Comtech EF Data DMD-2050E Spezifikationen PDF herunterladen (Seite 81)

DMD2050E Universal Satellite Modem Theory of Operation

MN-DMD2050E Revision 2 3–39

The link budget summary for the different MODCOD combinations is as follows:

No.

Modulation & FEC

Allocated BW

(MHz)

PEB

(MHz)

Leased BW

(MHz)

Savings

Compared

to Original

PSD

Ratio

(dB)

8-QAM, LDPC

2/3

0.3584 1.1468 1.1468 -20% 2.1

QPSK, LDPC 3/4

0.47785

0.6734

30%

2.1

QPSK, LDPC 2/3

0.53735

0.5777

40%

2.1

QPSK, LDPC 1/2

0.7168

0.5184

0.7168

25%

2.1

Based on this analysis, QPSK, LDPC 2/3 with Carrier-in-Carrier provides the maximum savings of

40%.

In addition to 40% reduction in Leased Bandwidth, using Carrier-in-Carrier also reduced the

required HPA Power by almost 40%:

HPA Power

Traditional Link

(QPSK, TPC 3/4)

CnC Link

(QPSK, LDPC 2/3)

HPA Power Reduction

HPA @ 4.6 m

0.7 W

0.5 W

40%

HPA @ 2.4 m

1.5 W

1.1 W

36%

3.11.9.2 Asymmetric Data Rate Link

As occupied (or allocated) bandwidth of a Carrier-in-Carrier circuit is dictated by the larger of the

two carriers, it is strongly recommended that the smaller carrier be spread as much as possible

using a lower order modulation and/or FEC, while meeting the PSD ratio spec. Spreading the

smaller carrier using a lower order modulation has multiple benefits:

• Lower order modulation is always more robust;

• Lower order modulation uses less transponder power – this reduces total transponder, and

increases available link margin;

• Lower order modulation uses less transmit power on the ground – this can significantly

reduce the BUC/SSPA size by not only reducing the transmit EIRP, but also reducing the

BUC/SSPA backoff

Consider the following example:

Satellite & Transponder

IS-901 @ 342º W, 22/22 (EH/EH)

Earth Station 1 Africa – 4.5 m

Earth Station 2 Africa – 3.0 m

Data Rate 3000 Mbps / 1000 Mbps

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 341 342

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