ND SatCom SKYWAN IDU 7000, Manual

Page: 56 / 154

52 Network Design and Engineering Guide 2010-10-26
General Carrier Design
From User Traffic to Satellite Link Carriers
Figure 2-33 Gross Container Information Content
3. So far we have determined the information content of a gross container. This information
is protected by adding redundancy bits which allow the receiver to detect and correct a
certain ratio of bit errors generated during the transmission over the satellite link. This pro-
cedure is called “Forward Error Correction” (FEC). The technology applied in Sky WAN
®
networks is called Turbo-Phi representing the most advanced method for forward error
correction of TDMA burst signals. The FEC rates are selectable per carrier with possible
rates of 1/3, 2/5, 4/9, 1/2, 2/3, 3/4, 4/5, 5/6, 6/7 for QPSK modulation and 2/3, 3/4, 4/5,
5/6, 6/7 for 8PSK modulation.
The lower the FEC rate the lower the requirement on Signal-to-Noise ratio on the satellite link
for an error-free transmission. A lower FEC rate reduces power requirements on both, the earth
station and the satellite transponder. On the other hand the redundancy bits increase the data
rate and thus increase the bandwidth requirement for the carrier.
Figure 2-34 Add Turbo-Phi Coding
4. The result of step 3 is a coded gross container which includes the error correction bits.
This information is transported over the satellite link by modulating the carrier using phase
shift keying. Sky WAN
®
IDU 7000 supports quadrature (QPSK) and 8 (8PSK) phase shift
keying. Generally the number of PSK symbols relates to the number of coded bits by:
Symbols = coded Bits/modulation factor, where the modulation factor is given by the
number of bits represented by one symbol: QPSK = 2, 8PSK = 3. To ensure proper syn-
chronization of the demodulator the symbols derived from the information bits are inter-
spersed with additional synchronization symbols started by a preamble followed by
midambles and finally finishing the burst with a postamble. Four different preamble pat-
terns are used for reference, request, ranging, and data bursts, respectively, to exclude
reception of unexpected bursts. Preamble length is 64 symbols, midambles and postam-
ble lengths are 16 symbols each.
5. Finally the modulated burst has to be put into a time slot of the carrier. Since an absolute
precise synchronization between transmission times of all stations is not feasible, there
are transmission gaps at the start and the end of each time slot allowing a transmission
time inaccuracy of some microseconds: