I think a little more detailed precise answer to John's question might
help:
John Andrews wrote:
> each complex sample that enters the
> USB bus is the following,
>
> x[i] = (inphase_component) + j (quadrature_component), and
> x[i] = m(t)cos( 2*pi*FREQ_OFFSET*t + PHI ) + jm(t)sin(
2*pi*FREQ_OFFSET*t +
> PHI ), where m(t), is the actual message signal, FREQ_OFFSET is the
> frequency offset, and PHI is the phase.
>
> Is that correct?
I think you're confusing the baseband and passband signals a little, and
the equations aren't quite right.
The complex-baseband signal (your message) is the data that is transferred
across the USB channel.
x[i] = (in-phase) + j*(quadrature)
= (x_i) + j*(x_q)
These are samples of your message signal, after modulation (mapping to a
complex QAM-constellation, for example), coding, pulse-shaping, etc.
The signal is up/down converted on the USRP device such that the
transmitted RF signal is
r(t) = x_i*cos(2*pi*f_c) - (x_q)*sin(2*pi*f_c)
(where f_c is your RF carrier frequency, and I'm ignoring phase offsets
and noise)
Notice the subtraction there (which comes from the trig identities) and
that all the terms are real (it's a real passband signal).
Hope that helps a little.
Patrick Sisterhen
National Instruments_______________________________________________
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