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LTE NB-IoT NPDCCH Function | LTE NB NPDCCH Physical Layer Processing


This page describes LTE NB-IoT NPDCCH function with location and its basics with respect to LTE-NB IoT Standard. It covers LTE NB NPDCCH Physical Layer Processing and its applications in the system.

Function:
The NPDCCH is used to carry DCI (Downlink Control Information). NB-IoT device extracts following information from NPDCCH channel. Following table-1 lists useful informations related to NPDCCH.
• DCI format N0 with size 23 bits carrying UL grant information
• DCI format N1 with size 23 bits carrying DL scheduling information
• DCI format N2 with size 15 bits carrying SI update (or indicator of paging)

NPDCCH information Value
Subframe Any
Basic TTI 1 ms
Repetitions 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048
Subcarrier spacing 15 KHz
Bandwidth 90 KHz or 180 KHz
Carrier Any

Location:
• NPDCCH subframe is divided into two elements known as NCCEs (Narrowband Control Channel Elements). NCCE0 occupies lower six subcarriers where as NCCE1 occupies upper six subcarriers.
• After cell selection as well as SI acquisition process, NB-IoT device comes to known exact location/mapping of NPDCCH REs in NPDCCH subframe.

NB-IoT NPDCCH mapping examples

• In order to avoid LTE DL control region in the in-band deployment mode, NPDCCH is not being mapped to first few OFDM symbols in the subframe. Due to this index of starting OFDM symbols within NPDCCH subframe depends on LTE DL control region size. Following table-2 lists possible number of REs per NCCEs which ranges from 50 to 80.

Deployment mode # LTE CRS antenna ports # OFDM symbols for LTE control region # NRS antenna ports # REs per NCCE
Stand-alone, Guard band NA 0 1 80
Stand-alone, Guard band NA 0 2 76
In-band 2 1 1 68
In-band 2 1 2 64
In-band 2 2 1 62
In-band 2 2 2 58
In-band 2 3 1 56
In-band 2 3 2 52
In-band 4 1 1 66
In-band 4 1 2 62
In-band 4 2 1 60
In-band 4 2 2 56
In-band 4 3 1 54
In-band 4 3 2 50

LTE NB NPDCCH Physical Layer Processing

A DCI can either be mapped to one NCCE known as AL-1 (Aggregation Level-1) or mapped to both NCCEs in the same subframe known as AL-2. AL-2 is useful in order to increase NPDCCH coverage.
➤DCI is attached with 16 bit CRC.
➤CRC attached DCI is masked with RNTI (Radio Network Temporary Identifier).
➤After CRC attachment and RNTI masking TBCC encoding and rate-matching is applied to generate code word with length equal to available number of encoded bits.
➤QPSK modulation is applied to NPDCCH as a result code word length ranges from 100 to 160 for AL1 and from 200 to 320 for AL-2.
➤Baseband waveform is generated using QPSK symbols as input.

Reference: 3GPP 36 series



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