punctured RS column . . . First punctured RS column Parity bytes in last FEC section . . Parity bytes carried in section 2 Parity bytes carried in section 1 Last data padding column . . First data padding column Last IP datagram Padding bytes . . . 2nd IP dg cont.. 3rd IP dg 1st IP dg cont. 2nd IP datagram 1st IP datagram IP header (20B) Payload (0-1480B) Application data table RS data table 1 191 1 64 1 Nbr of rows 256, 512, 786 or 1024 MPE-FEC in TX (1/2) Heidi Joki

  12. 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 Heidi Joki

  13. 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 • Hamming distance depends on the amount of transmitted RS columns Heidi Joki

  14. 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 Heidi Joki

  15. Heidi Joki

  16. 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 Heidi Joki

  17. MAC sublayer: MPE and MPE-FEC sections (Header includes 4B Real time parameters) MPE header (12B) IP datagram CRC-32 (4B) MPE-FEC header (12B) Column (max 1024B) CRC-32 (4B) MPEG-2 Transport Stream ... TS Header (4B) Payload (184B) TS Header (4B) Payload (184B) ... 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 Heidi Joki

  18. 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 Heidi Joki

  19. 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 Heidi Joki

  20. Simulation model of Finnish WingTV consortium Heidi Joki

  21. 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 Heidi Joki

  22. 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). Heidi Joki

  23. 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 Heidi Joki

  24. Different DVB-T modes Hardware channel simulator and noise generator: COST 207 TU channel Fd C/N Only the TS error statistics were saved into the file MPEG-2 Test Signal DVB-T Modulator Channel Simulator Noise Generator DVB-T/H Receiver MPEG-2 Source Logic Analyzer TS error Data: 100111… The error pattern Provided by Nokia Heidi Joki

  25. 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. Heidi Joki

  26. 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 Heidi Joki

  27. CRC erasure decoding vs. TS erasure decoding Heidi Joki

  28. 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) Heidi Joki

  29. 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. Heidi Joki

  30. 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 Heidi Joki

  31. 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 Heidi Joki

  32. 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. Heidi Joki

  33. 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 Heidi Joki

  34. 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 Heidi Joki

  35. Thank You! Questions? For more information contact Heidi.Joki@utu.fi Heidi Joki

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