beautypg.com

A.3 versafec (short-block ldpc), A.3 versafec, Short-block ldpc) – Comtech EF Data CDD-562AL User Manual

Page 211

background image

CDD-562AL/564A/L Multi-Channel Demodulator

Revision 0

Forward Error Correction

MN-CDD562AL564AL

A–3

A.3

VersaFEC

(Short-block LDPC)

VersaFEC is a system of short-block LDPC codes that have been designed as a low-latency alternative to
both Comtech’s existing LDPC codes, and to the system set forth in the DVB-S2 specification (EN
302307) ratified by ETSI.

Since the introduction of Comtech’s LDPC, considerable research has been carried out to find ways to
reduce the block size of LDPC (and hence its latency) while preserving the coding gain performance very
close to the Shannon bound.

The set of VersaFEC codes was developed with two distinct purposes:

1) To provide an expanded choice of combinations of modulation and coding that maintain the

same coding gain performance as Comtech’s existing LDPC, while significantly reducing
latency. The existing LDPC approach (and DVB-S2 short-block) uses blocks that are 16 kbits
in length. VersaFEC, on the other hand, uses block sizes that vary between 2k and 8.2 kbits,
and at the same time reduces latency still further through a novel design approach.

2) To provide combinations of modulation and coding (ModCods) that are suitable not only for

Constant Coding and Modulation (CCM) applications, but are also the basis for Comtech
Comtech EF Data and Comtech AHA Enterprise Products Group’s patent-pending Adaptive
Coding and Modulation (ACM) system. The VersaFEC ACM feature requires activation of the
appropriate FAST codes.

There are 12 ModCods in the VersaFEC set (see Table A-1).The modulation types (BPSK,
QPSK, 8-QAM and 16-QAM) and the code rates have been chosen to give a continuous
progression of performance in terms of both Eb/No and spectral efficiency – an essential aspect
of a well-engineered ACM system.

Table A-1. The VersaFEC ModCod Set

ModCod Modulation

Code

Rate

Spectral

efficiency,

bps/Hz

Block

size,

bits

Typical

Eb/No, for

BER = 5 x 10

-8

Latency at

64 kbps, in

milliseconds

Min. Data

Rate (kbps)

Max. Data

Rate

(Mbps)

00

BPSK

0.488

0.49

2k

2.4 dB

26

16.0

1.462

01

QPSK

0.533

1.07

4.1k

2.2 dB

53

16.0

3.200

02

QPSK

0.631

1.26

4.1k

2.7 dB

59

16.0

3.785

03

QPSK

0.706

1.41

4.1k

3.4 dB

62

16.0

4.233

04

QPSK

0.803

1.61

4.1k

3.8 dB

66

16.0

4.818

05

8-QAM

0.642

1.93

6.1k

4.6 dB

89

16.0

5.782

06

8-QAM

0.711

2.13

6.1k

5.2 dB

93

16.0

6.401

07

8-QAM

0.780

2.34

6.1k

5.6 dB

97

16.0

7.021

08

16-QAM

0.731

2.93

8.2k

6.3 dB

125

16.0

8.776

09

16-QAM

0.780

3.12

8.2k

7.0 dB

129

16.0

9.361

10

16-QAM

0.829

3.32

8.2k

7.5 dB

131

16.0

9.946

11

16-QAM

0.853

3.41

8.2k

8.0 dB

132

16.4

10.239

This manual is related to the following products: