ND SatCom SKYWAN IDU 7000, Manual

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2010-10-26 Network Design and Engineering Guide 83
Outdoor Unit and Satellite Link Design
Considerations for SkyWAN
®
Link Budget Calculations
ment must be calculated to transmit signals with sufficient quality to all reachable remote sta-
tions on their respective home channels. For a master and backup master station this must
include all stations in the network because the reference burst has to be sent to every station
in the network. For slave stations at least master and backup master station must be reachable
for the transmission of request and ranging burst. In a fully meshed
Sky WAN
®
network of
course all stations in the network must be reachable by any Sky WAN
®
station. The required
EIRP for each station is then determined by the highest power requirement of all these satellite
links.
The power requirement and PEB for the satellite transponder must be calculated individually
for each
Sky WAN
®
carrier. Here the power must be calculated which is needed to reach from
the satellite all Sky WAN
®
stations which are members of the carrier’s downlink population. The
maximum power requirement of these downlinks determines the required power and PEB for
this carrier. Finally more than one set of transponder parameters is required to calculate
Sky-
WAN
®
satellite links, specifically if MRB-DUB mode is used.
3.4.2 Downlink Optimization
Figure 3-9 Downlink Considerations
The performance of the satellite downlink is given by the maximum available EIRP of the sat-
ellite, the earth stations figure of merit and their location within the satellite beam footprint. The
maximum EIRP is only available in the center of the beam, stations located at the beam edge
will receive a weaker signal. The difference is called the “footprint disadvantage”. Note that the
maximum EIRP is not identical to the saturation EIRP which is specified in the footprint dia-
grams. The difference is due to the required output backoff of the transponder.
Assuming that the satellite is transmitting with maximum EIRP, the earth stations will receive
the signal with a quality Eb/No
max
which will determine the maximum possible modulation and
coding of the downlink carrier (cf. discussion Power Equivalent Bandwidth in previous section).
Note that Eb/No
max
does not depend on the downlink carrier bandwidth since the satellite
EIRP
max
always refers to the full bandwidth of the satellite transponder. Therefore, decreasing
the bandwidth of individual carriers by splitting up capacity into multiple carriers will not in-
crease Eb/No
max
and the possible modulation and coding. For a given satellite beam,
Eb/No
max
can only be increased by increasing the earth station’s G/T. As the noise part of
G/T is mainly fixed by the LNA type, the only possibility to increase the figure of merit is to in-
crease the antenna gain by using larger antennas.