Since many physical layer things (e.g, sub carrier spacing, OFDM symbol length) varies in 5G/NR, you would see many physical layer parameters are specified by equations and variables instead of specific constants values. It would be good to have a few fundamental timing units that are frequently used in various part of physical layer specification. In this note, I am trying to collect a list of basic timing units being used in NR specification.

38.211 - 4.1 General defines following Time Units.

This (Tc) is basic timing unit in NR. The NR basic timing unit is defined to be the shortest/smallest sampling time possible which is the case of widest subcarrier spacing in NR as of release 15 specification (NOTE : from release 17, even wider subcarrier spacing is specified like subcarrier spacing 960. But this Tc definition is still remain same).

This (Ts) is another type of timing unit specifically for the case where the subcarrier spacing is 15Khz.  This corresponds to the sampling rate for LTE 20Mhz BW

This (Κ, Kappa) is the ratio between Ts and Tc. This is mostly used in the specification of NR PRACH format as shown here.

38.211 - 4.3.1   Frames and subframes defines following Time Units.

T_f stands for frame duration (the length of one radio frame) which is defined as follows. You would notice this is same as the radio frame duration of LTE.

T_sf stands for subframe duration (the length of one radio frame) which is defined as follows. You would notice this is same as the subframe duration of LTE.

Reference

[1]