[srsran-users] 5G NR "Phase compensation"

[Sylvain Munaut]

Hi,

I'm trying to understand what the 5G "phase compensation" thing is and
what it's for.

I see in the implementation (
lib/phy/lower/modulation/phase_compensation_lut.h ) the reference to
TS38.211 Section 5.4.
But I must admit I don't find that section to be super explicit, so I
have a couple of questions :

1) What is it for ? What's the purpose of that "special" up conversion
instead of the way it's done in 4G for instance ?

2) My understanding of where this comes from ( from the eq in 5.4 to
the implementation ) is that :
   - In the formula the phase of the exponential used to up-convert
"resets" at every symbol
   - But of course in the downconversion done in hardware by the
mixer, that's not the case, phase would be continuous, so that's where
the phase shift at every symbol comes from.

Is my understanding correct ?
And if it is, something I still don't get is that the same kind of
discontinuity would also happen between sub-frames ( i.e going from
symbol N_sym-1 of subframe 0 to symbol 0 of subframe 1 for instance ).
But I don't see that being accounted for, AFAICT it just applies a
static LUT to each symbol in a subframe.

Thanks for any light anyone could shine on the matter.

Cheers,

     Sylvain