could any body help me on how to calculate the BW/ remote in an iDirect Star network? if I give the customer 256/128kbps with 1:10/1:20 contention in the downstream and upstream respectively (QPSK with .66FEC in upstream and .793 FEC in downstream and 40% spacing) what is the consumed BW (in MHz) by the customer? please elaborate
256k is the outlink information rate. FEC = 0.793 so transmission bit rate is 256/0.793 = 323 kbit/s QPSK has 2 bits per symbol so symbol rate = 323/2 = 162 kps (symbols per second) The allocated bandwidth of the carrier is approx 1.4 x symbol rate. Transponder bandwidth needed = 227 kHz.
Similarly the return link (upstream).. 128k is the return link information rate. FEC = 0.66 so transmission bit rate is 256/0.66 = 191 kbit/s QPSK has 2 bits per symbol so symbol rate = 191/2 = 96 kps (symbols per second) The allocated bandwidth of the carrier is approx 1.4 x symbol rate. Transponder bandwidth needed = 134 kHz.
Note: The satellite operator will decide the actual bandwidth needed, to protect adjacent transponder leases. They will almost certainly accept a 1.4 figure (as above), but might dispute 1.2 (i.e. a rather close spacing) if you were to suggest it. A factor of 1.35 is often used. I would suggest an extra 10 kHz or so for frequency errors, which is relevant for such very small carriers.
The average bandwidth associated with each customer is 1/10 or 1/20 of the above if shared evenly over say one month. It depends how many are sharing and whether or not you force some fair sharing policy. If the system is active, at any instant one customer is using the full download capacity and one customer using the full upload capacity. If you have a free-for-all and one customer is dominant and tries to send/receive all of the time then that customer will get more than their 1/10th or 1/20th share.
