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Channel spacing - terrestrial - point to point

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Nov 15th, 2010 at 8:16am  
How does a designer choose the channel spacing in a point to point terrestrial network ?
                                 
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« Last Edit: Dec 3rd, 2010 at 12:52pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Nov 15th, 2010 at 10:16am  
Research existing available equipment and also the applicable terrestrial radio regulations and national and international standards.

Do you want your equipment to be compatible with other manufacturers equipment ? If so, follow the popular standards.

In satellite communications, transponder bandwidth is a limited resource and special effort is made to reduce channel spacing, 1.2 in the case of some iDirect equipment.  1.4 is a more regular starting point, involving less aggressive filtering, less distortion of the signal and less adjacent carrier interference.  You choose; the tighter the spacing the more efficient use of the bandwidth but the penalties of distortion and interference will require more Eb/No power for same BER. The usual trade off...

Further comments welcome..

Best regards, Eric.
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Reply #2 - Nov 17th, 2010 at 4:09am  
Eric thank you for your response. Is there any direct calculation which shows the penality of distortion and interference will require more Eb/No power for same BER ? Please try to provide document.
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Eric Johnston
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Reply #3 - Nov 17th, 2010 at 8:05am  
I am not aware of specific documnents. Maybe some expert on modulation/coding can help ?

Regarding filtering versus difficulty in decoding the carrier, this is the sort of thing that some university graduate will have studied and reported on.  Some companies may also have released such studies.  Ask iDirect about their exceptional 1.2

Regarding adjacent carrier interference and difficulty in decoding the carrier, the adjacent carrier interference varies according to the level of the adjacent carrier and frequency error.  The overall effect of such interference also varies according to the rest of the link budget C/Ns and on how you allocate the C/Ns in the link budget.

The link budget is not some precise calculation, all the C/Ns have probability statistics associated with them and as a designer you have to make judgements about what is reasonable.   A spacing of 1.4 is normal and safe. Spacings of 1.35 may work OK and save some money. I would not use a spacing of 1.2 unless the modem manufacturer recommended it, the frequency stability of my system was adequate and the adjacent carrier levels similar and under my control.  

Best regards, Eric.
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Reply #4 - Nov 19th, 2010 at 3:52am  
Thank you Eric for prompt response. Please give me clarification of 1.2 spacing,1.3 spacing and 1.4 spacing. I understood that 1.2 times the (X frequency). What is the X frequncy. Please elaborate the fundamental more. Then i can easy to analyze.
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Reply #5 - Nov 19th, 2010 at 7:51am  
I meant 1.4 x symbol rate.

Type symbol rate channel spacing into Google and read much more.

This result shows channel spacing for a variety of terrestrial wireless systems:
http://www.rfcafe.com/references/electrical/wireless-comm-specs.htm

Best regards, Eric
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« Last Edit: Nov 19th, 2010 at 11:20am by Admin1 »  
 
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Reply #6 - Nov 20th, 2010 at 10:07am  
Thanks...
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Reply #7 - Dec 2nd, 2010 at 5:31am  
Dear Eric,

You are talking the carriers are in multi channel environment. If i have a single carrier and i want to select single carrier at a time what could be the channel spacing i have to take in such kind of environment.

Regards,
G.S.SRIKANTH
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« Last Edit: Dec 3rd, 2010 at 12:53pm by Admin1 »  
 
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Reply #8 - Dec 3rd, 2010 at 12:14pm  
Yes, I was referring to channel spacing in a multi carrier environment.  If you are transmitting/receiving one carrier only you will still need to be careful to avoid adjacent carrier interference to/from other operators, now and in the future.

If no one is operating on the adjacent frequencies follow whatever rules are set by your terrestrial radio frequencies regulatory agency. They may licence adjacent frequencies to other people who may start operating next week and either interfere with you or you interfere with them. It is most likely they will apply a 1.4 x symbol rate rule, but you need to check as in some cases the centre frequencies and carrier symbol rates may already be planned and specified, so all you can do is to choose a presently unused a carrier frequency slot.  Talk to your radio frequency regulatory agency - they are there to help you and everyone else get the best use out of the available spectrum.

Best regards, Eric.

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