78 Network Design and Engineering Guide 2010-10-26Outdoor Unit and Satellite Link DesignFundamentals of Link Budget CalculationThe required SkyWAN® IDU 7000 Eb/No levels for different carrier modulation and coding val-ues are both contained in the TDMA calculation tool and the SkyWAN® link budget tool. For abrief discussion we have a look at 3 different values for a satellite link with a bit error rate< 10 -7, gross container size > 200 Byte:Table 3-1 Eb/No Values for different FEC Coding and ModulationsLet’s assume the result of the traffic estimation and TDMA calculation showed that a carrierwith a modem data rate = 1000 kbps is required. For a TDMA structure with a frame time of100 ms, a container size of 417 Byte and a carrier spacing factor of 1.2 the resulting bandwidthand power requirements to achieve the above stated Es/No levels are shown in table 3-2Table 3-2 Carrier Power and Bandwidth for TDMA structure exampleGoing from QPSK - 1/3 to 8PSK - 6/7 for a satellite carrier- the bandwidth needed on the satellite transponder is reduced by 78%.- the required power on both the satellite and the transmitting earth station is increased by316%.One important result of a link budget calculation is the determination of the optimal channelmodulation and coding. The goal is to use the highest possible value for modulation and FECto save bandwidth costs without exceeding the maximum power available on both the satellitetransponder and the earth stations.Power Equivalent BandwidthAs already mentioned, the available power on a satellite transponder is limited. The availablepower is determined by the transponder’s saturation EIRP reduced by the necessary outputback-off. This back-off is needed to prevent carrier interference which would occur if the ampli-fier on the satellite is operated at saturation level. If a carrier uses not the full transponder band-width but only a fraction, then the available power for this carrier is the same fraction of thetransponder power. Hence the carrier power may also be expressed as a bandwidth: This isthe definition of the “Power Equivalent Bandwidth” (PEB).If the carrier PEB is higher than the carrier bandwidth, the PEB must be leased on the trans-ponder. Since this increases the space segment cost without increasing the user bandwidththat situation should be avoided by selecting an optimized carrier coding and modulation.Note that within a network using multiple carriers, it is possible that for some carriers PEB islarger than the carrier bandwidth whereas for other carriers PEB is smaller than the carrierModulation FEC Rate Eb/NoQPSK 1/3 2.4 dBQPSK 6/7 5.6 dB8PSK 6/7 8.8 dBModulation - FEC Rate Carrier Bandwidth [kHz] Relative Carrier PowerQPSK - 1/3 1904 100 %QPSK - 6/7 758 208 %8PSK - 6/7 428 416 %
