[srsran-users] Time alignment thresholds for Time Advance (srsENB)

[Jonathan Pichel Carrera]

Hi everyone,

I have some doubts about the implementation of the Time Advance mechanism
in the srsENB code.

>From what I see, when in *state_measure*, the TA state machine assigned to
each user collects *ta_err* values obtained from the PUCCH and PUSCH and
stores them in an internal buffer. Once enough time has passed in
*state_measure* and it has received a minimum number of values, it checks
if the average of the stored values exceeds a certain absolute error
threshold. This threshold is 16 times the sampling period. If it exceeds
this threshold, the ENB sends a TA command in differential mode (encoded in
6 bits) to the UE.

I understand that the *ta_err* is the time alignment error of the user's
transmission with respect to the assigned slot. So if *ta_err* = 16 Ts,
that means that the time slot has arrived 16 Ts later than the start of the
slot and is occupying the entire guard interval. If my understanding of the
code is correct, it raises a question in my mind about the assignment of TA
values:

Let's say a user has a TA absolute value of 1. Given that the spatial
resolution of the TA command is about 70 meters (78.12 to be exact), this
implies that the user is at a distance of 70-140 meters from the ENB.

Let's assume the user is exactly 70 meters away from the base station.
Disregarding multipath effects, in this situation the *ta_err* recorded by
the base station would be 0, since the user's transmission is being
received exactly at the allotted time.

1) The user starts to move away from the base station, so the ta_err will
be increasing (positive values). When he reaches 140 meters, his ta_err
will be exactly 16 Ts, and the base station will end up sending a
differential TA command of 1, so the absolute value of TA will be 2
(140-210 zone).

2) The user starts approaching the station again from 210 meters. Now his
ta_err will be decreasing taking negative values as his transmission will
arrive before the slot assigned by the ENB, since he is closer to 140
meters. When it arrives back at 70 meters, its ta_err will be exactly -16
Ts and the base station will order to return to the TA of 1 (70-140 meters
zone).

In the first case I understand that the behavior is correct, since the
terminal ends up using the TA of 1 through the entire 70-140 meter zone.
However, in the second case, the terminal will have passed through the
entire zone of TA=1 with a TA=2.

If this is so, doesn't this mean that during the second case the user's
transmission will enter the guard interval of the previous slot? Could this
cause problems in a multi-user environment, in case the previous slot is
being occupied by another user?

Thanks,
Jonathan
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