I/Q modulation and phase noise sensitivity

>> I-imag(hilbert(Q))
>
>If I change my code as follows, result get worse by precicely 3dB (but
>maybe because I got something wrong before):

I have a theory where the 3dB loss come from. Your method of modulation is
DSB-SC (Double Sideband Suppressed Carrier). By using an SSB demodulator,
like I suggested, to demodulate the signal, only one sideband is used and
so half of the transmitted power is thrown away. That may be the reason
for the 3dB loss.

I believe the correct way would be to implement a DSB-SC demodulator which
is, from what I understand, a bit more complicated. I have never done this
but I think it involves finding the original phase of the signal, using a
Costas loop for example. When phase-locked, the signal can be extracted
from the I-branch and the Q-branch should be 0.

That being said, if in your simulations you make sure that the phase noise
has an average of 0, the system should behave as if it's phase locked.
That would then imply that you can extract the original signal directly
from the I-branch and that the Q-branch can be ignored. I think it was
something like that you started out with...

In a real-world implementation though, you could of course never assume
that the DSB-SC signal is magically phase-locked so something like a
Costas loop is needed to recover the phase.

/ Tobias

>
>Thanks again!
>Peter
---------------------------------------
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sm0svx <107472@DSPRelated> wrote:

>I have a theory where the 3dB loss come from. Your method of modulation is
>DSB-SC (Double Sideband Suppressed Carrier). By using an SSB demodulator,
>like I suggested, to demodulate the signal, only one sideband is used and
>so half of the transmitted power is thrown away. That may be the reason
>for the 3dB loss.

>I believe the correct way would be to implement a DSB-SC demodulator which
>is, from what I understand, a bit more complicated. 

Why not use both a SSB transmitter and an SSB receiver?

Or more complicatedly, a vestigal-sideband system.


Steve