Terminology » Mobile Communication » LTE NB-IoT NPDCCH: Function and Physical Layer Processing

LTE NB-IoT NPDCCH: Function and Physical Layer Processing

lte nb-iot npdcch physical layer dci

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

Function

The NPDCCH (Narrowband Physical Downlink Control Channel) is used to carry DCI (Downlink Control Information). The NB-IoT device extracts the following information from the NPDCCH channel:

  • 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)

The following table lists useful information related to NPDCCH.

NPDCCH informationValue
SubframeAny
Basic TTI1 ms
Repetitions1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048
Subcarrier spacing15 KHz
Bandwidth90 KHz or 180 KHz
CarrierAny

Location

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

NB-IoT NPDCCH mapping examples

  • In order to avoid the LTE DL control region in the in-band deployment mode, NPDCCH is not being mapped to the first few OFDM symbols in the subframe. Due to this, the index of starting OFDM symbols within the NPDCCH subframe depends on the LTE DL control region size.

The following table lists the possible number of REs per NCCE, 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 bandNA0180
Stand-alone, Guard bandNA0276
In-band21168
In-band21264
In-band22162
In-band22258
In-band23156
In-band23252
In-band41166
In-band41262
In-band42160
In-band42256
In-band43154
In-band43250

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 a 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 are applied to generate a codeword with a length equal to the available number of encoded bits.
  • QPSK modulation is applied to NPDCCH as a result codeword length ranges from 100 to 160 for AL1 and from 200 to 320 for AL-2.
  • The baseband waveform is generated using QPSK symbols as input.

Reference: 3GPP 36 series LTE