A UE does not send its uplink frame at the moment it receives the downlink one. It sends earlier, by an amount the network controls. The purpose is that every UE in the cell arrives at the base station at the same time, whatever its distance. This page covers how much earlier, and how precisely the UE has to match it.

• Uplink Timing Advance
• Timing Error Limit
• Reference

Uplink Timing Advance

Two quantities set the advance, and they arrive from different places. The first is the timing advance the network sends, and it changes as the UE moves. The second is a fixed offset that depends only on the band, and the UE looks it up rather than being told it each time.

The picture puts the two frames one above the other, with the uplink frame drawn earlier than the downlink frame by that advance. Underneath the frames it gives the formula, the offset table the second term comes from, and the two ways the timing advance itself reaches the UE.

< 38.211 - Figure 4.3.1-1: Uplink-downlink timing relation >

The advance is a whole number of T_c, so one expression covers every numerology. What the numerology changes is the step the network can command, not the unit it is counted in.

• The advance has two terms : (N_TA + N_TA,offset) multiplied by T_c, where T_c is 0.509 ns.
• The offset is a property of the band : 25600 for an FR1 band without LTE-NR coexistence, 0 for FR1 FDD with it, 39936 for FR1 TDD with it, and 13792 for FR2.
• The default is 25600 : a UE not given n-TimingAdvanceOffset assumes that value on an FR1 band.
• RAR sets, MAC CE adjusts : RAR fixes N_TA outright with T_A from 0 to 3846, and MAC CE moves the existing value with T_A from 0 to 63.
• 31 is the centre of the MAC CE range : the correction carries (T_A - 31), so a value under 31 reduces the advance and a value over 31 increases it.

Timing Error Limit

The advance tells the UE when to transmit. It does not say how accurately the UE has to manage it, and that is a separate requirement. 38.133 states the allowed error as Te. The value depends on the subcarrier spacing of the SSB the UE measured, and on the spacing of the uplink it sends.

The table is read left to right, and three things index it. The frequency range comes first, then the subcarrier spacing of the SSB used as the timing reference, and then the subcarrier spacing of the uplink signal being transmitted.

< 38.133 v1.0.0 - Table 7.1.2-1: Te Timing Error Limit >

A wider subcarrier spacing allows a tighter limit, because the symbol is shorter and the same fraction of it is less time. The tightest FR2 entry is a quarter of the loosest FR1 entry.

• Every entry is a multiple of 64·T_c : the limits are counted in the same basic unit as the advance itself.
• Both spacings index the table : the SSB spacing and the uplink spacing are separate columns, and the pair selects the row.
• FR1 runs from 12 to 7 : 12·64·T_c at 15 kHz SSB with 15 kHz uplink, down to 7·64·T_c at 30 kHz SSB with 60 kHz uplink.
• FR2 runs from 3.5 to 3 : both of its entries are tighter than anything in FR1.
• The square brackets are a warning : in the version shown they mark values still under discussion, so check the current 38.133 before using any of them.

Reference

[1] 38.211 v19.4.0 : NR - Physical channels and modulation. Figure 4.3.1-1 is the uplink-downlink timing relation drawn above.

[2] 38.213 v19.4.0 : NR - Physical layer procedures for control. Clause 4.2, Transmission timing adjustments, holds both timing advance formulas.

[3] 38.133 : NR - Requirements for support of radio resource management. Table 7.1.2-1 gives Te and Table 7.1.2-2 gives N_TA,offset. The two images above are taken from v1.0.0 and v15.6, and the current published version is v20.0.0.