ND SatCom SKYWAN IDU 7000, Manual

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84 Network Design and Engineering Guide 2010-10-26
Outdoor Unit and Satellite Link Design
Considerations for SkyWAN
®
Link Budget Calculations
In a Sky WAN
®
network the maximum possible modulation and coding of a carrier will be de-
termined by the weakest station within a downlink population, i.e. the station with the lowest
Eb/No. This weak station may considerably decrease the possible coding efficiency for a carrier
which leads to an increased bandwidth requirement for the network. To prevent this it might
make sense to
- increase G/T for weak stations by using larger antennas for these stations.
- put the weak stations on a separate home channel.
The latter option allows to use a high modulation and coding for the strong stations and the low-
er coding efficiency only for a separate carrier for the weak stations. This increases the overall
network coding
efficiency. Remember that for Sky WAN
®
networks, modulation and coding may
be selected independently for each carrier.
3.4.3 Uplink Optimization
Figure 3-10 Uplink Considerations
In the downlink optimization discussion it was assumed that the satellite transmits with the max-
imum available EIRP to the earth stations. This is however only true, if the signal intensity re-
ceived from the earth stations reaches the maximum Input Power Flux Density (IPFD). The
IPFD is given by the transponder Saturation Flux Density (SFD) reduced by the Input Back-Off
(IBO), which is necessary to prevent excessive carrier intermodulation. To reach this flux den-
sity at the satellite, the earth station’s EIRP
max
must be large enough to compensate the path
loss and potential additional rain fade.
If the station cannot reach the required EIRP it may be increased by using larger antennas or
higher power classes for the station amplifier. Another option would be to reduce the bandwidth
of an individual carrier by splitting the required capacity into multiple carriers. Splitting however
only makes sense if the traffic flow topology allows the simultaneous use of all carriers by dif-
ferent
Sky WAN
®
IDUs (cf. discussion in chapter 2.4.2). Note that a Sky WAN
®
unit may trans-
mit on multiple carriers sequentially (by frequency channel hopping) but not simultaneously.
Therefore the transmit power requirement of a station is determined by the largest carrier.