Asymmetric Data Rate Link - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 High Speed Trunking Modem
DoubleTalk Carrier-in-Carrier Option
The link budget summary for the different ModCod combinations is as follows:
S. No. Modulation & FEC
1 S2 8PSK 5/6
2 S2 8PSK 8/9
3 S2 16APSK 2/3
4 S2 16APSK 5/6
Based on this analysis, S2 16APSK 2/3 with CnC provides the maximum savings of 31%.
8.4.6.1

Asymmetric Data Rate Link

As occupied (or allocated) bandwidth of a CnC 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 CnC 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 required transponder
power, and increases available link margin;
•
Lower order modulation uses less transmit power on the ground – this can significantly
reduce the BUC/SSPA size not only by reducing the transmit EIRP, but also by 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 8PSK, TPC 3/4 and required 27.8 MHz of leased
bandwidth, the CnC link was based on 8PSK 3/4 and 8PSK 3/5 and required 19.3 MHz of leased
bandwidth.
The savings summary is as follows (note TPC 7/8 is 20/23 actual):
Item
Data Rate (kbps)
Modulation
FEC
Occupied BW (MHZ)
Power Eq. BW (MHz)
Leased BW (MHz)
Hub HPA (W)
Remote HPA (W)
Allocated BW
(MHz)
18.2
17.9
17.35
13.9
IS-901 @ 342º W, 22/22 (EH/EH)
Africa – 4.5 m
Africa – 3.6 m
30 Mbps / 10 Mbps
Original Link
Hub to Remote To
Total
Remote Hub
30 Mbps 10 Mbps
8PSK 8PSK
TPC 3/4 TPC 7/8
19.1 5.5 24.6
22.0 5.8 27.8
27.8
503
219
8–12
Leased BW
PEB (MHz)
(MHz)
26.9 26.9
34.9 34.9
25.1 25.1
38.3 38.3
With Carrier-in-Carrier and LDPC
Hub to Remote to
Remote Hub
30 Mbps 10 Mbps
8PSK 8PSK
S2 3/4 S2 3/5
19.3 8.0
13.5 3.4
310
129
Revision 1
MN-CDM750
Savings Compared PSD Ratio
to Original (dB)
25% 0.0
4% 0.0
31% 0.0
-9.7% 0.0
Savings
Total
19.3
16.9
19.3 30.5%
38.3%
41.1%