Comtech EF Data CDM-425, Installation And Operation Manual

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CDM- Advanced Satellite Modem
Revision 0
Appendix N N–2 MN-CDM-425
•
The recommended minimum carrier spacing equals
(1 + Tx
α) x symbol rate
,
assuming equal symbol rates for all carriers and zero frequency error between carriers.
•
At lower symbol rates, particular care needs to be taken with the frequency accuracy and
stability of the RF carriers.
It is your responsibility to determine if sufficient guard
band is built into the frequency plan to avoid carriers moving towards each other
and overlapping at the edges.
Now that you can choose between six different values of Tx
α, it now
becomes important to match the Rx Alpha (Rx
α) accordingly. Comtech
EF Data has analyzed the effect of mismatching Tx
α and Rxα, and has
concluded that there is little benefit in providing six values of Rx
α. It
has been determined that just two values of Rx
α – 0.35 and 0.12 – are
required to match all six Tx cases.
In order to correctly set Rx
α
– this assumes that you have knowledge of the Tx
α
being
transmitted – follow these guidelines:
•
For a Tx
α
of 0.05, 0.10, 0.15, or 0.20, use Rx
α
= 0.12.
•
For a Tx
α
of 0.25 or 0.35, use Rx
α
= 0.35.
If these rules are followed, then the amount of degradation due to mismatch will be < 0.05 dB.
If these rules are not followed, then the amount of degradation due to mismatch can be > 0.3 dB.