Frequency Planning Guard Band & DID Number Iss

Have a look in the technicial specifications of your modems for something called "filtering roll-off". There may be several figures for different alpha values, or your modem may come with one alpha value only.

The sharper the roll off, the narrower the bandwidth, but the tighter filtering distorts the signal more and make the BER worse. The chosen value is a compromise. Ask your modem manufacturer if you want to to learn more.

Suggested carrier spacing is:
alpha=0.2 carrier spacing = 1.2 x symbol rate
alpha=0.25 carrier spacing = 1.27 x symbol rate
alpha=0.35 carrier spacing = 1.4 x symbol rate

If you are working with very small carriers allow for frequency error also.

If you lease a block of transponder bandwidth, the inter-carrier spacing, within your bandwidth is really up to you. If you can accept more adjacent carrier interference then put them closer together. If you have carriers with high and low power spectral densities adjacent to one another you might wish to move them further apart.

At the edges of your leased transponder bandwidth the satellite operator will decide and tell you what spacing to use between the edge of your bandwidth and the centre frequency of your edge carrier, as they must protect the adjacent customer from unacceptable interference.

If you operate your high power amplifier (HPA) near saturation the bandwidth of the carrier will enlarge and sidelobes appear, called spectral regrowth. Use a carrier spacing of 1.4 x SR if in any doubt.

I've no idea what "DID" means.

Best regards, Eric.

You need to read your iDirect documentation.

This extract from an old document may still be valid:
From "Technical Reference Guide iDS Release 7.x" page 107
"For example, consider an iDirect network with a user data (information) rate of 5Mbps on the downstream and three upstream carriers of 1Mbps each. FEC rate 0.793 with 4kbit TPC is used for all carriers in this example. Figure 32 on page 107 shows that an additional Upstream carrier may be added by reducing the channel spacing of the existing carriers.
"

You must start with the bit rate, FEC ratio and modulation method used on each of your carriers. I suggest you make your own spreadsheet to work out the centre frequencies of each carrier.

Example calculation (second carrier, "Upstream #1", lower figure):
Information bit rate = 1 Mbit/s
FEC = 0.793
Transmission bit rate = 1 / 0.793
Symbol rate = 1 / (0.793 * 2) , assuming QPSK
Carrier spacing = 1.2 x Symbol rate = 756.62 kHz.

At the edges of your leased transponder bandwidth your satellite operator may insist on a 1.4 factor being used.

The iDirect hardware/software may have a minimum incremetal step size applicable to setting the frequencies.

A larger gap between your outlink and the adjacent return link may be appropriate if the spectral densities differ significantly.

Read the manuals that came with your iDirect hub.  If it is 5 years old and has not been upgraded you need the old manuals. 

Contact iDirect tech support for latest updates, applicable to any new software updates or new equipments.

For general introduction to iDirect start here:
https://www.idirect.net/support-and-training/