DVB-S2 standard with normal and short frames Guy Lesthivent Alban - - PDF document

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DVB-S2 standard with normal and short frames Guy Lesthivent Alban Duverdier 1 Scope This document is aimed at summarizing the main features and performance of the channel coding scheme selected by the DVBS2 working group regarding short

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DVB-S2 standard with normal and short frames

Guy Lesthiévent – Alban Duverdier

1 Scope

This document is aimed at summarizing the main features and performance of the channel coding scheme selected by the DVBS2 working group regarding short frames. It is based on the draft version of the future standard registered as DVBS2-74r9 (DRAFT Digital Video Broadcasting: Second generation framing structure, channel coding and modulation systems for broadcasting, interactive services, news gathering and other broadband satellite applications). This work was performed with the help of the consulting office NOVACOM (Paris 15e) in the context of a CNES R&D study.

2 System overview

The system overview is presented by Figure 1.

1 = η

BPSK

2 = η

QPSK

3 = η

8PSK

4 = η

16APSK

5 = η

32APSK

BCH

K

user frame r-user frame BCH FEC r-user frame LDPC FEC

bits 16200

64800 =

LDPC

N

BCH FEC r-user frame

BCH

N

channel frame

η /

LDPC

N

⊕

EXOR

Systematic BCH encoder

scrambling sequence

Systematic LDPC encoder Mapping Header

( 2

BPSKmodulated)

&Pilot insertion IQ Randomization 90(S+1)+36 int(S/16) symbols

PLHEADER

1 slot (π/2BPSK) Slot 1 Slot... Slot 16

Pilot block (if requiered)

Slot S 36 symbols (unmodulated) 16 slots (selected modulation) S=720 S=360 S=240 S=180 S=144

S slots of 90 symbols

PLHEADER

Randomized sequence (2 PN generators on I and Q)

Figure 1: System overview

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3 Code features

The redundancy introduced by the BCH code followed by the LDPC code is presented by Figure 2.

redudancy BCH redudancy LDPC

16200

64800 =

LDPC

N

BCH

K

BCH BCH

K N −

BCH LDPC

N K =

LDPC LDPC

K N − Scrambled user frame

Figure 2: DVB-S2 information block after channel coding

The code parameters are given by Figure 3, where

corr

t is the bit error correction capability of the BCH code.

Figure 3: DVB-S2 code parameters

The trellis of the selected LDPC code is described by Figure 4.

… … … …

p2

…

pR-1

…

pi: LDPC parity bit nodes p0 p1 c0 c1 c2 cR-1

… ci: Check nodes ui: User / BCH parity bit nodes d°=2 d°=1 d°=ρ ρ ρ ρ-1 d°= ρ ρ ρ ρ d°=3 d°=3 d°=D>3

u0 u1 uKLDPC-1

LDPC LDPC

K N R − = Figure 4: DVB-S2 LDPC trellis

The trellis is check regular: all the check nodes but one have the same degree ρ that depends

n the LDPC code rate. On the contrary, the trellis is not bit node regular: all LDPC parity

bits but one have degree 2, BCH parity bits and a fraction of the user bits have degree 3, the user bits of the remaining fraction have a degree D greater than 3. The fraction of user bits of degree D and this degree depend on the code rate. The trellis is easily described through congruence relationships. As a consequence, the encoding process is very simple. Besides, on the decoding side, the Belief Propagation Algorithm can be efficiently implemented (high parallelism and reduced connectivity). Short frame Normal frame LDPC Rate

BCH

K

LDPC BCH

K N =

corr

t

BCH

K

LDPC BCH

K N =

corr

t ½ 7032 7200 12 32208 32400 12 3/5 9552 9720 12 38688 38880 12 2/3 10632 10800 12 43040 43200 10 ¾ 11712 11880 12 48408 48600 12 4/5 12432 12600 12 51648 51840 12 5/6 13152 13320 12 53840 54000 10 8/9 14232 14400 12 57472 57600 8

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4 Modulation

In addition to usual BPSK, QPSK and 8PSK, two modulation schemes have been added: 16APSK and 32APSK. The 16APSK constellation is given Figure 5 with

