ND SatCom SKYWAN IDU 1070, Manual

Page: 57 / 154

2010-10-26 Network Design and Engineering Guide 53
General Carrier Design
From User Traffic to Satellite Link Carriers
Figure 2-35 Modulated Gross Container
6. Not all timeslots carry bursts which have been constructed from user data. Depending on
the reference burst mode (see chapter 2.3.3 “ Sky WAN
®
MF-TDMA”) some carriers
include reference or request bursts. These signaling time slots do require a part of the car-
riers bandwidth but do not contribute to the user traffic capacity of the carrier. In the exam-
ple of a TDMA carrier presented in figure 2-36, out of 15 time slots in the frame only the
12 slots (indicated by a grey color) are used to transport user traffic. One slot is used for a
reference burst and two slots for request bursts. That means that for this carrier 20% of
the bandwidth is consumed for signaling traffic.
Note that ranging slots are not considered here, as they are needed only if there are
unregistered stations in the network. This is generally only the case for a brief period after
a network restart, once all stations are registered again at the active master, these slots
may be allocated as user traffic slots.
Figure 2-36 Signalling Time Slots
7. At this point we have constructed a TDMA carrier which has enough capacity to carry the
required user traffic data rate as well as the necessary signaling information. The band-
width of the carrier is specified by its symbol rate (symbols per second sps). The fre-
quency bandwidth which has to be leased on the satellite transponder is however larger. It
must be ensured that outside the leased frequency band the carrier signal does not inter-
fere with adjacent carriers. Usually the satellite operator requires that outside the leased
band the spectral power density of the carrier is at least 17 dB lower than the power den-
sity in the center of the band. Sky WAN
®
IDU 7000 applies a root-raised-cosine Nyquist fil-
ter technique to limit the required frequency bandwidth.
The filter bandwidth which is specified by the roll-off factor may be configured. Possible
values are 0.2, 0.3 and 0.4. With that filter the required frequency bandwidth is given by:
frequency bandwidth = symbol rate x (1 + roll-off factor)