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

[Xavier Arteaga]

Hi Sylvain,

> Although I'm a bit curious when you say "TS38.211 Section 5.4 says"
> ...
> they're telling us that" ?

You are right, the section does not say much and I could rephrase the
sentence.

After reading Section 5.4 paragraph once again, `... OFDM symbol *l* in a
subframe assumed to start at t = 0 ...`. I understand that all subframes
start with *t=0*, which implies that the phase of the first symbol within
each subframe is the same.

As you mentioned in your first message, our current implementation
compensates the phase relative to the beginning of the subframe using a
LUT. It looks that there is some sort of gap between subframes.

However, correct me if I am wrong, considering that subframes are 1ms long
and the NR-ARFCN raster is multiple of 1kHz, the phase is repeated for
every subframe and there is no "phase gap".

Regards,
Xavier

On Fri, 3 Mar 2023 at 10:32, Sylvain Munaut <246tnt at gmail.com> wrote:

> Hi Xavier,
>
> Thank you very much for your reply.
>
> > If the phase compensation was not in place, the UE would see a different
> phase in each OFDM symbol. The phase shift is relative to the difference of
> center frequencies between the gNb and itself and it would be difficult to
> detect.
>
> Oh, yeah, I think I see.
> And the f0 in that equation would be the center frequency that you
> tuned to (i.e. the center of the grid you're decoding / FFT you're
> doing ) which doesn't necessarily match the center of the cell or
> whatever the gNB used in its RF chain.
>
>
> > To conclude, as TS38.211 Section 5.4 says, the phase compensation is
> applied relative to the beginning of the subframe. This makes each of the
> OFDM symbol phases within a subframe coherent for transmission and
> reception, independently of the resource grid center frequencies.
>
> Ah yeah, you're not supposed to make any assumption about the phase of
> things across subframes anyway right.
>
> Although I'm a bit curious when you say "TS38.211 Section 5.4 says"
> because maybe I'm not looking at the right PDF but my copy of section
> 5.4 is really just one sentence and 3 equations with like no
> explanation whatsoever, is there some more detailled spec I'm missing
> ? :)   Or did you mean "they say" as in "through those equations
> they're telling us that" ?
>
> Cheers,
>
>     Sylvain
>
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