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LTE NB-IoT NPRACH Function | LTE NB NPRACH formats


This page describes LTE NB-IoT NPRACH function with location and its basics with respect to LTE-NB IoT Standard.It covers LTE NB NPRACH formats viz format 0 and format 1. It describes NPRACH repetition unit and preambles.

Function:
NB-IoT NPRACH is used by device to initialize connection and to allow serving eNB to estimate ToA (Time of Arrival) based on received NPRACH signal. ToA estimation gives round trip time between Base station and devices. NB-IoT uses SC-FDMA which is similar to OFDM. NB-IoT Base Station requires to align received multiple signals from NB-IoT devices and ToA estimation helps to fulfill this task.

NPRACH configuration information is provided in SI messages from eNB. Typical parameters include number of repetitions, number of NPRACH preambles, time periodicity, signal level threshold etc. Refer NB-IoT RACH Procedure which describes messages exchanged between NB-IoT device and eNB (i.e. Base Station).

NPRACH information Value
Subframe Any
Basic TTI 5.6 ms or 6.4 ms
Repetitions 1, 2, 4, 8, 16, 32, 64, 128
Subcarrier spacing 3.75 KHz
Bandwidth 3.75 KHz
Carrier Anchor

The table-1 above mentions basic parameters of NB-IoT NPRACH channel.

LTE NB NPRACH Formats and configurations

NPRACH formats

NPRACH is newly designed compare to LTE PRACH which uses 1.08 MHz. NPRACH uses less bandwidth which is 180 KHz. NPRACH preambles use single tone transmissions with frequency hopping. NB-IoT NPRACH supports two NPRACH formats viz. format-0 and format-1 as shown in the figure-1. Let us understand difference between them.
➤NPRACH format-0 uses CP duration of 66.67 µs and used for cell radius of upto 10 Km.
➤NPRACH format-0 uses CP duration of 266.7 µs and used for cell radius of upto 40 Km.

NPRACH frequency hopping

As shown in the figure-2, one NPRACH preamble consists of 4 symbol groups with each symbol group having 1 CP and 5 symbols. Frequency index of symbol groups follow frequency hopping pattern as shown. There are 4 possible deterministic hopping patterns within NPRACH repetition unit as mentioned with table-2 below. In order to support coverage extension, NPRACH preamble is repeated upto 128 times.

Index of tone used by first symbol group Deterministic hopping pattern within a repetition unit
0, 2, 4 {+1, +6, -1}
1, 3, 5 {-1, +6, +1}
6, 8, 10 {+1, -6, -1}
7, 9, 11 {-1, -6, +1}

The deterministic hopping patterns create 12 orthogonal preamble set within a repetition unit as mentioned in table-3 below.

NPRACH preamble Time index k(l) for symbol group "l"
0 0 1 7 6
1 1 0 6 7
2 2 3 9 8
3 3 2 8 9
4 4 5 11 10
5 5 4 10 11
6 6 7 1 0
7 7 6 0 1
8 8 9 3 2
9 9 8 2 3
10 10 11 5 4
11 11 10 4 5

Reference: 3GPP 36 series



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