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Frequency Planning Guard Band & DID Number Iss

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Ex Member
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Jan 30th, 2013 at 7:12am  
Hi everyone i am new bie in this forum.

1. Can anyone Explain me the guard Band calculation or formula . i mean how many B/W is needed one carrier to next carrier.

2. What is DID number? How it is work & why we need DID number?

Thanks.
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Eric Johnston
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Reply #1 - Jan 30th, 2013 at 10:01am  
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.  
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Ex Member
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Reply #2 - Jan 30th, 2013 at 10:56am  
Eric we have 3 MHz bandwidth last 5 yrs  and we take 4 inroute group with 9 M0D1 Card. But recently we increase 1 MHz bw and also one M0D1 card. but we thinking about Guard Band because we don't know about it. Previous days Idirect People configured our HUB system. now i am seeing from ibuilder that they keep guard band between 2 carrier. all guard band value are not same. some are 32 Khz and some are 17 khz etc. we are using x3 modem.

* DID means "Derived ID". but what is its necessary?

Eric, Do you have any Excel Program that represent inbound and outbound carrier like spectrum analyzer. if you have, can you share it with me.


Thanks for your Quick Reply Eric.
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Eric Johnston
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Reply #3 - Jan 30th, 2013 at 11:36am  
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.
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Reply #4 - Jan 30th, 2013 at 12:36pm  
Dear Eric,
Many many thanks to you. your give your more valuable time. Eric i want to learn idirect satellite communication System but i want to start from 0 level. what is your suggestion? can you give me any document for this?


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Eric Johnston
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Reply #5 - Jan 30th, 2013 at 1:34pm  
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:
http://www.idirect.net/Training-and-Services/Training/Tutorials.aspx
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