G.2 background – Comtech EF Data CDM-760 User Manual

Page 336

Appendix G

Revision 2

CDM-760 Advanced High-Speed Trunking Modem

MN-CDM760

G–2

G.2 Background

The CDM-760 system was originally based on DVB-S2 standards. New network specifications

such as DVB-S2-EB1 have been released by Comtech EF Data since the initial release of the CDM-

760. The DVB-S2 network specification in the CDM-760 offers 24 MODCODs that work over a

guaranteed Es/No range of 1.2 dB to 16.5 dB. By comparison, Comtech EF Data’s DVB-S2-EB1

Network Specification offers a suite of 40 MODCODs over this same Es/No range – nearly twice

the number of MODCOD selections for enhanced ACM performance

DVB standards also defined much of the system for both ACM and a related feature named

InBand Signaling. The CDM-760 supports duplex ACM operation between a pair of modems in a

point-to-point link (point-to-multipoint ACM operation is not currently supported). In ACM

mode, the symbol rate remains constant, and the modulation and coding (ModCod in DVB-S2

terms) changes to preserve the data integrity. Most links are designed with enough Es/No margin

to provide error free performance under faded conditions when there is higher attenuation in the

uplink or downlink path to/from the satellite.

Depending on the geographical region and link budget criteria, faded conditions can occur less

than 1% of the time. In such as case the operating Es/No of the link is higher than needed for more

than 99% of the year. ACM takes advantage of this link margin by increasing the ModCod during

unfaded conditions allowing the link to operate at a higher data rates during these periods. A

simplified example illustrates the point. Table G-1 shows the several ModCods, the Spectral

Efficiency (SE), and Es/No for the CDM-760 assuming Normal FEC Frame (64,800 bits).

Table G-1. ACM Example for Standard FECFrame (64,800 bits)

ModCod

Spectral Efficiency

Pilots ON

Guaranteed Es/No

(dB)

Data Rate (Mbps)

at 20 Msps

QPSK 1/2

0.9653

1.4

19.31

QPSK 3/5

1.1600

3.7

23.20

QPSK 2/3

1.2908

3.4

25.82

QPSK 3/4

1.4521

4.6

29.04

QPSK 4/5

1.5494

5.1

30.99

QPSK 5/6

1.6153

5.6

32.31

QPSK 8/9

1.7244

6.7

34.49

QPSK 9/10

1.7460

6.8

34.49

For example: If a link is designed to operate at an Es/No of 3.7 dB during a 3.0 dB fade, the 3.0

dB fade is commonly referred to as link margin. When fade conditions are not present, the link

margin is directly converted into a higher link Es/No: 3.7 dB + 3.0 dB margin = new Es/No of

6.7dB. This non-faded Es/No of 6.7 dB allows for a much higher ModCod to be used, offering a

higher Spectral Efficiency. The corresponding Spectral Efficiencies in this example range from

1.1600 to 1.7244.

The Data Rate (DR) and Symbol Rate are related by the symbol rate as follows:

DR (Data Rate) = SR (Symbol Rate) x SE (Spectral Efficiency)