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5G NR Uplink Timing Control Procedure | 5G NR Timing Advance

This page describes 5G NR Uplink Timing Control Procedure based on timing advance provided by gNB to UE. The uplink timing control based on timing advance command is applied to PUSCH, PUCCH and SRS in 5G NR. The page mentions messages in which timing advance value is conveyed to the UE by gNB (i.e. 5G NR Base Station).

Introduction:
In wireless cellular systems it is required to adjust timing of the uplink frame in order to have alignment with downlink frame in time scale. As we know uplink frame is transmitted by UE towards gNB where as downlink frame is transmitted by gNB towards UE. gNB is the 5G NR base station. The timing control procedure is initiated by MAC layer and conveyed to the PHY layer for time adjustment.

5G NR uplink downlink frame

The uplink timing control is similar to LTE. UE transmits random access preamble to gNB. The gNB estimate transmission timing correction for the UE and conveys the same to UE using "Random Access Response (RAR)" message. This message contains "timing advance command" used by UE to make adjustment in the transmit timing. Both of these are RRC messages.

5G NR Timing Advance

As shown in the figure-1, one set of frames in the uplink and one set in the downlink is transmitted on a RF carrier. Uplink frame number-i for transmission from UE shall start TTA = (NTA + NTA offset)*Tc before the start of the corresponding downlink frame at the UE where NTA offset depends on frequency band as mentioned in the table below. Here Tc = [1/(ΔFmax * Nf)], Here ΔFmax is subcarrier spacing and Nf is FFT size. Tc is known as basic time unit for 5G NR system.

• Tc = 0.509 ns for ΔFmax = 480 KHz and Nf = 4096., The table below mentions NTA offset for different frequency ranges (FR1/FR2) and topologies (TDD/FDD).

5G NR TA offset

➤ Value of 39936 implies 20.327 µs and 25600 implies 13.030 µs
➤ Value of 13792 implies 7.020 µs

When and how timing advance information reaches UE

• During physical random access procedure, PRACH is transmitted by UE to gNB. The random access response (RAR) message is replied back to UE from gNB. This RAR message contains TPC command which has following meanings.
Refer MAC layer overview >> which explains MAC header, payload.

Contention based RA vs Contention free RA

Following MAC RAR message is response from gNB to the PRACH message transmitted by UE. MAC RAR message is fixed in size. The "Timing advance command" field indicates index value of TA which is used to control amount of timing adjustment which MAC need to apply. The size of "Timing Advance Command" is 12 bits. This message contains "timing advance command" field.

5G NR MAC RAR Message

MAC subheader for RAR message is as follows.

MAC subheader BI

BI (Backoff Index) parameter is 4 bits in size and has mapping (index to ms) as mentioned in the following table.

Index Backoff parameter value (ms)
0 5
1 10
2 20
3 30
4 40
5 60
6 80
7 120
8 160
9 240
10 320
11 480
12 960
13 1920
14 Reserved
15 Reserved


Timing Advance Command MAC CE:
• This message is identified by 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 TA (0, 1, 2, ...,63) used to control the amount of timing adjustment that MAC entity has to apply. The length of the field is 6 bits.

Timing advance command MAC CE

Following is the contents of random access response grant field.

5G NR Random Access Response Grant Fields

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


TPC command value (in dB)
0 -6
1 -4
2 -2
3 0
4 2
5 4
6 6
7 8


References

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


5G NR Numerology | 5G NR Terminology


5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences


5G TECHNOLOGY RELATED LINKS

This 5G tutorial also covers following sub topics on the 5G technology:
5G basic tutorial
5G Frequency Bands
5G millimeter wave tutorial
5G mm wave frame
5G millimeter wave channel sounding
Difference between 4G and 5G
5G testing and test equipments
5G network architecture
5G NR Physical layer
5G NR MAC layer
5G NR RLC layer
5G NR PDCP layer

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