Satellite Communications/Transponder characteristics
QUESTION: hey there,
what is the relationship between transponder bandwidth and symbols rate?
does it have any connection with the roll-off-factor?
ANSWER: In satellite communication practically, the roll-off factor will be set to the lowest value (0.20) in order to reduce carrier spacing for accommodating increased number of carriers in the same occupied bandwidth. Commercially,Satellite operators don't allow these low roll-off factors due to interference problems.They normally allow you to use it but will charge you for an occupied bandwidth of symbol rate x 1.35,1.40 or even higher, regardless of the roll-off factor you are applying.The symbol rate and the available transponder bandwidth are, in practical situation, factors that are considered in Link Budgets for percentage utilization estimates.
A good example of roll-of is when you look at single carrier per transponder operation. In DVB-S QPSK in a 36 MHz transponder where 0.35 is used, the maximum symbol rate is 36/1.35 = 26.7 MHz. However because of the filtering effect of the transponder, the nearest 27.5 Mbaud is a value commonly used. In DVB-S2, where 20% roll-off is allowed, one can do 36/1.2 = 30 Mbaud in a single carrier. So applying the lower roll-off increases the possible throughput significantly.
When applying the higher symbol rate, more power is needed from the satellite (or if the same power is kept the resulting carrier-to-noise will be slightly lower). Secondly, a lower roll-off causes a bit more degradation.
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QUESTION: Very appreciate your quick answer. Thanks!
If I understand your answer correctly, one can use the formula BW=Rs*ROF (roll-off-factor) to describe the relationship between symbols-rate and transponder bandwidth. In addition, the roll-off-factor in DVB-S is 1.35.
In the other hand when I'm looking in Annex C of the DVB-S protocol datasheet (page 19 of v1.1.2 "examples of bit rates versus transponder bandwidth") I see a table that uses the next relation: BW/Rs=1.28 (column 3 in table C.1).
How these two get along together??
Hi Mr Ziv,
Nice of you for the apreciation.
The Roll of Factor is defined for each standards separately and also the power levels also decide the bandwidth requirements. As I already mentioned every application requires separate calculation based on the Baseband characteristics and types of modulation schemes etc. However to answer specifically on the point you mentioned, the standards for DVB-S and DVB-S2 are different. The table below shows example comparison (From DVB- S2 Standards):
Satellite EIRP 51dBw 51dBw 53.7dBw 53.7dBw
System DVB-S DVB-S2 DVB-S DVB-S2
Modulation&Coding QPSK2/3 QPSK3/4 QPSK7/8 8PSK2/3
SymbolRate(Mbaud) 27.5(α=0.35) 30.9(α=0.2) 27.5(α=0.35) 29.7(α=0.25)
C/N(in27.5MHz)(dB) 5.1 5.1 7.8 7.8
UsefulBitrate(Mbit/s) 33.8 46(gain=36%) 44.4 58.8(gain=32%)
NumberofSDTVProgrammes 7MPEG-2 10MPEG-2 10MPEG-2 13MPEG-2
15AVC 21AVC 20AVC 26AVC
NumberofHDTVProgrammes 1-2MPEG-2 2MPEG-2 2MPEG-2 3MPEG-2
3-4AVC 5AVC 5AVC 6AVC
From the above table you may see that roll of is varying for each standard and also depends on modulation also.
Hope this answers your query.