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DMD50 Universal Satellite Modem
3.14.9.2
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
Earth Station 1
Earth Station 2
Data Rate
While the traditional link was based on QPSK, TPC 3/4 and required 3.9 MHz of leased bandwidth,
the Carrier-in-Carrier link was based on QPSK, LDPC 3/4 and QPSK, LDPC 1/2 and required 2.8
MHz of leased bandwidth.
The savings summary is as follows:
Item
Data Rate (kbps)
Modulation
FEC
Occupied BW (MHZ)
Power Eq. BW (MHz)
Leased BW (MHz)
Hub HPA (W)
Remote HPA (W)
If this link was designed using QPSK, LDPC 3/4 in both directions, it would have required:
Occupied BW
Power Eq. BW
Leased BW
Hub HPA
Remote HPA
MN-DMD50 Revision 4
IS-901 @ 342º W, 22/22 (EH/EH)
Africa – 4.5 m
Africa – 3.0 m
3000 Mbps / 1000 Mbps
Original Link
Hub to
Remote To
Total
Remote
Hub
3000
1000
QPSK
QPSK
TPC 3/4
TPC 3/4
2.8
0.9
3.7
3.3
0.6
3.9
3.9
26.0
10.6
2.8 MHz
3.0 MHz
7.2% increase in Power Eq. BW
3.0 MHz
7.2% increase in Leased BW
20.3 W
8.3 W
30% increase in Remote power
With Carrier-in-Carrier and LDPC
Hub to
Remote to
Remote
Hub
3000
1000
QPSK
QPSK
LDPC 3/4
LDPC 1/2
2.8
1.4
2.5
0.3
20.3
6.4
Theory of Operation
Savings
Total
2.8
2.8
2.8
27.5%
22%
40%
3–32
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