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Re: commutating antennas (was Re: Canted S-band antennas on satellite)

>If you are using a digital mode where phase is of importance, the PLL
>will be pulled with this phase modulation. The demod PLL's will have
>to be designed to track thru this modulation. I'm sure Karn will
>suggest that proper application of coding/interleaving can make the
>data losses be tolerable.

Yes, I can suggest this. But as Franklin has already pointed out, if
the phase change rate is slow with respect to the data rate, then the
carrier phase tracking PLL in the demodulator can easily follow it.

But if there is a sudden RF phase discontinuity when antennas are
switched, then a PLL will take some time to respond. During that time,
demodulated data will be unreliable. Then you'd fall back to coding
and interleaving to ride it through.

Widening the PLL loop bandwidth would permit faster phase tracking,
but it would also let in more noise and result in a noisier carrier
reference and poorer overall demodulator performance.

You might try to size the antenna array such that the phase jumps are
close to an integral number of RF wavelengths. Then the PLL would not
have to jump much at all.

If the antenna switching phase jumps cannot be controlled, and if they
must occur often relative to the symbol rate (i.e., more than once
every several hundred channel symbols for BPSK modulation), then
differentially-coherent detection could be used. Then a phase jump
could only corrupt at worst two adjacent channel symbols, and again
the interleaving and coding could easily handle it.

This is the approach I've taken with my prototype demodulator/decoder
for my proposed AO-40 FEC format. There are probably some rapid
carrier phase jumps associated with the off-axis spin fading, as RF
from the S-band antenna glints randomly off various objects on the
surface of the spacecraft as it spins.

But I do pay a penalty of 3-4 dB for this capability compared to a
fully coherent BPSK demodulator on a non-fading channel, so it's still
a good idea to try to eliminate uncontrolled phase jumps in any
commutating antenna array, if at all possible.


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