LTE NB-IoT NPRACH: Function, Formats, and Preamble Structure

This page describes the LTE NB-IoT NPRACH (Narrowband Physical Random Access Channel) function, its location, and its basics with respect to the LTE-NB IoT Standard. It covers LTE NB NPRACH formats, specifically format 0 and format 1. It also describes the NPRACH repetition unit and preambles.

Function of NPRACH

The NB-IoT NPRACH is used by a device to initialize a connection and to allow the serving eNB (evolved Node B, i.e., base station) to estimate the ToA (Time of Arrival) based on the received NPRACH signal. ToA estimation helps determine the round trip time between the base station and the devices.

NB-IoT utilizes SC-FDMA (Single-Carrier Frequency-Division Multiple Access), which is similar to OFDM (Orthogonal Frequency-Division Multiplexing). The NB-IoT Base Station needs to align multiple received signals from NB-IoT devices, and ToA estimation aids in accomplishing this task. NPRACH configuration information is provided in System Information (SI) messages from the eNB. Typical parameters include the number of repetitions, the number of NPRACH preambles, the time periodicity, and the signal level threshold.

Table 1: Basic Parameters of NB-IoT NPRACH Channel

The table above mentions basic parameters of the NB-IoT NPRACH channel.

LTE NB NPRACH Formats and Configurations

NPRACH is a newly designed channel compared to the LTE PRACH, which uses 1.08 MHz. NPRACH uses less bandwidth, specifically 180 KHz. NPRACH preambles utilize single-tone transmissions with frequency hopping.

NB-IoT NPRACH supports two formats: format-0 and format-1, as shown in the figure above. Let’s understand the difference between them:

• NPRACH format-0: Uses a CP (Cyclic Prefix) duration of 66.67 µs and is used for a cell radius of up to 10 Km.
• NPRACH format-1: Uses a CP duration of 266.7 µs and is used for a cell radius of up to 40 Km.

NPRACH Preamble Repetition Unit and Frequency Hopping

As shown in the figure above, one NPRACH preamble consists of 4 symbol groups, with each symbol group having 1 CP and 5 symbols. The frequency index of the symbol groups follows a frequency hopping pattern. There are 4 possible deterministic hopping patterns within an NPRACH repetition unit, as mentioned in the table below. To support coverage extension, the NPRACH preamble is repeated up to 128 times.

Table 2: Deterministic Hopping Patterns

The deterministic hopping patterns create 12 orthogonal preamble sets within a repetition unit, as shown in the following table.

Table 3: Orthogonal Preamble Sets

Reference: 3GPP 36 series LTE