5G NR PRACH: Function, Contents, and Physical Layer Processing

This page delves into the function, contents, mapping, and physical layer processing of the 5G NR PRACH (Physical Random Access Channel).

5G NR PRACH Function, Contents and Mapping

• PRACH’s Role: PRACH is used to carry the random access preamble from the User Equipment (UE) to the gNB (i.e., 5G NR base station).

• Uplink Timing Adjustment: It assists the gNB in adjusting the UE’s uplink timings, among other parameters.

• Zadoff-Chu Sequences: Similar to LTE, 5G NR random access preambles are generated using Zadoff-Chu sequences.

• Sequence Lengths and Formats: Unlike LTE, 5G NR random access preamble supports two different sequence lengths with various format configurations, as illustrated in the figure. These diverse formats cater to a wide range of deployment scenarios.

  • Long Sequence (Length 839): Employs four preamble formats, akin to LTE. These are designed for large cell deployments in Frequency Range 1 (FR1, sub-6 GHz range) and use subcarrier spacings of 1.25 kHz or 5 kHz.
  • Short Sequence (Length 139): Utilizes nine preamble formats and is designed for small cell deployments, including indoor coverage. These preamble formats are used in both FR1 (sub-6 GHz) and FR2 (mmWave) ranges. In FR1, it supports 15 or 30 kHz subcarrier spacing, while in FR2, it supports 60 or 120 kHz.

5G NR PRACH Physical Layer Processing

• FFT Usage: PRACH utilizes the same Fast Fourier Transform (FFT) as used for data.
• OFDM Baseband Signal Generation: The generation of the OFDM baseband signal for PRACH is defined in 3GPP TS 38.211 section 5.3.2.
• Troubleshooting PRACH Issues: Engineers often encounter situations (test cases) where the UE does not receive a response from the gNB for the PRACH message it transmitted. In such cases, they need to analyze the test case across various layers, including the radio link, physical layer (L1), and upper layer messages. This helps pinpoint the root cause of the problem.