Specifications

DMD20/20LBST Universal Satellite Modem Theory of Operation
MN-DMD20-20LBST Revision 14 3–37
3.15.8.2 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
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
Original Link
With Carrier-in-Carrier and LDPC
Savings
Hub to
Remote
Remote To
Hub
Total
Hub to
Remote
Remote to
Hub
Total
Data Rate (kbps)
3000
1000
3000
1000
Modulation
QPSK
QPSK
QPSK
QPSK
FEC
TPC 3/4
TPC 3/4
LDPC 3/4
LDPC 1/2
Occupied BW (MHZ)
2.8
0.9
3.7
2.8
1.4
2.8
Power Eq. BW (MHz)
3.3
0.6
3.9
2.5
0.3
2.8
Leased BW (MHz)
3.9
2.8
27.5%
Hub HPA (W)
26.0
20.3
22%
Remote HPA (W)
10.6
6.4
40%
If this link was designed using QPSK, LDPC 3/4 in both directions, it would have required:
Occupied BW
2.8 MHz
Power Eq. BW
3.0 MHz
7.2% increase in Power Eq. BW
Leased BW
3.0 MHz
7.2% increase in Leased BW
Hub HPA
20.3 W
Remote HPA
8.3 W
30% increase in Remote power