U NIVERSITY OF TURKU Modeling of DVB-H Link Layer Heidi Joki Deparment of Information Technology...

UNIVERSITY OF TURKU

Modeling of DVB-H Link Layer

Heidi Joki

Deparment of Information TechnologyUniversity of Turku

Supervisor: Professor Jorma VirtamoInstructor: Jarkko Paavola, M.Sc.

10.5.2005 Heidi Joki 2

UNIVERSITY OF TURKU

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 => 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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MPEG2 TV serviceMPEG2 TV service

MPEG2 TV serviceMPEG2 TV service

MUX

DVB-H IP Encapsulator

MPE MPE-FEC

Timeslicing

IP

DVB-H IP Decapsulator

MPE MPE-FEC

Timeslicing

DVB-T Modulator

8k 4k DVB-H TPS2k

DVB-T Demodulator

8k 4k DVB-H TPS2k

Channel

TS

RF

RF

TS IP

Transmitter

Receiver

New to DVB-H

The DVB-H system


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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)IP header (20B) Payload (0-1480B)

Last punctured RS

column

...

First punctured R

S colum

n

Parity b

ytes in last FE

C section

..

Parity b

ytes carried in section 2

Parity b

ytes carried in section 1

Last data padding column

..

First data padding colum

n

Last IP datagram

Padding b

ytes

...

2nd IP dg cont.. 3rd IP

dg

1st IP dg cont. 2nd IP

datagram

1st IP datagram

1

1

Nbr of rows256, 512,

786 or 1024

191 1 64Application data table RS data table

MPE-FEC header (12B) Column (max 1024B) CRC-32 (4B)

Network Layer:IP datagram

LLC sublayer:MPE-FEC frame

MAC sublayer:MPE and MPE-FEC sections

(Header includes 4BReal time parameters)

MPE header (12B) IP datagram CRC-32 (4B)

TS Header (4B) Payload (184B) TS Header (4B) Payload (184B)... ...

MPEG-2 Transport Stream

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• 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

Time-slicing

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MPE-FEC in TX (1/2)

IP header (20B) Payload (0-1480B)

Last punctured RS

column

...

First punctured R

S colum

n

Parity bytes in last F

EC

section

..

Parity bytes carried in section 2

Parity bytes carried in section 1

Last data padding column

..

First data padding colum

n

Last IP datagram

Padding bytes

...

2nd IP dg cont.. 3rd IP

dg

1st IP dg cont. 2nd IP

datagram

1st IP datagram

1

1

Nbr of rows256, 512,

786 or 1024

191 1 64Application data table RS data table

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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

• Hamming distance depends on the amount of transmitted RS columns

eu ttd 12

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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-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

msb lsb

delta_t

address

table frme

MAC_address_4

MAC_address_3

MAC_address_2

MAC_address_1

msb

msblsb

lsb

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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

MPE-FEC header (12B) Column (max 1024B) CRC-32 (4B)

MAC sublayer:MPE and MPE-FEC sections

(Header includes 4BReal time parameters)

MPE header (12B) IP datagram CRC-32 (4B)

TS Header (4B) Payload (184B) TS Header (4B) Payload (184B)... ...

MPEG-2 Transport Stream

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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• 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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MPE and FECsections

IP encapsulationto ADT

Add paddingto ADT

RS(255,191)encoding

MPE and MPE-FEC sectionsheader+payload

CRC-32 encodingTS packets header + payload

TS demux / PID filtering

MPE- and MPE-FEC

section parsing

CRC-32decoding

Section decapsulation

RS(255,191) decoding

IP parsing and filtering

TS channel model

IP IP datagram or RS data column

MPE and FECsections

TS

TS

EIT

IP

Measurements

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

10.5.2005 Heidi Joki 24

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The error pattern

MPEG-2Source

DVB-TModulator

ChannelSimulator

NoiseGenerator

DVB-T/H Receiver

MPEG-2 TestSignal

Different DVB-T modes

Hardware channel simulator and noise generator:COST 207 TU channel Fd C/N

LogicAnalyzer

Only the TS error statistics were saved into the file

TS error Data:

100111…Provided by Nokia

10.5.2005 Heidi Joki 25

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Simulation parametersThe 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

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 vs. TS erasure decoding

17 18 19 2010

-3

10-2

10-1

FER with and without erasure info decoding, 1024 rows

SNR [dB]

Fra

me

Err

or R

atio

EIT

real32real64

TS PER

17 18 19 2010

-3

10-2

10-1

FER with and without erasure info decoding, 256 rows

SNR [dB]

Fra

me

Err

or R

atio

EIT

real32real64

TS PER

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

10-3

10-2

10-1

10-4

10-3

10-2

10-1

Input SER

Out

put

SE

R

Input vs. Output SER (TS SER vs. MPE-FEC SER)

1024 rows

256 rows

• 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.

10.5.2005 Heidi Joki 30

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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

10.5.2005 Heidi Joki 31

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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

1. 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.

2. Consider all unreliable bytes, marked with ‘1’ or ‘2’ in the EIT, as erasures.

3. If the amount of unreliable bytes is less than 64, use the remaining Hamming distance for error decoding. Perform the erasure (and error) decoding.

4. 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 [email protected]

U NIVERSITY OF TURKU Modeling of DVB-H Link Layer Heidi Joki Deparment of Information Technology...

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