2 1,R

R real numbers such that 1 12 4

2 2 2 1

= + R R and

1 2 / R

R = γ is a function of the code rate.

text

1100 1101 1111 1110 0000 0100 1010 0001 1001 1011 0011 0111 0110 0010 1010 1000

I Q LSB MSB R1 R2

Figure 5: DVB-S2 16APSK constellation

The 32APSK constellation is given Figure 6 with

2 1,R

R ,

R real numbers and 1 16 12 4

2 3 2 2 2 1

= + + R R R where

1 2 1

/ R R = γ and

1 3 2

/ R R = γ are function of the code rate.

text

10001 10011 10111 10101 00000 10000 10010 00010 00011 00111 00110 10110 10100 00100 00101 00001

I Q γ γ γ γ1

1 1 1

=R2 / R1 γ γ γ γ2

2 2 2

=R3 / R1 LSB MSB R1 R2 R3

11000 01000 11001 01001 01101 11101 01100 11100 11110 01110 11111 01111 01011 11011 01010 11010

Figure 6: DVB-S2 32APSK constellation

Code rate γ 2/3 3.1 3/4

2.85

5/6 2.6 8/9 2.58 Code rate γ1 γ2

3/4

2.84 5.26

4/5

2.7 4.8 5/6

2.6 4.5

8/9

2.5

4.2

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5 Header and pilot insertion

A header of 90 symbols long has been devised to ensure fast and secure frame detection and

synchronization. The header comprises a unique word (SOF) followed by 64 symbols which

carry 7 control bits, Reed-Müller encoded, as described.

Figure 7: DVB-S2 header

They inform the receiver about:

the modulation type,
the LDPC code rate,
the frame type (normal or short),
the use of pilot symbols.

For modes requiring pilots, pilot blocks of 36 symbols are inserted every 16 slots to help receiver synchronisation.

HEADER

SOF

Service information

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6 Performances

The performances of the code have been validated by DVB-S2 for normal frames in an AWGN channel (ideal carrier synchronization and no phase noise). Table 1 presents them.

Ideal Es/No performance at Quasi Error Free PER=10-7 (AWGN channel) Mode Spectral Es/No (dB) Eb/No (dB) BPSK 1/2 0,495114

2.00

1,05 BPSK 3/5 0,594976

0.77

1,49 BPSK 2/3 0,662042 0.10 1,89 QPSK 1/2 0,988857 1.00 1,05 QPSK 3/5 1,188303 2.23 1,48 QPSK 2/3 1,322251 3.10 1,89 QPSK 3/4 1,487472 4.03 2,31 QPSK 4/5 1,587195 4.68 2,67 QPSK 5/6 1,654662 5.18 2,99 QPSK 8/9 1,766451 6.20 3,73 QPSK 9/10 1,788612 6.42 3,89 8PSK 3/5 1,779989 5.50 3,00 8PSK 2/3 1,980633 6.62 3,65 8PSK 3/4 2,228122 7.91 4,43 8PSK 5/6 2,478560 9.35 5,41 8PSK 8/9 2,646012 10.69 6,46 8PSK 9/10 2,679207 10.98 6,70 16APSK 2/3 2,637197 8.97 4,76 16APSK 3/4 2,966726 10.21 5,49 16APSK 4/5 3,165621 11.03 6,03 16APSK 5/6 3,300181 11.61 6,42 16APSK 8/9 3,523142 12.89 7,42 16APSK 9/10 3,567341 10.13 4,61 32APSK 3/4 3,703293 12.73 7,04 32APSK 4/5 3,951568 13.64 7,67 32APSK 5/6 4,119537 14.28 8,13 32APSK 8/9 4,397854 15.69 9,26 Table 1: DVB-S2 performances for normal frames (DVB-S2 validation)

For short frames, CNES validation is given for the 8PSK 2/3 (belief propagation algorithm for decoding, 6 bits for I/Q and branch metric quantification, maximum of 48 iterations). 8PSK 2/3

1,E-06 1,E-05 1,E-04 1,E-03 1,E-02 3,4 3,5 3,6 3,7 3,8 3,9