Rain Fade; Table 3-3 Relation Between Modulation, Coding And Carrier Peb - ND SatCom SKYWAN IDU 7000 Manual

bandwidth. As long as the summary PEB for all carriers is still smaller than the summary band-
width of all carriers, no extra space segment has to be leased for the carriers with high power
requirement.
Example:
Available transponder EIRP:
Transponder bandwidth:
Required carrier EIRP:
-> Carrier PEB:
For an example we assume the values given above. Now we make a calculation for a Sky-
®
WAN carrier with a modem rate of 1000 kbps, QPSK modulation and 1/3 coding. We find a
carrier bandwidth of 1904 kHz (cf. previous example) and a power requirement for the satellite
of 30 dBW. Using the results from the previous example, we can now derive the required band-
width and PEB for other modulation and coding selections.
Modulation Carrier
- Coding EIRP on
Satellite
[dBW]
QPSK - 1/3 30.0
QPSK - 6/7 33.2
8PSK - 6/7 36.4
Table 3-3 Relation between Modulation, Coding and Carrier PEB
In this example, with QPSK 1/3 coding the carrier uses a much smaller fraction of the trans-
ponder's power than the transponder's bandwidth. Increasing the coding will increase the pow-
er requirement and decrease the carrier bandwidth. Indeed, at QPSK - 6/7 the satellite link
power and bandwidth is optimally equilibrated as the carrier bandwidth and PEB are identical.
Any further increase in modulation and coding will increase again the space segment cost, as
the PEB increases even though the carrier bandwidth would still decrease. So the optimal mod-
ulation and coding for this downlink would be QPSK - 6/7.

Rain fade

In Ku-Band strong rain falls can attenuate the signal substantially resulting in a temporary drop
of the Eb/No levels below the minimum requirement for a good quality satellite link. This may
lead to a temporary outage of services. To prevent this the link budget has to include a rain
margin for the satellite link power requirement. The amount of this margin depends on:
- The probability of intense rain at the geographical position of the earth station.
- The required availability for the satellite link.
- The carrier frequency.
As the rain fade increases with the carrier frequency, it is basically negligible for C-Band but
has to be taken into account for Ku-Band or higher carrier frequencies. The probability of in-
tense rain can be derived from the ITU-T rain zones which represent the maximum rain inten-
sity at a specific location which is not exceeded in 99.9% of a typical year. Rain zones A (polar
and desert regions) to Q (tropical Africa) are defined by the ITU-T. Finally, a required satellite
2010-10-26
50 dBW
36 MHz
40 dBW
3.6 MHz
PEB [kHz] Carrier Band-
width [kHz]
360 1904
758 758
1571 428
Network Design and Engineering Guide
Outdoor Unit and Satellite Link Design
Fundamentals of Link Budget Calculation
Space Segment to
lease on Transpond-
er [kHz]
1904
758
1571
79