5G NR Uplink Timing Control and Timing Advance

This page describes the 5G NR Uplink Timing Control Procedure based on the timing advance provided by the gNB to the UE.

The uplink timing control based on the timing advance command is applied to PUSCH, PUCCH, and SRS in 5G NR.

This page mentions the messages in which the timing advance value is conveyed to the UE by the gNB (i.e., 5G NR Base Station).

Introduction

In wireless cellular systems, it’s crucial to adjust the timing of the uplink frame to align it with the downlink frame on a time scale. As we know, the uplink frame is transmitted by the UE towards the gNB, while the downlink frame is transmitted by the gNB towards the UE.

gNB is the 5G NR base station. The timing control procedure is initiated by the MAC layer and conveyed to the PHY layer for time adjustment.

The uplink timing control is similar to LTE. The UE transmits a random access preamble to the gNB.

The gNB estimates the transmission timing correction needed for the UE and conveys this information to the UE using a “Random Access Response (RAR)” message. This message contains a “timing advance command,” which the UE uses to adjust its transmit timing. Both RAR messages are RRC messages.

5G NR Timing Advance

As shown in Figure 1, a set of uplink frames and a set of downlink frames are transmitted on an RF carrier. The start of uplink frame number i for transmission from the UE shall occur $T_{TA} = (N_{TA} + N_{TAoffset}) * T_c$ before the start of the corresponding downlink frame at the UE, where $N_{TAoffset}$ depends on the frequency band, as mentioned in the table below.

Here, $T_c = [1/(\Delta F_{max} * N_f)]$, where $\Delta F_{max}$ is the subcarrier spacing and $N_f$ is the FFT size.

$T_c$ is known as the basic time unit for the 5G NR system.

• $T_c$ = 0.509 ns for $\Delta F_{max}$ = 480 KHz and $N_f$ = 4096.

The table below mentions $N_{TAoffset}$ for different frequency ranges (FR1/FR2) and topologies (TDD/FDD).

• A value of 39936 implies 20.327 µs, and 25600 implies 13.030 µs.
• A value of 13792 implies 7.020 µs.

When and How Timing Advance Information Reaches the UE

• During the physical random access procedure, the PRACH is transmitted by the UE to the gNB. The Random Access Response (RAR) message is sent back to the UE from the gNB. This RAR message contains a TPC command, which has the following meanings.

The following MAC RAR message is the response from the gNB to the PRACH message transmitted by the UE. The MAC RAR message has a fixed size.

The “Timing advance command” field indicates the index value of $T_A$, which is used to control the amount of timing adjustment that the MAC needs to apply. The size of the “Timing Advance Command” is 12 bits. This message contains the “timing advance command” field.

The MAC subheader for the RAR message is as follows:

The BI (Backoff Index) parameter is 4 bits in size and has the following mapping (index to ms):

table_start

Timing Advance Command MAC CE

• This message is identified by the MAC PDU subheader with LCID as specified in Table 6.2.1-1.
• It has a fixed size and consists of a single octet defined as follows.
• TAG Identity (TAG ID): This field indicates the TAG Identity of the addressed TAG. The TAG containing the SpCell has the TAG Identity 0. The length of the field is 2 bits.
• Timing Advance Command: This field indicates the index value $T_A$ (0, 1, 2, …, 63) used to control the amount of timing adjustment that the MAC entity has to apply. The length of the field is 6 bits.

The following are the contents of the random access response grant field.

The following table mentions the meaning of the TPC command field in dB value. This field is used for uplink power control in 5G NR.

table_start

References

• 3GPP TS 38.211
• 3GPP TS 38.213
• 3GPP TS 38.321
• 3GPP TS 38.133